Online proceedings - EDA Publishing Association

7-9 October 2009, Leuven, Belgium
Heat transfer and film thickness measurements in a
closed loop spray cooling system with R134a
Eduardo Martínez-Galván, Juan Carlos Ramos, Raúl Antón
TECNUN - University of Navarra
Paseo de Manuel Lardizábal, 13
San Sebastián, Guipúzcoa 20018 Spain.
Björn Palm, Rahmatollah Khodabandeh
Royal Institute of Technology, KTH, Department of Energy Technology
Stockholm, Sweden.
Abstract - Experimental measurements in a spray cooling test
rig have been carried out for different heat fluxes in the heater
and different volumetric spray fluxes of the refrigerant. Results
of the thermal parameters and the sprayed refrigerant liquid
film thickness over the heater are presented. The film thickness
measurements have been made with a high speed camera
equipped with a long distance microscope. It has been found
that there is a relation between the variation of the heat
transfer coefficient and the film thickness along the spray
boiling curve.
I. INTRODUCTION
Spray cooling is a powerful heat transfer technique, due to
the combined effect of forced convection and nucleate
boiling, used to remove large amounts of heat keeping lower
operating temperature. Spray cooling could be applied in:
electronic cooling, combustion technology, to cool human
skin during laser therapy, metallurgical processes, etc.
Another mechanism which also combines forced
convection and phase change is jet cooling. Estes and
Mudawar [1] made a comparison between both these
techniques and found that jet cooling produces a larger
temperature gradient in the heater surface than spray cooling,
because the latter disperses the fluid over the heater more
uniformly than the former. They also show in their
experiments that for the same flow rate the spray cooling
technique achieves a larger critical heat flux (CHF) than that
achieved using the jet cooling technique. Finally, they also
found that the CHF had a larger dependence on sub-cooling
with jets than with sprays. In jet cooling, at low sub-cooling,
the great amount of vapour generated leads to separation of
the liquid film.
Some spray characteristics which have a large effect on
the CHF are the mean droplet density, the Sauter mean
diameter and the mean droplet velocity. The most efficient
combination of these parameters is a large value of the mean
droplet velocity with a small value of the Sauter mean
diameter and the mean droplet density [2], in other words,
sprays with low density, low droplet diameters and large
velocity would have a high CHF.
The experiments made by Estes and Mudawar [3] showed
spray cooling to be more efficient at a low Weber number,
which implies a light spray. Chen et al. [2] results are in
agreement with the ones from Estes and Mudawar [3].
However, the latter concluded that the volumetric flux is a
better parameter than the droplet velocity to characterize the
spray, because it takes into account the commutative effect
of multiple drops impingement.
Estes and Mudawar also concluded that in order to
increase the CHF there are three alternatives: increasing the
flow rate, increasing the sub-cooling or decreasing the drops
size.
There is a technique employed to measure the film
thickness which uses the diffraction phenomenon through
the liquid film, but the test heater must be transparent in
order to measure with a laser beam from the bottom of it [4].
As this technique is based in the refractive index, which is
temperature dependent, and in spray cooling there is a large
temperature gradient in the film, it could not be very suitable
to measure the film thickness.
Hsieh and Tien [5] also made film thickness
measurements in the non-boiling regime for R134a coolant
which ranged between 0.93 and 1.35 mm for spray mass
fluxes between 1.33 and 1.4 kg/(m 2·s). They also
characterized the drop velocity and velocity distribution
within the spray and found that the velocity reduction
through the centreline was similar to a conventional jet.
Another parameter that has an important effect on spray
cooling is the coolant sub-cooling. Hsieh et al. [6] made
some experiments with two liquids: R134a and water. The
Weber numbers (We) and the sub-cooling degree were the
parameters that they analyzed for several heat fluxes. They
found that We had a stronger effect over the spray cooling
performance than the sub-cooling degree.
In this paper, the results of thermal and film thickness
measurements in a spray cooling system with refrigerant
R134a are presented. This refrigerant has been selected by
its high dielectric properties, because the application of this
study is electronic cooling. The film thickness measurements
were motivated to understand better the heat transfer
mechanisms which take place in the spray cooling. The film
thickness has been measured directly from images obtained
by means of a high speed camera with a long distance
microscope. It has been found that there is a relation between
©EDA Publishing/THERMINIC 2009 180
ISBN: 978-2-35500-010-2