Developments in Turbomachinery Flow

Forward Curved Centrifugal Fans

Nader Montazerin, Ghasem Akbari, Mostafa Mahmoodi

• Publisher: Woodhead Publishing
• Year: 2015
• Print ISBN: 9781782421924
• eBook ISBN: 9781782421931
• Topic: Technology & Engineering
• Subtopic: Fluid Mechanics
• Index: Technology & Engineering

Developments in Turbomachinery Flow: Forward Curved Centrifugal Fans explores the forward curved squirrel cage fan as an excellent instrument for fluid mechanics research in turbomachines.The book explores phenomena such as jet/wake interaction, circulation, separation and noise in turbomachines, also addressing the characteristics that are specific to this fan and applications in other centrifugal turbomachines.Chapters begin with a general introduction that includes research techniques and a survey of older research, and then proceed into a detailed description of improvements for different parts of the fan, including the inlet, the rotor and the volute. Final sections include a comprehensive discussion on geometrical modifications that could improve performance without impacting cost.- Explores how forward curved centrifugal fans have wide industrial application- Discusses the strong trade-off between fan aerodynamics and construction cost- Provides a wide selection of optimums for geometrical configuration- Introduces new geometries that can improve performance and reduce noise for the same construction cost- Presents fluid phenomena as a potential research field for this fan

1

General introduction of forward-curved squirrel-cage fan

Abstract

This chapter begins with an overall introduction to the forward-curved fan. It then discusses its applications and shows why it is important to understand the performance, flow field and noise of such fans. Two general approaches, namely, experimental techniques and simulations, are used to derive the results that are presented in this book. Both these approaches are discussed in some detail towards the end of the chapter.

Keywords

Laser Doppler anemometry (LDA); Measurement; Particle image velocimetry (PIV); Performance; Simulation; Squirrel-cage fan

1.1. Introduction

Forward-curved squirrel-cage fans are widely used for residential and automobile air conditioning and also for cooling of computers and electronic appliances. High volumetric flow rate, low noise and simple construction are basic characteristics of such fans. They are easy to manufacture, operate quietly and pump a relatively large volume of air. The squirrel-cage fans are slower than other fans for the same flow rate, or are smaller for the same head. This feature and low manufacturing cost make them very popular in the comfort industry and household appliances.

Such merits are achieved at the cost of low efficiency, which is related to the poor aerodynamics of squirrel-cage fans. These fans are usually designed for a lower first cost and, therefore, the efficiency is not higher than 50–60%. Although individual fan power consumption is low, since there are a large number of such appliances in use, the overall energy consumption becomes significant. Even small improvements in their performance, when multiplied by their widespread application, are important for energy saving.

The squirrel-cage fan is a strange piece of machinery. It is widely used but far less studied. It works with considerable volume of separated flow inside it. In fact, as shall be shown in this book, in some cases larger separation is more beneficial to fan characteristics. It is found that the squirrel-cage fans are excellent devices for examination of the flow phenomena of turbomachines.

Evaporative coolers were the initial drive for the authors to pay attention to such fans. A geometrical optimization was performed as the initial step of fan operation enhancement. It was discovered rapidly that interrelation between different dimensions makes the fan very unpredictable. Three-dimensional flow and separation are obstacles to accurate modelling. A combination of modelling and experiment was then followed.

Fans are judged through their characteristic curves. This is the criterion for the customer to select a fan. Every modification in fans does not necessarily affect their characteristics. An intermediate step to improve centrifugal fans is to reach uniform flow through the blades and volute and to reduce turbulence intensities. Other industrial criteria are also cited that include compactness and lower sound-pressure levels.

Available research on the squirrel-cage fan includes attempts to understand the complicated flow structure within the fan, and in this way covers the following four different viewpoints:

1. Parametric performance studies on the classic fan configuration for different geometries. In these studies the rotor diameter is set as a base for fan design and performance is optimized with variations in rotor or volute parameters.

2. The second group of research tries to understand flow fields inside the fan and relates the results to observations on performance. This includes velocity measurements with Pitot tube, hot-wire and laser Doppler anemometers. Such measurements are only possible for limited points within the fan and for certain points on the performance curves. The results are successful in showing the flow separation after the inlet, jets and wakes after the blades, and the nonuniform flow through the volute.

3. The third group includes attempts to increase efficiency by modifying the common configuration, for example, adding suction just after the inlet for a smaller separation as well as using an outward or annular inlet. Complication in such manufacturing is an important feature that should be considered in each new configuration.

4. Although small fan noise is an inherent advantage of such fans, the fourth group of studies on noise has suggestions for geometric parameters that would result in a quieter fan. These relate fan noise to flow field and geometry through simultaneous noise and turbulence measurements.

1.2. Fan geometry

A schematic of the forward-centrifugal fan is given in Figure 1.1. It is a long time that people do not attempt to model forward-blade fan performance with the Euler energy equation. Flow-field studies in this book and from other researchers have...

Table of contents

• Cover image
• Title page
• Table of Contents
• Related titles
• Copyright
• Dedication
• List of figures
• List of tables
• About the authors
• Preface
• 1. General introduction of forward-curved squirrel-cage fan
• 2. Inlet configuration
• 3. Rotor
• 4. Volute
• 5. Noise in forward-curved centrifugal fans
• 6. Contribution of jet–wake–volute interactions to flow characteristics and turbulence models
• Index