Flow boiling in a vertical diverging channel: Mesoscopic approach

Abstract

The present study aims to numerically investigate the flow boiling phenomenon inside a vertical diverging channel by using the Multi-Relaxation Time based Pseudopotential Lattice Boltzmann method. The effects of operational and geometrical parameters on the flow boiling characteristics in the diverging channel are explored in detail and compared with those in the straight channel with the same inlet width. The coalescence of bubbles and its effects on heat transfer under variable pitch distances between two discrete microheaters in the flow direction are examined thoroughly. The evolution of the flow pattern from bubbly to slug flow under variable wall superheats is performed by using multiple discrete microheaters to determine the critical wall superheat for the initiation of slug bubbles in both channels. The results indicate a lower critical superheat for the initiation of slug bubble formation in the straight channel than in the diverging channel. The simulation results suggest that the diverging channel exhibits a higher bubble growth rate with better heat transfer capability than the straight channel does and, for the range of diverging angles studied, the effects are more pronounced with an increase in the diverging angle. The bubble departure frequency and heat transfer are enhanced by increasing the wall superheat and flow rate. The current study also revealed that there is an optimum pitch distance for the maximum average heat flux under a given condition. The findings of the current study help to understand the evolution of thermo-hydrodynamics characteristics of flow boiling under variable geometrical and operational modifications, which helps to develop a better design strategy for cooling applications.