Burners for Supersonic Ram-Jiets -
INSTABILITY IN A TWO-INCH RAM-JET BURNER
development I
J. B. FENN, H. B. FORNEY,
AND
R. C. GARMON
EXPERIMENT INCORPORATED, RICHMOND, VA.
S
Rough burning and its effects have plagued investigaINCE the inception of connection with the investitors of ram-jet burners from the beginning. The present gation of rough burning, intensive study of highhowever, a n additional work was undertaken in an attempt to determine the output ducted burners in causes of cornbustion instability in a ducted burner opermeasurement technique was connection with the developused. This comprised the ating under ram-jet conditions. ment of ram-jet propulsion, Three general types of instability were isolated and assodetermination of the static it has been the common exciated with a characteristic frequency of pressure fluctuapressure fluctuations with reperience of practically all tion. In each instance a simple theory was developed workers in the field t h a t under spect t o frequency and amwhich describes the observed behavior. plitude. Both crystal and certain conditions the comAs a result it becomes possible to design burners in which inductance bridge-type gages bustion process is unstable. these types of instability may be minimized or eliminated. * were employed together with The symptoms of this inappropriate amplifying and stability have been various, recording eauiDment. The including, in general, the remainder of the experimental details will be elucidated in the following to a greater or lesser degree: following discussion. Marked increase in burner noise 1. 2. Marked decrease in thrust or efficiency CAUSES O F ROUGH BURNING 3. Pressure fluctuations in the burner of constant or varying Early Observations. As has already been indicated, combustion frequency and amplitude instability known as rough burning was encountered from the Frequently, the pressure fluctuations become so violent that they very beginning of the development of ram-jet burners. However, culminate in extinction of the burner or actual destruction of its because experience with ram-jet burners and their behavior was 1 component parts. so limited and because of the urgency t o develop a successful Unfortunately, the conditions necessary and sufficient for the burner quickly, no particular study was devoted to the pheonset of burner instability have never been adequately defined or nomenon itself. Rather, it w~ recognized as something t o be described. This is due partly to the complexity of the phenomavoided. As a result of considerable empirical research of a trial enon and partly t o the fact that certain empirical developments and error nature, some burner cqnfigurations were developed in burner design have succeeded in eliminating this instability which were reasonably immune to instability over a range of to the extent that it no longer comprises an immediate threat to operating conditions. Nevertheless, during the course of this the successful operation of some ram-jets. As a result, all burner early development some general observations were made. Some instabilities have been more or less lumped together and classiof these observations have been reported ( 3 ) but are repeated fied by the descriptive but nonetheless vague term, “rough here as a matter of record. burning.” On the other hand, the term ‘kmooth burning” It was noted that ordinarily a burner configuration which was has come to connote t h a t which is desirable in the performance of capable of high efficiency was much less susceptible to rough a satisfactory burner. Moreover, there has been a tendency to burning than one which could not achieve high impulse or comdiscard or ignore burners which are rough and to devote the bustion efficiencies. This generally applies only t o burners bulk of attention and enterprise toward the development of operating with premixed and vaporized air-fuel streams. For burners which have demonstrated themselves capable of smooth example, a burner comprising a simple 0.75-inch oxyhydrogen burning. pilot cone is much more prone to instability than one comprising Believing that in the long run successful and intelligent burner the same cone with the addition of four radial gutters. Corredevelopment and design will depend, among other things, on an spondingly, with a 14-inch tail pipe and comparable inlet condiunderstanding of the causes and effects of burner instability, tions the former attains a n impulse efficiency of 68% as conthis laboratory has devoted considerable time to a study of rough trasted with 90% in the latter case (4). Although by increasing burning as observed in a 2-inch ducted burner. The present the tail pipe length the impulse efficiency obtainable with poor report summarizes the work which has been carried out off and igniters could be greatly increased, the tendency toward rough on over the last 2 or 3 years. No brief is held for the generality burning was not decreased. The instability seems to be associor the applicability t o other burners of the results which have ated with the performance of the igniter itself rather than with been obtained. However, it is hoped that some clues to a n the over-all performance of the burner. understanding of the phenomenon will be recorded. I n this same connection it was observed in the cape of piloted burners that the tendency toward rough burning was increased EXPERIMENTAL PROCEDURES as the amount of heat input by the piloting device was decrcased. Moreover, a sudden susceptibility of a n ordinarily smooth burner -4 description of the general featui-s of the 2-inch burner to roughness not infrequently was traced to failure of pilot heat facility has already been reported together with a discussion of input occasioned by leaks in the pilot fuel feed lines. the methods and techniques used in the measurement of the Both of the above observations seemed to indicate that i n variables associated with the performance of a burner ( 4 ) . In
-
1663
1664
INDUSTRIAL AND ENGINEERING CHEMISTRY
+e"-
1,875'' I T
Figure 1.
WI F,,,,,,;',;,j t PI101
40"-
&
