Chenzhen Ji

Combustion Instabilities in a Bifurcating Tube: Open- and Closed-Loop Measurements

Combustion instabilities characterized by large-amplitude self-sustained oscillations are detrimental to gas-turbine and aeroengine systems. To mitigate these unwanted oscillations, a dynamic actuator is generally implemented. In this work, combustion instability in a bifurcating tube with a loudspeaker implemented is investigated. The bifurcating system is different from conventional Rijke-type and T-shaped ones. It can produce nonharmonic “hot” and “cold” oscillating flows, which provides a useful platform to aid experimental research and teaching on combustion instability. To monitor the flow and acoustic fields in the bifurcating branches, two arrays of pressure sensors and an infrared thermal-imaging camera are implemented. When the loudspeaker attached near the open end of the bottom stem is not actuated, the tube is corresponding to an open-loop thermoacoustic system. To gain insights on the distinguishing characteristics of the flow and acoustic fields, two-dimensional numerical simulations and ex...

Experimental evaluation of the acoustic damping effect of single-layer perforated liners with joint bias-grazing flow

As one of the most commonly used acoustic dampers, perforated liners are receiving wide spread interest for reducing engine noise and stabilizing combustion systems. It is a cylindrical sheet with perforated orifices fitted along the bounding wall of the combustor. In this work, the damping performance of seven single-layer perforate liners with different open area ratios are experimentally investigated. For this, a cold-flow pipe with lined section is designed. To simulate the practical engine, both grazing (mean flow through the pipe) and bias flows (air flow through the perforated holes) are applied. The effect of the joint grazing-bias flow on the liner damping behavior is studied. It is shown experimentally that the grazing flow can reduce the maximum acoustic power absorption, while the bias flow can increase the liners damping effect. Furthermore, the power absorption coefficient is varied periodically over forcing frequency. And the local maximum value is decreased with increased frequency. Finally, it is found that the increase of open area ratio does not necessarily increase the maximum power absorption or the effective frequency range. In order to simulate the liner damping behavior, a time-domain numerical model is used. It is shown that the liner thickness needs to be considered to correct the predicted damping trend so that the estimated acoustic power absorptions agree well with the measured ones over the interested frequency range.