Experimental Investigation On The Characteristics Of Bubble Confined Behaviors During Flow Boiling In Mini/Micro-channel

The thermal management on the high-heat-flux devices or electronic elements in vehicle is significant for the safe operation of the machinery. Two-phase mini/micro-channel heat sink is widely used in the thermal management on vehicle due to the excellent heat dissipation capability and the precise temperature control performance. The bubble behaviors during flow boiling in mini/micro-channel mainly determine the heat transfer performance, and in addition, the bubble can be widely used in MEMS system as one of the effective drive and control components for liquid flow, so the bubble behaviors in mini/micro-channel flow boiling need to be studied systematically and in detail.The bubble behaviors during subcooled flow boiling in rectangular mini/micro-channel were experimentally investigated using the degassed deionized water as the working fluid. The confined bubble flow or elongated bubble flow appeared at certain operating conditions and the confined/elongated bubble formed only in two ways:the continued growth of a single bubble or the coalescence of several adjacent bubbles. The growth rate of the elongated bubble was much larger than that of the bubble before elongation, in addition, the operating condition, the original shape of nucleated bubble and the neighbouring bubble also significantly affected the bubble growth.The bubble behaviors were mainly determined by the various forces acted on the bubble interface. The orders of magnitude of the forces acted on the evaporating meniscus of the elongated bubble were theoretically analyzed, including the gravitational force, the surface tension force, the viscous shear force and the evaporation momentum force. Based on the consensus that the gravitational force should be neglected in microscale, the orders of magnitude of other forces were compared with that of the gravitational force, it was found that the effect of the evaporation momentum force on the bubble behavior should be neglected in mini/microscale at heat flux conditions used in engineering.The growth process of a single bubble from nucleation to elongation was investigated in detail, three stages with different features were distinguished, they are respectively the free growth stage (also can be called unconfined growth stage), the confined growth stage and the elongation stage. During the bubble free growth stage, the equivalent diameter of the bubble abided by the power law model (Deq=ktn) for the bubble growth in the conventional channel flow boiling, while in the bubble confined growth stage, the bubble growth rate deviated from the bubble growth rule during flow boiling in conventional channel.A preliminary understanding on the confined bubble growth was obtained by conducting the boiling experiment in mini/micro-channel without liquid flow, the onset of the bubble confined growth stage was indicated by the first cooccurrence of the slowing of the maximum local void fraction growth rate and the obvious decrease in the bubble aspect ratio. During the flow boiling in mini/micro-channel, the curvature of the bubble top interface began to decrease after the onset of the bubble confined growth, so the bubble top interface flattened before touching the confining wall. The variations of the bubble maximum local void fraction and the bubble aspect ratio were no longer smooth during the bubble confined growth stage, with ripples appeared in their variation curves, which was one of the bubble confinement characteristics. The bubble equivalent diameter increased linearly with time(Deq=kt+n) during the confined growth stage, deviated from the bubble growth rule in the free growth stage, which was the other bubble confinement feature. It was the negative feedback between the bubble top velocity and the wall confining force resulted in and sustained the fluctuations in the maximum local void fraction and the bubble aspect ratio during the bubble confined growth until the bubble is fully confined by the mini/micro-channel wall in its height direction. The bubble confinement characteristics were affected by the operating conditions, which was illustrated by the different proportion of the bubble confined growth stage in the whole growth process and the differrent critical maximum local void fraction at the onset of bubble confinement.The heat transfer capability of the confined bubble and the incipient elongated bubble was larger than that of the unconfined bubble, thus the maximum local heat transfer coefficient generally occurred in the confined bubble flow region along the mini/micro-channel. In addition, the bubble confined growth and elongation resulted in the bubble sliding at the upstream subcooled boiling region.