Heat Transfer And Interfacial Characteristics In Microchannels With/without Phase Change

Microchannel plays a critically important role in the development of modern science and technology, particularly micro/nano technology. Heat and mass transfer in microchannels has attracted worldwide researchers as one of frontiers in the energy and thermal fluid science discipline, showing great significance in both understanding fundamentals of transport phenomena at microscale and promoting development of new technologies such as micro energy systems, compact heat transfer devices/equipments, and cooling technology, etc. The present work carried out a series of experiments, simulations and theoretical investigations on liquid single-phase convection and flow boiling two phase flow in microchannels.Microchannels with different sizes were fabricated by employing MEMS technology. An integrated single-phase/phase-change experimental system was established with the microchannel test sections, flow and heating devices, visualization system, measuring and data acquisition instruments. Based on the liquid single-phase convection experiments, mathematical and physical models were constructed for the three-dimensional fluid-solid conjugated convection simulation in a microchannel. The effects of flow development, conjugated heat transfer and property variation on microchannel flow and convective heat transfer were distinguished and evaluated. Property variation at high heat flux caused coupled hydrodynamic and thermal development, altered the flow and heat transfer performance, and showed its importance on heat transfer enhancement. The overall trend and local characteristics of flow and thermal re-development were described in the single-phase property variation study. The heat transfer enhancement was discussed on the specific mechanisms of variable properties. The impacts of inlet velocity, temperature and heat flux on heat transfer enhancement were distinguished. The property variation effect induced marked non-linear feature at high heat flux.Different stages in a microchannel flow boiling process were identified and discussed. The non-uniform phase change in microchannels showed strongly transitional features. Due to the influence of flow distribution, local pressure fluctuation and conjugated heat transfer, different oscillation phenomena appeared in the stages of transition, vigorous boiling and vapor single-phase flow. Local interface competition and diversified bubble dynamics phenomena were also observed. At local heating condition, evaporation and condensation occurred at the upstream and downstream caps of an elongated bubble. Diversified phase-change modes showed dominating effect on the interface oscillation. Signal analysis explored that the film-driven oscillations of both evaporating and condensing interfaces generally operated at higher frequencies than their counterparts driven by nucleation or dropwise condensation. The upper and lower caps of one elongated bubble oscillated at the same characteristic frequency (band).