dessalement arabian journal of science

8/2/2019 Dessalement Arabian Journal of Science

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T. Khir, R. K. Rahim, N. Ghaffour and A. Ben Brahim

April 2005 The Arabian Journal for Science and Engineering, Volume 30, Number 1B

47

EXPERIMENTAL STUDY ON FORCED CONVECTIVE

BOILING OF AMMONIAWATER MIXTURES IN A

VERTICAL SMOOTH TUBE

Tahar Khir *

Mechanical Engineering Department

Jeddah College of Technology, Jeddah, K. S. A.

Rahim K. Jassim

Mechanical Engineering DepartmentJeddah College of Technology, Jeddah, K. S. A.

Noureddine Ghaffour

Middle East Desalination Research Center (MEDRC)

Muscat, OMAN

AmmarBen Brahim

Chemical Engineering Department.

Gabes Engineering School, Gabes, TUNISIA

:

.6

:(kW/m2 18.521 -8.211)(kg/m2.s 2688 - 707)(kg/s 0.076(% 61 - %55 - % 42).(0.02 -

. -

. .%

* Address for correspondence:

Refrigeration and A/C Department

Jeddah College of Technology

P.O. Box 42204

Jeddah 21541 Saudi Arabia

E. mail: [email protected]


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The Arabian Journal for Science and Engineering, Volume 30, Number 1B April 2005

48

ABSTRACT

As part of our work on the absorption solar refrigeration cycle and with theobjective of optimizing the efficiency of flat plate solar collectors in the

generation phase, an experimental study has been conducted on the forced

convective boiling heat transfer of NH3H2O mixtures flowing inside a 6 mm

inner diameter vertical smooth tube. Using a water-heated double pipe typegenerator, the local heat transfer coefficients are measured inside the inner tube

for a range of heat flux (8.21118.521 kW/m2), mass flux (7072688 kg/m

2.s),

mass flow rate (0.02 0.076 kg/s), and ammonia mass concentration (42%, 55%,and 61%). Three theoretical models are used to predict the boiling heat transfer

coefficients. Experimental data were compared with the available correlations.

The obtained results confirm the good performance of Mishra et al. and Bennett

and Chens models in predicting the convective boiling heat transfer coefficient

of ammonia water mixtures. These models are able to predict the boiling heat

transfer data within an accuracy of 20 %.

Key Words: Two phase flow, mixtures, convective boiling, heat transfer,

Ammonia-water


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49

NOMENCLATURE

A: tube area m

Bo: boiling number

d: tube inside diameter m

db: bubble diameter m

Cp: specific heat of fluid J.kg-1.K-1

F: convective boiling factor

G: mass flux kg.s-1.m-2

g: acceleration of gravity m.s-2

H: enthalpy J.kg-1

h: heat transfer coefficient W.m-2.K-1

hm: mass transfer coefficient m.s-1

Hlg: latent heat of vaporization J.kg-1

m : mass flow rate kg.s-1

P: pressure Pa

Pr: Prandtl number

pr: reduced pressure

q: heat flux W.m-2

Ra: surface roughness 10-6 m

Re: Reynolds number

S: suppression of nucleate boiling factor

Sc: Schmidt numberT: temperature C, K

X: liquid mass fraction kgAmL/kgmel

x: quality

Y: vapor mass fraction kgAmg/kgmel

z: total heated length of the tube m

Z: distance along tube m

Greek symbols

m: liquid mass diffusivity m2.s-1

th: thermal diffusivity m2

.s-1

: variation (according to variable)

: thermal conductivity W.m-1.K-1

tt: LockhartMartinelli parameter

: dynamic viscosity kg.m-1.s-1

: density kg.m-3

: thermal losses rate


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50

: surface tension N.m-1

Subscripts

Am: ammonia nb: nucleate boiling

av: average npb: nucleate pool boiling

c: critical state o: outer, outlet

cv: convective sat: saturation condition

g: gas (vapor) SC: subcooled

hw: heating watertp: two-phase

i, in: inner, inlet W: water

l: liquid w: wall

mix: mixture z: at location z


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51

EXPERIMENTAL STUDY ON FORCED CONVECTIVE BOILING OF AMMONIA

WATER MIXTURES IN A VERTICAL SMOOTH TUBE

1. INTRODUCTION

Ammonia and ammoniawater mixtures have been widely used in refrigeration and air conditioning cycles.However, the high pressure, the toxicity, and the aggressiveness of ammonia on copper alloys have reduced its use in

thermal processes. Chlorofluorocarbon refrigerants (CFC) have taken the place of ammonia. However, given the

problems of global warming and potential ozone depletion that the use of these fluids provokes, ammonia appears again,in our days, as one of the alternatives permitting better protection of the environment.

Exhaustive research on forced convective boiling of ammoniawater mixtures inside tubes has not been conducted.

As part of a research program conducted on absorption solar refrigeration cycles, an experimental study on forced

convective boiling of ammoniawater mixtures inside a vertical tube has been performed. The experimental variable

ranges have been defined according to the operating conditions of a flat plate solar collector used as generator, in order toanalyze the heat transfer inside the tubes.

In this paper, a review of previous works on convective boiling of binary mixtures is given, with mainly heat

transfer correlations. The experimental device and the obtained results are presented. Finally a comparison between theexperimental results and the predicted data is discussed.

2. PREVIOUS RESEARCH ON BINARY MIXTURES

Several research works have investigated the forced convective boiling of refrigerant binary mixtures. The firststudies in this field were conducted by BennettChen [1] on aqueous ethylene/glycol solutions. A correlation has been

developed to predict the heat transfer coefficient, where the effects of diffusive resistance on nucleate boiling and on two

phase forced convection were taken into account separately. Mishra et al. [2] have performed an experimental study onforced convective boiling of the CFC refrigerant mixtures R22/R12 inside horizontal tubes. The experimental results

showed a lower heat transfer coefficient for the binary mixture, with respect to the simple linear interpolation between

the values corresponding to the two pure components. Hihara and Saito [3] have performed experiments on the CFC

refrigerant mixtures R22/R114 in a horizontal tube. The experimental results have confirmed that heat transfer

coefficients for mixtures are much lower than those for pure R22 and R114. A study on forced convective boiling insidehorizontal tube was investigated experimentally by Murata et al. [4], using mixtures of the HCFC/HFC refrigerants

R123/R134a. The heat transfer coefficient for the mixture was found to be lower than that for an equivalent purerefrigerant with the same physical properties. The reduction in heat transfer coefficient for mixtures is attributed to the

mixture effect on nucleate boiling and to the heat transfer resistance in the vapor phase. An experimental study on binarymixtures of the CFC refrigerants R12/R114 in up-flow forced convective boiling has been conducted by Celata et al. [5].

The degradation of heat transfer coefficients with the mixture composition appears to be dependent on both saturationpressure and mass flux. The results obtained confirm the good performance of the BennettChen correlation in predicting

heat transfer coefficient in the case of refrigerant mixtures.

3. CORRELATIONS

3.1. Pure Fluids

Two methods are used to express the heat transfer coefficient in forced convective boiling (htp). The first method is

derived from the well-known Chens correlation. This method is used by Celata et al. [5] to predict the convective

boiling heat transfer coefficients for R12/R114 binary mixtures. As reported by the authors, the convective boiling heattransfer coefficients can be expressed as the arithmetic summation of the two-phase convection contribution (hcv) and the

nucleate boiling contribution (hnb).

tp cv nbh h h= + (1)

( )1cv l tt h h F = (2)

nb npbh h S= ; (3)

hl is the single liquid phase heat transfer coefficient given by:


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53

( )

0.5

20.146b

l g

dg

=

with the angle = 35

3.2. Mixtures

Bennett and Chen [1] have proposed an extension of Equation (1) to binary mixtures. The hcv was calculatedaccording to Equation (2) and using a factorFmix taking into account the effective mass transfer on the thermal driving

force. By the same method, the coefficient hnpb was calculated using an expression proposed by Forster and Zuber [8] for

pool boiling taking into account the greater thermal gradient in the vapor generating zone near the wall due to the forcedconvection. A suppression factorSmix was defined as a function of the two-phase Reynolds number.

This method is chosen to be applied in this study because it presents the advantage to be usable for binary mixtures

of refrigerants without many restrictions. In the same way, this correlation includes physical properties of mixtures and

some predictable quantities of state; the number of empirical parameters is limited with regard to other methods. Asreported by Celata et al. [5], the final BennettChen correlation in mixture case, is expressed as:

tp cvmix nbmixh h h= + (15)

where cvmix l mixh h F= and nbmix npb mixh h S=

( )(Pr )mix l sat nbF Ff T T = (16)

[ ]0.444

(Pr ) (Pr 1) 2l lf = +

lg

(1 )1

bulk

sat

sat l m sat Pnb

dTT Y q

T H h T dX

=

hm is the mass transfer coefficient given by:

0.8 0.40.023 Remm tpi

h Scd

= (17)

where Sc is the Schmidt number expressed as: ( . )L mSc =

( )0.5

lg

( )1

mixsatl

th m

SS

dTCp Y X

H dX

=

(18)

th and mare the thermal and the mass diffusivity.

Mishra et al. [2] have correlated their experimental data, obtained using R12/R22 mixtures, by:

( )1m n

tp l tt h h E Bo= , (19)

As reported by Celata et al. [5] the values ofE, m, and n in Equation (19) are taken as follows:

(a) 12 : 23 27%22 : 77 73%

RR

E=5.64, m=0.23, n=0.05;

(b)12 : 41 48%

22 : 59 52%

R

R

E=21.75, m=0.29, n=0.23.

4. EXPERIMENTAL APPARATUS

The experimental loop is presented schematically in Figure 1. It mainly consists of a feed reservoir FR, of capacity

of 50 liters, a boiler BL, a vapor generator G, a rectifier REC, an absorber ABS, and a chilling unit CHL.


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Figure 1. Schematic of the experimental loop

An ammoniawater mixture, with defined initial concentration, is introduced in the reservoir FR. A piston pump

with variable speed drive PP sends this mixture towards the adjustable power electrical pre-heater PH, permitting control

of the value of the mixture inlet temperature. Afterwards, the mixture is sent towards the vapor generator G constituting

of two stainless steel coaxial tubes (type 304) with a heated length of 1 m; the inside and outside diameters are 6 and 9

mm respectively. Ammoniawater mixture flows inside the inner tube and the heating water flows in the outer annulus.The inner and outer diameters of the external tube are 28 and 32 mm respectively. The heating water temperature is

controlled by a thermostat connected to the electrical boiler BL. At the generator outlet, the mixture is introduced in the

phase separator PHS. Then the generated vapor is cleansed in the rectifier REC. The pure vapor stemming from therectifier is condensed in CD and after that stored in the reservoir SRG. The weak solution is cooled in the heat exchanger

HEX and then stored in the reservoir SRL. The absorption phase is made later. Chilled water (CHL) is sent towards the

condenser, the heat exchanger and the absorber by a centrifugal pump CP. All the elements in contact with the ammoniawater, mixture are made from stainless steel. For the purpose of reducing the thermal losses towards the surroundings,

the generator elements are insolated by a woodglass layer of 5 cm of thickness.

The test channel is shown in Figure 2.

The inlet and the outlet pressures are measured in (P) with two absolute pressure transducers type SEDEME and

provided with integrated output regulators. The temperatures are measured with 0.5 mm K type insulated thermocouples.

BL

P

CH

L

P

P

P

PHS

CD

HEX

ABS

V

E

R

F

PPSg PH

FR

F

Vv

Pressure measure Temperature measure

CPRT

T

TF

Sg

REC

G

P

P

RSL

RSG

F: Filter - Sg: Sight glass - Vv: Valve

Vv

Heatingwater

Tmix,o

Tmix,in

T

TT

TT

TT

T

T

T

T

T

E

To vacuum

pump

Servicevalve

P

T


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55

The temperature of the outside wall is measured at seven locations (T) along the tube length, with fifteen thermocouples

oriented to measure the temperature at two sides of the tube perimeter as shown in Figure 2. Seven thermocouples are

used to measure fluid bulk temperatures. The volumetric flow rates of the ammoniawater mixture and the poor solution

are measured with the help of two turbine flow meters TF. Generated vapor flow rate is measured, at the condenser

outlet, with the help of a volumetric technique provided by a precise stopwatch. Water volumetric flow rates in variouscircuits are measured by the rotameters RT. The errors on experimental parameters are indicated in Table 1. The ranges

of experimental parameters are indicated in the Table 2.

Figure 2. Schematic of the generator tube

All measurements were recorded with a HewlettPackard data acquisition/control unit connected to a micro-

computer, permitting storage of all of the sensor outputs after conversion to engineering units.

Table 1 : Errors in experimental Parameters

Experimental parameters Errors

Temperature

Pressure

Mixture volume flow rate

NH3 vapor volume flow rate

Water volume flow rate

0.1 C0.2 %1.5 %2 %3 %

21 Thermocouples

150

150

150

150

150

150

50

1000

Water

NH3- H20

GasketsP

P

T

A A

0 5

T

T

A

69


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56

Table 2 : Range of Experimental Parameters

Experimental parameters Variation ranges

Heat flux q kWm-2

Mass flux G kgm-2s-1

Pressure P 105Pa

Boiling temperature T C

Inlet temperature Tin,mixt C

Mixture flow rate m kg/s

Mass fraction XNH3 %

8.211 14.781 18.521

707 1590 2688

1.5 20

70 90

40

0.02 0.045 0.076

42 55 61

5. EXPERIMENTAL RESULTS

5.1. Evaluation of Thermal LossesThe thermal losses on the generator tube have been evaluated according to series measurements in single liquid phase.

The thermal losses along the tube are defined as the relative error calculated considering, on the one hand, the heat flux q

absorbed by the mixture inside the generator tube and, on the other hand, the heat flux qhw supplied by the heating water

as follows:

hw

hw

q q

q

= (20)

( ). .. .

mix mix omix inmix

i

m Cp T T q

d Z

=

(21)

( ). .

. .

hw hw inhw ohw

hwi

m Cp T T

q d Z

=

, (22)

where: Tinmix, Tomix: The inlet and the outlet temperature of the mixture respectively,

Tinhw, Tohw: The inlet and the outlet temperature of heating water respectively.

For mixture flow rates varying from 0.02 to 0.06 kg/s and heating water flow rates varying from 0.027 to 0.08 kg/s,the thermal losses on the generator tube are lower than 8 %.

5.2. Measurements on AmmoniaWater Mixtures

The decrease in the heating water temperature along the generator is less than 3 C. In order to estimate the

variation of the supplied heat flux, the generator length is divided into different sections. For each section (10 cm in

length), a linear decrease of the heating water temperature is assumed. The difference between the heating water inlet

and the outlet temperatures Thw, for each section, is practically constant. The error in Thw, engendered with this

procedure, is about 4 %. For constant values of the specific heat and water flow rate through the annulus, the variationin the supplied heat flux along the generator is consequently less than 4 %.

The average useful heat flux, absorbed by the mixture inside the generator tube, is given by:

mixav omix inmix

i

mq H H

d Z=

, (23)

whereZis the total heated length of the generator tube (Z= 1 m).

The local heat flux, absorbed by the mixture at each locationz, is determined according to the enthalpy variation from

the tube generator inlet to the levelz, as follows:


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57

[ ]mix z inmixi

mq H H

d z=

. (23 )

Considering the errors on the mass flow rate mixm and the enthalpies, the heat flux accuracy was evaluated to

within 5 %.

The boiling heat transfer coefficient htpis given by:

( )tp

w z

qh

T T=

. (24)

The inside wall temperature Tw, is calculated from the value of the external wall temperature using Fourier's law

related to the radial thermal conduction. Tz is the local mixture temperature determined according to a polynomial

interpolation of the temperature values measured in the longitudinal axis of the generator tube (Figure 2). Considering

the uncertainties on q, Tw, and Tz, the accuracy of the heat transfer coefficient htp, was evaluated within 12 %.

The sub-cooled liquid length is given by:

( )( )

mix mix sat inmixSC

i

m Cp T T z

d q

=

; (25)

Tsat indicates here the mixture temperature at bubble line. Taking into account the uncertainties on the different

variables in Equation (25), the error onzSCis estimated to be 7 %.

Assuming the thermodynamic equilibrium, the quality at locationz,xz is calculated as follows:

z lz

l

H Hx

H H

=

(26)

where

Hz: the mixture enthalpy at locationz,

Hl: the saturation liquid mixture enthalpy,

Hg: the saturation vapor mixture enthalpy.

Three experience series are performed using three different values of heat and mass fluxes. In order to ensure an

optimal generated vapor flow rate, the ammonia mass fraction is taken as 42, 55, and 61 %. According to the ammonia

mass concentration, the boiling temperature varies between 70 and 86 C. In order to avoid vaporization before the

generator, the mixture inlet temperature that was taken is 40 C.

In Figure 3, the quality x is plotted as a function of the length along the tube, for various ammonia mass

concentration, q, and G. The quality x increases linearly with z and its value depends on the ammonia massconcentrationXNH3. For the all performed tests the quality is lower than 0.60.

The heat transfer coefficient is plotted in Figure 4 as a function of the length along the tube for various ammonia mass

fractions. The onset of sub-cooled boiling is marked by a fast increase of heat transfer coefficient htp. The length of the

sub-cooled liquid region zSC is conversely proportional in the ammonia mass fraction. In the single phase liquid, theammonia concentration does not have sensible influence on the heat transfer coefficient. In the two-phase region, the

heat transfer coefficient increases slightly.The influence of the heat and mass fluxes on the heat transfer coefficient is shown in Figure 5; the values ofhtp are even

greater when q and G are increased.

6. DATA ANALYSIS

6.1. Calculation ProcedureThree models are used to predict the convective boiling heat transfer coefficients. The first two models are both

based on the BennettChens method defined by Equation (15). Different correlations are used to calculate the nucleate

pool boiling coefficient hnpb, the convective boiling factorFand the suppression factorSas follows:


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58

BennettChen:F, S, and hnpb are calculated by equation (8), (9), and (10) respectively, (model I). Jung et al.: F, S, and hnpb are calculated by equation (12), (13), and (14) respectively. This model is called Jung

BennettChen (model II),

The third model is based on the Mishra et al. method [2]; two correlations are used according to the values ofE, mand n in Equation (19) as follows:

E= 5.64 ; m = 0.23 et n = 0.05. This model is called Mishra 1 (model III-1),

E= 21.75 ; m = 0.29 et n = 0.23. This model is called Mishra 2 (model III-2).

0

0,1

0,2

0,3

0,4

0,5

0,6

0 0,2 0,4 0,6 0,8 1

q = 8,211 - 14, 781 kW/m2

G = 707 - 1590 kg/m2.s

XNH3 Tsat

55% 77 C

42% 86 C

z (m)

x

Figure 3. Variation of the quality (q) as a function of the tube length

0

5

10

15

20

25

0 0,2 0,4 0,6 0,8 1

q = 8,211 kW/m2

G = 707 kg/m2.s

Tsat = 70 -- 86 C

XNH3

61 % 55 %

42 %

htp

(kWm

-2K-1)

z (m)

Figure 4. Heat transfer coefficient as a function of the tube length

q

G

z

x

X T

.

.

.

.

.

. . . .

.


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59

The physical properties of ammoniawater mixtures are evaluated using correlations experimental validated in

previous research works [913].

The heat transfer coefficient htp is obtained according to the following procedure:

1. For a given q, G and initial ammonia concentrationXNH3, determine Tsat and then calculate zSC from Equation(25).

2. From zSC and according to a chosen step (5 cm), calculate Tz according to a polynomial interpolation of thetemperatures measured in tube axis.

3. Calculate the local ammonia concentration in the liquid phaseXz and in the vapour phase Yz.4. Calculate the local enthalpyHz and the saturation enthalpiesHlandHg.5. Calculate the local qualityxz from eq. (26).6. Calculatettfrom Equation (7).7. Calculate hlfrom Equation (4).8. CalculateFand Saccording to models I and II; then calculateFmix and Smix from Equation (16) and (18) respectively.9. Calculate hnpb for the models I and II from Equation (10) and (14) respectively.10. Calculate htp by Equation (15) for the models I, and II and Equation (19) for the models III-1 and III-2.

Software elaborated in TurboPascal language allows to perform the previously calculus into all the details.

6.2. Discussion

The evolution of the boiling heat transfer coefficient htp as a function of the qualityx is shown in Figures 6a and 6b,

for an ammonia mass fraction of 55 %. The model III-1 of Mishra et al. predicts the heat transfer coefficient htp with a

precision of 12 %; contrary to the model III-2, which shows a wide undervaluation of htp as shown in Figure 6a. Themodels I and II present wide divergences from the experimental results especially for qualitiesx above 0.3.

0

10

20

30

40

50

0 0,2 0,4 0,6 0,8 1

Tsat = 77 C

X NH3= 55 %

q = 8,211 kW/m2

G = 707 kg/m2.s

q =14,781 kW/m2

G = 1590 kg/m2.s

q = 18,521 kW/m

G =2688 kg/m2.

htp(kWm

-2K-1)

z (m)

Figure 5. Influence of the heat and mass fluxes on the heat transfer coefficients


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60

For increased values ofq and G, the models I and II present more divergences in predicting the heat transfer coefficients

as shown in figure 6b.

0

5

10

15

20

25

30

35

40

0 0,1 0,2 0,3 0,4 0,5 0,6

htp(kWm

-2K-1)

q= 8,211 kW/m2

G = 707 kg/m2

.sTsat = 77 C

XNH3 = 0.55

Bennett-Chen (I) Mishra 1 (III-1)

Jung-Bennett-Chen (II) Mishra 2 (III-2)

Experimental results

x

Figure 6. (a) Boiling Heat transfer coefficients as a function of the quality

0

20

40

60

80

100

0 0,1 0,2 0,3 0,4 0,5 0,6

q = 18,521 kW/m2

G = 2688 kg/m2.sTsat = 77 C

XNH3 = 0,55

htp(kWm

-2K-1)

x




Figure 6. (b) Boiling Heat transfer coefficients as a function of the quality


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61

For vapor qualities above 0.25, a saturated nucleate boiling zone develops gradually along the generator tube

[14,15]. In the same way, Bjorge et al. [16] have considered that the saturated flow boiling occurs for values above 0.05.

In this zone the values of the heat transfer coefficients htp depend essentially on the dominant contribution of nucleate

boiling, characterized by the coefficient hncmix in Equation (15). The contribution of the forced convective boiling,

characterized by the coefficient hcvmix, is, consequently, less important. It seems that Chen [6] and Jung et al. [7]overpredict the coefficient hcvmix in their correlations. In fact the correlations related to the convective boiling factor

calculation are modified for the purpose to decrease the contribution of the forced convective boiling.

The convective boiling factorFdepends on the inverse of the LockhartMartinelli parameter 1/tt and, as a result,

on the qualityx. For the models I and II, the factorFis calculated from Equation. (8) and (12) respectively. These can beexpressed as follows:

( )1t

ttF r s= + , (27)

where r,s and tare parameters defined in Equations (8) and (12).

The proposed modifications consist in correction of the exponent t(equation 27) in the aim to converge the huge

differences between the theoretical and the experimental results related to the heat transfer coefficients. This correctionhas been defined according to the variation ranges of the heat and mass fluxes; the retained values of t are:

Chen [6]t= 0.736

Jung et al. [7] t= 0.85

This work t= 0.45 for 8.5 q 15 kW/m2 and 800 G 1600 kg/m2.s

t= 0.3 for 15 < q 18.5 kW/m2 and 1600 < G 2688 kg/m2.s .

The obtained results after these modifications are shown in Figure 6c for the same experimental conditions already

considered in Figure 6b.

0

20

40

60

80

100

0 0,1 0,2 0,3 0,4 0,5 0,6

q = 18,521 kW/m2

Tsat = 77 C

G = 2688 kg/m2.s XNH3 = 0.55

htp(kWm

-2K-1)

x




Figure 6. (c) Boiling Heat transfer coefficients as a function of the quality

The modified models I and II present a good result in predicting the heat transfer coefficients. The maximum error,

with regard to the experimental results, is less than 12 %.

A comparison between the experimental and the predicted values of heat transfer coefficients is shown in Figures 7a

and 7b. The data predicted from models I and II (without modification) present deviations of more than 40 % as shownin Figure 7a. After the convective boiling factor modification, the predicted heat transfer coefficients from models I and

II agree with the experimental results with an accuracy of 15 % as shown in Figure 7b.


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62

0

10

20

30

40

50

60

70

80

90

0 10 20 30 40 50 60 70 80 90htp,exp (kWm

-2K

-1)

htp,t

h

(kWm

-2.K

-1)

q : 8,211 - 18,521 kW/m2

G : 707 - 2688 kg/m2.s

Tsat = 77 C

XNH3 = 0,55

+ 20 %

- 20 %

Bennett-Chen (I)

Jung-Bennet-Chen (II)

Mishra 1 (III-1)

Figure 7. (a) Comparison between the predicted and the experimental heat transfer coefficients

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70

htp,exp (kWm2K

-1)

htp,t

h

(kWm

-2K-1)

q : 8,211 -- 18,521 kW/m2

G : 707 -- 2688 kg/m2.s

Tsat = 77 C

XNH3 = 0,55

+ 20 %

- 20 %

Bennett-Chen (I with modification) Jung-Bennett-Chen (II with modification)

Mishra 1 (III-1)

Figure 7. (b) Comparison between the predicted and the experimental heat transfer coefficients


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April 2005 The Arabian Journal for Science and Engineering Volume 30 Number 1B

7. CONCLUSION

The heat and mass fluxes have a significant influence on the values of the forced convective boiling heat transfercoefficients of ammoniawater mixtures; contrary to the ammonia mass concentration, which has a less influence on the

values ofhtp.

The investigated variation ranges in this study concerningXNH3, q, and G, have confirmed the validity of the Mishra

et al. model to predict the convective boiling heat transfer coefficients of ammonia-water mixtures. This model predictshtp within 12 % of error.

BennettChen's model presents a large divergence in predicting the forced convective boiling heat transfer

coefficients, especially for qualities above 0.25. In order to reduce this divergence, the correlations related to the

convective boiling factor are modified. After modifications, the BennettChen model predicts data with an accuracy of

20 % for more than 90 % of the considered data.

In the same way, the adaptation of the JungRadermacher correlations to BennettChen's method seems to give

satisfactory results in predicting the forced convective boiling heat transfer coefficients of ammonia-water mixtures. The

mean deviation, obtained according to this method, is lower than 18%.

REFERENCES

[1] D. L. Bennett and J. C. Chen, Forced Convective Boiling in Vertical Tubes for Saturated Pure Components and BinaryMixtures,Applied International Chemical Journal, 26 (1980), pp. 454-461.

[2] M. P. Mishra, H. K. Varma, and C. P. Sharma, Heat Transfer Coefficients in Forced Convection Evaporation ofRefrigerants Mixtures,Letter of Heat and Mass Transfer, 8 (1981), pp. 127-136.

[3] E. Hihara and T. Saito, Forced Convective Boiling Heat Transfer of Binary Mixtures in Horizontal Tube, Proceedings,9

thInternational Heat Transfer Conference, 2 (1990), pp. 123-128.

[4] K. Mutara and K. Hashizume, Forced Convective Boiling of Nonazeotropic Refrigerant Mixture Inside Tubes, Journalof Heat Transfer, 115 (1993), pp. 680-689.

[5] G. P. Celata, M. Cumo, and T. Setaro, Forced Convective Boiling in Binary Mixtures, InternationalJournal of Heatand Mass Transfer, 36 (1993), pp. 3299-3309.

[6] J. C. Chen, Correlation for Boiling Heat Transfer to Saturated Fluids in Convective Flow, Industrial andEngineeringChemical Process Design and Development, 5 (1966), pp. 322-329.

[7] D. S. Jung and R. Radermacher, Prediction of Heat Transfer Coefficient of Various Refrigerants during Evaporation,ASHRAE Transactions, 97 (1991), Pt. 2.

[8] H. K. Forster and N. Zuber, Dynamic of Vapour Bubbles and Boiling Heat Transfer, American Institute of ChemicalEngineers Journal, 1 (1955), pp. 531-535.

[9] ASHRAE,Handbook of Fundamentals Atlanta, GA., American Society of Heating, Refrigerant and Air ConditioningEngineer, Inc., 1989.

[10] Y. S. Touloukian, P. E. Liley and S. C. Saxena, Thermophysical Properties of Matter, vol. 3, New YorkWashington:IFI/Plenum, 1970.

[11] R. C. Reid, J. M. Prausnitz and B. E. Poling, The Properties of Gases and Liquids, New York: McGraw-Hill, 1977.[12] R. H. Perry and D. Green, The Chemical Engineers HandbookChapters 3.14 and 18.6. New York: Mc-Graw Hill, 1984.[13] P. C. Jain and G. K. Gabel, Equilibrium Property Data Equations for Aqua Ammonia Mixtures, ASHRAE

Transactions, 77: (1979), pp. 149-151.

[14] V. P. Carey,LiquidVapor Phase-Change Phenomena, London: Hemisphere Publishing Corp., 1992.[15] J. G. Collier, Convective Boiling and Condensation London; McGraw-Hill, 1981.[16] R. W. Bjorge, G. R. Hall, and W. M. Rohsenow, Correlation of Forced Convective Boiling Heat Transfer Data,

International Journal of Heat and Mass Transfer, 25 (1982), pp. 753-757.

Paper Received 30 June 2003; Revised 17 March 2004; Accepted 1 June 2004.

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