Abstract
The study of buoyancy effects in enclosed jet systems is a much neglected field of combustion aerodynamics, research. Although theoretical analyses exist,little experimental data has been collected and this experimental investigation sets out to study the influence of various input momentum parameters on the deflection experienced by an enclosed buoyant jet. Due to the difficulty of modelling the jet under buoyant forces of sufficient magnitude,a technique has been used which was originally developed for free jet buoyancy studies. A slurry of finely ground Magnetite was used as a primary jet source whilst water was the fluid used for the general stream. Density ratios (Primary to secondary) of up to 2.7 have been used in the study whilst the velocity ratio was varied between 5 and 50, giving a range of Curtet numbers between 0.3 and 1.4. The resulting turbulent jet was photographed with a high speed 16mm. cine camera. From the film, measurements of jet enclosed angle, length of potential core, axial velocity and jet deflection were made. At the lower densities the jet angle was found to be constant for each density ratio, thus supporting the common assumption of constant entrainment rate, in jet systems where the variation in density of the jet fluid is small. However as the density ratio increased then it was found that the constant entrainment assumption was conditional on higher velocity ratios. Generally the assump tion only held for Curtet Numbers less than 0. 35. The axial velocity was found to decay significantly faster in the enclosed buoyant jet than predicted by the free jet theory. That this decay was a result of mixing and not input momentum was shown in the fact that varying the velocity ratio had a much greater effect than varying the density ratio. Greater mixing levels exist in the enclosed jet as compared to the free jet, a fact further borne out by the similarity of the potential core velocity ratio relation ship to that found in jets in cross streams where increased mixing is also experienced, although possibly not for the same reasons. An expression has been derived which can be used to predict, with some caution, the path of a jet issuing horizontally into an enclosed space with an initial density difference, up to the point where buoyancy control ceases, and the jet is no longer deflected. This point was found by experiment to be at s = 2.3/k. where k is an empirical constant from the expression for the axial velocity ratio. An attempt was made to model a flame envelope near the mixing tube wall and using the assumption that the envelope was enclosed by the half jet angle it was found that for velocity ratios less than approximately 10, this half jet contour would strike the mixing tube wall whilst the jet was still under the influence of the buoyancy forces.