Study On Flow And Heat Transfer Enhancement Characteristics In Silicon-Based Sinusoidal Wavy Microchannels

In this thesis, a series of silicon-based sinusoidal wavy microchannels were designed and fabricated by the Micro-Electrical Mechanical Systems (MEMS) technology. An experimental system was set up for investigating the flow friction and heat transfer characteristics in microchannels. Various effects on flow friction and single phase heat transfer in sinusoidal wavy microchannels with different structures were discussed. Also, the boiling flow pattern and boiling instability in microchannels were investigated with the aid of high-speed microscopic visualization technology.The flow friction experiment showed that: (1) the flow friction in wavy microchannels increased when compared with the smooth straight microchannels with the same hydraulic diameter; (2)both the phase shift and the wavelength had impact on the flow friction in wavy microchannels. For the microchannels with the same wavelength, the flow friction increased as the phase shift increased. However, the influence of wavelength was complicated. For the microchannels with the phase shift of 0, the flow friction normally increased as the wavelength decreased with the exception for the microchannel with the smallest wavelength. For the microchannels with the phase shift ofπ, the flow friction decreased as the wavelength decreased due to the decrease of the throat size in the microchannels; (3) with the decrease of the wavelength, the influence of phase shift on the flow friction decreased. When the wavelength was 0.5mm, there was nearly no difference in the flow friction between the microchannels with 0 andπphase shifts.The heat transfer experiment showed that: (1) when Re<200, there was no obvious enhancement in heat transfer in sinusoidal wavy microchannels as compared with the straight microchannel. However when Re>200, the Nusselt numbers of sinusoidal wavy microchannels exceeded those of the straight microchannel; (2) the Nusselt numbers in the sinusoidal wavy microchannels increased with the increase in the Reynold number, and moreover, the increasing rate of Nu with Re was more rapid for the sinusoidal wavy microchannels with the larger phase shifts; (3) the influence of wavelength on the heat transfer was complicated. For the microchannels with the phase shift of 0, the Nusselt number was larger in the sinusoidal wavy microchannel with a smaller wavelength when Re>350. For the microchannels with the phase shift ofπ, the Nusselt number was larger in the sinusoidal wavy microchannel with a larger wavelength except microchannel #6.The flow boiling experiment showed that: (1) under the stable boiling mode, both the bubbly flow and the plug flow were observed in the microchannels with the phase shifts of 0 andπ/4, while only the bubbly flow could be observed in the microchannels with the phase shifts ofπ/2 or more. Under the unstable boiling mode, flow patterns were complicated. Wavy flow, annular flow and mist flow were observed in the microchannels with the phase shifts of 0 andπ/4. In microchannels with the phase shifts ofπ/2, 3π/4 andπ, injection phenomena appeared in the throat part, while the wavy flow, annular flow and mist flow could hardly be observed in this situation; (2) under the stable boiling mode, no fluctuations in the fluid temperatures, wall temperatures and fluid pressures were detected. However, there appeared the periodic fluctuations in the fluid temperatures, wall temperatures and fluid pressures under the unstable boiling mode. The mass flux, heat flux and structure of microchannel all influenced the fluctuation period and amplitude of various measurements; (3) the stability of boiling could be improved by increasing the mass flux or reducing the heat flux. There existed a line with a positive slope which could divide the mass flux-heat flux map into the stable boiling region and the unstable boiling region, and the location of this line was influenced by the phase shifts of microchannels.