UNSTEADY FLOW PHENOMENON IN A HEATED MICROCHANNEL AT HIGH HEAT FLUXES

摘要

An experimental study was performed on the unsteady flow phenomena of the two-phase flow in a single microchannel with the inside diameter of 500 μm. Depending on the inlet liquid temperature, two types of unsteady flow were identified. The first type is the long period/large amplitude liquid/two-phase alternative flow, occurring at the liquid temperature of the water tank of 100℃. It behaves the large amplitude oscillations of fluid/watt temperatures and mass flux which have clear switch points from liquid to two-phase flow with a relative long section of the two-phase flow between them. It is conformed such that slow transition flow can be self-sustained with the feedback control of the mass flux and the entrance liquid temperature of the microchannel, incorporating the system geometry, dimensions, and the microchannel itself. The second type of unsteady flow appears when the liquid temperature of the water tank varying from room to 80℃, is named as the long period/small amplitude periodic subcooled flow boiling. The outlet temperature displays the positive signal pulse periodically, followed by negative pressure drop signal pulse. The flow is indicated to exhibit the liquid/two-phase alternative flow with a short section of boiling and bubble emission between them. The oscillation time period can be up to 50 sec at heat flux just beyond the boiling incipience, and is in the range of 5 to 10 sec at high heat flux approaching the critical heat flux. For a given pressure drop and inlet liquid temperature, the mean mass flux only has a slight decrease with increasing the heat flux after the boiling incipience, providing the evidence that the periodic flow possesses a short section of two-phase flow for a full cycle.