14-12-2012, 02:57 PM
Spray cooling heat transfer: The state of the art
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Abstract
Spray cooling is a technology of increasing interest for electronic cooling and other high heat flux applications, and is characterized by
high heat transfer, uniformity of heat removal, small fluid inventory, low droplet impact velocity, and no temperature overshoot. The
mechanisms by which heat is removed during spray cooling are poorly understood, however, due to its dependence on many parameters
that are not easily varied independently, and predictive capabilities are quite limited. This paper provides an introduction to spray cooling
for electronic cooling applications, reviews some proposed spray cooling heat transfer mechanisms, and summarizes the data regarding
the effects of non-condensable gas, surface enhancement, spray inclination, and gravity. Some models of spray cooling are also
presented.
Introduction
Future electronic systems and power electronics will
require increasing use of high heat flux removal technologies.
In a study on the limits of device scaling andswitching speeds,
Zhirnov et al. (2003) conclude that ‘‘even if entirely different
electron transport models are invented for digital logic, their
scalingfor densityandperformancemaynotgomuchbeyond
the ultimate limits obtainable with CMOS technology, due
primarily to limits on heat removal capacity’’. High heat flux
thermal designs are necessary to maintain lower operating
temperatures, which increases the reliability of components
andcanresult inhigherperformance.Somedesiredcharacteristics
of these technologies are low cost, minimal power input,
and adaptability to a wide range of heat fluxes. Use of liquid
cooling will become unavoidable as the power dissipation
levels increase in future electronic systems.
Nozzle types
Various types of sprays can be produced by specific nozzle
types. Hollow-cones sprays are typically produced by
forcing liquid tangentially into a swirl chamber or by
grooved vanes directly upstream of an orifice. The swirling
liquid exits the orifice as a ring of droplets. Full-cone
sprays are produced by forcing liquid through stationary
vanes that add turbulence. The shape of the spray can vary
from circular to square to oval.
Objectives
The purpose of this paper is to introduce the reader to
the capabilities of spray cooling for electronic cooling
applications, review current understanding of spray cooling
mechanisms, and to outline areas where additional research
is needed. Although heat transfer at high temperatures
where a vapor film forms between the hot wall and the
liquid has been studied extensively (e.g., for cooling of steel
in strip mills), this regime is generally not of interest to electronic
cooling and is not discussed in this paper.
Spray cooling of flat surfaces
The mechanisms by which heat is removed during spray
cooling are very complex due to its dependence on many
factors. The droplets produced by spray nozzles have
unique droplet size distributions, droplet number density,
and velocities that change with the liquid flow rate (pressure
drop across the nozzle) and nozzle geometry. Unfortunately,
it is very difficult to vary each of the above
parameters independently so their effects are not easily
measured. Other factors affecting spray cooling heat transfer
are impact angle, surface roughness, gas content, the
presence of other nozzles and walls, and heater surface
orientation.
Thin film evaporation
Pais et al. (1989) and Tilton (1989) suggested that the
high heat transfer observed in spray cooling was due to
the efficiency by which liquid molecules escape into the
vapor/ambient at the surface of a thin liquid film. The suggested
a thin liquid layer forms on the heated surface
through which heat is conducted. Because the top of the
film is assumed to be at the saturation temperature, thinner
films result in higher heat transfer. Large heat transfer at
small superheats requires the existence of an ultrathin
liquid film on the surface. For example, a 1.4 lm thick
layer of water is required to transfer 1000 W/cm2 of heat
at a superheat of 20 C. Their analytical model suggested
that the optimum heat transfer would occur by using the
smallest possible droplets and the highest percentage of
surface saturation to obtain the thinnest liquid film. They
also suggested that the impact velocity should be carefully
chosen such the maximum droplet spread is achieved without
droplet rebound from the surface.
Summary of flat plate spray cooling
The complexity of the spray cooling process has resulted
in significant disagreement regarding the fundamental
mechanisms of spray cooling heat transfer. Although it is
evident that single-phase heat transfer is dominant at low
temperatures, the relationship between the spray parameters
and the heat flux is not understood. Reliable correlations
based on spray parameters are needed. The
partitioning of energy between single-phase and two-phase
mechanisms at higher wall temperatures and the mechanisms
by which CHF occurs also needs further
investigation.