The characterization of the thermal properties of fluids requires that two methods of heat transfer be taken into account; thermal conductivity and convection. Thermal conductivity transfers heat at a molecular level, while convection moves heat through the bulk movement of the fluid itself. Convection is the dominant force of heat transfer in fluids, and in order to measure solely the thermal conductivity of a fluid the effects of convection must be minimized. Convection occurs due to the density change that arises when fluid warms, the denser, colder fluid sinks while the warmer, lighter fluid rises, creating circulation in the sample. One way to completely suspend the effects of convection is to place the sample in a state of microgravity. Since gravity is the force that pulls the denser fluid down, convection will not occur in its absence.
Microgravity experiments have long been used to understand how materials used by astronauts will behave in space, but they also have provided valuable data in materials science on factors that are often difficult to measure in the presence of gravity. Nagai et al. (2000) from the Hokkaido National Research Institute in Japan performed an interesting project using the Hot Disk Transient Plane Source thermal conductivity system to measure the thermal conductivity of silicone oil and mercury in a normal environment and in a microgravity environment. The microgravity conditions were created using parabolic flight, two drop shafts, and a drop tower. This project is a fascinating example of the use of the TPS on liquids and in a gravity free situation, and indicates that the TPS is an excellent tool to determine thermal properties in microgravity research.
Figure 1. A research aircraft climbing in flight to simulate a microgravity environment. By flying the plane upwards in an arc formation and throttling back the engines at the top, the plane experiences around 20 seconds of freefall that are gravity free1.
Thermal Conductivity Measurement of Silicone Oils
Nagai et al. (2000) used four different silicone oils with varying viscosities. Thermal conductivity was measured by placing the Hot Disk disk sensor in air bubble free samples in a sealed container. A liquid sample cell is also available when using the Hot Disk TPS for measuring the thermal conductivity of liquids (Figure 2). This cell seals 2-3 ml of the fluid around the sensor, enabling measurements to take place on tiny volumes while obtaining the maximum probing depth. For microgravity experiments, the entire Hot Disk TPS system and sample container were placed in a capsule in the drop shafts/tower, or fixed into the plane used for parabolic flight. The researchers analyzed their data in minute detail, points were plotted representing each second of measurement.
Figure 2. Liquid cell sample holder available for use with the Hot Disk TPS.
Nagai et al. (2000) determined that the thermal conductivity results were the same at ground and microgravity conditions for the silicone oils with higher viscosities. The silicone oil with the lowest viscosity returned results that differed between the ground and microgravity experiments. The researchers concluded that this was due to thermal convection occurring in the sample, as it will occur more rapidly in low viscosity liquids. However, since the Hot Disk TPS is able to produce rapid, accurate results, short test times are able to measure the thermal conductivity of a sample effectively before convection kicks in.
Thermal Conductivity Measurement of Mercury
Nagai et al. (2000) used the same sample set up used for the silicone oil for mercury. The thermal conductivity of mercury was also measured over a temperature range. The researchers determined that convection did occur in measurements performed on the ground. Data measured in microgravity produced a thermal conductivity value that was 3% lower than in regular gravity conditions. Thermal conductivity increased with temperature.
This research illustrates the ability of the Hot Disk TPS to effectively measure the thermal conductivity of liquids prior to convection occurring due to a fast test time. This rapid testing capability is one of the factors that sets the TPS method apart in the field of thermal conductivity testing equipment as it enables data to be collected on materials that are difficult to measure. The capacity of the Hot Disk TPS to relay accurate measurements in microgravity situations that match literature values showcases the incredible range of applications this machine can be applied to; it is a versatile tool that can meet complicated testing needs.
Note: For comprehensive results and an in-depth discussion, please follow the link to the scientific paper in the reference section.