Experimental and theoretical study of the flow in the manifold of a curtain coater, 2010 TAPPI Advanced Coating Fundamentals Symposium

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Curtain coating is a contactless coating process which generates hope for increased quality and productivity for coaters. Nevertheless, it is demanding and its requirements differ from traditional processes regarding coating colour components. This process requires a perfect stable curtain to obtain a good quality of coating and many problems originate from the internal flow when the fluid goes through cavities and slots before reaching the inclined plane. Owing to few information in the literature on modelling in the manifold, we performed 3D CFD simulations in the manifold in a laboratory curtain coater in order to analyze the phenomena taking place in the first cavity, find parameters that spark off defects and propose some solutions to reduce the non uniformity of the flow. It has been found that the Reynolds number is a relevant parameter both for Newtonian and Non-Newtonian fluids and increasing it leads to disturbances in the manifold. The power law index has also a deep impact on flow uniformity since its decrease leads also to perturbations whereas it has been demonstrated that the yield shear stress has no impact on vortex formation for coating colours. To maintain vortex-free operation, a critical Reynolds number at the inlet exists whatever the fluid (Newtonian or Non-Newtonian) and it is equal to 20 with the studied geometry. Thereafter, geometrical changes were simulated with a higher radius of the inlet pipe or an end-fed manifold and the results revealed that it improved flow uniformity. Finally, simulation results have been validated thanks to flow visualization experiments with tracers using a transparent replica of the coater.