Experimental Reasearch And Theoretical Analysis On Flow And Heat Transfer In Microchannels With Different Surface Roughness

Miniature and portable mechanical devices and systems are today encountered with the development of science and technology, and the development of micro-fluidics and micro-heat transfer are promoted by the mainstream technology of MEMS (Micro-Electro-Mechanical Systems). Along with the wide applications of micro-flow transducers (micro-flowmeter, micro-pressure sensor, micro-surface stress transducer et al.) and micro-flow system and devices, so the mechanism and theoretical research on micro flow and micro heat transfer form a hot topic of current scientific research. Microchannels are commonly characterized by higher relative surface roughness and inner laminar flow. However, existing published results of flow and heat transfer in microchannels show that there is a great deal of discrepancies between classical values and experimental data, surface relative roughness is a main factor account for these differences. In this thesis, emphasis is put on the effect of surface relative roughness on the flow and heat transfer experimental characteristics in rectangular microchannels, and a regular perturbation is used to analyze the influence of wall surface on laminar flows in microchannels to obtain the analytical and numerical solutions.The famous conclusion of conventional-sized channels by Nikuradse and Moody is that there is no effect to laminar flow friction in channels when the surface relative roughness less than 5%. Study of literatures show that the discussion concerning the incompressible laminar fluid flow in conventional-sized coarse channels can not give satisfactory explanation to the results of microchannel fluid flow. It is found that the hagen-poiseuille number is unconstant but slowly increased with the Reynolds number, the Darcy friction factor and hagen-poiseuille number are increased with the surface roughness in laminar area at the same Reynolds number. Experiments concerning the flow resistance in stainless rectangular microchannels with relative roughness of 0.33%～5.36% gives 0～88% higher Darcy friction factors in laminar regime. The liquid polarity has little effect on friction factors for water, alcohol andhexane in microchannels with hydrodiameter larger than 250μm.Another experimental investigation is performed on heat transfer performance of water in stainless rectangular microchannels with relative roughness of 0.40%, 2.32% and 5.36%, the measured Nusselt number is increased with surface roughness, and the experimental results also show that the heat transfer is augmented about 6%～66%, 33%～84%,and 53%～88% respectively for laminar regime(Re>400).An experimental study is conducted to visualize flow field to confirm the transitional Reynolds number of laminar to turbulent flow, distilled water and Eosin Y(water soluble) were used as working fluid and dye stream respectively. The red trace was a thin straight line in microchannels at the low Reynolds number, which is the same as the famous Reynolds transitional phenomenon. The exit, middle and entrance region become diffuse and transitional flow occurs successively with increasing Reynolds number. The transition Reynolds number is about 1700 for relative roughness smaller than 3%, but the transition Reynolds number is about 1500 with relative roughness larger than 3%, which showed that transition occurs earlier than traditional values.At last a regular perturbation method is used to analyze the influence of wall roughness on the laminar flows in straight long microtubes. By expansion of perturbation series for physical quantities, the leading-order and the first-order linear (partial) differential equations are derived, and consistent solutions of whole fields are approached. A relationship between friction and viscous dissipation under the condition of rough surfaces is setup by analog of Poiseuille flow cases, and the dependences between friction or pressure drop and wall parameters as well as Reynolds number are found. The rough wall elements are modeled by one-dimensional harmonic functions and complex periodic functions respectively, and the finite difference method is used to solve equations. The computing results show that the main factors affecting the flow regime are relative roughness, spatial wave number (rough element interval) of wall rough curve and the Reynolds number. If the wave number is kept constant and increased the relative roughness, the peak of the disturbed non-dimensional stream function increased and the disturbed area in flow field is invariable. If the relative roughness keep constant and the spatial wave number decreased, both the peak of disturbed stream function and disturbed area in flow field increased simultaneously. The disturbed flow field is affected by Reynolds number a little.