Nucleate boiling heat transfer during water–IN–oil emulsion drop impact onto a heated solid surface

Abstract

The phenomenon of collision of fuel droplets with the walls of the combustion chamber largely determines the formation of the air-fuel mixture. The main attention in the study is paid to establishing the influence of the hydrodynamic parameters of the drop-wall interaction on the heat transfer during nucleate boiling of a drop of an n-decane-based water-in-oil emulsion. A drop of emulsion impacts onto a quartz glass surface heated to a temperature from 140 °C to 260 °C at Weber numbers (We) from 20 to 1000 and Reynolds numbers (Re) from 250 to 8000. The contact (evaporation) time of liquid droplets is measured experimentally. As a result, it becomes possible to derive an empirical model of the contact time τc, taking into account the dimensionless temperature of the heat exchange surface θ, as well as the main driving forces of the drop-wall interaction (inertia, viscosity and surface tension) at Re from 40 to 3000 and We from 1 to 1085. A complex mechanism of nucleate boiling in an emulsion drop is proposed. It involves injecting an intense vapor flow into a growing bubble from a near-wall microlayer and from the entire surface of the bubble, which is in contact with the n-decane and microdrops of water. An empirical model is introduced to predict the heat flux density at the solid-liquid interface during the contact of emulsion drops with a heated surface at nucleate boiling, q″, and tested at Re from 50 to 8000 and We from 20 to 1000. The empirical models allow evaluating the considered characteristics of heat transfer during the formation of an air-fuel mixture in the combustion chamber of the engine, taking into account the effects of the nucleate boiling nonlinearity and the collision of drops with the wall.