Measurements of Density Pulsations in the Outlet Nozzle of a Combustion Chamber by Rayleigh-Scattering Searching Entropy Waves

Abstract

The development of measurement techniques, which enable temporal and spatial highly resolved density investigations even in harsh environments, is essential. Rayleigh scattering is a noninvasive optical measurement technique permitting such investigations. A Rayleigh-scattering measurement system is set up, providing a new insight into fluid mechanical processes in turbomachines. In this paper, Rayleigh scattering is used for the detection of density oscillations in the optical accessible convergent-divergent outlet nozzle of a small scale combustion test rig at various power consumptions and equivalence ratios. Until now, this part of the combustion chamber is sparsely investigated due to the challenging measurement conditions. The temporal density oscillation inside the nozzle can be shown up to 4 kHz as well as its spatial distribution. Systematic errors of the setup are investigated. Spectra of pressure and density oscillations are compared. Measurements with nonreacting air flow are conducted to study flow induced density fluctuations. Entropy noise related correlations between density and pressure fluctuations are found. Therewith, the builtup Rayleigh-scattering system enables investigations of the presumed region of indirect noise generation.

Analysis of chemiluminescence, density and heat release rate fluctuations in acoustically perturbed laminar premixed flames

“…(5) requires measurements of density perturbations. Different spectroscopic techniques can be envisaged to this purpose [38][39][40][41]. These perturbations are inferred here with a Laser interferometric vibrometer (LIV).…”

Spatial correlation in the ambient core noise field of a turbofan engine

“…4 A related topic is the noise generated by propagation of an entropy disturbance through a nozzle considered theoretically by Marble and Candel, 5 Leyko et al, 6 and Howe. 7 Experimental studies of indirect combustion noise generated by flow of an entropy disturbance through a nozzle have been conducted by Bake et al 8 and Rausch et al 9 A theoretical study of the generation of indirect combustion noise in an auxiliary power unit (APU) has been conducted by Xu, Tam, and Parrish. 10 Combustion chamber instability can be caused by a feedback mechanism which couples combustion chamber acoustics with convectively transported fluctuations of entropy as noted by Polifke, Paschereit and Döbbeling.…”

Interferometric determination of heat release rate in a pulsated flame

“…(1) to determine the heat release rate. Information on density fluctuations can be given by Rayleigh scattering [35][36][37][38] and hence the heat release rate can be estimated. Rayleigh scattering is caused by the elastic scattering of light on particles or molecules smaller than the wavelength of the light source applied [39,40].…”

“…The technique is influenced by parasitic light, e.g. of flames, the chemical composition of the gas [38], and needs thermal equilibrium [41,42]. Moreover it requires very clean gaseous flow conditions as the Mie scattering signal in the visible range is several orders of magnitude larger than the Rayleigh scattering if dust or particles are present within the flow [39].…”