Impeller Design of a Centrifugal Fan with Blade Optimization. Carderock Division, Naval Surface Warfare Center, Code 5. West Bethesda, MD 2. USA2. Combustion Research and Flow Technology, Inc. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. A method is presented for redesigning a centrifugal impeller and its inlet duct. The double- discharge volute casing is a structural constraint and is maintained for its shape. The redesign effort was geared towards meeting the design volute exit pressure while reducing the power required to operate the fan. Given the high performance of the baseline impeller, the redesign adopted a high- fidelity CFD- based computational approach capable of accounting for all aerodynamic losses. The present effort utilized a numerical optimization with experiential steering techniques to redesign the fan blades, inlet duct, and shroud of the impeller. The resulting flow path modifications not only met the pressure requirement, but also reduced the fan power by 8. A refined CFD assessment of the impeller/volute coupling and the gap between the stationary duct and the rotating shroud revealed a reduction in efficiency due to the volute and the gap. The calculations verified that the new impeller matches better with the original volute. Model- fan measured data was used to validate CFD predictions and impeller design goals. The CFD results further demonstrate a Reynolds- number effect between the model- and full- scale fans. Introduction. A heavy- duty air cushion vehicle usually employs centrifugal lift fans to pressurize the air cushion and power the steering thruster. Process design of fans and blowers (project standards and specifications) table of content scope 2 references 2 definitions and terminology 2. The design of the lift fan system is subject to meet payload, machinery spacing, and ruggedness requirements . The current low- specific- speed (. The impeller is a double- width, double- inlet (DWDI) centrifugal type with two nonstaggered blade rows. Each impeller blade row has backward- swept blades mounted between a common back plate and shrouds. In order to effectively manage the craft fuel consumption, a reduction in fan. Since the DDV is a structural constraint and required to be maintained in its shape, the baseline impeller and a dual bellmouth (or inlet duct) assembly are therefore redesigned to improve the fan performance. In addition to the baseline impeller, there is an existing reference impeller (named the B#2 impeller) which provides further performance comparisons in reference to the baseline. In this paper, a systematic numerical study was carried out of the aerodynamic characteristics of the existing impellers. Of the design guides and examples that were in the 6th. Fan Coils and Blower Coils Chapter 14: VAV Diffusers. Math and Maintenance for Pumps and Blowers. Overview of Blower Technologies and. Comparison of High-Speed Turbo Blowers.
The study revealed that although the existing impellers were high performing to start with, there was some margin for improvement. In particular, both impellers were susceptible to flow separations near the leading edge of the blade and near the shroud region where the hub transitioned into the common backplate for the impeller system. Subsequently, a piecemeal approach was taken in the redesign effort and the hub, shroud, and bellmouth as well as the impeller blades were redesigned to improve the performance of the fan system. A variety of different techniques were utilized in the redesign process: for example, the hub was modified by streamline tracing; the bellmouth/shroud was modified by altering the local curvature near the blade whereas a formal genetic algorithm- (GA- ) based optimization procedure was used to redesign the blade profile. Experiential steering was used to alter the optimized two- dimensional blade profile into a three- dimensional swept blade that further enhanced the performance of the impeller. Figure 1: The baseline impeller B#1. Figure 2: Component representation for a half of the centrifugal fan. A detailed study was also carried out of the coupled impeller- volute system. The interaction between the impeller and its associated volute can significantly alter the performance of the impeller. Several groups have reported their findings on the performance of impeller- volute systems. However, the majority of the prior related investigations in the literature dealt with centrifugal impellers and single discharge volutes. For example, Kaupert and Staubli . Hillewaert and Van den Braembussche . Although all three investigations . The current DDV further complicates the flow pattern, shortens the pressure recovery path compared to the single discharge volute, and produces double pressure peaks at two peripheral tongue locations. The significance of the feedback depends, however, on each individual design configuration. Without predefined knowledge of the volute feedback to the impeller performance, impellers from these past efforts . In our case, since we are primarily interested in performance of the lift fan system, we have catalogued the performance degradation with the addition of a hard- constrained volute. We have carried out the impeller- volute coupling calculations with the use of the frozen impeller approximation which provides a conservative estimate of the performance when compared to fully unsteady simulations. Lastly, a rigorous design validation study was undertaken with a carefully designed test rig for the 1/5 scale model. Both fans with the existing impellers and the fan system with the redesigned impeller were tested to verify improvement in performance. In the following sections, we provide details of the strategy and methodology for redesigning the impeller using the impeller- only CFD calculations. Refined CFD calculations coupling the impeller, the volute, and the shroud gap that were used to assess the design and quantify the volute feedback to the impeller performance are discussed after the design procedure. Following that we provide details of the model- scale fan test . We end the paper with a detailed summary of the redesign process and the lessons learned therewith. Impeller Aerodynamics for the Existing Impellers. In order to establish a design strategy within a constrained design window, two existing impellers B#1 and B#2 were first analyzed with a second- order accurate CFD method which solves a full compressible form of the Navier Stokes equations with preconditioning to obtain an efficient time- marching numerical scheme . The flow field formulation was implemented within a 3. D unstructured code CRUNCH. The CRUNCH CFD code employs a multielement, cell- vertex- based unstructured framework which allows for a combination of tetrahedral, prismatic, and hexahedral cells. The standard high Reynolds number formulation of the . These turbulence equations, with supplemental low Reynolds number correction terms, are given in . Considering the computational efficiency, the wall- function approach was used for the current calculations. Figure 3 depicts the blade (left figure) and shroud (right figure) arrangements for the 1. B#1 impeller in black and the 1. B#2 impeller in gray. The baseline volute shown in Figure 3 is connected to the impeller with a sudden expansion in the flow path area. Figure 3: Blade/shroud arrangements for impellers B#1 and B#2. Fan aerodynamic performance at the design point requires air at a temperature of 2. This results in the following nondimensional parameters: . At the design point, 5. The goal of the design study is to achieve a reduction in the power coefficient shown in (3) while maintaining the lift- flow characteristics of (1) and (2). Figure 4 shows the assembly of the bellmouth and impeller for one half of the fan. Due to the geometrical symmetry, the CFD calculations only cover one single blade passage for the gridding system used, as shown in Figure 5. To accurately capture the boundary layer and loading on the blade surface, the grid on the blade portion is structured and all other surfaces are either structured or unstructured as shown in Figure 5. The unstructured cells help to reduce the overall size of the grid thereby reducing turnaround time for the calculations. Although a relatively small gap exists between the rotating shroud and the nonrotating bellmouth, the impeller- only design CFD calculation does not include the effect of the shroud gap flow. Figure 4: Bellmouth/impeller assembly for the B#1 impeller. Figure 5: Gridding for the impeller B#1. For the incompressible flow calculation, a uniform inflow condition was imposed at the bellmouth inlet to maintain the required flow rate and a mass- averaged back pressure was applied at the impeller exit. A periodic boundary condition was enforced for the passage boundaries between the blades and a no- slip condition was used at the blade, shroud, backplate, and shaft surfaces. Although the inlet was controlled with a velocity condition, the inlet pressure was predicted as part of the simulation since the pressure pertains to the upstream propagating characteristic. As a consequence, the pressure rise was determined from the difference between the inlet and exit pressures and is a function of the impeller design. The performance- related parameters, that is, shaft power, output power, and total- to- total efficiency, for the impeller flow field are as follows: Shaft. PWR=. The impeller torque was calculated by integrating the forces from the blade, hub, shroud, and backplate. The convergence of the solution is determined by the variation of the calculated impeller torque and the mass- averaged total and static pressure variations at the inlet and outlet planes. Impeller B#2 was used to investigate the grid density requirement. Figure 6 shows the computed percent change in Shaft. PWR versus the design power with the number of cells for the structured and unstructured grids ranging from 1. The result shows that a grid density of 2. Calculations were also performed to investigate the effect of using the wall- function procedure. Calculations were made for both B#1 and B#2 impellers with an approximately 2. The predicted Shaft.
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