ISSN Online: 2377-424X
ISBN Print: 0-89116-559-2
International Heat Transfer Conference 8
THE MEASUREMENT OF THERMOPHYSICAL PROPERTIES
Resumo
Uncertainties in data for the thermophysical properties of materials can cause considerable error in heat transfer calculations. Measured data, in particular for the thermal conductivity of fluids, can often disagree by over 100 percent. Exact engineering calculations and highly successful theoretical results, for example, the Nusselt numbers for fully developed laminar flow in a pipe, are critically dependent upon the provision of thermophysical property data and hence are as "inexact" as the properties assumed know.
This lecture restricts itself to a consideration of those thermophysical properties of major significance in the determination of the heat transfer from surfaces surrounded by fluids. There are three such properties: thermal conductivity, thermal diffusivity, and viscosity. Of these the first is the most difficult to determine precisely since this property becomes infinite at the critical point and even at modest temperatures in organic fluids exhibits significant radiation emission characteristics. The measurement methods most widely used for thermal conductivity and diffusivity measurements in fluids are described along with their important sources of error. Consideration of the available measurements in a wide variety of fluids illustrates the need for a critical re-examination of existing data and further measurement and interpretive programmes, especially near fluid phase boundaries and the critical region for industrially important fluids.
This lecture restricts itself to a consideration of those thermophysical properties of major significance in the determination of the heat transfer from surfaces surrounded by fluids. There are three such properties: thermal conductivity, thermal diffusivity, and viscosity. Of these the first is the most difficult to determine precisely since this property becomes infinite at the critical point and even at modest temperatures in organic fluids exhibits significant radiation emission characteristics. The measurement methods most widely used for thermal conductivity and diffusivity measurements in fluids are described along with their important sources of error. Consideration of the available measurements in a wide variety of fluids illustrates the need for a critical re-examination of existing data and further measurement and interpretive programmes, especially near fluid phase boundaries and the critical region for industrially important fluids.