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|Title:||EXPERIMENTAL AND NUMERICAL STUDY OF CONTINUOUS GALVANIZING GAS JET WIPING VIA A MULTI-SLOT AIR KNIFE|
|Authors:||Yahyaee Soufiani, Ali|
|Abstract:||This thesis investigates a novel configuration of a multi-slot jet, through numerical simulation and experimental measurements, for coating thickness reduction and noise elimination in the continuous galvanizing gas jet wiping process. Gas jet wiping is an effective hydrodynamic method for controlling the final zinc coating thickness on a moving steel substrate during continuous hot dip galvanizing (CHDG). In this process, an impinging jet, which is referred to as an air knife in the industry, is used to wipe excess zinc alloy from the steel substrate and control the final coating thickness through the combined effects of a pressure gradient and shear stress distribution on the moving strip emerging from the molten zinc bath. In this study, a novel configuration of a multi-slot air-knife, comprising one main jet with two auxiliary jets, symmetrically located on each side of the main jet, was investigated as an alternative to the conventional single slot jet geometry. For this purpose, computational fluid dynamics was used to determine the wall pressure profile and wall shear stress distributions produced by the multi-slot jet, and these results were used in an analytical model to estimate the final zinc coating thickness on the substrate. An operating region, which was relatively robust to air knife geometry changes, was determined through numerical simulations. Based on the CFD results, a modified geometry for the multi-slot air knife was proposed which led to lighter coating weights compared to the single slot jet. Numerical simulations over a wide range of gas wiping parameters was then performed in order to evaluate the wiping efficiency of the modified design of multi-slot jet at different operating conditions. It was shown that for higher jet to wall distances (Z/D ≥ 8) and at high strip velocities (Vs ≥ 1 m/s), lighter coating weights can be obtained through use of the multi-slot jet design compared to that of the conventional single slot jet. Moreover, a cold laboratory-scale model of the continuous galvanizing gas jet wiping process was designed and manufactured with the objective of validating numerically modelled coating weights for the prototype multi-slot air knife. Experimental measurements under a variety of knife geometries and process conditions agreed with the coating weight predictions of the analytical model. It was determined that the final coating weight was significantly affected by the auxiliary jet width, Da, where lighter coating weights at higher strip velocities (up to 5.4 % at Vs = 1.5 m/s) could be achieved by using the multi-slot air knife prototype versus the conventional single-slot configuration. The effects of various operating conditions, such as: main jet Reynolds number (Rem), auxiliary jet Reynolds number (Rea) and jet-to-substrate distance (Z/D) on the final coating weight were also determined experimentally. The results showed that the final coating weight produced by the multi-slot air knife, with a relatively low flow for the auxiliary jet (i.e. Rea/Rem ≤ 0.5), was lower than the final coating weight utilizing a similar main jet Reynolds from the single slot jet design. Finally, the acoustic properties of the multi-slot prototype design were experimentally investigated. It was observed that the auxiliary jets had the ability to either attenuate or eliminate the tonal noise produced by the main jet. The measurements were performed for various main jet Reynolds number (Rem), auxiliary jet Reynolds number (Rea), jet to strip distance (Z/D) and strip velocities (Vs). It was found that the high intensity tonal noise observed for the single jet was eliminated when using the multi slot-jet working with the same main jet condition and with relatively low auxiliary jet flows (i.e. Rea /Rem ≤ 0.5). The coating weight measurements carried out under the same operating conditions also showed that multi-slot jet resulted in lighter coating weights compared to the single slot jet.|
|Appears in Collections:||Open Access Dissertations and Theses|
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|Yahyaee-Soufiani_PhD- thesis.pdf||PhD thesis||7.34 MB||Adobe PDF||View/Open|
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