The icing of cylinders in conditions of simulated freezing sea spray

(1)

MECHANICAL ENGINEERING REPORT M D-50

THE ICING OF CYLINDERS IN CONDITIONS OF

SIMULATED FREEZING SEA SPRAY

BY

J. R. STALLABRASS AND P. F. HEARTY DIVISION OF MECHANICAL ENGINEERING

OTTAWA

JULY 1967

This Report May Not Be Published In Whole Or In Part Without The Written Consent Of

The National Research Council

(2)

REPORT

Division of Mechanical Engineering Low Temperature Section

Pages - Preface - 3 _{Report: MD-50}

Text - 11 _{Date: July 1967}

Tables -

i

_{Lab. Order: 16329A}

Figures - 17 _{File: M2-17-131-5}

Subject: _{THE ICING OF CYLINDERS IN CONDITIONS OF}

SIMULATED FREEZING SEA SPRAY

Submitted by: T. R. Ringer Authors: J. R. Stallabrass

Section Head P. F. Hearty

Approved by: D. C. MacPhail

Director

SUMMARY

Tests carried out in an icing wind tunnel on cylinders of various diameters demonstrate the effect of air temperature and cylinder _{diameter on} the percentage of spray that freezes on the cylinder.

(3)

TABLE OF CONTENTS

Page

SUMMARY (i)

1.0 INTRODUCTION 1

2.0 THE ICING PROCESS 1

2. 1 Droplet Trajectories

i

2. 2 Supercooling and Ice Nucleation 3

2. 3 Heat Balance 4

4 Icing Efficiency 5

3.0 TEST APPARATUS 6

1 Icing Wind Timnel 6

3. 2 Description of Cylinders 6 4.0 TEST PROCEDURE 7 5.0 TEST RESULTS 8 6. 0 DISCUSSION 9 7.0 CONCLUSIONS 10 8.0 REFERENCES 11

TABLE I TEST RESULTS

LIST OF ILLUSTRATIONS

Figure

Plan View of Icing Wind Tunnel

i

Streamline Flow around Circular Cylinder 2a

Water Drop Trajectories in the Vicinity of a Circular Cylinder 2b

(4)

LIST OF ILLUSTRATIONS (Cont'd)

Figure

Dimensions of Ice on Vertical arid Horizontal Cylinders 4

Ice Accretions on 1k-Inch Horizontal Cylinder after 1 Hour

Exposure 5

Ice Accretions on 3-Inch Horizontal Cylinder after 1 Hour

Exposure 6

Exposure 7

Expo sure 8

Ice Accretion on 1k-Inch Vertical Cylinder after 1 Hour

Exposure (Run No. 11) 9

Ice Accretion on 1-Inch Vertical Cylinder after 1 Hour

Effect of Temperature on Icing Efficiency 15

Effect of Cylinder Diameter on Icing Efficiency 16

Effect of Temperature on Ice Thickness 17a

(5)

1.0 INTRODUCTION

As an initial investigation in a program of research into the problem of the icing of fishing trawlers resulting from the freezing of sea spray blown over the vessel, tests have been made in an icing wind tunnel to study the

ice formation and rate of growth on cylinders of various diameters.

Earlier British tests on a model fishing trawler in _{an icing wind}

tunnel, the main concern of whichwas with the loss of stability due to ice

accre-tion, are reported in Reference 1. The present tests, however, are concerned with the more basic aspects of the icing process itself. As such, these tests

form a useful introduction to some of the concepts of icing technology for those

not familiar with the subject. In addition, the results help to provide _some

in-dlication of the percentage of spray that freezes _{on superstructure and rigging.}

Lack of such information has been noted in References 2 and 3.

Certain aspects of the icing process are discussed in the

follow-ing Section, and the concept of an Icfollow-ing Efficiency is introduced. 2.0 THE ICING PROCESS

2. 1 Droplet Trajectories

Consider an obstacle placed in_{an airstream having a velocity V}

relative to the obstacle (or alternatively, _{an obstacle moving with velocity V} through stationary air); the air will be _{displaced by the presence of the body} _and

will flow around it. The manner in which the air flows around the obstacle may

be visualized by the use of streamlines, i.e. curves whose direction in each

point coincides with the direction of the velocity of the fluid. Streamline flow

around a cylinder is shown in Figure 2a.

THE ICING OF CYLINDERS IN CONDITIONS OF SIMULATED FREEZING SEA SPRAY

(6)

If a cloud of droplets is in suspension in the airstream, the droplets follow the streamlines where the latter are straight, but where the streamlines are curved in the vicinity of the obstacle the droplets follow

trajec-tories that are somewhat less curved. These trajectrajec-tories are the result of two forces acting on the drops:

the drag force that tends to make the drops conform to the local air velocity and direction (i. e. to follow the streamlines), and the inertia force that tends to make the drops continue iii a

straight Une at a constant velocity.

Figure 2b illustrates the manner in which the droplet trajec-tories depart from the streamlines as they approach a cylinder.

The drag force depends on the density of the air, the projected frontal area of the drop, the drag coefficient of the drop, and the local relative

velocity between the drop and the air.

The inertia force depends on the mass of the drop and its rate

of change of velocity.

Since droplet mass is proportional to density times the cube of the diameter, while projected frontal area is proportional to the square of the

diameter, it follows that change in droplet size has a greater effect on its inertia than on drag (other factors remaining the same). Thus larger droplets are de-flected less from their original direction by the presence of an obstacle than are

smaller droplets.

By a similar reasoning it may be shown that because of a large deflection in the direction of air flowing around a large or bluff obstacle, drag effects are great and consequently droplet deflection is large, while for flow around a small or slender obstacle the small deflection in the air flow results in little deviation in the paths of the droplets.

(7)

The effect of droplet deflection may be expressed by a single non-dimensional number called the Collection Efficiency. The collection

efficiency is defined as the ratio of the mass of the droplets actually impinging

on the obstacle in unit time to the mass of the droplets that would have impinged in the same time had no deflection of the drops taken place (i. e. that would have

passed through the projected frontal area of the obstacle if the obstacle were

removed).

Thus the larger the droplet size the larger the collection effi-ciency, while the larger the obstacle the smaller the collection efficiency. If

the collection efficiency is denoted by Em the velocity of the air relative to the obstacle by V, the projected frontal area of the obstacle by A, and the mass of water droplets in unit volume of air by m, then the mass of water impinging on the obstacle in time, t, is given by

M =E VAmt

_w _m

Values of E_m are not easily calculated; however, solutions are

available for simple shapes of obstacles such as cylinders, spheres, and flat

plates (see, for instance, Ref. 4). 2. 2 Supercooling and Ice Nucleation

Droplets of pure water can exist in a supercooled state (i. e. at temperatures below their equilibrium freezing point) for extended periods of time

down to temperatures of about -35°C to -40°C (Ref. 5). Freezing of these

droplets occurs only when minute particles known as freezing nuclei are present. The freezing nucleus (which could be another ice crystal) promotes the organi-zation of the water molecules into the specific geometrical arrangement of the

ice crystal lattice.

The supercooling properties of sea spray droplets are unknown, but indications are that below about -18°C (0°F) the spray is frozen (Ref. 1) and reportedly strikes the ship as small, dry ice crystals that will not adhere.

(8)

However, that such crystals are dry is doubtful, since traces of brine were still contained in sea ice that had been cooled to 180°C (Ref. 6, p. 15).

When supercooled droplets strike an obstacle, the temperature of which is below the droplets' equilibrium freezing temperature, nucleation also occurs, the surface acting as a catalyst. Whether all or only part of the impinging water actually freezes depends on whether sufficient heat is removed from the water/surface system to balance the latent heat available for release when the water turns into ice.

2. 3 Heat Balance

The complete heat balance at the icing surface can be expressed

by the following equation

qfq -q

k w i

where qf = Heat of fusion of accreted ice

= Kinetic heating due to water impingement

= Heating of impinging water from initial temperature to 0°C Cooling of resultant ice from 0°C to final ice temperature = Kinetic heating due to air

= Viscous heating in boundary layer

q = Convective heat loss

q = Evaporative heat loss

q_a = Heat conducted to or from the icing surface through the underlying structure

Heat loss or gain due to radiation.

In the case of sea spray icing, several of the above terms may be considered negligible and the heat balance reduces to

+q

0

a

(2)

(9)

The solution of this equation determines the fraction of the

im-pinging water that freezes on the object. If the temperature is close to the

freezing point, and q are small, and correspondingly, q1 will also be

small; thus the freezing fraction is small. If the quantity of water impinging increases, the term q will also increase, but q and will remain substan-tially constant; the net result is that qf increases, but not in proportion to the increased water impingement, consequently the freezing fraction again decreases. Thus the fraction of impinging water that freezes decreases both as the

temp-erature increases and as the water concentration increases. The _{excess water}

will run off or he blown off the surface, and the _{mass of ice forming on the}

obstacle in time, t, is

M. =nM

_i _w nE VAmt_m

where n is the freezing fraction. 2.4 Icing Efficiency

In most practical situations, fraction n is an unknown quantity,

not readily amenable to calculation. Further, as ice forms on the obstacle, the

shape presented to the flow of droplets changes, as also may the frontal area. Thus, in equation (3), the quantities n, E, and A may all change with time, so

that the instantaneous rate of icing, R = dM./dt, also becomes a function of time.

If in equation (3), A is considered constant (i. e. equal to the frontal area of the ice-free obstacle), then it is possible for Em and even the Icing Efficiency, N nE, to assume values greater than unity if the frontal

area of the iced obstacle exceeds sufficiently the ice-free area A.

This concept of Icing Efficiency will be used in this Report, not

in the form of its instantaneous value at some specific time, but rather_{as a}

(10)

one-hour average icing efficiency that will be denoted by the sumbol Nih.

M.

Thus N

-1h VAm

where M. is the mass of ice formed in one hour

j-and

A=LD

L cylinder length

D = cylinder diameter.

This Report therefore presents values of the one-hour average icing efficiency derived from actual measurements, and thus gives some measure

of the percentage of spray that would be expected to freeze on various exposed

cylindrical surfaces of a ship.

3.0 TEST APPARATUS 3. 1 Icing Wind Thnnel

The icing tests were conducted in a closed circuit, refrigerated

icing wind timnel (Fig. 1), in which the air is continuously recirculated by a

1, 000-hp electric fan. At one point in its circuit the air passes through a cooling

heat exchanger to maintain it at the desired temperature. The icing condition is produced by an array of water spray nozzles located ahead of the 4-ft. square test section. These spray nozzles are of the air-atomizing type of design. The

tunnel is capable of speeds up to 180 mph, and a temperature range of about -30°C to room teperature.

3. 2 Description of Cylinders

Five cylinders, 1

in., 3 in., 6 in., 12 in., and 18 in. in diameter,

were used for the tests in the icing wind tunnel. The 1-in. diameter cylinder was

(11)

the tunnel walls. It was also used to support the larger diameter cylinders.

The 3-in, cylinder was 3 ft. long and was mounted concentrically, using wooden discs, over the 1f-in. cylinder. The 6-in., 12-in., and 18-in., cylinders (also

3 ft. long) were mounted similarly, but were made of rolled sheet aluminum

instead of tubing. All cylinders were locked to prevent rotation.

4.0 TEST PROCEDURE

Tests were rim on each cylinder in both a horizontal and a vertical

orientation.

With each cylinder in turn mounted in the wind tunnel, the

air-speed was set at 50 mph (corresponding to a wind force of 9 on the Beaufort

Scale), arid the air temperature at a convenient value between -16°C and -5°C. The icing sprays were set to give a water concentration of 3. 2 grams per cubic

metre of air (gm/rn3) with a median volume diameter of the droplets of about

200 microns*.

The cylinder was allowed to ice-up for one hour; the shape of the

ice accretion was then noted together with salient thicknesses and dimensions. A 3-ft. length of the ice accretion was then removed from the cylinder and

weighed.

The water concentration was estimated from the quantity of water being sprayed into the airstream, its area of coverage, and the air velocity. A

check of this estimate was made using a hot wire liquid water content meter at

an air velocity of 100 mph (the minimum speed at which the meter can be operated),

but with the same water flow rate. The meter reading of 1. 6 gm/rn3 confirmed

the estimate of 3. 2 gm/rn3 for the 50 mph case.

* _{One micron = one millionth of a meter; therefore 200 microns} _{= 1/5 mm.}

The volume median diameter denotes that half the volume of water in a given

sample is contained in drops larger than the quoted value, _{and half in drops} smaller.

(12)

Also, to confirm the estimate of droplet size, periodic samples were taken using an oiled slide droplet sampler (Ref. 7). The results of one such sample are shown in Figure 3, which indicates a median volume diameter of 185 microns and a maximum drop size of 460 microns. Drops of smaller than 40 microns in the sample were not counted since their contribution to the total

volume of water was considered negligible (the 91 droplets in the range 40-50 microns contributed only two percent to the total volume of water in the sample).

5.0 TEST RESULTS

In all, 21 test runs were made. Four runs at different

tempera-tures were made on each size of cylinder, two with the cylinder horizontal and

two with it vertical. Details of each run are given in Table I, and sketches of

ice shapes are presented in Figure 4. Figures 5 to 14 show the ice accretions at the conclusion of many of the runs. The effect of greater blow-off and run-off

of water at the higher temperatures is evident in a smaller ice accretion, and the effect of gravity results in an asymmetrical accretion and icicles on the horizontal cylinders, and a variation of thickness with height on the vertical cylinders.

The extra run (No. 21) was made to ascertain whether the water droplets produced by the nozzles were supercooled to the temperature of the air by the time they reached the cylinder. The water temperature was normally in

the range of 16°C to 20°C prior to entering the spray rig, thus requiring a

reduc-tion in temperature of between 20°C and 40°C during their flight to the cylinder

(a distance of 17 ft.). Run No. 21 was made with an initial water temperature of 42°C, other conditions being essentially the same as run No. 12. The icing

effi-ciency was substantially the same for both of these runs. It is reasoned that in both cases the droplet temperature was also substantially the same (i. e. at the

ambient temperature) on impact with the cylinder.

In spite of the differences in the shape of the ice accretions on a horizontal and on a vertical cylinder (as the result of gravity effecting the run-off),

(13)

no significant difference in icing efficiency was apparent. It was therefore possible to combine the results from both orientations in Figures 15 and 16,

which show the effect of temperature and of cylinder diameter on the icing effi-ciency. As these tests were made using fresh water, it is suggested that all

temperatures should be reduced by about 2°C to make them applicable to sea

water.

6.0 DISCUSSION

These tests constitute a rather tentative introduction to the subject of superstructure icing resulting from sea spray. No very definitive information is currently available regarding the significant parameters of the spray (i. e. the water content and the drop size distribution); not only are these difficult to mea-sure but they are a fi.mction of a large number of other parameters such as state of the sea, wind force, ship speed and heading relative to the wind, size and design of hull, and sampling location relative to the ship, etc. _{The water} concen-tration and drop size used in these tests were therefore chosen rather intuitively

within the capabilities of the existing spray-producing facility of the icing wind

tunnel, and are felt to represent conditions that might exist under certain

combi-nations of the contributing parameters noted above. No great significance should

be read into the actual rates of icing of the test cylinders, since their relationship to rates of icing at sea is unknown.

However, the icing efficiencies derived from these tests, and presented in Figures 15 and 16, are considered to give _{a useful guide to the}

fraction of water that actually freezes on rails, masts, lines, etc. over an

ex-tended period of time. As noted in Reference 2, "there is virtually no knowledge

as to the rate at which spray strikes the exposed surfaces of a ship or of the per-centage that freezes". These results therefore provide some indication of the

latter.

The manner in which the icing efficiency increases with decreasing cylinder size, even to values greatly in excess of 100 percent for small diameter

(14)

cylinders (Fig. 16), demonstrates very forcefully the absolute necessity for keeping the number of rails, stays, etc. to a minimum, and that, where it is not possible to dispense with them completely, one larger diameter strut should be

made to do the job of several smaller ones.

It is noticeable from Table I that the ice thickness is not signifi-cantly affected by the cylinder diameter, but only by the temperature. On the other hand, the width of the ice accretion (dimension A, Fig. 4) is affectedby

cylinder diameter as well as temperature, and it is this dependence on cylinder

diameter that is reflected in the icing efficiency. Figures 17a and 17b

demons-trate the effects of temperature on ice thickness, and of cylinder diameter on the ice width expressed as the ratio of width to cylinder diameter.

The fact that the ice thickness appears to be independent of cylin-der diameter (a relationship that does not apply when the freezing fraction is unity) leads to the speculation that, for sea spray ice, the rate of ice growth might give an approximate measure of the spray concentration providing the air temperature and velocity are known. Tests at other water concentrations and

speeds are required to ascertain the reliability of such a measure.

7.0 CONCLUSIONS

Simple ice accretion tests on a number of vertical and horizontal cylinders mder conditions intended to represent sea spray icing were carried out

in a force 9 wind (50 mph). The dependence of the icing efficiency on the cylinder

diameter and on the air temperature was demonstrated; the icing efficiency being shown to increase with both decreasing diameter and decreasing temperature.

It was shown, for the conditions of the tests, that the ice thickness

is a function of the temperature, but is independentof the cylinder diameter. The frontal projection of the ice accretion, on the other hand, is afunction of both

temperature and cylinder diameter, such that for small diameter cylinders the ice width may grow to many times the cylinder diameter.

(15)

These tests indicate clearly the necessity for reducing rigging, spars, etc. to a minimum as one very necessary measure for diminishing the

hazard of icing at sea. 8.0 REFERENCES

1. Trawler-Icing Research.

BSRA Report 221, British Shipbuilding Research Association, London, England, 1957.

2. Lee, A. Ice Accumulation on Trawlers in the Barents Sea.

Marine Observer, Vol. 28, No. 181, 1958, pp. 138-142.

3. Fein, N. A Survey of the Literature on Shipboard Ice Formation.

Freiberger, A. Naval Engineers Journal, Dec. 1965, pp. 849-855.

4. Langmuir, I. A Mathematical Investigation of Water Droplet Trajectories.

Blodgett, K. B. United States AAF II-t 5418, 1946.

5. Bigg, E.K. The Supercooling of Water.

Proc. Phys. Soc., Sec. B, Vol. 66, Pt. 8, Aug. 1953, pp. 688-694.

6. Poi.mder, E.R. The Physics of Ice.

Pergamon Press, Oxford, England, 1965, p. 15.

7. Golitzine, N. Method of Measuring the Size of Water Droplets in Clouds,

Fogs and Sprays.

NRC, DME Mech. Eng. Report ME-177, National

(16)

TABLE I

TEST RESULTS

Run No.

Dia, of _{Cyl. (in.)}

Position of Cyl. Air Temp. °C Wt. of Ice (lb.) Icing Eff. (%)

Dimensions of Ice Formation at Mid-Span (in.) (see Fig. 4)

A B C 1 1.5 horizontal -15 33-3/4 170 14 3-1/2 4-1/4 2 1. 5 horizontal -8 18 90 6-1/2 2 3 3 3 horizontal -14 37 95 13 3-1/4 3-3/4 4 3 horizontal -7 23 58 9 2 3-1/4 5 6 horizontal -16 53-1/2 67 15-1/2 3-1/4 5 6 6 horizontal -7. 5 31 38 11-1/2 2 3-3/4 7 12 horizontal -14 80 50 25 3-1/4 4 8 12 horizontal -10 54-1/2 38 19 2 4 9 18 horizontal -13 93 39 27 3-1/4 4-1/2 10 18 horizontal -9 65 27 22 1-1/2 3 11 1.5 vertical -15 34 170 9 3-1/2 5-1/4 12 1.5 vertical -6. 5 14 70 4-3/4 1-5/8 3 13 3 vertical -10 30 75 8-1/4 2-1/2 4 14 3 vertical -6. 0 22 55 6-1/2 2 3-1/4 15 6 vertical -11 45 56 11 2-1/2 3-1/4 16 6 vertical -6. 0 30 37, 5 9-1/2 1-3/4 3-1/4 17 12 vertical -11 63 40 15 2-1/2 4 18 12 vertical -5. 5 31 20 13 1-1/2 2-1/2 19 18 vertical -15 84 35 20 3 5 20 18 vertical -4. 5 38 16 18-1/2 1-1/4 3 21 1. 5 vertical -6 13 66 5 1-1/2 2-3/4

(17)

FAN

DROPLET SETTLING CHAMBER

AIR FLOW TEST CYLINDER IN VERTICAL POSITION TEST SECTION 4.5 x 4.5 ft. REFRI GERATION COILS

PLAN VIEW OF ICING WIND TUNNEL

(18)

(19)

TANGENT TRAJECTORY

___--vo0

N N

WATER DROP TRAJECTORIES STREAMLINES

COLLECTION EFFICIENCY Em

WATER DROP TRAJECTORIES IN THE VICINITY OF A CIRCULAR CYLINDER

(20)

80 60 40 20 o o VOLUME

MEDRAN DIAMETER = I85

H

DROPLETS SMALLER 4O WERE INCLUDED THAN NOT n 00 200 300 400 500

DROPLET DIAMETER (MICRONS)

ANALYSIS OF WATER DROPLET SAMPLE

80 60 40 20 o I00

(21)

VERTICAL O_2O% LESS THAN MfDSECTION - MIDSECTION IO_2O% GREATER THAN MIDSECTION A A VERTICAL HOR IZONTAL

(22)

CROSS-SECTION OF ICE

IR TEMPERATURE -15°C. (RUN NO. I)

fI! I

AIR TEMPERATURE -8°C. (RUN NO. 2)

(23)

AIR TEMPERATURE: -14°C. (RUN NO.3)

(24)

AIR TEMPERATURE: -7.5°C. (RUN NO. 6)

(25)

AIR TEMPERATURE: -14°C. (RUN NO. 7)

AIR TEMPERATURE; -fO°C. (RUN NO. 8)

(26)

FRONT

AIR TEMPERATURE: -15°C.

ICE ßCCRETIONS ON If-INCH VERTICAL

CYLINDER AFTER

RUN NO.

II

(27)

BACK

AIR TEMPERATURE-65°C.

CE ACCRETIONS ON I-i-INCH VERTICAL

CYLINDER IFTER

RUN NO.

(28)

AIR TEMPERATURE: -6.7°C.

ICE ACCRETIONS ON 3-INCH VERTICAL

CYLINDER AFTER

RUN NO. 14

(29)

BACK

AIR TEMPERATURE: -6.0°C,

ICE ACCRETIONS ON 6-INCH VERTICAL CYLINDER AFTER

(30)

RUN NO. 18

BACK

FRONT

IR TEMPERTURE -5.5°C.

ICE ACCRETIONS ON 12-INCH VERTICAL CYLINDER

AFTER

I

(31)

BACK

AIR TEMPERATURE -4.5°C.

ICE ACCRETIONS ON 18-INCH VERTICAL

CYLINDER AFTER

(32)

18 16 4 20 00 80 60 40 20 o I 5 Dl HORIZ VERT. DA ' 3 6 12 18

u

A A y 3 DI& 2 6 DIA. DIA. 8 DIA. -16 -(2 _-8 _-4 _o IR TEMPERATURE °C.

(33)

80 60 40 20

i

loO o 80 z o 60 40 20 o t -10°C - 5°C - 15°C o 3 6 9 12 15 ¡8

DIAMETER 0F CYLINDERS (IN.)

EFFECT OF CYLINDER DIAMETER ON ICING EFFiCIENCY

(34)

4 3 DIMENSION B (FIG 4) D D D DIMENSION B X C D HO R IZONTA L VERTICAL DIMENSION C (FIG. 4) o X X D AIR TEMPERATURE-°C.

EFFECT OF TEMPER/\TURE ON iCE THICKNESS

O

(35)

V.'u 8 6 4 2 o -15°C. 0 1.5 3 6 9 2 5 '8 CYLINDER DIAMETER-IN.

EFFECT OF CYLINDER DLAMETER ON ICE WIDTH

\

-5

VERTICAL CYLINDER -5°C.