- Title
- Impingement of plane turbulent jets and their application in industrial coating control
- Creator
- Tu, Cat Vo
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 1995
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Extensive investigations of the characteristics of single and dual impinging plane turbulent jets were conducted and an analysis provided for coating film flow behaviour under the influence of impinging jets. The special characteristics of the fluctuating pressure on the impingement plate were also studied for both normal and oblique impinging jets. The findings of the present study provide an insight into the behaviour of impinging turbulent jets and their use in the modern industrial hot-dipped metal coating. The main findings are: Velocity fields from a normal jet plane impingement onto a flat plate show that the decay of the centreline velocity agreed well with the equation determined from Hiemenz solution for stagnation flow. However the wall shear stress in the impingment region, which is linearly proportional to the distance from the stagnation position, was found significantly below that predicted from Hiemenz solution. The influence of the impingement plate on the jet flow development was found to be confined to a region relatively close to the impingment surface. The wall jet growth and wall friction were found higher than the values for wall jets in parallel nozzle studies and this is likely to be related to the difference in the initial jet conditions. For normal impingement, two distinct groups of normalised shear stress profiles corresponding to small H/D (≤5) and large H/D(>7 approximately) were found. The distinction is related to H/D only and independent of the Reynolds number. Reynolds number was found to have insignificant effect on the stagnation pressure, impingment pressure profile and the profile half-width. However the maximum wall shear stress was a function of both H/D and Re. The present results are consistent in magnitude with previously published values which were measured over a narrower range of H/D and Re. In the study of impingement pressure fluctuations, the pressure spectra and spectrograms were obtained for normal and oblique jet impingement at various Reynolds numbers and nozzle distances. The profile of the RMS impingement pressure was independent of Re and the results suggest that the change in the profile with H/D is not related to the transition in the wall jet flow but is most likely associated with the penetration of the mixing layers towards the nozzle centre line as the nozzle distance varies. There are regions consisting of distinguishable dominating frequencies in the pressure spectrograms and both the nozzle distance and nozzle exit velocity can influence these features in the impingement and wall jet regions. The pressure spectrum at the stagnation position was well related to the velocity spectra along the jet centre line over a wide frequency range. At higher frequencies the pressure fluctuation was found to decay significantly. Comprehensive study of the mean pressure and wall shear stress due to the impingement of asymmetric dual jets onto a flat surface was conducted. For dual jet impingement, it was found that there were significant changes in the distribution of both pressure and wall shear stress with nozzle distance and correlations were made to provide a description of their behaviour. The force acting on the impingement surface was found to be inversely proportional to distance as predicted by simple hovercraft theory. Even though the behaviour of peak pressures of dual jet system is similar to that of single jet impingement, their magnitudes are between 10% to 30% lower and the half-width of the pressure distribution was higher than the single jet results. Both stagnation points moved from the intersection point of the nozzle centreline in the direction away from the cushion region. The effect of Re on impingement pressure profiles was insignificant but there was small effects of Re on the wall shear stress distributions. An analysis was presented for the coating film flow under the influence of external forces imposed by impinging jets. This analysis was used to calculate the final film thickness. Asymptotic solutions to the simplified solution for single jet impingement in extreme cases of the withdrawal speed, U, suggest that the final film thickness is linearly proportional to U/PP at low speed and proportional to (U/PP)1/2 at high U. With the beneficial effect of the cushion force, the dual nozzle system can operate much closer to the strip to reduce the film thickness, energy consumption and noise level. Furthermore, actual trials showed improvement in coating thickness distribution across the strip width could be obtained.
- Subject
- plane turbulent jets; industrial coating control
- Identifier
- http://hdl.handle.net/1959.13/1320858
- Identifier
- uon:24224
- Rights
- Copyright 1995 Cat Vo Tu
- Language
- eng
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