Design of a dusty-gas shock-tube facility with preliminary experimental results

May 1987

DESIGN OF A DUSTY-GAS SHOCK-TUBE FACILITY

WITH

EXPERIMENTAL RESUL TS

by

W. Czerwinski, R. L. Deschambault, and G. D. Loek

. TECH , CHE UNIVER, .TEIT

DELFT

BIBLIOTHEEK

KluY'lerweg

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DELFT

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4 NOV. 1987

UTIAS Technical Note No. 263

CN ISSN 0082-5263

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.

DESIGN OF A DUSTY-GAS SHOCK-TUBE FACILITY

WITH PRELIMINARY EXPERIMENTAL RESULTS

by

W. Czerwinski, R. L. Deschambault, and G. D. Loek

Submitted April 1987

©

Institute for Aerospace Studies 1987

May 1987

UTIAS Technical Note No. 263

Acknowledgements

We are pleased to express our thanks to Professor l.I. Glass. Many thanks are extended to Messrs. D. Wilmut, A. Morte, D. Yan, A. Perrin, V. Levinson, Z.H. Cao, and W. Davies for their technical assistance provided during various phases of the design, construc-tion, installaconstruc-tion, instrumentation development, and calibration of the UTIAS dusty-gas shock-tube facility. Furthermore, we would like to thank Messrs. J. Bradbury, H. Schumacher, K. Bopp, and J. Toni-gold of the UTIAS machine shop for their efforts with the construc-tion and installaconstruc-tion of the shock tube and some of its ancilliary equipment.

The financial assistance received from the Defence Research Establishment Suffield, Ralston, Alberta, the Natural Sciences and Engineering Research Council of Canada, the U.S. Air Force under grant AF-AFOSR-87-0124, and the U.S. Defence Nuclear Agency under contract DNA OOl-85-C-0368, is acknowledged with many thanks.

Summary

The design and development of a new dusty-gas shock-tube facility at UTIAS is described, and initial

experimental results are included. This advanced gas dynamic facility is used to study experimentally the behavior and structure of shock waves propagating in a two-phase gas-partlcle mixture that is initially at atmospheric pressure. Shock waves with amplitudes as high as 3 MPa and flat-tops up to a metre in length can be produced. The dust particles will normally be inert glass spheres varying in size from 1 to 60 microns in diameter, unless combustion is to be studied. Such particles will be injected and suspended 'homogeneously' in air in the horizontal shock-tube channel by means of an initial air flow with induced turbulence. The design and construction features of the shock-tube components, operating and control systems, and safety features are presented.

Instrumentation techniques employed to monitor the initial operation and performance of the dusty-gas shock-tube are described, along with more advanced instrumentation which is needed for conducting meaningful experiments. Preliminary experimental results are presented to illustrate the utility of the dusty-gas shock-tube facility.

- - - --~--- ---Table of Contents Acknowledgements Summary Table of Contents Notation 1. INTRODUCTION

2. SHOCK-TUBE DESIGN AND CONSTRUCTION

2.1 General Description 2.2 Driver 2.3 Channel 2.4 Transition Section 2.5 Test Section 2.6 Dump Tank 2.7 Recoil System 2.8 Dust-Air lnjection 3. CONTROL SYSTEMS

3.1 Driver Gas Mixing

into Channel

3.2 Diaphragm Breaking Mechanisms 3.2.1 Driver plunger system 3.2.2 Driver ignition system

3.3 Control, Pressure, and Vacuum Systems

4. SHOCK TUBE HAZARDS AND SAFETY PRECAUTIONS

4.1 Safety Precautions for Personnel 4.2 Driver Block House

4.3 General Safety Features

iv Page H Hi iv vi 1 3 3 3 4 5 5 6 6 7 9 9 10 10 10 10 11 11 12 12

Table of Contents (continued)

Page

5. OIAPHRAGM MATERIAL, PREPARATION AND CALIBRATION 13

5.1 Cold-Gas Driver 13

5.2 Combustion Driver 14

6. GENERAL OPERATING PROCEDURE 14

6.1 Clean-Gas Mode 14 6.2 Dusty-Gas Mode 15 7. INSTRUMENTATION 16 7.1 Wave-Speed Measurements 7.2 Pressure Histories 7.3 Laser-Doppler Velocimetry 7.4 Light Extinction Technique 7.5 Stagnation Probe

7.6 Beta-Ray Density Gauge

8. EXPERIMENTAL RESULTS

8.1 Wave-Speed Measurements 8.2 Pressure Histories

8.3 Partiele Velocity Measurements 8.4 Light Extinction Measurements

9. CONCLUDING REMARKS 10. REFERENCES Table Figures 16 17 18 19 19 20 20 20 20 21 23 23 24

Notation

Alphanumeric Symbols C contact surf ace DC direct current

F farad, unit of electrical capacitance

Hz frequency, cycles per second

ID inside diameter

m meter, unit of length

mil 0.001 inch (0.0254 mm), unit of leng th M shock Mach number

s

N Newton, unit of force

OD outside diameter Pi pressure of state (i) Pa pascal, unit of pressure R rarefaction wave

RCM Random Choice Method S shock wave

s second, unit of time Ti temperature of state (i)

V volts, unit of elect ri cal potential

Greek Symbol

~ mass loading ratio, i.e., ratio of mass of dust particles per unit volume to the mass of

clean gas per same unit volume

Prefixes M 106 (mega) k 10 3 (kilo) c 10- 2 (cent!) m 10- 3 (milli) 1.1. 10-6 (micro) Subscript

(i) thermodynamic state i

- - - -- - - - -- -- -- - --- - - .

1. INTRODUCTION

In the field of gas dynamics the study of steady and unsteady two-phase flows has become increasingly important. A two-phase flow is defined as a flow field containing two distinct phases. Examples relevent to dusty gas dynamics are moving gases that contains numerous small particles or liquid droplets in suspension. Several examples of such flows found in nature are: rain, sleet and snow storms; clouds; forest-fire and other smoke; dust storms; and dispersed pollutants. Man-made examples include: solid-propellant rocket motors, combustors which use solid-particulate or liquid droplet sprays, and pipe-line transportation of particulate material suspended in either gases or liquids. The effects of dust on an explosion in a dusty-gas environment such as a military battle ground, coal mine or grain elevator are very important to computational fluid dynamicists, because these scientists need fundamental data to validate their complex computer codes which attempt to simulate these flows numerically. Hence, the aim of this work is the design a suitable shock-tube facility to experimentally investigate the basic physics of dusty-gas flows induced by shock waves. The 'dust' consists of small solid par-ticles or liquid droplets which can be either chemically inert or reactive.

Theoretical and experimental studies of gas-particle flows in general are not at all new. They have been rather extensive in scope, as is obvious from books by Soo [1] and Rudinger [2]. However, theoretical studies far outnumber experimentalones, albeit a fair number of experiments have been done. The behavior of water droplets af ter being engulfed by a shock wave has been studied experimentally as early as the 1950s. Shortly thereafter shock waves

travelling through an inert dusty gas and over an inert dusty layer were researched by Rudinger [3], vom Stein and Pfeifer [4], Bracht and Merzkirch

[5], Konig and Frohn [6], Outa, Tajima and Suzuki [7], Sommerfeld and Gronig [8], Suzuki and Adachi [9], and Sommerfeld [10]. It is apparent from this literature that experimental facilities are increasing in number, and that such shock-tube studies with shock induced flows in dusty gases are becoming increasingly common and more sophisticated.

Most of the recent shock-tube experiments at the University of Toronto Institute for Aerospace Studies (UTlAS) involve oblique-shock-wave phenonmena and blast-wave simulation in gases without dust. For example, see Hu [11] and his comprehensive list of references for the former work, which was done in

the UTIAS 10 cm by 18 cm hypervelocity shock tube [12], and see Zhang and Gottlieb [13] for the latter work. A number of numerical studies have also been completed recently at UTIAS for shock-induced dusty-gas flows relevent to shock tubes [14 to 23]. The new UTIAS dusty-gas shock-tube facility will be used to verify such predictions.

In order to describe dusty-gas flows in shock tubes in more detail, consider two companion cases of shock-tube flows without and with dust. A conventional constant-area shock tube consists of two chambers called a driver and a channel, which are separated by a thin plastic or metal diaphragm. The shock-tube driver, having a normally closed end opposite to the diaphragm, generally contains an initially high pressurized gas. The shock-tube chanriel, normally ending with a dump tank, contains a test gas usually at atmospheric pressure (or less). When the diaphragm is broken the high-pressure driver gas produces a shock wave that propagates into the lower pressure gas in the

channel. An expansion or rarefaction wave is generated simultaneously, which propagates back into the high pressure gas in the driver, as sketched in

figure la. The flows induced in regions 2 and 3 between the two outward movlng waves are separated by a contact surface (as shown in the figure), which is the dividlng surface between the gases originally in the driver and channel. Since the channel gas is compressed and heated by the shock wave and the driver gas is expanded and cooled by the rarefaction wave, the density and temperature change discontinuously across the contact surf ace but the pressure and velocity of the gas remain equal. This is depicted in figure lb. Between the incident shock front and contact surface, a short quasi-steady flow region (2) of a compressed and heated gas with constant state properties is produced for testing purposes, as illustrated in Figure lb. The test section in the channel is normally located well downstream of the diaphragm location such that the shock wave and contact surface become well separated and the length or duratlon of the test-gas flow becomes sufficiently long to conduct worth-while experiments. Such experiments could involve shock structure, the flow around a model placed in the channel, or an oblique shock-wave reflection from a wedge or half cylinder.

Now consider the other case when the shock tube contains a dusty gas in the channel. A similiar wave pattern and similar spatial distributions of the flow properties occur for this new case, as illustrated in figure 2. However, important differences do occur and are noted [14]. The speeds of the shock wave and contact surface both decrease gradually as they travel down the chan-nel, and the characteristics in the rarefaction-wave tail are curved toward the head of this wave (figure 2a). Furthermore, nonequilibrium processes occur at the shock front and through the contact surface, because the gas and particle temperatures and velocities differ (figure 2b). Two relaxation

processes in the form of heat transfer and viscous drag occur automatically in these nonequilibrium regions, continuously bringing the gas and particle

temperatures and velocities closer together.

The theory and analysis of such nonequilibrium flows in a dusty-gas shock tube can be found in previous analytical and numerical studies. Otterman and Levine [24] used a particle-in-cell method to compute the unsteady dusty-gas flow in a shock tube for the first time; Outa, Tajima and Morii [25] were the first to use the method of characteristics; Miura and Glass [14] were the first to employ the ralldom-choice method (ReM) with an operator-splitting technique; and Sommerfeld [10] also applied the RCM later. However, good

,experiments have neither been conducted to thoroughly check numerical predic-tions nor been aimed at providing comprehensive data of lasting value. The best experiments to date are those of Sommerfeld [10].

Past experiments have not been directed at simultaneous measurements of the gas and particle velocities and gas density. Such measurements would

permit definitive predictions of the drag coefficient of the particles in dusty-gas flows. Measurements in the past usually provided only the particle velocity, and steady-state equations (instead of unsteady ones) were simply used to obtain the gas velocity and density which in turn were employed to estimate the drag coefficient. This clearly needs rectification, because viscous drag is the dominant mechanism that establishes the dynamic relaxation length. Furthermore, measurements of heat-transfer rates have not even been attempted, in spite of the fact that heat transfer is the dominant mechanism for establishing the thermal relaxation length. Besides these basic experi-mental studies, and the usualones concerning normal shock-wave structure,

investigations of more general dusty-gas flows are required. Some examples include: contact-surface transitions, oblique shock-wave reflections,

2

boundary 1ayers on side wa11s and flat plates, boundary conditions for

particles ref1ecting from or sticking to rigid wa11s, to mention a few. Such investigations are required to conc1usive1y verify comp1icated analyses, generate new information, and, of course, enhance our understanding of such f10ws in general.

The new UTIAS dusty-gas shock-tube faci1ity, which is described in this report, has many nove1 features that make it idea1 for studying dusty-gas f10ws induced by shock waves, such as the ones just mentioned. However, successfu1 studies wi11 be comp1eted on1y as more advanced instrumentation is coupled to this shock tube. Such instrumentation, on1y partia11y complete at this time, wi11 a1so be described herein.

Pre1iminary experimenta1 data of shock wave structure in dusty-gases is a1so inc1uded in this report. The gas is air and the particles are glass beads or fog (water and oi1 mixture). Pressure measurements and partic1e ve10cities from a laser Dopp1er velocimeter are presented and described.

2. SHOCK-TUBE DESIGN AND CONSTRUCTION 2.1 General Description

The UTIAS dusty-gas shock tube is a faci1ity designed to simu1ate the flow fie1ds associated with exp10sions in a dusty enviornment. It consists of a horizonta11y-mounted driver with p1unger or combustion diaphragm breaking mechanisms, transition sections, recoi1 sections, dust-gas admit/iso1ation system, associated dusty-gas mixer recircu1ation and dust injection systems, horizonta11y-mounted channe1 (driven section), test section, and dump tank. A list of the principle components can be found in Table 1. A side view of the dusty-gas shock tube faci1ity is shown in figure 3 disp1aying an assemb1y of all the major components. Photographs and general technica1 drawings of the principle components appear in figures 4 to 28. Explanations of the details 1eading to these particu1ar designs fo110w in the rest of this chapter.

2.2 Driver

The driver is achamber which is separated from the rest of the shock tube by a diaphragm. It is pressurized with various gases to the point where the diaphragm bursts spontaneous1y or is broken by a p1unger, thereby prod-ucing a shock wave in the channe1. The driver, sketched in figure 4, normally has an interna1 diameter of 21.2 cm with a wa11 thickness of 6.9 cm. Two overall views of the driver appear in figure 5. The 1ength of the driver is 2.44 m, in a single section. · It is opened and c10sed by the operation of a pneumatic jack shown in figure 5. In order to achieve incident shock Mach numbers up to 5 or greater in the test gas, the driver was designed to operate at pressures up to 70 MPa with a safety factor of 3. It is constructed of hot rolled SAE 1020 carbon steel (yie1d strength 200 MPa, tensile strength 380 MPa) and is chromium plated to a depth of 0.025 mmo

The driver section can be configured for two modes of operation: cold-gas or combustion runs. In cold-gas runs a spring-loaded p1unger is p1aced a10ng the central axis of the driver. This p1unger is triggered to break plastic diaphragms at some predetermined time. It is activated by a solenoid

and is synchronized with the dusty-gas admit/isolation system (described in section 2.8). The plunger can be seen in figure 6. The diaphragm cannot be broken until the dusty-gas mixer is isolated from the shock tube and the shutter valves are open to the channel. In practical situations the use of cold air in the driver with thin plastic diaphragms produces relatively weak incident shock waves, i.e., shock Mach numbers less than 2. To increase the shock strength one could use a driver gas with a higher sound speed, such as helium, which extends the range to shock Mach numbers as high as 3.

Very strong shock waves can be produced by employing constant-volume combustion-driver techniques. During combustion-driven runs the plunger and

its associated driver endplate are replaced with an ignition wire and an end-plate incorporating a rupture disk. This wire is located along the central axis of the driver. Scribed stainless steel diaphragms, broken by the high combustion pressures, are used to drive strong shocks through the dust-air mixture. These diaphrams break at pressures ranging from 6 MPa to 70 MPa. In this case a stoichiometrie mixture of oxygen and hydrogen diluted with 75% helium is introduced into the driver through a mixing tube. This mixture is ignited and the resulting chemical reactions (which produce water) heats the helium to a high temperature (and as a consequence raises its sound speed). This simultaneously increases the pressure in the driver eightfold. By vary-ing the thickness of the diaphragm and the depth of the scribe, Mach numbers from 3 to 5.5 can be produced. If an accident al gaseous detonation does take place in the driver, the rupture disk will burst at 100 MPa and stop the driver from being destroyed. More details about the properties of different diaphragms will be discussed in chapter 5.

The use of an area contraction at the diaphragm station, illustrated in figure 4 and shown pictorally in figure 7, can be very advantageous in the generation of stronger shock waves. Stronger shocks can be produced with the same pressure ratio across the diaphragm if the driver cross-sectional area is larger than the channel cross sectional area. Therefore, the driver can be designed to operate at a lower maximum pressure than would normally be allowed in a simple constant-area shock tube. The internal diameter can be altered from the maximum value of 21.2 cm to a smaller diameter of 13.6 cm. This is accomplished by inserting a steel liner to reduce the diameter. In this configuration the shock tube has a constant area. The length of the driver can also be reduced by the insertion of a movable endplate. This gives the capability to control the time when the rarefaction wave reflects from the driver endplate. This helps the simulation of blast-wave pressure profiles.

2.3 Channel

The shock wave propagates from the driver af ter the diaphragm is burst into achamber called the channel. The channel extends 9.14 m to the test section with a rectangular cross section of 19.7 cm by 7.6 cm (see figures 3, 8 and 9). This channel is able to survive a transient maximum pressure load of 3.4 MPa with a safety factor of 2. The channel is constructed of four plat es (see figures 9 and 8) and uses a circular rubber rope along the side-walls in a longitudinal sealing groove. The seal is completed with a butt-seal with the endwall D-rings. The use of hot-rolled carbon steel type SAE 1020 standard plates enabled economical construction and good machinability. Another 3.05 m section is designed for the downstream side of the test section to complete the channel.

Four individual sections are used in the present channel which, along with the test section, comprises a total length of 13.1 m between inlet/outlet isolation valve sections (see figure 3). The sections are bolted together with angle brackets. The design drawings specify a tolerance of ± 0.06 mm on the 19.6 cm dimension and ± 0.04 mm on the 7.6 cm dimension. The natural surface finish of the material is 63 microinches (rated fine). All interior surf aces are plated with 0.025 mm of chromium.

Instrumentation ports are located 0.61 mapart throughout the length of the channel, at the midpoint of the sidewall (figure 8). The instrumentation ports are identical in dimension so that standard instrumentation plugs can be used. The channel section just upstream of the test section has an additional six ports on the other three plates. The channel sections are individually track mounted on stands, as shown in figure 8.

The 7.6 cm wide by 19.7 cm high cross-section of the channel (figure 9) was chosen for several reasons. The test section incorporates two 23-cm-diameter circular windows, affording a large field of view, including the top and bottom walls of the channel. Eventually optical measurements of the flow field using shadowgraph and schlieren photographic techniques will be done in the shock tube. Boundary-layer growth behind the incident shock may he an important factor, this implies a wider channel would be desirable. The final choice was mainly a compromise between these factors, available materials and construction costs.

The study of two-phase flows in the dusty-gas shock tube requires that particulate matter (either solid or liquid) be suspended homogeneously in the test gas. This can be accomplished by injecting the dust particles into a turbulent flow. There is the further constraint that the initial pressure of this flow be at least one atmosphere (about 101 kpa) to prevent rapid settling of the particles by gravity. This was an important factor in determining the operating pressures of the channel.

2.4 Transition Section

The transition sections are 0.5 m in length and are located between the driver recoil section and channel and between the channel recoil section and dump tank. A photograph is shown. in figure 10 and the design is illustrated in figure 11. The internal cross-section is a direct transition from the circular section at the driver to a 19.7 cm by 7.6 cm rectangular cross-section at the channel. The gradual transition reduces flow disturbances arising from the change in cross section between the driver and the channel. The design is such that the area remains fairly constant along the transition, being about 8% larger in the centre of the transition. It is constructed of ductile cast iron (ASTM A339-55 grade 80-60-03) and is plated with a 0.025-mm chromium layer. The transition section was manufactured as a casting and is able to withstand transient pressures of 3.4 MPa with a safety factor of 2.

2.5 Test Section

The test section is 0.9 m in length and incorporates two interferometric quality glass windows for optical observation of flow phenomena. A photograph is shown in figure 12 and the design drawing is shown in figure 13.

The design specifications of the test section are identical to those of the channel. The method of construction, however, was different. The four plat es are welded together. (This is not a recommended means of construction, because the welding causes a warping of the side walis.) It incorporates ten instrumentation ports, and can hold two 23-cm-diameter interferometric quality windows. Note that the large high-quality interferometric windows in the test section limit the maximum pressure in the channel since they were designed to operate at pressures up to 3.4 MPa.

Recent modifications to the test section include a mechanism to position and remove the large test section windows easily (see figure 14). A second set of high quality windows have been added to the test section (see figure 15). These windows are made of quartz and are 12 mm in diameter. They will be used primarily for laser Doppier velocimeter measurements. A third set of 6 mm diameter optical glass windows have also been added for use by a light extinction device to measure particle concentration.

2.6 Dump Tank

The shock wave that moves along the channel cannot be allowed to enter the laboratory; hence, the channel must be closed. If it was allowed to reflect from the end of the channel, the pressure behind the reflected shock would be almost as high as the driver pressure. This can be prevented by

terminating the channel with a large volume dump tank. The dump tank also serves to reduce the overall pressure in the shock tube af ter a run to a reasonable value. This avoids subjecting the test section windows to high pressure loads for any appreciable length of time.

The dump tank has been designed according to the Unfired Pressure Vessel Code [26]. Photographs of the facility appear in figures 3, 8 and 16, and a schematic diagram of the dump tank is illustrated in figure 17. It has an internal diameter of 96.5 cm, a height of 2.44 mand a total volume of 1.64 m3 • The operating pressure is 1.7 MPa with a safety factor of 3. Access to the inside of the tank is provided by a 45.7 cm diameter quick-opening manway

(see figure 18). The dump tank consists of two hemispherical caps 0.305 m in depth which are welded to a cylinder 1.83 m in length. The inside walls have been painted with white TREMCLAD rust paint. Four instrumentation ports are provided for various gauges, a vent release and a dust ejector. Four adjustable wheel assemblies, welded to the bottom hemispherical cap, support the dump tank on the channel section tracks.

2.7 Recoil System

Very high axial loads can occur in combustion driven shock tubes. At the driver end plate the load results from the pressure exerted by the combustion gas until it is relieved by the incoming rarefaction wave. The load at the dump tank is due to the incoming flow being decelerated by the walls of the dump tank. In the present design, peak pressures of the order of 70 MPa are expected in the driver. Hence, with a driver diameter of 21.2 cm the axial load would be 2.5 MN. When the facility is configured as a constant-area shock tube the driver diameter is 13.6 cm, and this results in an axial load of lol MN.

These axial loads dictate that the shock tube cannot be constructed as a solid assembly; such loads would require massive endplates and flanges. A more cost effective solution allows the driver and dump tank to recoil, thus

isolating the channel from large axial loads. !he shock-tube recoil system is shown in a schematic diagram in figure 19. The entire system is comprised of: a) two telescopic recoil sections of IS-cm stroke which allow the driver and the dump tank to move independently of the channel; b) steel spring shock absorbers to absorb the recoil energy and to restrict the recoil distance; and c) a reinforced floor foundation and connecting struts to anchor the channel to the laboratory floor. This system would eliminate the relative motion of the test section with respect to a fixed optical system used to investigate the flow field.

A photograph of the driver recoil section is shown in figure 20 and a design drawing is shown in figure 21. !he internal diameter is 14 cm and it .

has a length of 0.46 m. The upstream end is attatched to the driver diaphragm station and the downstream end is connected to the driver transition section.

An O-ring provides a pressure seal at the telescopic joint. The joint is open approximately 7.5 cm in its neutral position allowing the driver to recoil in either the forward or backward direction. All surfaces exposed to the gas flow have a chromium plating and the section is track mounted on an individual stand. The dump tank recoil section is identical in design and construction. It is located just downstream of the channel transition section. The steel spring shock absorbers are very simple in design and construction and require no maintenance in contrast to the liquid spring shock absorbers of the UTIAS hypervelocity shock tube [12].

The shock-tube channel cannot move during testing; hence, it must be ridgidly secured. A massive steel-reinforced concrete anchor, 0.9 m by 4.0 m by 0.9 m, is embedded in the laboratory floor. Strong 0.9 m steel beams shown in figure 22 support and anchor the shock-tube channel. These steel beams are bolted into the concrete footing through heavy steel pads. Hence, the channel is rigidly connected to the floor foundation.

2.8 Dust-Air Injection into the Channel

A homogeneous dust-air mixture is required in the channel of the dusty-gas shock tube. One way this can be accomplished is by the recirculation of

dust and air through a closed loop through the channel. A dusty-gas mixer and recirculation system is used to set up a uniform flow field in the channel of the shock tube. Dust particles (glass beads under 50 ~ in size) are injected into the mixer through a feeder mechanisme The dust-air mixture is allowed to recirculate through the channel until some form of uniformity is achieved.

At the rectangular cross-section ends of both transition sections are located the high-speed dusty-gas channel isolating inlet/outlet and shutter valve systems. A typical section is shown in figure 23. These components allow the channel of the shock tube to be part of a closed loop with the dust mixer recirculation system. Two high-speed shutters isolate the driver and

the dump tank from the recirculation loop of channel and mixer. !wo inlet/ out let valves adjacent to the shutter valves allow the dust-air mixture to recirculate. Sketches of the shutter valve and channel isolation valve are shown in figure 24. The isolation valves have plugs with O-ring seals. When open they protrude into the channel and are flush with the channel walls when

closed. All the valves are actuated through the use of high pressure 620 kPa air lines.

The operating system requires synchronization with the plunger (for

cold-gas runs) and ignition system (for combustion runs). The shock wave must not enter the dust-air mixer recirculation system and the shutter valves

should be open. The shutters are able to withstand weak shock waves but the mixer/recirculation system is not designed to be under pressure. Hence it 1s important that the plunger/ignition systems on the shock tube remain inhibited until two things occut': the channel is isolated (inlet/outlet valves closed), and the dusty-gas loop is opened (shutter valves open).

Microswitches mounted ne ar the shutter and isolating valve shafts can sense the position of the various valves. The signals from the mict'oswitches are 'anded' electrically to the fire control system, hence the diaphragm plunger cannot be activated until all valves are in the correct position. A premature diaphragm breakage is a potential problem for damaging the valves.

The initial design of the dusty-gas mixer can be found in figure 25 and a photograph is given in figure 26. This piece of equipment is a cylindrically shaped tank, 0.15 m3 _{in volume, with a horizontally mounted motor on one side.} The dust feeder mechanism is- located on top of the unit, and a viewing port is

available to visually inspect conditions in the interior of the mixer. The motor is connected to a impeller which produces a flow through the dusty-gas

loop. The return loop consists of 7.62 cm ID stee~ pipe and is firmly mounted on the channel (figures 23 and 26). The De motor (Honeywell one horsepower continuous duty motor) is manipulated by a motor control system designed at UTIAS. The motor control system allows for precise regulation of the impeller speed and ensures stability over a long period of time. This is accomplished by monitoring the speed of rotation of the shaft with a tachometer. The informat10n is used in a negative feedback loop to control the motor's speed precisely. In the case of overload the control unit cuts off automatically. The motor speed is displayed on a digital indicator in RPM. The rotation speed can be controlled from 700 to 2400 RPM, resulting in flows up to 2 mis through the channel. Unfortunately, this flow speed was not sufficient to produce a homogeneous mixture, because most of the dust particles settled on the channel floor near the inlet valve at the driver transition section. A more powerful recirculating flow was required.

In an effort to develop a bet ter dust injection system, a number of

different types were tried. Some of these are sketched in figure 27. The one at the top of the page (27a) is a repeat for reference. It was noticed that the dust from the feeder would fall almost straight through the internal flow in the mixer, because of the low velocity therein. Hence the volume was reduced substantially by 75% and the feeder was connected directly to the recirculation loop, to obtain mixer 2 (27b). This helped to get more dust'

into the recirculating dusty-air flow, but the flow speed in the shock-tube channel was still unacceptably low (2 mis), and dust settled out along the first part of the channel floor.

A 3/4 horsepower centrifugal pump was used directly to recirculate the dust-air mixture through the channel (27c). This produced flow fields of the order of 6 mis in the test section. For this case, the dust particles settled out only at the outlet valve side of the recirculation system and enabled a quasihomogeneous cloud of dust to be produced in the test section for about 5

to 10 seconds. This was enough time to fire a 'shock wave through the dust-air mixture with some success. Other improvements include the moving of the out-let and associated shutter valve closer to the test section, which shortened the recirculation loop by six meters, and this helped considerably in produc-ing a stable cloud of dust in the channel. However, the dust-air flow in the channel was stratified, with more dust in the lower part of the recirculating flow. Higher recirculating flow velocities were definitely needed.

With this initial success it was decided to use a larger centrifugal blower with a 3 horsepower motor. Flow speeds were now higher at 9 mIs in the chann~l, but these were not much higher than those for the 3/4 horsepower

motor. The dusty-gas flow created in this manner was still stratified, and higher recirculation velocities could not be obtained using this methode

The latest design, which has been contructed and is presently being tested in the dusty-gas shock tube, injects a dust-air mixture under high pressure from a large reservoir. A schematic diagram is shown in figure 28. Under high pressure the main air flow and the other flow through the dust chamber creates a dust-air mixture prior to entering the channel. This dust-air flow is initiated by opening the fast on/off valve (Parker H2002 single solenoid valve). A dust rate valve controls the dust flow from the dust chamber so that the loading ratio for each experiment can be controlled beforehand. The fast acting valve is synchronized with the plunger system from the control console. This valve closes when the dust-air mixture reaches the dump tank and a more or less uniform mixture is achieved along the

channel. The shock tube is fired shortly thereafter, before the particles can settle due to gravity.

This system creates a fairly homogeneous dusty-gas mixture with a flow speed ranging between 24-32 mis, it can last for approximately 2 seconds if required. The dust cloud is exited through a 5 inch I.D. relief pipe in the dump tank. Filter bags collect the dust particles in accord with UTIAS safety regulations.

3. CONTROL SYSTEMS 3.1 Driver Gas Mixing

In this facility two types of driver techniques are employed, cold-gas and constant-volume combustion. Since the cold-gas techniques usually involve the use of only one gas component, driver gas mixing is not a concern.

However, for combustion runs a stoichiometric mixture of hydrogen and oxygen diluted with 75% helium is required in the driver. To ensure uniform

combustion and to reduce the possibility of detonation a thorough mixing of the constituents is required.

,To guarantee complete mixing of the component gases used in constant-volume combustion, a mixing tube as described by Boyer [12] is implemented. The mixing tube, shown in figure 4, is a high pressure stainless steel tube capped at one end with small orifices spaced every 15 cm. The tube runs the entire leng th of the driver, and hydrogen, oxygen and helium are injected under moderate pressure through this tube into the driver. The induced tur-bulent motion helps mix the constituent gases; this promotes uniform burning of the hydrogen and oxygen and uniformly heats the helium.

Boyer [12] reported the position of the mixing tube is not critical; hence, in the present dusty-gas shock tube the mixing tube was mounted along the bot tom of the driver. The orifice diameter is such that the total orifice area is equal to the minimum cross-sectional area of the loading line. The mixing tube is constructed of type 302 stainless steel tubing with an OD of 1.43 cm and an ID of 0.79 cm. The tube is capped at one end and is rigidly mounted in the driver with steel clamps.

3.2 Diaphragm Breaking Mechanisms 3.2.1 Driver Plunger System

To initiate the rupture of mylar or cellulose acetate diaphragms in the cold-gas method, a plunger system is used. Initially the diaphragms are

tested to see at what pressure they break spontaneously. Some typical data is shown in figure 29 for the cases of mylar and cellulose acetate using air and helium driver gases. For actual runs the pressure is set slightly above 80% of the breaking pressure and the plunger is used. In case of premature rupture the shutter valves are capable of withstanding the resulting shock pulse of moderate strength waves produced by these plastic diaphragms.

3.2.2 Driver Ignition System

From previous experience [12], a heated wire technique was chosen to initiate constant-volume combustion in the driver. In this method a tungsten wire is placed lnside the driver along the central axis. It is heated through the discharge of an 8.5 ~F, 20 kV capacitor. A general layout is shown in figure 30. The tungsten wire is attached to the mixing tube strut at the downstream end of the driver. This is also the electrical ground for the ignition system. The shock tube is electrically grounded to UTIAS's elect ri-cal network through a mechaniri-cal pump used to evacuate the driver. At the upstream end of the driver, near the endplate, the ignition wire is connected to an insulated high voltage connector which is vacuum and pressure sealed from the outside.

Power for the system is supplied by a Del model RIU-87S high voltage power supply. Once the capacitor is energized, energy is released through an ignition coil initiating a spark across a standard automotive spark plug. The spark jumps across a gap discharging the capacitor which in turn heats the ignition wire. With this system shock Mach numbers up to 5.5 can be generated in the channel of the dusty-gas shock tube.

3.3 Control, Pressure, and Vacuum Systems

All controls for the operation of the shock-tube facility are located on the control console in the control room. All valves have solenoid controlled pneumatic actuators for remote operation. Control switches operate these valves and signal lights indicate the valve position. A photograph of the control panel is shown in figure 31.

The driver gases are stored in standard high pressure cylinders, the cylinders being stored outside the blockhouse (figure 32). The gases are fed

to the driver through stainless steel tubes with a design operating pressure of over 100 MPa. Gas regulators are used to obtain the desired pressure to the driver. The pressure can be monitored from the control panel on Winter's high pressure gauges. Check valves are used in. the gas lines to prevent backflows and accidental mixing of the driver gases.

The various driver gases are connected to a common loading line through 3.4 MPa solenoid controlled service valves and are actuated at the console. The loading line conducts the gas es through a 140 MPa regulating valve to the high pressure valving system at the driver. This system consists of 410 MPa vent, isolating and pump valves. The isolating valve is backed by a 410 MPa check valve to isolate the gas charging system. The gas loading line and driver are evacuated by a Welch Scientific Duo Sealmodel 1402 mechanical vacuum pump. The pump is mounted on the driver support stand and is operated

remotely from the control console. A Heise model 710A digital pressure indicator is used to monitor the pressure in the driver.

The driver ignition and plunger systems are also controlled from the console. Warning lights, dump tank status, shutter valve and channel

isolating valves status indicators, and channel pressure indicators complete the control panel.

The control console is now being modified to include the operation of the fast on/off valve for the dust injection system. This fast acting valve is synchronized with the plunger and ignition systems so that it is open for a specified period of time and then closed just before the shock tube is fired.

4. SHOCK TUBE HAZARDS AND SAFETY PRECAUTIONS 4.1 Safety Precautions for Personnel

The operation of a shock tube at high pressure presents some potential hazards, especially in the operation with explosive gas mixtures. Structural failure during the the operation of the facility would be quite disastrous to unprotected personnel. An existing reinforced concrete blockhouse, previously used to house the UTIAS implosion chamber [27], was used to house the driver. The 30 cm wall provides adequate protection for a combustion phenomenon known

as over-detonation [28], where the pressures inside the driver can attain values three or four times those of normal detonations. In such a case a burst pressure safety factor of at least 20 would be required. Furthermore, the laboratory exposed sections were designed with a safety factor of at least 2.

Many dangers exist in the operation of facilities involving the use of high-pressure vessels. The combustion of explosive mixtures of hydrogen and oxygen, the use of high-voltage equipment, as weIl as various components succumbing to structural failure such as windows bursting, Burdon tube type gauges blowing out, the rupture of high-pressure gas lines, or failure due to timing control problems of shutter valves and the admit/isolation system, could be disasterous to shock-tube personnel. In the following sections special devices which help protect personnel from potentially fatal accidents are described. The driver blockhouse, warning systems, safety interlocking of various controls, fail-safe valve operation, check valves and rupture disks all help to provide passive protection to personnel from accidents as aresuit of structural failure or carelessness.

4.2 Driver Block House

The high-pressure driver up to the first transition section of the dusty-gas shock tube is located in the driver blockhouse. The enclosure originally housed the Hypervelocity Launcher/Explosively-Driven Implosion Chamber [27]. A schematic diagram of the blockhouse can he found in figure 33. It is 6.1 m by 6.1 mand houses the driver, associated vacuum' pump, gas lines, and high-voltage electronics for combustion runs. One wall is

constructed of 0.305 m thick reinforced concrete and protects personnel at the control panel. The remaining three walls are constructed of a light steel frame and panels. In case of a driver structural failure these walls are designed to act as a pressure relief diaphragm.

During operation of the shock tube in the combustion mode two exhaust fans ventilate the blockhouse. !wo air intakes on the opposite wall ensure a good cross-draft which helps to minimize the accumulation of toxic and/or explosive gases in the driver blockhouse. The blockhouse can be accessed through two 5 cm thick steel doors. The shock tube passes through a 26 cm diameter opening in the front wall. !wo close fitting steel plates (2 cm thick) effectively shield the driver opening from the main lab. All of the piping and control lines are floor level, and they enter the control room via steel pipes.

A rupture disk, shown in figure 34, is incorporated into the driver end plate for combustion-driven runs. Failure can occur either through fatigue of the rupture disk under normal operating conditions or el se when detonation accidently occurs in the driver. Simple fatigue can be avoided by replacing

the rupture disk when it shows signs of excessive wear. To confine debris from the accident al bursting of a rupture disk, a duct is attached to the driver endplate during high-pressure operation. An external steel and wood barricade at the end of the duct is used to absorb the debris from the burst rupture disk.

4.3 General Safety Features

The dusty-gas shock-tube facility has a number of passive safety features in addition to the blockhouse and alarm systems. These features are qescribed in the this section.

Interlocking control system. All control switches in the main console are interlocked to prevent the shock-tube facility from being fired in an unsafe mode. This includes the prevention of the driver ignition system from being charged unless all remote valves are closed, warning lights activated, and the blockhouse isolated.

Fail-safe valve operation. The remote control valves of the dusty-gas shock tube facility are solenoid controlled and pneumatically operated. In the event of a power or air supply failure all valves will automatically close, with the exception of the driver and dump-tank vent valves which automatically open upon failure.

Timing and fire control. Af ter the high-voltage power supply is activated the fire control circuit is disarmed for a 10 minute 'gas-mixing' periode It is not possible to fire the shock tube during this time.

Ignition power supply cutout. Af ter the shock tube has been fired, during a combustion run, the fire control system disables the high-voltage power supply from recharging the capacitor.

Vacuum line protection. All vacuum lines are protected with relief valves so that inadvertent admission of high pressure to these lines cannot occur. These devices blow out with an overpressure of 101 kPa. In addition, a 2 cm thick plexig1ass shield covers all gauge faces on the console to protect personnel from a high pressure failure.

Check va1ves. Check valves are used in all gas lines from the high-pressure bottles to the console to prevent accidental lnxing of the gases in these 1ines.

Console power switch. The main console power switch provides power to all operating controls of the shock tube. In the event of any malfunction, the

throwing of this single switch shuts off all power supp1ies and initiates the fail-safe valve operation sequence. In the event of a power fai1ure, a relay disables the switch such that when power is restored shock-tube systems remain inactive unti1 it is determined safe to reactivate them.

5. DIAPHRAGM MATERIAL, PREPARATION AND CALIBRATION

Nominal operation of a shock-tube faci1ity depends on the abi1ity to consistently repeat test conditions for several runs. This is directly linked to the choice of diaphragm materials. The materials should have uniform

bursting and opening characteristics so that satisfactory performance of the shock tube can be achieved.

In the dusty-gas shock tube two different methods are employed to drive shocks through the channel test medium, namely cold-gas and constant-volume combustion drivers. The cold-gas driver methods are used primarily to drive shocks over a range of low Mach numbers, from 1 to about 3. Some important characteristics include a material which breaks quick1y so that the shock wave produced is as sharp as possib1e. Constant-volume combustion is used to drive stronger shocks through the test medium, with Mach numbers ranging from 3 to about 5.5. For the case of combustion driven runs it is important that the diaphragm open at the peak driver pressure, this prevents the possibility of detonation of partially burned driver gases which form from the premature opening of the diaphragm. In this way the highest possible driver temperature and pressure behind the shock wave and performance advantages of constant-volume combustion operation can be fully realized. Otherwise, premature rupture of the diaphragm leads to very near constant-pressure combustion, which drives very strong shock waves into the channel but suffers from severe attenuation of the shock and a rapidly decaying pressure field [29].

5.1 Cold-Gas Driver

Materials such as mylar and cellulose acetate are used as diaphragm

material for cold-gas runs. These materials come in a variety of thicknesses: 1, 2, 3, 5, 10 and 14 mils. The driver gases used are helium and air. With the proper choice of diaphragm thickness and driver gas, incident shock Mach numbers from 1 to about 3 can be produced in the shock tube.

The mylar (type A milky) material breaks f,airly consistently from run to run and tears into very large petals (figure 35a). Unfortunately, when the diaphragms are pressurized to slightly above 80% of their burst pressure and braken with the plunger, the material does not break fast enough to produce a sharp frozen shock with a flat top pressure profile. Cellulose acetate

(available Erom art supply outlets) has very consistent breaking or shattering characteristics, eVe(l when burst with the plunger, and tends to shatter into very small pieces when ruptured, as is seen in figure 35b. The thickness of the diaphragm determines its bursting pressure. Thicker diaphragms may be assembled from several layers of the material.

The disadvantage of cellulose acetate diaphragms is that when ruptured a large amount oE debris consisting of small pieces of the diaphragm are swept down the tube. The mylar diaphragms tend to tear into large petals and throw very llttle debris down the channel.

5.2 Combustion Driver

Previous experience with the 10 cm by 18 cm Hypervelocity Shock Tube was used to fabricate diaphragms for combustion-mode operation. Details of the research and development of these diaphragms can be found in the UTIAS report by Boyer [12].

The diaphragms are made of stainless steel and are machine scribed in the farm of a 90 degree cross in the center. By knowing the properties of the material and the depth of the scribe, a desired bursting pressure can be

attained. The depth of the scribe is important, if too deep the petals formed upon opening may be torn loose and cause considerable damage to the facility. If the scribe is too shallow, the petals may not open fully and this leads to poor rupture and consequently poor shock-tube performance.

6. GENERAL OPERATING PROCEDURE 6.1 Clean-Gas Mode

In this mode a choice of two different driver techniques are available, cold-gas and constant-volume combustion drivers. The specific applications of the different techniques have already been discussed in chapters 2, 3 and 5. The operating procedure for the clean-gas mode will now be discussed.

The first step in the operation of the dusty-gas shock-tube facility in the clean-gas mode is to pre pare the driver. For the case of cold-gas runs, the plunger mechanism is inserted into the driver. Two sizes of struts are available to support the plunger when it is installed in the driver, depending on whether or not the facility is being utilized as a constant-area shock tube.

A diaphragm of mylar or acetate is mounted in the driver diaphragm

station and the driver is bolted shut. Once all the instrumentatlon has been set up (trigger level settings, reset counters and so on) the driver is

evacuated i f a driver gas other than air is being used. All valves are closed, the shutter valves are leEt open and the dusty-gas mixer is isolated from the channel in the clean-gas mode.

Before the driver is pressurized, the blockhouse door is closed and personnel are moved away from the test section. The driver gas is then admitted slowly until the diaphragm bursts. Alternatively, if the value of the bursting pressure is criticalor the timing of the event is important, the plunger mechanism is used. In the case a slightly thicker diaphragm is used and the driver is pressurized to the desired bursting pressure. A manual plunger release is available at the front panel (see figure 31) which breaks the diaphragm when depressed.

There are two vent mechanisms built into the facility. One is located at the driver and another at the dump tank. During normal operation the execess pressure in the shock tube is released through the vent at the dump tank. It is fairly large and quick. In emergencies, the driver gases can vented before the diaphragm is broken, releasing the pressure in the driver. This vent is part of the 'fail-safe' safety system where all the valves close during a power interruption except the driver vent.

For the case of combustion runs in a clean gas, a similiar operating procedure is followed. The driver is prepared by placing a 0.38 mm tungsten wire at the cent ral axis. A scribed stainless steel diaphragm which is designed to open at predetermined pressure is inserted at the diaphragm

station. A stoichiometric gas mixture of hydrogen and oxygen diluted with 75% helium is admitted into the driver. From previous experience [12, 301 the bursting pressure of the scribed diaphragms and the proper amounts of the combustible gases needed to break the diaphragms is known with precision. When the gases have been thoroughly mixed the tungsten wire is heated through the discharge of a large 8.5 ~F, 20 kV capacitor, this initiates combustion and ruptures the diaphragm.

6.2 Dusty-Gas Mode

In the dusty-gas mode preparation of the shock tube proceeds much like that of a clean-gas run. The only difference is that the test gas will contain some prescibed amount of dust. The procedure for the dust injection has been described in section 2.8. The following describes the procedure when the recirculation system is used.

Af ter the driver is pressurized, the shutter valves are closed which isolate the channel from the driver and the dump tank. The iso lat ion valves are open and allow the dust-air mixture to be recirculated through the shock tube channel section. The flow in the channel is oriented so that the dust-air mixture is moving towards the dump tank. The dust is fed into the

recirculation loop via a rotating, vibrating hopper. The dust falls through a thin slit into the mixer and is circulated through the system by means of an impeller or centrifugal blower. The flow is allowed to recirculate for a period of time until the dust-air mixture reaches some form of equilibrium and homogeneity in the shock tube. The mixture is monitored through optical

diagnostic techniques.

Cold-gas runs are initiated with the use of the driver plunger diaphragm breaker. The shutter valves are able to withstand premature rupture of cold-gas driver diaphragms. This is not of concern during combustion-mode runs because it is virtually not possible for the stainless-steel diaphragms to,

possible moment and is interlocked with the rest of the shock tube safety systems.

Af ter the run, dust and residual diaphragm debris is accumulated in the bottom of the dump tank. Dust and debris in the channel is pushed out by the use of wooden block covered with cloth. I t has the same dimensions as the channel and is pulled through by means of ropes. This removes 85% of the loose dust. The dump tank manway is opened and the dust and debris are then vacuumed out and separated for later use. The broken diaphragm is removed and clean dry air is blown throllgh the channel for a period of twellty minutes. Further cleaning consists of a clean-gas run using mylar diaphragms and swabbing the channel interior with solvents. Once all cleaning is achieved, the shock tube is prepared for another run.

7. INSTRUMENTATION

To fully analyse shock structure in dusty gases a nllmber of fundamental shock, gas, and particle properties need to be measured: frozen shock Mach nllmber, particle velocity, gás pressure, particle concentration, gas velocity, gas density, and particle temperatur~. All these proper ties vary dramatically

throllgh the shock front and nonequilibrium zone following the shock front, and they must be recorded as a function of time. From these seven fundamental measurements the gas temperatllre, unsteady drag coefficient, and gas-particle heat transfer coefficient can be calculated. Again, they are all functions of

time. '

Instrumentation is available at the UTIAS dusty-gas shock-tube facility to measure the first four of the above li~ted fundamental parameters, namely: shock Mach number, gas density, particle velocity, and particle concentration. This instumentation is described in this chapt'er. Further, instrumentation is

being developed or bought to measure the gas velocity and gas density. This instrumentation is also briefly described here.

No feasible method of measuring the particle temperature (and hence the gas-particle heat transfer coefficient) has been developed, and this is

presently beyond the scope of the dusty-gas shock-tube research at UTIAS.

7.1 Wave-Speed Measurements

The spee~of the shock front as it moves down the channel is important to monitor. First of all, this speed might decay somewhat, and the degree of

decay is important. Secondly, and more importantly, this shock speed provides the frozen shock properties (just behind the shock front), which are needed as part of the general data gathering process.

The channel is equipped with a large number of instrumentation ports. To measure the speed of the shock wave as it propagates along the channel, small pin trigger gauges (Channel Industries PK14-12 shock pins) are mounted in universal plugs which can be inserted into the instrumentation ports. The signals from the pin gauges are processed by a UT lAS designed and built signal conditioning system. The processed signals are fed into a 20 channel Time Interval Counter designed at DRES (Defense Research Establishment Suffield) and reconstructed at UTIAS. With this instrument 20 time intervals of the

- - - --- --- --- - ---- -- - - -- --- ---- -- -- - - - --- - -- -- - --

-shock wave speed can be measured at various positions along the channel. The theory of operation and design of the system is described below.

The PK14-12 pin gauges are piezoelectric devices, that is they produce an electric potential when deformed by pressure. The gauges are quite small and

inexpensive and are used mostly in field trials of large chemical explosions. The devices do not provide a linear response of voltage with pressure; hence, they are used basically to trigger electronic equipment upon the arrival of a shock front. In our experiments the shock Mach numbers range from 1 to 5.5. In this range the pin gauges provide outputs of 5 mV and up. In the lower range of Mach numbers it is more difficult to obtain a reliable system to properly trigger our 20 channel time interval counter. However, this has been rectified to a large degree with some recent electronic modifications.

It was necessary to design and build an electronic interface from the pin gauge to the time interval counter. This was done in three steps, as can be seen in the electronic circuitry of Fig. 36. Firstly, the signal had to be amplified; secondly, a trigger level is required; and thirdly, a digital pulse is generated for the time interval counter. The signal from the PK14-12 pin transducers is the input to a two stage amplifier, which consisted of two fet input low-noise operational amplifiers (Texas Instruments TL071), The first stage, set up as an inverter, has a gain of 10, whereas the second stage, also set up as an inverter, has a gain of 100, resulting in an overall noninverting gain of 1000. This circuit was placed as close as possible to the pin gauge

to minimize any noise pickup from external sources. The signal was then passed on to a high-speed voltage comparator (National Semiconductor U1311A), which allowed the setting of any desired trigger level by adjusting a trimpot. Hence, by setting the trimpot for a voltage of 0.3 V allowed signals below 0.3 V to be ignored by the comparator. Once the comparator is fired the signal is sent to J-K flip flop circuit (Texas Instruments 74LS109) which changes state when it is triggered by the comparator. This change of state is locked so that any subsequent triggering of the comparator will not reset the flip flop. The time interval counter requires a pulse rather than a level change, hence the signal from the flip flop is sent to a monostable multivibrator (Texas Instruments 74LS221) which generates a 500 ns pulse for use by the counter.

The wave-speed measurements can be adversely affected by the electrical field produced by the dust-air mixture flowing down the shock-tube channel. This electrical pulse can trigger the system prematurely. Good shielding will

reduce this problem, but we have found it is still a problem to be resolved. 7.2 Pressure Gauges

Several commercial pressure transducers are available to measure pressure histories in the shock tube, and these are some of the easiest measurements

that can be made. These gauges include Kistler piezotrons (models 206, 603A, 601B1, 211B11), and a PCB model 113A21 high-pressure transducer. The output signal is stored in Hewlett Packard 1744A storage oscilloscopes and recorded on polaroid film. Pressure ports are available in the driver, channel and dump tank, so that all phases of shock tube operation can be monitored and possibly compared to theory. Digitizing equipment is also available (Houston Instruments HIPAD digitizer) to store the pressure traces in several computers at UTIAS for analysis at a later date. These pressure gauges have not been found sensitive to the electrical pulse produced by the moving dust-air mixture in the shock-tube channel.

7.3 Laser-Doppler Velocimetry

Laser Doppier Velocimetry (LDV) is an optical technique to measure the velocity of partieles moving through a fluid. This involves two focused laser beams which cross to form a small control volume. As a part iele passes

through the control volume light is scattered from the two beams giving rise to oscillating signals which interfere at a photomultiplier tube. The

frequency of this signal can be related to the velocity of the partiele. An overall view of the LDV system is shown in figure 37.

LDV measurements of flow veloeities of 20-100 mis require a high powered Argon-ion laser. In the dusty-gas shock-tube facility a 3 watt Argon-ion (Spectra-Physics 900 series) and necessary opties are used. Optical stands for the laser, transmitting opties, and recieving opties have been built and are in place on either side of the test section.

The transmitting opties split the laser beam into two beams which are focused at the control volume (0.001 mm3) by a 250 mm converging lens. The scattered light from the control volume is received at the photomultiplier tube and the frequency of the interference signal is measured using a signal processor counter (TSI model 1990) which has been borrowed from DRES. The frequency data is stored digitally into hard memory in a microcomputer.

Extensive computer software has been written in the form of a data aquisitiön system to record partiele velocity as a function of time.

Since this investigation involves measuring shock structure over very short time intervals, the data aquisition system must sample at a fast rate. The present system samples data at 43 kHz, which is fast enough to obtain a sufficient number of data points in an experiment. For each particular experiment the receiving opties, signal processor counter, amount of seeding and laser power must be optimised for the best result. Af ter each experiment a velocity-time record can be graphically displayed on the microcomputer.

A lot of attention was paid to the alignment and calibration of the instrumentation parameters of the LDV system. For good quality signals it is important that the laser beams cross at their beam waist, where the beam diameter is a minimum. To verify that the beams did cross at the correct point, the control volume was rotated 360 degrees inside a 50 ~ pin hole. Furthermore, the angle between the beams was measured and compared within experimental uncertainty with the figure given by TSI.

The LDV was calibrated using a calibrating wheel. This is done by

placing glass beads, used as scattering particles, of various sizes on a glass slide attached to the rotating wheel. This is shown in figure 38. The

velocity measured using the LDV was compared with the tangential velocity of the wheel and shown to be equal, within experimental error. The rotating wheel was used as a calibrating device both outside of the shock tube and through the special LDV windows (figure 15). The TSI signal processor and data aquisition system were checked with a frequency generator as shown in figure 39a. The microcomputer and printer are shown in figure 39b.

A second calibration was performed in the shock-tube channel with a steady flow of 1-10 ~ diameter water-based fog particles. This flow was set up by the 3 horsepower centrifugal blower and recirculation system. The velocity history is shown in figure 40. A relatively constant velocity of