Sudden Releases of Hydrogen into a Tunnel

This paper presents work undertaken by the HSE as part of the Hytunnel-CS project, a consortium investigating safety considerations for fuel cell hydrogen (FCH) vehicles in tunnels and similar confined spaces. The sudden failure of a pressurised hydrogen vessel was identified as a scenario of concern due to the severity of the consequences associated with such an event. In order to investigate this scenario experimentally, HSE designed a bespoke and reusable ‘sudden release’ vessel. This paper presents an overview of the vessel and the results of a series of 13 tests whereby hydrogen was released from the bespoke vessel into a tunnel at pressures up to 65 MPa. The starting pressure and the volume of hydrogen in the vessel were altered throughout the campaign. Four of the tests also included congestion in the tunnel. The tests reliably autoignited. Overpressure measurements and flame arrival times, measured with exposed-tip thermocouples, enabled analysis of the severity of the events. A high-pressure fast-acting pressure transducer in the body of the vessel showed the pressure decay in the vessel, which shows that 90% of the hydrogen was evacuated in between 1.8 and 3.2 ms (depending on the hydrogen inventory). Schlieren flow imagery was also used at the release point of the hydrogen, showing the progression of the shock front following initiation of the tests. An assessment of the footage shows an estimated initial velocity of Mach 3.9 at 0.4 m from the release point. Based on this, an ignition mechanism is proposed based upon the temperature behind the initial shock front.