Thermal conductivity of liquids
Thw-l2 transient hot wire liquid thermal conductivity meter
In the last two decades, there have been contradictory claims in the literature about the behavior of apparent thermal conductivity of two- or three-phase structures containing nanometer-sized solid particles suspended in fluids. Many of the studies have handled these multiphase systems as if they were a single-phase fluid, and the thermodynamic characteristics of the systems have differed despite the fact that they were assigned the same name. The so-called nanofluids have been the subject of several studies using a range of different experimental techniques. The studies of seven of the simplest particulate/fluid systems: Cu, CuO, (hbox Al 2hbox O 3), and (hbox TiO 2) suspended in water and ethylene glycol are critically evaluated in this paper. The apparent thermal conductivity of the nanofluid shows no behavior that is unpredictable or inconsistent with a simple model of conduction in stationary, multiphase systems when results for exactly the same thermodynamic system are obtained with proven experimental techniques. The wider range of behavior recorded in the literature for these systems can thus be due to poor thermodynamic system characterization and/or the use of experimental techniques of unknown validity.
Thw thermal conductivity, thermal diffusivity testing of liquids
A fluid is a material that deforms (flows) continuously under an applied shear stress in physics. Liquids, gases, plasmas, and synthetic solids are all examples of fluids, which are a subset of the phases of matter. Thermal energy transfer is less efficient in the fluid state than in the solid state since the intermolecular spacing is much greater and the motion of the molecules is more random. As a result, the thermal conductivity of gases and liquids is usually lower than that of solids. Thermal conduction in liquids is caused by atomic or molecular diffusion. The diffusion of molecules from a higher energy level to a lower energy level causes thermal conduction in gases.
The kinetic theory of gases can be used to understand how temperature, pressure, and chemical species affect a gas’s thermal conductivity. In the absence of convection, air and other gases are normally strong insulators. As a result, many insulating materials (such as polystyrene) work by containing a large number of gas-filled pockets that prevent large-scale convection. The heat must be transmitted through several interfaces as the gas pocket and solid material alternate, resulting in a rapid decrease in heat transfer coefficient.
Basics of thermal conductivity measurement with lambda
The thermal conductivity of some common liquids at temperatures ranging from -25 to 100 °C is mentioned in this table. Both measurements are in watts per meter kelvin (W/m K). When an entry is given at a temperature above the usual boiling point of the material, the pressure is taken to be the saturation vapor pressure at that temperature.
1. T. E. Daubert, R. P. Danner, H. M. Sibul, and C. C. Stebbins, Physical and Thermodynamic Properties of Pure Compounds: Data Compilation, Taylor & Francis, Bristol, PA, extant 1994 (core with four supplements) (also available as database).
Thermal conductivity of liquids
Three concentric cylinders are used in this experiment. An electric heater is located in the inner cylinder (the heat source). Around the heat source, the test liquid or gas forms a second, thin cylinder. A heat sink is formed by the third cylinder, which is cooled by water. The entire setup is placed on a base plate with a detailed diagram of the experiment configuration.
Heat is transferred from the heat source to the heat sink by conduction through the test liquid or gas. The temperature of the inside and outside edges of the cylinder of test liquid or gas is measured by thermocouples.
The electric heater and thermocouples are connected to sockets on the Heat Transfer experiments base unit, which also serves as the source of cold water for the heat sink’s feed and drain. While the end caps of thermally insulating material minimize heat loss, students must first calibrate the equipment to account for heat losses and improve experiment accuracy. When switching from one fluid to another, one end cap may be removed to make the unit easier to clean.