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

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

Osmotic Virial Coefficients of Hydroxyethyl Starch from Aqueous Hydroxyethyl Starch–Sodium Chloride Vapor Pressure Osmometry

Hydroxyethyl starch (HES) is an important industrial additive in the paper, textile, food, and cosmetic industries and has been shown to be an effective cryoprotectant for red blood cells; however, little is known about its thermodynamic solution properties. In many applications, in particular those in biology, HES is used in an aqueous solution with sodium chloride (NaCl). The osmotic virial solution thermodynamics approach accurately captures the dependence of osmolality on molality for many types of solutes in aqueous systems, including electrolytes, sugars, alcohols, proteins, and starches. Elliott et al. proposed mixing rules for the osmotic virial equation to be used for osmolality of multisolute aqueous solutions [Elliott, J. A. W.; et al. <i>J. Phys. Chem. B</i> <b>2007</b>, <i>111</i>, 1775–1785] and recently applied this approach to the fitting of one set of aqueous HES–NaCl solution data reported by Jochem and Körber [<i>Cryobiology</i> <b>1987</b>, <i>24</i>, 513–536], indicating that the HES osmotic virial coefficients are dependent on HES-to-NaCl mass ratios. The current study reports new aqueous HES–NaCl vapor pressure osmometry data which are analyzed using the osmotic virial equation. HES modifications were measured after dialysis (membrane cut off: 10 000 g/mol) and freeze-drying using vapor pressure osmometry at different mass ratios of HES to NaCl for HES up to 50% and NaCl up to 25% with three different HES modifications (weight average molecular weights [g/mol]/degree of substitution: 40 000/0.5; 200 000/0.5; 450 000/0.7). Equations were then fit to the data to provide a model for HES osmotic virial coefficient dependence on mass ratio of HES to NaCl. The osmolality data of the three HES modifications were accurately described over a broad range of HES-to-NaCl mass ratios using only four parameters, illustrating the power of the osmotic virial approach in analyzing complex data sets. As expected, the second osmotic virial coefficients increase with molecular weight of the HES and increase with HES-to-NaCl mass ratio