Experimental study with analytical validation of thermo-hydraulic performance of plate heat exchanger with different chevron angles

Transcription

1 Exerimental study with analytial validation of thermo-hydrauli erformane of late heat exhanger with different hevron angles Arunahala U C, Ajay R K and Sandee H M Abstrat The Plate heat exhanger is rominent among all alternatives to the onventional shell and tube heat exhanger beause of simliity in oeration, maintenane, and high surfae area density. Currently lot of researh is going on to otimize the design by onsidering thermo-hydrauli erformane. In the resent study, an extensive analytial model is develoed using MATLAB to study the thermo-hydrauli harateristis of Plate heat exhanger with high beta (60 /60 ), low beta (30 /30 ) and thermally mixed (30 /60 ) late onfigurations. Further, the exerimental investigation is arried out to validate the analytial model and it reveals that the maximum deviation of ±10% onsidering both hot and old fluid outlet temerature with reset to different flow rates in ase of all the three late onfigurations. To inororate the effet of rise in ressure dro with different hevron late onfigurations, the term thermohydrauli effetiveness is introdued. In all ases, it is found to be highest for high beta lates and least for low beta lates. Whereas for thermally mixed lates, the values lies between high and low beta lates being more loser towards high beta lates. This indiates that the thermal erformane of thermally mixed lates is nearly math with the high beta lates but the ressure dro being lesser than the latter. A generalized orrelation to estimate the Nusselt number has been develoed with a relative standard error of ±18%. The Nusselt number redited from the develoed orrelation is found to be in good agreement with exerimental results omared to the values redited from different orrelation in the literature. Keywords Chevron angle, orrelation, Thermal mixing, thermohydrauli effetiveness. P I. INTRODUCTION LATE heat exhangers (PHE) are built of thin lates whih are either smooth or have some form of orrugations on the surfae. In ase of gasketed PHE, number of thin retangular metal lates sealed around the edges by gaskets Arunahala U C is with the Manial Institute of Tehnology, Manial , South India (hone: ; fax: ; arun.handavar@manial.edu). Ajay R K was a Post graduate student with the Manial Institute of Tehnology, Manial , South India ( ajaykalbavi@gmail.om). Sandee H M was a Post graduate student with the Manial Institute of Tehnology, Manial , South India ( sandeemnitte@gmail.om). and held together in a frame. The major advantages of PHE is its simliity in leaning and hange of heat duty, less fouling due to high turbulene et. But the limitation of gasketed PHE is narrow working range of ressure and temerature. These are most ommonly used in food roessing industries, syntheti rubber industries, aer mills, oolers and losed iruit ooling system for large etrohemial and ower lants. Desite of its better erformane, ontinuous researh is ongoing to otimize the thermal and hydrodynami harateristis by means of late geometry modifiation, flow distribution, use of nanofluids and thermal mixing. The heat transfer and ressure dro harateristis of three different late strutures viz. smooth arallel lates, inlined disrete ribs and inlined ontinuous ribs were omared exerimentally in air to air PHE under ounter flow onditions by Li et al. [1]. For the flow range of 800 < Re < 1500, the inlined disrete rib lates showed an enhanement in heat transfer of 110% to 310% and 5% to 15% omared to smooth arallel lates and inlined ontinuous rib lates resetively. The enhanement in heat transfer in inlined disrete rib lates is attributed to the high degree of turbulene and generation of longitudinal vorties due to zig-zag flow arrangement of fluid in the hannel. Whereas the ressure dro for inlined disrete rib lates is less than the inlined ontinuous rib lates due to redued blokage effets. Nilueng and Wongwises [2] have erformed a omarison between heat transfer and ressure dro harateristis of PHE (β = 25 ) having different surfae roughness (0.936μm, 1.189μm, 1.378μm and 3.312μm) at old fluid side for 1300 < Re < 3200 with water as the working fluid. It was observed that the heat transfer oeffiient is roortional to the surfae roughness with a rise of 4.46%, 8.18% and 17.95% for three higher roughness lates omared to late roughness of 0.936μm. This is due to the fat that the surfae roughness inreases turbulene and also auses reirulation at the trough. The orresonding rise in ressure dro was noted as 3.9%, 8.25% and 19.24%. Searate Nu orrelations for hot water flowing over smooth surfae with a deviation of ±1.61% and old fluid flowing over rough surfae (±1.21%), and a ommon orrelation for f (±0.95%) were develoed. 47

2 Visual basi based rogram was develoed to evaluate thermal and hydrodynami arameters for a given orrugated PHE by Gulben et al. [3]. In this rogram, the orrelations (Tovazhyanski et al. [4], Chisholm and Wannairahhi [5], Talik and Swanson [6], Kumar [7] and, Muley and Manglik [8]) were used to alulate effetive area, effetive number of lates and total ressure dro (both sides). Later the ode was validated using a samle PHE roblem from Kaka and Hongtan [9]. Martin [10] develoed a searate orrelation for f and Nu for 300 Re by referring the exerimental results of Foke et al. [11]. The orrelations were develoed by onsidering Re, ressure dro (fre 2 ) and β. The results for the range of 0 β 80 were omarable with the exerimental values of Foke et al. [11]. The orrelations were also omared with the Leveque equation [12] and a deviation of ± 20% (for f) and ±10% (for Nu) was notied for the same flow onditions and range of β. Han et al. [13] have erformed an exerimental and numerial analysis of a orrugated PHE (β = 30 ) in ounter-urrent mode in the flow range of 50 lh to 90 lh using water as working fluid. In the CFD analysis, the three dimensional temerature, ressure and veloity fields were obtained for the model built using GAMBIT. Further, exeriments were onduted and a deviation of 6% for water exit temerature and 35% in ressure dro was observed over the flow range as omared to the numerial results whih is due to the resene of dead zone where the fluid temerature will be same as inlet temerature and also uneven distribution of flow ours. Sine uniform flow distribution was onsidered in CFD, onsiderable deviation in ressure dro is observed. Thus the erformane of PHE is signifiantly influened by this henomenon. The heat transfer and ressure dro harateristis of orrugated hannels with V-shaed two lates (β = 20, 40 and 60 ) laed in staggered arrangement were studied by Nahon [14] using air as working fluid for the range 2000 < Re < The exerimental results of V-shaed lates were omared with arallel late, whih showed the highest temerature for arallel late being 36% greater than lates with β = 60. A numerial and exerimental study of heat transfer and fluid flow in a ounter flow orrugated PHE (14 lates with β = 60 ) for 400 < Re < 1300 was studied by Jain et al. [15] with water as working fluid. The CFD analysis was arried under the similar onditions. A orrelation was develoed for estimating Nu and f and the error observed was 13% and 14.5% resetively. The deviation of 18% in Nu and 33% in f was notied in the CFD analysis. The higher deviation in numerial value of f and Nu was due to the exlusion of ort and flow distribution areas in the analysis. Exerimental erformane of mixed hevron lates (β = 30 /60 ) PHE was studied by Muley et al. [16] with vegetable oil (130 < Pr < 230) as hot fluid and water (2.4 < Pr < 4.50) as old fluid in the range of 2 < Re < The test was arried out for 12, 20 and 24 late aks with U-tye ounter flow arrangement of fluid. It was observed that the mixed late onfiguration enhaned Nu by 1.5 to 5.2 times and f by 1.7 to 37 times omared to arallel late PHE. The mixed late had 32% more heat transfer area as omared to arallel late PHE. It was also observed that higher heat transfer and ressure dro was obtained due to the late surfae generated swirl flow and the eriodi disrution and starting of boundary layers. From the results obtained, orrelation for Nu and f were develoed whih shows a deviation of ± 14% and ± 5% for 2 < Re < 1000 and ± 4% and ± 7.5% for Re Exerimental study of ommerial PHE for symmetri onfiguration (30 0 /30 0 and 60 0 /60 0 ) and mixed onfiguration (30 0 /60 0 ) of three lates ( = 1.17) were arried out by Khan et al. [17] for the range 500 < Re < 2500 and 3.5 < Pr < 6.5 with U-tye flow arrangement. From the results, the Nu was found to inrease with β being nearly four to nine times greater than arallel lates. For the artiular ase of Re = 2000, the Nu was found to be highest for β = 60 /60 being 36%, 57% and 87% greater than 30 /60, 30 /30 and arallel lates resetively. A orrelation for Nu was develoed for three different ases and orresonding error obtained were of ±2%, ±1.8% and ±4% resetively. A similar study was onduted by Khan et al. [18] for mixed late onfiguration (30 /60 ) with water as the working fluid for U-tye ounterflow arrangement. Heat transfer oeffiient was alulated using modified Wilson s lot (a general orrelation for onvetive oeffiient where mass flow rate is varied as a funtion of Re and Pr instead of veloity) and a orrelation for Nu was develoed based on the same with a deviation of ±10%. In-detail literature review revealed that the erformane analysis is based on late onfiguration, flow diretion and flow intensity. Further, as an outome of exerimental study, lenty of orrelations are made available whih is based on artiular hevron angle. Hene the resent work is foused on develoment of an analytial model to asertain the thermohydrauli erformane of PHE as well as to develo a single orrelation to take into aount various hevron angle lates. II. ANALYTICAL ANALYSIS The detailed analytial analysis to determine the energy arameters of the late heat exhanger for different inut onditions is resented using a MATLAB rogram. The shemati of the orrugated late of PHE is shown in Figure 1. The mean hannel saing is, b L / N t (1) t The effetive area available for heat transfer of a single late, A L L (2) ht w The total effetive area available for heat transfer, A A N 2 (3) e ht t The hydrauli diameter of the lates, 4 A D (4) h P w 48

3 Fig. 1 Shemati of hevron late Where, N is the total number of lates, t is thikness t of the late (m), L PHE ak length (m), is enlargement fator, A is flow area (m 2 ) and P is the wetted erimeter (m). w The Nusselt number and frition fator is determined using different orrelations (Kaka et al. [9]). The overall heat transfer oeffiient, 1 U (5) 1 1 t h h k h w The thermal effetiveness, 1 ex NTU 1 C 1 C ex NTU 1 C Where, h and h are the onvetive oeffiient of hot h and old fluid (W/m 2 K), k is the thermal ondutivity of w late material (W/mK) and C is the ratio of heat aaity. The total ressure dro is the sum of hannel side and ort side ressure dro. 2 4 f L G v 2 The ort side ressure dro is: 1.4 N G 2 2 By onsidering total ressure dro, the thermo-hydrauli effetiveness is alulated as: QW (9) th Q max Where, G and G are the hannel side and ort side mass veloity (kg/m 2 s), Q and Q max (6) (7) (8) are the atual and maximum heat transfer (W) and W is the uming ower (W). Based on the above mentioned roedure, a ode was develoed. The inlet temeratures, mass flow rates and the hevron angle of the late onfiguration are rovided as inuts to the model. Sine, it is an iterative roess, aroriate initial assumtions are made for the hot and old fluid outlet temeratures. The heat transfer oeffiient and erformane arameters are obtained as outlined in the flow hart (Figure 2). III. EXPERIMENTAL ANALYSIS The details of exerimental setu, roedure and methodology for the develoment of orrelation and the exerimental unertainty analysis are disussed here. A. Exerimental setu The exerimental setu onsists of a late heat exhanger with 20 orrugated (30-30 /30-60 /60-60 ) stainless steel lates. Water is used as the working fluid and the flow attern is a single ass U tye ounter flow. The shemati diagram of the exerimental setu is as shown in Figure 3. The different sensors used are re-alibrated. The inlet and outlet temeratures of the hot and old fluid are measured using four K-tye thermooules ositioned at the inlet and outlet orts of hot and old fluid. The hot water obtained by heater is Fig. 2 Flowhart of analytial model 49

4 (a) high beta (b) low beta () thermal mixing Fig. 4 Different Plate onfigurations used for exeriments Fig. 3 Shemati of late heat exhanger test setu reirulated in the loo by um and the old water flow is one through basis whih is sulied from overhead tank. The ressure dro aross the hot and old fluid is measured using a differential ressure transduer. The flow rates of both the fluids are measured using two searate rotameters. The temerature and ressure dro readings are monitored ontinuously using a real time data logging system. B. Exerimental roedure The exerimental roedure for determining thermal and hydrodynami arameters is exlained below: 1) The old water suly is started and one the hot water in the tank reahes required temerature, it is irulated using um. Both the flow rates are adjusted to required value. 2) The data logger is started as soon as the hot and old water flow is initialized. The temerature and ressure dro variations are observed in the transient grah. 3) One the steady state is reahed, both ressure and temerature reading are atured. 4) Above set of roedure is reeated 4 times to hek the reeatability in ase of different hot and fluid flow ombinations, varied hot water inlet temerature and hevron angle. The different late onfigurations used in the exerimental analysis are shown in Figure 4. The error involved in measurement of different arameters are given in Table 1. C. Correlation develoment The terms involved in the Nu orrelation are Reynolds number, Prandtl number and sine of hevron angle of the late ombination. The general form of the orrelation is: q m n Nu C Re Pr sin (10) A total of 169 data oints are used and the analysis is done in MS EXCEL. The final form of orrelation is: 0.21 Nu 0.05 Re 0.52 Pr 1.99 sin (11) Instrument T ABLE I PROBABLE ERRORS IN INSTRUMENTS thermooule flow meter differential ressure transduer Error ± C ±0.01 lm ±0.001 Pa IV. RESULTS AND DISCUSSION The validation of analytial model by onsidering different late onfigurations (30-30 /30-60 /60-60 ) and the goodness of fit of the develoed orrelation have been disussed in this setion. A. Validation of analytial model The exerimental outlet temeratures of the hot and old water are omared with the outut of the analytial model as a art of validation. For eah old water flow rate (5, 10, 15 and 20 lm), the hot water flow rate is varied from 5 to 35 lm in stes of 5 lm. The late onfiguration used in this study are high beta (β = 60 0 /60 0 ), low beta (β = 30 0 /30 0 ) and mixed late onfiguration (β = 30 0 /60 0 ) resetively. The exeriment is arried out under variable onditions of hot and old water inlet temeratures. Figure 5 shows the variation of exerimental and analytial outlet temerature for the high beta late onfiguration with reset to the variable hot and old water flow rate. The maximum deviation notied is 10% in ase of old water flow rate of 10 lm/hot water flow rate of 5 lm. The minimum deviation is 0.1% for old water flow rate of 5 lm/hot water flow rate of 35 lm. In the resent exeriment, the range of Reynolds number is 198<Re<2194. However, the orrelations available are based on a wide range of Re. Hene deviation in analytial values is observed at multile oints. For the low beta late onfiguration, the maximum and minimum deviation notied are 5% and 0.8% resetively as shown in Figure 6. The random variation in the analytial and exerimental values is due to the variable inut temeratures. Thermal erformane omarable with high beta lates but lower ressure dro an be ahieved by thermal mixing. This is learly evident from Figure 7. 50

5 Similarly, the analytial and exerimental values of old water outlet temeratures for different onfigurations are found to be in well agreement with ±10% deviation as deited in Figure 8, Figure 9 and Figure 10. Fig. 5 Hot water outlet temerature for high beta onfiguration Fig. 8 Cold water outlet temerature for high beta onfiguration Fig. 6 Hot water outlet temerature for low beta onfiguration Fig. 9 Cold water outlet temerature for low beta onfiguration Fig. 7 Hot water outlet temerature for mixed late onfiguration Fig. 10 Cold water outlet temerature for mixed late onfiguration 51

6 Figure 11 shows the variation in analytial thermo-hydrauli effetiveness for the old water flow rate of 5 lm orresonding to the hot water flow rate from 5 lm to 35 lm. As disussed earlier, the thermo-hydrauli harateristis of thermal mixed lates lies near that of high beta lates. It is also observed that the ondition of equal hot and old heat aaity rate, thermo-hydrauli effetiveness is minimum beause of LMTD. The trend of thermo-hydrauli effetiveness is also studied for the old water flow rate of 10 lm, 15 lm and 20 lm onsidering the same onditions of hot water flow rate. As shown in the figure the highest value of thermo-hydrauli effetiveness is observed in ase of high beta, followed by thermally mixed and lowest in as e of low beta late onfiguration. Subsequently the di in the value of effetiveness is observed in all ases for the ondition of equal heat aaity rates. With reset to exerimental results, maximum deviations of analytial and resent work orrelations should be omared. Figure 13, Figure 14 and Figure 15 shows the variation of results of exerimental, analytial and develoed orrelation for different flow rates and different late onfiguration. The trend of the develoed orrelation follows the trend of the exerimental results while the trend of the analytial model shows deviation at low flow rates. Hene it an be onluded that the develoed orrelation has good agreement with the exerimental data. B. Validation of develoed orrelation In the revious disussion, three sets of orrelations based on hevron angle and for eah onfiguration, based on Re value, onstants of orrelations are hanged. Instead of using multile orrelations and onstants, a single orrelation whih inludes hevron angle as an additional variable for determining Nusselt number is develoed. The validation of exerimentally determined Nusselt number vs Nusselt number of resent orrelation is deited in arity lot (Figure 12). It is observed that the maximum number of oints lies within the standard relative error (±18%). For the develoed orrelation, the oeffiient of orrelation was found to be Fig. 12 Parity Plot Fig. 11 Thermo-hydrauli effetiveness at onstant old water flow rates 52

7 Fig. 13 Outlet temerature omarison for high beta onfiguration Fig. 14 Outlet temerature omarison for low beta onfiguration Fig. 15 Outlet temerature omarison for mixed late onfiguration V. CONCLUSION Based on the intensive study, the following onlusions have been derived: 1. Analytial model is found to be in well agreement with the exerimental results with a maximum and minimum deviation of ±10% and ±1% resetively in temerature. 2. The thermo-hydrauli effetiveness initially dereases with inrease in flow rate and attains least value when the hot and old water flow rates are the same. This is beause at this oint the value of LMTD is least. Further it inreases with flow rate. 3. A generalized orrelation for Nusselt number has been develoed with a standard relative error of ±18%. REFERENCES [1] Li, X. W., Meng, J. A., and Li, Z. X., An exerimental study of the flow and heat transfer between enhaned heat transfer lates for PHEs. Exerimental Thermal and Fluid Siene, 34(8), [2] Nilueng, K., and Wongwises, S., Exerimental study of single hase heat transfer and ressure dro inside a late heat exhanger with a rough surfae. Exerimental Thermal and Fluid Siene, 68, [3] Gulben, G., Aradag, S., Sezer-Uzol, N., and Atamturk, U., 2010, January. Develoment of a Comuter Program for the Numerial Investigation of Heat Transfer in a Gasketed Plate Heat Exhanger. In ASME th Biennial Conferene on Engineering Systems Design and Analysis ( ). Amerian Soiety of Mehanial Engineers. 53

8 [4] Tovazhnyanski, L. L., Kaustenko, P.A., and Tsibulnik, V.A., Heat Transfer and Hydrauli Resistane in Channels of Plate Heat Exhangers, Energetika, 9, [5] Chisholm, D., and Wanniarahhi. A.S., Maldistribution in Single-ass Mixed-hannel Plate Heat Exhangers, ASME HTD Comat Heat Exhangers for Power and Proess Industries, 201, [6] Talik, A.C., and Swanson, L.W., Heat transfer and ressure dro harateristis of a late heat exhanger using a roylene-glyol/water mixture as the working fluid, Pro. 30 th Natl. Heat Transfer Conferene,12,. 83 [7] Kumar, H., The Plate Heat Exhanger: Constrution and Design, 1 st U.K Natl. Conferene on Heat Transfer. Inst. Chem. Sym. Series, No. 86, [8] Muley, A., and Manglik, R.M., Exerimental Investigation of Heat Transfer Enhanement in a PHE with β=60 Chevron Plates, Heat and Mass Transfer, Tata MGraw- Hill, New Delhi, India. [9] Kakaç, S., and Hongtan, L., Heat Exhangers Seletion. Rating and Thermal Design, CRC, 2, Florida, [10] Martin, H., A theoretial aroah to redit the erformane of hevron-tye late heat exhangers. Chemial Engineering and Proessing: Proess Intensifiation, 35(4), [11] Foke, W. W., Zahariades, J., and Olivier, I., The effet of the orrugation inlination angle on the thermohydrauli erformane of late heat exhangers. International Journal of Heat and Mass Transfer, 28(8), [12] A. Leveque, Les lois de la transmission de ehaleur ar onvetion, Ann. Mines, Set. 12, [13] Han, X. H., Cui, L. Q., Chen, S. J., Chen, G. M., and Wang, Q., A numerial and exerimental study of hevron, orrugated-late heat exhangers. International Communiations in Heat and Mass Transfer, 37(8), [14] Nahon, P., Heat transfer harateristis and ressure dro in hannel with V orrugated uer and lower lates. Energy onversion and management, 48(5), [15] Jain, S., Joshi, A., and Bansal, P. K., A new aroah to numerial simulation of small sized late heat exhangers with hevron lates. Journal of Heat Transfer, 129(3), [16] Muley, A., Manglik, R. M., and Metwally, H. M., Enhaned heat transfer harateristis of visous liquid flows in a hevron late heat exhanger. Journal of Heat Transfer, 121(4), [17] Khan, T. S., Khan, M. S., Chyu, M. C., and Ayub, Z. H., Exerimental investigation of single hase onvetive heat transfer oeffiient in a orrugated late heat exhanger for multile late onfigurations. Alied Thermal Engineering, 30(8), [18] Khan, T. S., Khan, M. S., and Ayub, Z. H., Exerimental Investigation of Single-Phase Heat Transfer in a Plate Heat Exhanger. Journal of Thermal Siene and Engineering Aliations, 7(4), /4. 54