Bubble dynamics under an impinging planar water jet

Abstract

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.

The limiting factor in many industries is the maximum operating temperature and/or the maximum heat flux to be dissipated from the surface. Liquid Jet Impinging Cooling (LJIC) is one of the most effective cooling means because of its high heat transfer coefficient. LJIC is extensively used in steel quenching, electronic chips cooling and emergency rapid core cooling in nuclear reactors. Though, the surface heat flux has no straight forward formula. The present study adapts the mechanistic modeling approach to quantify the surface heat flux under different conditions. The mechanistic model assumes that the heat is transferred from the surface through multiple mechanisms, namely: boiling, forced convection and transient conduction. The total wall heat flux is calculated as the algebraic sum of these different heat flux components. The boiling heat transfer component depends on bubbles behaviour on the surface (bubble dynamics). Bubble growth rate, departure diameter, release frequency and number of bubbles on the surface are the major bubble dynamics parameters that are affected by the jet. This paper presents the result of an experimental investigation of bubble dynamics under a planar water jet. The experimental data are collected using high speed imaging of the boiling process at different degrees of superheating. Results reported here are for a 0.85 m/s jet. The jet was found to suppress nucleation close to the impinging zone and deform growing bubble from the spherical shaper. Away from stagnation, bubble diameter was found to depend on the square root of growth time, i.e. D~tg^1/2