T.W. von Backström

Numerical simulation of the flow field near an axial flow fan operating under distorted inflow conditions

Abstract The results of numerical solutions of the Navier-Stokes equations, augmented by the k-ϵ model of turbulence, for an axial flow fan operating under distorted inflow conditions are presented. The fan is of the low solidity/low hub-to-tip ratio type: this type of fan has attained some importance with the advent of large directly air-cooled power stations in certain arid regions of the world. The numerical model is the SIMPLEN algorithm, which has been published by one of the authors previously. In the numerical predictions, blade element theory is used to model the thrust and torque exerted by the fan blades on the air. Experiments have shown that flow across the intake of a fan has a detrimental effect on its performance in that an increased amount of power is necessary to deliver the same flow rate as with no crossflow. The numerical results generally confirm the results of the experiment, although the increase in power is underestimated: an increase of only approximately half of the measured increase is predicted.

Assessment of the wind power potential at SANAE IV base, Antarctica: a technical and economic feasibility study

This paper presents a study for the utilization of wind energy at the South African research station, SANAE IV, in Antarctica (71°40’ S 2° 50’ W). A procedure to evaluate the feasibility of utilising wind power for Antarctic stations is given. The analysis is based on the technical and economic aspects of installing and operating a wind turbine at remote locations. Special attention has been given to conditions encountered at Antarctica, like site accessibility, low temperatures, icing and snow, long transportation distances and environmental issues. The aspect of externalities is incorporated into the economic analysis. The Northern Power Systems NW100/19 wind turbine is found to be the best-suited wind turbine for use at SANAE IV, given the harsh climatic conditions, like frequent windstorms and extreme temperatures. The wind turbine features a yearly energy output of 430 MWh with a capacity factor of 0.49, at a mean wind speed of 10.8 m/s. The study shows that a wind turbine installation at SANAE IV is an attractive solution to reduce fuel consumption and therefore emissions of the diesel electric generators considerably. The use of a wind turbine at SANAE IV could lead to a savings in externalities of about R110 000, -per annum.

Effect of cross-flow on the performance of air-cooled heat exchanger fans

Abstract In large-scale applications such as arrays of axial fans in air-cooled heat exchanger systems, edge–proximity and wind-induced cross-flow may decrease the flow through some fans by causing the flow to enter them at off-axis angles. In this study, such off-axis inflows were introduced by inserting inlet pipe sections between the plenum chamber of a standard test facility and one of three different scale model test fans of 1542 mm diameter. Fan power consumption turned out to be completely independent of off-axis inflow angle up to 45°. Fan total-to-total pressure rise was found to be independent of off-axis inflow angle, and the decrement in fan pressure rise was equal to the dynamic pressure based on the cross-flow velocity component at the fan inlet. Analysis showed that for model fans to represent the cross-flow behaviour of their prototypes, they should have the same ratio of dynamic pressure to pressure rise, and the same dimensionless characteristic slope at their operating points. The performance of a row of fans operating at off-axis inflow conditions representing a cooling system was well predicted by a simple model assuming that the fans farther from the edges induce cross-flows over the fans closer to the edges.

Critical evaluation of heat transfer coefficients applicable to solar chimney power plant collectors

A solar chimney power plant consists of a translucent collector which heats the air near the ground and guides it into the base of a chimney at its centre. The buoyant air rises in the chimney, and electricity is generated through one or more turbines in or near the base of the chimney. Two comprehensive studies are relevant, namely those of [6] and [2].

A proposed reference framework for unmanned aerial vehicle and system airworthiness requirements

Various programmes are underway internationally to establish legislative instruments for regulating civil and military unmanned aerial vehicles and systems. An analysis of a selection of these programmes revealed that the approaches used for airworthiness regulation are not harmonised and are usually limited to specific unmanned aerial vehicle types, indicating the need for a generic framework for airworthiness requirements. A functional Reference Framework for unmanned aerial vehicle and system airworthiness requirements was developed using Annex 8 of the Chicago Convention as a reference basis, supplemented with airworthiness procedures and functional requirements derived from manned aircraft regulations, unmanned aerial vehicle and system airworthiness material, and flightworthiness guidelines for reusable launch vehicles. Various airworthiness elements were identified for which further research is required to develop appropriate airworthiness requirements. This paper summarises the development of the framework and proposes the Reference Framework as a functional basis for generating comprehensive South African civil and military airworthiness requirements for unmanned aerial vehicles and systems.

TECHNICAL NOTE DERIVATION OF A MODIFIED POWER-LAW APPROXIMATION

The accuracy of the power-law approximation of the exponential functions appearing in the solution of the one-dimensional convection-diffusion equation is improved by an additional term derived in this note. An application in which this modification leads to a significant improvement in accuracy is described. Time trials, however, indicate that the use of partial exponential approximations can be faster than both the original and modified power-law approximations.

EXTENSION OF THE SIMPLEN ALGORITHM DIFFERENCING SCHEME TO CYLINDRICAL POLAR COORDINATES

Abstract The differencing scheme used with the SIMPLEN algorithm for the solution of convection-diffusion problems on nonstaggered grids, formulated by one of the authors in two previous papers for Cartesian coordinates, is extended to a cylindrical polar coordinate system. The scheme is shown to be equivalent to its Cartesian counterpart: The interpolation functions derived for the radial coordinate direction reduce to those for Cartesian coordinates when the cell interfaces are positioned at the geometric mean radial positions between grid points. Three case studies involving laminar, incompressible flow are presented to validate the scheme: stagnation in three-dimensional flow, flow near a rotating disk, and flow through an abrupt pipe expansion. Excellent agreement between the numerical solutions and the corresponding analytical solutions and experimental data is demonstrated.

An efficient scheme for describing airfoils using non-uniform rational B-splines

In the well developed field of airfoil design there are a number of established schemes for describing airfoils, each with its own limitations and suitability to a particular application. Although less parsimonious, B-splines are often used to find solutions unconstrained by the characteristics of these schemes. They have proven useful to improve on existing designs and are useful in a numerical optimization process. As part of a numerical design project of a small wind turbine, a parametric scheme using Non-Uniform Rational B-Splines (NURBS) offering intuitive control over airfoil shape and unambiguous definitions for chord length and angle of attack is needed. These features are achieved at the lowest number of variables possible to ensure numerical optimization efficiency. Two previously uninvestigated NURBS arrangements are defined and investigated. The quality of the resulting airfoil reproductions are geometrically and aero-dynamically investigated and based on the results, the use of a thickness-camber scheme is motivated.Copyright

Performance investigation of a turbocharger compressor

The compressor section of a diesel locomotive turbocharger was re-designed to increase its maximum total-to-total pressure ratio and efficiency. Tests conducted on the prototype compressor showed possible rotating stall in the diffuser section before the designed higher pressure ratio could be achieved. It was decided to simulate the prototype compressor’s operation by using one-dimensional theory [1], followed by a three-dimensional CFD analysis of the compressor. This publication focuses on implementation of the impeller, vaneless annular passage and vaned diffuser one-dimensional theories. A verification process was followed to show the accuracy of the one- and three-dimensional modelling methods using two well-known centrifugal compressor test cases found in the literature [2–5]. Comparing the test case modelling results to available experimental results indicated sufficient accuracy to investigate the prototype compressor’s impeller and diffuser. Conclusions drawn on the prototype compressor’s performance using the one- and three-dimensional modelling methods led to a recommendation to redesign the impeller and diffuser of the prototype compressor.Copyright

MINIMIZATION OF THE EXIT LOSS OF A ROTOR-ONLY AXIAL FAN

The main loss source in rotor-only axial fans is that associated with the kinetic energy flux at the Tan exit. Both the axial and the tangential velocity components contribute to this loss. In this paper the loss is minimized by sequential quadratic programming under the constraints of constant volume flow and work input. It is shown that, when observing certain simple traditional fan design rules, the exit kinetic energy flux can be reduced by only 1.8 percentage points compared to Tree vortex Tans.