Design of compressor impeller under high Reynolds number conditions

Purpose A parametric method for designing the hub, casing and blades of the miniature centrifugal compressor impeller was developed. The relationship model of the size, aerodynamic and performance parameters of the centrifugal impeller was established. Based on the selected design parameters, the miniature centrifugal-type impeller was designed, and the work efficiency was calculated. Design/methodology/approach In this study, a micro-centrifugal compressor impeller with a diameter of less than 25 mm was designed. A parametric design method was developed, and the functional relationship between the geometric and gas fluidity parameters was established. Findings The results of this study showed that the performance parameters of the designed micro-centrifugal impeller satisfied the design requirements. The proposed method is useful as a reference for designing and analysing compressor impellers under high Reynolds number conditions. Originality/value A parametric design method was developed, and the functional relationship between the geometric and gas fluidity parameters was established. Under the Reynolds number conditions, the flow characteristics of the gas in the compressor were analysed; the shear-stress transport turbulence equation was solved using the finite volume method. In addition, the effects of the Reynolds number on the velocity, pressure, mass flow and efficiency of the micro-scale centrifugal compressor were evaluated. The results showed that the performance parameters of the designed micro-centrifugal impeller satisfied the design requirements. The proposed method is useful as a reference for designing and analysing compressor impellers under high Reynolds number conditions.

Friction Factor Comparison Between Mixing Length Theory and Empirical Correlation

Equivalent sand grain roughness is required for estimating friction factor for engineering applications from empirical relation via Haalands equation. The real surfaces are different from the sand grain profile. The correlations for friction factor were derived from use of discrete roughness elements with regular shapes such as cones, bars etc. The purpose of the paper is to derive analytical expression of friction factor for a 2 dimensional semi-cylindrical roughness (not exactly a 3 dimensional sand grain but for the circular profile of cross- section) using Navier Stoke equation and mixing length theory. This is compared with the modified series mathematical representation of Haalands equation for friction factor in terms of equivalent sand grain roughness. The comparison is valid for high Reynolds number where the velocity profile is almost flat beyond boundary layer and approximately linear all throughout the boundary layer. The high Reynolds number approximation for Haalands equation is derived and the series form of the friction factor compares approximately with the series form derived from first principles, where in the exponents of the series expansion are close.

High Reynolds number wind turbine blade equipped with root spoilers. Part I: Unsteady aerodynamic analysis using URANS simulations

A wind turbine blade equipped with root spoilers is analysed using 2D Computational Fluid Dynamics (CFD) to assess the unsteady impact of passive devices. Several metrics such as lift and drag coefficients, pressure and instantaneous velocity field around the aerofoil, Power Spectral Density and Strouhal number are used in the 2D unsteady analysis. The spoiler is found to efficiently rearrange the flow, adding lift throughout the positive angles of attack. However, the drawback is a high drag penalty coupled with high unsteadiness of the aerodynamic forces.

Investigation of internal intermittency by way of higher-order spectral moments

The analysis of turbulence by way of higher-order spectral moments is uncommon, despite the relatively frequent use of such statistical analyses in other fields of physics and engineering. In this work, higher-order spectral moments are used to investigate the internal intermittency of the turbulent velocity and passive-scalar (temperature) fields. This study first introduces the theory behind higher-order spectral moments as they pertain to the field of turbulence. Then, a short-time Fourier-transform-based method is developed to estimate these higher-order spectral moments and provide a relative, scale-by-scale measure of intermittency. Experimental data are subsequently analysed and consist of measurements of homogeneous, isotropic, high-Reynolds-number, passive and active grid turbulence over the Reynolds-number range $35\leq R_{\lambda } \leq ~731$ . Emphasis is placed on third- and fourth-order spectral moments using the definitions formalised by Antoni (Mech. Syst. Signal Pr., vol. 20 (2), 2006, pp. 282–307), as such statistics are sensitive to transients and provide insight into deviations from Gaussian behaviour in grid turbulence. The higher-order spectral moments are also used to investigate the Reynolds (Péclet) number dependence of the internal intermittency of velocity and passive-scalar fields. The results demonstrate that the evolution of higher-order spectral moments with Reynolds number is strongly dependent on wavenumber. Finally, the relative levels of internal intermittency of the velocity and passive-scalar fields are compared and a higher level of internal intermittency in the inertial subrange of the scalar field is consistently observed, whereas a similar level of internal intermittency is observed for the velocity and passive-scalar fields for the high-Reynolds-number cases as the Kolmogorov length scale is approached.

Quasi-periodic forces and conditionally averaged characteristics of unsteady cavitation flow around a hydrofoil

Large-eddy simulations are performed to investigate the cavitating flow around two dimensional hydrofoil section with angle of attack of 9° and high Reynolds number of 1.3×106. We use the Schnerr-Sauer model for accurate phase transitions modelling. Instantaneous velocity fields are compared successfully with PIV data using the methodology of conditional averaging to take into account only the liquid phase characteristics as in PIV. The presence of two frequencies in a spectrum corresponding to the full and partial cavity detachments is analysed.