| Summary: | 博士 === 國立中央大學 === 機械工程研究所 === 88 === The dissertation primarily focuses on the passive techniques of heat transfer enhancement, derived from special configuration of the surface geometry. This includes three methods of the passive techniques: additives for liquids or additives for gases, rough surfaces and polymer solutions.
First, additives for liquids or additives for gases; Present a numerical study of stagnant conductivity effect on the thermal developed non-Darcian mixed convection in horizontal packed-sphere channels. The effect of stagnant conductivity is characterized by the conductivity ratio of solid sphere-to-fluid (ks /kf). The results show that the buoyancy effect affects significantly the secondary flow structure and enhanced heat transfer when ks /kf and Peclet number are all low. But under a fixed Rayleigh number, the effect of buoyancy will be suppressed when the value of ks /kf or Peclet number increases. Based on the criterion of 5% deviation of Nusselt number from that for forced convection, the critical Rayleigh number for the onset of buoyancy effect is shown to increase more than 1 order of magnitude when ks /kf increases from 1.3 to 26.8 or 38.8 at low Peclet number.
Second, rough surfaces; In microchannels, the effect of the surface roughness has been simulated by a numerical study to investigate the laminar flow and heat transfer behaviors on the forced convection by different parameter''s change. A result show that both parameters include larger surface relative roughness, smaller roughness geometry slip coefficient which have all more momentum transfer on near the wall induce heat transfer enhancement. Another result as shown heat transfer''s effect are also dependent on thermal boundary conditions and microchannel''s aspect- ratio, when microchannel''s aspect-ratio , constant axial wall heat flux with uniform peripheral wall heat flux which had larger local Nusselt number Nul value than constant wall temperature peripherally as well as axially. Larger microchannel''s aspect-ratio had larger local Nusselt number Nul value than aspect-ratio , which appeared on the constant wall temperature peripherally as well as axially. But, for constant axial wall heat flux with uniform peripheral wall heat flux, local Nusselt number Nul value is not regularly increasing to increase microchannel''s aspect-ratio.
Last, polymer solutions; A numerical study has been performed on the heat transfer mechanism of Newtonian and non-Newtonian fluids in 2:1 horizontal rectangular ducts. The effects of temperature which dependent on fluid''s viscosity, shear thinning, and buoyancy-induced secondary flow are all considered. Experimental data for Newtonian fluid, water, and non-Newtonian fluid, 1000 weight parts per million (wppm) aqueous Separan AP-273 solution (0.1%), were chosen for the comparison with the numerical results. For water, the present numerical results are all in good agreement with the experimental data. The heat transfer enhancement is caused by the buoyancy-induced secondary flow. For Separan AP-273 solution (0.1%), the present numerical results agree with the experimental data in the region near the entrance, but the present modeling underestimates the value of Nul in the fully-developed region. In the region near the entrance, the heat transfer enhancement is caused mainly by the axial velocity distortion, which is mainly due to the temperature dependence of viscosity. The effect of buoyancy-induced secondary flow which is much weaker in the case for Separan AP-273 solution (0.1%) rather than that for water. It is mainly caused by the relatively high viscosity of fluid around the central zone of rectangular duct.
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