Two-phase Transport Model Research Articles

Two-phase flow in small-scale ribbed and finned passages for compact evaporators and condensers

Complex small-scale finned and ribbed passages are being used with increasing frequency in compact high-performance evaporators and condensers. The complexity and small scale of these passages causes the flow and transport mechanisms to differ substantially from those in larger simpler tubes more commonly used in power, process and refrigeration applications. Special features of two-phase and transport in ribbed and finned passages are described. Results of recent investigations which have illuminated some of the very different mechanisms that arise in these circumstances are also summarized. The effects of liquid shedding from discontinuous structures, the strong role of surface tension forces, and the existence of recirculation zones and lateral nonuniformity of the flow are especially important in these small-scale passages. The impact of these features on modeling of two-phase transport in evaporators and condensers is also discussed.

Gas sorption and transport in semicrystalline poly(4-methyl-1-pentene)

The solubility of CO 2 and CH 4 plus permeability coefficients for these and other gases were measured for a series of poly(4-methyl-1-pentene) polymers with crystallinity levels ranging from 20% to 75%. The crystallinity was varied through thermal treatment and by addition of a comonomer that permitted thermally-induced crosslinking. An extrapolation, assuming a two-phase model, of the sorption results to 100% crystallinity indicates a finite solubility for CO 2 and CH 4 in the crystal (about 25–30% of the solubility for the amorphous phase). Regression analysis of the permeation data using a two-phase transport model indicated that diffusion occurs at finite rates in the crystal. This is contrary to the usual situation for polymers like polyethylene but is consistent with the unusual fact that the density of the poly(4-methyl-1-pentene) crystal is slightly less than that of the amorphous phase. The diffusion rate in the crystal appears to become nil as the dimensions of the gas molecules exceed the estimated gap sizes between chains in the crystal.