Theoretical And Experimental Study On Gaseous Flow And Heat Transfer In Microchannels

The background of the current investigation is based on the application of micro channel heat sink in the thermal management of the aerospace craft operation system. The research starts from the basic theory of micro scale fluid flow and heat transfer. Then the mathematical models for governing fluid flow and heat transfer in micro scale are proposed.. According to the established mathematical model, key factors, which affect flow and heat transfer characteristic, are systematically analyzed. The single micro-tube experiment platform is set up and a series of experiments have been done for flow characteristic analysis. Inspired by the obtained conclusions, the design concept of the new type micro channel heat sink is proposed and the experiment instruments are fabricated, which verifies a better heat transfer performance than conventional multiple straight micro channel heat sink.The mathematical models for governing flow and heat transfer in microchannels are proposed for different flow regions. For slip flow region, Navier-Stokes equations, which are based on the conventional theory, with Maxwell slip model are used to describe the flow. Navier-Stokes equations with second-order slip boundary condition can accurately describe the flow in transit flow region by proper choosing of the slip coefficients in slip model. A powerful, easy-to-use analytic technique for non-linear problems, that is, the homotopy analysis method (HAM) was provided for solving these strong non-linear differential equations. By analyzing solutions and comparing with the results of other investigators, the above opinion has been validated. Meanwhile, recent researches show that, at the nano-scale level, an interesting phenomenon, namely "velocity inversion", is obtained by solving the molecular-based model. In the current paper, gas flow in nanochannels is also analytically investigated by using the homotopy analysis method (HAM). It is found that the inverted velocity profile can also be predicted by solving the conventional control equations, which are the Navier-Stokes equations, combined with high-order accurate slip boundary conditions.Based on the conclusions obtained above about the mathematic models for micro flow and heat transfer, proper equations are chosen to describe the physical process in microchannels. Numerical and analytical analyses were provided to study the effects of compressibility, rarefaction, entrance region and viscous heating on fluid flow and heat transfer characteristics in microchannels. It is indicated that the effect of compressibility can not be neglected in microchannels. The conventional criteria for compressibility effect, that is Mach number larger than 0.3, can not be used as a criteria in micro-scale. The pressure drop is better than Mach number to be used as criteria for compressibility effect. The "developed flow region", which is defined in conventional theory, must be re-defined in microchannel as there is no so-called "developed flow region" in micro-scale. In slip flow region, a new non-dimensional parameter relative slip length Ls/Dh is found to be very useful to describe friction characteristic for compressible flow with slip in micro-scale. The correlation for fRe with this new non-dimensional parameter Ls/Dh is suggested. The suggested correlation for fRe with Ls/Dh is validated by comparing with experimental data. This correlation can be used for both slip and non-slip flow, and for both compressible and incompressible flow.For entrance effect on fluid flow, it is found that the velocity profile in cross-section in microchannels is different from that in macrochannels, that is, maximum velocity occurs not in the channel core but near the walls due to the surface effect. Meanwhile, another feature of the velocity profiles is the presence of the very large velocity gradients near the walls. These phenomena result in the reduction of the thickness of hydrodynamic boundary layer. So the hydrodynamic entry length in microchannels is much larger than that in conventional channels. The correlation between L/D and Reynolds number and height-to-width ratio, which is useful for designing and optimizing the microchannel heat sinks and other microfluidic devices, is established. For entrance effect on heat transfer, it is found that the temperature gradient near the walls is very large. Thermal entry length in microchannels is much larger than that in conventional channels. Comparing with conventional channels, the heat transfer performance in microchannels is better in entry region and worse in the fully developed region. Also the effects of Reynolds number, hydrodynamic diameter, length diameter ratio, height-to-width ratio and wall temperature on thermal entry length were analyzed. The correlation among these parameters is established.Based on the superposition principle, an analytical solution for steady convective heat transfer in a two-dimensional microchannel in the slip flow region is obtained, including the effects of velocity slip and temperature jump at the wall, which are the main characteristics of flow in the slip flow region, and viscous heating effects in the calculations. The cases of constant heat flux boundary conditions, one wall with adiabatic boundary and the other wall with constant heat flux input and non-symmetric constant heat flux boundary condition are studied. The effect of viscous heating can be seen as a volume energy source, the temperature of fluid increase linearly along the flow direction. In addition, it is noted that the effect of viscous heating distorts the temperature distributions at the cross-sections. The velocity gradient is larger near walls, since the effect of viscous heating is more significant there. According to these calculations, it is found that the position in the inflexion of the temperature profiles does not change with Br and Kn. The position of the inflexion is a constant, given byη= 1/(?). For non-symmetric constant heat flux boundary condition, it is indicated that uniform heating will get better heat transfer performance.The single micro-tube experiment platform is set up and a series of experiments have been done for flow characteristic analysis. It is found that the friction constant is the function of Mach number, and the correlation is obtained. By analyzing the experimental data, it can be concluded that the entrance length in microtube is larger than that predicted by conventional theory. Also, the value of friction constant in the developed region is smaller than that predicted by conventional theory. These agree with the conclusions obtained in numerical study part. Meanwhile, the theoretical prediction suggested can give reasonable prediction on temperature field under the effect of viscous heating for incompressible (low Re number) flow.Inspired by the above conclusions, the new type micro channel heat sink with new structures is proposed. The test sections with these new structures are fabricated by using dry etching method. Then, the microchannel heat sink experimental system is established. A series of experiments have been conducted for flow and heat transfer performance analysis. The conclusion on the effect of viscous heating on flow and heat transfer characteristics, which is obtained above, is verified by experimentally investigation about the multiple straight microchannels. That is that temperature change is due to the viscous heating effect and compressibility effect. The relationship of these two effects is competitive. By experimental study on two new type microchannel heat sink——two stage microchannel heat sinkⅠ(250μm-100μm) and two stage microchannel heat sinkⅡ(100μm-40μm), it can be concluded that the new type microchannel heat sink proposed in this paper has better heat transfer performance and lower pressure lose. It can be used as a high efficiency heat exchanger to the real applications. In practice, the geometry parameters of this new type microchannel heat sink can be optimized according to the purpose of the heat exchanger to realize the maximum the heat transfer and minimum the pressure loss to save energy.