EXPERIMENTAL STUDIES OF HEAT TRANSFER COEFFICIENT AND PRESSURE DROP IN INCLINED CONDENSING UNITS

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1 th Internatinal Cnference n Heat ransfer, Fluid Mechanics and hermdynamics EXPERIMENAL SUDIES OF HEA RANSFER COEFFICIEN AND PRESSURE DROP IN INCLINED CONDENSING UNIS Adelaja A.O.*, Dirker J.** and Meyer J.P. *** *Authr fr crrespndence Department f Mechanical and Aernautical Engineering, University f Pretria, Pretria, 2, Suth Africa, *adekunle.adelaja@up.ac.za **jac.dirker@up.ac.za ***jsua.meyer@up.ac.za ABSRAC his paper presents the experimental heat transfer cefficient and frictinal pressure drp values during the cndensatin f R34a inside a smth cpper tube fr vapur qualities f, and, mass fluxes f 2 kg/m 2 s t 3 kg/m 2 s, saturatin temperatures f 3 C and 4 C and tube inclinatin angles ranging between -9 (vertical dwnward) and +9 (vertical upward). he effects f the inclinatin angle cupled with mean vapur quality, mass flux and saturatin temperature were investigated n the heat transfer cefficient and pressure drp. An entrpy generatin analysis was cnducted t determine at which inclinatin angle entrpy generatin is minimised. It appears as if entrpy generatin was at its lwest fr vertical upward flw, but that fr sme cases this was als achieved fr slightly dwnward flw. INRODUCION Cndensatin heat transfer and pressure drps are very imprtant parameters fr cnsideratin in the effective and efficient design f cndensing heat exchangers. Fr heat exchanger design and prcess engineers the interest is fcussed n enhanced heat transfer cefficient and reduced pressure drp in the equipment, be it either the cndenser r the evapratr. Enhanced heat transfer imprves the thermal perfrmance f the heat exchanger. Reducing the pressure drp, hwever, causes a reductin in the pumping pwer requirement f the cycle resulting in reductin in the cmpressr r pump design size which crrespnds t lwer cst. Mst techniques prpsed t enhance the heat transfer cefficient are unfrtunately accmpanied by increased lcal pressure penalty factrs. his is apart frm the increased cst f prductin f the enhanced tubes in case f passive methds, r the additinal external pwer which had t be applied in active heat enhancement techniques []. Fr instance, fr spiral micrfins, the heat transfer enhancement is between 8% and 8%, the pressure penalty factr is between 2% and 8% while fr crss grve tubes the heat enhancement is between 25% and 3% while the pressure penalty factr is between 6% and % [2]. hus, the aims f new cnvectin heat transfer methds are the reductin f pressure drp, increase f the verall thermal efficiency, and reductin f the capital cst f such systems. It is knwn that the inclinatin angle f the cndenser tubes in cndensatin devices have an impact n the NOMENCLAURE A [m 2 ] Area d [m] Diameter ΔP [kpa] Differential pressure g [ms -2 ] Gravitatinal acceleratin G [kgm -2 s] Mass flux k [Wm - K - ] hermal cnductivity L [m] Length ΔP fric [Pa] Frictinal pressure drp Q [W] Heat transfer rate R [KW - ] hermal resistance s gen, P [J/kgK] Specific entrpy generatin due t pressure drp s gen,q [J/kgK] Specific entrpy generatin due t heat transfer [K] emperature [K] Mean temperature x [-] Vapur quality z [-] Axial directin Special characters α [Wm -2 K - ] Heat transfer cefficient Inclinatin angle ε [-] Vid fractin ρ P [kgm -3 ] Density Subscripts Cu i j lcal m ut sat test w Cpper Inner Measurement lcatin Specific psitin Mean Outer Outlet Saturatin est Wall tw-phase flw pattern distributin which influences the heat transfer cefficient and the effective pressure drp. Inclined tubes are especially f interest in the A- and V-frame cndensers. Several studies [3-8] have been cnducted n the heat transfer cefficient f in-tube cndensatin f R34a at different inclinatin angles, but few have been cnducted dedicated t pressure drp [9-]. the best f the authrs knwledge the applicatin f these design parameters in term f the influenced f the inclinatin angle t aid prcess and heat exchanger design engineers and researchers in their activities is scarce in pen literature. Akhavan-Behabadi et al. [] cnsidered in their study cndensatin heat transfer cefficients f R34a in micrfin tubes f an inner diameter f 8.92 mm at 48

2 th Internatinal Cnference n Heat ransfer, Fluid Mechanics and hermdynamics mass fluxes (G) between 59 kg/m 2 s and 7 kg/m 2 s, saturatin temperatures ( sat ) between 26 C and 32 C, mean vapur qualities between.2 and.8 and inclinatin angles (β) ranging between -9 and +9. hey reprted enhanced heat transfer which was ptimised at an upward inclinatin f 3. Sapali and Patil [2] cnducted an experimental investigatin f heat transfer cefficient f R34a and R44a inside a hrizntal smth tube with an inner diameter f 8.56 mm and a micrfin tube with an inner diameter f 8.96 mm fr mass fluxes between 9 kg/m 2 s and 8 kg/m 2 s and saturatin temperature between 35 and 6. he heat transfer enhancements btained in the micrfin tube were between 5% and 5% fr R34a and between 3% and % fr R44a. In mst f the wrks n micrfin tubes the area rati (AR) is greater than unity. Lyulin et al. [3] experimentally investigated the cndensatin heat transfer cefficient f pure ethanl in a smth tube at a saturatin temperature f 58 C. hey stated that the heat transfer cefficient reduces with increased temperature difference between the saturatin and wall temperatures. hey frmally reprted ptimised inclinatin effect n heat transfer cefficient at between -5 and -35. Lips and Meyer [3, 4 and 9] in their experiments n cndensatin heat transfer cefficient and pressure drp f R34a in a smth cpper tube with an inner diameter f 8.38 mm perated at mass fluxes between 2 kg/m 2 s and 6 kg/m 2 s, mean vapur qualities between. and.9, and inclinatin angles between -9 and +9 fr sat = 4 C btained ptimised inclinatin angles between -5 and -3. he heat transfer changed by up t 43% in terms f inclinatin with respect t the hrizntal tube inclinatin heat transfer cefficient. hey als bserved that the pressure drp increased with mass flux and vapur quality fr -9 β < +3 but decreased with mean vapur quality fr +3 β +9. Meyer et al. [5] and Adelaja et al. [6-8] extended the wrk f Lips and Meyer in varius experimental results at different saturatin temperatures. In additin t the cnclusins f Lips and Meyer, they stated that bth heat transfer cefficient and pressure drp decreased with saturatin temperature. It is, hwever, interesting t have a hlistic view f the heat transfer and pressure drp in an inclined smth tube t serve as a guide fr heat exchanger design, prcess engineers and researchers wh are nt nly interested in heat transfer enhancement, but als in the reductin in pressure drp. In this study, it is shwn that in the gravity dminated flw regime, there culd be heat enhancement accmpanied by reduced penalty factr withut an increase in heat transfer surface area. his culd be achieved by inclining the equipment relative t the hrizntal causing the tw-phase flw refrigerant subject t the influence f gravity. EXPERIMENAL SE-UP he present study was cnducted at the test facility in the labratries f the Department f Mechanical and Aernautical Engineering, University f Pretria, Suth Africa (Figure ). he test set-up allwed fr duble tube cndensatin heat transfer and pressure drp data t be cllected ver a wide range f perating cnditins. he setup cmprised f three flw lps: the refrigerant, the cling water and the ht water Figure Schematic diagram f the test rig. lps. In the refrigerant lp, there were tw high-pressure lines. he first line cntained the pre, test and pst cndensers, while the secnd line cntained the bypass cndenser which was emplyed t assist in regulating the amunt f refrigerant trugh the test cndenser. In the high pressure test line, superheated vapur frm the kw hermetically sealed scrll cmpressr entered int the pre-cndenser and was partially cndensed. he vapur quality achieved at the utlet f the pre-cndenser was equal t the inlet vapur quality t the test cndenser because the cnnectin interface between the tw cndensers was adiabatic. he refrigerant entered the test cndenser, which carried the refrigerant in an inner tube and water in annulus in a cunter flw arrangement, at a prescribed mass and inlet vapur quality. Here, the refrigerant vapur cndensed further in the test tube at a prescribed heat flux as cntrlled via the lcal cld water flw rate. In this study this heat flux was maintained between 23 W and 27 W. Mre infrmatin n the test cndenser is supplied in the next sectin. he partially cndensed liquid-vapur mixture left the test cndenser and was fully cndensed in the pst-cndenser. he pst cndenser ensured that the tw-phase mixture became fully liquid befre reaching the electrnic expansin valve (EEV). Liquid refrigerant frm the tw high-pressure lines were cmbined and flwed t the evapratr where warm water frm the ht water flw lp was used t evaprate the liquid refrigerant befre the vapur passed t the cmpressr via the accumulatr. All heat exchangers (including all cndensers and the evapratr) where f the duble pipe kind and were perated in cunter flw arrangements with the refrigerant flwing in the inner tube and water flwing in the uter annular space. All water and refrigerant flw rates were measured by Crilis flw meters. Fr the water lps, a 5 kw cling and 7 kw heating dual functin heat pump supplied cld and ht water via a distributin system. he cld and ht water temperatures were thermstatically cntrlled t be between5 C and 25 C, and 4 C respectively deepening n the test cnditins. hermal buffering was btained by using tw 5 litre insulated water reservirs t stre the ht and cld water. β g 49

3 th Internatinal Cnference n Heat ransfer, Fluid Mechanics and hermdynamics ES SECION he test sectin had a heat transfer length f L =.488 m. It was a duble pipe cunter flw cndenser cnstructed frm hard drawn cpper with water as a clant in the annulus. he refrigerant vapur cndensed inside the inner tube. he inner tube had an inner diameter f d i = 8.38 mm and an uter diameter f d = 9.55 mm while the annulus had an uter diameter f 5.9 mm. he cnnectins t the test cndenser were made f flexible pressure hses. hese enabled the test sectin t rtate abut tw fixed hinges. ensure that the flw thrugh the test sectin was fully develped, a straight calming sectin, 5 diameters lng was situated at the entrance. All temperature measurements were dne with -type thermcuples calibrated against a high precisin Pt- resistant temperature detectr t an accuracy f. C. On the test cndenser, thermcuples were arranged such that the tube wall temperature ( w, ) culd be measured. At each lcatin fur thermcuples were installed and spaced 9 t each ther arund the tube (i.e. tp, bttm and tw sides). wenty-eight such thermcuples were installed in small pt hles drilled at seven equal-distant psitins n the inner tube. w abslute pressure transducers were cnnected between the inlet and utlet f the test sectin s that the abslute pressure recrding used was the average f the tw pressure readings. he pressure was used as the saturatin pressure (accurate t ±% f the full scale). he measured value when used with the cndensatin curve prvided by REFPROP [4] gave the saturatin temperature which was verified by direct measurement. he difference in the tw values was fund t be arund. C. he pressure drp acrss the test sectin was measured by a differential pressure transducer (FP 2 Senstec) which was calibrated t an accuracy f ±.5 kpa. At the inlet and utlet f the test sectin were tw sight glasses which enabled flw visualizatin and als served as insulatrs against axial heat cnductin. A high speed camera was installed at the utlet sight glass and was used t recrd and dcument the flw patterns. EXPERIMENAL PROCEDURE AND DAA REDUCION he test cnditins described in able give the range f experimental variables used in this study and their cntrl fluctuatins. he mean vapur quality, x m, is based n the arithmetic average between the entrance and exit qualities, x in and x ut respectively f the R34a as btained by using the energy balance principle fr the pre-cndenser and testcndenser. After steady state cnditins were reached, the different sensr signals were recrded cntinuusly thrugh the Labview and Natinal Instrument data acquisitin system fr abut a perid f minutes. In rder t avid nise measurement, the average f the pints was used fr the calculatins f the fluid prperties, heat transfer cefficients, and ther parameters f interest. he cnvective heat transfer cefficient in the test cndenser was calculated frm Newtn s law f cling: able Experimental variables and fluctuatins. Parameter Range Fluctuatins sat 3 C and 4 C ±.6 C G 2 and 3 kg/m 2 s ± 5 kg/m 2 s x m,, ±. -9 t +9 ±. Q A test ( w, i sat Here Q test is the heat transfer rate in the test cndenser as btained via the energy equatin n the water side based n the measured inlet and utlet water temperatures and the water mass flw rate. A is the inner surface area f the inner tube f the test cndenser. sat is the mean f the saturatin temperature between the inlet and utlet f the sectin. w, is the calculated i mean inner wall temperature based n the measured mean uter wall temperature f the tube w, thrugh the thermal resistance f the wall f the cpper tube, R w ) () w i w Qtest R (2),, w Here Rw lnd di / 2kCuL with k Cu referring t the thermal cnductivity f the cpper wall. w, was calculated using the trapezidal numerical integratin: where j w, w, L 6 j j z z j w, w, j j (3) is the average temperature at the j th lcatin f the seven different statins and z j+ z j is the distance between the measurement lcatins. RESULS AND DISCUSSION Heat ransfer Cefficient In Figures 2 t 4 sme heat transfer cefficient results are presented. Generally, it was fund that the heat transfer cefficient increases with mass flux and mean vapur quality, but decreases with saturatin temperature. It was further fund that relative t the value at the hrizntal inclinatin, there is enhancement in the heat transfer cefficient between the inclinatin angle f -5 and -3. Figure 2 presents the result f experimental data fr sat = 4 C fr different G and x m =. Fr, the flw pattern is within the gravity influenced regime (stratified-wavy) while fr G = 3 kg/m 2 s, the flw is annular-wavy. On clser inspectin it can be seen that the highest heat transfer cefficient ccur between -5 and -3 during which there is a reductin in the liquid film thickness thus reductin in the thermal resistance f the film t heat transfer. 5

4 th Internatinal Cnference n Heat ransfer, Fluid Mechanics and hermdynamics α [W/m 2 K] G values [kg/m 2 K]: 3 2 x m = Figure 2 Inclinatin effect n heat transfer cefficient fr different G fr sat = 4 C, x m =. α [W/m 2 K] Figure 3Inclinatin effect n heat transfer cefficient fr different x m fr and sat = 4 C. α [W/m 2 K] x m = sat values [ C]: 3 4 Figure 4 Inclinatin effect n heat transfer cefficient fr different sat fr and x m =. Figure 3 shws the influence f the vapur quality n the heat transfer cefficient at. Higher vapur qualities result in higher heat transfer cefficients. he figure als indicates that the highest heat transfer cefficients are present during slightly dwnward inclined tubes irrespective f the vapur quality, and that the lwest heat transfer cefficients were btained fr vertical dwnward flw (β = -9 ). Figure 4 gives a cmparisn n the effect f the saturatin temperature at and x m =. It reveals that the ptimal inclinatin angle ccur during dwnward flw fr β = -3 fr sat = 3 C and fr β = -5 fr sat = 4 C. In general lwest heat transfer cefficients where achieved at vertical dwnward flw fr bth f these temperatures. Frictinal Pressure Drp Figures 5 t 7 cntain the frictinal pressure drp, P fric, in the test sectin as btained frm experimental differential pressure measurement by using the vid fractin mdel f Bhagwat and Ghajar [4]. he effects f the static pressure difference which changes accrding t the inclinatin angle are thus remved, alng with the lesser effects f the mmentum pressure differences. In Figure 5 it can be seen that bth the mass flux and inclinatin angle affect the frictinal pressure drp. Fr bth mass flux cases there was a lcal minimum in the frictinal pressure drp at an inclinatin angle f. Hwever, in general it appears as if the upwardly inclined tube cases had lwer frictinal pressure drps. hese pressure drp variatins can be linked t the flw pattern map, but is nt discussed here due t space limitatins. In Figure 6 it can be seen that the vapur quality has a significant effect n the frictinal pressure drp. Higher vapur qualities resulted in higher frictinal pressure drps. In general, lwer pressure drps were btained fr upwardly inclined flw. Fr vapur qualities f and the lwest frictinal pressure drp was achieved during vertical upward flw, while fr a vapur quality f the frictinal pressure drp appears t be present in hrizntal tubes. ΔP fric [kpa] x m = G values [kg/m 2 s]: 3 2 Figure 5 Inclinatin effect n pressure drp fr different G fr sat = 4 C and x m =. 5

5 th Internatinal Cnference n Heat ransfer, Fluid Mechanics and hermdynamics ΔP fric [kpa] Figure 6 Inclinatin effect n pressure drp fr different x m fr G = 2 kg/m 2 s and sat = 4 C. ΔP fric [kpa] x m = sat values [ C]: 3 4 Figure 7 Inclinatin effect n pressure drp fr different sat fr G = 2 kg/m 2 s and x m =. In Figure 7 it can be seen that higher frictinal pressure drps were btained fr a saturatin temperature f 3 C, as cmpared t a saturatin temperature f 4 C. Fr bth cases a slight lcal minimum in the frictinal pressure drp was bserved fr hrizntal tube rientatins. In all the cases cnsidered here, vertical dwnward flw resulted in the highest frictinal pressure drp. his was als the rientatin that has the lwest heat transfer cefficient. ENROPY GENERAION ANALYSIS he cmbined effect f inclinatin n the heat transfer cefficient and frictinal pressure drp is presented in in this sectin. In rder t make a cmparisn f the verall effects, entrpy generatin due t these tw phenmena are cnsidered fr the tw-phase flw passing thrugh the test sectin. It must be nted that an entrpy analysis dne in islatin fr a heat transfer tube, may nt indicate the ptimum system perating pint, as this shuld be determined by cnsidering the s gen [JkgK] Figure 8 Inclinatin effect n ttal entrpy generatin fr different x m fr and sat = 4 C. s gen [JkgK] Figure 9Inclinatin effect n ttal entrpy generatin fr different x m fr G = 3 kg/m 2 s and sat = 4 C. entire system peratin in which such a tube may be emplyed. It is dne here nly fr discussin purpses. he specific entrpy generatin assciated with the frictinal pressure drp, s gen, P, can be expressed as fllws: s gen, P P P Here the tw-phase density, ρ P, f the mixture can be written as a functin f the liquid and vapur phase densities, ρ l and ρ v, respectively and the vid fractin, ε: sat (4) (5) tp l G = 3 kg/m 2 s he specific entrpy generatin assciated with the heat transfer ccurring at the tube wall ver a temperature difference, s gen,q, can be expressed as fllws: v 52

6 th Internatinal Cnference n Heat ransfer, Fluid Mechanics and hermdynamics s gen, Q h fg x in xut w, i sat Here, h fg is the enthalpy f vaprisatin and bth temperatures are expressed in kelvin. he ttal entrpy generatin inside the test sectin, s gen, is expressed by adding s gen, P, and s gen,q. Figures 8 and 9 shw the influence f the inclinatin and vapur quality n the ttal entrpy generatin in the tube fr mass fluxes f 2 kg/ 2 s and 3 kg/m 2 s respectively fr a saturatin temperature f 4 C. It can be seen that fr sme f these test cases (x m = fr bth mass fluxes) there is an pprtunity fr entrpy minimizatin at an inclinatins angle f apprximately -3 t -5. Hwever, in general terms, based n the data it appears as if entrpy generatin is at its lwest at vertical upward flw. Fr the 2 kg/ 2 s cases, higher vapur qualities resulted in higher entrpy generatin rates, while fr the 3 kg/ 2 s cases, a clear dependence n the vapur quality culd nt be btained. Similar type trends were bserved at a saturatin temperature f 3 C. CONCLUSIONS his study cnsidered the heat transfer and frictinal pressure drp in an inclined tube fr cndensatin f R34a. hese aspects are f particular imprtance t heat exchanger design engineers. It was fund that slightly dwnward inclined tubes exhibited the highest heat transfer cefficient, while frictinal pressure drps where generally lwer fr vertical upward flws. In certain cases a lcal minimum in the frictinal pressure drp was btained at hrizntal tube rientatins. An entrpy generatin analysis was cnducted t determine the verall effect f the heat transfer enhancement and the pressure drp penalty n cmpnent level. It was fund that entrpy generatin was minimised fr sme cases at slightly dwnward inclined flw, but fr mst cases the lwest entrpy generatin was btained fr vertical upward flw. FURHER NOE hese results are based n the Bhagwat and Ghajar [4] vid fractin crrelatin. Other vid fractins crrelatins shuld als be cnsidered. ACKNOWLEDGMENS he funding btained frm the NRF, ESP, University f Stellenbsch/ University f Pretria, SANERI/SANEDI, CSIR, EEDSM Hub and NAC is acknwledged and duly appreciated. REFERENCES [] Siddique, M., Khaled, A-R.A., Abdulhafiz, N.I. and Bukhary, A.Y, Recent Advances in Heat ransfer Enhancements: A Review Reprt, Internatinal Jurnal f Chemical Engineering, Vl. 2, 2, Article ID 646, pp (4) [2] Cavallini, A., Del Cl, D., Dretti, L., Lng, G.A., and Rssett, L., Heat transfer and pressure drp during cndensatin f refrigerants inside hrizntal enhanced tubes, Internatinal Jurnal f Refrigeratin, Vl. 23, 2, pp [3] Lips, S., Meyer, P.J., Experimental study f cnvective cndensatin in an inclined smth tube. Part : inclinatin effect n flw pattern and heat transfer cefficient, Internatinal Jurnal f Heat and Mass ransfer, Vl. 55, 22a, pp [4] Lips, S., Meyer, P.J., Stratified flw mdel fr cnvective cndensatin in an inclined tube, Internatinal Jurnal f Heat and Fluid Flw, Vl. 36, 22b, pp [5] Meyer, J.P., Dirker, J., Adelaja, A.O., Cndensatin heat transfer in smth inclined tubes fr R34a at different saturatin emperatures, Internatinal Jurnal f Heat and Mass ransfer, Vl. 7, 24, pp [6] Adelaja, A.O., Dirker, J., Meyer, J.P., Cndensing heat transfer cefficients fr R34a at different saturatin temperatures in inclined tubes, Prceedings f the ASME23 Summer Heat ransfer Cnference (H ), Minneaplis, MN, 23, pp [7] Adelaja, A.O., Dirker, J., Meyer, J.P., Cnvective cndensatin heat transfer f R34a in tubes at different inclinatin angles, Internatinal Jurnal f Green Energy, (In press). [8] Adelaja, A.O., Dirker, J., Meyer, J.P., Experimental studies f cndensatin heat transfer in an inclined micrfin tube, Prceedings f the 5 th Internatinal Heat ransfer Cnference, IHC-5, Kyt, Paper number 936, 5 August, 24. [9] Lips, S., Meyer, P.J., Experimental study f cnvective cndensatin in an inclined smth tube. Part II: inclinatin effect n pressure drps and vid fractins, Internatinal Jurnal Heat and Mass ransfer, Vl. 55, 22c, pp [] Adelaja, A.O., Dirker, J., Meyer, J.P., experimental investigatin n pressure drp and frictin factr in tubes at different inclinatin angles during the cndensatin f R34a, Prceedings f the 5 th Internatinal Heat ransfer Cnference, IHC-5, Kyt, Paper number 9363, 5 August, 24. [] Akhavan-Behabadi, M.A., Kumar, R. and Mhseni, S.G., Cndensatin heat transfer f R 34a inside a micrfin tube with different tube inclinatins, Internatinal Jurnal f Heat and Mass ransfer, Vl. 5, 27, pp [2] Sapali, S.N., Patil, P. A., Heat transfer during cndensatin f HFC 34a and R44a inside f a hrizntal smth and micr-fin tube, Experimental hermal Fluid Science, Vl. 34, 2, pp [3] Lyulin, Y., Marchuk, I., Chikv, S. and Kabv, O., Experimental study f laminar cnvective cndensatin f pure vapur inside an inclined circular tube, Micrgravity Science echnlgy, Vl. 23, 2, pp [4] REFPROP, NIS hermdynamic Prperties f Refrigerants and Refrigerant Mixtures (REFPROP), versin 8., NIS Standard Reference Database 23, Natinal Institute f Standards and echnlgy, Gaithersbury, MD, 25.. [5] Bhagwat, S.M. and Ghajar, A. J., A flw pattern independent drift flw mdel based vid fractin crrelatin fr a wide range f gas-liquid tw-phase flw, Internatinal Jurnal f Multiphase Flw, Vl. 59, 24, pp