Determination of Heat Transfer Coefficients of Boiling Liquids

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P U R D U E U N IV E R S IT Y

THIS IS TO CERTIFY THAT THE THESIS PREPARED UNDER MY SUPERVISION

by

Richard Garter Potter

ENTITLED

Determination of Heat Transfer Coefficients of Boiling Liquids

COMPLIES WITH THE UNIVERSITY REGULATIONS ON GRADUATION THESES

AND IS APPROVED BY ME AS FULFILLING THIS PART OF THE REQUIREMENTS

FOR THE DEGREE OF

Doctor of Philosophy

C û. M . CJLqjla.

P r o f e s s o r in C h a r g e o f T h e s is

:e a p o f S c h o o l o r D e p a r t m e n t

August 16

19

TO THE LIBRARIAN:-----

w THIS THESIS IS NOT TO BE REGARDED AS CONFIDENTIAL.

PROFESSOR EST CHARGE

REGISTRAR FORM 10—7-47— 1M

DETERMINATION OP HEAT TRANSFER COEFFICIENTS OF BOILING LIQUIDS

A Thesis

Submitted to the Faculty

of

Purdue University

by

Richard Carter Potter

In Partial Fulfillment of the Requirements for the Degree

of

Doctor of Philosophy

August, 1950

ProQuest N um ber: 27714174

All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is d e p e n d e n t upon the quality of the copy subm itted. In the unlikely e v e n t that the a u thor did not send a c o m p le te m anuscript and there are missing pages, these will be noted. Also, if m aterial had to be rem oved, a n o te will ind ica te the deletion.

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To Ann Stimson Potter

ACKNOWLEDGMENT

I wish, to take this opportunity to express my thanks to Mr* William Van Camp who obtained the experimental data on ethanol and to Professor C* R* St* Clair who supervised that activity* I am also indebted to Dr. W* L* Sibbitt, who directed my portion of the experimental work and to certain members of the staff of the School of Electrical Engineering who offered advice and suggestions on the design of the electrical circuit and the instrumentation#

ABSTRACT

The surface coefficients of heat transfer for boiling ethanol and carbon tetrachloride with a small horizontal wire at atmospheric pressure were determined* Data were also obtained for saturated water boiling at a pressure of approximately 300 psi*

A correlation of

the data based on physical properties for ethanol and carbon tetrachloride showed a close grouping of experimental points and favorable comparison with other data on boiling liquids* The data on water were not correlated but two experiments under the same conditions showed excellent reproducibility of results*

TABLE OF CONTENTS

Page INTRODUCTION .....................................

1

SURVEY OF THE LITERATURE

5

...............

10

RESULTS AND CONCLUSIONS.................... Re suit s

.......................

Conclusions

10

............

DESCRIPTION OF THE APPARATUS

19 ............

21

...................

21

Power Supply and Instrumentation ..............

27

Boiler

EXPERIMENTAL PROCEDURE...................

3i(.

APPENDIX A.

TABLES OF D A T A .......................

36

APPENDIX B.

CALCULATIONS ........................

50

APPENDIX C.

NOMENCLATURE ANDABBREVIATIONS

.....

55

APPENDIX D.

ACCURACY OF RESULTS

...............

57

APPENDIX E.

BIBLIOGRAPHY......................

59

APPENDIX F.

EQUIPMENT ............................

6l

LISTS OP TABLES AND FIGURES List of Figures Figure le

2.

3»

4e

5.

6.

7.

8.

9*

Page Typical curve of heat flux vs temperature difference between heated surface and bulk liquid for boiling liquids .........

6

Surface coefficient of heat transfer vs temperature difference between surface and bulk liquid— saturated carbon tetrachloride at temperature of 168 F

11

Surface coefficient of heat transfer vs temperature difference between surface and bulk liquid— saturated ethanol at .... temperature of 176°F

12

Surface coefficient of heat transfer vs temperature difference between surface and bulk liquid— saturated water at temperature of °F

13

Heat flux vs temperature difference between surface and bulk liquid— saturated carbon tetrachloride at temperature of l68° F .................................

14

Heat flux vs temperature difference between surface and bulk liquid— saturated ethanol at temperature of 176°F . .. ....

15

Heat flux vs temperature difference between surface and bulk liquid — saturated water at temperature of 444 °p ........

l6

Dimensionless plot of heat transfer in boiling carbon tetrachloride and .... ethanol

17

Cross-section of boiler

23 .......

25

10.

Heater wire assembly (original)

11*

Heater wire assembly (modified) ... . . .....

25

12.

Boiler shell with heater wire assembly (modified) mounted in position ..........

26

Bath heater circuit wiring diagram ..........

28

13*

List of Figures - Continued Figure ......

14*

Test circuit wiring diagram

is#

General view of experimental apparatus

16 .

Close-up of instrument panel •••••••«.

List of Tables Table Is

2

,

Heating of Saturated Water at F Run No* 1 Run No# 2 ............. .. Heating of Saturated Carbon Tetrachloride at 168° F - Run No. 1 ............... at l6if° F - Run No. 2 ...............

3.

Heating of Saturated Ethanol at 176°F Run No. 1 ...... •*•• .............

4.

Heating of Saturated Ethanol at 176°F ................... . Run No. 2 ...

.

5

»

Heating of Saturated Ethanol at 176°F Run No. 3 ................. ........ .

6

Heating of Saturated Ethanol at 176°F Run No. 4 .... ....... ....... .

7.

Heating of Saturated Ethanol at 176 F Run No. 5 ....

8,

Heating of Saturated Ethanol at 176°F Run No. 6 . ...... . ..... .. .........

9*

Data of McAdams, et al (II4.) on Heating of Saturated Water at Atmospheric Pressure.

#

,0

.

I DETERMINATION OF HEAT TRANSFER COEFFICIENTS OF BOILING LIQUIDS

INTRODUCTION

Previous investigations of the boiling of liquids to determine heat transfer coefficients have resulted in a considerable body of uncorrelated data.

This investigation

was attempted in an effort to extend the store of informa­ tion on this subject.

It was hoped that these additional

data would be sufficient to make possible an acceptable correlation with other data on boiling liquids. Boiling is defined as evaporation inside a liquid body with consequent formation of vapor under the free surface of the liquid.

If vapor bubbles form on the heated surface and

rise from it to pierce the free liquid surface, the condi­ tion is termed ordinary nucleate boiling.

Another type of

boiling, film boiling, occurs when the temperature differ­ ence between the heated surface and the liquid is so great that a film of superheated vapor forms between the heated surface and the liquid.

This film of vapor acts as an in­

sulator between the heated surface and the liquid and re­ tards the flow of heat; when finally the vapor film becomes large enough it breaks away from the heated surface, rises as a large bubble, and pierces the free liquid surface just as in nucleate boiling.

Either nucleate or film boiling may

2 also take place In conjunction with the forced circulation of liquid past the heated surface; this is called forced convection boiling*

Boiling in a stationary pool of liquid

is termed free convection boiling or just free boiling* Nucleate boiling is the type most commonly occurring in industrial equipment and this study is limited to such boil­ ing. The law governing the transfer of heat during the boil­ ing process takes the simple symbolic form q” = h 6S

q" = Heat transferred per unit time per unit area of heated surface, usually called "heat flux." h

= Coefficient of heat transfer*

9S = Temperature difference between the heated surface and the bulk of the liquid* Thus* in determining the coefficient of heat transfer, it is only necessary to measure qM and ©s*

The coefficient,

h, for a particular condition is then calculated from these measured values* Correlation of data is aimed at establishing a relation between any two of the three quantities presented in the general law and certain measurable or known physical charac­ teristics of the apparatus and the fluid.

For instance, if

one is to design an i tern of heat transfer equipment he has

3 only to establish the fluid and its state and either q" or Q3 in order to determine any of the other quantities in­ volved in the heat transfer relation. An experimental investigation of this sort involves basically three items: 1»

Boiler

2#

Source of heat energy

3*

Instrumentation

The details of these items are described under Descrip­ tion of the Apparatus, The boiler shell was a heavy-walled stainless steel container of small volume, about 250 cubic centimeters, equipped with a condenser*

The stainless steel was used to

reduce the corrosion known to be produced in varying degrees by liquids at high temperatures and pressures*

The volume

was small because it originally was intended to use the boiler for the measurements of heat transfer coefficients of new fluids of very limited quantities*

Although data

were obtained only for free convection and nucleate boiling at atmospheric pressure and about 300 psi a, the boiler was designed to operate at higher pressures and could be adapted to forced convection boiling.

The heated surface was a

small diameter, 0,010 inch, platinum wire* The measured electrical energy dissipated in the test length of platinum wire was changed to heat energy, qff, by the well-established factor converting watts to B per hour.

k

Energy was supplied to the wire by a small portable direct current generator; it was measured by observing the poten­ tial drop across a standard resistor and the resistance of the heated surface#

The potential drop was measured with

a K-2 type potentiometer and the resistance with a Kelvin bridge#

The change in the Kelvin bridge reading between

data points was used in the calculation of

5 SURVEY OF THE LITERATURE

The literature on the subject of boiling is quite ex­ tensive and seems to be concerned with two broad are as, which can be called microscopic and macroscopic#

The first

is concerned with the mechanism of boiling, such as growth, formation and size of vapor bubbles, while the latter deals primarily with the determination of heat transfer coeffi­ cients, as described in the Introduction#

Both areas of

study are aimed in the same direction and are actually in­ separable; the macroscopic studies being primarily empiri­ cal, however, in an effort to obtain presently useful in­ formation# Since this investigation is classified in the macro­ scopic area, this survey is restricted to similar experi­ mentation# One of the pioneers in the study of boiling using electrically heated wires was Nukiyama (15)»^

His contribu­

tion was principally one of exploration of the now familiar boiling curve shown in Figure 1# Insinger and Bliss (8 ) studied boiling with an elec­ trically heated hollow tube immersed in various liquids# The tube was chromium-plated and mounted in a vertical posi­ tion#

Particular pains were taken in their investigation

^ Numbers in parentheses refer to Bibliography, Appendix E#

FIGURE

I

UNSTABLE F IL M BOILING STABLE FILM

BOILING

NUCLEATE B O ILIN G

N ATURAL CONVECTION

e TY P IC A L BETWEEN

CURVE OF

HEATED

HEAT

SURFACE

FLUX

VS

AND BULK

TEMPERATURE L IQ U ID

FOR

DIFFERENCE BOILING

LIQUIDS

to) insure a clean heating surface•

They found that time is

not an important variable viaere contamination is practically eliminated#

The equation which fitted their data, only in

the region of nucleate boiling was Log Y = O .363 + 0.923 log X - 0.047 (log X )2

1 rg r )V2

•

10

10

Jakob (9) has summarized work done by him and his co­ workers, Fritz and Linke, on the subject of boiling#

He

investigated the effect of conditions of the heating sur­ face on the heat transfer rate, and also presented a corre­ lation based on physical considerations#

This he has shown

to be equivalent to the Insinger and Bliss relation, when it was modified to a straight line relation, Y = i|. X^#^ * The influence of pressure has not been definitely established#

Insinger and Bliss found that a fairly satis­

factory correction to atmospheric pressure boiling data could be made by multiplying the value of h obtained from their equation by (p/pa)^*^ p ss existing pressure pa = atmospheric pressure * The meaning and units of symbols and abbreviations used are in Nomenclature and Abbreviations, Appendix C#

8

Warner (17) obtained data on various polyfluoro com­ pounds, at pressures from atmospheric to 200 psi, using an electrically heated iron wire as the heating surface.

The

liquids' temperatures were all below their boiling points. He correlated his data with the Insinger and Bliss equation by multiplying Y by the ratio T^p/T], Tfcp = liquid boiling point temperature in degrees R T%

= ambient liquid temperature, degrees R. Cichelli and Bonilla (2) measured the temperature dif­

ference required to cause boiling from a horizontal chromium plated surface at medium and high heat fluxes for water and several organic compounds.

The pressures ranged from at­

mospheric to near the critical.

The coefficient of heat

transfer rose as the pressure was increased, until nucleate boiling became unstable near the critical pressure.

No cor­

relation was presented for their data. McAdams and co-workers (13) conducted an investigation of boiling using horizontal platinum wires as the heating surfaces.

They restricted the substance used to water but

varied the pressure over a considerable range, up to near the critical pressure.

The data presented were similar to

previous data on the effect of pressure, in that the co­ efficient increases with pressure, but no correlation was presented.

Their fwork was extremely valuable in this in­

vestigation since many ideas for the construction of this apparatus were obtained from it.

9 Cryder and Finalbargo (3) conducted boiling experiments with water and several organic liquids.

They determined h

values for the liquids at boiling points both above and be­ low atmospheric pressure, using an electrically heated brass tube.

They expressed their results by the equations log h = a + n log Qs + bt log h/ha = b (t - tn )

where subscript n indicated normal boiling point conditions, t was the temperature of the boiling liquid in degrees F, and a, b, and n were experimentally determined constants# Bonilla and Perry (1) determined h values for boiling binary mixtures with a horizontal plate heater, and found them roughly equal to those for boiling outside of single horizontal tubes. Cryder and Gilliland (1*.) made measurements of h values of water and several other compounds with an electrically heated brass tube.

They obtained a correlation with a very

complex set of relations.

10 RESULTS AND CONCLUSIONS

Results The determined values of h plotted vs 9g for saturated carbon tetrachloride (CCl^) and ethanol (C2H6O) heated at temperatures of l68°P and 176°P> respectively, are shown in Figures 2 and 3 #

Those for saturated water at a tempera­

ture of )(lfi{°F are shown in Figure I*.» The values of q" plotted vs 0 g for the same liquids and conditions are shown in Figures 5» 6 , and 7*

The methods of calculating qM, Qs,

and h are shown in Appendix B* Runs were attempted with this apparatus with water at higher pressures and temperatures but the conductivity of water increased so greatly at those conditions that the heater wire could not be kept clean.

Electrolysis occurred

between the platinum and the boiler shell, even though the platinum was made anodic in an effort to prevent this* In the low ranges of heat flux, the plots of q” vs Qa follow the usual pattern, that is, straight lines on logarithmic co-ordinates.

This is the region where free

convection heat transfer prevails.

The striking difference

between the data for ethanol and that of other investiga­ tors is the high value of 8 g attained before definite nucleate boiling began— that region where q,f increases sharply for a small increase in Gg#

Il

FIGURE 2 SURFACE CO EFFIC IENT OF HEAT TRANSFER VS TEMPERATURE D IFFE R E N C E B ETW EEN SURFACE AND BULK LIQUID CARBON T E T R A C H L O R ID E AT TEMPERATURE OF 168e F (RUN NO, I) # 6 4 * F (RUN NO. 2 ) 8000 LJE / ITI ED j5LIRFACIE-HC RIZ ONTAL n CYL-IN UL, R

6000 5000 4000 3000

6»

2000

O

1500 C) A

1000

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800 600 500

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Heating of Saturated Ethanol at 176°P ** Run No

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