Nucleate boiling, especially near the critical heat flux (CHF), can provide
excellent economy along with high efficiency of heat transfer. However, the
performance
of nucleate boiling may deteriorate in a reduced gravity environment and the
nucleate
boiling usually has a potentially dangerous characteristic in CHF regime. That
is, any
slight overload can result in burnout of the boiling surface because the heat
transfer will
suddenly move into the film-boiling regime. Therefore, enhancement of nucleate
boiling
heat transfer becomes more important in reduced gravity environments. Enhancing
nucleate boiling and critical heat flux can be reached using micro-configured
metal-graphite
composites as the boiling surface. Thermocapillary force induced by temperature
difference between the graphite-fiber tips and the metal matrix, which is
independent of gravity, will play an important role in bubble detachment. Thus boiling heat
transfer performance does not deteriorate in a reduced-gravity environment.
Based on the existing experimental data, and a two-tier theoretical model,
correlation formulas are derived for nucleate boiling on the copper-graphite and
aluminum-graphite composite surfaces, in both the isolated and coalesced bubble
regimes. Experimental studies were performed on nucleate pool boiling of pentane
on cooper-graphite (Cu-Gr) and aluminum-graphite (Al-Gr) composite surfaces with
various fiber volume concentrations for heat fluxes up to 35 W/cm2. It is
revealed that a significant enhancement in boiling heat transfer performance on the composite
surfaces is achieved, due to the presence of micro-graphite fibers embedded in the matrix.
The onset of nucleate boiling (the isolated bubble regime) occurs at wall superheat of
about 10oC for the Cu-Gr surface and 15oC for the Al-Gr surface, much lower than
their respective pure metal surfaces. Transition from an isolated bubble regime to a coalesced
bubble regime in boiling occurs at a superheat of about 14oC on Cu-Gr
surface and 19oC on Al-Gr surface.
According to a two-tier configuration and its mathematical model [1], and based
on the existing experimental data, correlations for the boiling heat transfer
performance in the isolated bubble regime and in the coalesced bubble regime are obtained as
follows: The boiling heat flux in the low heat flux boiling region (isolated bubble
regime) is mainly contributed by micro bubbles, with negligible heat conduction across the
microlayer, and can be expressed as where Cs and m are constants determined by
the experimental data.
Zhang, N., Chao, D.F., Yang, W-J., Heat Transfer Performances of Pool Boiling on Metal-Graphite Composite Surfaces, Proceedings of the Fifth Microgravity Fluid Physics and Transport Phenomena Conference, NASA Glenn Research Center, Cleveland, OH, CP-2000-210470, pp. 1459-1461, August 9, 2000.