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Fluid mechanics of impingement region in an experimental simulation of a twin-wire paper machine Gune, Nandakumar Vithal


An experiment with impinging jet flows in narrow channels having moving walls was carried out. The channel was formed by placing two counter-rotating cylinders near each other. This geometry represents a simplified case of the impingement zone of a formation wedge on a twin-wire paper machine. The variables considered in this study were jet velocity, roll velocity, jet width and, roll gap. The working fluid was water. Wall shear stress signals and wall static pressure signals through the flow zone were recorded using signal recovery and averaging techniques. Flow velocity at a location below the mid-nip was measured using hot film anemometry from which mean velocity at the mid-nip was deduced. Flow visualisation experiments were also conducted. A very short duration synchronised flash (synchronised with roll rotation) was used to take the flow pictures. It was observed that under certain conditions the impinging jet loses its momentum as it encounters an adverse pressure gradient in the flow zone. This results in a reversal of flow leading to a pond development above the nip. A relation has been obtained which sets conditions of roll speed, jet speed, jet width and roll gap under which stable operation (no backflow) can be achieved. Flow pictures showed the presence of large quantities of entrained air. Entrained air was seen to pass through the flow zone as columns of air bubbles when the jet velocity was higher than the roll velocity. For jet velocity less than roll velocity, the passage of entrained air through the flow zone resulted in formation of rows of air bubbles. Wall static pressure measurements revealed development of large positive pressures and pressure gradients in the impingement region. The pressure build up was believed to be due to loss of kinetic energy of the impinging jet, interaction of the wall boundary layers on either wall with flow in the core region (boundary layer interaction) and, reduction of cross sectional area of the flow geometry. Wall shear stress levels were lower than pressure levels encountered in the flow. Hence it was concluded that the flow is mainly a pressure driven type, and that the shear stress values show variation of velocity gradients and subsequently velocity of the flow as affected by pressure changes. A flow model is proposed to explain the reversal of flow leading to pond development. Relevance of this study to the analysis of a formation wedge of a twin-wire paper machine is explained. The study indicates that fluid mechanics of such a formation wedge should strongly affect the formation of paper web and machine runnability. The formation of streaks and machine direction basis weight variation observed in industrial situations may possibly be explained by the observed entrained air passage through the experimental flow nip.

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