Modeling and CFD simulation of an isothermal heat flow microcalorimeter

Abstract

The paper deals with the modelling and simulation of an isothermal heat flow microcalorimeter that is able to measure heat flows of several μW in microliter volume test samples. The microcalorimeter model is obtained and analyzed using the computational fluid dynamics (CFD) techniques. The only unknown model parameter is a thin layer of thermal grease between the microcalorimeter rim and the aluminum block that is used to stabilize the reference temperature. The thickness of the thermal grease layer cannot be physically measured, but has a significant effect on the microcalorimeter sensitivity. To determine the thickness of the thermal grease layer, a calibration procedure that involves two numerical environments (ANSYS Fluent and Simplorer) is proposed. A detailed, finite volume model of the microcalorimeter is created in ANSYS Fluent and the time-varying boundary conditions are defined in Simplorer. Finally, the calibrated model is used to study the effects of the thickness of the thermal grease layer or air gap on the microcalorimeter sensitivity and its dynamical behavior. The results clearly show that even a small change in the thickness of the thermal grease layer can have a significant effect on the microcalorimeter sensitivity and its response speed.