CRITERIA FOR LOCAL EQUILIBRIUM IN A SYSTEM WITH TRANSPORT OF HEAT ANDMASS

Nonequilibrium molecular dynamics is used to compute the coupled heat and mass transport in a binary isotope mixture of particles interactin g with a Lennard-Jones/spline potential. Two different stationary stat es are studied, one with a fixed internal energy flux and zero mass fl ux, and the other with a fixed diffusive mass flux and zero temperatur e gradient. Computations are made for one overall temperature, T=2, an d three overall number densities, n=0.1, 0.2, and 0.4. (All numerical values are given in reduced, Lennard-Jones units unless otherwise stat ed.) Temperature gradients are up to del T=0.09 and weight-fraction gr adients up to del w(1)=0.007. The flux-force relationships are found t o be linear over the entire range. All four transport coefficients (th e L-matrix) are determined and the Onsager reciprocal relationship for the off-diagonal coefficients is verified. Four different criteria ar e used to analyze the concept of local equilibrium in the nonequilibri um system. The local temperature fluctuation is found to be delta T ap proximate to 0.03T and of the same order as the maximum temperature di fference across the control volume, except near the cold boundary. A c omparison of the local potential energy, enthalpy, and pressure with t he corresponding equilibrium values al the same temperature, density, and composition also verifies that local equilibrium is established, e xcept near the boundaries of the system. The velocity contribution to the Boltzmann H-function agrees with its Maxwellian (equilibrium) valu e within 1%, except near the boundaries, where the deviation is up to 4%. Our results do not support the Eyring-type transport theory involv ing jumps across energy barriers; we find that its estimates for the h eat and mass fluxes are wrong by at least one order of magnitude.