EFFECT OF INLET SECTION ON PULSE COMBUSTOR PERFORMANCE A.W. Mazlan [1] , U. Mohd Haffis, B. Azwan Fakulti Kejuruteraan Mekanikal, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia Email: mazlan@fkm.utm.my [1] Abstract Experimental investigation on a valveless pulse combustor that applies Helmholtz resonation concepts is presented. The pulse combustor is attached with a J-shape inlet. Part of the burnt gases mixes with inlet air in the J-shape inlet due to pulse action. Since the J-shape is in the same direction as the tailpipe, it also produces additional thrust. The thrust produced is from 22.07 N to 31.88 N. The J-shape inlet increases the minimum required fuel supply. It also reduces the noise that is produced by the pulse combustor. It is shown that the optimum range of fuel supply for the pulse combustor is in the lean region. The effect of the J-shape inlet to the operating frequency is the same as lengthening the tail pipe with a frequency of 125 Hz with the J-shape and 157 Hz without. Keyword: Valveless pulse combustor, Helmholtz resonation, J-shape inlet, operating frequency 1. INTRODUCTION Pulse combustor is a periodically burning combustion device. It can be designed based on 3 acoustic principals; Rijke tube, Hemlholtz resonator and Quarter wave resonator (Zinn, 1984). As shown in Figure 1, a pulse combustor consists of 3 main sections; inlet, combustion chamber and tail pipe . Figure 1: Pulse combustor In a Helmholtz type pulse combustor, when combustion occurs, the increased pressure gives an impulse that makes the combustor and the gas within it to resonate. The combustion will create overpressure condition in the combustion chamber that will drive out the exhaust gas and create an expansion wave at both inlet and exhaust exit. Due to the expansion wave, the pressure in the combustion chamber drops to sub atmospheric pressure, allowing some of the hot exhaust gas to travel back into the combustion chamber and also causing fresh fuel-air mixture to be drawn into the chamber through the inlet section [Geng et.al 2006]. Illustration of the operating principle of a pulse combustor is shown in Figure 2. The combustion will be repeated at a resonant frequency of the pulse combustor and the frequency depends on the pulse combustor geometry. This acoustic resonation is referred to as Helmholtz resonation and the frequency of this resonation can be determined using equation 1 based on the diagram in Figure 3 (Litke et. al. 2005). Figure 2: Pulse combustor operating cycle (1) Vl A π 2 a f  a = the speed of sound in air Expansion Induction Self ignition Combustion Tail pipe Combustion chamber Inlet