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along the measure zone over the heater as a function of the
dimensionless heat flux at two different volumetric fluxes.
As previously mentioned, the larger the heat flux, the thicker
the film. But also as the volumetric flux increases, so does
the average film thickness over the heater.
Dimensionless average film thickness
20
18
16
14
12
10
8
6
4
2
0
0.00 0.05 0.10 0.15 0.20 0.25 0.30
Dimensionless heat flux
Q"=0.026 (m3/(m2s))
3 /m 2·s)
Q"=0.021 (m3/(m2s))
3 /m 2·s)
Figure 10. Dimensionless average film thickness as a function of the
dimensionless heat flux.
In Figures 11 and 12 the dimensionless average film
thickness and the Nusselt number curves are shown as a
function of the dimensionless heat flux for two different
fixed volumetric fluxes. These plots have been made in order
to see the relation of both variables during the different heat
transfer regimes.
As expected, in the forced convection zone the
dimensionless average film thickness can be considered
constant. This is because in this zone there are no vapour
bubbles in the liquid film.
After the forced convection regime, the increase in the
liquid film thickness is due to the nucleate boiling over the
heater. Figures 11 and 12 show that the nucleate boiling
regime could be divided into three zones considering the
variation of the average film thickness and the Nusselt
number with the heat flux.
In the first zone of the nucleate boiling regime, the sites
over the heater where the vapour nucleation takes place
increase as the heat flux increases. Due to that in this zone
both the average film thickness and the Nusselt number
increases with the heat flux.
Dimensionless Film thickness
20
18
16
14
12
10
8
6
Forced convection
Incrase of
heat
transfer
coefficient
Q"=0.026 (m 3 /m 2· s)
Maximun heat
transfer
coefficient
Decrase of
heat
transfer
coefficient
14000
12000
10000
4
2000
2
Nucleate boiling
0
0
0.00 0.05 0.10 0.15 0.20 0.25 0.30
Dimensionless Heat Flux
Figure 11. Dimensionless average film thickness and Nusselt number as
functions of the dimensionless heat flux for a volumetric flux of 0.026
m 3 /m 2·s.
In the second zone of the nucleate boiling regime, the
amount of nucleation sites is the optimum. Here the Nusselt
8000
6000
4000
Nusselt number
7-9 October 2009, Leuven, Belgium
number is the maximum and the film thickness seems to be
constant.
Finally, before the CHF is reached, there is a third zone in
the nucleate boiling regime: as the heat flux increases, so
does the film thickness but, unlike the other zones, the
Nusselt number decreases. Since the heat flux increases
beyond the second zone it produces a more vigorous boiling
over the heater surface with more vapour generation and,
therefore, an increase of the film thickness and a decrease of
the heat transfer coefficient. The CHF is reached when the
vapour generated over the heater surface forms dry zones
where the heat transfer coefficient drops sharply and the
heater temperature shoots up.
Dimensionless Film thickness
20
18
16
14
12
10
8
6
4
2
Forced convection
Incrase of
heat
transfer
coefficient
Q"=0.021 (m 3 /m 2·s)
Maximun
heat transfer
coefficient
Nucleate boiling
0
0
0.00 0.05 0.10 0.15 0.20 0.25 0.30
Dimensionless Heat Flux
Decrase of
heat transfer
coefficient
14000
12000
10000
Figure 12. Dimensionless average film thickness and Nusselt number as
functions of the dimensionless heat flux for a volumetric flux of 0.021
m 3 /m 2·s.
C. Uncertainty analysis.
The uncertainties from the thermal experimental
measurements are the following: ± 1.12 ºC for the
temperatures; ± 0.8% of the reading in the voltage; ± 1.1% in
the reading of the refrigerant volumetric flow; and ± 0.04 bar
for the pressure. In the measurement of the local film
thickness the maximum uncertainty is ± 29.6% and the
average is ± 11%. In the heat flux values the maximum
uncertainty is ± 10.3% at the lowest voltage and the
minimum uncertainty is ± 2.1% at the highest voltage.
IV. CONCLUSIONS.
Spray cooling experiments with R134a have been carried
out with measurements of the thermal parameters and of the
film thickness.
The spray boiling curves obtained are consistent with the
results presented in [2] and [3] about the spray density: a
lighter spray has a higher efficiency than a denser spray.
Moreover, the values of the spray cooling efficiency in the
CHF point obtained in our experiments are in good
agreement with the results presented in [8].
Although the efficiency of a dense spray is lower than for
a light spray, its CHF is larger. This is because the increase
of the volumetric spray flux entails an increase in the liquid
forced convection heat transfer capacity.
A technique based on high speed photography (or video)
has been used to measure the refrigerant film thickness over
the heater surface.
It has been found that there is a relation between the
variation of the heat transfer coefficient and the refrigerant
8000
6000
4000
2000
Nusselt number
©EDA Publishing/THERMINIC 2009 184
ISBN: 978-2-35500-010-2