EFFECT OF INLET SWIRL ON FLOW SEPARATION IN DIFFUSER

Abstract

The diffuser is a critical component of turbomachinery, such as the inlet portion of jet engines, gas turbines, axial compressors, etc. The annular diffuser is employed to efficiently convert kinetic energy into pressure energy within the shortest possible length. When the fluid passes through the diffuser, it is retarded, and diffusion comes into the picture. The diffusion process in the diffuser is very complex. There is a chance of developing an unfavorable adverse pressure gradient that may lead to flow separation due to severe loss of the gas turbine performance. A good diffuser design increases the efficiency of the power plant and minimizes the requirement for fuel, which leads to the benefit of society. The performance of an annular diffuser depends upon a large number of geometrical and dynamical parameters. Despite knowing the importance of an annular diffuser for turbomachinery, the literature present in the open forum is scanty due to the confidential nature of the application. Based on the available literature survey, the effect of inlet swirl on the performance of an annular diffuser has been studied. In addition, the effect of geometrical parameters has also been reported. The emphasis of the current work is to systematically investigate the various geometrical configurations such that higher performance can be achieved. In the present research work, experimental and numerical investigations have been carried out to study the effect of fully developed swirling and non-swirling flow to characterize the flow behavior and performance of the annular diffuser. The geometric design of the annular diffuser is calculated on different area ratios (i.e. 2 to 4) and varying casing angles. The flow conditions at the inlet are varied with different inlet swirl angles (0°-25°) to evaluate the effect of flow development inside the diffuser. The flow behavior of the annular diffuser is analyzed at Reynolds number 2.5 × 105 .

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The performance characteristics are assessed based on longitudinal velocity profiles, swirl velocity profiles, static pressure recovery coefficient, total pressure loss coefficient, and effectiveness. The velocity profiles were measured at several locations along the length of the diffusers. The numerical investigations were carried out using Fluent, a commercial

Computational Fluid Dynamics code. The obtained simulated outcomes using steady- state Reynolds-averaged Navier-Stokes equations with a two-equation turbulence

model were validated with the experimental data. The RNG k-ɛ model was used to capture the turbulence effects. The obtained results are analyzed, and it reveals that at a lower swirl angle, the separation is near the casing, whereas at a higher swirl, the point of separation shifts towards the hub side. Further, the introduction of adequate swirl intensity at the inlet is found to provide a substantial improvement in static pressure at the casing wall. The point of flow separation tends to shift away from the casing wall and can be completely vanished with a high degree of inlet swirl. However, it may appear at the hub. The maximum static pressure recovery coefficient and minimum total pressure loss coefficient are observed at the optimum value of the inlet swirl angle.