Influence of sensor orientations on the thermal conductivity measurements of liquids by transient hot disk technique

Abstract

Using transient hot disk technique to measure the thermal conductivity of liquids, the sensor is traditionally placed in the vertical orientation. In this study, the horizontal orientation method was analyzed. Effects of convection were studied and compared when using both the vertical and the horizontal orientation transient hot disk methods to measure the thermal conductivities of liquids. The temperature field and the velocity field of the testing fluid in both methods were obtained when numerical simulations were utilized to get quantified results of convection effects. Moreover, experiments were executed to validate the numerical simulation settings. The velocity magnitude and the Rayleigh number distribution were monitored in the direction which was perpendicular to the sensor face. It was found that the horizontal method had a lower maximum velocity and convection intensity. Besides, the testing region which was affected by heating process was smaller in horizontal method. The temperature distribution in horizontal orientation method showed asymmetry around the sensor face, the heat transfer ability of up face was higher than the down face. Meanwhile, the flow intensity in the horizontal orientation method grew faster than the vertical orientation method with long testing time. Judging from the average temperature rise of the sensor and the flow intensity in the fluid region, it is proposed that the horizontal orientation method is a better way for thermal conductivity measurements of liquids when testing time is not too long. Experiments were performed to measure the thermal conductivities of water, ethanol and glycol in both vertical and horizontal methods. It was found that the thermal conductivities measured in horizontal method were more accurate than the vertical method for all three liquid samples compared with the values in the literature. For ethanol, the relative error in the horizontal method is 11.6 % lower than the results in the vertical method.