OCR Text |
Show 10 of the images is caused by shallow depth of field of the optics used. Photographic Results The pulse combustor was photographed at the lowest and highest achievable firing rate at several exhaust back pressures, or boost pressures. The purpose of operating at these conditions was to examine the characteristics that occur at the extremes of the stable operating region. Also, photographs were taken at midpoints at the operating regime such that the pulse combustor was operating stably. Comparing the CH radical images from the extreme operating conditions to the stable operating condition yields an understanding of why unstable operation occurs. Figures 4 shows a typical result from the CH filming experiments. These sequences were taken at 4-inches pressure boost, with the P/C operating in the stable range, about 76,700 Btu/hr. The photographs shown in this figure were reproduced from sequential frames of a 16 millimeter high-speed movie. Each frame represents 0.001 seconds of time. The sequences begin at near zero operating pressure in the combustion chamber, and show the progressing of the cycle beginning with increasing negative pressures. An oscilloscope trace was superimposed upon the film during the experiment. Two dots are shown at the lower left of each frame. One dot is a reference point at zero pressure. The other is the combustion chamber pressure. Due to the optics involved in superimposing the oscilloscope image on the film, the oscilloscope image was reversed. Consequently, negative pressure is shown by the dot rising above the reference dot. When the pressure is positive, the moving dot falls below the reference dot. Also, due to the fnct that the pressure transducer was located 8 inches f;urn the combustor, there is a time lag in the measurement. Pressure waves traveling at the speed of sound (1100 ft/sec) require 0.001 seconds to travel to the transducer. An additional correction was needed because the transducer was sampling air-valve chamber pressure. This pressure lags combustion chamber |