The Lewis Number Under Supercritical Conditions

An effective Lewis number has been defined for two-fluid mixtures under conditions of (1) supercritical temperature and pressure and (2) large gradients of temperature and composition. The Lewis number is a measure of the ratio between characteristic lengths for diffusion of heat and diffusion of mass. The traditional definition of the Lewis number for a fluid is straightforward under subcritical conditions, in which the molar flux depends only on mole-fraction gradients and the heat flux depends only on the temperature gradient. Under supercritical conditions, the traditional definition of the Lewis number does not account for additional heat- and mass-transfer effects and thus leads to inaccurate estimates of heat- and mass-transfer scales. Accurate estimates of these scales are needed for designing combustors that operate under supercritical conditions; for example, combustors in rocket, gas turbine, and Diesel engines.