ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT amplified by the combustion process, having a frequency equal t o a resonant frequency of the gas in the burner. A program was carried out in the General Electric Co. by the authors to investigate the combustion instability phenomenon commonly known as screech. Tests were performed using burners ranging from 6 to 18 inches in diameter, as well as burners of square and rectangular cross section. Preheated air entered the burner at temperatures between 400" and 1600" F. and velocities between 100 and 450 feet per second. Many different burner lengths and flame holder configurations were employed. Excellent correlation was found between the measured screech frequencies and the theoretically predicted transverse resonant frequencies of the gas in the burner. The occurreiice of screech depends on burner configuration, flow parameters upstream from the combustion zone, and combustion properties of the gas. An oscillation of the flame front accompanied screech; the measured frequency of this osciIIation was equal to the measured screech frequency.
ORGAN-PIPE OSCILLATIONS IN DEEP-PORTED BURNER ABBOTT A. PUTNAM AND WILLIAM R. DENNIS Baffelle Memorial Insfifufe, Columbus,
D
Ohio
ATA are presented on acoustic oscillations produced by a burner using a hexagonal bank of hypodermic tubes as deep ports. Most of the tests were made with ethane as the fuel, but some tests were run with methane and propane, for comparison purposes. Both the diameter and length of the combustion chamber were varied during the tests. The combustion chamber could be considered as a driver which forced the slugs of gas in the ports to oscillate. Burning of the incremental pulses of combustible mixture periodically issuing from the ports furnished energy to drive the oscillations when the pulses burned in phase with the oscillating component of the pressure in the chamber. The phasing depended on a time-lag factor which was a function of the velocity of the gases through a space, similar to dark space, between the burner ports and the mean flame surface, and the width of the space itself. The oscillations not only ceased when there was a failure to satisfy the timing criterion, but also ceased when the air/fuel ratio approached either rich or lean blowoff limits of the conventional type. This latter cessation apparently was connected with the
fact that the flame burned from fewer and fewer ports as the limits were approached, and thus less driving energy was available.
INACCURACIES IN HIGH-SPEED OSCILLATORY PRESSURE MEASUREMENTS *
R. 8. LAWHEAD
Norfh American Aviafion, Inc., Propulsion Field laboratory, Chafsworth, Calif.
P
ICKUP calibration, sensitivity to mechanical vibration, and probe effects are some of the problems encountered in the measurement of oscillatory pressures. A dynamic calibration of a condenser-type pressure pickup may be made by using a small cylinder chamber excited by a n acoustic driver unit giving root mean square prepsure amplitudes to 0.05 pounds per square inch. By alternately measuring the pressure variations a t the end wall of the chamber with the test pickup and a calibrated microphone and comparing the results, it is possible to obtain a frequency response-calibration a t frequencies to 4000 cycles per second. In some applications, particularly those involving combustioni.e., rocket engines-the pressure pickups may be subjected to severe mechani cal oscillations that can cause spurious output signals. The existence of such mechanical effects can not always be detected from ordinary laboratory shake table tests. Therefore, actual environmental testing, with the pickup mounted alternately in a normal and blind pressure tap, is recommended. Occasionally it is necessary to use some sort of pressure tap in which the pickup diaphragm is not flush with the chamber wall for the measurement of high-amplitude, high-frequency oscillating pressures. This may introduce large changes in amplitude and phase which must be accounted for in interpreting such data. For small amplitude oscillations, equations analogous to those of electrical transmission lines give an adequate approximation. However, a t large amplitudes these equations no longer apply. Experimental data have been obtained to show:
1. The effect of probe length and volume on the amplitude and phase of the indicated pressure 2. The introduction of spurious resonances by pressure probes 3. The attenuating effect of undegassed liquid trapped in the pressure probes This work was conducted as part of 4ir Force Contract A F 33(038)-19430.
Reprints .of this symposium may be purchased ;for F$1.25 -each from-the Reprint Department, American Chemical Society, 1 155 Sixteenth-St., N.W., Washington 6, D. C.
E N D OF S Y M P O S I U M AND E N D OF E N G I N E E R I N G , DESIGN, AND PROCESS D E V E L O P M E N T - S E C T I O N
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 41, No. 6
