A CFD Method for Simulation of Gas-Liquid Flow in Cooling Systems : An Eulerian-Eulerian Approach

Sammanfattning: When designing modern engines it is important to construct a cooling system that cools the engine structure efficiently. Within the cooling system there is always a certain amount of air which can accumulate and form air pockets in critical areas, such as the water jacket, which can lead to wall degradation. A Computational Fluid Dynamics (CFD) method in STAR-CCM+ from CD-adapco, was derived at Volvo Cars in order to study the accumulation of air bubbles in the water jacket. The method was derived by investigating and evaluating already existing methods. The method initially considered as the best suited was the Eulerian-Eulerian approach. The method was validated against three simpler geometries where experimental data was available. The Eulerian-Eulerian approach treats both phases, liquid and gas, as continuous phases. The idea with the method is to solve the Navier-Stokes equation, the continuity equation and the energy equation for both phases using the Eulerian approach, therefore called Eulerian-Eulerian. The interaction between the two phases was important to model properly which was done by including several interaction models within STAR-CCM+. By tuning different coefficients, which were investigated by a thorough parameter study, the method resembled the experimental data in a satisfying way. The best suited mesh for these simpler geometries was a directed mesh. However, the mesh in the water jacket was automatically generated by STAR-CCM+ and the simpler cases were therefore validated with an automated mesh as well. To capture the experimental data the convection scheme for volume fraction had to be of second order when simulating with automated mesh. This resulted in convergence issues when implementing the method on the water jacket. Instead first order convection scheme, which did not present as satisfying results as second order, had to be implemented. Simulations of the water jacket were performed with two different velocities, that were 10 m/s and 19 m/s, and different flow split ratios for the three outlets. Air with volume fraction 0.1 was injected at the inlet during the first 0.5 s followed by 0.5-1.1 s of further simulation without injecting air. Increased velocity resulted in increased flow through of gas, whereas no big difference could be seen between the different outlet flow split ratios. At two different zones lower pressure was found which resulted in gas holdup. To be able to validate the results from the water jacket, experiments would be necessary to perform in order to provide experimental data for comparison. Velocity profiles from the derived two-phase method resemble the velocity profiles from the one-phase simulation from Volvo, which indicated that the two-phase method did not affect the solution in a remarkable way. Granted that the zones of lower pressure and gas holdup normally coincides, the pressure field from the one-phase simulation could be directly studied, which would lower the computational costs significantly.