Impact of Oxidation on Pool Boiling Heat Transfer Performance over Flat Plates Exposed to Extended Operating Conditions

Phase change heat transfer is utilized in a number of high heat flux applications. In order to ensure reliable functionality at moderate temperatures, one has to guarantee a stable long-term operation. In this experimental study, boiling heat transfer (BHT) performances of several substrates under continuous operation at pre-determined heat fluxes were studied. Tests were performed on bare copper surfaces in saturated deionized (DI) water and HFE-7100 under atmospheric conditions. Measurements were conducted at heat fluxes of 30 W/cm
²
and 60 W/cm
²
for DI water, whereas at 6 W/cm
²
and 12 W/cm
²
for HFE-7100. In order to identify the temporal change in surface conditions for each substrate, subsequently repeated heating tests were conducted before a 24-hour operation at constant heat flux. Besides the computation of heat transfer coefficients, contact angle (CA) measurements, high-resolution microscopic images, and scanning electron microscope (SEM) analyses were carried out to characterize the impact over surfaces. Microscopic images showed that the use of DI water leads to an intensified oxidization on the test surface. HFE-7100 does not allow oxide layer formation on the copper surfaces. Critical heat flux (CHF) at the surface operated only at 60 W/cm
²
in DI water increased, while the boiling curves shifted to the left by decreasing surface temperatures over time. The surfaces immersed in HFE-7100 showed a great consistency with preliminary tests on heat transfer and repeatability tests.