Study On Flow And Heat Transfer Enhancement Characteristics In Novel Silicon-based Microchannels

In this thesis, a series of novel silicon-based microchannels, including corrugated microchannel, intercrossed microchannel and internal-rib microchannel, were designed and fabricated by the Micro-Electrical Mechanical Systems (MEMS) technology. For investigating the single-phase/two-phase flow and heat transfer characteristics in microchannels, an experimental platform was set up. On this platform, two kinds of experiments with different objectives were conducted. The first one was for studying the single-phase flow and heat transfer characteristics of each type of novel microchannels. To get better understandings, the experimental results of flow characteristics were compared with the numerical predictions based on the traditional fluid dynamics theory. The second one was for analyzing the boiling instability in microchannels with the aid of high-speed microscopic visualization technology and simultaneous measurement technology.Following findings were obtained from the experimental study on the single-phase flow and heat transfer characteriscts in microchannels: (1) under the condition of same hydraulic diameter, the corrugated microchannel consumes more pumping power and has larger flow friction constant than the straight microchannel does. Furthermore, with the increasing of Reynolds number, the gap of flow friction constant between these two types of micochannles becomes more significant; the intercrossed microchannel consumes almost same amount of pumping power and has similar flow friction as the straight microchannel does. Compared with the straight microchannel, both corrugated microchannel and intercrossed microchannel can greatly enhance the convection heat transfer; (2) under the condition of same mass flux, both corrugated microchannel and intercrossed microchannel can reduce the wall temperature efficiently; (3) under the condition of same pumping power, both corrugated microchannel and intercrossed microchannel have smaller thermal resistance as compared with straight microchannel; (4) the simulation results of flow friction based on traditional fluid mechanics agree well with the experimental results in this thesis. The numerical simulation can describe the pressure field and velocity field inside the internal-rib microchannel, corrugated microchannel and intercrossed microchannel. Specifically, in the internal rib microchannel, the pressure gradient and energy loss achieve their maximum values at the turning point of the rib. There exists vortex in the intercostals grooves. Moreover, with the increasing of Reynolds number, the intensity of the vortex is strengthened and the location of the vortex shifts to the downstream. The flow in both corrugated microchannel and intercrossed microchannel is periodicly fully developed.The boiling instability in microchannels is another research objective in this thesis. The flow boiling in microchannels can be divided into stable boiling mode and unstable boiling mode based on whether or not the fluid temperature, wall temperature and fluid pressure fluctuate with time. Following results can be obtained from the present boiling experiments: (1) in the stable boiling mode, the flow patterns include bubbly flow and plug flow. While in the unstable boling mode where the temperature and pressure measurements change periodically with time, the flow patterns include wavy flow, annular flow and mist flow. Additionally, the reversed flow phenomenon occurs in the latter mode; (2) for the unstable boiling mode, both fluctuation period and amplitude of temperature and pressure are affected by mass flux, heat flux, hydraulic diameter and structure of microchannel. Specifically, the decreases of mass flux and hydraulic diameter cause the increases of fluctuation period and amplitude of temperature and pressure; the increase of heat flux, however, increases the amplitudes of temperature and pressure, but has little influence on the period of temperature and pressure. The corrugated microchannel can increase both the fluctuation period and amplitude of pressure, and the period of temperature, but decrease the amplitude of temperature; the intercrossed microchannel can increase the fluctuation periods of temperature and pressure, but has little influence on their amplitudes; (3) the static instability characteristics study through the pressure drop-mass flux curve obtained from the experiment shows that the static instability of the above microchannels will be increased by increasing the heat flux and reducing the hydraulic diameter, while weakly affected by structure of microchannel; (4) the stability of boiling can be improved by increasing the mass flux and reducing the heat flux. There exists a line, which obeys the equation that q/G equals to a specific constant. This line can divide the mass flux-heat flux range into stable boiling region and unstable boiling region. This line is strongly affected by the hydraulic diameter and weakly affected by structure of microchannel.