Numerical study of seed bubble-triggered evaporation heat transfer in a single microtube

Abstract

High boiling incipience temperature and flow instabilities in silicon-based microchannels with smooth surface are challenging issues. This work numerically investigates the seed bubble-triggered evaporation heat transfer in a microtube, with a length of 5.0 mm and diameter of 106 lm. Acetone was the working fluid. Seed bubbles were assumed to be generated periodically at the microtube upstream. The fixed grid allocation technique was proposed to successfully perform the parallel compu- tation via a set of computer core solvers. It is found that the seed bubble-guided heat transfer consists of a start-up stage and a steady operation stage. The start-up time equals to the residence time of the first seed bubble growing and traveling in the microtube. The seed bubble frequency is a key parameter to influence the performance. Low-fre- quency seed bubbles cause alternative flow patterns of liquid flow and elongated bubble flow, corresponding to the apparent spatial-time oscillations of wall and bulk fluid superheats. High-frequency seed bubbles result in quasi- stable elongated bubble flow, corresponding to quasi-uni- form and stable wall and fluid superheats. There is a sat- uration seed bubble frequency beyond which no further performance improvement can be made. There are residual fluid superheats specifying the required minimum super- heats to sustain the evaporation heat transfer between the two phases. Elongated bubbles with thin liquid films are responsible for the heat transfer enhancement. Contrary to wall temperatures, the transient local Nusselt numbers are slightly changed due to the fact that heat transfer is more closely related to the dynamic elongated bubble flow evolution within millisecond timescale in the microchan- nel. The heat transfer coefficients can be 2.0 to 3.5 times of that for the superheated liquid flow before seed bubble injections.