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Description
The purpose of shaft power matching of contra-rotating axial flow fans (CRAFF) is to ensure the best utilization of the motors at their rated power while not to overload them at the operation. Usually it requires a lot of time and effort due to lack of theoretical guidance and, thus, becomes a major problem for the application of CRAFF. Through analyzing velocity triangle, the authors have found a nondimensional parameter k, defined as the tangential ratio of two relative outflow angles for two impellers, to be critical for successful shaft power matching and aerodynamic design. The authors then performed a numerical simulation of the flow field for a design of CRAFF, of which the performance prediction was validated very well by test data. Based on the analysis of the flow filed data, the result obtained from the velocity triangle analysis is further improved and some values for k and deviation angle [delta] are recommended. In order to determine the approximate optimum of blade pitch angles of two impellers, a guideline for shaft power matching is then presented. Finally, a comparison of the blade pitch angles obtained from the guideline with that obtained from the optimum using numerical simulation are carried out for six designs of CRAFF. The errors are only within 2 degrees.
INTRODUCTION
A contra-rotating axial flow fan (CRAFF) consists of two impellers rotating at the same speed but in opposite directions. It generally delivers a large flow rate at high pressure and high efficiency and is compact in structure. Therefore, CRAFFs are widely used in applications such as mines and tunnels where strong ventilation is required while space is limited. Figure 1 shows the structure of a typical CRAFF.
As early as the 1940s, contra-rotating impeller design was used in industrial axial flow fans (Cory 1992). Eck (1973) rated these fans with good aerodynamic performance. Since the 1980s, more studies have been focused on the aerodynamics and acoustics of contra-rotating axial flow fans and compressors (Roy et al. 1992; Sharma and Adekoya 1996). Computational fluid dynamics (CFD) was also used to study CRAFF in recent years. Li et al. (2002a, 2002b) numerically studied the flow field of a CRAFF design and predicted the aerodynamic performance at the design point. Wang et al. (2003) predicted the hydrodynamic performance of a contra-rotating axial flow pump and qualitatively analyzed the characteristics of the flow field. However, no comparison with experimental data was provided in the paper. Wang et al. (2004a, 2004b) qualitatively discussed the variation of pressure head with flow rate for contra-rotating pumps based on velocity triangle analysis but did not address the variation of shaft power with flow rate. In order to match the shaft power Kanemoto et al. (2002) designed a special motor with intelligent control, which can automatically adjust the power of the individual motor for each impeller. Obviously the high cost associated with the motor prohibits its broad applications for industry.
[FIGURE 1 OMITTED]
It should be noted that due to the strong interaction between two contra-rotating impellers, there are some special aerodynamic issues related to CRAFF. These issues are as follows:
* The relative velocity at the inlet of the rear impeller is usually very large. Thus, its total pressure loss and the aerodynamic noise could be significant, especially if not designed properly.
* CRAFFs generally have a smaller operational range of flow rate than other common fans do, which would limit its broad application.
* The shaft power of two individual impellers is very sensitive to the flow rate and the rear motor is especially susceptible to overloading at small flow rate, which could cause serious accidents.
On the other hand, there are some manufacturing and application issues on CRAFF:
* The ventilation system in mines commonly requires different flow rates and different pressure head as the tunnel length and working space change. To meet this requirement, the same series of CRAFF products was designed with different sizes, which often require motors of different rated power.
* In order to reduce cost, all different-sized fans in the same series have only one set of blueprints, and blade profiles are scaled up or down, but the blade pitch angles are varied to match the rated power of motors. A double-arc blade profile with uniform thickness is often used for low manufacturing cost.
* It is very important that if the blade shape and rotating speed of two impellers are determined, in order to avoid the overloading of the rear motor and to best use the power of the two motors at the design operation, it is usually necessary to adjust each blade pitch angle to search for an optimum combination. Due to lack of theoretical guidance, this requires a large amount of performance tests, which are generally very time consuming and cost prohibitive.
Even though it can now predict the performance of the fans very well, CFD is still not very practical for optimization due to its lengthy iterations. Therefore, it is still of practical significance to come up with an effective guideline for shaft power matching, with which the optimum combination of blade pitch angles can be achieved easily and quickly. This is the main goal of the research presented here.
There are four main parts in this paper: (1) The authors analyzed the work done by two impellers of CRAFFs using the velocity triangle method and discovered an important parameter k for aerodynamic design and application, which is called the coefficient of shaft power matching. (2) A numerical prediction of aerodynamic performance for a CRAFF design was completed and validated by test data. (3) Based on a detailed analysis of the numerical work above, the results obtained from the velocity triangle method were improved and an effective guideline for shaft power matching is presented. (4) In order to validate the design guideline, comparisons of the approximate optimum of blade pitch angle obtained from the guideline with that obtained from CFD were completed for six designs of CRAFF.
THEORETICAL ANALYSIS OF SHAFT POWER MATCHING
Velocity Triangle Diagram
The method of velocity triangle analysis is still the main tool used in the design of axial flow fans. The method is used in this paper to analyze the characteristics of the shaft power of the impellers.
Figure 2a shows the geometric relationship of a blade profile with double arc. Figures 2b and 2c show the velocity triangle diagrams of the blade profiles of the front and rear impellers, respectively, in which the absolute velocity of inlet flow of the front impeller is assumed to be axial. The symbols shown in Figure 2 are defined in the nomenclature of this paper. After the blade is fixed on the hub of the impeller at a proper angle, i.e., blade pitch angle based on the requirement of the aerodynamic performance of the fan, the shape of the impeller channel is defined. A geometrical relationship about [theta] could be easily obtained from Figure 2a as follows:
[theta] = [[beta].sub.2g] - [[chi].sub.2] = [[beta].sub.2] + [delta] - [[chi].sub.2] (1)
where [[beta].sub.2g], [[beta].sub.2], [delta], and [[chi].sub.2] are the outlet geometric angle, relative outflow angle, deviation angle, and stretched angle of the rear circular arc, respectively. Obviously [theta] varies along the blade height direction. It is noted that in practice the term of blade pitch angle is usually referred to as [theta] at the hub section of the blade, but in this paper [theta] at the mid-blade height is used as the blade pitch angle in order to determine the shaft power matching.
Theoretical Analysis of Total Pressure and Shaft Power for Two Impellers
First, it is noted that if there exists a characteristic section for a blade at which the axial flow velocity and the shaft power are just near the average of those on the whole blade at different flow rates, the total pressure and shaft power of the impeller might be analyzed using the velocity triangle method. Here it is assumed as the... |
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