Public defense of Achref Rabhi´s licentiate thesis
The public defense of Achref Rabhi’s licentiate thesis in Energy and Environmental Engineering will take place at Mälardalen University Västerås, room R1-343 and via Zoom or Teams at 09.00 on March 19, 2021.
Title: “Numerical Modeling of Subcooled Nucleate Boiling for Thermal Management Solutions Using OpenFOAM”.
The examining committee consists of: Associate Professor Christophe Duwig, KTH Royal Institute of Technology, Docent Henrik Ström, Chalmers University of Technology, Professor Anatoliy Malyarenko, Mälardalen University.
The faculty examiner is: Associate Professor Christophe Duwig.
Reserve: Professor Hailong Li, Mälardalen University.
Serial number: 304
The advancement of engineering equipment and industrial processes is usually limited by the ability to dissipate the released heat. Hence, optimal heat removal will enable the next generation of these equipment and processes. Despite that they were extensively used for cooling, single phase cooling means and methods hit their maximum performance at around 100 W/cm2, which can be far below current released heat fluxes. Then a novel heat removal method allowing to dissipate extreme heat fluxes is required, while operating at lower cost. Two-phase cooling and boiling flows allowing heat removal by phase-change are a promising solution to dissipate high heat fluxes, while keeping the surface temperature uniform, few degrees above the saturation point. Such flows can operate at lower pressure drop, and several orders of magnitude higher heat transfer coefficient can be achieved when operating in their subcooled nucleate boiling heat transfer regime.
This thesis is focused on the Computational Fluid Dynamics (CFD) modeling of the low and moderate pressure subcooled boiling flows in minichannels using the open-source tool OpenFOAM. High fidelity multidimensional CFD simulations are carried out to model water flow boiling upward a narrow rectangular channel (3 mm x 100 mm x 400 mm) heated from one side by a constant heat flux. Several present boiling sub-models are evaluated and analyzed to determine their prediction capabilities. It is shown in this thesis that predictions associated with the available boiling sub-models are not accurate. Convective boiling key parameters as the thermophysical properties of the involved materials (density, specific heat capacity and thermal conductivity of the working fluid and the heated surface), liquid-vapor thermodynamic properties (surface tension, contact angle and pressure) and the heated surface micro-properties (average surface roughness and total number of the present micro-cavities) have to be taken into account in a boiling sub-model, in order to improve the predictions accuracy. A sensitivity analysis for the interfacial moment transfer is carried out using CFD simulations of refrigerant boiling flows upward narrow annular pipes. This configuration is often met in industrial applications related to power generation. The results show that all the interfacial forces acting on bubbles have significant effects on the boiling field, except the lift force which has a minor effect and can be neglected in the simulations.
Within this work, the available models of interfacial forces in the literature are implemented and evaluated. The most accurate models are highlighted and proposed as recommendations for future simulations. Using the improved CFD predictions data of subcooled nucleate boiling of water at atmospheric pressure flowing upward a narrow rectangular channel heated from one side, a boiling sub-model (ONB) is studied and evaluated. The Onset of Nucleate Boiling (ONB) marks the transition between the single phase forced convection heat transfer regime to the two-phase subcooled nucleate boiling heat transfer regime. The convective dependencies of the ONB are determined and taken into account to develop a new mathematical model for this boiling parameter. The new developed model for the ONB takes into account the inlet Reynolds number, the flow regime, the inlet subcooling, the applied heat flux and to the thermophysical properties of the involved materials. This model is characterized by a wide validity range, acquired from the extended range of operating conditions used in the CFD simulations. The new ONB model predictions fall within an accuracy of ±2.7% instead of ±30% of the majority of the models from the literature.
The work proposed in this thesis consists on a knowledge foundation for the high fidelity CFD simulations of boiling flows. Propositions of boiling sub-models leading to better CFD simulations accuracy are provided. A methodology of building highly accurate boiling sub-models based on CFD simulations data is presented. The work presented here can be used further to develop a hybrid 1D-3D simulation tool, allowing for the optimization of two-phase cooling operation and schemes, and to address the thermal management of complex industrial and energy systems.