Effects of orientation and heater length on critical heat flux from discrete and continuous heaters
Abstract
The effects of flow orientation on critical heat flux (CHF) on a series of nine in-line simulated microelectronic chips in Fluorinert FC-72 were investigated. The chips were subjected to coolant in upflow, downflow, or horizontal flow with the chips on the top or bottom walls of the channel with respect to gravity. Changes in angle of orientation affected CHF for velocities below 200 cm/s, with some chips reaching CHF at heat fluxes below the pool boiling and flooding-induced CHF values. Increased subcooling was found to slightly dampen this adverse effect of orientation. Critical heat flux was overwhelmingly caused by localized dryout of the chip surface. However, during the low velocity downflow tests, low CHF values were measured because of liquid blockage by vapor counterflow and vapor stagnation in the channel. At the horizontal orientation with downward-facing chips, vapor/liquid stratification also yielded low CHF values. Previously derived correlations for water and long, continuous heaters had limited success in predicting CHF for the present discontinuous heater configuration. Because orientation has a profound effect on the hydrodynamics of two-phase flow and, consequently, on CHF for small inlet velocities, it is shown that downflow angles should be avoided, or when other constraints force the usage of downflow angles, the inlet liquid velocity should be sufficiently large. A critical heat flux model is presented that accounts for both heater length and orientation effects in near-saturated flow. Formulation of the model was based on flow visualization and photomicrography of the vapor-liquid interface on 10, 30, and 110-mm long heaters subjected to vertical, inclined, and horizontal flow. The photographs revealed the formation of a wavy vapor layer on the heater prior to CHF with surface wetting occurring at the wave troughs. The distance between these troughs increased in the stream-wise direction causing a reduction in the number of wetting fronts available for liquid replenishment of the heated surface. Lift off of the most upstream wetting front was found to catastrophically cause CHF. The model predicts the CHF data with a mean absolute error of 14.6%.
Degree
Ph.D.
Advisors
Mudawar, Purdue University.
Subject Area
Mechanical engineering
Off-Campus Purdue Users:
To access this dissertation, please log in to our
proxy server.