Aerospace and defense research

Abstract

An experimental study was conducted on the performance of synchronized sweeping jets (SJ) for film cooling applications. Various SJ designs were examined, each with a nozzle spacing between 8.5 and 6.0 Dh and an aspect ratio of unity. The synchronized SJ design was compared to an individual SJ as well as a pair of unsynchronized SJ of matching pitch. The frequency response, pressure ratio, spreading angle, and average velocity map were determined in order to characterize each of the designs. Cooling performance of each configuration was evaluated in a wind tunnel at various freestream turbulence levels (Tu = 0.5% and 11.0%) and blowing ratios (M = 1.0, 2.0, 3.0, 4.0). The cooling effectiveness of each design was calculated using wall temperature measurements. Although the synchronized SJ design had similar characteristics to the individual and unsynchronized SJ pair, they were found to have a lower cooling performance at higher blowing ratios. The cause of the decreased cooling effectiveness of the synchronized SJs was attributed to the inner alternating streamwise vortices acting as liftoff vortices for the cooling flow.

Reference

Spens, A., & Bons, J. P. (2021). Experimental investigation of synchronized sweeping jets for film cooling application. AIAA Scitech 2021 Forum. https://doi.org/10.2514/6.2021-2003

Abstract

Metal additive manufacturing (AM) of heat exchanger enables custom and conformal designs for a wide range of applications. However, one challenge with metal AM is the resultant surface roughness formed when using this process, which is nonexistent during traditional manufacturing processes. The goal in this study is to explore how this roughness impacts the pressure drop and flow field of a commonly used heat exchanger surface called an offset strip fin (OSF). Two OSFs of the same geometry are tested: one with an average fin roughness of 34 μm from metal AM and the other with an average fin roughness 2.5 μm, used as a baseline. The roughness from the metal AM process increased pressure losses and transitioned the flow to turbulent-like behavior at lower Reynolds numbers when compared with the smooth fin.

Laser Doppler velocimetry (LDV) measurements captured the row number in the fin array where transition from laminar to turbulent-like flow occurred. The location of transition from low to high turbulence levels occurred earlier in the fin array as the Reynolds number was increased for the smooth and rough fins. Wake profiles of time-averaged axial velocity were similar between the rough and smooth fins, with the rough fins having higher levels of turbulence intensity (TI) and less symmetric wake profiles. Overall, this study indicates that a pressure loss penalty is associated with using metal AM OSF due to the resultant surface roughness and an earlier transition to turbulent-like flow.

Wake profiles of time averaged axial velocity and turbulence intensity of the (a) smooth and (b) AM rough OSF geometries at Reynolds number of 300

Reference

Saltzman, D., & Lynch, S. (2021). Flow field measurements in a metal additively manufactured offset strip fin using laser doppler velocimetry. Journal of Fluids Engineering. https://doi.org/10.1115/1.4049245

Abstract

This paper presents the experimental results regarding the detonation wave dynamics, power production, and combustion instabilities of a disk-shaped pressure gain combustor (DPGC). Ethylene-air mixture was used for the DPGC experiments, and the experiments can be classified into two stages. For the first stage of the experiments, the DPGC was tested under the atmospheric back condition with a test duration of two seconds. The continuous detonation has been proven to be successfully established through the evidence from the dynamic wall pressure measurements. Detonation wave dynamics have been evaluated among various testing conditions at the air mass flow rate of ~0.15 kg/s from fuel-lean to rich conditions. The results show that the characteristic wave frequencies continuously change during the tests, which could be associated with the variations of the combustor thermal environment upon the existence of the detonation waves. Meanwhile, the reactant injection pressures have also been found to be affected, showing the air and fuel plenum pressures change with similar trends to the wave frequency shifts. The detonation wave onset frequencies are selected out from the pressure spectrograms at 0.1 s from the ignition for further discussion. The result shows that the detonation wave velocities produced in this combustor fall in between the classical CJ detonation wave velocities and the sonic velocities in the burnt gas.

For the second stage of the experiments, the DPGC was coupled with a turbocharger, aiming to use the shaft power extracted by the turbocharger turbine to evaluate the DPGC power production. A demonstration case is exhibited in this paper, which shows a power of 21.71 kW was extracted by the turbine from the DPGC exhaust when the combustion air flow rate was ~0.19 kg/s and the equivalence ratio was ~0.83. Coexisting with the detonation wave characteristic frequencies, low and intermediate frequency combustion instabilities showing sinusoidal pressure oscillations have been found in the combustor and the reactant injection plenums once the DPGC was coupled with the turbocharger. The sources of these combustion instabilities have been discussed, concerning the unsteady wave propagation and the Helmholtz resonance. The experimental results suggest that the DPGC has the superiority in compactness.

Reference

Huang, X., Chang, P.-H., Li, J.-M., Teo, C. J., & Khoo, B. C. (2021). Wave dynamics, power production and combustion instabilities of a disk-shaped pressure gain combustor. AIAA Propulsion and Energy 2021 Forum. https://doi.org/10.2514/6.2021-3664