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Article Excerpt
INTRODUCTION
A blower is a gap pump that is capable of providing moderate to high pressure rise and flow rate. Blowers are utilized widely in all kinds of engineering applications, such as building ventilation, air-supply in processes, and consumer electronics. There are three main types of blowers used for moving air: axial, centrifugal, and mixed flow. Among the three main types, centrifugal blowers (shown in Figure 1) are commonly applied to general heating, ventilating, and air-conditioning applications. They are usually only applied to larger systems, which may be low-, medium-, or high-pressure applications, and large, clean-air industrial operations for significant energy saving. Depending on the blade design, centrifugal blowers can be categorized into forward-curved, backward-curved, radial, and airfoil types. While the first three types are relatively easy to manufacture, centrifugal blowers with airfoil type blades exhibit the following characteristics (ASHRAE 1996; Cengel and Cimbala 2006):
[FIGURE 1 OMITTED]
* highest efficiency of all centrifugal blower designs
* blade of airfoil contour curved away from direction of rotation
* air leaves impeller at velocity less than tip speed
* scroll-type design for efficient conversion of dynamic pressure to static pressure
* maximum efficiency requires close clearance and alignment between wheel and inlet
* highest efficiencies occur at 50% to 60% of wide open volume; the pressure characteristics around the peak efficiency point are still efficient
* power reaches maximum near peak efficiency and becomes lower, or self-limiting, toward free delivery
Including the centrifugal blowers we focus on in this study, blowers can be analyzed theoretically, then some basic performance characteristics can be derived and calculated from simple equations (Logan 1993; Fox and McDonald 1998). The horsepower, which is the useful power actually delivered to the fluid, can be written as Equation 1:
[W.sub.horsepower] = [rho] * g * Q * H (1)
where [rho] is the fluid density, g is the gravitational acceleration constant, Q is the volume flow rate, and H is the net head of the blower. The external power supplied to the pump is the brake horsepower (bhp). Equation 2 provides the definition of brake horsepower:
bhp = [W.sub.shaft] = [omega] * [T.sub.shaft] (2)
where [omega] is the rotational speed of the shaft and [T.sub.shaft] is the torque supplied to the shaft. The blower efficiency, usually abbreviated as [[eta].sub.pump], then can be defined as follows:
[[eta].sub.pump] = [[W.sub.horsepower]/[bhp]] = [[[rho] * g * Q * H]/[[omega] * [T.sub.shaft]]] (3)
The idealized velocity tangential and normal components are shown in Figure 2. The fluid is assumed to enter the impeller wheel at radius [r.sub.1] with uniform absolute velocity [V.sub.1] and to leave the impeller wheel at radius [r.sub.2] with uniform absolute velocity [V.sub.2]. The rate of work done on an impeller wheel can be written as where U is the tangential speed of the impeller wheel at radius r and is the mass flow rate. The head added to the flow, which is the dimension of length, can be written as
[FIGURE 2 OMITTED]
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)
where U is the tangential speed of the impeller wheel at radius r and m is the mass flow rate. The head added to the flow, which is the dimension of length, can be written as
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (5)
If the fluid enters the impeller with purely radial absolute velocity, [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. The increase in head becomes
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (6)
Observing the exit velocity triangle in Figure 2, it can be found that
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (7)
Then
H = [[[[U.sub.2.sup.2] - [U.sub.2] * [V.sub.n2]cot([[beta].sub.2])]/g] = [[U.sub.2.sup.2]/g] - [[[U.sub.2] * cot([[beta].sub.2])]/[2[pi] * [r.sub.2] * w * g]]Q, (8)
where Q = 2[pi] * [r.sub.2] * w * [V.sub.n2] represents the ideal flow rate for an impeller of width w. The theoretical derivations in Equations 1-8 estimate the performance of ideal blowers. But, these equations cannot investigate several effects on actual blower performance, such as number of blades and scroll contour. More powerful tools are necessary to take more design parameters into account to better estimate the blower performance and facilitate the design process.
Computational fluid dynamics (CFD) packages have progressed to the point where they can produce accurate predictions for thermal and fluid applications (Gonzalez et al. 2002; Kim and Seo 2004; Medvitz et al. 2002; Tsai and Wu 2007; Velarde-Suarez et al. 2001). Subhash and Majumder (2007) used FLUENT (Fluent 2005a) to solve a large eddy simulation (LES) model for the film cooling problem. After the time-dependent analysis, they obtained the temperature, pressure, and velocity distributions. Numerous researchers have used CFD simulations to study turbo-machinery. Calvert and Ginder (1999) studied transonic fans and compressors. In their study, the evolution of transonic compressor designs and methods was outlined, followed by more detailed descriptions of current compressor configurations and requirements and modern aerodynamic design methods and philosophies. Detailed quasi-three-dimensional and three-dimensional computational fluid dynamics analyses were employed to refine the design. Lin and Huang (2002) numerically simulated the internal flow for a forward-curved centrifugal fan used in notebook PC cooling. In their study, an integral solution, including design powered by CFD, prototype manufacturing, and experiment verification,...
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