Acoustic attenuation experiments on subscale, cold-flow rocket motors.

Abstract

Acoustic dissipation studies were undertaken using a subscale cold-flow rocket motor model to determine the effect of purely geometric variables on the acoustic performance leading to axial mode combustion instability. Cold air served as the working fluid to simulate the internal flow and to obtain a critical nozzle throat. By using interchangeable parts, it was possible to vary the nozzle throat diameter and to simulate the geometry of a cylindrical-bore grain at various motor burn times. The model was acoustically driven by three separate methods: pulse decay, steady-state decay, and steady-state resonance. The methods are compared, and the applicability of each is discussed. Test results show that the transmission of acoustic energy through the nozzle was the largest source of loss for the axial mode and that the losses increase linearly with / (ratio of nozzle throat area to grain port area). Acoustic bulk and wall losses were negligible compared with the nozzle losses. The results also showed that the nozzle losses are independent of the way Jis changed and thus can be scaled. The test equipment has been established as a semi-automated facility to evaluate models with more complex geometries, to test chamber damping devices, and to gather design and research data. Nomenclature a* = critical nozzle velocity, m/sec c = velocity of sound at stagnation conditions, m/sec c p = specific heat at constant pressure, joules/kg °C D = diameter of the grainport channel, m /o = first mode resonant frequency, cps f lt f 2 = half-power frequencies, cps J = ratio of nozzle throat area to grain port area L = length of motor chamber, m M = Mach number p = acoustic or a. c. pressure (p-to-p), newton/m 2 po = initial acoustic or a.c. pressure (p-to-p), newton/m 2 p r = Prandtl number Q = quality factor t = time, sec V = d.c. or mean chamber gas velocity, m/sec a = temporal damping constant, sec" 1 7 = ratio of specific heats K = thermal conductivity of gas, joule/m sec°C I JL = viscosity, newton sec/m 2 TT '= 3.1416 p = d.c. or mean gas density, kg/m 3 PO = stagnation gas density at nozzle, kg/m 3 p* = d.c. gas density at sonic nozzle throat, kg/m 3 co = angular frequency, r ad/sec UQ = angular resonant frequency, rad/sec