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EnginSoft - Conference Abstracts
EnginSoft Users' Meeting 2008 |
CFD modeling of two phase cryogenic flow in an internal orifice |
De Giorgi Maria Grazia - Università del Salento, Dipartimento di Ingegneria dell'Innovazione, Centro Ricerca Energia Ambiente |
Abstract |
The study is concerning the modelling of cavitating cryogenic flows, present in many engineering applications as superconductivity technology, liquefied natural gas plants, aerospace components, and many other fields. The cavitation phenomenon is a phase change from liquid to vapour at a constant temperature due to a pressure drop, below the saturation pressure at the liquid temperature. This phenomenon appears with bubbles nucleation and considering that the pressure inside the bubble is greater than the freestream pressure, bubbles could grow and the incipient cavitation changes to a developed cavitation. The bubbles size is proportional to the permanence time of vapour bubbles in the low pressure zone, when these bubble reach regions at high pressure, they implode. In many cases, this collapse is a undesirable effect, principally, because it can affect the performance of engineering application in which it appears or, sometimes, it erodes the walls of engine. In the case of cryogenic fluids the cavitation phenomenon assumes different characteristics on respect to the standard fluids as water. Liquid phase for cryogenic fluids exists in low temperature ranges, as 150 K for liquid oxygen and 5 K for liquid helium. Moreover cryogenic fluids are characterized by large compressibility as compared with fluids, such as water, at room temperature, by a small difference in density between vapour and liquid phases and by a small latent heat of vaporization. The aim of this paper is a numerical investigation of this phenomenon, using a multiphase formulation that accounts for the energy balance, variable thermodynamic properties of the fluid and nucleation transport equation. Usually the cavitating flows are modelling with the basic hypothesis of isothermal flow, however it’s known, for cryogenic fluids, that there is a temperature decrease in the vapour cavity. For that reasons it is necessary to consider the thermal in the modelling of such fluids. In addition this investigation stressed, also, the necessity to implement a cavitating flow model taking into account the nucleation theory. The numerical simulations reproduced the operating conditions obtained by an experimental apparatus of University of Salento, in which it has been possible to observe the two-phase cryogenic flow passing through an internal nozzle. The numerical simulations have been performed by the commercially available code Fluent release 6.3, modified by linking subroutines in order to implement the thermal and nucleation effects in the cavitation model. The numerical results have been compared with experimental results. |
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