Flow boiling heat transfer and associated bubble behaviors over backward- and forward-facing steps

The influence of backward-facing step and forward-facing step on flow boiling heat transfer performance was experimentally studied. The experiments were carried out with expansion ratio of 2, 2.5 and 3, and channel Reynolds number of 3048 to 6097. The heated surface was immobilized at the downstream next to the step. FC-72 was used as the working fluid with the inlet subcooling of 35 K. The bubble behaviors were analyzed by the high-speed camera. The results reveal that the backward-facing step geometry has a negative effect on the heat transfer performance compared with the flat channel. The effect of forward-facing step geometry is related with the dimension of the recirculation regions: the heat transfer performance is enhanced if the reattachment point falls on the heated surface, and is deteriorated if the recirculation region is larger than the length of the heated surface. The bubble behaviors were analyzed to illustrate the dimension of the recirculation region and the mechanism of flow boiling heat transfer over a step.