Fine Structures Roll up in the Flow of Film Boiling at High Density Ratios

TL;DR

This study uses high-resolution simulations to analyze the complex flow structures, including interface roll-up and vortex shedding, in high-density ratio film boiling, revealing the influence of instabilities and flow dynamics.

Contribution

It provides detailed insights into the flow structures and instabilities in film boiling at high density ratios using advanced numerical methods.

Findings

Flow dominated by Rayleigh-Taylor and Kelvin-Helmholtz instabilities

Interface roll-up occurs at high density ratio (1000) but not at lower (100)

Vortex shedding and enstrophy spikes are observed during roll-up

Abstract

Film boiling has practical applications in the current technology including steam power plants, cooling of electronic devices and emergency cooling systems. A finite difference/front tracking method is used to simulate film boiling at high density ratios on a horizontal plate subject to a constant wall heat flux. The grid resolution is relatively high (768 grids per width of the domain). The flow is dominated by Rayleigh-Taylor instability as well as Kelvin-Helmholtz instability. The flow structure includes the roll up of the interface between the gas and liquid. This happens at high density ratio (1000) where the difference between the gas and the liquid velocities across the interface is large. The jump in tangential velocity is an order of magnitude smaller at a lower density ratio (100). Hence, there is no roll up at lower density ratio. The flow is also influenced by vortex development as a result of the baroclinic term in the vorticity transport equation. The density gradient is large at the interface at high density ratio which tends to increase the baroclinic term. The plot of pressure gradient and density gradient shows that they are not parallel in the roll up regions. As a result, vortices in small scales develop that shed in the gas phase. The plot of the enstrophy with time shows that it shows smooth variation that match for two grid resolutions, however at a specific time enstrophies become spiky, and they depart from each other at two grid resolutions. The spiky behavior of enstrophy is due to vortex shedding in the roll up region.