Investigation On Phase-Change Heat Transfer Characteristics Of Liquids In Minichannels

Experimental in on phase-change heat transfer characteristics in circular minichannels with three different hydraulic diameters(1.0 mm,1.6 mm and 2.0 mm) were carried out in a wide range of experimental conditions. The average pressure drop characteristics of single-phase flow and two-phase flow and the transient pressure drop characteristics in the aspects of the time-domain and the frequency-domain were studied. And a strong correlation were found between the flow pattern distribution and frequency-domain transient two-phase flow characteristics. The characteristic frequency of the bubbly flow is intermediate-frequency signal, which is related with the process of the bubble nucleation and growth.The characteristic frequency of the intermittent flow is a complex signal combined intermediate-frequency with high-frequency. The intermediate frequency signal is thought to be dependent on bubble nucleation and growth as mentioned before and the high-frequency signal is thought to be dependent on the process of bubbles merger and collision. What is more, the characteristic frequency of the annular flow is low-frequency low-amplitude signal, which appears when the dry-out point occurs at the thin film area. The low-frequency high-amplitude signal is considered to be the characteristic frequency of the dry-out region, which is mainly corresponded to the periodical process of backflow and refill. The influence of heat flux,mass flux and the channel diameter on the boiling curve and the local heat transfer coefficient were studied. The boiling curve was divided into single-phase heat transfer area, nucleate boiling dominant region and forced convection evaporation dominant region according to the heat transfer mechanism. Furthermore, a strong correlation between the flow patterns of distribution and local heat transfer coefficient was found.A VOF method was used in 1 mm channel phase-change heat transfer numerical simulation to obtain the effect of heat flux on the phase-change heat transfer process. The analysis on the flow field of the process of the bubble merger and growth were conducted. The recirculation zone and vortex structures were discovered around the bubble, which is considered to be the consequence of the momentum exchange of the gas-liquid interface.