This study concerns the theoretical modeling of critical heat flux (CHF) in
reduced gravity flow boiling. Throughout the study, the near-wall conditions at heat fluxes
approaching and exceeding CHF will be examined with the aid of photomicrography. The proposed
work will aim for (a) developing a test apparatus for exploring flow boiling and CHF in
reduced gravity, (b) developing photographic methods for investigating vapor layer interfacial
instabilities, (c) constructing a phenomenological model for flow boiling CHF in reduced gravity,
and (d) providing detailed plans for testing of the same hardware in a future Space
Shuttle follow-up study.
During the initial stages of the study, experiments will be performed with
vertical upflow at 1g to insure proper operation of the flow module, test heater, instrumentation, and
flow loop, and to validate previous observations of CHF for vertical upflow. Experiments will then
be repeated for different flow orientations to explore any differences in the CHF trigger
mechanism in the presence of a smaller body force perpendicular to the heated wall. Finally, the
test apparatus will be prepared for future microgravity experiments onboard the Space Shuttle during
a follow-up study.
Much of the rationale for the CHF modeling is based upon recent work at Purdue
University which has successfully yielded a theoretically-based model for flow boiling CHF,
albeit for vertical upflow at 1g. High-speed motion analysis revealed bubbles coalesce into
a wavy vapor layer at fluxes well below CHF. Hydrodynamic instability of the vapor layer
interface facilitates bulk liquid contact with the wall, painting a thin liquid sub-film upon the
wall. The sub-film is consumed by a combination of vigorous boiling and interfacial evaporation. CHF
commences when the momentum of the vapor issued from the sub-film exceeds the pressure
force exerted upon the perturbed interface in the regions of interfacial contact with the
wall. This causes the wavy interface to lift away from the wall, precluding any sustained wetting and
resulting in wall dryout.
The role of interfacial instability is paramount to establishing the necessary
conditions for the dryout. Since interfacial wavelength is sensitive to surface tension forces,
inertia, and body force, the proposed experiments will help isolate the individual effects of
these forces by measuring the instability features (mainly wavelength and amplitude) at
different flow velocities and different orientations. At high g's, interfacial instability is dominated by
a balance between surface tension and body forces (Taylor instability). On the other hand,
instabilities below 1g are far more complex, exhibiting sensitivity even to minute body forces for small
flow velocities, but becoming completely dominated by a balance between surface tension forces and
inertia (Helmholtz instability) for high velocities.
Particular attention will be given in this study to (a) the sensitivity of
interfacial instabilities at low flow velocities to small body forces, and (b) the nonlinear attributes of
interfacial instability such as vapor wave stream-wise stretching and merging of adjacent waves. This
information will be used to construct a phenomenological CHF model applicable to reduced
gravity conditions which can be used as a design tool to guard against burnout of
electronic and power devices in space thermal management systems.
Mudawar, I., Zhang, H., Investigation of Critical Heat Flux in Reduced Gravity using Photomicrographic Techniques, Proceedings of the Fifth Microgravity Fluid Physics and Transport Phenomena Conference, NASA Glenn Research Center, Cleveland, OH, CP-2000-210470, pp. 1445-1447, August 9, 2000.