This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.A micro-sized shell and tube heat exchanger (MTHE) was fabricated, and its performance in heat transfer and pressure drop was experimentally studied. The single-phase forced convection heat transfer correlation in the tube side of the MTHE was proposed and compared with previous experimental data in the Reynolds number range of 500-1800. The averaged deviation of the correlation in calculating the Nusselt numbers is about 6.59%. The entrance effect in the thermal developing region was discussed in detail. In the same range of Reynolds number the pressure drop and friction coefficients were found to be considerably higher than those predicted by the conventional correlations. The product of friction factor and Reynolds number is also a constant, but about one fold higher than the conventional. The reasons resulting in these physical phenomena have been preliminary discussed.Tianjin Science and Technology Committee Key
Project fund, under Grant No. 08JCZDC203

3rd Micro and Nano Flows Conference (MNF2011)

Dai, C

Li, B

Wang, Q

English

Brunel University Research Archive

3rd Micro and Nano Flows Conference Thessaloniki, Greece, 22-24 August 2011 - 1 -  An Experimental Study on Heat Transfer and Pressure Drop of MTHE  Chuanshan DAI, Qiuxiang WANG, Biao LI         Tel.: +86-02227401830; Fax: +86-02227401830; Email: csdai@tju.edu.cn School of Mechanical Engineering, Tianjin University, Tianjin 300072, China   Abstract A micro-sized shell and tube heat exchanger (MTHE) was fabricated, and its performance in heat transfer and pressure drop was experimentally studied. The single-phase forced convection heat transfer correlation in the tube side of the MTHE was proposed and compared with previous experimental data in the Reynolds number range of 500-1800. The averaged deviation of the correlation in calculating the Nusselt numbers is about 6.59%. The entrance effect in the thermal developing region was discussed in detail. In the same range of Reynolds number the pressure drop and friction coefficients were found to be considerably higher than those predicted by the conventional correlations. The product of friction factor and Reynolds number is also a constant, but about one fold higher than the conventional. The reasons resulting in these physical phenomena have been preliminary discussed.   Keywords: Micro-tube heat exchanger,  Heat transfer,  Pressure drop,  Entrance effect   1. Introduction  During the last two decades more and more compact systems in MEMS have resulted in a great demand for developing efficient heat transfer equipment to accommodate these high heat fluxes. One of the simplest arrangements that can be used for the heat removal involves using single-phase forced convection or flow boiling in small straight circular tubes or rectangular channels.  A number of theoretical and experimental investigations devoted to this problem were performed in these years. One of the early studies concerning circular microtube configurations was reported by Choi et al (1991). The conclusion was that the Reynolds analogy does not hold for microchannels. Bruno’s (2004) experiments indicated that the derived formula of friction factor and heat transfer coefficient were consistent with those of large diameter tubes. Also experiments of Wahib and Biorn (2004) showed good agreement between the experimentally measured data and classical correlations .Primal et al (2008) found that the Nusselt numbers agreed with those predicted by the Gnielinski correlation within about 5% accuracy. It was found that the measured Nusselt numbers and friction factors were in good agreement with the theoretical values expected from Poiseuille flow in Boris (2010) studied. Krzysztof et al (2008) found that the critical Reynolds number is Re=2000, which is concurrent with macrotubes. However, the results obtained by Mala and Li’s (1999) , Hsieh et al. (2004), Jiang et al. (2006) and Liu et al (2006) showed that the pressure drops in microchannels were higher than the predictions by classical correlations.  In a summary that the heat transfer and fluid flow in microtubes or microchannels is a very complex issue. Many international scholars have devoted to this issue by both theoretical and experimental investigations. However, there is a little common accepted conclusion so far.  The aim of the present study is for further understanding the single phase convection heat transfer and pressure drop performance in MTHE.   2. Experimental setup and procedure  A micro sized shell and tube heat exchanger (MTHE) was made using 61 copper 3rd Micro and Nano Flows Conference Thessaloniki, Greece, 22-24 August 2011 - 2 - tubes, with outside diameter of 1.0 mm and wall thickness of 0.16 mm. The tubes, each with the total length of 154 mm (active heat transfer length of 140 mm), were with equilateral triangles arrangement having a tube pitch of 2.0 mm. The tube bundle was placed inside a copper shell and the ends of the tubes were welded to a tube plate. Connections were added at the ends of the MTHE to serve as inlet and outlet ports for the tube-side fluid. The total length of the MTHE was 178 mm and the outside diameter of the shell was 28 mm only. TMPPTP   HeaterwaterbathT TTest  MTHE  Cooler waterbathSafety valvePumpFilterBypass valveLevelglassM    MassflowmeterTemperaturePressure sensor Fig. 1. Schematic of the experimental setup  The schematic diagram of the experimental test loop is shown in Figure 1. The circulation medium, R142b pumped by a metering pump, firstly entered into the mass flow meter and then passed into the tube side of the MTHE where heated by the hot water circulated through a temperature controlled water bath from the shell side. The heated working fluid, R142b, came from the MTHE were cooled to an appropriate temperature in the cool thermostatic water bath, then pumped by metering pump for constituting a closed cycle. The MTHE was mounted horizontally. Two pressure sensors were mounted on the inlet and outlet of the MTHE, respectively, to measure the pressure drop of the working fluid. The four temperatures at both the inlet and outlet of working fluid and warm water were measured by T-type thermocouples. The mass flux rate of the working fluid in the MTHE was measured by mass flow meter.  After confirming the experimental system reached a hydrodynamic and thermal steady state, temperatures, pressures, and flow rates were recorded every 3s for approximately 5 min. The working fluid rate was then changed incrementally while the warm water flow rate was fixed. At each increment, measurements were recorded at stable conditions. In seven different shell side conditions, more than 100 different working fluid flows have been measured.  3. Results and discussion  It should be noted that both the internal and external heat transfer performances of the MTHE are unknown, Wilson Plot Method (WPM) were used here. In the method, the following assumptions have been made: firstly, the micro copper tubes have the uniform surface roughness and the same circular cross section; secondly, the fluid flow in the tube side has no maldistribution problem, and thirdly, the axial thermal conduction of the tubes is negligible.  3.1 Heat transfer More than 116 measurements of the overall heat transfer coefficients at different flow velocities on tube side have been done in seven different shell side conditions. In each stabilized shell side heating and flowing condition, the overall heat transfer coefficients vary mainly with the flow of the tube side.  The inverse of the overall heat transfer coefficient ( 1K ) was plotted versus the inverse of the tube side fluid flow rate with a power index of n ( 1 nU ). Assuming the exponent n initially to be 0.8 in each measurement, the best linear fit to the obtained data for every stabilized shell side condition can be determined by using the least squares method.  It is found that the fitting linear curve having the exponent of n = 1.08 has the minimum deviation for all experimental data. But the intercepts are different, which are as shown in Fig.2. The intercept A decreases gradually with increasing the external heat transfer coefficient, h0. The experimental Re number ranges from 500 to 1800.   3rd Micro and Nano Flows Conference Thessaloniki, Greece, 22-24 August 2011 - 3 - 0 2 4 6 8 10 12-0.00050.00000.00050.00100.00150.00200.00250.00300.00350.00400.00450.0050y=4.6e-4+2.70e-4x 0.0164 0.0260 0.0370 0.0435 0.0610 0.0680 0.08051/K  (m2 KW-1)1/U1.08  (m1.08s-1.08)m0  (kgs-1)y=0.5e-4+2.55e-4x Fig. 2. The 1/K versus the inverse of the tube side flow rate with an exponent of 1.08 (1/U1.08)  Therefore, the correlation for the single phase forced convection heat transfer was proposed as equation 1. 1.08 0.4 0.140.0043 ( )sNu Re Pr   500 1800Re    (1) The comparison of the experimental Nusselt number, Nue, with the calculated, Nuc, is shown in Figure.3. It shows that the experimental Nu number, Nue, agree well with the predicted values by the correlation. The averaged negative and positive errors are 15% and 15% respectively. The mean absolute error (MAE) defined as below is about 6.59%, where M is the number of experimental condition. 1 100%e ceNu NuMAEM Nu              (2) 0 5 10 15 20 25 30 35 40 45051015202530354045m0  (kgs-1) 0.0164 0.0260 0.0370 0.0435 0.0610 0.0680 0.0805Nusselt number experimental valuesNusselt number calculated values+15%-15% Fig. 3. Comparison of the experimental Nue with the calculated Nuc  3.2 Experimental result comparison  In order to compare the present experimental results with those in the previous literature, some correlations for heat transfer in convectional tubes, microtubes, and microchannels are summarized and listed in Table 1.  Figure 4 shows the present experimental data and the correlations proposed by Sider and Tate (1936), and Dittus and Boelter (1930) for convectional tubes. It is shown that both the correlations could underestimate the Nusselt number in the Re number range of 500-1800. Further more Presenter Time Correlations Applicable scope Scale Sieder-Tate 1936 0.14131.86( )sRePrNu ld     0.48 167000.0044 9.75sPr  ， conventional Dittus-Boelter 1930 0.80.023 nf fNu Re Pr  4 510 1.2 100.7 120 60ffRelPrd    ， conventional Primal 2008   0.144 1.25 0.4 s4.526 10Nu Re Pr        2300 6000Re   microchannel Debray 2001 3/4 1/30.0593Nu Re Pr  3000 20000Re   microchannel Yu et al. 1995 1.2 0.20.007Nu Re Pr  250 20000Re   microchannel Wu and Little 1984 1.09 0.40.00222Nu Re Pr  3000 20000Re   microchannel Choi et al. 1991 1.17 1/30.000972Nu Re Pr  2000Re   microchannel Choi et al 1991  6 1.96 1/33.82 10Nu Re Pr   2500 20000Re   microchannel  Table 1: Correlations from the literature for both conventional and microchannels  3rd Micro and Nano Flows Conference Thessaloniki, Greece, 22-24 August 2011 - 4 - studies are needed in order to determine whether the conventional correlations can be used for the microtube heat exchanger, for example, the outer diameter is 1mm or even less.  400 600 800 1000 1200 1400 1600 1800 2000051015202530 Present Nuc ExperimentalNusselt number NuReynolds number ReDittus-BoelterSider-Tate Fig. 4. Comparison of the experimental correlations proposed by various authors for conventional tubes  400 800 1200 1600 2000 2400 2800020406080100120 Present Nuc ExperimentalNusselt number NuReynolds number ReNu1Nu2Nu3Nu4Yu.(1995)Nu1---Primal (2008)Nu2---Choi,1  (1991)Nu3---Choi,2 (1991)Nu4---Wu and Little (1984)Debray (2001) Fig. 5. Comparison of the experimental correlations proposed by various authors for conventional tubes  The comparison of the present experimental Nusselt number with those from the others correlations can be shown in Figure.5.It is that the experimentally measured Nusselt numbers are in good agreement with the result of Debray et al (2001) .The correlation proposed by Wu and Little (1984) 、 Choi et al (1991) and Primal’s (2008) will give a lower Nu number. The correlation suggested by Yu et al. (1995) give the highest Nusselt number among all of them .  The points are much scattered beyond our expectation as shown above. This could be probably caused by the relative surface roughness. In a fact, there has been no a convincing explanation so far about impact of the relative surface roughness on micro scale heat transfer.  3.3 The entrance effect  When fluid comes from a large space into a single pipe, the local surface heat transfer coefficient at the entrance region should be higher than that in the fully development section.  0.025 0.050 0.075 0.100 0.125 0.150110100 Experimental curve fitNusselt number NuL/diRePrShan & Bhatti(1987) Shan & London (1978)Hausen (1959) Fig. 6. The averaged Nu as a function of the inverse of Gz（axis title is Gz-1）  The experimental averaged Nusselt numbers, Nu, versus the inverse of the Graetz number, Gz-1 from the present experiments are also plotted in Figure 5. The results were also compared with those approximately averaged Nusselt numbers by integration of the Hausen formula, the Shah and London formula, and the Shah and Bhatti formula, respectively. It is found to be that the average experimental Nusselt numbers are much higher than the others. With increasing the inverse of Gz, the difference between the experimental averaged Nu and the predicted by Hausen’s formula decreases. The experimental averaged Nu gradually decreases to a stable value, which is more close to the curve of Hausen rather than those of the others. The experimental demarcation point separating from the entrance and developed regions also seems larger than that of the convectional tube of 0.05, which is about 0.1 or even larger according to our experimental data. This means that the entrance effect can have greater influence on micro scale than the conventional scale.  3rd Micro and Nano Flows Conference Thessaloniki, Greece, 22-24 August 2011 - 5 - 3.4 Pressure drop  The variation of the experimental pressure drop with the mass flux of R142b is shown in Figure 6. The pressure drop increases monotonically for the micro-tubes with the increase of the mass flux. Compared with the results for the conventional channels or tubes, the pressure drop through the micro tubes is relatively large. This is probably because that the inner surface roughness might have a larger effect on the microtube than the conventional tube.  0 100 200 300 400 500 600 700 800 900010002000300040005000 0.0164 0.0260 0.0370 0.0435 0.0610 0.0680 0.0805Pressure drop (Pa)Mass flux G  (kgm-2s-1)mo  (kgs-1) Fig. 7.  Experimental pressure drop at different mass flowrate  The real MTHE may be sensitive to the external force considering the tiny geometric dimension of the heat exchange tube. The vibration caused by the cross flow is much lager than by the vertical mobility. Therefore the main reason for scattered experimental data may be that the vibration caused by the cross flow in the shell side of the MTHE. 400 800 1200 1600 20000.00.20.40.6Darcy number fReynolds number Remo  (kgs-1) 0.0164 0.0260 0.0370 0.0435 0.0610 0.0680 0.0805Hagen-Poiseuille Fig. 8. Darcy friction factor f as a function of the Reynolds number  The correlation of Darcy friction factor f as a function of the Reynolds number is shown in Figure 7. With increasing the Reynolds number, the Darcy friction factor f correspondingly decreases, but at a higher level than the predicted by Hangen-Poiseuille correlation in all experiments. The correlation of Darcy friction factor f and the Reynolds number can be expressed as below 1.094368.6f Re      500 1800Re    (4)  3.5 Uncertainty analysis The detailed experimental uncertainties were listed in table 2. Parameter Uncertainties (%) Temperature  0.67 Pressure  0.31 Mass flow rate (working fluid) 0.17 Mass flow rate (heat source) 0.1 Overall heat transfer coefficient 8.19 Convective heat transfer coefficient(tube - side) 8.40 Friction  factor 1.32 Table 2: The variable uncertainties of parameters  The surface roughness on the inner side of the microtube and the wall shape specific distribution are not still clear. This experimental investigation can only give a preliminary understanding on the fluid flow and heat transfer characteristics in the microtube.   4. Conclusions  According to our experimental results, the following conclusions of this research may be summarized: 1. A single-phase forced convection heat transfer correlation in the tube side of the MTHE was proposed in a Reynolds number range from 500 to1800. and with a standard deviation of 6.59%. The experimental results were compared with correlations from the literature. 2. It is shown that the averaged Nusselt number is about 1.89-5.82 times larger than that of the conventional scale heat exchanger, and in the fully developed region, the averaged 3rd Micro and Nano Flows Conference Thessaloniki, Greece, 22-24 August 2011 - 6 - Nusselt number is about 1.44 times larger. According to our results, the thermal entry length in a microtube is longer than that of the conventional tubes for laminar flow.  3. In the Reynolds number range of Re = 500-1800, the pressure drop and friction coefficients were found also considerably higher than those predicted by the conventional correlations.  The reasons resulting in these physical phenomena have been preliminary analyzed. The experimental uncertainties and their evaluation were given.  Acknowledgements  This work is supported by Tianjin Science and Technology Committee Key Project fund, under Grant No. 08JCZDC20300.  References  Boris S., Simon Y., Ching M., Nobuhide K, 2010, Flow visualization and local measurement of forced convection heat transfer in a microtube, J. Heat Transfer, 132,  031702-1-9 Bruno A., Barbara W., 2004, Liquid flow friction factor and heat transfer coefficient in small channels: an experimental investigation, Exp. Therm. Fluid Sci. 28,  97-103. Choi S.B., Barron R.F., Warrington R.O., 1991, Liquid flow and heat transfer in microtubes, in Micromechanical Sensors, Actuators and Systems, ASME DSC ,32, 123-128.  Debray F., Franc J.P., Maitre T., 2001, Measured coefficient de transfert thermique par convection forcée en mini-canaux, Mechanics & Industries, (2), 443-454. Dittus F.W., Boelter L.M.K., 1930, Heat transfer in automobile radiators of tubular type, Univ. California. Berkley. Publ. Eng.2 (13) 443-461. J. Jiang, Y.L. He, M.S. Shi, 2006, Experimental study on flow and heat transfer characteristics in rectangular microchannel, J. Therm. Sci. Tech. 5(3), 189-194. Krzysztof D., 2008, Experimental investigation of Poiseuille number laminar flow of water and air in minichannel , Int. J. Heat Mass Transfer, 51, 5893-5990. Liu Z.G., Zhao Y.H., 2006, Experimental investigation on flow and heat transfer in rough microtube, J. Beijing Polytechnic University, 32(2),  101-108. Mala G.M., Li D., 1999, Flow characteristics of water in microtube, Int. J. Heat Fluid Flow, (20), 142-148. Primal F., 2008, A minichannel aluminum tube heat exchanger-part1 evaluation of single-phase heat transfer coefficients by the Wilson plot method, Int. J. Refrigeration,  669-680. Sieder E., Tate G., 1936, The study on the laminar forced convection heat transfer of oil fluid in the tubes, Ind. Eng. Chem. 28 , 1429-1438. S. S. Hsieh, C. Y. Lin, C .F. Huang, 2004, Liquid flow in a microchannel, J. Micromech. Microeng. 14 (4), 436-445. Wahib O., Biorn P., 2004, Experimental investigation of single phase convective heat transfer in circular microchannels, Exp. Therm. Fluid Sci. 28, 105-110. Wu P., Little W. A., 1984, Measurement of heat transfer characteristics of gas flow in fine channel heat exchangers used for microminiature refrigerators, Cryogenics, 24(8), 415-420. Yu, D. Warrington R.B.,1995, Experimental and theoretical investigation of fluid flown and heat transfer in micrortube, in: Proceedings of the 1995 AWME/JSME Thermal Engineering Joint Conference, Maui, Howaii, 1, 523-530    

A minichannel aluminum tube heat exchanger-part1 evaluation of single-phase heat transfer coefficients by the Wilson plot method,

Experimental and theoretical investigation of fluid flown and heat transfer in micrortube, in:

Experimental investigation of Poiseuille number laminar flow of water and air in minichannel ,

Experimental investigation of single phase convective heat transfer in circular microchannels,

Experimental investigation on flow and heat transfer in rough microtube,

Experimental study on flow and heat transfer characteristics in rectangular microchannel,

Flow characteristics of water in microtube,

Flow visualization and local measurement of forced convection heat transfer in a microtube,

Heat transfer in automobile radiators of tubular type,

Liquid flow and heat transfer in microtubes,

Liquid flow in a microchannel,

Measured coefficient de transfert thermique par convection forcée en minicanaux,

Measurement of heat transfer characteristics of gas flow in fine channel heat exchangers used for microminiature refrigerators,

The study on the laminar forced convection heat transfer of oil fluid in the tubes,

An experimental study on heat transfer and pressure drop of MTHE

An experimental study on heat transfer and pressure drop of MTHE

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