A descirption of the V.K.I. piston driven shock tube

(1)

I\) o

FOR FLUID DYNAMICS

TECHNICAL MEMORANDUM 20

IECHNISWE

HOGESCI OOL fJELfr

Vl!EGTUIGaOU\v KUNDf

2

n

r·-

.

o

_I _...1,.;₄' _.'

"''''''1

_/

BI UGli

EK

A DESCRIPTION OF THE V.K.I. PISTON DRIVEN SHOCK TUBE

by

M. J. LEWIS

RHODE-SAINT-GENESE, BELGIUM

(2)

(3)

TECHNICAL MEMORANDUM 20

A DESCRIPTION OF THE V.K.I. PISTON DRIVEN SHOCK TUBE

by

M.J. LEWIS

(4)

(5)

This report describes the mechanical construct ion of the riston driven shock tube. The gas supply system, the electronic control system and the operating procedure for

(6)

(7)

1. INTRODUCTION

2. MECHANICAL ASSEMBLY

2.

i • General Construct ion and

Principle

2

<

2 .

The Reservoir

2 .

3.

The Barrel

2 ,

4.

The Shock Tube Channel

2.5 .

The Dump Tank and Vacuum

2.6 .

The Piston

2.7.

The Diaphragms

2.8.

General ConsideJ"at ions

3, THE GAS SUPPLY AND CONTROL SYSTEM

4. ELECTRONIC INSTRUMENTATION

5

0

PERFORMANCE ACKNOWLEDGEMENTS REFERENCES FIGURES Operating Pumps 2 2 3 3 4 5 6

7

8

11 13 16

11

18 19

(8)

(9)

1. INTRODUCTION

Analysis of the high temperature flow fields associated with re-entry vehicles, rocket nozzles and

combustion phenomena demands theoretical models which embody thermodynamic, chemical kinetic and radiative processes

(Refs. 1-3). Such models rely on an accurate knowledge of the fundamental properties of gas particles at high temperatures, such as chemical reaction rates and thermodynamic and

radiative transition probabilities. These properties, because of the inadequacy of present day theories, of ten have to be determined in a laboratory; this calls for a device for producing a high temperature gas specimen with a known thermodynamic and chemical state.

Shock tubes have a wide range of application for this purpose (Ref.

4).

However, because of the limited shock Mach number range of the simple shock tube, and hence the

limited attainable temperature, various shock tube derivatives have been developed which are capable of producing very strong shocks and very high temperatures (Ref.

5).

Typical of this new generation in the piston driven shock tube developed by R.J. Stalker (Ref.

6).

This uses a light driver gas preheated during an adiabatic compression by a heavy pist on.

This report describes the V.K.I. piston driven

shock tube, its operation and its associated instrumentation. The theoretically predicted performance of this device, using helium or hydrogen as the driver gas and air as the test gas,

~s given by Enkenhus (Ref.

7).

A calibration, using helium as the driver gas and argon as the test gas, is presented

~n Ref.

8.

A summary of this performance is presented in this report.

(10)

2. MECHANICAL ASSEMBLY

The piston driven shock tube assembly, shown in

Fig. 1, has three main components; a reservoir containing

the gas which propels the piston, a barrel in which the piston travels and compresses the driver gas, followed by a conventional shock tube. The complete assembly is mounted on two large

girders attached to three main supports. The supports of welded steel tubing are mounted on wheels which may rolIon guide rails.

Air is used to drive the piston which compresses the helium or nitrogen driver gas in the barrel. Helium enables

very high shock Mach numbers (10 to 30) to be achieved whil st

nitrogen can be used to study the lower range of shock Mach numbers. Any desired test gas may be used in the shock tube channel.

Two metallic diapragms are used. The first retains the piston in its initial position and separates the reservoir from the barrel. The second and main diaphragm separates the high and low pressure sections of the shock tube.

In operation, the piston is released by shearing the piston diaphragm as described in section 2.6. The piston compresses the driver gas nearly isentropically raising its temperature and pressure. Near peak pressure, when the piston is almost at rest near the end wallof the barrel, the ma1n diaphragm bursts and a shock wave is transmitted into the test gas.

(11)

The reservoir, a steel cylinder, designed to with-stand a maximum operating pressure of 300 kg/cm2 , is 2.4 m in length, with an internal diameter of

273

mm, and walls 42 mm in thickness. The reservoir is strapped to two trolleys which are free to rolIon guide rail mounted on the main steel girders. A hydraulic jack moves the reservoir backwards or forwards on the rails to give access to the piston retaining diaphragm. The re~ervoir is held to the barrel by an internally threaded sleeve which mates with teeth on the barrel.

The barrel, fabricated fr om high strength steel, is a cylinder

2.387

m in length having an internal diameter of 92 mm and walls

77.5

mm in thickness. It is supported by, and is free to slide on, two greased plain bearings held by two sleeves which are bolted to the main girders. Two rubber dampers, limiting the longitudinal motion of the barrel to a maximum of

=

15 mm, are attached between one sleeve and a clamp on the barrel. This clamp also prevents rotation of the barrel.

The barrel and reservoir assembly with a piston

mounted is shown in Fig. 2~ A cylindrical adaptor, screwed into a recess at the end of the barrel, supports the piston which is prevented from moving by a diaphragm. Six screws clamp the

diaphragm between a retaining plate and a supporting collar and

~ '0' rings on all mating surfaces ~n the piston driven shock

tube provide suitable gas seals. To avoid confusion these rings are not shown on the diagrams.

(12)

hold the supporting collar to the end adaptor. Holes drilled

in the adaptor enable gas to be bIed from the reservoir

through a control valve and across the diaphragm. This gas

lS used to shear the piston diaphragm and release the piston

as explained later.

The high pressure end of the barrel, the shock tube driver section and the main diaphragm assembly are shown in Fig. 3. The driver section is a steel cylinder which slides into the barrel. It is retained in position by a breech which screws into the end of the barrel and mates against a flange on the driver section. The main diaphragm, a steel plate, is held to the end of the breech by a plate and four screws

which prevent the diaphragm from pulling out under load.

The internal diameter of the driver and breech is

38

mmo Two

driver inserts are available which give a shock tube driver length of either 105 mm or 200 mmo

A "safety nozzle" is attached to the end of the driver insert by four screws and its function is to prevent the piston from damaging the barrel at the end of the piston stroke. The nozzle is 30 mm in length and has an orifice

diameter of 17.7 mmo The principle of this safety feature is

described in Ref.

9.

The high pressure end of the barrel is designed to

withstand a maximum pressure of 6,000 kg/cm2 •

The shock tube channel shown in Fig.

4

consists

of two steel tubes (38 mm internal diameter and 70 mm

(13)

2.13 m in length is held to the barrel breach by a "V" clamp, and is free to move longitudinally on four roller bearings attached to the main girders. "U" bolts prevent any vertical or lateral motion of the tube whilst allowing it to roll freely on the bearings. The second tube 0.9 m in length is connected by a flexible tube to a dump tank. The flexible

tube, which is bolted to the dump tank, allows small vibrations of the shock tube assembly. When the bolts are removed the

channel may slide back on the bearings to gain access to the barrel breach and main diaphragm.

The working section 0.44 m in length is supported by the two channel tubes and held by two "V" clamps. Two strip windows 0.3 m in length and 6 mm wide, and two 10 mm diameter holes are available in the working section. Small windows or pressure transducers may be mounted in these holes, which are diametrically opposite each other and perpendicular to the strip windows.

Twelve small ports are available in the shock tube channel, six on each side. Thin film platinum gauge shock detectors are mounted in six of these ports, the other six being closed by blanking plugs. All fittings are mounted in such a way as to interfere least with the inner circular cross section of the shock tube channel.

A dump tank fabricated from welded steel plate is attached to the end of the shock tube. It is supported by four legs which are mounted on one of the main supports. A Leybold vacuum diffusion pump (Type PD 1000/635) and a rot.ary "backing" pump (TypeD12 156/16) are attached to the

(14)

dump tank. This system is capable of evacuàting the shock

tube channel and dump tank to pressures of less than 10- 5 torr.

To decrease the effects of outgassing and to limit the risk of contaminating the shock tube test gas, the

interior of the shock tube channel and dump tank has been nickel plated.

2.6. The Piston

---The piston is shown in Fig.

5.

It consists of two steel shafts which screw together and support an aluminium

sleeve and a nylon ring. A nylon forebody screws into the front

end of the ~iston and this forebody incorporates a small conical

plug which fits into the safety nozzle

*

entrance without binding.

The nylon ring and forebody support the piston such that metal to metal contact does not occur and, should the nylon fail, the aluminium sleeve prevents steel to steel contact. Expanding conical collars are built into both nylon supports. These,

working on the "bicycle pump" principle, provide gas tight seals without requiring accurate machining and without producing too much friction.

The piston is held in its initial position by a diaphragm which is clamped to the piston by a supporting collar. Initially gas at the reservoir pressure acts on the diaphragm support collar at B (see Fig.

5).

To release the piston, gas is bIed from the reservoir through a control valve to flange A on the piston. This increases the shear

load on the diaphragm. The diaphragm thickness ~s chosen

such that it will withstand the shear load due to the gas acting on surface B, but not the load due to the gas on

(15)

surfaces A and B.

The diaphragm which holds the piston in its initial position is an aluminium plate 107 mm in diameter. Small aluminium samples were tested in shear and the operating limits for this diaphragm were estimated. These are given in Fig.

6,

as a function of the reservoir pressure

Po.

For a given reservoir pressure and allowing for any gas pressure in the barrel ~, curve A in Fig.

6

represents the maximum diaphragm thickness which can be used if the piston is to release. Curve B gives the minimum thickness required to prevent the piston escaping prematurely. Satisfactory operation of the piston release mechanism occurs when the region between curves A and B is used to determine the required diaphragm thickness, provided a 10

%

safety margin is allowed.

The main shock tube diaphragm made from cold rolled steel is 131 mm in idameter and is unscribed. The high

pressure under operating conditions acts upon a circular area of the diaphragm 38.1 mm in diameter. Statie hydraulic strength tests were carried out on unscribed diaphragms 1 mm in thickness. These burst at approximately 300 kg/cm2 • This pressure was smaller than the pressure required to burst similar diaphragms in the shock tube. This is probably bacause in the shock tube the diaphragm is subjected to a dynamic pressure. Typical results of tests in the shock tube with and without diaphragm bursts are shown in Fig.

7,

where the maximum displacement ö of the diaphragm under

~

Any pressure in the barrel has to be substracted from the reservoir pressure before Fig.

6

is used.

(16)

load is given as a function of the statie and dynamic pressures acting upon it.

Under working conditions good bursting of the unscribed diaphragms occurs with negligible fragmentation. A photograph

of two diaphragms 1 mm and 1.5 mm in thickness) af ter use

is shown in Fig.

8.

The mm diaphragms burst at approximately

360 kg/em 2 and the 1.5 mm diagrams at approximately 600 kg/cm 2 .

The construct ion of the shock tube was designed for ease of operation and short turn round times between each experiment al run.This has proved to be the case. However, the shock tube channel has to be cleaned thoroughly when spectros-copie experiments are being ean-ied out and in these circum-stances only four runs per day are possible. Without

eleaning,eight runs per day are feasible.

Three serious problems were eneountered in preliminary experiments. It was found that the complete assembly, which was initially bolted directly to the floor of the laboratory, vibrated in operation with a frequency close to the natural frequeney of the floor. This set up dangerous vibrations in the floor. The interaction was removed by mounting the assembly on wheels. Secondly, the original nylon seals on the piston were plain cylindrieal bearings. These produced two serious problems in that the piston was either too tight in the barrel and stuck there in operation, or serious

leakage oeeurred past the seals and the piston would hit the end wall. The present piston seals have solved both the above problems, and they may be used many times before

replacement is necessary. Finally, because of the uncertain

(17)

bursting characteristics of the main diaphragm, it was difficult to control the piston during the final stages of bompression. The safety nozzle described earlier helps

in this respect. However, to obtain good diaphragm bursting, without the piston hitting the end wallof the barrel too hard, the main diaphragm must burst when the piston is about 20 mm from the end of the barrel. The pressure at bursting must then be about

5%

less than the final pressure which would be achieved if the diaphragm did not burst.

A summary of the important dimensions of the shock tube is given in Table 1.

(18)

TABLE 1

Important Shock Tube Dimensions

Reservoir volume

_{O. 136}

_m3

Barrel length (diaphragm to diaphragm)

2.42

m

Barrel internal diameter

₉₂

mm

Piston weight

_{1 1 . 3}

kg

Piston length (diaphragm to front face)

0.324

m

Driver length

₁₀₅

or

200

mm

Driver internal diameter

₃₈

mm

Channel internal diameter

38

mm

Overall channel length

_3.7

m

From main diaphragm to

Shock detector

0.415

m Shock detector

2

0.865

m Shock detector

3 1 .320

m Shock detector

4

1.765

m Shock detector

₅

_2.215

m Shock detector

6

2.964

m

Temperature and pressure

}

2.667

m

measurement positions

(19)

3.

THE GAS SUPPLY AND CONTROL SYSTEM

The gas supply and control system is shown

schematically in Fig.

9.

The method of operating the system

is as follows

1. Choose the diaphragms suitable for the working pressures expected and carry out and complete the mechanical assembly.

2. §l~~~~_~y~~y~~iQ~

1. At the vacuum pump close valves 1, 2,

3, 4, 5

and

6.

(The valves are closed when the "0" mark is seen on

the levers).

2. Start the mechanical backing pump.

3.

Check that valves

7

and 11 are closed.

4.

To evacuate the shock tube channel and barrel,

open valves 2,

3, 5, 8

and

9.

5.

Verify the barrel pressure by closing valve

8.

6.

Close valves

8

and

9.

These operations are only necessary when initial pressures less than 1 torr are required in the shock tube channel. Under these circumstances the system must be

thoroughly evacuated prior to filling the channel with the test gas to reduce the effects of test gas contamination.

1. Turn on the water supply to the d~ffusion pump.

2. When the channel pressure reaches 2 x 10- 1 torr,

open valve 1 and close valve 2.

(20)

1. Close valves 1 and 5. and stop the diffusion pump. (The cooling water must be left running for at least 30 minutes afterwards).

2. Switch off the Penning vacuum gauge.

3. Fill the channel with the test gas through valve 6.(The pressure in the test section may be checked with the Thermotron gauge by opening valve 5).

4. Open valve

9

and f i l l the barrel using valve 7. 5. Close valves

9

and 7.

6. Verify that valve 12 is closed.(Green light on, and valve pin horizontal).

7. Check that valves 1 . 2 . 4 . 5 . 6 . 7 . 8 and

9

are closed.

8. Fill the reservoir to the required pressure by closing valve 10 and opening valve 11.

9.

Close valve 1'. The tunnel is now ready to fire. 10. Fire the tunnel by operating valve 12.

, 1. Af ter firing, open valve 10 and vent the reservoir to atmosphere.

12. Stop the rotary pump, close valve 3 and open valve 4.

13. To vent the shock tube, open valve 2.

The ~low .off" safety valve mounted on the diffusion pump prevents any danger of overloading valve 1. The safety valve vents at a gauge pressure greater than 1 kg/cm2•

(21)

4.

ELECTRONIC INSTRUMENTATION

The instrumentation of the piston driven shock

tube may be divided into two separate parts; that used in the compression section and that in the shock tube. This is neces-sary because of the different time scales. The time scale for

the piston motion is of the order of milliseconds whereas

measurements in the shock tube involve microseconds.

A schematical diagram of the electronic instrumentation

is shown in Fig. 10. A piezo-electric accelerometer mounted

on the barrel gives a signal from the initial impulse of the piston accelerating from its initial position af ter shearing its diaphragm. This signal is amplified and used to trigger

a square wave pulse of 10 s duration. The start of this

signal may b~ used to trigger oscilloscopes or other

instruments which monitor the piston motion of the compression cycle. Instrumentation can also be triggered from 0 to 500 ms af ter the initial signal fr om the accelerometer by using a delay unit.

Thin film gauges are used to measure the shock wave motion along the shock tube channel and to trigger oscillos-copeswhich monitor the shocked gas conditions at the working section. A transient electronic s1gnal is generated as the

shock wave traverses each thin film gauge. These signals are

amplified and differentiated and the resulting peaked ele~

trical pulses are fed into a "gate" amplifier. This gate will only allow signals from the thin film gauge amplifier to pass when the signal fr om the accelerometer amplifier is present. This system minimises the chance of stray triggering due to laboratory electrical noise. With the gate open, the signal from the first shock detector is used to trigger a

spiral control amplifier. This amplifier is connected to the

(22)

where each complete turn of the spiral corresponds to a

.

* .

.

t~me of 100 ~s . The ~mpulses from each gauge, ~nclud~ng

gauge 1, attenuate the cathode voltage of the oscilloscope for 10 ~s** intervals of time. These impulses produce a series

of gaps in the spiral. The start of each gap corresponds to the shock wave arriving at a particular shock detector. The impulse fr om shock detector 5 is also used to trigger any oscilloscope which is monitoring gas properties at the working section.

A l i s t of the equipment which is available for use on the shock tube is given in Table 2.

*

This will later be reduced to 20 ~s to suit the present advanced performance of the shock tube.

~

(23)

TABLE 2

Shock Tube Eguipment

Charge Amplifiers Piezo-electric Gauges Oscilloscopes (2) Kistier 568 M5 Kistier 6221 and 601 A* Tektronix 531 A, (2) 502 A Hewlett-Packard 130 BR Photomultipliers (3) EMI 9558 C

Spectrometer Spex, Czerny-Turner 1600

-

*

This gauge is being replaced by aKistler 603 B which is more suitable for shock tube work.

(24)

5. PERFORMANCE

The theoretical performance of the shock tube using helium or hydrogen as the driver gas and air as the test gas

is presented in Ref.

7.

A calibration of the shock tube is

presented in Ref. 8 using argon as the test gas and helium as

the driver gas. A summary of this measured performance is given in Table 3.

TABLE 3

Performance with Argon as the Test Gas

Shock Mach numbers 10 to 30

Initial channel pressures 100 to 0.2 torr

Temperatures behind the shock front 10,000 to 14,000 K

Pressures behind the shock front 20 to 0.2 kg/cm 2

Hot flow duration at test section 150 to 20 ~s.

Similar shock Mach numbers would be achieved in air

inferring lower temperatures behind the shock front. The

possible re-entry vehicle stagnation conditions that may be

simulated in the piston driven shock tube, using air or

(25)

ACKNOWLEDGEMENTS

The VKI expresses grateful thanks to the Naval Ordnance Laboratory, Silver Spring, Maryland, USA, who kindly donated the barrel of the shock tube.

The author is greatly inde~ted to Messrs. J.L. Royen, R. Borres, F. Toubeau and F. Vandenbroek of the VKI staff.

Messrs. Royen and Borres designed and built the electron ic instrumentation. Mr. Toubeau, together with Mr. Royen, designed the mechanical system and Mr. Vandenbroek assembIed the shock tube.

Many thanks are also expressed to Miss L. Abbott for typing the manuscript.

(26)

REFER~NCES

1. Selected reports from "Proceedings of the Sixth International Shock Tube Symposium".

Ernst Mach Institute, Freiburg, Report No.4/68, April 1967.

2. Selected reports fr om "AGARD Conference Proceedings" C.P. No.12, 1967.

3. WOLF, F., SPIEGEL, J.

"Status of Basic Shock Layer Radiation Information for Inner-Planet Atmospheric Entry".

AIAA Journal Spacecraft, Vol.4, pp 1166-1173, Sept. 1967.

4. GAYDON , A. G., HURLE, I. R. :

"The Shock Tube in High-Temperature ChemiC'al Physics". Reinhold Pub. Co., N.Y., 1963.

5. ENKENHUS, K.R.

"Intermittent Facilities"

VKI Short Course on Advanced Shock Tube Techniques, Lecture Series II, Vol. I, Feb. 1969.

6. STALKER, R.J.

"The Free Piston Shock Tllbe"

Aeronautical Quarterly, ~, Part 4, pp 351-370, 1966. 7. ENKENHUS, K. R. :

"Theoretical Performance Study of the Free-Piston Shock Tube".

VKI TN 42, Jan. 1968.

8.

LEWIS, M.J., SMITH, J.

"Performance Data fr om the VKI Piston Driven Shock Tube" •

VKI TN 69, 1971-9. KNOOS, S.,

"Piston-Retardation Insert in a Free-Piston Compression" AIAA Journal, Vol. 8, No.1, pp. 169-171, Jan. 1970.