Prediction of the behavior of a CNG tank exposed to flames

The increasing request of vehicles fuelled by Compressed Natural Gas (CNG) requires that the safety issues related to this kind of fuel should be analyzed more and more thoroughly. So far, the minimum requirement for a CNG tank is to set up a Pressure Relief Device (PRD) triggered by temperature. This work aims to develop a procedure able to perform thoroughly tank behavior analyses, in presence of fire, in order to reduce the number of expensive experimental tests and to improve the design criteria of tank safety devices. The procedure consists in the solution of the unsteady solid-fluid conjugate heat transfer problem, by means of numerical simulations, in order to evaluate the transient behavior of both the temperature field inside the solid walls of the tank and the thermo-fluid-dynamic field of the compressed gas inside it, when it is exposed to a heating source. Moreover, the melting process of the fusible plug, inside the PRD, is simulated. In order to validate this procedure, experimental tests have been carried out on a suitable test rig. Several heating conditions, even unlikely, but able to jeopardize safety, have been considered. The condition characterized by a flame located on the opposite side of the PRD has been recognized to be the most critical with respect to explosion risk. In this case, the relationship between time and tank pressure has been numerically evaluated. The fusible plug, inside the PRD, has been checked to be completely melted of the fusible plug before the pressure reaches the safety limit.