Pool boiling on vertical and horizontal heated plates with a dielectric fluid: Influence of vertical canal width

Abstract

A closed system testbench has been developed to study pool boiling with a dielectric fluid at atmospheric pressure. The experimental setup consists in immerged horizontal and vertical adjacent heated plates, representing the corner of a parallelepipedal heating element. Both plates are 64x40mm2. The plates can be heated separately or simultaneously, with surface heat flux ranging from 0 to 20 W/cm2. Additionally, the canal width in front of the vertical heated plate is varied from 0.5 to 15 mm to study the influence of confinement. The opposite wall of the canal is made of transparent glass to allow optical visualization of the boiling on the vertical plate. A condenser at the top of the system controls the fluid subcooling. The wall temperatures are measured in steady-state regime in the middle of the plates, heat transfer coefficients are also determined. This configuration leads to complex flows, from natural convection to film boiling, including nucleate boiling. For low heat flux, smaller vertical canal widths enhance natural convection mixed with phase change phenomena: this conveys significantly higher heat transfer coefficients than in the vertical free configuration case. At higher heat flux, confinement does not enhance heat exchange on the vertical plate. When the vertical plate is confined, bubble coalescence and critical heat flux occur for lower heat flux compared to the free configuration. Visual observations on boiling regimes on the vertical plate are given depending on confinement. Finally, simultaneous heating of both plates is compared to isolated plate heating: boiling on the vertical plate improves heat exchange on the horizontal one, whereas boiling on the horizontal plate has no effect on the vertical one.