Direct measurements of entrainment by acoustically pulsed axisymmetric air jets flowing into surrounding air have been made for a range of orifice sizes, Strouhal numbers, and excitation powers. The entrainment was considerably increased, by up to 5.8 times at distances greater than 15 diameters axially downstream of the orifice exit plane. The entrainment of the excited jet varied linearly with downstream distance. The jet response varied nonlinearly with excitation strength, indicating that there may be a practical upper limit to the acoustic augmentation of entrainment. The response depends on Strouhal number and appears to be optimum at about 0.25.

The mixing of an acoustically pulsed air jet with a confined hot crossflow has been assessed by temperature profile measurements. These novel experiments were designed to examine the affects of acoustic driver power and Strouhal number on jet structure, penetration and mixing. The results showed that excitation produced strong changes in the measured temperature profiles. This resulted in significant increases in mixing zone size, penetration (at least 100% increase), mixing, and the length to achieve a given mixed state was shortened by at least 70%. There was strong modification to the jet-wake region. The increase in jet penetration and mixing was saturating near 90 W the largest driving power tested. The jet response as determined by penetration and mixing was optimum at a Strouhal number of 0.27. Overall, pulsating the jet flow significantly improved the jet mixing processes in a controllable manner.

A small combustor of normal design employing acoustic control of the dilution-air flows has been successfully tested up to “half-load” conditions. It has been shown that this technique can be used to control the exit plane temperature distribution; also the ability to trim the temperature profile has been convincingly demonstrated. The acoustic driver power requirements were minimal, indicating that driver power at “full-load” will not be excessive. The nature of the acoustically modulated dilution-air flows has been clearly established as that of a pulsating jet flow with superimposed toroidal vortices. The pressure loss of the unit and its combustion efficiency were insignificantly affected by the acoustic drive. The work contributes to the design of combustors such that a desired exit plane temperature distribution may be achieved.

Novel direct measurements of the entrainment coefficient in the initial zone of a pulsating air jet have been successfully accomplished. A series of pulsating air jets flowing from different nozzle orifice sizes into surrounding air were investigated for the effects of jet axial length, excitation power and Strouhal number. The entrainment coefficient of the excited jet varied strongly with axial distance downstream of the orifice exit plane and with pulsation strength. The acoustic drive considerably increased the entrainment coefficient by up to 4.6 times at 10 diameters downstream of the nozzle. There was only a tendency for the entrainment coefficient to increase with the Strouhal number and hence an optimum Strouhal number for jet response was not found.

Successful NOx measurements at the end of the primary zone of a small tubular combustor of conventional gas turbine design, employing acoustically controlled primary zone air-jet mixing processes, have been made at scaled 1/4 and 1/8 load operating conditions. Testing at 1/8 load significantly increased the effective strength of the acoustic drive, which strongly improved the mixing by the acoustically driven primary zone air-jets. The acoustic drive caused partial blockage of the combustor primary zone airflow. This increased the equivalence ratio and the gas temperature, and made the gas temperature distribution more uniform, except for lean conditions at 1/8 load, in the plane of the NOx measurements. This explained the measured greater NOx “with-drive,” and the distinctly more uniform NOx distribution, which confirmed that mixing was acoustically augmented. The acoustically produced changes were greater at 1/8 load. The acoustic drive significantly changed the combustor operating characteristic so far as mean NOx was concerned, and under lean conditions at 1/8 load mean NOx was reduced, indicating that a value of 10 ppm is possible (a 50 percent reduction).