Three Dimensional Thermo-Fluid Analyses on Convective Heat Transfer and Friction Loss in Micro/Mini Channel Based on High Order LES Model

Abstract

Several researches dealing with the single-phase forced convection heat transfer in micro tubes have been published in the past years. Most of their tests results are significantly departed from those of the traditional forced convection heat transfer coefficients in larger tubes. Some recent work reported that measurement accuracy is one of the most important factors that may cause this discrepancy. Since the diameter of the sensor for measuring micro tubes surface temperature is comparable to the size of the micro-tubes, the tubes surface temperature can not be accurately measured due to the effect of sensor wire thermal shunt. In this work some recent experimental results on heat transfer and frictional losses in mini/micro channel of semi-circular configurations using water as working fluid are investigated numerically by comparing the measured and predicted data. The flow conditions considered here cover a wide range of Reynolds number (300–4000), which corresponds to laminar, transitional and turbulent flow. Since the flow considered here is turbulent in nature emphasis is put on the physics based turbulent model. In this study, a high order LES turbulent model in which in a dynamic eddy viscosity model, transfer of information between the sub-grid and large scale eddies is improved by solving an additional transport equation for turbulent kinetic energy in the grid scale level. Here, sub-grid-scale turbulent stresses are closed using a dynamic turbulent kinetic energy transport model. The sub-grid scale length scale is represented by the minimum of the universal length scale lu and the grid scale. The universal length scale lu, which represents the blending of the length scales of cascade of eddies starting from the near wall small scale all the way to the sub-grid scale, is defined on the basis of turbulent Reynolds number Ret. A test filter was used for the dynamic procedure, which is applicable to stretched grid near the body surface. Also the thermal convection problem is coupled with thermal conduction within the material to obtain the overall solution. Predicted results agreed well with the measured data. The results helped to have a good understanding of how the flow and thermal phenomena attributed to the overall heat transfer and frictional loss mechanism. The comparison of measured and predicted data based on single phase N-S equations showed a very good agreement and the visualization of the three-dimensional results of computation led to a good understanding of the physics based mechanism associated with the laminar to turbulent transitional phenomena inside the micro/mini channels.