1

EXPERIMENTAL INVESTIGATION OF DIRECT-CONTACT HEAT TRANSFER IN ISO-PENTANE/WATERSYSTEMS

M.A. Zablouk*

department of chemical engineering,university of technology, Iraq,mumtaz_zablouk@yahoo.com

F.Z. Hannadepartment of chemical engineering,

university of technology, Iraq,dr_fadihanna@yahoo.com

M. Hayekdepartment of mechanical engineering,notre dame university, p.o.box 72-zoukmikael,Lebanon, mhayek@ndu.edu.lb

ABSTRACTThe present experimental investigation deals with thephenomenological study of direct-contact heatexchange for iso-pentane/water system. The testsection consisted of a cylindrical perspex column, inwhich distilled water was to be confined. Liquid iso-pentane drops were injected into the hot water filledcolumn through special distributors located at thebottom of the column. Various operating and designparameters were investigated and their effects on theoverall performance of the heat transfer process werededuced. The experimental runs were planned usingthe central composite rotatable design method. It hasbeen found that the volumetric heat transfer coefficientvalues fall with an increase in the inlet temperature ofwater, also small-diameter nozzles associated withfaster nozzle velocities, and smaller droplets, yieldhigher volumetric heat transfer coefficient. In addition,iso-pentane was found to yield a slightly highervolumetric heat transfer coefficient compared with n-pentane.

Keywords: direct-contact heat transfer, bubble column,heat exchanger, iso-pentane.

INTRODUCTIONDirect-contact heat exchange is a process whereby twofluids pass in direct-contact with each other to facilitateheat transfer. The heavier fluid enters at the top of thecolumn and flows downward. A lighter fluid is admittedat the bottom and flows upward in the column.Obviously the flow is driven by gravity effects. One ofthe fluids (generally either one of them) can play therole of the continuous fluid, while the other takes therole of the dispersed fluid. The latter is usuallydispersed by some type of spray header (droplet spraynozzle) (Brickman & Boehm, 1994).Direct-contact heat transfer between two immiscibleliquids has the advantages of (a) eliminating metallicheat transfer surfaces, which are prone to corrosionand fouling, (b) relative simplicity of design, whichutilizes natural buoyancy to create a counter flowthrough a column, (c) higher heat transfer rates(relatively high performance), (d) ability to operate atrelatively small temperature driving forces, (e) lowcapital cost, and (f) lower pressure drop. The practicalapplications are found in water desalination, productionof electricity from low or moderate temperature heatsources in geothermal brines or solar pond powerplants coupled with Rankine cycle, ocean thermalenergy conversion, thermal energy storage system,emergency cooling of chemical reactors, andproduction of steam generation for the Rankine power

cycle from the direct-contact vaporization of water withlead-bismuth eutectic in Pb-Bi/ water reactor (PBWR)(Brickman & Boehm, 1995), (Battya et.al., 1982, 1983),(Seetharamu & Battya, 1989), (Buongiorno et.al.,2001).Terasaka and Tsuge (Terasaka & Tsuge, 1993) studiedthe bubble volumes and shapes formed from aconstant-flow nozzle submerged in a liquid. Theyphotographed the bubble shapes during bubbleformation with a high-speed video camera, usingdifferent liquids in N2 gas such as tap water and 68Wt% glycerol. (Sideman et al. 1965) investigated thespray column with fixed dispersed phase flow rates anddifferent diameters of orifices using the n-pentane / seawater system. The results show that the smaller thedroplets, the smaller the optimal volume, and the largerthe volumetric heat transfer coefficient. (Sideman andGat. 2004) measured the volumetric heat transfercoefficient and column heights required to vaporizepentane in water. Volumetric heat transfer coefficientswere in the range of 8,000 to 20,000 kJ/m hr. C, andthe results show that the coefficients decrease withincreasing driving force. (Ghazi,1991) investigated thedirect heat transfer for air- water system and concludedthat water temperatures appear to have little or noeffect on the overall heat transfer coefficient.(Brickman and Boehm, 1994) studied the liquid-liquiddirect-contact heat exchangers for the purpose offinding the design that brings the temperaturedifference between the two fluids to as a small value aspossible, using oil-water system. They confirmed that alonger column and smaller droplet size yield anincrease in effectiveness. (Özbelge and Shahidi, 1999)studied the direct-contact heat transfer between waterand oil in co-current flow through a horizontalconcentric annulus. They compared conventional heatexchangers with direct-contact heat exchangers(DCHX), and concluded that DCHXs have manyadvantages, especially in an annular geometry,because the unequal shear stresses exerted on theliquid drops, located in the annulus, by the inner andouter pipe surfaces induce the deformation of dropsand their break-up into droplets, thus an enhancementof heat transfer is obtained. (Dammel and Beer, 2000)studied the rise and evaporation of furan drop(dispersed phase) in a second less volatile immiscibleliquid of aqueous glycerol (continuous phase) of higherdensity and a temperature above the saturationtemperature of the dispersed phase, and investigatedthe motion of the interfaces. (Nagayoshi et al., 2002)conducted a study on heat and mass transfercharacteristics of air bubbles and hot water in directcontact. Hanna et al., (2003) conducted a parametric

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analysis on a three phase direct-contact systemconsisting of n-pentane and water. (Ribeiro & Lage,2004) used an air-water system for measuring liquidtemperature, bubbling height, evaporation rate, gashold-up and bubble size distributions in a direct-contactevaporator for four gas superficial velocities in bothhomogeneous and heterogeneous bubbling regimes,revealing some interesting features of transient non-isothermal bubbling. (Ribeiro & Lage, 2005) alsocharacterized the gas-liquid direct-contact evaporatorsby bubbling of a superheated gas through a solution tobe concentrated).(Adams and Pinder, 1972) obtained the average heattransfer coefficients for the evaporation of the drops ofiso-pentane and cyclopentene in different glycerin-water solutions. (Chughtai & Inayat, 2010) conducted anumerical simulation of direct-contact condensationfrom a supersonic steam jet in subcooled water andcompared the findings of the computational fluiddynamics simulations with published experimental datawhich showed good agreement thus validating theirmodel.

EXPERIMENTAL SET-UPThe experimental system is shown schematically inFig.1. It consisted of a test section, hot water supplysystem, cooling supply system, dispersed phase supplysystem, and dispersed phase distributor.

1.Direct – contact heelexchanger

2.Submerged cold coil3.Cylindrical storage tank4.Cooling bath5.Jaconet bath6.Water bath7.11,12,14,26 ball valves8.Water rat meter9.17.18 serocr (thermocouples10. water distnbutcx13.pump

15.Iso-pentane rotamerer16.Iso-nentane distbetar19.21.Mub-channel reoo rare20.Set of thermocouples22.Conderser23.Coleation starage tank24.Digital camera25.Computer (p3)27.Discharge line of water28.Steel bar cameracalibration

Fig.1. Schematic diagram of the experimental apparatus

The test section consists of a cylindrical Perspexcolumn of 17 cm inside diameter and 1m length, inwhich the test fluid, water, is to be confined. A

cylindrical Perspex-made water jacket with a variableworking temperature was used as a fixed-temperature water bath. The jacket contributed tominimizing the heat losses from the test column bycirculating water around the test column. The topand bottom ends of the test column were closed withPerspex plates. The water was supplied at the top ofthe test column through the distributor located at thecenter of the test column and flows downward to exitthrough the bottom by a constant-temperaturecirculating bath equipped with a thermostaticallycontrolled electric heater. The optimum water heightin the column was found to be around 85cm. Thewater flow rate was measured by using a calibratedrotameter. The temperature of inlet water wasmeasured by calibrated sheath copper - constantanthermocouple (9), inserted at the top of the inletwater line, and the temperature of outlet water wasmeasured by a similar thermocouple (17), insertedbelow the distributor section.The iso-pentane entered the test column at constanttemperature of 30oC from the bottom through adistributor and flows upward in droplet form. Its flowrate was measured by using a calibrated rotameter.The flow rates of the continuous phase (water), anddispersed phase (iso-pentane) were manuallycontrolled by using a stainless steel ball valvesmounted in the liquid lines.The temperature of the iso-pentane was measuredby a calibrated, sheath copper - constantanthermocouple (18) inserted before the distributor.The thermocouples (9), (17), and (18) wereconnected to a multi-channel temperaturerecorder. Six calibrated sheath copper-constantanthermocouples were inserted along the length ofthe column on its left side with a spacing of 17 cmfrom each other. These thermocouples wereconnected to a multi-channel temperature recorderof the same type above. The iso-pentane vaporwas collected at the top of the test column, andcondensed in a vertical condenser by circulatingcooling ethanol/water mixture. The condensatewas collected in a storage tank to be reused in theexperiments. A digital camera with a speed of 30Frames per second was used in the experimentalwork to obtain the two-phase bubble size, and thelevel of any two-phase bubble along its path fromthe position of the initial drop.Hot water was supplied to the test column by aconstant temperature circulating bath equipped witha thermostatically controlled electric heater. Coldethanol-water mixture was supplied to the condenserat the top of the test column by a constanttemperature circulating bath equipped with athermostatically controlled chiller.The dispersed phase was stored in a vessel of QVFglass, which was a cylindrical vessel of 25 litercapacity. This vessel was supplied with submergedcold coil of the chiller unit. The dispersed phase wasiso-pentane fed by a pump from the cylindrical

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vessel located approximately 0.5m above the groundlevel.Two types of distributors were used in theexperiments as shown in Fig.2. Distributor (A) wasmade of Teflon coated Aluminum plate of 5 mmthickness in which 25 orifices of 1 mm diameter eachwere drilled according to a square in-linearrangement of 2 cm pitch. Distributor (B) was basedon the same design with 15 orifices of 2 mmdiameter each and a pitch of 2.5 cm.

Disrbrtor (A) No. of orifice = 25Orifice diameter = 1mmPitch = 2cm

Disrbrtor (B) No. of orifice = 15Orifice diameter = 2mmPitch = 2.5cm

Fig.2. Design details of iso-pentane distributors

DATA ANALYSISData were acquired by direct measurement Enlargedconsecutive pictures were developed from the fastand high resolution camera films For thesemeasurements, all the drops, as well as the vaporphase bubbles, were taken as of ellipsoidal shape.The instantaneous equivalent spherical diameterwas calculated from the measured horizontal andvertical diameters as well as the diameter of a steelball suspended inside the test section for calibrationpurposes (Raina et.al., 1984), (Simpson et.al., 1974)

d = D + M − 1M d d (1)By assuming no heat losses from the test column,and knowing inlet and outlet temperatures of waterand iso-pentane, the logarithmic mean temperaturedifference could be calculated, as well as the heattransfer rate, and the volumetric heat transfercoefficient using the following relationship:q = m C (T − T ) m. λ + (T − T )m. C= U VΔT F (2)in which the superheating effects were neglectedand the correction factor F is taken equal to 1.

RESULTS AND DISCUSSIONThe experimental results are given in Figs. 3 to 8 inwhich the meanings of code numbers are as given inTable 1. Each figure includes two charts: one fordistributor (A) labeled A and another one fordistributor (B) labeled B.

Table 1. Working range of coded and correspondingreal variables.

Codedlevel

Inlettemperature

of water( C) o

Watervolumetricflow rate(cm3 /s)

Iso-Pentane

volumetricflow rate(cm3 /s)

-1.732-101

1.732

3031.734

36.338

9.818.1329.4240.83

49

0.96361.1671.4451.7221.927

Fig.3 shows the influence of inlet temperature ofwater on the droplet radius. Droplet radii increasewith increasing inlet temperature of water for the twodistributors (A) and (B), because of the increase ininlet temperature of water leading to an increase inthe logarithmic mean temperature difference (TLm)and an increase in the vapor portion in the two-phase bubbles moving along the length of column.This is attributed to the increase in size of the two-phase bubbles. Similar conclusion was reached by(Simpson et al., 1974) and (Sideman and Taitel,

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vessel located approximately 0.5m above the groundlevel.Two types of distributors were used in theexperiments as shown in Fig.2. Distributor (A) wasmade of Teflon coated Aluminum plate of 5 mmthickness in which 25 orifices of 1 mm diameter eachwere drilled according to a square in-linearrangement of 2 cm pitch. Distributor (B) was basedon the same design with 15 orifices of 2 mmdiameter each and a pitch of 2.5 cm.

Disrbrtor (A) No. of orifice = 25Orifice diameter = 1mmPitch = 2cm

Disrbrtor (B) No. of orifice = 15Orifice diameter = 2mmPitch = 2.5cm

Fig.2. Design details of iso-pentane distributors

DATA ANALYSISData were acquired by direct measurement Enlargedconsecutive pictures were developed from the fastand high resolution camera films For thesemeasurements, all the drops, as well as the vaporphase bubbles, were taken as of ellipsoidal shape.The instantaneous equivalent spherical diameterwas calculated from the measured horizontal andvertical diameters as well as the diameter of a steelball suspended inside the test section for calibrationpurposes (Raina et.al., 1984), (Simpson et.al., 1974)

d = D + M − 1M d d (1)By assuming no heat losses from the test column,and knowing inlet and outlet temperatures of waterand iso-pentane, the logarithmic mean temperaturedifference could be calculated, as well as the heattransfer rate, and the volumetric heat transfercoefficient using the following relationship:q = m C (T − T ) m. λ + (T − T )m. C= U VΔT F (2)in which the superheating effects were neglectedand the correction factor F is taken equal to 1.

RESULTS AND DISCUSSIONThe experimental results are given in Figs. 3 to 8 inwhich the meanings of code numbers are as given inTable 1. Each figure includes two charts: one fordistributor (A) labeled A and another one fordistributor (B) labeled B.

Table 1. Working range of coded and correspondingreal variables.

Codedlevel

Inlettemperature

of water( C) o

Watervolumetricflow rate(cm3 /s)

Iso-Pentane

volumetricflow rate(cm3 /s)

-1.732-101

1.732

3031.734

36.338

9.818.1329.4240.83

49

0.96361.1671.4451.7221.927

Fig.3 shows the influence of inlet temperature ofwater on the droplet radius. Droplet radii increasewith increasing inlet temperature of water for the twodistributors (A) and (B), because of the increase ininlet temperature of water leading to an increase inthe logarithmic mean temperature difference (TLm)and an increase in the vapor portion in the two-phase bubbles moving along the length of column.This is attributed to the increase in size of the two-phase bubbles. Similar conclusion was reached by(Simpson et al., 1974) and (Sideman and Taitel,

3

vessel located approximately 0.5m above the groundlevel.Two types of distributors were used in theexperiments as shown in Fig.2. Distributor (A) wasmade of Teflon coated Aluminum plate of 5 mmthickness in which 25 orifices of 1 mm diameter eachwere drilled according to a square in-linearrangement of 2 cm pitch. Distributor (B) was basedon the same design with 15 orifices of 2 mmdiameter each and a pitch of 2.5 cm.

Disrbrtor (A) No. of orifice = 25Orifice diameter = 1mmPitch = 2cm

Disrbrtor (B) No. of orifice = 15Orifice diameter = 2mmPitch = 2.5cm

Fig.2. Design details of iso-pentane distributors

DATA ANALYSISData were acquired by direct measurement Enlargedconsecutive pictures were developed from the fastand high resolution camera films For thesemeasurements, all the drops, as well as the vaporphase bubbles, were taken as of ellipsoidal shape.The instantaneous equivalent spherical diameterwas calculated from the measured horizontal andvertical diameters as well as the diameter of a steelball suspended inside the test section for calibrationpurposes (Raina et.al., 1984), (Simpson et.al., 1974)

d = D + M − 1M d d (1)By assuming no heat losses from the test column,and knowing inlet and outlet temperatures of waterand iso-pentane, the logarithmic mean temperaturedifference could be calculated, as well as the heattransfer rate, and the volumetric heat transfercoefficient using the following relationship:q = m C (T − T ) m. λ + (T − T )m. C= U VΔT F (2)in which the superheating effects were neglectedand the correction factor F is taken equal to 1.

RESULTS AND DISCUSSIONThe experimental results are given in Figs. 3 to 8 inwhich the meanings of code numbers are as given inTable 1. Each figure includes two charts: one fordistributor (A) labeled A and another one fordistributor (B) labeled B.

Table 1. Working range of coded and correspondingreal variables.

Codedlevel

Inlettemperature

of water( C) o

Watervolumetricflow rate(cm3 /s)

Iso-Pentane

volumetricflow rate(cm3 /s)

-1.732-101

1.732

3031.734

36.338

9.818.1329.4240.83

49

0.96361.1671.4451.7221.927

Fig.3 shows the influence of inlet temperature ofwater on the droplet radius. Droplet radii increasewith increasing inlet temperature of water for the twodistributors (A) and (B), because of the increase ininlet temperature of water leading to an increase inthe logarithmic mean temperature difference (TLm)and an increase in the vapor portion in the two-phase bubbles moving along the length of column.This is attributed to the increase in size of the two-phase bubbles. Similar conclusion was reached by(Simpson et al., 1974) and (Sideman and Taitel,

4

1964) who used a system consisting of butane dropsin brine.

Fig.3. Variation of droplet radius with inlettemperature of water

Fig.4. shows the effect of inlet temperature of wateron the volumetric heat transfer coefficient Thevolumetric heat transfer coefficient decreases withincreasing inlet temperature of water due to theincrease in the logarithmic mean temperaturedifference (driving force), and the increase in theoverall resistance to heat transfer. Similar results arereported by (Sideman et al., 1965), (Sideman & Gat,1966) and (Sideman & Taitel, 1964).

Fig.4. Variation of volumetric heat transfer coefficientwith inlet temperature of water

Values of the droplet radius are plotted against thewater volumetric flow rate in Figs.4 for distributors(A) and (B), respectively. The droplet radiusdecreases with increasing water volumetric flow ratefor both distributors. Such a decrease is attributed tothe increase in the deformation of the two-phasebubbles in the iso-pentane (break-up of two phasebubbles), thus the rate of coalescence decreases.This will result in a smaller two-phase bubble size,and larger interfacial area for direct-contact heattransfer. Similar results were reported by (Shahidiand Özbelge, 1995) using a system of twoimmiscible liquids.

Codedlevel

-1.732-101

1.732

Drop

let

Radi

us R

d(m

m)

Drop

let

Radi

us R

d(m

m)

Inlet temperature of water Tci(0C)

Inlet temperature of water Td(0C)

Inlet temperature of water Td(0C)Vo

lum

etric

hea

t tra

nsfe

r coe

ff.U

v(KW

/m3 .0 C

Inlet temperature of water Td(0C)

Drop

let

Radi

us R

d(m

m)

Volu

met

ric h

eat t

rans

fer c

oeff.

Uv(K

W/m

3 .0 CInlet temperature of water Tci(

0C)

Drop

let

Radi

us R

d(m

m)

Inlet temperature of water Tci(0C)

Inlet temperature of water Tci(0C)

5

Fig.5. Variation of droplet radius with volumetricflowrate of water

The results shown in Fig.5, representing the variationof the volumetric heat transfer coefficient with thewater flow rate, indicate, clearly, two operatingranges. In the first range, the volumetric heat transfercoefficient decreases with increasing watervolumetric flow rate because of increasing rate ofcoalescence compared with the rate of break-up inthis range. In the second range, the volumetric heattransfer coefficient increases with increasing watervolumetric flow rate because of the formation ofsmaller droplets, which have less tendency tocoalesce and high tendency to break-up. Increasingin break-up of the two-phase bubbles will increasethe effective heat transfer area per unit volume of theheat exchanger, and, as a result, the volumetric heattransfer coefficient. Such a behavior is reported by(Sideman et al., 1965).

Fig.6. Variation of volumetric heat transfer coefficientwith volumetric flowrate of water

Fig.6 shows the influence of iso-pentane flow rate ondroplet radius. Droplet radius decreases withincreasing iso-pentane flow rate for both distributors(A) and (B), because a more significant deformationof iso-pentane droplets in the flow direction (break-up of two phase bubbles), leads to small two-phasebubbles size, (Özbelge & Shahidi, 1999).

Water volumetric flow rate Qc (cm 3/s)*((yhhyhjhjjjffff(cm3/sQQQ0(cm3/temperature of

water Td(0C)

Drop

let

Radi

us R

d(m

m)

Water volumetric flow rate Qc (cm 3/s)*((yhhyhjhjjjffff(cm3/sQQQ0(cm3/temperature of

water Td(0C)

Drop

let

Radi

us R

d(m

m)

Volu

met

ric h

eat t

rans

fer c

oeff.

Uv(K

W/m

3 .0 C

Water volumetric flow rate Qc (cm 3/s)*((yhhyhjhjjjffff(cm3/sQQQ0(cm3/temperature of

water Td(0C)Vo

lum

etric

hea

t tra

nsfe

r coe

ff.U

v(KW

/m3 .0 C

Water volumetric flow rate Qc (cm 3/s)*((yhhyhjhjjjffff(cm3/sQQQ0(cm3/temperature of

water Td(0C)

6

Fig.7. Variation of droplet radius with volumetricflowrate of iso-pentane

The effect of iso-pentane volumetric flow rate on thevolumetric heat transfer coefficient is shown in Fig.7.The volumetric heat transfer coefficient increaseswith increasing iso-pentane flow rate for bothdistributors (A) and (B). This increase is attributed tothe fact that smaller bubbles are formed in higheriso-pentane flow rate. These small bubbles havelarge interfacial areas leading to high volumetric heattransfer coefficients. Similar trends are reported by(Sideman et al., 1965), Nagayoshi et.al., 2002) and(Shahidi & Özbelge, 1995).

Fig.8. Variation of volumetric heat transfer coefficientwith volumetric flowrate of iso-pentane.

A complete second-order polynomial regressionanalysis of the results was conducted using standardstatistical software. Three variables were consideredin the investigations, namely, the inlet temperaturewater, the continuous phase, e.g. water, volumetricflow rate, and the dispersed phase, e.g. iso-pentane,volumetric flow rate for both distributor designs (A)and (B). The resulting correlations for the dropletradius and the volumetric heat transfer coefficientare:

Volu

met

ric h

eat t

rans

fer c

oeff.

Uv(K

W/m

3 .0 CVo

lum

etric

hea

t tra

nsfe

r coe

ff.U

v(KW

/m3 .0 C

Drop

let

Radi

usR d

(mm

)Dr

ople

t Ra

dius

Rd

(mm

)

Iso-pentane volumetric flow rate Qd (cm3/s)volumetric flow rate Q0 (cm 3/s)

*((yhhyhjhjjjffff(cm3/sQQQ0(cm3/temperature ofwater Td(0C)

Iso-pentane volumetric flow rate Qd (cm3/s)volumetric flow rate Q0 (cm 3/s)

*((yhhyhjhjjjffff(cm3/sQQQ0(cm3/temperature ofwater Td(0C)

Iso-pentane volumetric flow rate Qd (cm3/s)volumetric flow rate Q0 (cm 3/s)

*((yhhyhjhjjjffff(cm3/sQQQ0(cm3/temperature ofwater Td(0C)

Iso-pentane volumetric flow rate Qd (cm3/s)volumetric flow rate Q0 (cm 3/s)

*((yhhyhjhjjjffff(cm3/sQQQ0(cm3/temperature ofwater Td(0C)

7

• Distributor (A):Rd= 2.627572- 0.463522Td - 0.0762151Q0 +1.023141Qd +0.016831T2

d +0.000137 Q20 +0.013779

Q2d +0.003062TdQc -0.047992TdQd -0.038164QcQd (3)

Uv= 63.3236 - 3.63413Tci – 0.405121Qc +17.3598Qd

+0.052008T2ci +0.001891Q2

c +0.699334Q2d

+0.010389TciQc - 0.442591TciQd - 0.052263QcQd (4)

• Distributor (B):Rd=45.71144 -3.3060Tci – 0.078543Qc +5.910858Qd

+0.06331T2ci - 0.001891Q2

c – 0.963606 Q2d

+0.002706TciQc-0.139228TciQd -0.016319QcQd (5)

Uv=42.62726-2.38412Tci – 0.13685Qc +8.774618Qd

+0.033075T2ci +0.000876Q2

c +0.376376Q2d

+0.003593TciQc -0.208444TciQd -0.031041QcQd (6)

A comparison between the volumetric heat transfercoefficient obtained for n-pentane and iso- pentaneis illustrated in Fig.8. The figure shows that using iso-pentane yields a slightly higher volumetric heattransfer coefficient compared with n-pentane. This isprobably because of the decrease in driving forcedue to the lower boiling point of the former.

Fig.9. Volumetric heat transfer coefficient for pentaneand iso-pentane

Tables 2 & 3 show the regression analysis results fordroplet radius and volumetric heat transfer coefficientfor both distributors. (A) and (B) which show verygood absolute average error, correlation coefficientand standard deviation.

Table 2. Results of regression analysis for dropletradius and volumetric heat transfer coefficient -Distributor (A).

DropletRadius

Vol. HeatTransferCoefficient

Absolute average errorCorrelation coefficient

Standard deviation

1.1742%0.999120.064836

2.52428%0.998040.076714

Table 3. Results of regression analysis for droplet radiusand volumetric heat transfer coefficient - Distributor (B).

DropletRadius

Vol. HeatTransferCoefficient

Absolute average errorCorrelation coefficient

Standard deviation

2.135710.995560.184608

3.78032%0.993780.096821

CONCLUSIONSParametric analysis of direct-contact heat exchangerin a counter-current column of immiscible liquid hasbeen carried out. The results compare favorably withsimilar results in the literature. The followingconclusions can be made:1. The shape of the two-phase bubbles observed in

the present study changed from nearly sphericalto ellipsoidal and sometimes to spherical-capshapes.

2. When the Box-Wilson technique is used, arelationship is found between the processvariables TCi, QC, and Qd, and the designproperties Rd, and Uv. The experimental resultsagree quite well with a polynomial type ofcorrelation.

3. The volumetric heat transfer coefficient values fallwith an increase in the inlet temperature of water.

4. Small-diameter nozzles associated with fasternozzle velocities, and smaller droplets, yieldhigher volumetric heat transfer coefficient.

5. Iso-pentane yields a slightly higher volumetricheat transfer coefficient compared with n-pentane

Recommendations for Future WorkIt is highly recommended that further analyticalinvestigations are to be carried out in order todevelop techniques that can be used for optimizationpurposes. It is also recommended to conduct anumerical simulation similar to the one conducted by(Chughtai and Inayat, 2010) on their system.

Volu

met

ric h

eat t

rans

fer c

oeff.

Uv(K

W/m

3 .0 C

volumetric flow rate Qc (cm 3/s)*((yhhyhjhjjjffff(cm3/sQQQ0(cm3/temperature of

water Td(0C)

8

NOMENCLATURECP specific heat at constant pressure, J/kg oC,dd droplet diameter of the two-phase bubble, m,dh horizontal diameter of the bubble, m,dv vertical diameter of the bubble, m,Do initial drop diameter, m,F correction factorm mass flow rate, kg/s,M density ratio of liquid density to vapor density of

iso-pentane,q heat transfer rate, kW,Q volumetric flow rate, m3/s,Rd droplet radius of the two-phase bubble, m,T temperature, oC,Uv volumetric heat transfer coefficient, kW/m.oC,V optimum volume of heat exchanger, m3

GREEK LETTERS latent heat of vaporization, J/kg,T lm logarithmic mean temperature difference, oC

SUBSCRIPTSc continuous phase,d dispersed phase,i inlet condition,

o outlet condition

REFERENCESAdams A.E.S and K.L. Pinder. 1972. Average heattransfer coefficient during the direct evaporation of aliquid drop, Canadian Journal of ChemicalEngineering 50:707-713.Battya P, V.R. Raghavan and K.N. Seetharamu.1982. Parametric analysis of direct contact sensibleheat transfer in spray columns, Letters in Heat andMass Transfer 9:265-276.Battya P., V.R. Raghavan and K.N. Seetharam.1983. A theoretical analysis of direct contactevaporation in spray columns, InternationalCommunication Heat Mass Transfer 10:533-543.Brickman, R.A. and R.F. Boehm. 1994. Maximizingthree-phase direct-contact heat exchanger output,Numerical Heat Transfer 26:287.Brickman R.A. and R.F. Boehm. 1994. Optimizationconsiderations of balanced, liquid-liquid contact heatexchanges, International Mechanical EngineeringCongress 1-7.Brickman R.A. and R.F. Boehm. 1995. Optimizationanalyses of balanced liquid-liquid contact heatexchanges, proceedings of the ASME / JSMEThermal Engineering Joint Conference 4:373-379.Buongiorno J., N.E. Todreas and M.S. Kazimi. 2001.Thermal design of a lead-bismuth cooled fast reactorwith in-vessel direct-contact steam generation, Ph.D.Thesis, Department of Nuclear Engineering,Massachusetts Institute of Technology, 17-28.Dammel F. and H. Beer. 2000. Numerical simulationof the direct-contact evaporation of drop rising in asecond liquid due to buoyancy, European Congresson Computational Methods in Applied Sciences and

Engineering 1-18.Ghazi H.S. 1991. Direct-contact heat transfer for airbubbling through water, Energy ResourcesTechnology 113:71-74.Hanna F.Z., A. Kendoush and M. Zablouk. 2003.Evaporation of n-pentane in three-phase direct-contact heat exchanger, Ph.D. Thesis, Departmentof Chemical Engineering, University of Technology,Baghdad, Iraq.Nagayoshi K. et al. 2002. A study on heat and masstransfer characteristics of air bubbles and hot waterby direct contact, Journal of Heat & Mass Transfer38:449-457.Özbelge T. and M.K. Shahidi. 1999. Enhancement ofdirect-contact heat transfer in concentric annuli,Kluwer Academic Publishers 649-663.Raina G.K. and R.K. Wanchoo, P.D. Grover. 1984.Direct-contact heat transfer with phase change:motion of evaporating droplets, Journal of AmericanInstitute of Chemical Engineering 30:835-837.Ribeiro C.P. and P.L.C. Lage, 2004. Direct-contactevaporation in the homogeneous and heterogeneousbubbling regimes. Part I: experimental analysis,International Journal of Heat and Mass Transfer47:3825-3840.Ribeiro C.P. and P.L.C. Lage, 2005. Gas-liquiddirect-contact evaporation: a review, ChemicalEngineering and Technology 28:1081-1107.Seetharamu K.N and P. Battya, 1989. Direct-contactevaporation between two immiscible liquids in aspray column. ASME Journal of Heat Transfer111:780-785.Shah I,R. and M.H. Inayat, 2010. Numericalsimulation of direct-contact condensation from asupersonic steam jet in subcooled water. Chin. J.Chem. Eng. V18(4):577-587.Shahidi M.K. and T. Özbelge. 1995. Direct-contactheat transfer between two immiscible liquids flowingin a horizontal concentric annulus, InternationalJournal of Multiphase Flow 21:1025-1036.Sideman S. and Y. Taitel. 1964. Direct-contact heattransfer with change of phase: evaporation of dropsin an immiscible liquid medium, International Journalof Heat and Mass Transfer 7:1273-1289.Sideman S., G. Hirsch and Y. Gat. 1965. Direct-contact heat transfer with change of phase: effect ofthe initial drop size in three-phase heat exchangers,Journal of the American Institute of ChemicalEngineering 11:1083-1087.Sideman S.and Y. Gat. 2004. Direct-contact heattransfer with change of phase: spray-column studiesof a three-phase heat exchanger, Journal of theAmerican Institute of Chemical Engineering 12:296-303.Simpson H.C., G.C. Beggs and M. Nazir. 1974.Evaporation of butane drops in brine, Desalination15:11-23.Terasaka K. and H. Tsuge. 1993. Bubble formationunder constant-flow conditions, ChemicalEngineering Science 48:3417-3422.