Elongated bubbles in microchannels. Part I: Experimental study and modeling of elongated bubble velocity

The velocity of elongated vapor bubbles exiting two horizontal micro-evaporator channels with refrigerant R-134a was studied. Experiments with tube diameters of 509 and 790 μm, mass velocities from 200 to 1500 kg/m² s, vapor qualities from 2% to 19% and a nominal saturation temperature of 30 °C were analyzed with a fast, high-definition digital video camera. It was found from image processing of numerous videos that the elongated bubble velocity relative to that of homogeneous flow increased with increasing bubble length until a plateau was reached, and also increased with increasing channel diameter and increasing mass velocity. Furthermore an analytical model developed for a diabatic two-phase flow, has been proposed that is able to predict these trends. In addition, the model shows that the relative elongated bubble velocity should decrease with increasing pressure, which is consistent with the physics of two-phase flow.     

The electric double layer effect on the microchannel flow stability and heat transfer

The effect of the electric double layer (EDL) on the linear stability of Poiseuille planar channel flow is reported. It is shown that the EDL destabilises the linear modes, and that the critical Reynolds number decreases significantly when the thickness of the double layer becomes comparable with the height of the channel. First results coming from direct numerical simulations on the non-linear effects show also that the by-pass transition is much more rapid in the presence of EDL. There is an acceptable qualitative correspondence between the estimated transitional Reynolds numbers and some experiments, showing that early transition is plausible in microchannels under some conditions. Several questions remain however unanswered such as the surface conduction effect on EDL.     

Experimental study on the heat transfer coefficient of water flow boiling in mini/microchannels

The heat transfer coefficients of the evaporative water flow in mini/microchannels are studied experimentally to explore the novel heat dissipation for high power electronics. Two sets of parallel channels which are 61 channels with hydraulic diameter of 0.293 mm and 20 channels with hydraulic diameter of 1.2 mm are investigated respectively. The inlet and outlet temperatures of fluids, and the temperatures beneath the channels are measured to calculate the heat dissipation of the evaporative water in channels. The experiments are carried out with the mass flow rates range from 11.09 kg/(m² s) to 44.36 kg/(m² s) for minichannels and 49.59 kg/(m² s) to 198.37 kg/(m² s) for microchannels. The effective heat flux range from 5 W/cm² to 50 W/cm², and the resulted outlet vapor qualities range from 0 to 0.8. The relations of the heat transfer coefficient with heat flux and vapor quality are analyzed according to the results. The experimental heat transfer coefficients are compared with the prediction of latest developed correlations. A new correlation takes the effect of Bond number is proposed, and be verified that it is effective to predict the heat transfer coefficient of both minichannels and microchannels in a large range of vapor qualities.