Comparative study of two phase flow boiling of refrigerant mixtures and pure refrigerants inside enhanced surface tubing

INT. COMM. HEAT MASS TRANSFER VoL 19, pp. 137-148, 1992 Printed in the USA

0735-1933/92 $5.00 + .00 CopyrightO1992 Pergamon Press plc

COMPARATIVE STUDY OF TWO PHASE FLOW BOILING OF REFRIGERANT MIXTURES AND PURE REFRIGERANTS INSIDE ENHANCED SURFACE TUBING

S. M. Sami and J. Schnotale Mechanical Engineering, School of Engineering University of Moncton, Moncton, N.B., EIA 3E9, Canada

(Communicated by J.P. Hartnett and WJ. Minkowycz) ABSTRACT In this paper, the characteristics of two phase flow boiling of pure refrigerant; R-22 as well as nonazeotropic refrigerant mixtures R-22/R-II4 and R-22/ R-152a inside horizontal enhanced surface tubing is presented. Correlations were proposed to predict the heat transfer characteristics of non-azeotropic refrigerant mixture flow boiling inside enhanced surface tubing. In addition, it was found that the enhancement of the heat transfer coefficient is dependent on the mixture components and their concentrations.

Introduction

In the past few years, substitutes,

a number of fluids that may serve as

either as pure fluids or as constituents of mixture

have been developed or identified chlorofluorocarbons,

[1-3] in response to the

CFCs, phase-out by the year 2000.

N o n - a z e o t r o p i c mixture may produce environmentally sound superior working fluids depending on its components and their

137

138

S.M. Sami and J. Schnotale

concentrations.

Vol. 19, No. 1

At a given composition or concentration,

the

non-azeotropic mixture condenses and boils over a temperature range by an isobaric thermal process.

Therefore,

a non-

azeotropic mixture has a temperature distribution parallel to that of the surrounding fluid with which heat transfer takes place during evaporation and condensation processes.

This leads

to an improved thermodynamic performance.

Research on the thermodynamic and heat transfer characteristics of non-azeotropic

fluids is still in its infancy.

Recent published research work on the convective boiling heat transfer of non-azeotropic refrigerant mixture has been confined to smooth surface tubing

[4-6].

Therefore,

this study has been

carried out to enhance our understanding of the flow boiling and heat transfer characteristics

of non-azeotropic refrigerant

mixture inside enhanced surface tubing.

Experimental Apparatus and Measurements

Figure i, shows a schematic diagram of the experimental setup,

which is a water / water vapour compression heat pump

composed mainly of 8 KW compressor, pre-condenser,

pre-evaporator,

test evaporator.

oil separator,

condenser,

adjustable expansion device,

and a

The oil content in the refrigerant loop was

estimated to be about 1%.

The horizontal 3.5m evaporator test

section was constructed to eliminate entry length effects.

The

test section was composed of a double fluted tube evaporator, where the refrigerant

flows inside an inner double fluted tube

with 0.0324m envelope diameter and water flows countercurrently

Vol. 19, No. 1

TWO

in the o u t e r dimension

annulus.

0.0226m,

transfer

area

with

bore

number

0.514 m 2.

of flutes The o u t e r

into 20 s u b s e c t i o n s

stations.

This was n e c e s s a r y

transducers employed were

was

to m e a s u r e

measured

measurements

All entering cooled

the

outlet

mass

measurements

using

transfer

calibrated

of p r e s s u r e were

Temperatures

Temperature

obtained

at a sink w a t e r

On the other hand,

was e m p l o y e d

to control

to a c h i e v e

the w a t e r

the q u a l i t y

The t e m p e r a t u r e

was c o n t r o l l e d

was

±I K.

were

of 16 ° C.

J and K.

of

of the

difference

a constant

across

quality

at

of the evaporator.

orifice

a liquid

flow rate.

differential

measured

heat

drops.

to the evaporator.

A calibrated line a f t e r

the local

pressure

was w i t h i n

heat

measuring

the r e f r i g e r a n t

pre-evaporator

the e v a p o r a t o r

temperature

The a c c u r a c y

type

outside

and m a d e

transducers

temperature

refrigerant

effective

pressure

accuracy

recorded

bore

Differential

by t h e r m o c o u p l e s

with

The outer annulus

to m e a s u r e

(0-800 kPa).

2.5%.

0.0212m,

tube was a smooth

were

139

of copper

4 and total

with

All p r e s s u r e s

transducers

diameter

0.0508m.

subdivided

pressure

t u b e was made

inside

an inner d i a m e t e r

characteristics.

BOILING O F R E F R I G E R A N T S

The inner

as follows:

diameter

copper

PHASE FLOW

installed

receiver

Both

pressure

pressure

transducer

also m e a s u r e d

the m a s s

flow m e a s u r e m e n t s

liquid

was u s e d to m e a s u r e

orifices'

rate was

in the

(0-250

by a c a l i b r a t e d was

refrigerant the r e f r i g e r a n t

taps were kPa).

connected

Water m a s s

orifice.

3% of the nominal

flow

The a c c u r a c y flow.

to a

of

140

S.M. Sami and J. Schnotal¢

VoL 19, No. 1

Refrigerant composition for such particular mixture has been determined with the aid of an electronic scale.

In addition,

a

liquid sample of each mixture was expanded to superheated vapour and analyzed by the gas chromatography to accurately determine the overall composition of the mixture prior to the testing. Data collection was carried out using an AT/PC 286 equipped with a data acquisition system with a capacity of 112 channels. This enabled us to record at a single scan local properties such as pressure drops,

pressures,

temperatures,

flow rates and power.

All tests were performed under steady state conditions.

The data

collection were scanned every one second and stored every i0 seconds. The primary parameters observed during the course of this study were mixture overall composition, quality for pure refrigerants R-22,

mass flux, heat flux, and

as well as non-azeotropic

refrigerant mixture R-22/R-II4 and R-22/R-152a at various concentrations.

Mass flow rates ranged form 50 to 90 g/s.

Input

quality was kept constant at 0.25 and boiling took place reaching saturation condition at the exit of the evaporator. In order to develop the proposed correlations describing the flow boiling heat transfer characteristics,

the thermodynamic

properties of pure and non-azeotropic refrigerant mixture should be known.

The Carnahan-Starling-DeSantis

(CDS) equation of state

[7] was used to evaluate the mixture characteristics. rules suggested by Reid et al.

The mixing

[8] were employed with caution to

determine the transport properties of the mixed refrigerants.

V o L l % No.l

TWO PHASE FLOWBOILINO OF REFRIGERANTS

141

R@sults and Discussion

In the following,

the results of the heat transfer

characteristics such as, the heat transfer coefficients at different conditions will be presented and discussed.

Evaporator

pressure varied from 180 to 600 kPa, refrigerant temperature ranged from 1 to 5 ° C.

The Reynolds number was in the range of

9.8 x 103 to 2.2 x 104 .

The overall heat transfer coefficient based on the outside surface area of the test section A o is; _

O,

(I)

Where LMTD is the mean logarithmic temperature difference based on the inlet/outlet temperatures of water/refrigerant

flow

and Qr, represent heat transfer to refrigerant.

Assuming no fouling and R~ is the thermal resistance in the copper wall of the tube,

the refrigerant heat transfer

coefficient h r can be calculated as follows;

hr~

~o

Where h. is the water heat transfer coefficient and is calculated using the Wilson plot technique as described in Khartabil et al.

[9].

R, is the wall resistance evaluated using

the actual thickness and the outside diameter of the tube.

142

S.M. Sami and J. Schnotale

During the course purposes,

of this study

the enhanced

surface tube

for data resolution (doubly

treated as a plain tube with an envelope point,

the heat transfer

constant

mass

frigerants

and 3, illustrate coefficients

at several

for R-22/R-II4

increases

mixture

R-22/R-II4,

As expected,

in enhanced

by Jung et al.

heat transfer

of the smooth tube. coefficient

This

is mixture

the heat transfer

R-f14

in Figure

Another boiling,

series

involving

2, shows. for refrigerant

compared to those It is

leads to a

higher than that

and the variation

is a function

of

of the mass

Furthermore,

the results

of concentration

of

the rate of nucleate boiling and

rate.

of tests were conducted refrigerant

of the mass

surface

dependent

compositions.

the heat transfer

2

in the heat transfer

factor

decelerates

Figures

for smooth tube.

2, shows that the increase

in the mixture

consequently

tubing

is significantly

increase

enhancement

Figure

coefficient,

surface

composition

flux for various mixture depicted

flux.

[4] correlation

that

refrigerant

the heat transfer

figure that the enhanced

coefficient

of mixed re-

heat transfer

as a function

heat transfer

At a

to that of a smooth tube.

flow boiling

plotted

At this

compositions.

at higher mass

of the average

from this

overall

has been

is introduced.

coefficient

surface

the measured

samples

evident

diameter.

factor

the heat transfer

flux and concentrations. coefficient

fluted tube)

of tests were run with non-azeotropic

R-22/R-II4

predicted

enhancement

inside the enhanced

Series mixture

flux,

VoL 19, No. 1

mixture

with two phase flow

of R-22/R-152a

in various

VoL 19, No. 1

TWO PHASE FLOW BOILING OF R E F R I G E R A N ~

concentrations.

The maximum permissible

concentration

was limited to 30% because of flammability the heat transfer coefficients

reasons.

Values of

in Figures

4 and 5.

the heat transfer coefficient

increases with increasing the mass flux. higher concentrations

of R-152a

obtained through these runs were

plotted and compared to smooth tube values AS observed with R-22/R-144,

143

The data suggest that

of R-152a in the mixture,

increase in the heat transfer coefficient.

lead to a slight

On the other hand,

it

appears that the mixture produces a heat transfer coefficient slightly higher than that of R-22. the enhancement

Also,

this figure shows that

factor seems to have weak dependence

on the mass

flux. Finally, observations azeotropic

based on the experimental

evidence and our

of the boiling characteristics

refrigerant

mixtures,

of pure and non-

a generalized

correlation

determined by a regression analysis of the data of heat transfer coefficients, in the evaporative by Bo Pierre [10] is proposed;

region using the form proposed

(3)

N u = O . 0 2 5 C ° ' 3 R e ° ' ~ l K f °'4

Where C is the mixture composition. Figure

6, shows that the proposed generalized

predicts the heat transfer coefficient in the convective evaporation

correlation

for pure and mixed fluids

region with a mean deviation of

±20%. The authors are persuaded that the findings presented this paper,

represent

a significant

contribution

in

to our knowledge

144

S.M. Sami and J. Schnotale

of the phenomena,

%/ol. 19, No. 1

taking place during forced convective boiling

of mixed refrigerants

inside enhanced surface tubing. Conclusions

During the course of this experimental

study, the behaviour

of forced convective boiling of non-azeotropic mixture has been investigated.

refrigerant

A generalized correlation

has

been proposed to predict the average heat transfer coefficient inside enhanced surface tubing. Acknowledqment The research work presented through grants acknowledge

in this paper was possible

from N. B. Power and NSERC.

the continuous

The authors wish to

support of the University

of Moncton.

Nomenclature Cp

Specific heat

(kJ kg_ I K_I)

Db

Equivalent

G

Mass flux

diameter of the annulus

g

Gravitational

h

Heat transfer coefficient

htq

Latent heat of vaporization

K

Thermal conductivity

L

Test section length

x

Quality based on mass

acceleration

(m s_2) (kW at2 K_I) (kJ kg_ I)

of liquid

(kW nil k_1)

(m) (-) Greek Symbols

Viscosity

P

Density

(Db=Doi-D~)

(kg nt2 s_1)

of liquid

(kg m_3)

(Pa s)

(m)

Vol. 19, No. 1

TWO PHASE FLOW BOILING OF REFRIGERANTS

145

Dimensionles s Number8 kf

Bo Pierre boiling number

(Ax hfq/L g)

Pr

Prandtl

(Cp ~/K)

number of liquid

Re

Reynolds number

Nu

Nusselt

number

(G D b /~) (h Db/K) Re fe r en ce s

i.

D.P. Wilson and R.S. Basu, Thermodynamic Properties of a New Stratospherically Safe Working Fluid Refrigerant 134a, ASHRAE Transactions, 94, Part 2. (1988).

2.

S.M. Sami, Non-Azeotropic Mixture as Potential CFC Substitutes for Heat Pumps, Intl Conf. on Heat Pumps in Cold Climates, Hotel Beaus~jour, Moncton, NB, (Aug. 13-14, 1990).

.

L.J.M. Kuiper, The CFC Issue: International Actions, Periodical IrA, Vol. ~, No. 2, (June 1989).

4.

D.S. Jung, M. McLinden, R. Radernacher and D. Didion, A Study of Flow Boiling Heat Transfer with Refrigerant Mixture, Intl Jl. Heat Mass Transfer, Vol. 32, No. 12, p. 1751, (1989).

5.

D.S. Jung and R. Radermacher, Prediction of Pressure Drop during Horizontal Annular Flow Boiling of Pure and Mixed Refrigerants, Intl Jl. Heat Mass Transfer, Vol. 32, No. 12, p. 2435, (1989).

6.

S.M. Sami and T.N. Duong, Experimental Study of the Heat Transfer Characteristics of Refrigerant Mixture, Intl Comm. Heat Mass Transfer Jl., Vol. 18, No. 4, p. 547, (1991).

7.

G. Morrison and M. McLinden, Application of a Hard Sphere Equation of State to Refrigerants and Refrigerant Mixture, NBS Tech. Note 1226, NBS, Gaithersburg, Maryland, (1986).

.

R.C. Reid, J.M. Prausity and B.E. Poling, The Properties of Gases and Liquids (4th Edition), McGraw Hill, N.Y., (1987).

9.

H.F. Khartabil, R.W. Christensen and D.E. Richards, A Modified Wilson Plot Technique for Determining Heat Transfer Correlations, 2nd UK Natl. Conf. on Heat Transfer, University of Stratholyde, Glasgow, England, (14-16 Sept. 1988).

10.

Bo Pierre, Flow Resistance with Boiling Refrigerants, Jl., 58-77, (1964) .

ASHARE

146

S.M. Sami and J. Schnotale

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VoL 19, No. 1

t

I. ~

?, IIII, LIE

3. g l U E

I, U ~ A l u l l . l 1 6 ~ l l J a

2. CJ'lUlfl(llllnI,B) 4. mllll ml'

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FIG. 1 Schematic view of the experimental

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FIG. 2 Boiling heat transfer coefficient versus mass flow rate.

Vol. 19, No. 1

TWO PHASE FLOW BOILING OF REFRIGERANTS

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MIXTURE COfl~NTRATIO~ MASS

,

FTU~CTeONR22/(R22+RI14)

FIG. 3 Boiling heat t r a n s f e r coefficient versus mixture mass c o n c e n t r a t i o n for R - 2 2 / R - 1 4 4 m i x t u r e s at d i f f e r e n t m a s s f l o w rates.

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FIG. 4 coefficient versus mass

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147

148

Vol.19,No.1

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FIG. 5 Boiling heat transfer coefficient versus mixture mass concentration for R-22/R-152a mixtures at different mass flow rates.

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FIG. 6 _ Calculated versus measured boiling heat transfer coefficient.