Local Measurement and Characterization Via Fluorescing Materials For Phase Change Heat Transfer Applications

Abstract

Better understanding of phase change phenomena can be obtained through local measurements of the heat transfer process, which cannot be attained by traditional thermocouple point measurements. Infrared (IR) technology, which has been used by many researchers in the past, cannot be used under certain circumstances due to spectral transparency issues present in some materials. In the current study, the optical properties of fluorescing materials are proposed as a novel tool for heat transfer measurements. Two fluorescing materials were examined within the framework of the current dissertation: Namely Quantum dots and Ruthenium based temperature sensitive paint, which tend to fluoresce upon excitation by blue or Ultraviolet (UV) light. The light intensity emitted by those fluorescing materials tends to drop with temperature, which can be utilized to obtain the surface temperature distribution at a pixel resolution, for a given monochromic camera. Advantages of the fluorescing materials include feasibility, applicability to various surface geometries, and the ability to resolve submicron features. The main objective behind the current research work was to develop and assess the optical measurement technique of fluorescing materials, where phase change heat transfer applications, including ethanol drop evaporation and pool boiling, were used to quantify the advantages and limitations of the current temperature measurement technique. Furthermore, a thermofluid study was conducted in order to examine the mechanism of rapid vapor patch formation near critical heat flux (CHF) conditions. Results from the current research work show a correlation between the fluid velocity gradient near the wall and surface heat flux, where both tend to follow similar trend with surface super heat. Thus, it’s believed that the incomplete wetting of previous vapor patches near CHF is associated with restricted capillary motion near the surface, where the wetting liquid fails to reach the dry areas with the increased bubble generation activity, due to the local heating caused by the mushroom bubble ebullition.