The discharge of complex fluids through an orifice: A review

The discharge of fluids with complex rheological properties from industrial tanks, pipes, dispensers, or packaging containers through an orifice is a complex process often encountered in industry, which, if not properly characterized, can result in slow discharge and significant residual fluid left in the container. The discharge is strongly affected by the orifice geometry and fluid rheology. In this article, we review and summarize the current state-of-the-art in the discharge of complex fluids through orifices. While general correlations to predict the orifice flow are available, a more in-depth analysis of the fluid dynamics of the orifice discharge process as well as innovative surface modification techniques are required to improve industrial equipment design, process operation, and consumer packaging. Future perspectives that have the potential to address discharge issues and decrease the amount of waste are also discussed, including the utilization of high-precision multi-dimensional flow monitoring techniques such as Particle Image Velocimetry (PIV) and Magnetic Resonance Imaging (MRI), computational methods such as Computational Fluid Dynamics (CFD), and recent innovative surface modification techniques reducing fluid-container interaction, thus avoiding the no-slip boundary condition that causes viscous fluids to adhere to the container.