Non-linear analyses of flow boiling in microchannels

Recently, four unstable boiling cases with different fluctuating amplitudes were observed in parallel silicon microchannels having a hydraulic diameter of 186 μm. These were: the liquid/two-phase alternating flow (LTAF) at two different heat fluxes, the continuous two-phase flow (CTF) at medium heat flux and medium mass flux, and the liquid/two-phase/vapor alternating flow (LTVAF) at high heat flux and low mass flux. In this paper, data of these unstable boiling cases are analyzed using the following methods: correlation coefficient, attractor reconstruction, correlation dimension and largest Lyapunov exponent. The processes responsible for appearance of chaotic oscillations in microchannels, such as nucleation, stability of bubbly flow, vapour core stability and vapour-phase flow stability, are discussed. It is shown that under certain conditions, the microchannels system works as a thermal oscillator. It was found that heat supplied to the microchannels increases the heating surface temperature while the appearance of the two-phase flow inside the channels decreases the heating surface temperature. The mechanism involving an increase in heating surface temperature is supported by phenomena of blocking the liquid flow in microchannels by the two-phase flow.