Effect of the Thermocouple on Measuring the Temperature Discontinuity at a Liquid–Vapor Interface

Abstract

The coupled heat and mass transfer that occurs in evaporation is of interest in a large number of fields such as evaporative cooling, distillation, drying, coating, printing, crystallization, welding, atmospheric processes, and pool fires. The temperature jump that occurs at an evaporating interface is of central importance to understanding this complex process. Over the past three decades, thermocouples have been widely used to measure the interfacial temperature jumps at a liquid-vapor interface during evaporation. However, the reliability of these measurements has not been investigated so far. In this study, a numerical simulation of a thermocouple when it measures the interfacial temperatures at a liquid-vapor interface is conducted to understand the possible effects of the thermocouple on the measured temperature and features in the temperature profile. The differential equations of heat transfer in the solid and fluids as well as the momentum transfer in the fluids are coupled together and solved numerically subject to appropriate boundary conditions between the solid and fluids. The results of the numerical simulation showed that while thermocouples can measure the interfacial temperatures in the liquid correctly, they fail to read the actual interfacial temperatures in the vapor. As the results of our numerical study suggest, the temperature jumps at a liquid-vapor interface measured experimentally by using a thermocouple are larger than what really exists at the interface. For a typical experimental study of evaporation of water at low pressure, it was found that the temperature jumps measured by a thermocouple are overestimated by almost 50%. However, the revised temperature jumps are still in agreement with the statistical rate theory of interfacial transport. As well as addressing the specific application of the liquid-vapor temperature jump, this paper provides significant insight into the role that heat transfer plays in the operation of thermocouples in general.

The influence of container geometry and thermal conductivity on evaporation of water at low pressures

“…The temperature jumps were larger at lower pressures and were strongly dependent on the heat flux from the vapor side 26 , 27 . The measured temperature jumps were inconsistent with the prevailing kinetic theory of gases that would predict a 10–20 times smaller temperature jump under the same condition 27 , 28 . To tackle such a discrepancy, statistical rate theory (SRT) of evaporation was applied 29 , 30 .…”

“…If experiments are performed in the limit of evaporation that is mainly governed by heat transfer, as was the case in most of previous studies, the conclusions about these concepts may not be accurate since the interfacial phenomena have not played a role in evaporation from the interface. This is perhaps the reason that the SRT expression for evaporation flux agrees with a wide range of temperature jumps 28 , or that the evaporation coefficients obtained from the experiments are highly scattered between zero and one 49 .…”

“…Heat transfer in the fluids was assumed to take place by conduction and convection mechanisms and radiative heat transfer was ignored as discussed in the supporting information of ref. 28 . The heat fluxes were determined by Fourier’s law of conduction and the heat generated by viscous dissipation was neglected.…”

Temperature Discontinuity at an Evaporating Water Interface

“…Possible factors affecting the measurement of the interfacial temperature discontinuity, including radiation and evaporative cooling of the thermocouple bead, were closely examined and concluded to be negligible. Various scientists conducted these experiments and reported temperature discontinuity of 0.14–28 K. ,,− Although the majority of experiments indicated that the liquid side of the interface is colder than the vapor side, few experiments , showed the opposite direction of temperature discontinuity. This temperature discontinuity at an evaporating water interface remains still a mystery.…”

Single‐Material Graphene Thermocouples

“…A multitude of different types of thermocouples has been developed, covering a wide range of temperatures and work environments. [ 7–10 ] Typically, in order to achieve on‐chip thermometry with conventional thermocouples, two separate fabrication runs are required.…”