OCR Text |
Show amount .of exc~ss air requi red for comp 1 ete combust10n Wh1 ch, in turn, improVEs tb thermal efficiency of the system. \ ,9,10' Another very attractive feature of many pulse combustors is their ability to self-aspirate· that is, their ability to "pump" the required combust i on air and generated combustion products through the system wi thout the use of any aux i 1 i ary fans. Pu1 se combustors can burn any kind of fuel and they generally produce low concentrations of CO and soot Finally, for reasons which are yet not understood, gas burning pulse combustors ~roduce 1 ow concentrat(iff~.Af NO , genera 11 y 1 n the 30-60 ppm range ,~} . x The increase in the rates of mass momentum and heat transfer processes whi ch resu 1 ts from the presence of pu 1 sati ons in the generated exhaust products can be also taken advantage of in a variety of applications. For example, the increase in convect i ve heat transfer rates reduces both the heat transfer area and size of the equipment requi red for transferri ng a given amount of heat. Thi s, in turn, reduces capi ta 1 investment costs. Dryi ng is another area where the presence of pulsations in the drying medium has been shown to be advantageous. In this case the pulsations increase the rates of heat and moisture transfer to and from the wet material resulting in a higher drying rate and operational costs savings. Additional industrial processes where similar improvements in process effi ci ency and cost reductions could be attained by the use of a pulsating flow will undoubtedly be uncovered in the future as the applications of pulse combustion becomes more widespread. I n what fo 11 ows the paper wi 11 di scuss the types of pulse combustors which have been developed to date, past and potential applications of these devices, and needed research and development activities in this area. PULSE COMBUSTORS Existing pulse combustor designs can be generally divided into three classes depending upon their acoustic properties. These are: the quarter wave or Schmidt combustor, the Helmholtz combustor and the Rijke combustor. The operating principles of the quarter wave and Helmholtz type combustors are very similar and they will be discussed by referring to Figure 1 which describes a quarter wave type pulse combustor. Thi s combustor is based upon the operating principles of the acoustic qua rter-wave tube wh i ch cons i sts of a pipe closed at one end and open at the other. The fundamenta 1 acoustic mode of th is tube has its pressure maximum and minimum at the closed and open ends of the tube respectively. For purposes of discussion, th~ 56 combustor shown in Fig. 1 can be divided into three subsections; namely, the inlet, the combustion chamber and the exhaust pipe. The inlet section contains either mechanical or aerodynamic fuel and air valves. The mechanical valves open and close when the pressure in the combustor is lower or higher than the pressures upstream of the valves, respect i ve 1 y. On the other hand aerodynami c valves, which are used less frequently, offer high and low resistances to flows out or into the combustor, respectively. The combustion chamber extends over that portion of the tube where most of the combustion process takes place and the exhaust pipe serves to move the combustion products from the combustor to the outside or into an appropriate processing chamber, depending upon the speci fi c application of the pulse combustor. To start operation, a small fan is used to supply a low flow rate of air into the combustor. Next, the spark plug is turned on and the fuel valve is open. The fuel mi xes with the available air and is ignited by the spark plug, resulting in a sudden increase in pressure. This, in turn, results in closure of the mechanical valves and movement of the combustion products out of the combustion chamber and into the exhaust pipe. The inert i a of the gases 1 eavi ng the combustor decreases the combustor pressure to a 1 eve 1 lower than the pressures upstream of the mechan~ca1 valves. Consequently, the mechan1ca1 valves open and admit new charges of fuel and oxidizer into the combustor. The decrease in combustor pressure also resul ts in some of the combustion products returning from the tail pi pe. The new charges of fuel and air mi x wi th one another, wi th the hot combustion products and with burning pockets from the initial combustion cycle. This results in ignition of the fresh charges and a new pressure increase in the combustor. This periodic combustion process can now repeat itself indefinitely without the use of a spark plug. The fresh charges of fuel and air enter the combustor duri ng the phase of negati ve pressure. In order to satisfy the above mentioned Rayleigh's criterion for driving pressure oscillations the combustion heat release must occur during the phase of pos it i ve pressure. Referri ng to the pressure time diagram in Figure 1, it follows that in order to achi eve pu 1 se combustion ope rat ion the duration of the mixing and reaction processes must equal, approximately, half the period of the oscillations. If, however the durat i on of these processes is too 1 o~g or too short, pu1 se combustion operation wi 11 not be .attained .. Fina1~y, . to achieve pulse combust10n operat10n, 1t 1S desirable that the combustion process take place in a region where the amplitude of the pressure oscillations is significant. |