Multiphase Science and Technology

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

ABSTRACT

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/cm². 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/cm²· 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/cm² ). 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 × 10⁴ sites/cm².