SATURATION BOILING ON MPC: EFFECTS OF THICKNESS, INCLINATION ANGLE, TRANSIENT BUBBLE GROWTH, AND NUCLEATION SITE DENSITY

摘要

Saturation pool boiling experiments of PF-5060 dielectric on micro-porous copper (MPC) near atmospheric pressure (～0.085 MPa) have investigated the effects of the inclination angle (θ) and thickness (δ_(MPC)) on the nucleate boiling (NB) heat transfer coefficient (h_(NB)) and critical heat flux (CHF). In the experiments, δ_(MPC) varied from 80 to 230 μm and θ varied from 0° (upward facing) to 180° (downward facing). In addition to enhancing h_(NB), there was no (or a slight) excursion in the surface temperature (＜5 K) prior to boiling incipience. Increasing δ_(MPC) from 95 to 230 μm increased the CHF from 22 to 26 W/cm~2. The highest [maximum nucleate boiling (MNB)] heat transfer coefficient (h_(MNB)) near the end of fully developed NB region occurred at lower superheat than at the CHF. Both h_(NB) and h_(MNB) increased with increasing δ_(MPC), peaking at ～171 μm, and then decreased with a further increase in δ_(MPC). They were highest in the upward-facing orientation. The h_(MNB) value (up to ～7.8 W/cm~2•K) decreased slowly with increasing θ to 90°, and then decreased faster to the lowest value at 180°. In the upward-facing orientation h_(NB) was correlated in terms of the applied heat flux (h_(NB) = Aq~B); coefficient A increased linearly from 0.427 to 1.143 as δ_(MPC) increased from 80 to 230 μm, while B increased from 0.522 to a peak of 0.686 at δ_(MPC) ～ 162 μm, and then decreased to 0.573 at δ_(MPC) = 230 μm. The h_(NB) correlation agreed with the data to within ±12%. Also developed was a correlation for the transient bubble growth on MPC at low heat flux (＜0.48 W/cm~2). This correlation agreed to within ±5% with measurements from footings recorded using a video camera at 210 frames per second. The bubble's departure diameter (D_d = 431 ± 7 μm) and detachment frequency (f_d = 36 ± 2 Hz) in the upward-facing orientation were independent of δ_(MPC). The active nucleation site densities on MPC, based on these values of D_d and f_d and the boiling curves in the experiments, ranged from ～725 to 1.07 × 10~4 sites/cm~2.