Effect Of Compressibility And Roughness On Flow And Heat Transfer Characteristics In Microtubes

Due to the widespread applications of microchannels in modern industries, the study of characteristics of microscale momentum and heat transfer becomes an important subject of recent investigation. At the present time, experimental data from different authors conflict quantitatively and even qualitatively, and the conclusions and explanations do not coincide. In this thesis, emphasis is put on the effect of compressibility and tube roughness on the flow and heat transfer characteristics in microtubes.Compared to conventional sized tubes, the compressible fluid flow in microtubes has higher Mach number M in laminar regime. It has been proven that the effect of compressibility on macrotubal flow friction resistance can be neglected, however, results of theoretical analysis and numerical simulation reveal that the gas compressibility in microtubes can lead to a variation of the velocity profile, bring on a larger velocity gradient at the tube wall and consequently increase the friction factor. A new tube-cutting method is employed to measure the pressure and Mach number distribution along a microtube of 108.3μm. Experiments were also performed concerning the average Fanning friction factors of five kinds of microtube whose diameter ranges from 80.0 to 166.6μm. It can be found that the pressure distribution in a microtube became nonlinear at higher Mach number and the product of measured average Fanning friction factors C_f and the Reynolds number Re is higher than 16.Method of order-of-magnitude estimate and numerical analysis are used to investigate the effect of reversible work and viscous dissipation on gas flow and heat transfer in microtubes. As to the compressible tubal flow, whether or not taking into consideration the effect of reversible work and viscous dissipation correspond to adiabatic and isothermal flow respectively. They are combined to alter the temperature profile in microtubes and lead tosubstantially different heat transfer characteristics. With the effect of reversible work and viscous dissipation, altering heat flow direction is observed under constant wall temperature condition and the total heat exchange rate of compressible pipe flow is also significantly influenced at high M. Due to flow acceleration, the Eckert number is not a constant along the microtube and therefore the inlet Eckert number is no longer an accurate criterion to determine the magnitude of reversible work and viscous dissipation for compressible fluid flow in microtubes.Microtubes are commonly characterized by higher relative roughness and inner laminar flow. Study of the literatures show that the discussion concerning the incompressible laminar fluid flow in conventional-sized coarse tubes can not give satisfactory explanation to the results of microtubal fluid flow. Computations are carried out to investigate the effect of two and three-dimensional roughness elements on the flow resistance in microtubes and a preliminary theoretical model is brought forward based on the thought that the influence of the roughness would be much the same as reducing the diameter. It can be concluded that the form drag induced by the presence of roughness elements can produce friction factors 17-23% higher than those predicted by Moody chart at e/d-Z%. Experiments concerning the flow resistance for deionized water flow in stainless steel microtubes with relative roughness of 3.3~ 3.9% also gives 15-37% higher Darcy friction factors in laminar regime. It can be concluded that the higher frictional resistance for incompressible fluid flow in rough microtubes can be partly attributed to the form drag induced by roughness elements.