MICROCHANNEL DEFINITION FOR TWO-PHASE FLOW BOILING: DEFINING A CRITICAL DIMENSION
The definition of microchannel size in the literature for two-phase flow boiling is arbitrary. There is no universally accepted definition. The transition from macrochannel to microchannel cannot be based solely and arbitrarily on the physical dimensions of the channel. The purpose of this paper is to define a critical dimension for a microchannel. It is proposed that there should be no difference between the temperature on the tip of the bubble and the average mixed temperature of the fluid at the point of bubble departure with the temperature of both equal to the saturation temperature. In this situation, the bubble should be stable after detachment. The point of bubble departure from the heated surface is determined from the force balance (buoyant force, surface tension force, and friction force) on the bubble, and the temperature distribution away from the heated wall. The hydraulic diameter was determined by solving the resulting equations. The properties of CO2 at saturation temperature (from -30.15°C to 20°C and mass flux ranging from 200 to 1000 kg/(m2s)) were used. The following trends were found: when temperature is constant, DH critical decreases as G increases; and when G is constant, the DH critical increases as temperature decreases. Preliminary experiments performed at CERN seem to support our model's microchannel definition. Visualization experiments are planned for further validation of our model. In conclusion, this model may allow us to predict whether microchannel conditions exist or not before experiments are conducted.