CFD modelling of underexpanded hydrogen jets exiting rectangular shaped openings

Abstract

Underexpanded jet releases from circular nozzles have been studied extensively both experimentally and numerically. However, jet releases from rectangular openings have received much less attention and information on their dispersion behaviour is not as widely available. In this paper, Computational Fluid Dynamics (CFD) is used to assess the suitability of using a pseudo-source approach to model jet releases from rectangular openings. A comparative study is performed to evaluate the effect of nozzle shape on jet structure and dispersion characteristics for underexpanded hydrogen jet releases. Jet releases issuing from a circular nozzle and rectangular nozzles with aspect ratios ranging from two to eight are modelled, including resolution of the near-field behaviour. The experimental work of Ruggles and Ekoto (2012, 2014) is used as a basis for validating the modelling approach used and an additional case study, in which jets with a stagnation-to-ambient pressure ratio of 300:1 are modelled, is also performed. The CFD results show that for the 10:1 pressure ratio release the hazard volume and hazard distance remain largely unaffected by nozzle shape. For the higher pressure release, the hazard volume is larger for the rectangular nozzle releases than the equivalent release through a circular orifice, though the distance to lower flammability limit is comparable across the range of nozzle shapes considered. For both of the release pressures simulated the CFD results illustrate that a pseudo-source approach produces conservative results for all nozzle shapes considered. This finding has useful practical implications for consequence analysis in industrial applications, such as the assessment of leaks from flanges and connections in pipework.