Understanding and analysis of wear in homogenizers for processing liquid food Fredrik Innings a , Erik Hultman a , Fredrik Forsberg a , Braham Prakashb b a Tetra Pak Processing Systems, Ruben Rausings gata, SE-221 86 Lund, Sweden b Luleå University of Technology, SE-971 87 Luleå, Sweden ABSTRACT The tribological research pertaining to homogenizers in processing liquid food has received much less attention compared to its potential in enhancing the efficiency and durability of the homogenization process equipment. Homogenization is a process used to disrupt fat globules in dairy products to reduce the formation of creamy layer (separation) and also to enhance the viscosity of certain products. This process takes place in a narrow gap in the homogenizer machine and this region is highly prone to wear. The occurrence of wear during processing not only impairs the homogenization effect but also leads to increased downtime of the machine. The aim of this work was to understand the occurrence of wear and wear mechanisms in the homogenization gap using both experimental and analytical approaches. Two experimental test rigs were used to study the differences in wear during operation, i.e., with and without particles. The trajectory of the particles was simulated without the influence of cavitation using a CFD-code to investigate whether the particles are the cause of the wear. The homogenizer gap has been simulated for a worn geometry to see how the occurrence of wear affects the particle trajectories. The results show that the presence of particles accelerates wear and most important parameters are the particle hardness and mass. When cavitation and particles are combined they create a synergistic effect on the wear. This can be explained by the fact that cavitation can accelerate particles in random directions through the imploding action of cavities. A change of impact angle increases the amount of wear significantly. CFD- simulations and calculations show that the particles do not fully follow the streamlines and therefore create wear on the gap surfaces. Keywords: Cavitation; cavitation-erosion; homogenizer; wear; food processing INTRODUCTION High pressure homogenization is a process used to disrupt fat globules in dairy products and beverages. This is done to reduce creation of a cream layer (separation), impart a more appetizing colour and satisfy consistency at ingestion. The homogenization is done in a homogenizer machine, in which an electrical motor drives a piston pump and the rotational speed of the motor controls the flow rate. The homogenization takes place in the homogenization device seen in Figure 1. By applying a pressure on the hydraulic actuator the product to be homogenized flows through the narrow gap, which is created between the forcer and the seat. Forcing of the fluid through the gap creates the desired velocities, pressure fluctuation and shear forces needed to homogenize the fluid. Typical distances between forcer and seat varies between 20-100 μm, a typical flow rate for a 3 piston machine is 5500 l/h, this will generate high velocities due to the area contraction. The high velocity is accompanied by a low static pressure according to Bernoulli’s equation, a low static pressure can lead to cavitation formation if the static pressure is below the vapour pressure. The cavitation intensity depends on the Thoma number (Th), since this will control the velocity in the gap. The action of cavitation emits sound and light and thus it is possible to measure the intensity. Experiments show that for Th-values from 0 to 0.05 the light and sound intensity emitted by the cavitation bubbles is much higher than in between 0.05 and 0.15 where the intensities drop very quickly [1]. The phenomenon of cavitation is known to cause severe damage to hydrodynamic systems by the formation and implosion of vapour bubbles. Cavitation can be induced in many ways and most focus is on research in the field of acoustic and hydrodynamic cavitation. The difference between these two cases is that in the acoustic case the medium is static and the cavitation is induced by ultrasound. In the hydrodynamic case the liquid is in motion and the balance between velocity, static pressure and pressure losses determines whenever or not the pressure drops below the threshold for cavitation, the vapour pressure.