Reverse roll coating flow

The Galerkin finite element method of solution formulated by Coyle et al, i s applied to analyse the coating flow field between two reverse rotating ro lls. A wide range of operating conditions that are of practical importance were studied. Many quantitative results that can benefit a practising engin eer are presented and physical insight is gained by the analysis of the dat a. It is shown that the reverse roll coating process is affected by various factors, among which the roll speed ratio is the most significant. Regardl ess of other parameters chosen, a critical speed ratio always exists at whi ch the metered film thickness experiences a minimum. At this critical speed ratio, the dynamic contact line is exactly located at the centre of the ga p between the two rolls. The significance of the Reynolds and the Capillary numbers increases with increasing speed ratio. For speed ratios beyond the critical value, a thinner metered film can be achieved by means of either increasing the film thickness of the entering layer or by decreasing the pr incipal roll radius. Furthermore, the computational predictions indicate th at changing the roll radius ratio has no obvious effect on reverse roll coa ting and that gravity effects, quantified by the Stokes number, can be igno red under normal operating conditions. Copyright (C) 1999 John Wiley & Sons , Ltd.