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Kondle Alvarado-IHTC14-22512

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1 Copyright © 2010 by ASME Proceedings of International Heat Transfer Conference IHTC14 August 8-13, 2010, Washington, DC, USA IHTC14-22512 LAMINAR FLOW FORCED CONVECTION HEAT TRANSFER BEHAVIOR OF PHASE CHANGE MATERIAL FLUID IN MICROCHANNELS WITH STAGGERED PINS Satyanarayana Kondle Department of Mechanical Engineering, Texas A&M University,College Station, TX. 77843 Charles Marsh U.S. Army Corps of Engineers Engineer and Research Development Center Champaign, Illinois. 61822-1076 Jorge L. Alvarado Department of Engineering Technology and Industrial Distribution, Texas A&M University, 3367 TAMU,College Station, TX. 77843-3367 Gurunarayana Ravi Department of Mechanical Engineering, Texas A&M University, College Station, TX. 77843-3367 ABSTRACT Microchannels have been extensively studied for electronic cooling applications ever since they were found to be effective in removing high heat flux from small areas. Many configurations of microchannels have been studied and compared for their effectiveness in heat removal. However, there is little data available in the literature on the use of pins in microchannels. Staggered pins in microchannels have higher heat removal characteristics because of the continuous breaking and formation of the boundary layer, but they also exhibit higher pressure drop because pins act as flow obstructions. This paper presents numerical results of two characteristic staggered pins (square and circular) in microchannels. The heat transfer performance of a single phase fluid in microchannels with staggered pins, and the corresponding pressure drop characteristics are also presented. An effective specific heat capacity model was used to account for the phase change process of PCM fluid. Comparison of heat transfer characteristics of single phase fluid and PCM fluid are made for two pins geometries for three different Reynolds numbers. Circular pins were found to be more effective in terms of heat transfer by exhibiting higher Nusselt number. Circular pin microchannels were also found to have lower pressure drop compared to the square pin microchannels. Keywords: Microchannels, Phase Change Material (PCM) fluid, staggered square and circular pins, and aspect ratio. INTRODUCTION Microchannels have been studied for electronic cooling applications because of their capacity in removing high heat flux from small areas. Many configurations of microchannels have been studied and compared for their effectiveness in heat removal [1-8]. However, there is little data available on the use of pins in microchannels. Also, except for few publications, there are virtually no optimization studies in this area. Recent studies of microchannels have also shown that a phase change material (PCM) fluid improves the heat removal rate while maintaining lower wall temperature. Experiments have shown that PCM fluids exhibit an enhanced heat capacity due to the phase change material’s latent heat of fusion [9-18]. Hao and Tao [18-19] did an extensive study and evaluated the performance of PCM particle flow in circular microchannels. They modeled the particle flow separately from the mean flow using source terms in momentum and energy equations. They also considered the particle-particle interactions and particle-depletion layer effects near the wall. Said [20] performed a computational fluid dynamics (CFD) analysis of PCM slurry flow in microchannels with thick walls taking into account conjugate heat transfer. Results from the previous efforts on PCM slurry flows have been promising with the main advantage being the lower wall temperature for same heat flux removal compared to conventional single phase fluids. The current work focuses on use of PCM fluid in microchannels with staggered pins, both circular and square pins. Also, the heat transfer characteristics without the presence of pins (called as “no pins”) is studied and compared with square and circular pins configurations.2 Copyright © 2010 by ASME SPECIFIC HEAT MODEL Simulating the complete phase change process is difficult with the currently available multi-phase models in commercial software, and even if possible, it would be computationally expensive. Two types of effective modeling approaches were found in the literature. One uses a heat source term in the energy equation. The other considers a specific heat model. While both the methods are found to be effective, the use of the specific heat model is simpler and easier to implement into a computer code. The latter approach was used in the current simulation. The model assumes that the phase change can be approximated by change in the specific heat of the bulk fluid within the melting temperature range of the phase change material. Yamagishi et al. [10] has found that this model gives results that are comparable with experimental findings. Also, using this type of model for the specific heat is simple and straight forward to implement in commercial software like Fluent. The following equations were used to account for effective specific heat. For T < T1 or T > T2 [15]: (1) For T1 < T < T2 [15]: (2) Where, T1 = Temperature at which phase change starts T2 = Temperature at which phase change ends GEOMETRY AND MESH Microchannels containing staggered square and circular pins were selected for computational simulation. The height of the microchannel was taken as 500 µm comprising of pins of 300 µm in height with a base height of 200 µm (same for both square and circular pins). The width and length of the microchannel were 1 cm and 2 cm, respectively. It should be noted that the width of the microchannel does not have any significance on the results since symmetry was used to model only a section of the width, as shown in Figure 2. The assumption of symmetry is valid since there are about 50 pins along the width. The wall boundary condition at the bottom wall is either constant heat flux or constant wall temperature. The fluid and solid interfaces on both sides are specified as symmetric, having no heat flux, or flow perpendicular to the symmetry boundary


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