An all-pressure fluid drop model applied to a binary mixture: heptane in nitrogen

The differences between subcritical liquid drop and supercritical fluid dro p behavior are shown to be a direct consequence of the length scales near t he fluid drop boundary. Under subcritical, evaporative high emission rate c onditions, a him layer is present in the inner part of the drop surface whi ch contributes to the unique determination of the boundary conditions; it i s this film layer in conjunction with evaporation which gives to the soluti on its convective-diffusive character. In contrast, under supercritical con ditions the boundary conditions contain a degree of arbitrariness due to th e absence of a physical surface, and the solution has then a purely diffusi ve character. Results from simulations of a free fluid drop under no-gravit y conditions are compared to microgravity experimental data from suspended, large drop experiments at high, low and intermediary temperatures and in a range of pressures encompassing the sub- and supercritical regime. Despite the difference between the conditions of the simulations and the experimen ts, the time rate of variation of the drop diameter square is remarkably we ll predicted in the linear curve regime. Consistent with the optical measur ements, in the simulations the drop diameter is determined from the locatio n of the maximum density gradient. Detailed time-wise comparisons between s imulations and data show that this location is very well predicted at 0.1 M Pa. As the pressure increases, the data and simulations agreement becomes g ood to fair, and the possible reasons for this discrepancy are discussed. S imulations are further conducted for a small drop, such as that encountered in practical applications, over a wide range of specified, constant far fi eld pressures. Additionally, a transient pressure simulation crossing the c ritical point is also conducted. Results from these simulations are analyze d and major differences between the sub- and supercritical behavior are exp lained. In particular, it is shown that the classical calculation of the Le wis number gives erroneous results at supercritical conditions, and that an effective Lewis number previously defined gives correct estimates of the l ength scales for heat and mass transfer at all pressures. (C) 2000 Publishe d by Elsevier Science Ltd.