Study On Nitrogen Inerting Characteristic Of Military Aircraft Fuel Tank

Aircraft fuel tank ullage explosion resulting from ignition of vapors is a major cause of military loss in combat. The on-board inert gas generator system (OBIGGS) processes high pressure air from engine bleed air into nitrogen rich gas for fuel scrubbing and ullage wash which can maintain a safe ullage by preventing the oxygen concentration from exceeding safety oxygen concentration so that the vapor in tank ullage can not be ignited and the survivability of aircraft is improved effectively. In addition, the nitrogen rich gas is used for fuel tank pressurization during descent.The paper analyses the hazard of fire and explosion of fuel tank and discusses the explosion suppression principle of nitrogen inerting. The OBIGGS is introduced briefly in the paper and compared with other fire and explosion suppression systems. The paper focalizes fuel scrubbing, ullage wash and descent pressurization with inert gas and three analytic models are established based on conventional gas mixture relationships, conservation equations, ideal gas law and the gas solubility coefficient for evaluating the inert gas requirement.The overseas test data is collected and organized and the correlative test parameters simulating aircraft climb, cruise and descent are determined which are the initial conditions and boundary conditions demanded in computing with analytic model. The computed programs are compiled with MATLAB. The agreement between computed and test result is very well, which validates the analytic model.The inert rule of fuel scrubbing is investigated by computing with fuel scrubbing model. The changes of ullage oxygen concentration versus time is evaluated when the conditions of fuel tank, such as initial conditions (include temperature of fuel, volume of fuel tank and the ratio of fuel to ullage volume), fuel depletion rate, climb rate of aircraft, inert gas flow rate and the oxygen concentration of inert gas etc, are changed. At last, the inert gas requirement of one aircraft fuel tank is evaluated as a computational example.