Low-pressure flashing mechanisms in iso-octane liquid jets

Abstract

This paper examines a flashing liquid regime that takes place at very high ratios of injection to discharge pressures in flow restrictions. Typically, the flashing phenomenon has been observed in laboratory experiments where a liquid flows through a short nozzle into a low-pressure chamber at a pressure value considerably lower than the liquid saturation pressure at the injection temperature. By using two visualization techniques, the schlieren and the back-lighting methods, it was possible to identify some compressible phenomena associated with the liquid flashing process from the nozzle exit section. The schlieren method was used to capture the image of a shock-wave structure surrounding a liquid core from which the phase change takes place, and the optical technique allowed us to observe the central liquid core itself. The work corroborates previous physical descriptions of flashing liquid jets to explain an observed choking behaviour as well as the presence of shock waves. According to the present analysis, flashing takes place on the surface of the liquid core through an evaporation wave process, which results from a sudden liquid evaporation in a discontinuous process. Downstream of the evaporation discontinuity, the two-phase flow reaches very high velocities, up to the local sonic speed that typically occurs at high expansion conditions, as inferred from experiments and the physical model. That sonic state is also a point of maximum mass flow rate and it is known as the Chapman–Jouguet condition. The freshly sonic two-phase flow expands freely to increasing supersonic velocities and eventually terminates the expansion process through a shock-wave structure. This paper presents experimental results at several test conditions with iso-octane.