Bubbles Transient Growth in Saturation Boiling of PF-5060 Dielectric Liquid on Dimpled Cu Surfaces

Abstract

Investigated is the transient growth of vapor bubbles in saturation boiling of PF-5060 dielectric liquid on 10 × 10 mm, uniformly heated Cu surfaces with circular dimples, at an applied heat flux of 0.5 W/cm2. At such low heat flux, the surfaces are populated with growing discrete bubbles, emanating mostly from the manufactured dimples. The 300, 400, and 500 μm diameter and 200 μm deep dimples are manufactured in a triangular lattice with a pitch-to-diameter ratio of 2.0; thus, the total number of dimples increases with decreasing the dimple diameter. Captured video images of growing discrete bubbles at a speed of 210 frames per second (fps) confirm that the bubble diameter increases proportional to the square root of the growth time, and the bubble departure diameter and detachment frequency increase with increasing the dimple diameter. The total volumetric growth rate and diameter of the bubbles at departure increase with increasing the dimple diameter, ∼1.81, ∼4.75, and ∼8.2 mm3/s and ∼738 μm, ∼963 μm, and ∼1051 μm for the 300, 400, and 500 μm diameter dimples, respectively. The corresponding bubble detachment frequency is ∼8.6 Hz, ∼10.2 Hz, and ∼13.5 Hz, respectively. The fraction of the active dimples for bubble nucleation on the surfaces with 300, 400, and 500 μm dimples, at an applied heat flux of 0.5 W/cm2, is ∼0.85, ∼0.64, and ∼0.53, respectively. On these surfaces, the estimated bubble volume at departure is ∼0.21 mm3, ∼0.47 mm3, and ∼0.61 mm3, and the corresponding rate of energy removed by a single bubble is ∼1.99 mW, ∼5.24 mW, and ∼9.02 mW, respectively. These results help explain the measured enhancements in nucleate boiling and the critical heat flux (CHF) on the dimpled Cu surfaces.