A novel method of measuring microtube wall temperatures by thermocouples to investigate sCO2 flows

Abstract

Experimental study on convective heat transfer through microtubes encounters many challenges such as measuring the wall temperature of the tube with a reasonable uncertainty, relatively high heat loss from the outer surface of the tube, and difficulty in measuring the fluid bulk temperature at the tube outlet. These issues become exacerbated with flow at low Reynolds numbers. The present study presents a new method to measure the tube wall temperature with a small solder cast where a thermocouple is embedded. The net heat flux to flows, determined with the heat loss on the outer surface and the axial heat conduction, is integrated to obtain the local mean enthalpy of fluid and then the fluid mean temperature along the tube. Thus, convection heat transfer coefficients are calculated without measuring the mean temperature at the outlet. The new method is successfully validated with laminar water flows through a horizontally configured microtube, 0.501 mm in inner diameter. All Nusselt numbers are found to lie between 4.36 and 4.36 + 10%. Finally, the method developed in this study is applied to supercritical CO2 flowing upward at 8.0 MPa through a microtube, 0.509 mm in inner diameter. For both high and low mass flow rates, a local maximum of the convection heat transfer coefficient is observed at a location where the film temperature is close to the pseudo-critical temperature. On the other hand, the second local maximum of the convection heat transfer coefficient appears only at low mass flow rates.