Thermoreflectance for the Instantaneous Measurement of Temperature at a Wall-Vapor Interface

Abstract

Annular flow is a two-phase internal flow regime consisting of a vapor core with entrained droplets, surrounded by a thin liquid film. This type of flow is very common in heat exchanger systems. At high heat flux, the liquid film may dry out, resulting in a sharp reduction in the heat transfer coefficient of the flow. During the transition to complete dryout, cyclic re-wetting and dryout events are observed. Our long term goal is to characterize the heat transfer during these cyclic dryout events and to better understand what triggers the transition to dryout completion. To characterize heat transfer during these cyclic dryout events, a non-invasive and instantaneous optical thermometry technique based on thermoreflectance was developed. This technique infers the temperature at the wall-vapor interface by measuring the reflectance of an incident laser beam—an optical quantity dependent on the vapor density and incident beam angle. By combining this technique with thermoreflectance at a wall-liquid interface and pressure measurements, the time-varying heat-transfer coefficient during cyclic dryout events was estimated. Due to the small change in refractive index (±0.0015) in the temperature and pressure range of interest, a highly rigid and accurate experimental setup has been designed to reduce noise in the measurements. Potential sources of error between experimental heat transfer coefficient data and a literature model were identified. The R245fa working fluid used in this work does not have well-documented optical properties. A refractometer was developed as part of this work to provide accurate refractive index values as a function of fluid density. The method measures the critical angle of an incident point source of light at the interface of a glass substrate and the fluid, at various fluid densities. The resulting data from a validation process of the refractometer using 2-propanol—a fluid with well-known optical properties—show the promise of this method. Several sources of error that may cause discrepancies with published data are discussed.