US1982665A - Explosion turbine - Google Patents

Description

PatentedDec. '4, 1934 PATE-N1' OFFICE 1,932,665 A l EXPLOSION TURBINE Hans Holzwarth, Dusseldorf, Germany, 'assigner' to Holzwarth Gas Turbine Co San Francisco,

Calif., a corporation ofDelawar ApplioationApril 19, 1929,`l Serial No. 356,446

In Germany April 30, 1928 4 Claims.o (Cl. 6041)` sideration. As'alreadyvstated, however,`th fac- My invention relatesto explosion turbines of theI-Iolzwarth type wherein charges of fuel and.

air are periodically fedito one or more explosion chambers associated with the rotor of the turbine to. be exploded in such chambers under constant volume and then discharged at predetermined instants into an expansion nozzle which directs the combustion gases against the blades of the rotor, each explosion and expansion being'followed by a lscavenging period during which the residual explosion gases are driven out of the explosion chamber through the same expansion nozzle; and more particularly to the constructional relationship of certain parts of such turbine wherebya stable and eiicient operation of a plant of this type for indenite periods is obtained.

My present invention has for its primary object the construction of an explosion turbine,- of the type referred to, which will be stable in operation,

that is, will operate yfor an indenitetime without difiiculty or stoppagefrom premature ignition.

It has for its secondary object the constructionl of such 'a turbine which will be not only stable but as efficient as otherwise possible; It 4is obvious that, whatever may be'the degree `of efliciency of such an engine, any substantial lack of stability will be fatal to its operation as a whole,-

1 and that therefore its construction, must be de# signed first for stability and only secondarily rfor an efficiency as high as possible under the cir-'f cumstances. While, therefore, in carrying out my A invention, other things being equal, considerations of stability must dominate any relevant constructional detail of the engine, even at the cost of maximum possible eiiiciency, it is in some cases possible that such a maximum eii'iciency mayr nevertheless beobtained. I have found that the relative size of -the smallest cross-section of the nozzle through -which the residual gases escape as they are expelled from vthe combustion chamber is an important element of such an engine and largely determines its stability of operation, and as I' have already pointed out heretofore in my pending application Serial No. 186,094, the relative size of the smallest cross-:section of the nozzle through which the gases expand is an important element in operating such an engine efficiently. Asmy invention relates to an engine inwhich bothv the expanding gases and the scavenged gases escape through the samenozzle it is obviousJ that there is a close inter-relation between the i'actors governing stabilityand' the factors governing eiliciency, and that' for the proper designing of a commercial engine of this typeboth sets oi factors must be taken into contors making for stable operation'must in any highest possi/ble eiliciency can be permitted to prevail only-in so far as they do notinterfere with the former set of factors.' l

As it is usually the primary endeavor of lthe mechanical. engineer soto design an engine as to obtain the highest possible efciency I will rst give a brief description of the features which should be given consideration when designing an'engine of this type from the standpoint of highest efiici-I ency, that is, other things being equal, from the standpoint of the avoidancev -as far as possible of heat losses during the expansion of the gases.

It is believed also that in'this way the problemA which remained even after I.had discovered the principles'y of construction for obtalning'highest eflciency will be better understood; it is the primary object of my invention to solve this problem.

to the whirling of the gases in the nozzle chamber connecting the outlet end of the explosion chamber with the nozzle, can-be very' greatly reduced by observing lthe following constructional formula i V l where f. the smallest nozzle cross-section measured in square centimeters and V=the volume .of the combustion chamber measured- -in cubic meters, it being assumed' that the nozzle. valve is as large as practicable and isopened as quickly as possible.V

As explained in the above cited application, a nozzle dimensioned in accordance with this formula has a' minimum crosssection which is considerably below that recommendedL by the prior art, and in fact such constructional relationship is quiteV contraryto the teaching of the prior 'art whichcalled for a nozzle opening as Alarge as practicable to permit rapid discharge of the gases.

The above constructionalv relationship is the result of my discovery that itis more important to prevent whirling `in the vnozzle channel than to discharge thegases in a ottime l! excessive heat losses Jin such Ychannel are to. be avoided;

.residual gases in each and that the prevention of such whirling is eiected by reducing the minimum nozzle crosssection to such an extent that upon opening of the nozzle valve (which, as is known, should be opened tained, I make the nozzle channel leading to the nozzle as small as possible, and in a further development of the idea, I have departed fromthe teaching of the explosion turbine artand also from the steam turbine art by employing, for each combustion chamber, a single discharge nozzle having a single unobstructed outlet, as disclosed in my copending application Serial No. 186,095, filed April 23, 1927. In this way, for a given total eiective area of nozzle outlet, I obtaina nozzle having a much smaller circumferential length than, for instance, a nozzle whose outlet is divided into a number of passageways by vanes or blades but has the same given total eifectivearea. I am thus enabled to construct the nozzle channel between the nozzle valve and the point of minimum cross-section of the nozzle as small as possible, so that the advantages in heat economy obtained by constructing the turbine in accordance with the formula f -17-40 to 100 may be more completely realized, as with al smaller nozzle channel the pressure is more rapidly built up therein and made equal to that prevailing 4.in thev explosion chamber upon opening of the l nozzle valve. y

I have observed in the operation of a number of explosion turbines built by me in accordance with the rules and formulas available' in this art, even those built in accordance with the formula hereinabove explained, that after the turbine has been running for a short time, pre-ignition sets in.

whereupon the operation of the turbine becomes unstable and nally ceases. This condition pointed to the fact that a very hightemperature, suillcient to ignite an incoming charge, had been reached in the explosion chamber. Both prac.- tice and theory have demonstrated that this rise' in temperature cannot ordinarily be prevented by Dumping more or cooler cooling medium through --the cooling Jackets with which the vexplosion chambers are usually provided. ""'I'he present invention aims pre-ignition and to provide anexplo'sion-turbine to `prevent such which is stable in operation and at the same time 'suffers a minimum of heat losses, so that the etliciency thereof is maintained at a high level.

I have determined that this pre-ignition is due to incomplete scavenging of the combustion chamber of the residual combustion gases, such residual gases progressively raising the explosion temperature until they are suillciently hot to ignite the incoming charge eve'n though admixed 4with a large proportion of scavenging air. That the presence of lcomparatively small amounts of successive fuel and air charge in the explosion chamber could so detrimentally aiiectthe operation of theturbine was hardly to be'expected, as explosion engines in general-are known tooperatesatisiactorily with 'in my United States my invention by observing a certain minimumrelationship discovered by me, and capable of being expressed mathematically, between 'certain elements of the explosion turbine and the time allotted to the scavenging portion of the explosion cycle of the combustion chambers. I have found that the duration of the scavenging period is of the highest importance and that the same is organically linked up with the Vdimensions of certain elements of the explosion turbine, and that this minimum relationshipI must be adhered to before a turbine stable in operation can be constructed. That this relationship existed was never before known, nor can the mathematical expression thereof be deduced from the hitherto Aknown relationships between the parts of an explosion turbine. VThis new relationship discovered by me must moreover be observed before advantage can be taken of-the optimum relationship exhaust pressures andthe dimensions of the' nozzle.

Fig. 1 Ashows an explosion chamber a of substantially cylindrical form which is periodically charged withI a mixture of air and fuel .through the valvesb and c, the mixture being exploded 4therein at predetermined instants by means of a spark plug or other ignition element After the explosion a nozzle valve d is, opened and thel combustion gases are permitted to escape into the nozzle channel e which directs them into an exe pansion nozzle 'a and they are then ldischarged 4against the blades h of the turbine rotor. It will be understood that the valves c, b, and d are timed and controlled in any suitable manner, as for instance by a controlling mechanism in the form of a pressure oil distributor m shown more in de-.

' tail in my'Patent No. 877,194.

After^the explosion and discharge lof the gases from the combustion chamber the latter is scavenged of the residual combustion gases. .This

scavenging may be eiIected by means of a stream of scavenging air charged by a separate scavenging valve (not shown), butI prefer to eiiect such scavenging by meansot the air designed to support the combustion-o! the fuel. To this end, the

combustion chamber a is made of elongated form, as illustrated, and the nozzle valve d kept open during the initial charging period for the combustion air. Due to the elongated formof the chamber this combustion air pushes' the residual combustion gases before it in the manner of a. piston. The nozzle valve is so timed that it is closed at the moment that the advance portion of such air reaches the outlet end o! the combustionchamber. The piston eiIect may be increased by making the inlet end of the explosion -chamber conical ,as shown in my copending application Ser` No. 376,135, tiled July 5. 1929 and Letters Patent No. 1,810,768. By so employing the charging air to effect 1,982,665' scavenging ofthe chambeni reduce the time for a complete explosion cycle by the time required to charge a separate stream of scavenging air.

- In the design of an explosion turbine which is -to have a givencapacity itis` necessary first to determine the number of explosion cycles that 4are to take place in each combustion chamber per minute. It is, of course, generally desirable tohave as many explosions per minute as pos-- sible in o'rder to increase 'the capacity of `the machine and thus increase the power output per ton of weight.

In, addition to the number of cycles tobe eml ployed per minute, it is necessary to determine l5 also the size of the combustion chamber, the size of the nozzle valve, the minimum cross-sectional area of the nozzle,l and the various pressure relations to be employed. The considerations involved in the dimensioning 'of these several elements are'not simple and are. not of a Y purely mathematical nature. These consideranozzle.

gases referred to under 9.

5. The form or shape of the combustion chamber, its ratio of length to diameter, andthe appendages upon its inlet and outlet ends.

6. The degreeof whirling of the residual combustion'gas'es with the incoming charge of scavenging' air.

7. Tl'le extent of heat exchange between the combustion gases and the wallsof the combustion chamber, the nozzle channel and the nozzle.

3. The temperature of the combustion gases.

9. The permissible amount of residual combustion gases in the'new explosive charge.

0f the above, only the factors 1, 2 and 3 can in general be determined mathematically from known data. With respect to the factor 5, certain rules of construction for explosion chambers be applied upon which factors Sand 'l are in part dependent. The number, size and form of the combustion chamber will in part depend upon the capacity of the turbine, which in a measure controls also the temperature of the combustion gases indicated under 8, and will depend in part also on the `nature of the fuel employed, which in turn bears a relation also to the permissible' amount of residual combustion My researchesihave indicated that the size of the minimum cross-section of the nozzle, listed under 4, is-of very great importance. As above indicated, by reducing this minimum nozzle crosssection below the size heretofore employed in this art, I reduce the amount of heat lost in the nozzle channel due to whirling, and though I 'increase theloss dependent on the time factor, yet the resultant is an enormous increase in heat econ.-` omy. On the other hand I have found that pre-f ignition is caused by the presence of residua combustion gases in the new charges fed to the vexplosion chambers, and that it is necessary to insure as complete scavenging as possible. The completeness of the scavenging will depend, at least in part, upon the time allowed for the same 'and' upon the size of the minimum cross-section attainedI has been 4by reducingv the. scavenging period 'by making the minimum nozzle `cross.

.sectionj relatively large to permit discharge of theresidual gases within va minimum, of time.

vThis reduction inI scavenging timeand increase in nozzle cross-section I havefound to have more or less criticalv limits from the standpoint both l of stable operation and of eiciency, first' because of my discovery that it is incomplete scavenging that' is responsible for the gradual building up of thetemperature within thel combustion chamber which ultimately.l causes pre-ignition, and

that therefore suliicient time must be allowed to permit substantially all of the residual combustion gases to escape from the combustion chamber; and secondly, because as the minimum nozzle cross-section is increased,`the volume of the 'nozzle channel is likewise increased so that, as explained above in connection with my copending application Serial No. 186,094, the heat losses due to whirling in such channel are enormously in creased. Different considerations thus require different sizes of minimum nozzle cross-section and different scavenging periods. mined, as indicated below, that a definite rela- -tion exists between the scavenging period, the

size of the explosion chamber, and the minimum nozzle cross-section, which relation, defines the conditions for both stable and eilicient explosion the drawing is indicated as taking place at the point 1 and consumes the time 1-1; secondly,

I have deterthe so-called saddlel from the instant l' to the instant 2, during which the nozzle valve is kept closed to insure complete combustion of the exploded charge, the valve being quickly opened and depends primarily upon the thermal content of the explosive mixture. I have found that optimum conditions are maintained when a mixture having a heat content of 400 to 450 kg. cal..p=.`

according to the example taken, 0.06 second for.

the explosion, 0.01 second for the saddle and 0.13 second for the charging. There thus remains v0.8 second for the expansion of the exploded gases and the scavenging of the residual gases remaining in the combustion chamber. The present invention is concerned primarily with the splitting or apportioning of this residual period of the explosion cycle in such manner that a stable operation of the turbine is assured while at the same time a high efficiency is obtained. y

The scavenging'period 3-4 is indicated as Z in Fig. 2. The

period with the ratio Pa until, upon reaching-the critical pressure relationship, such amount attains its highest value. the case of a uniformly cylindrical nozzle this critical pressure relationship is expressed by the the expansion ratio of the nozzle, F=the outlet area of the nozzle, and f=the minimum crosssection of -the nozzle (Fig. 3).

The graph of this formula is shown in Fig. 4

where the relationship between the values and p (the latter based on the assumption of an exhaust pressure p0=1.06) on one hand and on the` other, is indicated. It is clear from the graph that the greater the expansion ratio of the nozzle thesmaller is the pressure ratio and consequently the smaller is the necessary scavenging air pressure, and the more rapidly will the chamber therefor be freed of residual comy bustion gases.

The value yfor cannot be arbitrarily xed. In the first place it determines in large measure the velocity at'which the gases strike the turbine blades, and thus affects the' speed of the turbine; it determines also the dimensionsof the turbine rotor and the efiiamount of gasescaping from theA chamber a per unit of time rises during this "cated above.

ciency of the energy transference. from the gases to the rotor. On the other hand, the ratio determines the scavenging pressure to the extent*v that the.latter must not be below the pressure pL derived from the above formula, or. from the graph of Fig. 4, if an unnecessarily prolonged scavenging period, and hence arreduced number of cycles and increased loss of heat, or else an incomplete scavenging of the explosion chamber andthe consequent danger of pre-ignition,v are to beavoided. In this way the magnitude of a .90 number of the factors bearing on the operation of an explosion turbine may becalculated.

In accordance with my invention, the nozzle valve is held open during the charging of the air which effects scavenging of the explosion chamber for a period of time which is sufficiently long to insure proper scavenging. I have found that the duration of this period can be expressed in terms of the size of the explosion chamber and of the nozzle and that a distinct relationship exists between the minimum time which must be allowed for scavenging to insure substantially complete expulsion of `the residual gases inthe explosion chamber'and avoidance of pre-ignition,`

and the ratio where f and V represent the-magnitudes indi 110 This relationship is of paramount importance and must be observed if a stable turbine operation is tobe assured, and may be expressed as follows:

Z being the scavenging time in seconds (Fig. 2). This equation expresses mathematically the relation between the nozzle and the combustion chamber to insure substantially complete scavenging and avoidance of pre-ignition. As the other time divisions of the explosion cycle may be computed mathematically or fixed arbitrarily,

the relation between the scavenging time, the 105 chamber volume, and the minimum nozzle crosssection in effect states the interdependence of the same with the cycle or explosion frequency of the engine. Pre-ignition can be avoided on operation Y at the highest possible number of cycles only by 1n() operating with a scavenging air pressure which is not below the minimum determined by the nozzle ratio and by allowing for the scavenging a time interval which is not below about Referring now to the example given above,

wherein it was assumed that each chamber was to be operated at 60 cycles per minute and where- 140 in it was found that 0.8 second was left for both vthe expansion and scavenging periods, the values and Z may be determined as follows: It is known that be observed:

=f(T).* For best efficiency the following equationmust Assuming f n ,,100 A and substituting in the scavenging equation fz; 4o,

I obtain Z=0.4 sec.

The value of T can be obtained from Equation If this should be greater thanOA sec., so that Z-l-T 0.8, then the number of cycles per minute must be reduced, or else must be made greater than 100 and the heat economy of the turbine-sacriced. If T is less than 0.4 sec., say 0.2 sec., then either a lower value for may be selected, with consequent increase in heat economy in the nozzle channel, or the number of cycles per minute may be increased, so that greatl er capacity per unit of machine weight may be obtained. In this way the expansion and scavenging periods are determined and also the. ratio V 'Ihe actual value of ,f and V will' obviously depend upon the size ofthe machine andthe numberl equal to about 3.5 f gives very satisfactory results;

while with suitable mechanism the opening period may be reduced to 0.006 sec., whereby the whirling period in the nozzle channel is greatly reduced and the heat losses in such channel limited to about 10%. 1 l

As already indicated hereinabove, stability is a matter ofj primary importance to `which any conflicting considerations of eiicienc'y must yield,

asY a stable engine of a comparatively lower degree of eiciency is far preferable from a commercial standpoint to an unstable engine of higher eiciency. It is therefore obvious that in the construction of my engine, wherein the volume of the combustion chamber is V and the time allowed for scavenging Z, the smallest nozzle'cross-section 1 10000 y ,)Rf 1*] VT rem/s+ 273 p, where p2 pressurev in ats. at point 2 of Fig.

2 temperature in C. at point 2 of 2. p3 pressure in ats. at point 3 of Fig. l

(f) must be of a size substantially corresponding tothe formulaV .VZ- 40. y l

As alreadyexplainedJ is'not an absolute size but is the `minimum size of the smallest nozzle crosssection, and after this minimum has once been established it is obvious that4 from the standpoint of complete and eillcient scavenging this crosssection may be increased if necessary. If now the eiliciency formula f I--40 to 100,

which calls for an optimum or maximum value for the smallest nozzle cross-section, calls in any particular instance for such across-section larger than that called for by the scavenging formula, such cross-section should be, chosen as 'is called for by the efiiciencyv formula. If, on .the other hand, the eiciency formula calls for a. smaller cross-section than that called for by the scavenging formula, the result of the scavenging formula must control.

I claim:

1. In combination, an explosion chamber, air and fuel charging valves at the inlet end of said chamber, an outlet valve at the outlet end of said chamben a discharge element, a channel between,

said discharge element and the outlet .end of said chamber, means for exploding a combustible mixture fed by said charging valves, said outlet valve being adapted to be opened a definite time after I said explosion to "discharge the`v combustion gases into said discharge element, said air valve being adapted to be opened when the pressure in said chamber is a definite amount vabove the counter pressure beyond said discharge element, means for holding said outlet valve open for a' period sufficient to insure'substantially complete scavenging of said chamber of the residual combustion gases, and means for closing said valve at the end of such period, said period, measured in seconds, being substantially 40 times the ratio of the volume of the explosion chamber, measured in cubic meters, to the minimum cross-sectional area of the' discharge element, centimeters.

2. In combination, an explosion chamber, air and fuel charging valves at chamber, an outlet valve at the outlet end 4of said chamber, a discharge element, a channel between said discharge elementand the outlet end of said chamber, means for exploding a combustible mixture fed by said charging valves, saidoutlet valve being adapted to be opened a definite time after said explosion to discharge the combustion gases into said element, saidy air valve being adapted to be opened when the pressure in said chamber is a definite amount above the counter-pressure beyond said discharge element to charger air into such chamber at` a pressure above the critical pressure, means for holding said outlet valve open for a/'period suilicient to insure .substantially complete scavenging of ysaid chamber of the residual combustion gases, and means for closing said valve at the end of such period, said period, measured in seconds, being at least 40 times the ratio of the volume of the explosion chamber, measured in cubic meters, to the minimum cross-'sectionalarea of the discharge element, measured theinlet end 'of saidmeasured in square explosion engine which comprises charging air and {uel into an explosion chamber forming part of such engine, exploding said air andfuel, dis-v v charging the resulting combustion gases from said chamber into a discharge element, and charging and fuel into an explosion chamber forming part of such turbine, exploding said air and fuel, discharging the resulting combustion gases from said chambeiginto a. discharge element, and charging scavenging air into said chamber at a pressure above the critical pressure to drive out the resid- Kual combustion gases, said scavenging air being charged for a suiiciently long period to insure substantially complete scavenging of said chamber, said period measured in seconds being at least about-40 times the ratio of the volume of the explosion chamber, measured in cubic meters, to the minimum cross-sectional area of the discharge element, measured in square centimeters, said ratio being as closely as possible within the limits 1/100 to 1/40. Y

HANS HOLZWARTH.

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