SUBCOOLED BOILING OF HFE-7100 ... - Eurotherm 2008

5th European Thermal-Sciences Conference, The Netherlands, 2008
region (III), as the bubbles continue to coalesce and grow larger near the heated surface, the boiling
heat transfer rate progressively decreases and the surface temperature increases, as indicated by the
gradually decreasing slope of the boiling curves in region III. Eventually, the formation of batches
of vapour on and near the heated surface causes a surge in surface temperature by as much as 45 K,
marking CHF; CHF is indicated by the last points on the boiling curves.
Figure 2: Boiling curves on plane Cu and Cu with 5 mm tall corner pins for subcooled HFE-7100.
4.1 Natural convection
Figure 2 shows the natural convection data on plane surfaces and Cu surfaces with corner pins.
Since the 10 x 10 mm footprint area is uniformly heated, the removed thermal power by natural
convection, PNC, is proportional to the surface superheat, ΔTp, raised to the 1.2 power. The results
in Figure 2 show that the heat removal rate by natural convection from the surfaces with 3 x 3 mm
corner pins is on average 67.5% higher than from plane surfaces having the same foot print area (10
x 10 mm) at the same surface superheat, ΔTp. These results are important to the cooling of
computer chips in the standby mode, in which the thermal power dissipation is typically < 3 W/cm 2 .
At such heat fluxes, the surfaces with corner pins will be ~ 20 K cooler than plane surfaces.
Figure 3: Natural convection data and correlations for plane Cu and Cu with 3 x 3 mm corner pins.
4.2 Effect of liquid subcooling and inclination angle
The obtained boiling curves for θ = 0 o are shown in Figures 4a – 4d for saturation and 10 K, 20 K,
and 30 K subcooling. The CHF values increase with increased subcooling and so does the
corresponding surfaces temperature. For the surface with 5 mm pins, CHF is as much as 93 W/cm 2