International Journal of Scientific & Engineering Research, Volume 5, Issue 7, July-2014 ISSN IJSER

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1 ISSN Det. of Mechanical Engineering, College of Engineering Trivandrum, Thiruvananthauram, Kerala, India Det. of Mechanical Engineering, Raiv Gandhi Institute of Technology, Kottayam, Kerala, India Abstract In the resent work, coer erforated late matrix heat exchanger (MHE surfaces with aer sacers have been evaluated exerimentally for heat transfer and flow friction characteristic by conducting single blow transient test. The transient exeriment has been conducted with erforated lates having different sacer to late thickness ratios (s/l (0.5, 1.0 and.0. The test is conducted with erforated lates having rectangular and triangular erforations. The heat transfer and friction characteristics like Colburn factor ( and Fanning friction factor (f are lotted against Reynolds Number for various erforated lates. MHE is shown as Fig. 1. The gas in between the lates ensure uniform flow distribution (by continuous reheadering and create turbulence which enhances heat transfer. Small erforations (diameter ranging from 1.5 mm to less than 0.4 mm are made into the lates so that a large heat transfer coefficient and high surface area density (u to 6000 /m is achieved. Keywords Perforated late matrix heat exchanger; single blow transient test; Colburn factor; friction factor; area goodness factor I. INTRODUCTION Heat exchangers are among the most vital comonents of any cryogenic refrigeration / liquefaction system and for these alications, heat exchangers should ossess very high effectiveness. The erformance of refrigerators, liquefiers and searation units is strongly deendent on the effectiveness of the heat exchangers used. If the effectiveness of the heat exchanger is below a certain critical value (less than 85% [1], most cryogenic rocesses would cease to function. A decrease in heat exchanger effectiveness from 97% to 95% reduces the liquefaction by 1% []. The low values of attainable Fig. 1: Perforated late matrix heat exchanger coefficient of erformance (COP [3] and the resulting high The ratio of the late thickness (length of the hole in the cost of refrigeration make it economically sensible to use a late to the diameter of the hole is on the order of 0.75, more effective and exensive heat transfer equiment. Aart therefore, the thermal and hydrodynamic boundary layers do from having a high effectiveness, cryogenic heat exchangers not become fully develoed within the erforations, which also need to be very comact, i.e. they must accommodate a results in high heat transfer coefficients and corresondingly large amount of surface area in a small volume. This hels in high friction factors [5]. The flow within the small holes in the controlling heat exchange with the surroundings by reducing erforated late is generally laminar. Due to their small exosed surface area. Besides, a small mass means a smaller hydraulic diameter and the low density of gases, the surfaces cooling load and a faster cooling time for refrigerators. This are usually oerated in the Reynolds number range 500 < Re < requirement is articularly imortant for small refrigerators 1500 [6]. The sacers, being of low thermal conductivity oerating at very low temerature. The requirement to attain material, also hel in reducing axial conduction and high effectiveness and high degree of comactness together in consequent deterioration of erformance. The sacers erform one unit led to the invention of matrix heat exchangers [4]. multile roles such as reducing the longitudinal heat Perforated late matrix heat exchangers (MHE essentially conduction through the walls, reducing the flow consist of a stack of erforated lates made of high thermal maldistribution by reheadering the flow in each sacer, conductivity material alternating with sacers made of low interruting the boundary layer and thus enhancing the heat thermal conductivity material. The stack of alternate high and transfer coefficients. low thermal conductivity materials is bonded to form a monolithic block. A schematic diagram of erforated late 015 htt://

2 ISSN deending on a number of geometric arameters such as late orosity, sacer thickness late thickness, erforation diameter, shae of erforations etc. The overall heat transfer coefficient deends on the convective heat transfer area, which consists of (i unerforated region of any late facing the flow (ii the unerforated region at the back of each late, in the wake region of the flow and (iii the tubular ortion of each late as referred in Fig.. Many researchers [11, 1, 13] have exressed the heat transfer coefficient as a function of orosity of matrix alone. In this aer, the results obtained with different s/l ratios for both rectangular and triangular erforated late geometry at same orosity using transient single blow tests are resented. An imroved maximum sloe method has been used to reduce the test data. II. EXPERIMENTAL DESCRIPTION Fig. : Convective heat transfer area of erforated late Conventional effectiveness NTU aroach methods cannot be used for MHEs because of the discontinuity in the heat Exerimentations were carried out in an oen circuit wind tunnel articularly designed for measuring the heat transfer and flow friction characteristics of erforated late matrix heat transfer surface in the axial (longitudinal direction [7]. New exchanger surface. The schematic diagram of the rating and sizing methods have been develoed [8] that treat the MHE as a distinct set of late - sacer airs. The convective heat transfer in MHE has been considered by a number of authors [9, 10]. The single blow transient test is most aroriate for determining the heat transfer coefficients investigational system is shown in the Fig. 3. The wind tunnel of 50 mm diameter is couled to the suction side of a centrifugal blower. A bell mouth is used at the inlet of the tunnel for distortion free suction. The flow straightners comose the flow uniform. The flow from beginning to end of of erforated late MHEs. Lot of methods have been the tunnel is controlled by means of a butterfly valve. For develoed in literature for calculating the heat transfer uniform heating the matrix, sirally coiled nichrome coil is coefficients from the single blow test data and to take into used as heater, which is controlled by an autotransformer. The account the non-ideal conditions that exist during the testing. rate of flow is measured using Fluke 99 Air flow meter. The Fig. 3: Exerimental setu to conduct single blow transient test The single blow test consists of flow through a wind tunnel with facility to vary flow arameters to observe changes in the flow. In this kind of test usually only one fluid is allowed through the wind tunnel or channel and hence the named as single blow transient test and the characteristics of late surfaces of the heat exchanger is studied. The convective heat transfer and flow friction characteristics are strongly temerature at the inlet and outlet of the test matrix is measured using two T- tye thermocoules of size 3 gauge. The temerature difference between the values measured by thermocoules is within the measurement accuracy (0.1K. A data acquisition system, Agilent 34970A was used to record the temerature history for the eriod of transient testing method. The lates were detained ointly by mechanical ressure, and sealed on the outside by means of adhesive taes to ut a sto to air leakage. The entire stack was insulated with 015 htt://

3 ISSN a mm thick olyurethane insulation. The lates were stacked randomly. The exeriment was conducted for Reynolds number in the range of 50 to Reynolds number is based on the III. EXPERIMENTAL METHODOLOGY erforation diameter. A varying inut was rovided at the inlet section and the exit temerature resonse curve was obtained. The exerimentation course of action consists of varying the temerature of air flow to the test section abrutly and observing the thermal resonse at the outlet of the test section The sloe of this curve was calculated and the maximum sloe and corresonding time at the maximum sloe were noted. In this exeriment, modified maximum sloe method [14] was (erforated late matrix heat exchanger. A sudden used to estimate the heat transfer coefficients. Based on the temerature erturbation is rovided at the inlet section of the fluid flow and the temerature history of the single blow transient test method is obtained using the data acquisition system. This exerimental method normally use single fluid flow throughout the wind tunnel and therefore known as single blow transient test method. The heat transfer coefficient is obtained from the exit temerature resonse. Different methods such as the maximum sloe method, curve matching modified maximum sloe method, the number of transfer unit (ntu values were obtained. The corresonding Colburn factor ( was calculated from ntu using governing Eq. (1-6. The governing equations for the solid and fluid can be obtained in any late by an energy balance across a control volume as follows, where n denotes number of lates and denotes th late. Eq. (1-7: For n=1, method etc, have been oted in literature to find out the heat transfer coefficient from the test data. In the current work, a 1 modified maximum sloe method in which both the maximum ntu ( 1 1 sloe and the time at which maximum sloe is used concurrently to forecast the heat transfer coefficient as well as the longitudinal heat conduction arameter exerimentally. The longitudinal heat conduction arameter is exerimentally ( forecasted in this work. The contact resistance between the late and sacers will differ from exeriment to exeriment. Heat transfer coefficient of the test core is forecasted by matching the sloe and time at maximum sloe of the ntu ( theoretical and exit temerature variations. The exerimentation is done to examine the effect of erforation diameter and orosity on the heat transfer characteristics. The details of late sets are tabulated in table 1 ( 1. The erforated lates, shown in figure 4, used were 50mm diameter and made of coer using hoto chemical milling rocess. Paer sacers was used the aer sacers were made For =n, using die unch sets. The s/l ratios used were 0.5, 1.0 and.0. Thirty number of late - sacer air is stacked together to form n the test matrix. Porosity of the erforated lates are ket ntu ( n n1 n n1 constant as 0.3. TABLE I. Plate set SPECIFICATION OF PLATE SET USED FOR EXPERIMENTS s/l ratio Perforation shae Rectangular 1.0 Rectangular 3.0 Rectangular Triangular Triangular 6.0 Triangular where ( T tin ( T t ( n1 n n n in,, k 0 in A ( t t ( T t ( mc 0 in s c, smc ( MC 1 ex( ntu ( ha ntu ( mc Fig. 4 : Rectangular and triangular erforated late The basic erformance data for a erforated late matrix heat exchanger surface are often shown as curves of the Colburn factor ( = St.Pr /3, and the Fanning friction factor (f, lotted 015 htt://

4 ISSN versus Reynolds number. The exit temerature resonse is used for the exerimental determination of ntu and. Dimensionless temerature and dimensionless time are determined. The sloe is determined from the dimensionless temerature and time lot. The maximum sloe and time at which the maximum sloe occurs are used in this method for the determination of ntu and longitudinal heat conduction factor. The heat transfer characteristics data is resented in the form of Colburn factor (, vs Reynolds number (Re and the flow friction data is resented in the form of fanning friction factor (f, vs Reynolds number (Re. The fanning factor (f was determined from isothermal ressure dro data and both factors ( and f were determined from the following exressions (Eq. 8: the increase in overall volume without any increase in heat transfer area. An increase in sacer thickness leads to an increase in weight of the heat exchanger and a decrease in its comactness. But the overall longitudinal conduction decreases with an increase in sacer thickness. Ac ntu A Pr 3 and f d 4LG Fig. 5: vs. Re lot for rectangular erforation with 0.3 and l/d 0. IV. RESULTS AND DISCUSSION The exeriment was conducted for different s/l ratios for triangular and rectangular shaed erforated lates having same orosity. The, f and /f lots against Re for rectangular erforated lates having orosity 0.3 and l/d ratio 0.5 were lotted in fig. 5, 6 and 7. The, f and /f lots against Re for rectangular erforated lates having orosity 0.3 and l/d ratio 0.5 were lotted in fig. 8, 9 and 10. The values of as well as f are found to be decreasing as Reynolds number increases. As the values of Re increases, the residence time available for the fluid inside the matrix for transferring heat to the lates reduces and hence the heat transfer coefficient decreases. Since the friction factor is inversely roortion to the square Fig. 6: f vs. Re lot for rectangular erforation with 0.3 and l/d 0. of the velocity, the friction factor reduces with increase in Reynolds number. It is found that the area goodness factor, /f is above the value of 0.1 as in fig. 7 and 10. This shows that at higher Re, erformance of the heat exchanger increases. The area goodness factor (/f is inversely roortional to the square of flow area. An increase in /f indicates a decrease in flow area and this leads to a more comactness for the heat exchanger. This asect is highly desirable for a heat exchanger. The effect of s/l ratio on the heat transfer and flow friction characteristics has been studied. Three triangular erforated late and rectangular erforated late with different s/l ratio with same orosity are selected for analysis. The and f decreases as the sacer thickness increase. Jet imingement heat transfer decreases with an increase in the distance Fig. 7: /f vs. Re lot for rectangular erforation with 0.3 and l/d 0. between the sacer to imingement surface ratio. Lytle et al. [15] rovided a correlation for average local heat transfer coefficients as The value of decreases as the sacing between the late increases for MHE. An increase in sacer thickness results in 015 htt://

5 ISSN Fig. 8: vs. Re lot for triangular erforation with 0.3 and l/d 0.5 VI. CONCLUSION The effect of shae of erforations on heat transfer and flow friction characteristics is studied for various erforation diameter and orosity. 1. It is observed that Colburn factor decreases as orosity and erforation diameter increases.. As Reynolds number increases, decreases as the residence time for the heat transfer decreases. 3. As Reynolds number increases, the value of f also decreases. 4. An increase in /f indicates a decrease in flow area and this leads to a more comactness for the heat exchanger. Nomenclature A Heat transfer area, m A c Cross sectional area, m C Secific heat of the matrix, J/kgK c Secific heat of fluid, J/kgK d Perforation diameter, m f Fanning friction factor Colburn factor /f Area goodness factor M Mass of the matrix, kg m Fluid flow rate, kg/s ntu Number of Transfer Units of late G Mass velocity of the fluid in the erforation Fig. 9: f vs. Re lot for triangular erforation with 0.3 and l/d 0.5 L Length of the heat exchangers, m Pr / k Re Reynold Number S Thickness of sacer, m T Temerature of the matrix, K T o t Temerature of the fluid, K t in Fluid temerature maintained at the entry of the bed, K Fluid static ressure dro Fluid density Fig. 10: /f vs. Re lot for triangular erforation with 0.3 and l/d 0.5 Subscrits Plate f fluid in Inlet V. ERROR ANALYSIS Based on the uncertainties in the measurements, the uncertainty in the estimation of was 10% and for friction factor,.4 %. This was calculated by the Kline and McClintock method. In this work, velocity, static ressure, fluid temerature etc. are the measure qualities and the error in this work is due to the uncertainty in the measurements and the uncertainty in the value of hysical roerties. The uncertainty bands that may be laced on the reduced data ( and f are functions of the uncertainties in the measurements (ressure difference, temerature and temerature difference, ambient ressure and time made during the exeriments. References [1] Barron, R. Cryogenic Systems Oxford University Press, , [] Atrey, M.D. Thermodynamic analysis of Collins helium liquefaction cycle. Cryogenics, 38:1199, 06, [3] Moon, J.W., Lee, Y.P., Jin, Y.W., Hong, E.S. and Chang, H.M. Cryogenic Refrigeration Cycle for Re-Liquefaction of LNG Boil-Off Gas, Cryocoolers 14,.International Cryocooler Conference, Inc., Boulder, CO, 007. [4] McMahon, H.O., Bowen, R.J. and Bleyle Jr., G.A. A erforated late heat exchanger, Trans ASME 7, 63-63, [5] Webb, R. L. Enhancement of Single-hase Heat Transfer, Wiley, New York, , htt://

6 ISSN [6] Hubbell, Richard H., Cain and Christina L., New heat transfer and friction factor design data for erforated late heat exchangers, Advances in cryogenic engineering, Volume 33, , [7] Venkatarathnam, G. and Sarangi, S. Matrix heat exchangers and their alication in cryogenic systems, Cryogenics, 30 ( , [8] Kays W.M. and London A.L. Comact heat exchangers, 3rd ed. New York: McGraw-Hill; 1984 [9] Chowdhury K. and Sarangi, S. Effect of finite thermal conductivity of the searating wall on the erformance of counterflow heat exchangers, Cryogenics, 3 (4 1 16, [10] Xing Luo,Wilfried Roetzel and Ulrich Ludersen, The Single-blow transient technique considering longitudinal core conduction and fluid disersion, International ournal of Heat and Mass Transfer , 001. [11] Mikulin, E.I., Shevich, Yu.A., and Potaov, V.N., Efficiency of Perforated Plate Array Heat Exchangers, Khim Neft Mashin No. 5, 13-15, [1] Ornatskii, A.P., Perkov, V.V. and Khudzinskii, V.M., Exerimental Study of Perforated Plate Heat Exchanger for Micro Cryogenic Systems, Promishelnaya Telo Tekhnika 5, 8-33,1983. [13] Hu, J., Gong, L., Zhu, T., and Guo, T., Heat Transfer Characteristics of Perforated Plate, Part-I, Mean Heat Transfer Coefficients, Proceedings of International Cryogenic Engg Conference, Beiing, Aril, [14] Krishnakumar, K. and Venkatarathnam G., On the use of time at maximum sloe in determining the heat transfer coefficients in comlex surfaces using the single blow transient test method, International Journal of Heat Exchangers, vol. 8, no. 1, 31-38, 007. [15] Lytle, D., & Webb, B. W. (1994. Air et imingement heat transfer at low nozzle late sacings. International Journal of Heat and Mass Transfer, 37, htt://