| With the rapid development of modern technologies, heat transfer phenomena are emerging in such industries as energy, aerospace, electronics, nuclear power et al. Enhancing heat transfer can not only improve the efficiency of energy systems, but also reduce the size and cost of the energy transportation system.MEPCM, i.e. microencapsulated phase change material, is formed by packing phase change material (PCM) into a microcapsule with a solid but flexible polymer shell; the size of the MEPCM particles ranges from 1 to 1000μm. MEPCM particles can be dispersed into fluids (water, glycol etc.) to form MEPCM suspensions. MEPCM particles are believed to be able to enhance fluid convective heat transfer considerably, which is due to the latent heat absorption by the PCM in the suspended MEPCM particles during the phase change process, the tremendously increased surface area per unit volume due to the microminiaturization of PCM and the interaction of MEPCM particles in the fluid.In recent years, one can observe a growing demand to dissipate extremely high heat fluxes in microstructures, e.g. in high-power electronics cooling and micro system thermal control. These facts inspired us to conduct the study on MEPCM suspension flowing heat transfer in rectangular mini/microchanels (Dh < 3 mm), with the aim to develop methods for a more efficient cooling technology for high-power electronics. Such a research work distinguishes itself from the conventional research work on MEPCM suspension flowing heat transfer in circular macro-size ducts.In the study, a comparative experimental study has been performed, in parallel with some numerical work, to investigate the laminar flow heat transfer characteristics of the MEPCM suspensions with various mass concentrations in rectangular minichannels. In the experiments, water and water-based MEPCM suspensions with MEPCM mass concentrations ranging from 5%-20% flow in three parallel rectangular minichannels (height H ch=4.2 mm, width W ch=2 mm, and hydraulic diameter Dh=2.71 mm). The investigated MEPCM particles have an average size of 4.97μm and an approximate density of 867.2 kg/m3. The core material is n-Octadecane, whose melting temperature is about 28 oC, and the shell material is Polymethylmethacrylat (PMMA).The main contents and conclusions of the study are listed as follows: (1) The minichannel heat transfer experimental setup has been newly built; the minichannel test section has been carefully designed and manufactured, which can achieve uniform flow distribution among 3 minichannels.(2) DSC thermal analysis has been performed on the provided MEPCM particles and its suspensions. The approximate phase-change temperature range is from 24°C to 29°C, and the corresponding melting enthalpy of MEPCM particles is ca. 147.1 kJ/kg. Rheological properties and bulk viscosities of MEPCM suspensions have been measured by using a rotating cylindrical rheometer, and the measurements showed that the viscositie of the suspensions increase rapidly with the increase in the concentration, and MEPCM suspensions showed some behaviours of non-Newtonian fluid.(3) Darcy friction factors and pressure drops of MEPCM suspensions larminar flow in the rectangular minichannels have been measured. The measurements showed that, the friction factors of the 5% concentration suspension agree well with the theoretical values of a continuous Newtonian fluid in the laminar flow region. For more concentrated suspensions ( c =10%-20%), their friction factors are appreciably smaller than the Newtonian fluid's theoretical values, and are approximately invariant with the suspension's concentration. The pressure drops of MEPCM suspensions in the minichannels increase over the entire range of the velocity as the mass concentration increases, and when MEPCM mass concentration is more than 10%, the pressure drop increases more distinctively.(4) Comparative experiments and the 3D numerical simulation have been conducted to explore the laminar flow heat transfer characteristics of MEPCM suspensions in the rectangular minichannels. It was found out that such parameters as MEPCM mass concentration, mass flow rate and wall heating flux play significant role in the cooling performance of MEPCM suspensions. Compared with water, at a low mass flow rate (M=0.05 kg/min), MEPCM suspension shows a better cooling performance with lowered wall temperatures, and the wall temperature rise decreases distinctively as a more concentrated suspension is used. However, as the mass flow rate increases, the relative cooling performance of MEPCM suspensions (to water) becomes worse with the wall temperatures becoming closer or even higher than those of water, and the wall temperature rise increases distinctively as a more concentrated suspension is used.It was also found out that the 5% concentration suspension shows a better cooling performance than water with lowered wall temperatures over the entire range of mass flow rates from 0.05 kg/min to 0.35 kg/min. (5) The effect of such parameters as mass concentratioin, mass flow rate and wall heating flux on the cooling performance of MEPCM suspensions has been analyzed theoretically, by considering the role of MEPCM particles in the thermal boundary layer of the suspension flow in the minichannel.(6) Heat transfer and pumping power of MEPCM suspensions flow in the rectangular minichannels have been considered comprehensively to evaluate the MEPCM suspension for wall temperature control application. For the minichannel heat exchanger with the present geometric configurations, the working efficiency of 5% concentration suspension is close to that of water with lowered wall temperatures; however, the working efficiency of the suspensions with a mass concentration higher than 10% is lower than that of water, and decreases distinctively as a more concentrated suspension is used, and only when at a low mass flow rate, using a high concentration suspension can achieve even lower wall temperatures. |
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