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Because aerodynamic stall is a major potential cause of mechanical failure in axial fans, effective stall-warning techniques have had wide application for many years. The study assesses the application of stall-detection to a class of heavy-duty fans for induced draft service in coal-fired power plants. With a multi-MW installed power, this class of fans operates at constant rotational speed, utilising variable-pitch in motion blades to adjust fan duty point. This load modulation system, in the presence of blade erosion or fouling, could result in fans becoming prone to stall as a consequence of the reduced pressure capacity. This article reports on a novel methodology, first proposed for the early detection of stall in low-speed axial-flow fans for tunnel ventilation. With respect to the original concept, which was based on the analysis of hydrodynamic pressure as measured on the fan ducting, the investigation demonstrates the potential of processing far-field pressure signals in identifying stall pre-cursors, and in so doing, establishing the stall incipience. This acoustic stall-detection system is characterised in a laboratory environment exploring the fan operating envelope. Time-resolved pressure measurements are carried out with pre-polarised pressure transducers fitted into (a) the fan casing over the fan blades (near field) and (b) in the inlet plenum 1 diameter upstream of the fan rotor (far field). The evolution of the rotating stall for the fan under scrutiny is first presented. The rationale of the signal processing technique is given, together with a sensitivity study on its metrical parameters. Finally, the signal visualization technique is used to compare the patterns related to the presence of stall in the pressure signals recorded in the near- and the far field. [ABSTRACT FROM PUBLISHER]

This paper presents a fully reversible blade parametric design methodology. The blades are for application in jet fans that ventilate and provide emergency smoke control in road tunnels. The blade design variables are tip solidity, twist, and camber distribution. The authors base the design methodology on a sensitivity analysis which they derived from a response sulfate approximation. They construct the latter using a computational analysis of four experimental cases which they generated using an experimental design approach. The sensitivity analysis calculates a rank and a weight for each design variable that affects the jet-fan performance parameters thrust and efficiency, and thus facilitates insight into each variable's relative importance. Finally, the authors present a redesign of" an existing reversible jet-fan blade by following the design guidelines which they obtained from the sensitivity analysis. The authors study the aerodynamic characteristics of the redesigned blade and compare it to that of the base- line design configuration. They then manufacture a redesigned blade prototype. Finally, the authors present thrust and power measurements for both the baseline jet fan, and the baseline jet .fan when fitted with a redesigned blade. [ABSTRACT FROM AUTHOR]

This paper reviews modelling and interpretation advances of industrial fan stall phenomena, related stall detection methods, and control technologies. Competing theories have helped engineers refine fan stability and control technology. With the development of these theories, three major issues have emerged. In this paper, we first consider the interplay between aerodynamic perturbations and instability inception. An understanding of the key physical phenomena that occurs with stall inception is critical to alleviate stall by design or through active or passive control methods. We then review the use of passive and active control strategies to improve fan stability. Whilst historically compressor design engineers have used passive control techniques, recent technologies have prompted them to install high-response stall detection and control systems that provide industrial fan designers with new insight into how they may detect and control stall. Finally, the paper reviews the methods and prospects for early stall detection to complement control systems with a warning capability. Engineers may use an effective real-time stall warning system to extend a fan's operating range by allowing it to operate safely at a reduced stall margin. This may also enable the fan to operate in service at a more efficient point on its characteristic. [ABSTRACT FROM AUTHOR]

There are controversial requirements involved in developing numerical methodologies in order to compute the flow in industrial fans. The full resolution of turbulence spectrumin such high-Reynolds number flow configurations entails unreasonably expensive computational costs. The authors applied the study to a large unidirectional axial flow fan unit for tunnel ventilation to operate in the forward direction under ambient conditions. This delivered cooling air to the tunnel under routine operation, or hot gases at 400°C under emergency conditions in the event of a tunnel fire. The simulations were carried out using the open source code OpenFOAM, within which they implemented a very large eddy simulation (VLES) based on one-equation SGS model to solve a transport equation for the modelled (subgrid) turbulent kinetic energy. This subgrid turbulence model improvement is a remedial strategy in VLES of high-Reynolds number industrial flows which are able to tackle the turbulence spectrum's well-known insufficient resolution. TheVLES of the industrial fan permits detecting the unsteady topology of the rotor flow. This paper explores the evolution of secondary flow phenomena and speculates on its influence on the actual load capability when operating at peakpressure condition. Predicted noise emissions, in terms of sound pressure level spectra, are also compared with experimental results and found to agree within the uncertainty of the measurements. [ABSTRACT FROM AUTHOR]

This paper describes work aimed at establishing the ability of a tunnel ventilation fan to operate without risk of mechanical failure in the event of aerodynamic stall. The research establishes the aerodynamic characteristics of a typical tunnel ventilation fan when operated in both stable and stalled aerodynamic conditions, with and without an anti-stall stabilisation ring, with and without a "nonstalling" blade angle and at full, half, and one quarter design speed. It also measures the fan's peak stress, thus facilitating an analysis of the implications of the experimental results for mechanical design methodology. The paper concludes by presenting three different strategies for tunnel ventilation fan selection in applications where the selected fan will most likely stall. The first strategy selects a fan with a low-blade angle that is nonstalling. The second strategy selects a fan with a high-pressure developing capability. The third strategy selects a fan with a fitted stabilisation ring. Tunnel ventilation system designers each have their favoured fan selection strategy. However, all three strategies can produce system designs within which a tunnel ventilation fan performs reliably in-service. The paper considers the advantages and disadvantages of each selection strategy and considered the strengths and weaknesses of each. [ABSTRACT FROM AUTHOR]

Tunnel ventilation fans, classically, must have the ability to both supply and extract air from a tunnel system, with the operator's choice dependent on the tunnel ventilation system's operating mode most appropriate at any given point in time. Consequently, tunnel ventilation fans must incorporate a reversible aerodynamic design which limits the maximum fan pressure rise. This paper presents three high pressure reversible fan concepts. These comprise a two-stage counter rotating fan, a single-stage high speed fan, and a two-stage fan with a single motor and impeller on each end of the motor shaft. The authors consider the relative merits of each concept. The third concept offers the most compact fan, transform, silencer, and damper package size. The authors discuss the mechanical design challenges that occur with a two-stage fan with a single motor and impeller on each end of the motor shaft. They present and consider a selectedmotor bearing arrangement and casing design formaintainability. Finally, the authors present both prototype fan and full-scale package aerodynamic and acoustic performance, before discussing the challenges presented by high temperature certification in accordance with the requirements of EN 12101-3: 2012. [ABSTRACT FROM AUTHOR]

This paper reports on quantitative tests of passive techniques for rotor-tip noise control in low-speed axial flow fans, based on blade-tip modifications involving the addition of antivortex appendages as end-plates. The end-plate thickness chordwise distribution is determined to control the chordwise evolution of the leakage vortex rotation number. The results confirm that the new end-plate configurations provide a mechanism by which leakage vortex bursting can be avoided. As such, the modified rotors represent an effective means of passive control of vortex breakdown. [ABSTRACT FROM AUTHOR]

An introduction is presented in which the editor discusses various reports within the issue on topics including noise mechanisms of air movement fan, gearless one-motor contra-rotating fan, and the conceptual design of reversible axial tunnel ventilation fans in high pressure.

This paper presents the findings of an investigation on the use of several blade-tip configurations (modified by the addition of various end plates at the blade tip) for passive noise control in industrial fans. Utilizing an experimental technique developed to investigate noise sources along the radius of the blades, together with cross-correlation and coherence analyses of the near field and far field, the modified blade-tip configurations are shown to reduce the rotor-only aeroacoustic signature of the fan as a direct consequence of changes induced in tip-leakage flow behavior. These changes in the nature of flow mechanisms in the region of the blade tip are correlated with the spanwise noise sources, and their role in the creation of overall acoustic emissions is thus clarified. The tip-leakage flow structures are analyzed to identify their contribution to overall noise and interaction with other noise sources. Coherence spectra are also analyzed to investigate the relevance of the noise sources. The cross-correlations reveal distinctive acoustic signatures that are described in detail. The methodology has been demonstrated to be effective in identifying (i) the blade-tip configuration with the best acoustic performance, and (ii) other significant noise sources along the blade span. The modified tip configurations are shown to have a significant effect on the multiple vortex behavior of leakage flow, especially with respect to the near-wall fluid-flow paths on both blade surfaces. The reduction in fan acoustic emissions is assessed and correlated with the control of tip-leakage flows achieved by the modified blade rips. [ABSTRACT FROM AUTHOR]

This study assesses the effectiveness of modified blade-tip configurations in achieving passive noise control in industrial fans. The concepts developed here, which are based on the addition of end-plates at the fan-blade tip, are shown to have a beneficial effect on the fan aeroacoustic signature as a result of the changes they induce in tip-leakage-flow behavior. The aerodynamic merits of the proposed blade-tip concepts are investigated by experimental and computational studies in a fully ducted configuration. The flow mechanisms in the blade-tip region are correlated with the specific end-plate design features, and their role in the creation of overall acoustic emissions is clarified. The tip-leakage flows of the fans are analyzed in terms of vortex structure, chordwise leakage flow, and loading distribution. Rotor losses are also investigated. The modifications to blade-tip geometry are found to have marked effects on the multiple vortex behaviors of leakage flow as a result of changes in the near-wall fluid flow paths on both blade surfaces. The improvements in rotor efficiency are assessed and correlated with the control of tip-leakage flows produced by the modified tip end-plates. [ABSTRACT FROM AUTHOR]

The application of improved blade tip geometries is studied with the aim of identifying an effective design concept for industrial fan passive noise control. The concept developed optimizes a datum blade by means of profiled endplates at the tip, reducing fan noise by changing the tip leakage flow behaviour. Experimental and computational investigations have been carried out on a family of axial fans, in fully ducted configuration, to establish the aerodynamic merits of the proposed blade tip design concept. The flow mechanisms in the fan tip region are correlated to specific blade design features that promote a reduction of the fan aero-acoustic signature in both tonal and broadband noise components. The tip vortical flow structures are characterized, and their role in creation of overall stage acoustic emissions clarified. The reported research identifies modification of tip geometry as markedly affecting the multiple vortex behaviour of blade tip leakage flow by altering the near-wall fluid flow paths on both blade surfaces. Blade tip endplates were also demonstrated to influence the rotor loss behaviour in the blade tip region. Improvement of rotor efficiency was correlated to the control of tip leakage flows. [ABSTRACT FROM AUTHOR]

This review aims to assist engineers in understanding and applying passive solutions for reducing industrial fan noise. The paper systematically reviews the extant literature on passive noise techniques, with a particular focus on experimental rather than theoretical research. The review provides an assessment of the current state of the art in industrial fan flow and noise control. It offers a vision for potential improvements in noise reduction via novel application of flow and noise control technologies. The review examines the interaction between aerodynamic cause and acoustic effect and the application of control technologies that current cause and effect theories have inspired. The purpose is to provide a vision for aerodynamics research during the next decade that will serve as a basis for systematically reducing industrial fan noise emissions. The review provides an assessment of recent flow and noise control advances and considers some opportunities for future research. The review reflects an emphasis on low-speed industrial fans. The authors consider high-speed turbomachinery noise control, with the objective of illustrating the linkages between the two technologies. The review concludes with a summary of the opportunities for future research and its application to flow and noise control in industrial fan design. [ABSTRACT FROM AUTHOR]