Experimental study and dynamic simulation of the continuous two-phase instable boiling in multiple parallel microchannels

Continuous vapor liquid two-phase flow means that both subcooled single phase and saturated flow boiling always exist in microchannels with the moving interface between them. The instable boiling phenomena and mechanisms are very complicated and needed to be deeply investigated and well understood. In the present study, both experiments and dynamics simulation of the continuous two-phase instable boiling in multiple parallel microchannels were systematically conducted. In the experimental aspect, characteristics of the unstable flow boiling in the microchannels were experimentally investigated using acetone as the working fluid and flow boiling processes were visualized with a high speed video camera simultaneously. The test section includes 16 parallel microchannels having a hydraulic diameter of 104.3 µm. The test mass flux ranged from 437.2 to 868.1 kg/m2 s and the test heat flux ranged from 0 to 100 W/cm2. The measured flow boiling heat transfer coefficients and two-phase frictional pressure drops are analyzed according to the observed flow phenomena and mechanisms. They were further compared to the existing flow boiling heat transfer and two-phase pressure drop prediction methods. The top three flow boiling heat transfer methods predict more than 65% of the heat transfer coefficients within ±30% while the top two pressure drop methods only predicted 43% of the measured data within ±30%. It shows that the continuous two-phase instable boiling mainly occurs at the mass flux larger than 607.6 kg/m2 s and the heat flux larger than 30 W/cm2. The backflow causes instability of the liquid and the two-phase interface which change the thermal resistances of the fluid. In the theoretical aspect, a dynamic simulation model was developed by using the thermal network method and was used to simulate the dynamic process of the instable boiling process. The simulated results were compared to the experimental data in this study and from the literature and the relative errors are within ±24.4%. According to the simulation and analysis, the mechanisms are that the axial thermal conduction in the channel walls generates a negative differential zone in the heat power characteristic curves, where the wall temperature decreases with increasing the heat flux in the negative differential zone. In order to suppress the oscillations in the boiling process, it is essential to eliminate the negative differential zone. Enhancing single phase forced convection heat transfer by 100% or enhancing flow boiling heat transfer by 50% can suppress the continuous two-phase instable boiling in the microchannels. Elongating the microchannel length and enhancing the axis thermal conduction can reduce the amplitudes of the oscillations by 60–80% at the conditions of G = 700 kg/m2 s, qeff = 70 W/cm2 and Tin = 30 °C.