János Vad

Three-dimensional flow in axial flow fans of non-free vortex design

Abstract Three-dimensional laser Doppler anemometer (LDA) measurements were carried out downstream of isolated axial fan rotors of non-free vortex design in order to investigate the role of radial velocity components in design. The structure of secondary flows due to non-free vortex operation was studied in detail. It is pointed out that the tangential gradient of radial velocity at midspan is nearly in direct proportion with the spanwise gradient of ideal total head rise prescribed in design. Design criteria have been established for the neglect of torsion of stream surface segments inside the blading. A linear relationship was proposed in order to estimate the pitch-averaged radial velocities at the rotor exit. Using this relationship, a proposal has been put forward for taking the radial velocity components into account in non-free vortex design with the assumption of conical stream surfaces through the blading.

Aerodynamic effects of blade sweep and skew in low-speed axial flow rotors at the design flow rate: An overview:

Abstract A detailed literature survey is presented herein in order to overview the aerodynamic impact of non-radial blade stacking techniques applied to axial flow fan and compressor rotors. The literature suggests a consensus that forward blade sweep and skew provides a means for the following advantages in the part load operational range of low-speed axial flow turbofan and compressor rotors: improvement of efficiency and performance, and extension of stall-free operating range. However, the published research results are rather diversified regarding the judgment of performance and loss modifying effects of sweep and skew at the design point. The current paper summarizes the major aerodynamic phenomena related to such blade stacking techniques, in order to contribute to a general reasoning of performance and efficiency modification at the design flow rate. Furthermore, it provides guidelines how to consider these phenomena in tailoring the blade geometry for potential efficiency gain and for achievement of the prescribed total pressure rise at the design point. The role of adequate computational fluid dynamics tools was considered in the paper to be essential in evaluation of aerodynamic effects of non-radial blade stacking, as well as in incorporation of sweep and skew in systematic blade design techniques.

Effects of blade sweep on the performance characteristics of axial flow turbomachinery rotors

Abstract Experimental studies were carried out in order to survey the performance and efficiency aspects of spanwise constant forward and backward sweep in axial flow rotors of low aspect ratio (AR) blading for incompressible flow, for part-load, near-design, and overload operational ranges. The experiments involved overall performance tests, laser Doppler velocimetry, and stationary total pressure probe measurements. The overall performance data and pitchwise averaged and resolved flow characteristics were evaluated in detail. For moderate and high flow rates, it was pointed out that positive or negative sweep tends to reduce or increase the blade load in the vicinity of the endwalls, respectively. It has been concluded that the loss-modifying effect of sweep can be judged by considering the three-dimensional viscous phenomena, and the influence of sweep on local blade efficiency depends on the balance of changes in blade load and losses. For low flow rates, forward sweep was found beneficial over the entire span from the aspect of improved stall margin and efficiency. The influence of AR on the performance reducing effect of sweep was studied on the basis of literature data.

Aerodynamic effects of forward blade skew in axial flow rotors of controlled vortex design

Abstract Comparative studies have been carried out on two axial flow fan rotors of controlled vortex design (CVD), at their design flowrate, in order to investigate the effects of circumferential forward skew on blade aerodynamics. The studies were based on computational fluid dynamics (CFD), validated on the basis of global performance and hot wire flow field measurements. The computations indicated that the forward-skewed blade tip modifies the rotor inlet condition along the entire span, due to its protrusion to the relative inlet flow field. This leads to the rearrangement of spanwise blade load distribution, increase of losses along the dominant part of span, and converts the prescribed spanwise blade circulation distribution towards a free vortex flow pattern. Due to the above, reduction in both total pressure rise and efficiency was established. By moderation of the radial outward flow on the suction side, being especially significant for non-free vortex blading, forward sweep was found to be particularly useful for potential reduction of near-tip loss in CVD rotors. Application of reliable CFD-based design systems was recommended for systematic consideration and control of both load- and loss-modifying effects due to non-radial blade stacking.

Forward Blade Sweep Applied to Low-Speed Axial Fan Rotors of Controlled Vortex Design: An Overview

An overview is given on the research maintained by the author about the design aspects of three-dimensional blade passage flow in low-speed axial flow industrial fan rotors, affected by spanwise changing design blade circulation due to controlled vortex design (CVD), blade forward sweep (FSW), and their combination. It was pointed out that, comparing the CVD method to the free vortex design, the fluid in the blade suction side boundary layer has an increased inclination to migrate radially outward, increasing the near-tip blockage and loss. It was concluded that the benefit of FSW, in terms of moderating loss near the tip, can be better utilized for the rotors of the CVD, in comparison to the free vortex design. Compared to the free vortex design, the FSW applied to the blades of the CVD was found to also be especially beneficial in loss reduction away from the end-walls, via shortening the flow paths on the suction side-in any case being elongated by the radially outward flow due to CVD-and thus, reducing the effect of wall skin friction. The necessity of correcting the swept blades was pointed out for matching with the prescribed CVD circulation distribution. [DOI: 10.1115/1.4007428]

Influence of Blade Sweep on the Energetic Behavior of Axial Flow Turbomachinery Rotors at Design Flow Rate

Experimental and computational studies were carried out in order to survey the energetic aspects of forward and backward sweep in axial flow rotors of low aspect ratio blading for incompressible flow. It has been pointed out that negative sweep tends to increase the lift, the flow rate and the ideal total pressure rise in the vicinity of the endwalls. Just the opposite tendency was experienced for positive sweep. The local losses were found to develop according to combined effects of sweep near the endwalls, endwall and tip clearance losses, and profile drag influenced by re-arrangement of the axial velocity profile. The forward-swept bladed rotor showed reduced total efficiency compared to the unswept and swept-back bladed rotors. This behavior has been explained on the basis of analysis of flow details. It has been found that the swept bladings of low aspect ratio tend to retain the performance of the unswept datum rotor even in absence of sweep correction.Copyright

Incorporation of forward blade sweep in preliminary controlled vortex design of axial flow rotors

A new controlled vortex design (CVD) method has been presented herein, incorporating forward blade sweep (FSW) in the preliminary design phase of axial flow rotors. Supplementing the traditional quasi-two-dimensional (Q2D) technique applied to CVD rotors of notable radial flow, the new method enables a more accurate consideration and control of blade aerodynamics along the three-dimensional suction side flow paths, where the majority of loss is generated away from the endwalls. For this purpose, the new method unifies Q2D and quasi-three-dimensional (Q3D) blade design approaches for CVD rotors. The incorporated Q2D and Q3D approaches can rely on traditional cascade correlations. The Q3D approach is to be aided by computational fluid dynamics. In contrast to the view dominating in literature that sweep is prescribed arbitrarily in preliminary design, the new method serves with the FSW angle distribution as design output. This means that the favourable blade stacking geometry is a result of the proposed preliminary design process. A design case study has been presented for demonstrating the consistency and advantages of the proposed method. At the design flow rate, fair agreement was found between the simulated and modelled three-dimensional flow features for the FSW rotor. Furthermore, moderation of total pressure loss was observed on the suction side away from the endwalls, leading to efficiency gain.

Radial fluid migration and endwall blockage in axial flow rotors

Abstract The article presents an analytical model for systematic investigation of the effects stimulating or retarding radial outward migration of fluid in the suction side (SS) boundary layer of the blades of axial flow rotors, contributing to increased near-tip loss and promoting tip stalling. With application of the model to an experimental case study, coupled with evaluation of detailed flow measurement data, it has been pointed out that the outward migration and near-tip accumulation of high-loss fluid are intensified by the controlled vortex design (CVD) style. This phenomenon is related to the SS outward flow associated with the vortices shed from the blade of spanwise changing circulation. It has been found that the outward migration, and consequently, the endwall blockage at the casing, becomes more pronounced as the intensity of shed vortices due to CVD increases. Based on experimental data, it has been pointed out that the increase in axial displacement thickness is approximately proportional to the radial gradient of blade circulation. Based on the purposeful use of the analytical model, forward blade sweep was found to be especially beneficial for the rotors of CVD, as a means for moderating outward migration along the suction surface.

Aerodynamic and aero-acoustic improvement of electric motor cooling equipment:

A radial flow rotor with radially aligned straight blades, used in electric motor cooling, has been considered as datum fan. The aerodynamic performance and acoustic behaviour of the datum fan have been measured, in order to establish a basis for redesign. As replacement for the datum fan, an axial flow shrouded rotor with skewed blades has iteratively been designed, involving computational fluid dynamics and computational aero-acoustics tools. The aim of redesign was reducing fan noise and moderating motor shaft power absorbed by the fan, while retaining the original cooling performance. Special flow features have been taken into consideration in three-dimensional axial rotor design, such as leakage flow in the axial clearance between the rotor shroud inlet and the perforated cover, and strong radial flow as well as deviation due to the motor shield located close downstream. The axial rotor has been manufactured via rapid prototyping. Measurements on the prototype confirmed the achievement of the redesign goal. The effect of axial clearance size on the operation of the axial rotor has been investigated by computational fluid dynamics, computational aero-acoustics and experimental means. It has been pointed out that the axial clearance size is a sensitive parameter in influencing fan aerodynamics and aero-acoustics, for which the major mechanisms, associated with the leakage flow, were qualitatively identified. Both the computational and experimental studies revealed the existence of an acoustically unfavourable clearance size, for which maximum noise emission can be expected. A semi-empirical model was outlined as starting point in prediction of cooling flow rate as a function of axial clearance size as well as other parameters.

Correlation of flow path length to total pressure loss in diffuser flows

This article studies the effect of changing the flow path length on development of total pressure loss in diffuser flows. The studies incorporate quasi-one-dimensional (conical diffuser), two-dimensional (linear blade cascade), and three-dimensional (annular blade cascade) flows. This article concludes that the consequence of decreasing the flow path length is to decrease total pressure loss, provided that the adverse streamwise pressure gradient remains subcritical. The loss calculation technique and measurement data published by Lieblein were post-processed in this article. On this basis, this article elaborates a method for estimating the total pressure loss as well as its modification due to change of flow path length in blade cascade flows. This method can contribute to preliminary axial flow rotor blade design. It was pointed out that, in comparison to free vortex design, controlled vortex design that is applied to straight axial flow rotor blades tends to represent an additional source of total pressure loss farther from the endwalls. This is due to elongation of the length of flow paths on the blade suction side, due to intensified radial outward flow. This adverse effect can be counterbalanced by incorporation of forward sweep in controlled vortex design, via shortening the flow paths on the suction side, without the beneficial features of the controlled vortex design being lost. This article presents a literature-based comparative case study to illustrate such loss-reducing effect of incorporating forward sweep in controlled vortex design, away from the endwalls.