Supercritical Pseudo-Boiling and its Relevance for Transcritical Injection

Abstract

In this paper, a new interpretation of cryogenic jet break-up in supercritical environments is introduced. It is firmly established that under these conditions a pure fluid will exhibit neither latent heat of vaporization nor surface tension. The jet undergoes a transition from a dense cryogenic fluid to an ideal gas as it mixes and blends with the surrounding warmer gas. Regarding the thermodynamic process, this transition is characterized by large changes in density and very small changes in temperature as energy is supplied. The state where density changes and the heat capacity are maximal is sometimes called `pseudo-boiling' in the literature. However, no clear definition of this process is available, its very existence debated. In this paper, the first quantitative pseudo-boiling analysis is presented. It can be shown that pseudo-boiling exists along a line which effectively structures supercritical uid states. An equation for this continuation of the coexistence line is given. Across this line, a continuous state transition can be identfied. The temperature at pseudo-boiling replaces the critical temperature as relevant parameter at supercritical pressures. By introducing a suitable definition for a supercritical uid boundary, supercritical jet break-up can be quantified thermodynamically. This suggests a novel, thermal, jet break-up mechanism. Experimental evidence from the literature is shown, further supported by CFD simulations. The pseudo-boiling effect is found to play a role for injection conditions of reduced pressures smaller than 3, and reduced temperatures lower than 1.2.