The option of using hydrogen as a fuel for propulsion of aircraft has been investigated in the recent past especially in combination with long endurance unmanned mission targets (Helinet, Helios). This application has proved to be challenging mostly due to the low volumetric density of hydrogen, which needs to be compressed at very high pressures to be confined in the narrow volumes allowed by aircrafts structures. Aerial, as well as automotive, applications of hydrogen pose also the issue of weight for the total storage solution adopted: high pressures may mean thick layers for vessels and consequently high weights for unit of mass of hydrogen stored. Composite materials have helped in reducing the weight but remain tough to be adopted for vessels large enough to store the hydrogen mass necessary for long trips. Liquid hydrogen has only been adopted so far for aerospace applications and just for boosting rather than for endurance. Instead, hydrogen can be efficiently used for fuelling auxiliary systems on board and for ground services, helping to reduce the environmental impact, also regarding the idling phase. Fuel cells that are supplied with hydrogen can provide the electricity needed by all the auxiliary equipment, from air conditioning systems, to controls and avionics, to lighting and security services. Although smaller quantities of hydrogen are needed on board to supply only the auxiliaries rather than for propulsion, still there is a need for pressurising the gas and so to have a pressurised feed line that runs into the congested environment of an aircraft where ventilation is anyway usually present. In view of the experience gained in the oil and gas offshore sector, where flammable and pressurised gases may be released due to failures in the feed lines, we propose an innovative approach to investigate the possible hazards deriving from the use of pressurised hydrogen in aeronautics. Hydrogen releases may happen due to failures all along the lines but, statistically, ruptures are more frequent in lower pressures sections that are potentially less protected. A hydrogen supply line can cover pressures that range from 350 bars of the storage vessel to the nearly ambient pressure when it supplies the fuel cells. This induces to take greater care of possible mid-pressure (10-15 bars) releases and of their consequences. Ruptures are seldom catastrophic, while more often they are represented by small diameter cracks. The release through these ruptures is supersonic and it soon slows down also due to the scattering with obstacles in the aircraft environment. Modelling of the entire phenomenon is a challenging task for Computational-Fluid-Dynamic analysts as some variables such as pressure, and therefore velocity, have too strong variations throughout the domain. Yet, CFD remains the best tool to predict the dispersion and possibly the dangerous (i.e. above the - very low- flammability limit of hydrogen) accumulation of gas. Our proposal is to split the phenomenon in two phases and to study them separately with a coupling based on the parameters that are more relevant to describe the evolution: velocity and concentration. First, the supersonic release of hydrogen from the rupture, and the consequent compressible effects, are modelled in a domain that is large enough to contain the deceleration of the gas up to dispersion-like rates: this domain is, however, smaller than the full domain where we wish to study the entire phenomenon. Second, data related to speed and concentration calculated on the surface of this domain are given as boundary conditions for the simulation of the dispersion phase. Preliminary applications of this method to hydrogen releases from 10 mm hole ruptures at a pressure of 10 bar have provided interesting results especially with the supersonic release phase, that is the most challenging for the CFD simulation due to the intrinsic characteristics of hydrogen as a very light gas. In particular, addressing the supersonic release phase may allow estimating the effect of impact of the jet release onto the first obstacle (in the form of thermal stresses). The final coupling of the two phases can provide a dispersion pattern within a congested environment that can be validated in field tests, in the same way as it is being done with this method applied to natural gas releases in offshore platforms.
Hydrogen leakages in a congested aircraft environment: a CFD simulation method / Uggenti, ANNA CHIARA; Moscatello, Alberto; Gerboni, Raffaella. - ELETTRONICO. - (2019), pp. 206-208. (Intervento presentato al convegno 9th EASN International Conference European Aeronautics Science Network tenutosi a Athens (Greece) nel 3-6 September 2019).
Hydrogen leakages in a congested aircraft environment: a CFD simulation method
Anna Chiara Uggenti;Alberto Moscatello;Raffaella Gerboni
2019
Abstract
The option of using hydrogen as a fuel for propulsion of aircraft has been investigated in the recent past especially in combination with long endurance unmanned mission targets (Helinet, Helios). This application has proved to be challenging mostly due to the low volumetric density of hydrogen, which needs to be compressed at very high pressures to be confined in the narrow volumes allowed by aircrafts structures. Aerial, as well as automotive, applications of hydrogen pose also the issue of weight for the total storage solution adopted: high pressures may mean thick layers for vessels and consequently high weights for unit of mass of hydrogen stored. Composite materials have helped in reducing the weight but remain tough to be adopted for vessels large enough to store the hydrogen mass necessary for long trips. Liquid hydrogen has only been adopted so far for aerospace applications and just for boosting rather than for endurance. Instead, hydrogen can be efficiently used for fuelling auxiliary systems on board and for ground services, helping to reduce the environmental impact, also regarding the idling phase. Fuel cells that are supplied with hydrogen can provide the electricity needed by all the auxiliary equipment, from air conditioning systems, to controls and avionics, to lighting and security services. Although smaller quantities of hydrogen are needed on board to supply only the auxiliaries rather than for propulsion, still there is a need for pressurising the gas and so to have a pressurised feed line that runs into the congested environment of an aircraft where ventilation is anyway usually present. In view of the experience gained in the oil and gas offshore sector, where flammable and pressurised gases may be released due to failures in the feed lines, we propose an innovative approach to investigate the possible hazards deriving from the use of pressurised hydrogen in aeronautics. Hydrogen releases may happen due to failures all along the lines but, statistically, ruptures are more frequent in lower pressures sections that are potentially less protected. A hydrogen supply line can cover pressures that range from 350 bars of the storage vessel to the nearly ambient pressure when it supplies the fuel cells. This induces to take greater care of possible mid-pressure (10-15 bars) releases and of their consequences. Ruptures are seldom catastrophic, while more often they are represented by small diameter cracks. The release through these ruptures is supersonic and it soon slows down also due to the scattering with obstacles in the aircraft environment. Modelling of the entire phenomenon is a challenging task for Computational-Fluid-Dynamic analysts as some variables such as pressure, and therefore velocity, have too strong variations throughout the domain. Yet, CFD remains the best tool to predict the dispersion and possibly the dangerous (i.e. above the - very low- flammability limit of hydrogen) accumulation of gas. Our proposal is to split the phenomenon in two phases and to study them separately with a coupling based on the parameters that are more relevant to describe the evolution: velocity and concentration. First, the supersonic release of hydrogen from the rupture, and the consequent compressible effects, are modelled in a domain that is large enough to contain the deceleration of the gas up to dispersion-like rates: this domain is, however, smaller than the full domain where we wish to study the entire phenomenon. Second, data related to speed and concentration calculated on the surface of this domain are given as boundary conditions for the simulation of the dispersion phase. Preliminary applications of this method to hydrogen releases from 10 mm hole ruptures at a pressure of 10 bar have provided interesting results especially with the supersonic release phase, that is the most challenging for the CFD simulation due to the intrinsic characteristics of hydrogen as a very light gas. In particular, addressing the supersonic release phase may allow estimating the effect of impact of the jet release onto the first obstacle (in the form of thermal stresses). The final coupling of the two phases can provide a dispersion pattern within a congested environment that can be validated in field tests, in the same way as it is being done with this method applied to natural gas releases in offshore platforms.Pubblicazioni consigliate
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https://hdl.handle.net/11583/2767492
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