Multiple Row Blades for Blowers

A development is undertaken to determine possible configurations for multiple row or tandem blades for blowers. For the individual rows of the airfoils, changes in blade number, blade camber, and chord length are analyzed. Modifications in the arrangement between two blade rows are made and tests are presented to determine the optimum design. Tests are shown regarding the effects of changes in blade number and blade solidity. The tests indicate that two row blades are capable of larger flow deflection with associated flow deceleration than single airfoils. The development makes it possible to reduce the number of stages for multistage blowers.

Optimization Design of the Over-All Dimensions of Centrifugal Compressor Stage

A new procedure employed in computer-aided design of centrifugal compressor stage to determine its over-all dimensions is described in this paper. By the use of the COMPLEX METHOD, the arbitrary number of variables to be optimized can be specified to remove the hidden danger of the local optima which stems from adopting a few, for example two or three, variables to be optimized. This procedure is available for any complicated implicit nonlinear objective function and ensures establishment of a true optimum solution. Numerical calculations have been carried out by using the computer program described here to check the ability of the optimization method. The results obtained by the calculations agree fairly well with that obtained by experiments.

Calculation of Complete Three-Dimensional Flow in a Centrifugal Rotor With Splitter Blades

The flow within a centrifugal rotor has strong characteristics of three-dimensional effect. A procedure called “stream-surface coordinates iteration” for the calculation of complete three dimensional flow in turbo-machinery is first described. Splitter blade techniques have been used in many rotors, especially in centrifugal compressors and pumps with high flow capacity. The difficulty of the calculation of the flow field for this type of rotor lies on that the mass flow ratio between the two sub-channels is unknown for the given total flow capacity. In the second part of this paper, an assumption about how to determine this mass flow ratio and a procedure to calculate the complete three-dimensional flow are presented. Finally, some design criteria about the splitter blades are put forward. Experimental data from two centrifugal pump impellers equipped with different splitter blades are also given to demonstrate the availability of the present calculation method.

Off-Design Performance Prediction for Radial-Flow Impellers

A numerical method was developed to consider the two-dimensional flowfield between impeller blades of a given geometry. Solution of the laminar Navier-Stokes equations in geometry-oriented coordinates was obtained for stream functions and vorticities. Velocities and pressures were calculated to determine the output fluid-energy head. The circumferential components of the normal and shear stresses along the blade were evaluated to give the input mechanical-energy head. Performance predictions were obtained for different load conditions. Comparisons were made with the measured velocity vectors of the flowfield of an air-pump impeller and with the measured performance of a production water pump, good agreements were reached.

Computation of the Jet-Wake Flow Structure in a Low Speed Centrifugal Impeller

The low speed flow through the shrouded de-Havilland Ghost centrifugal impeller is computed using an incompressible elliptic calculation procedure. The three dimensional viscous flow equations are solved using the SIMPLE algorithm in an arbitrary generalised coordinate system. A non-staggered grid arrangement is implemented in which pressure oscillations are eliminated using an amended pressure correction scheme. Flow computations are performed at ‘nominal’ low speed design and above design flow rates, and (on the coarse grids used in the calculations) good agreement is obtained with the experimentally observed jet-wake structure of the flow.

Quasi-3D Solutions for Transonic, Inviscid Flows by Adapative Triangulation

This paper describes an algorithm for computing two-dimensional transonic, inviscid flows. The solution procedure uses an explicit Runge-Kutta time marching, finite volume scheme. The computational grid is an irregular triangulation. The algorithm can be applied to arbitrary two-dimensional geometries. When used for analyzing flows in blade rows, terms representing the effects of changes in streamsheet thickness and radius, and the effects of rotation, are included. The solution is begun on a coarse grid, and grid points are added adaptively during the solution process, using criteria such as pressure and velocity gradients. Advantages claimed for this approach are (a) the capability of handling arbitrary geometries (e.g., multiple, dissimilar blades), (b) the ability to resolve small-scale features (e.g., flows around leading edges, shocks) with arbitrary precision, and (c) freedom from the necessity of generating “good” grids (the algorithm generates its own grid, given an initial coarse grid). Solutions are presented for several examples that illustrate the usefulness of the algorithm.

Analysis of Efficiency Sensitivity Associated With Tip Clearance in Axial Flow Compressors

The effects of tip clearance changes on efficiency in axial compressors are typically established experimentally. The ratio of change of efficiency with change of clearance gap varies significantly for different compressors in the published data. An analysis of this sensitivity range in terms of the blade and stage design parameters was initiated. The analysis revealed that the sensitivity range largely resulted from a derivation at constant flow of the efficiency decrement. It was also found that a generalized loss method of generating the sensitivities produced a much improved correlation of the change in efficiency with change in clearance over a variety of machines, configurations and speeds.

The Effects of Diffuser Geometry on Rotating Stall Behavior

Experiments were carried out in a model vaneless diffuser rig to investigate the rotating stall phenomenon and its relation to diffuser geometry. The experimental rig consisted of an actual impeller which was used to deliver the flow to the vaneless diffuser. Mass flow rate through the system could be adjusted by varying the rotational speed of the impeller at a fixed inlet opening or by changing the inlet opening at a fixed impeller speed. The flow exited to room condition. As such, the rig was designed to investigate the fluid mechanics of vaneless diffuser rotating stall only. Attention was focused on the effects of diffuser width and radius on rotating stall. Three diffuser widths and three outlet radii were examined. The width-to-inlet radius ratio varied between 0.09 and 0.142 while the outlet-to-inlet radius ratio varied between 1.5 and 2. Results showed that the critical mass flow rate for the onset of rotating stall decreases with decreasing diffuser width. The critical mass flow rate is affected also by the diffuser radius ratio; larger radius ratios resulted in smaller critical mass flow rates. The ratio of the speed of rotation of the stall cell to impeller speed is found to decrease with increasing number of stall cells. This relative speed also decreases with increasing diffuser radius ratio, but it is largely independent of the diffuser width.

A Radial Mixing Computation Method

A radial mixing calculation method is presented where both convective and turbulent mixing processes are included. The secondary flows needed for the convective mixing are derived from pitch averaged vorticity equations combined with integral methods for the 3D end-wall boundary layers, 3D profile boundary layers and 3D asymmetric wakes. The convective transport due to secondary flows is computed explicitly. The method is applied to a cascade and two single stage rotors. The three test cases show a very different secondary flow behaviour which allows the analysis of the relative importance of the different secondary flow effects. Turbulent diffusion is found to be the most important mixing mechanism, whereas convective mixing becomes significant when overall radial velocities exceed about 5% of the main velocities. The wake diffusion coefficient is found to be representative for the turbulent radial mixing and is the only empirical constant to be determined.

Prediction of the Pressure Distribution for Radial Inflow Between Co-Rotating Discs

The problem of radial inflow between two plane co-rotating discs with the angular velocity of the fluid at inlet equal to that of the discs is considered. An integral solution technique for turbulent flow, based on that of von Karman (1921), is described. Solutions are shown to be in good agreement with most of the available experimental data. For incompressible flow the pressure drop coefficient is a function of just two non-dimensional parameters; the radius ratio for the cavity and a throughflow parameter. For air flows compressibility can be important and an additional non-dimensional parameter is needed. Results for a wide range of conditions are presented graphically. These show the sensitivity of the pressure coefficient to the governing parameters and provide a quick method for estimating the pressure drop.