Online proceedings - EDA Publishing Association

The highest Nusselt number was obtained with a
volumetric flux of 0.021 m 3 /m 2·s, as can be seen in Figure 6.
Apparently at 0.021 m 3 /m 2·s the combination of liquid phase
forced convection and nucleate boiling is the optimum. That
is, for the same heat flux applied the Nusselt number is the
maximum.
Nusselt number
14000
12000
10000
8000
6000
4000
2000
0
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35
Dimensionless heat flux
Q"=0.026 (m3/(m2s))
3 /m 2·s)
Q"=0.025 (m3/(m2s))
3 /m 2·s)
Q"=0.021 (m3/(m2s))
3 /m 2·s)
Q"=0.021 (m3/(m2s))
3 /m 2·s)
Q"=0.018 (m3/(m2s))
3 /m 2·s)
Figure 6. Nusselt number as a function of the dimensionless heat flux at
different volumetric spray fluxes.
7-9 October 2009, Leuven, Belgium
because the high density of the spray prevents distinguishing
the liquid film from the spray cone. Although the measure
zone is outside the spray cone, we suppose that the heat
transfer regimens which take place in the impact area are the
same in these bots areas.
The film thickness has been measured directly from
photographs taken with a high speed camera. All the images
were taken at 4000 frames per second with 448 x 448 pixels
of resolution. In order to have a good average of the film
thickness, 5 sets of samples have been taken at different
times, each of them having 1000 images.
A re-scaling of the pixel grey intensity has been made by
using the MATLAB function “imadjust” in every picture in
order to facilitate the film thickness definition. An algorithm
has also been implemented to select the heater and the film
thickness boundaries.
In Figure 8 an example is shown where the film thickness
boundary (yellow line) can be seen as well as a green line
which is the heater boundary. The heater edge has been
previously determined from a picture without spray.
B. Liquid film thickness measurement.
The sprayed refrigerant film thickness has been measured
in a zone just outside of the spray cone but over the square
heater, as represented schematically in Figure 7 (a) and
shown in the photograph in Figure 7 (b). In this picture it can
also be seen a reference target placed close to the heater in
order to know how many micrometers that corresponds to
each pixel.
Figure 8. Example of a photograph used to measure the film thickness: the
yellow line is the film thickness limit and the green line is the heater limit.
Figure 9 shows some results of the local film thickness for
different heat fluxes and a volumetric flux of 0.026 m 3 /m 2·s.
It can be seen that the film thickness increases from the
corner of the heater to the spray cone.
Figure 9 also shows that as the heat flux increases, so does
the film thickness, as expected because high heat fluxes
generate more vapour inside the film and therefore a larger
film thickness.
1200
(a)
1000
176 (W/cm 2 )
176 [W/cm2] (W/cm 2 )
147 [W/cm2] (W/cm 2 )
Film thickness ( μm)
800
600
400
124 [W/cm2] (W/cm 2 )
99 [W/cm2] (W/cm 2 )
80 [W/cm2] (W/cm 2 )
61 [W/cm2] (W/cm 2 )
45 [W/cm2] (W/cm 2 )
32 [W/cm2] (W/cm 2 )
21 [W/cm2] (W/cm 2 )
200
5 (W/cm 2 )
12 [W/cm2] (W/cm 2 )
5 [W/cm2] (W/cm 2 )
(b)
Figure 7. Measurement zone of film thickness: (a) sketch of heater base and
spray cone impact area; (b) picture of the spray cone impinging over the
heater.
The film thickness measurements were made in this zone
0
0 500 1000 1500 2000 2500 3000 3500 4000 4500
Distance from the corner of the heater to the spray cone (μm)
Figure 9. Local film thickness at 0.026 m 3 /m 2·s of volumetric flux for
different heat fluxes.
The dimensionless film thickness is the ratio between the
film thickness and the Sauter mean diameter, d 32 , calculated
with equation (5).
Figure 10 shows the dimensionless average film thickness
©EDA Publishing/THERMINIC 2009 183
ISBN: 978-2-35500-010-2