Effect Of Axial Heat Conduction On Heat Transfer In Entrance Region Of Microchannel

Due to the excellent heat transfer performance,microchannel heat exchangers have become the best choice to remove the high heat flux produced by electronics with high power density.In order to further improve the heat transfer efficiency of microchannel heat exchangers,new microchannel heat sinks had taken advantage of the merit of the entrance effect to achieve heat transfer enhancement by the thermal boundary layer redeveloping mechanism.The axial heat conduction of the fluid in the channel has a significant influence on the entrance effect.A solid understanding of the effect of axial heat conduction on the heat transfer in the microchannel entrance region is of great academic significance and engineering application value to recognize the heat transfer augmentation mechanism and the optimal design of microchannel heat exchangers.The dissertation systematically studied the influence of the axial heat conduction on the heat transfer using the theoretical analysis and numerical simulation to quantify the heat transfer augmentation mechanism at the entrance region in microchannels.The fully developed region is a limiting case of the entrance region,and the exact solution can be obtained there because the velocity distribution is more stable.It is thus the theoretical analysis is first used to obtain the flow performance at the fully developed region in rectangular,elliptical,circular and parallel plate microchannels.The fully developed velocity distributions and the variation tendencies of the friction factor are thus obtained by solving the momentum equation.A double-eigenvalues method is used to solve the momentum equation in the rectangular microchannel.The method only needs to solve two homogeneous ordinary differential equations,which makes is simpler than the general Eigenfunction method.The separation of variables with two binomial expansions is used to solve the momentum equation in the elliptical microchannel.The exact solution amends the existing correlation and reveal the variation trend of the friction factor in the elliptical microchannel.What's more,f Re gradually decreases as Kn increases,and the reduction of f Re is the largest in parallel plate microchannel,which is 54.5%,followed by elliptical microchannel and microtube,and the smallest reduction is 43.5%in the square microchannel when Kn increases from0 to 0.1.Secondly,the fully developed velocity distributions are used as the inlet boundary condition to theoretical study the heat transfer characteristics of the thermally developing flow in microtube and parallel plate microchannel with both the isothermal and isoflux boundary conditions.The energy equation is solved to obtain the temperature distribution and the heat transfer coefficient at the thermal entrance region by the separation of variables with the help of Gram-Schmidt orthogonalization.The effects of the rarefaction(0<Kn<0.1)and the axial heat conduction(Pe>50)are considered.It is found that rarefaction effect will weaken the heat transfer performance,especially for small Pe.The weakening effect is more significant at the entrance region when compared with the fully developed region,and it is more obvious in the parallel plate microchannel.The axial heat conduction can enhance the heat transfer performance at the entrance region,especially for Pe ranging from 50 to 250.However,the effect of the axial heat conduction on the heat transfer characteristics can be ignored when Pe is over 500.In addition,the rarefaction effect will weaken the effect of the axial heat conduction on the heat transfer.The thermal entrance length is further obtained and it is found that the length in the microtube is almost 3 to 4 times that of the parallel plate microchannel.A two-dimensional numerical calculations are used to calculate the simultaneous developing flow and heat transfer in the microtube and parallel plate microchannel.A thermal resistance model is proposed to simulate the temperature jump boundary conditions,and the model is verified via comparing the result with the theoretical research results of the thermally developing flow.The distribution of velocity and temperature at the entrance region is thus obtained numerically.The friction factor and heat transfer coefficient are further derived.It is found that the slip velocity of the fluid near the wall gradually decreases along the flow direction,and f_appRe also gradually decreases along the flow direction at the same time.In addition,both f_appRe and Nu(x)decrease with the increase of Re at the entrance region,and the reduction is significant as Re less than 100.But when Re is greater than 500,the effect of Re gradually disappears.In addition,compared with the result of the thermally developing flow,the heat transfer performance of the simultaneous developing flow is better.Subsequently,the proposed thermal resistance model is further used to solve the three-dimensional heat transfer problem in rectangular microchannels,and the fully developed velocity distribution is used as the inlet boundary.The effects of Re(25?2000)and the aspect ratio(0.05?1)and Kn(0?0.1)on Nu of the thermal developing flow are considered.It is found that the heat transfer coefficient increases as the channel aspect ratio decreases.The effect of the aspect ratio on the heat transfer performance can be ignored at the channel inlet,and the effect gradually becomes obvious along the flow direction.Besides,the rarefaction effect will weaken the effect of geometric parameters on the heat transfer performance,while the influence of the rarefaction effect on the heat transfer increases as the aspect ratio decreases.Finally,the effect of axial conduction on the heat transfer performance under three boundary conditions(T,H1,H2)is further investigated numerically in the elliptical microchannel.It is found that the temperature distribution trends of the thermal entrance region under the boundary conditions of H1 and H2 are independent of the boundary condition.The effect of axial conduction on the heat transfer performance under H1 and H2 boundary conditions is almost the same.Compared with H1 and H2 boundary conditions,the axial heat conduction on the local Nu number is more obvious under T boundary condition.However,the temperature distribution under the H2 boundary condition is quite different from the results under H1 and T boundary conditions in the fully developed region.Local high temperature area can be observed at the end of the semi-major axis of the elliptical microchannel under the H2 boundary condition,which increases the synergy angle between the temperature field and the velocity field and results in the heat transfer deterioration.In addition,the thermal entrance region under the H2 boundary condition is longer than that under T or H1 boundary conditions,especially when the aspect ratio is less than 0.6,the thermal entrance length will increase dramatically as the aspect ratio decreases.The heat transfer correlations including the entrance effect are also obtained based on the results.