Colloquia

Summary

In coffee machines a venturi injector can be used to produce milk foam for various kinds of coffee. In this thesis, the process of creating suction and heating milk will be investigated using experimental and numerical methods. The goal of this research is to gain insight into the physics of the milk foaming process in a Venturi-driven milk foamer, with a specific focus on heat transfer between the two phases, the resulting phase change, and its impact on the pressure and flow fields.

This is achieved by developing a numerical model using a RANS-based Eulerian multiphase solver with an additional Boiling/Condensation solver in the simulation software Siemens Star CCM+. A multiphase model was built up and validated according to the thermodynamic expectation from steam tables. The accuracy of the model was found to depend on the rate of heat and mass exchange, which could be altered by varying the interactive length scale (mean bubble size). This parameter is calibrated using data obtained from an experiment. An additional experiment is conducted, which used pressure, suction mass flow rate, and temperature measurements to validate both the flow and thermal behaviour of the numerical Venturi-driven milk foamer.

The numerical model underestimates the ratio of suction mass flow to outlet mass flow by 30% compared to the experiment. Despite this, the numerical model gives good physical insight into the flow field, how condensation takes place in and after the neck, and how this impacts the results.