C-Therm Blog

High Temperature Thermal Conductivity Measurement of Liquids: Duratherm S

Heat transfer fluids are in wide use throughout a diverse array of industrial and recreational applications, from hot water in baseboard heaters to radiator fluid in vehicles to more exotic applications such as the alkali metal alloy used to cool some nuclear reactors. An ideal heat transfer fluid is chemically inert, non-toxic, and non-corrosive with a low viscosity, high thermal conductivity, high thermal effusivity, and high heat capacity. Depending on the application, thermal stability or electrical conductivity may be other important parameters to consider.
Silicone oils are a popular choice of heat transfer fluid for high-temperature applications. Generally, they have very high boiling points, enabling low-pressure use well beyond the boiling point of water. Silicone oils are non-flammable under typical conditions, unlike petroleum oils, which makes them safe for use at elevated temperatures in open air. They are also good lubricants and very good electrical insulators, which makes them attractive for a variety of electrical applications.
C-Therm’s TCi technology is proven both experimentally and theoretically to have high precision and accuracy in the measurement of liquids. We do this by minimizing convection effects through the use of a special low power setting and very short test times. To illustrate how the TCi technology, along with our optional thermal chamber accessory, may be used in assessing the properties of heat transfer fluids, we assessed a commercially-available high performance silicone oil-based heat-transfer fluid at various temperatures.
Figure 1. The C-Therm TCi sensor fitted to our high-temperature liquid cell
The heat-transfer fluid was filled to the desired level in our high-temperature liquid testing cell (Figure 1). The cell is sealed using an O-ring to eliminate the effects of evaporative coolingat very high temperatures by allowing the liquid to come to vapor-liquid equilibriumin a closed environment. In the case of flammable liquids, this sensor configuration also allows for the cell to be flushed with inert atmosphere prior to heating and testing, and thus mitigates risk of fire or explosion.  
Figure 2. Thermal Conductivity of a Commercially-available Heat-Transfer Fluid

Thermal conductivity was assessed simultaneously with effusivity from ambient to 300°C using a C-Therm TCi high-temperature sensor (Figure 2). Error bars showing the 5% acceptable accuracy are plotted on the reference series. In all cases, accuracy was better than 2.5%. 

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