Modelling the behaviour of a stationary horizontal liquid jet impinging on a vertical wall

Abstract

Predicting the flow patterns generated by liquid jets impinging on vertical surfaces is crucial for designing efficient industrial tank cleaning systems. The flow behaviour generated by a stationary horizontal water jet impinging on a vertical target surface (Perspex, glass or polypropylene) was investigated. The work was studied experimentally and numerically (modelling). The jets were generated by nozzles similar to those used in industrial cleaning, with diameters ranging from 2 to 4 mm. The jet velocity varied from 1.89 to 12.2 m s-1 for flow rates between 0.48 and 4 L min-1. Particle image velocimetry (PIV) was used to give detailed information about surface velocity distributions. Existing models for the impingement region gave reasonable predictions of the flow pattern. However, marked disagreement was found between the velocity field derived from these models and the velocities measured by PIV. The Nusselt thin film assumption, generally used when the flow is laminar and uniform, was found to be inaccurate as the flow is wavy and unsteady. The modelling of the draining film zone was based on the work of Mertens et al. (2005). This model was found to predict a negative film thickness due to the use of a quartic function for the height distribution. PIV measurements revealed that Mertens et al.’s assumption of a uniform velocity across the entire cross-section at a given vertical location was inaccurate. The draining film zone possesses a flat central region where the velocity is uniform, whereas the outer region, called the rope, is characterized by a relatively large thickness and a non-uniform velocity profile. Waves were also detected on the surface of the falling film, which led to interrogate the validity of the Nusselt falling film assumption used by Mertens et al. (2005) to derive the model. An alternative boundary condition based on capillary force alone was proposed for Mertens et al.’s model and this gave a good prediction of the narrowing pattern as well as the velocity field. The results highlighted the importance of the receding contact angle.