METHOD AND APPARATUS FOR THE CONTROL OF FLUID DYNAMIC MIXING IN PULSE COMBUSTORS

Abstract: In a method and apparatus for controlling combustion characteristics in a pulse combustor (20), the mixing characteristics of the combustion reactants and the combustion products are controlled as a function of the inlet geometry (22) of the inlet of the combustion chamber. The inlet geometry (22) may be fixed or variable for controlling the mixing characteristics. A feedback loop (44, 48) may be employed to sense actual combustion characteristics, and then vary the inlet geometry (22) to obtain the desired mixing characteristics necessary to achieve the desired combustion characteristics. In a preferred embodiment, the mixing time of the combustion reactants and products scales with the ratio of the inlet orifice radius to the injection velocity. Various embodiments relate to the varying of the mass flow rate of reactants while holding the radius/velocity ratio constant.

Description: METHOD AND APPARATUS FOR THE CONTROL OF FLUID DYNAMIC MIXING IN PULSE COMBUSTORS BACKGROUND OF THE INVENTION The present invention relates generally to the contr of pulse combustors. More particularly, the present inventi relates to a method and apparatus for controlling fluid dynam mixing of combustion reactants and combustion products in pul combustors. The Government has rights in this invention pursua to Contract No. DE-AC04-76DP00789 awarded by the U.S. Departme of Energy and AT&T Technologies, Inc. Generally / a pulse combustor includes a combusti chamber, an inlet for admitting combustion reactants (typical fuel and air) into the combustion chamber, and an outlet f expelling combustion products from the combustion chamber. Pul combustors operate cyclically in that a charge of combusti reactants is admitted into the combustion chamber and ignited form the combustion products, the initial ignition bei assisted, preferably by a spark plug. The combustion produ expand through the outlet thereby causing a partial vacuum in combustion chamber which vacuum assists in drawing a fresh cha of combustion reactants into the combustion chamber for the n cycle. The fresh charge ignites upon mixing with the combustio products from the previous cycle, so that the operation is self sustaining after the initial ignition. Compared to conventional combustion systems, puls combustors have the following attractive characteristics: two t three times higher heat transfer, an order of magnitude highe combustion intensity, one third lower emissions of oxides o nitrogen, forty percent (40%) higher thermal efficiencies, an possibly self-aspiration. This combination of attributes result in favorable economic tradeoff with conventional combustors i many applications. Moreover, the enhanced heat and mass transfe associated with oscillating flow fields in pulse combustors ma lead to significant improvements in industrial and chemical pro cesses. Potential drawbacks of pulse combustors, however, ar their inability to operate over a wide range of energy releas rates (i.e., they have limited turn-down ratios), and their sen sitivity to fuel properties, which may be highly variable geo graphically and temporally. Although there are several different types of puls combustors (i.e., the quarter-wave or Schmidt tube, the Rijk tube, the Helmholtz resonator, and the Reynst pulse pot), th underlying principle controlling their operation is the same that is, the periodic addition of energy must be in phase wit periodic pressure oscillations (Rayleigh's criterion). In spit of this apparent simplicity, the processes that occur in a' puls combustor are very complicated. They involve a three-dim sional, transient flow field that is highly turbulent and variable physical properties. They further involve a reson pressure field and a large transient energy release, the char 5 teristic times of which may be on the same order of magnitude the characteristic times for chemical reactions and fluid dyna mixing. Moreover, all aspects of the combustion system are hi ly coupled. There are many factors that, through their impact 10 the various characteristic times, can affect the operatio performance of a pulse combustor. For example, fuel properti turn-down ratios, heat transfer, and equivalence ratios all h rformance of a pulse combust g time scale appears to exhi performance of a pulse comb t time scales may all be comp able, and because the fluid dynamic mixing time scale controllable, fluid dynamic mixing may be used to compensate variations in other time scales, as well as to achieve a desi 20 operating condition. SUMMARY OF THE INVENTION It is an object of the present invention to elimin the aforementioned potential drawbacks in known pulse combust 25 by providing a method and apparatus for controlling the inject of combustion reactants into the combustion chamber, and ther control the combustion characteristics of the pulse combustor. It is a further object of the present invention t provide a method and apparatus for controlling the combustio process by controlling the fluid dynamic mixing characteristic of fresh reactants with hot combustion products from a previou cycle. This control may be either dynamic, through the use of a appropriate feedback mechanism, or static. To achieve these and other objects, the inventio relates to a method and apparatus for controlling combustio characteristics in a pulse combustor comprising a combustio chamber with combustion products therein, an outlet mechanism fo expelling combustion products from the combustion chamber, and a inlet mechanism having an inlet geometry for introducing combus tion reactants with a predetermined velocity and mass flow nat into the combustion chamber for mixing with the combustion pro ducts therein. The inventive method and apparatus control th mixing characteristics of the combustion reactants and th combustion products as a function of the inlet geometry of th inlet mechanism. As used herein, the term "as a function of" i intended to mean the relationship or nexus between a selecte inlet geometry and its consequent affect on the mixing character istics of the combustion reactants and the combustion products. Other embodiments of the present invention relate t the following improvements in pulse combustor technology: (1) method and apparatus for extending the turn-down-ratio of puls combustors; (2) a method and apparatus for tailoring the tempora location of the energy release rate to obtain the desired pu combustor operation; and (3) a method and apparatus for comp sating for fuel composition effects. Each embodiment employ particular inlet geometry to obtain a desired fluid dyna mixing. The inlet geometry may be fixed or variable. This invention effects changes in the fluid dyna mixing characteristics by modifying the reactanf injection geo try in such a way as to effect the fluid dynamic mixing t scale. In preferred embodiments, the invention can be utili either statically or dynamically. Static application of t invention may be obtained at the time of manufacturing or fi maintenance of the pulse combustor by adjusting the inject geometry to effect changes in the fluid dynamic mixing charact istics, hence achieving the desired operating conditio Dynamic application of this invention may be obtained by monit ing a suitable system parameter (for example, combustion cham pressure, frequency of operation, or chemiluminescense) a through a suitable feedback loop, controlling the inject geometry to effect changes in the fluid dynamic mixing charact istics and hence, achieving the desired operating conditions. Described herein are several geometries which can made to effect the fluid dynamic mixing characteristics achieve the desired pulse combustion operation. These geometr are not exhaustive, but rather serve as examples of how to mod the fluid dynamic mixing characteristics. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in detail with ref ence to the attached Figures, wherein: Fig. 1 is a theoretical representation of the struct of a pulse combustor; Fig. 2 is a graph of theoretical predictions of effect of mass flowrate on mixing time in a pulse combustor; Fig. 3 is a graph of experimental results showing effects of mass flowrate on energy release in a pulse combusto Fig. 4 is a schematic representation of a pulse comb tor incorporating the features of the present invention; Fig. 5 is a schematic representation of a multiple injection geometry according to one embodiment of .ihe pres invention; Fig. 6 is a schematic representation of a multiple injection geometry having injection jets of differing radii accordance with a second embodiment of the invention; Fig. 7 is a schematic representation showing the mix rate in a pulse combustor having multiple jets of differing ra in accordance with Fig. 6; Fig. 8 is a schematic representation of an inject system according to a third embodiment of the present inventi and Figs. 9a and 9b illustrate two embodiments of the p sent invention which utilize variable inlet geometries to cont fluid dynamic mixing. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An inherent characteristic of most pulse combustors the fluid dynamic mixing of fresh reactants with the combust products of the previous cycle. Fluid dynamic mixing is known be important to the operation of pulse combustors; largely a consequence of testing a variety of configurations that had d ferent mixing characteristics. Until recently the precise r of mixing, however, was not quantified either experimentally theoretically. To understand quantitatively the fluid dyna mixing characteristics of a pulse combustor, a theoretical mo was developed with reference to Figure 1, that has identified important system operating and geometric characteristics t determine fluid dynamic mixing times. Referring to Figure 1, a combustion chamber 10 inclu an inlet 12 and an exit 14. A charge of combustion reactants such as fuel and air at a temperature of T r is injected thro the inlet 12 into the combustion chamber 10 enclosing combust products P from a previous combustor cycle at temperature The inlet 12, which can be circular or noncircular, is prefera a circular orifice or jet having a radius T Q and the inject velocity of the combustion reactants is UQ (determined from mass flowrate of the combustion reactants, M, and the continu equation) . The mixing rate is the rate at which the combust reactants R-^ mix with the combustion products P at temperature to achieve a temperature for ignition m ^ χ . For methane, T m ^ χ approximately 1500 K, a temperature for which ignition, and th energy release, will occur approximately 1 ms later (i.e., chemical ignition delay time of 1 ms) . The mixing rate therefore a measure of the rate of energy release. For oth fuels, different values of T JJ ^ and chemical ignition delay ti are required. Chemical ignition delay time is the delay betwe the time when the reactants reach temperature Tmix, and the ti when rapid reactions begin. The mixing time is the time from t introduction into the combustion chamber of an element reactants to the time an element of reactants reaches t temperature Tmix. Some typical theoretical results are shown in Figu 2. As may.be seen, the mass flowrate of the reactants, M, has dramatic effect on the mixing characteristics of the reactant . Mass flowrate alone limits the range of combustor operations a explains the inability of most pulse combustors to operate at wide range of energy release rates and thus obtain desirab ranges of turn-down ratios. The theory also predicts that t initial slope of the mixing rate and the peak mixing rates bo decrease with decreasing mass flowrates. These effects have be verified experimentally, as the results shown in Figure 3 demo strate. It should be noted that the theoretical results shown Figure 2 are for a single inlet, while the experimental resul in Figure 3 are for a configuration with multiple inlets that m interact with each other. The results shown in Figure 3, ho ever, support the theoretical predictions shown in Figure 2. A method and apparatus in accordance with the inventi for controlling the mixing characteristics of the combusti reactants and the combustion products as a function of the inl geometry are illustrated schematically in the pulse combustor Figure 4. The combustion chamber 20 includes an inlet 22, outlet 24 and a spark plug 26 for igniting the initial charge reactants R . The reactants preferably are air from a blower upstream of the inlet 22 and fuel from a fuel supply inlet 3 Air passes through an air inlet valve 32 while fuel is dispers through a fuel inlet valve 34, the fuel and air mixing in mixing chamber 36. It is noted, however, that the invention applicable to systems employing pre ixed or non-premix reactants. The reactants Rτ_ enter the chamber 20 through the inl 22 having an inlet geometry which can be fixed or variable. T inlet 22 in Figure 4 has a variable inlet geometry in that movable center body 38 varies the inlet geometry of the inlet 2 thereby modifying the fluid dynamic mixing characteristics of t combustion reactants R-^ and products P in the chamber 20. T movable center body 38 is moved through a linkage 40 by actuator 42, preferably located upstream of the air inlet val 32. In dynamic or feedback controlled applications, t actuator 42 is controlled by a microprocessor 44 which receiv signals from sensors 46 in the combustion chamber 20. sensors may monitor actual combustion characteristics such pressure, frequency and/or chemiluminescence, and convey tho signals through a feedback loop 48 back to the microprocessor for comparison with desired combustion characteristics. A deviation between desired and actual combustion characteristi results through signal processing in selective actuation of t actuator 42 to vary the inlet geometry of the inlet 22 (by mov ment of the center body 38 relative to the inlet 22) and th modify the mixing characteristics of the combustion reactants and products P, which consequently modifies the combusti characteristics. In one preferred embodiment of the invention describ above, the concepts of the present invention are employed in control mechanism for adjusting the turn-down ratio of a pul combustor. In Figures 2 and 3, the mixing time scales with t ratio of inlet orifice radius over injection velocity, rg/ug, t scaling being approximately linearly as illustrated by t initial rising curves of Figures 2 and 3. Therefore an injecti system in which -C Q /U Q was held constant while the mass flowrat M, of reactants was varied would result in constant mixi characteristics over a wide range of turn-down ratios. O method and apparatus to accomplish this variation of mass flo rate uses a fixed injection geometry shown in Figure 5. Figure 5, six individual injection inlet orifices or jets a shown but more or less could be employed. The closed jets 50 a shown as darkened circles whereas open jets 52 are indicated open circles. Each jet has a fixed radius rg and thus a fi geometry. Injection jet 52' shows a reactant charge R2 pass therethrough. By successively closing individual jets as mass flowrate is reduced, it is possible to maintain a const value of injection velocity (U Q ). Likewise, as mass flowrate increased, individual jets may be opened. Since the jet rad ( T Q ) is fixed, the ratio g/ug is constant, ensuring that mixing characteristics are invariant. As discussed above Figure 4, an appropriate feedback system cooperating with individual jets could be used to determine pressure measureme in the combustion chamber and provide the mechanics to cont opening and closing of the jets in response to the determi pressure. Other possible control means include feedback syst coupled with either a method of monitorir.- - ' -iracteris ics combustion process signals such as determiι:i:.g the frequency the combustion cycle or a che iluminescarice measurement sys which determines precisely when energy release occurs. Another embodiment of the presen invention relates the optimization of temporal energy release rates in pulse c bustors. The strongest, most stable operation of a pulse comb tor occurs when the energy release rate is in phase with, and the peak of, the resonant acoustic pressure field. Since ene release rate is a function of the mixing characteristics, energy release rate can be tailored through the selection o variety of injection jet geometries. This theory is based on th discovery that mixing time is a function of the ratio rg/ug. A schematic of a geometric configuration that could b used to tailor the energy release rate is shown in Figure 6 Flow through all jets A, B, C and D is initiated simultaneous ly. Each jet (A, B, C, D) has a different radius (r Q1 , r Q2 , r Q3 r Q 4, respectively). Since the injection velocity of each jet i the same, while rg/ug varies, it is possible to vary the mixin time of each jet, since the mixing time scales with rg/ug. B selecting jet radii and summing the mixing rates of all of th jets, shown schematically in Figure 7, any desired tempora mixing profile may be obtained. Through the use of chambe pressure, cycle frequency or chemiluminescence measurements, a optimum mixing profile may be tailored for virtually any puls combustor apparatus using a feedback loop as discussed wit reference to Figure 4. Another embodiment of the present invention relates t accounting for variations associated with variable fuel proper ties in a pulse combustor. Fuel properties can also affect puls combustor performance. For example, in a Helmoltz-type puls combustor operating in a nonpremixed mode, changes of 1 ms in th chemical kinetics time scale out of a system acoustic time scal of 20 ms (accomplished by modifying the fuel's chemical proper ties) resulted in a dramatic effect on system performance. Spe cifically, theoretical models predict that for a fixed mass flo rate of fuel and air (operation at a constant firing rate) t mixing time scales with (rg) . Any of the above-described embo iments can be used to compensate for variations -in fuel prope ties. Minor variations in rg could also be used to compensa for these variations. One possible physical configuration f accomplishing this compensation for fuel composition effects illustrated in Figure 8. Figure 8 shows an injection syst wherein the injection orifice 60 has a variable geometry with variable radius r var attained through the use of an adjusta iris I. The variable radius r var of the injection orifice allows for the compensation of variations in fuel properti Thus, slight changes in Ug are compensated for by changes in to render rg/u constant. A feedback control means may be p vided for adjusting r in response to a determination of chamber pressure, cycle frequency, or reaction chemiluminesce which indicate energy release times. In two other embodiments of the present invent (Figures 9a and 9b), injection jets are formed by an inject orifice and a movable valving system that can modify the fl dynamic mixing characteristics of the combustor. As seen Figure 9a, a motor driven actuator 100 is used to mechanica adjust a stagnation plate valve 110 in response to a feedb loop 120 which monitors combustion characteristics. The pl valve 110 modifies the inlet geometry of the fuel/air inlet 1 and thus modulates the mixing characteristics of the premi fuel/air reactants with the combustion products in the combusti chamber 105. Preferrably, a microprocessor 130 is employed monitor the feedback signal and actuate the actuator 10 Accordingly, desired pressure oscillations in the combustor be maintained. Other system characteristics which may be mon tored and used to control fluid dynamic mixing are combusti cycle frequencies and chemiluminescence. Any type of movable valve geometry could be used th would modify the fluid dynamic mixing characteristics of t combustor. Figure 9b illustrates an embodiment similar to th shown in Figure 9a with the exception of a cone-shaped stagnati plate 140 which functions similarly to the movable stagnati plate seen in Figure 8a. Depending upon the particular pul combustor, a wide variety of stagnation plate shapes could used to vary the inlet geometry and thus provide desired mixi characteristics. The present invention has been described in deta including embodiments thereof. It will be appreciated, howeve that those skilled in the art, upon consideration of the prese disclosure, may make modifications and improvements on th invention and still be within the scope and spirit of this inve tion as set forth in the following claims.

Claims: WHAT IS CLAIMED IS: 1. A method for controlling combustion characterist in a pulse combustor comprising a combustion chamber with comb tion products therein, an outlet for evacuating combustion p ducts from the combustion chamber, and an inlet means having inlet geometry for introducing combustion reactants with a pre termined velocity and mass flow rate into said combustion cham for mixing with the combustion products within said combust chamber, said method comprising the step of: controlling mixing characteristics of the c bustion reactants and the combustion products as a function the inlet geometry of the inlet means. 2. The method of claim 1 wherein the step of contr ling the mixing characteristics as a function of the in geometry includes the steps of fixing the inlet geometry a controlling the mixing characteristics as a function of the fix inlet geometry. 3. The method of claim 1, wherein the step of contro ling the mixing characteristics as function of the inlet geomet includes the steps of varying the inlet geometry and controlli the mixing characteristics as a function of the variable inl geometry. 4. The method of claim 1 wherein the step of contro ling mixing characteristics includes the step of controlling mixing time. 5. The method of claim 1 urther comprising the ste of: sensing combustion characteristics in the combu tion chamber; and varying the inlet geometry in response to t sensed combustion characteristics to obtain desired mixi characteristics necessary to achieve desired combustion chara teristics. 6. The method of claim 5, wherein the step of sensi combustion characteristics includes sensing pressure in the co bustion chamber. 7. The method of claim 5 wherein the step of sensi combustion characteristics includes sensing frequency of combu tion in the combustion chamber. 8. The method of claim 5 wherein the step of sensi combustion characteristics includes sensing chemiluminescence. 9. The method of claim 1 wherein the inlet mea includes at least one orifice having a radius and the step controlling the mixing characteristics includes controlling t mixing characteristics as a function of a ratio of the radius the orifice to the velocity of the combustion reactants. 10. The method of claim 9 wherein the step of contro ling the mixing characteristics as a function of t radius/velocity ratio includes the step of controlling mixi time as a function of the radius/velocity ratio. 11. The method of claim 10 wherein the mixing ti varies linearly with the radius/velocity ratio. 12. The method of claim 11 wherein the step of contro ling the mixing characteristics includes the step of holding t radius/velocity ratio constant while varying the mass flowrate the combustion reactants to obtain constant mixing characteri tics over a range of turn-down ratios. 13. The method of claim 12 wherein the at least on orifice of the inlet means includes a plurality of selectivel closable inlet orifices having equal radii, and the step of con trolling the mixing characteristics to obtain constant mixin characteristics includes selectively closing a portion of sai plurality of inlet orifices to maintian constant velocity, there by holding said radius/velocity ratio constant to obtain constan mixing characteristics. 14. The method of claim 11 wherein the at least on orifice of the inlet means includes a plurality of orifice having unequal radii, and the step of controlling the mixin characteristics includes the step of varying the mixing time o each jet by equalizing the velocity of combustion reactants flow ing through each orifice of said plurality of orifices to obtai a desired mixing profile. 15. The method of claim 10 wherein the at least on orifice has a variable radius, and the step of controlling th mixing time includes the step of selectively varying the radiu of the orifice while holding the mass flow rate constant to var the mixing time as a function of the selected radius and thereb compensate for variations in combustion reactant properties. 16. The method of claim 15, wherein the mixing tim varies with the cube of the selected radius. 17. The method of claim 1 wherein the inlet mea includes a valve and an inhibitor of flow through said valve, a the step of varying the inlet geometry includes varying an ope ing degree of the valve by changing the relative position of t valve and the flow inhibitor in the valve. 18. The method of claim 17 wherein the flow inhibit is a movable splash plate. 19. The method of claim 17 wherein the flow inhibit is a movable cone. 20. The method of claim 17 further comprising the ste of: sensing combustion characteristics in the combu tion chamber; and varying the relative position of the flow inhibit and the valve in response to the sensed combustion characteri tics to obtain desired mixing characteristics necessary achieve desired combustion characteristics. 21. An apparatus for controlling combustion chara teristics in a pulse combustor comprising a combustion chamb with combustion products therein, an outlet for evacuati combustion products from the combustion chamber, and an inl means having an inlet geometry for introducing combusti reactants with a predetermined velocity and mass flow rate in said combustion chamber for mixing with the combustion product within said combustion chamber, said apparatus further com prising: X0 means for controlling mixing characteristics of th combustion reactants and the combustion products as a function o the inlet geometry of the inlet means. 22. The apparatus of claim 21 wherein the means fo controlling the mixing characteristics as a function of the inle geometry includes means for fixing the inlet geometry and mean for controlling the mixing characteristics as a function of th fixed inlet geometry. 23. The apparatus of claim 21, wherein the means fo controlling the mixing characteristics as function of the inle geometry includes means for varying the inlet geometry and mean for controlling the mixing characteristics as a function of th variable inlet geometry. 24. The apparatus of claim 21 wherein the means fo controlling mixing characteristics includes means for controllin a mixing time. 25. The apparatus of claim 21 further comprising: means for sensing combustion characteristics in th combustion chamber; and means for varying the inlet geometry in response t 5 the sensed combustion characteristics to obtain desired mixin characteristics necessary to achieve desired combustion chara teristics. 26. The apparatus of claim 25, wherein the means f sensing combustion characteristics includes means for sensi pressure in the combustion chamber. 27. The apparatus of claim 25 wherein the means f sensing combustion characteristics includes means for sensi frequency of combustion in the combustion chamber. 28. The method of claim 25 wherein the means f sensing combustion characteristics includes means for sensi chemiluminescence. 29. The apparatus of claim 21 wherein the inlet mea includes at least one orifice having a radius and the means f controlling the mixing characteristics includes means f controlling the mixing characteristics as a function of a rat , of the radius of the orifice to the velocity of the combusti reactants. 30. The apparatus of claim 29 wherein the means f controlling the mixing characteristics as a function of t radius/velocity ratio includes means for controlling mixing ti as a function of the radius/velocity ratio. 31. The apparatus of claim 30 wherein the mixing ti varies linearly with the radius/velocity ratio. 32. The apparatus of claim 31 wherein the means fo controlling the mixing characteristics includes means for holdin the radius/velocity ratio constant while varying the mas flowrate of the combustion reactants to obtain constant mixin characteristics over a range of turn-down ratios. 33. The apparatus of claim 32 wherein the at least on orifice of the inlet means includes a plurality of selectivel closable inlet orifices having equal radii, and the means fo controlling the mixing characteristics to obtain constant mixin , characteristics includes means for selectively closing a portio of said plurality of inlet orifices to maintain constan velocity, thereby holding said radius/velocity ratio constant t obtain constant mixing characteristics. 34. The apparatus of claim 31 wherein the at least on orifice of the inlet means includes a plurality of orifice having unequal radii, and the means for controlling the mixin characteristics includes means for varying the mixing time o i each jet by equalizing the velocity of combustion reactants flow ing through each orifice of said plurality of orifices to obtai a desired mixing profile. 35. The apparatus of claim 30 wherein the at least on orifice has a variable radius, and the means for controlling th mixing time includes means for selectively varying the radius o the orifice while holding the mass flow rate constant to vary th mixing time as a function of the selected radius and there compensate for variations in combustion reactant properties. 36. The apparatus of claim 35, wherein the mixing ti varies with the cube of the selected radius. 37. The apparatus of claim 21 wherein the inlet mea includes a valve and an inhibitor of flow through, said valve, a the means for varying the inlet geometry includes means for var ing an opening degree of the valve by changing the relati position of the valve and the flow inhibitor in the valve. 38. The apparatus of claim 37 wherein the fl inhibitor is a movable splash plate. 39. The apparatus of claim 37 wherein the fl inhibitor is a movable cone. 40. The apparatus of claim 37 further comprising: means for sensing combustion characteristics in t combustion chamber; and means for varying the relative position of the fl inhibitor and the valve in response to the sensed combusti characteristics to obtain desired mixing characteristics nece sary to achieve desired combustion characteristics.

Inventors: BRAMLETTE TILLMAN TAZWELL, KELLER JAY OWEN, BRAMLETTE, TILLMAN, TAZWELL, KELLER, JAY, OWEN
Applicants: US ENERGY, UNITED STATES DEPARTMENT OF ENERGY
Application year: 1990
Main classification: F23C11
IPC codes: F23C15