Fundamental Mechanism of Entropy Noise in Aero-Engines: Experimental Investigation

Abstract

Entropy noise caused by combustors increases rapidly with rising Mach number in the nozzle downstream of the combustion chamber. This is experimentally shown with a dedicated test facility, in which entropy waves are generated in a controlled way by unsteady electrical heating of fine platinum wires immersed in the flow. Downstream of the heating module called entropy wave generator (EWG), the pipe flow is accelerated through a convergent-divergent nozzle with a maximum Mach number of 1.2 downstream of the nozzle throat. Parameters like mass flux of the flow, nozzle Mach number, amount of heating energy, excitation mode (periodic, pulsed, or continuously), and propagation length between EWG and nozzle have been varied for the analysis of the generated entropy noise. The results are compared with the results of a one-dimensional theory found in early literature.

On the dissipation and dispersion of entropy waves in heat transferring channel flows

“…These resulted in improving the predictability of the acoustic conversion of entropy waves. For instance, the results of wellcontrolled experiments of Bake et al [19,20] in an entropy wave generator were initially in fair agreement with the classical prediction of Marble and Candel [12]. The causes of disagreement were explained and mostly resolved in the recent refinements of the original theory [15,11].…”

“…(4) also shows that in the absence of acoustic waves and by excluding broadband fluctuations of pressure, entropy waves reduce to temperature or density waves [10]. A temperature pulse is added to the air flow using a linear increment and an exponential decrement function [20]. Unlike the induced heat source function of Morgans et al [28], this has the capacity of practical implementation [20] and a lower potential for inducing numerical errors.…”

“…Due to the unknown span-wise and altitudinal position of the thermocouple in Ref. [20], the mass-weighted average of the flow temperature is shown in Fig. 4.…”

“…It is also noted that the very low Mach number of the flow in the current study makes it comparable to an incompressible flow. Nonetheless, to establish further validation, the computed history of the temperature increment was compared against the experimental results of Bake et al [20] (see Fig. 4).…”

“…A temperature pulse is added to the air flow using a linear increment and an exponential decrement function [20]. Unlike the induced heat source function of Morgans et al [28], this has the capacity of practical implementation [20] and a lower potential for inducing numerical errors. This high temperature region, with the amplitude of 1.1 times the absolute temperature of the base flow, then advects through the channel and develops an entropy wave.…”