Variable Property Effects in Momentum and Heat Transfer

Abstract

“Variable property effects” is a theoretical construct since a real fluid always is subject to variable properties when changes in temperature or pressure occur. Their influence compared to a corresponding situation but with artificially constant properties may be small and thus neglected in a first approximation. Those artificial “constant property results” may then be corrected with respect to the initially neglected effects due to the variability of the fluid properties. This concept assumes small variable property effects and therefore is not applicable when the flow itself is basically generated by a variable property (like natural convection, generated by density variations) or strongly affected by it (like strongly compressible flow, determined by density variations). Therefore we define “variable property effects” as those (small) artificial effects that would be present if fluid properties could change from constant to variable. They have to be added to a solution which is gained under the assumption of constant properties in order to account for the fact that real fluids always have properties which are temperature and pressure dependent. The variable property correction of a constant property solution can be accomplished in different ways. Basically there are three methods which are widely used in this context. They are