Pulsed combustion of fuel–air mixture in a cavity under the boat bottom: modeling and experiments

The physical and mathematical model for simulating combustion and detonation of fuel mixture in the semi-confined gas volumes above the free surface of water is applied for modeling the transient two-phase reactive flow in the gas cavity under the bottom of a ship/boat. With the proper organization of the combustion/detonation process in the gas cavity, thermal expansion of combustion products can provide an additional lifting force that reduces the area of contact of the boat bottom with water, as well as a propulsive force caused by the overpressure of combustion/detonation products on redans—vertical sections of the boat bottom. The model is validated on the set of laboratory experiments with pulsed combustion of propane–air mixture in a semi-closed gas cavity. The model is shown to predict satisfactorily the arising lifting and propulsive forces acting on the volumes, the time histories of pressure in the volumes, and the dynamics of flame and gas–water interface motion during combustion in the volumes. For further model validation in terms of its scaling capability, a set of preliminary experiments with a larger-scale (by a factor of at least 5) towed boat with a bottom gas cavity were conducted on open water. In the experiment, the hydrogen–air mixture was ignited and burned in the bottom gas cavity in a pulsed mode. These experiments confirmed that pulsed combustion of fuel–air mixture in a gas cavity under the boat bottom creates positive propulsive and lifting forces acting on the boat. Moreover, in some experiments a considerable increase in the propulsive force was registered due to flame acceleration causing a higher overpressure in the cavity. The elevated values of the propulsive force in these conditions can be treated in favor of a pulsed detonation mode, which will be studied later.