The fluid mechanics of the Aaberg exhaust hood
In this thesis an investigation of the fluid mechanics of the Aaberg exhaust hood is presented. The Aaberg exhaust hood is unique in its design as the speed of the air flow towards the exhaust inlet is enhanced by the entrainment of fluid into the hood's jet flow. The complex air flow pattern of the hood is governed by the Navier-Stokes equations. However, in this thesis modelling techniques have been developed in order to reduce the complexity of determining the fundamental air flow pattern. The modelling procedure adopted considers the hood's overall air flow to be composed of three component flows, namely, (i) the flow in the jet region, (ii) the jet-induced flow and (Ui) the suction flow. In practice the fluid flow pattern generated by the hood is such that the Reynolds number is very large, and hence the suction and jet-induced flows are modelled as potential flows with the boundary conditions governing the jet-induced flow coming from the solution of the shear-layer equations. This solution procedure enables the parameters which govern the hood's air flow to be identified and their effect on the flow produced by the hood may then be determined. Both two-dimensional and three-dimensional axisymmetric exhaust designs have been examined and for the latter case a new numerical model for the axisymmetric radial flow of a fluid from the space between two identical concentric discs, for laminar and turbulent flows, has been developed. Agreement between the turbulent radial jet model developed and the results of numerous other established theoretical and experimental investigations is very good. The inviscid models for the overall air flow have been developed in terms of the stream functionand. except for in the simplest case considered where an analytical solution is possible. the equations of motion which govern the fluid flow in the region of interest have been solved numerically using finite-difference techniques. The models developed illustrate all of the flow phenomena observed experimentally and comparisons made between the predictions of both the two-dimensional and three-dimensional axisymmetric mathematical models and (i) the available experimental data and (ii) the commercially available CFD code FLUENT. which solves the full turbulent Navier-Stokes equations. show good agreement. thereby confirming the credibility of the cost-effective modelling approach adopted in this thesis.