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Rotating stall is an unsteady flow phenomenon that causes instabilities and low efficiency in pumps. The purpose of this paper is to investigate the rotating stall characteristics and unsteady behavior of stall cells in a centrifugal pump impeller at low flow rates.

A developed large eddy simulation with dynamic mixed nonlinear model is performed to evaluate the unsteady flow in a centrifugal pump impeller. The rotating stall flow field through the centrifugal pump impeller is analyzed under three typical flow rates. Frequency spectrum analysis are carried out on the series of pressure fluctuation to get the rotating stall characteristics. The size and intensity of stall cells are also analyzed using time-averaged vorticity and static pressure.

The rotating stall cell first occurs in the suction side of the blade and exhibits an obvious life cycle including decay mergence, shedding, growing and development with a low frequency. With the decrease of flow rate, the amplitude of pressure fluctuations in the impeller tends to be larger, the propagated speed of stall cells and rotating stall frequency tends to be smaller, but the number of cells remains unchanged. The size of stall cells increases as the flow rate decreases, but intensity changes is very little.

The rotating stall characteristics in a centrifugal pump impeller under low flow rates are presented first using a developed large eddy simulation approach.

The paper aims to present an outline of the technology of the active anti-surge algorithm based on high-frequency pressure measurement. The presented system is fast, inexpensive and reliable and does not limit the machine-operating range. Many contemporary anti-surge systems are based on theoretical surge margin. This solution limits machine operating range by about 10-15 per cent in the region of the highest pressure ratios. It is also often sensitive to change in external conditions such as temperature or density, as the system reacts to limits calculated theoretically.

This paper presents results of pressure measurements obtained on the low-speed centrifugal blower DP1.12. The pressure signals were presented in the form of phase diagrams, and conclusions were drawn from their phase portraits to develop the surge indication parameter.

The presented safety system uses the signal to develop the so-called (rate of derivative fluctuation) RDF parameter. In nominal working conditions, this parameter keeps the value close to 1. When RDF reaches values over 3, the anti-surge procedure should be implemented. Experimental studies have shown that this algorithm assures enough time to incur actions suppressing unstable phenomena.

The system reacts to real machine working conditions and is hence reliable. The RDF algorithm could also be used to identify local flow instabilities, as well as off-design operation.

A method for optimizing net positive suction head required of axial‐flow pumps has been proposed by the present author, which is based on the two‐dimensional potential flow model and without considering the tip gap effect. The objective of the paper is to confirm if the method is just and feasible for the case of viscous fluid flow in impellers with tip gap.

A series of steady, three‐dimensional, noncavitating and cavitating, turbulent, incompressible flows of water through two axial‐flow pump impellers were calculated by using CFD code Fluent. The two impellers included a reference one with constant circulation at outlet and an optimized one with variable circulation designed with the author's method and code. In computations, the throttling and unthrottling approaches were used, respectively. Comparison of hydraulic performance, averaged flow variables at the impeller inlet and exit, flow in the tip gap, flow variables on blade surfaces and suction performance between the optimized and reference impellers was made.

It was confirmed that the optimized impeller has better hydraulic and suction performances. The method for optimizing with variable flow circulation profile along blade span at the outlet to impeller is proper and practical. Additionally, an unstable regime in the head curves of two impellers is presented. In the regime, a stall occurs on the pressure side of the blade and a hysteresis exists, which causes a hysteresis‐loop.

The effect of suction entry on flow is represented approximately by using a free‐vortex and uniform axial velocity. The diffusing component behind the impellers is not taken into account. The unsteadiness of flow is not considered, which would have a connection with stall pattern in an axial‐flow impeller.

The hydraulic and suction performances and flow variables of two axial‐flow pump impellers with tip clearance are obtained successfully with CFD. Stall and hysteresis as well as hysteresis‐loop in head curve are observed by using throttling and unthrottling approaches.

The stalling behaviour of an Orenda CT‐100 experimental two‐stage axial compressor was studied at the constant operating speed of 3,000 r.p.m. Rotating stall was observed to commence at a mass flow coefficient of 0·435 and as the mass flow was reduced several different patterns of rotating stall were observed. In two ranges of operation ‘steady’ patterns consisting of three rotating stalls were observed, in two other ranges of operation rotating stall was observed, but the stall patterns were continually changing, while for very low mass flow operation the flow over the whole of the annulus appeared to be stalled. Reversed flow within the stalls was found to be an important feature of rotating stall.

A stall model to predict the performance of a blade row operating under rotating stall conditions, is proposed.

The experiments were carried out on an isolated rotor row of an axial flow compressor of a radius ratio of 0.66 hub/tip. Wall static pressure tappings were used for measurement of blade row pressure rise. The mass flow rate through the machine was determined from the pressure drop at the intake. Detailed flow measurements were made using a hot wire “V” probe and transducers. An online data acquisition system was used in which data sampling was phase‐locked with respect to stall cell trailing edge.

Measurements indicate that a pressure depression occurs in the stalled region. The assumption of uniform static pressure at the exit of a stalled blade row is not supported by the present work. The assumption of uniform static pressure at the exit of a stalled row together with the assumption that flow in unstalled regions operates at fixed point on the unstalled characteristic leads to the conclusion that total‐to‐static pressure rise during stalled operation is independent of blockage. This view is not supported by the experiments carried out on an isolated rotor.

Additional experimental studies for axial compressors having different rotor and blade geometries and rotor speeds, are required.

The results can be used in the design and operation of axial compressor rotors.

A new stall model is presented in which the behavior during stalled operation with large blockage is different from that during, low blockage.

Analyses a stage‐by‐stage model to simulate the dynamic response of compression systems operating with axial multistage compressors. The compressor model is coupled with two different downstream volume (plenum) representations: the first is zero‐dimensional and the second is one‐dimensional. Both dynamic models utilize complete stage performance curves (direct unstalled flow, stalled flow and back flow) obtained through experimental investigation, or from theoretical analysis based on turbomachinery geometry and semi‐empirical considerations. Analyses in depth the results of the dynamic simulations, obtained utilizing both methods, compared to the experimental data reported in the literature, and finally compares one to the other through a fast Fourier transformer (FFT) analysis.

This study aims to propose a novel configuration for turbofan engine inlets to increase the overall effectiveness of the engine.

Conventional fan has been split radially into two blisk stages, namely, core blisk and bypass blisk. The two blisks are driven by a common shaft but rotate at two different revolutions per minutes (RPMs) on the same plane of rotation simultaneously through a planetary gear mechanism. To avoid any mechanical contact between the two stages, a minimum optimum distance is kept between them.

An apt reduction ratio of planetary gears allows the bypass blisk to rotate at a lower RPM. Thus, unlike conventional geared single fan configuration, transonic speed at the blade’s tip is prevented without decreasing the core stage’s RPM. Consequently, wave drag is eradicated without compromising the engine's core performance as surplus air can always be supplied to it. Compressor stall and surge can also be significantly reduced.

The concept is at its infancy. Extensive iterations and experimentations are required before implementing it practically.

The configuration fulfils to conceive a practical and industrially scalable method to extract better performance from existing engine architecture with minimal changes while reducing noise and emissions, meeting the short-term emission and noise goals unless electric or hydrogen-powered flight fully matured.

The present concept reduces engine noise and thereby helps in reduction of airport noise pollution. This concept also helps in reducing global warming by reducing emissions.

The paper presents a novel configuration for a turbofan engine’s inlet fan and discusses its engineering implications and initial feasibility in detail.

This paper aims to understand the aerodynamic blockage related to near casing flow in a transonic axial compressor using numerical simulations and to design an optimum casing groove for stall margin improvement using a surrogate optimisation technique.

A blockage parameter (Ψ) is introduced to quantify blockage across the blade domain. A surrogate optimisation technique is then used to find the optimum casing groove design that minimises blockage at an axial location where the blockage is maximum at near stall conditions.

An optimised casing groove that improves the stall margin by about 1% can be found through optimisation of the blockage parameter (Ψ).

Optimising for stall margin is rather lengthy and computationally expensive, as the stall margin of a compressor will only be known once a complete compressor map is constructed. This study shows that the cost of the optimisation can be reduced by using a suitably defined blockage parameter as the optimising parameter.

THE second annual Rotating‐Wing Aircraft Meeting was again held under the auspices of the Philadelphia Chapter of the Institute of the Aeronautical Sciences. The sessions were of considerable interest and were well attended. Many of the leading American aeroplane designers were present, including Grover Loening, Igor Sikorsky, and T. P. Wright.

This general paper deals mainly with the military aspect of some of the major problems resulting from turbine stall. Four which are highlighted are the increase in weapon system development time, operational limits are imposed which may prevent an aircraft from successfully accomplishing a mission, field maintenance problems are increased, overhaul costs and accident rates go up.