Note on a method of estimating the prandtl number of liquids

Abstract Ono of the dimensioluss numbers which outers into the theory of heat transfer in fluids is the Prandtl number. This is defined by the relation: where p is the specific heat at constant pressure, η is the viscosity and κ is the thermal conductivity of the fluid. In the case of gases a theorem of the kinetic theory shows that the modified Prandtl number, cvη/k, should be the same for all monatomie gases, and should have a numerical value of 0·4 independent of temperature. Actual values of this quantity lie between 0·4 and 0·6 for almost all gases, including those which are not monatomie. The true Prandtl number, cvη/k, is also nearly constant, and for almost all gases the values lie between 0·7 and 1·0. Among liquids, however, the values of the Prandtl number vary over a range from 3 to over 10,000. Unfortunately, reliable experimental data on cp, η and κ is available for only a small number of liquids, and this limits the extent to which equations, such as that of Dittus and Boeltor, can be used for the calculation of the film cooficient of heat transfer in liquids. During some work on industrial heat transfer the writer has examined the possibility of relating the Prandtl number with some other physical property whose value is much more generally known.