Abstract:In the field of axial flow turbomachines, the two-dimensional cascade model is often used experimentally or numerically to investigate fundamental flow characteristics and overall performance of the impeller. The core of the present work is a design method for axial fan cascades aiming to derive inversely the optimum blade shape based on the requirements of the impeller and not using any predefined aerofoil profiles.While most design strategies based on the aerofoil theory assume constant total pressure at all streamlines, i.e. free-vortex flow, this paper investigates the possibility of varying the total pressure along the blade and based on that, an analytical expression of the outlet blade angle is determined. When computing the blade profile at a specified radius, critical parameters reflecting on the flow characteristics are observed and adjusted (i.e. sufficient lift and controlled deceleration of the flow on the contour) so that the resulting profile is derived for minimum losses.The validation of this design strategy is given by the numerical results obtained when employed as an optimization tool for an industrial fan: 10-20 per cent absolute increase in the static efficiency of the optimized impeller.

Screw compressors are complex flow systems, but operate upon simple considerations: they are positive displacement machines consisting of meshing rotors contained in a casing to form a working chamber, whose volume depends only on the angle of rotation. Their performance is highly affected by leakages, which is dependent on various clearances and the pressure differences across these clearances. Nowadays, the manufacturing and profiling techniques have matured so much, that rotors of even the most complex shapes can be manufactured to tolerances in the order of few microns, resulting in high efficiencies. With manufacturing tolerances this tight, there is only small amount of improvement expected from further exploration of this venue, and a rather different direction for analysis may be more rewarding, i.e. other components of the screw compressor, like the suction and discharge areas. While the available literature includes several references on improvements of the compressor performance based on the analysis of the discharge port and discharge chamber, the investigation of the suction arrangement and inlet port remains fairly unexplored. This is the area of concern for the present paper, where the influence of the port shape and suction arrangement on the overall compressor performance is investigated. Two suction models were investigated for a standard screw compressor by means of CFD, which allowed in-depth analyses and flow visualizations, confirmed by the experimental investigation carried out on the actual compressor.

The core of the present work is an axial blade design procedure which stands as premises for an optimization tool for axial fans. The initial design assumptions made refer to the computation of the blade angles and the determination of the blade shape. The back-bone of the present work is a design solver which computes inversely the blade shape, based on the assumption that the angle distribution between the inlet and the outlet blade angles is parabolic. The flow-optimized blade shape of an established industrial fan, used in engine-cooling applications, was derived using this design solver. Intensive numerical investigations were carried out for both the industrial and the optimized impellers, and an appropriate performance indicator had to be found. The static-to-total efficiency is an accurate performance indicator and when computing it for both impellers, it was noticed that the optimized blade impeller performs better and a 24% difference in the efficiency curves was observed. Also, by computing the total-to-total efficiency curves, the optimized blade performed better and a 13% difference between the two impellers was found.

The present paper addresses a design method for pump inducers which allows the full calculation of the optimum impeller configuration for improved cavitation performance. This method permits a tight control of the blade geometry since it computes inversely the required blade shape to attain the desired stream-wise angle distribution. The blade profile is fully resolved by implementing a meridional analysis into the design process, where the radial head gradient (RHGR) is the design variable. Additional variables are the outlet blade angle and the diffusion factor (DF). Essential for the proposed method is the parabolic angle distribution assumption made to fully resolve the blade profile. Several new designs are derived for different outlet angles using the fore-mentioned design method and comparative multi-phase CFD analysis was performed, which allowed the determination of the optimum RHGR and outlet blade angle. It was shown that the models characterized by the highest hub loading delivered the best performance under the cavitating regime, with the smallest values for the inception cavitation number and the critical cavitation number. Because the overall suction performance of the pump, especially the breakdown point, is a direct measure of the inducer performance, the presented design method proves very advantageous since it delivers inducers optimized for the cavitation regime.

In the field of axial flow turbomachines, the two–dimensional cascade model is often used experimentally or numerically to investigate fundamental flow characteristics and overall performance of the impeller. The core of the present work is a design method for axial fan cascades aiming to derive inversely the optimum blade shape based on the requirements of the impeller and not using any predefined airfoil profiles. While most design strategies based on the airfoil theory assume constant total pressure at all streamlines, i.e. free–vortex flow, this paper investigates the possibility of varying the total pressure along the blade and based on that, an analytical expression of the outlet blade angle is determined. When computing the blade profile at specified radius, critical parameters reflecting on the flow characteristics are observed and adjusted, i.e. sufficient lift and controlled deceleration of the flow on the contour so that the resulting profile is derived for minimum losses. The validation of this design strategy is given by the numerical results obtained when employed as an optimization tool for an industrial fan: 10–20% absolute increase in the efficiency of the optimized impeller.