Abstract:
A pulsed combustor comprising at least one aerodynamic diode, a diffuser, fuel injectors, combustion chamber/duct, igniters, and muffler providing plug type flow with successive ignition and wave generation at the hot gas interface of the previous pulse. The downstream wave of each pulse propagates through the muffler whereas the upstream wave of each such pulse is at least partially reflected downstream with the diode(s) and diffuser minimizing reverse flow and maximizing pressure gain in the cumbustor.

Description:
This invention relates to pulsed combustors and more particularly to pulsed combustors for drying of particulate material, preheating processing. 
     BACKGROUND OF THE INVENTION 
     Pulse combustion drying is a recognized technique for effecting heat processing of particulate material. Illustrative of some prior art techniques are U.S. Pat. Nos. 3,618,655; 4,226,668; 4,226,670; 4,265,617 and 2,838,869. 
     Typically, pulse combustors require an acoustic resonator. Such combustors have high local combustion intensity and high local heat release. However, they also are subject to limited pressure gain and require a large resonator volume. 
     Broadly, pulsed combustors or burners resemble a two cycle gasoline engine, in which fuel and air are sucked into a combustion chamber, the mixture is caused to explode and then exhausted from the combustion chamber for its intended use. When the mixture explodes the pressure in the combustion chamber increases greatly and, typically, closes a flapper valve or the like thereby preventing both continued inflow of the mixture and outflow of products of combustion in the upstream direction. Where no valve is used a resonant chamber downstream of the combustion chamber provides more or less the same result. Under this condition, the combustion gas is exhausted through the resonator to the point of use, thereby creating a negative pressure which permits entry of a new quantity of the mixture in the combustion chamber and the cycle is then repeated. 
     SUMMARY OF THE INVENTION 
     The present invention is a pulsed combustor comprising at least one aerodynamic diode, a diffuser, fuel injectors, combustion chamber/duct, igniters, and muffler providing plug type flow with successive ignition and wave generation at the hot gas interface of the previous pulse. The downstream wave of each pulse propagates through the muffler whereas the upstream wave of each such pulse is at least partially reflected downstream with the diode(s) and diffuser minimizing reverse flow and maximizing pressure gain in the combustor. 
     The present invention eliminates the need for the large acoustic resonators of the prior art, alleviates the problems of noise and vibration in the burner itself, controls the shape and magnitude of the pressure pulses transmitted from the burner to succeeding components, and provides improved heat transfer and particle separation. Pulsed combustors in accordance with the present invention produce combustion which is controlled to provide very high pressure gain together with a very compact overall combustor system. 
     The very compact scale and high pressure gain provided by the present invention provides a highly attractive package for applications such as particle heating followed by cyclone separation. In such an application sufficient pressure gain is provide to permit highly efficient batch heating and subsequent inertial separation of the batch material. 
     Further, the plug type flow provided by the present invention minimizes the time necessary to purge the combustor of hot gases prior to the injection of fuel and ignition for subsequent pulses. The diffuser in addition to providing a transition from a preferable small diode outlet port to a much larger combustor cross section also results in a reflection of waves propagating upstream. 
    
    
     Other objects and advantages of the present invention will be apparent from the following description and appended drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic side elevation in cross section showing one embodiment of a pulsed combustor in accordance with the invention; 
     FIG. 2 is a side elevation showing multiple vortex diode configuration; 
     FIG. 3 is an end view taken on line 3--3 of FIG. 2; 
     FIG. 4 is a side elevation partly in cross section and on an enlarged scale of the flow diode shown in FIG. 1; and 
     FIG. 5 is an end view of the flow diode taken on line 4--4 of FIG. 4. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1 there is shown a pulsed combustor in accordance with the invention comprising an aerodynamic vortex diode 11 having an air inlet 12 and an air outlet 13. A duct 14 receives air from air outlet 13 and disposed within duct 14 is fuel injection means. Disposed at the air outlet 16 of duct 14 is a flow diode 17 more fully discussed hereinafter communicating with a diffuser 18 increasing in cross section in the direction of air flow. While the use of gas as a fuel is preferred, any other fuel may be used. 
     For the embodiment shown, mixing of air and fuel occurs in both duct 14 and diffuser 18. However, the point of fuel injection may be otherwise than as shown and mixing may occur in the combustion chamber 19 or elsewhere. Igniters 24, while shown as disposed at the outlet of the combustion chamber, may be disposed elsewhere in the combustion chamber if desired. 
     Muffler 25 more fully described hereinafter is disposed downstream of the combustion chamber 19. The muffler 25 is arranged and adapted to attenuate the waves generated in the combustion chamber optimally for an input wave amplitude and pulse frequency. Mufflers of the type used for attenuation in pulsed lasers are suitable. 
     As shown in FIGS. 2 and 3 multiple vortex diodes 26-29 may be utilized to optimize performance. The use of a plurality of vortex diodes is advantageous in that this permits a reduction in the size of each diode as compared to the use of a single diode and this has the additional benefit of reducing transit time. 
     Flow diode 17, shown on an enlarged scale in FIGS. 4 and 5 comprises a series of triangular members 35 with their bases 37 carried by the wall of duct 14, (which may be of any desired cross section) and their apexes 38 pointing downstream. Thus, the triangular members 35 are disposed at an angle to the wall such that their apexes 38 are spaced away from the wall. The angle at which the triangular members 35 are disposed from the wall should be between five and twenty degrees. The flow over the triangular members 35 is significantly different for flow in opposite directions. The triangular members are sized, shaped and disposed at an angle or angles providing a maximum ratio of high flow resistance in the upstream direction and low flow resistance in the downstream direction. 
     As air flows in the downstream direction the triangular members 35 generate vortices that direct high energy core or central flow to the walls whereas when air flows over the triangular members in the upstream direction the triangular members 35 generate different vortices that cause low energy flow at the walls to be separated therefrom. This generates high velocity vortex flow over at least a substantial portion of the duct cross section and especially in its central portion. This is effective to produce energy losses in upstream flow and thus a high resistance to such flow upstream of the flow diode. 
     Where combustor operating conditions remain fixed, the triangular members 35 may be fixedly attached to the duct walls. However, if desired, they may be movably carried by the duct wall whereby, in conventional manner, their positions within the duct may be varied as circumstance may require or as may be desired. 
     In the embodiment shown the members 35 provide the additional function of insuring rapid mixing of the air and injected fuel. 
     Flow diode 17 may be used singly, a plurality may be used spaced one from another a distance to maximize energy losses between them, and with or without vortex diodes. Maximum reduction of undesired upstream flow is, however, achieved when the flow diodes are used in combination with vortex diodes because of the uniquely beneficial effect the flow diodes produce at the inlet of and within the vortex diodes. 
     For a fifty megawatt thermal combustor, the peak pressure rise in the combustion chamber can be expected to be about thirty to forty pounds per square inch with an average pressure of about five to twenty pounds per square inch gauge versus a loss in pressure of about one pound per square inch present in prior art devices. 
     The use of particle entrainment and/or a muffler may be used to tailor the spectral content of the pressure into a staged particle heater/separator unit. The requirements for muffler technology are set by separator structural dynamics where used. 
     Since the high pulsed overpressure occurring during operation produces sonic backflow, choking must be provided at the innermost radius of the vortex diode. If this is not done the vortex pressure drop will not limit backflow. A suitable ratio of resistance is about fifty to one hundred. Where necessary to assist in providing the necessary ratio, steam injection to energize the boundary layers and/or transient steam counter flow as an ejector may be used. 
     In the operation of pulsed lasers a rapid energy deposition of a few microseconds can create overpressures of approximately one to five atmospheres. If good optical laser performance is to be provided on the next laser pulse, such overpressures must be damped to where the density perturbations are about 0.00004 in less than about five microseconds. These laser requirements, while more stringent, are analogous to those suitable for practice of the invention. 
     The present invention may be embodied in other specific forms without departing from the spirit and scope thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification as indicating the scope of the invention.