Two-Phase CFD Simulation of Turbulent Gas-Driven Liquid Film Flows on Heated Walls

• Main Author: Marati, Jagannath Rao
• Format: Others
• Language: English; en
• Published: 2016
• Online Access: https://tuprints.ulb.tu-darmstadt.de/5300/1/Marati_Dissertation.pdf; Marati, Jagannath Rao <http://tuprints.ulb.tu-darmstadt.de/view/person/Marati=3AJagannath_Rao=3A=3A.html> (2016): Two-Phase CFD Simulation of Turbulent Gas-Driven Liquid Film Flows on Heated Walls.Darmstadt, Technische Universität Darmstadt, [Ph.D. Thesis]

The emissions of oxides of nitrogen (NOx) released from combustion chambers have been the subject of numerous experimental, theoretical and numerical studies in recent years. These emissions are directly related to the quality of fuel air mixing prior to combustion in gas turbines. Presently, Lean pre-mixed pre-vaporizing (LPP) concept is considered to reduce NOx emissions in gas turbines. Therefore, their reduction relies on a more accurate prediction of transport phenomena and interaction between the liquid fuel with the turbulent gas flow field. In LPP concept, liquid fuel is sprayed onto a hot wall thereby forming a thin film. A high velocity co-current hot compressed gas stream blows over this film. The thin film evaporates, and the vapor mixes in the gas stream to form a combustible mixture. The study of the various mechanisms governing transport phenomena in such flows is an important step towards understanding the pre-mixing and pre-vaporization process. The shear force imposed by the gas flow at the gas-liquid interface causes the formation of interfacial waves, and the velocity and amplitude of the traveling waves increase with the rise in interfacial shear force. Furthermore, increase in the interfacial shear force leads to enhancement in the heat and mass transfer rates. The shear-driven flows are turbulent and characterized by strong fluctuations in the velocities of the two phases (air and liquid fuel). In order to have detailed insight of unsteady two-phase flows and thermodynamic processes, new numerical techniques and specific experiments are essential. The present study focuses on development of numerical model for description of unsteady two-phase flows in an externally heated channel. Within the framework of this dissertation, Computational Fluid Dynamics (CFD) is utilized to elucidate the fundamental mechanisms that govern transport processes in shear-driven liquid film flows on heated walls. The numerical studies are performed in OpenFOAM, an open source CFD code written in the C++ language. The open source code is further modified to perform detailed studies of heat transfer in two-phase flows. To predict the interfacial phenomena of two-phase flow, a Volume of Fluid (VOF) approach with an Eulerian-Eulerian method is adopted. The transport phenomenon in an unsteady two-phase flow behavior is studied in combination with Continuum Surface Force (CSF) model for the surface tension force at the gas-liquid interface. A Low-Reynolds number k-ε turbulence model combined with a near-wall grid adaptation technique is applied to both liquid and gas phases. The simulation results are verified with theoretical and experimental data from literature and also with in-house experimental data. Furthermore, the numerical simulations are performed by applying an artificial disturbance boundary condition at the inlet. The effect of gas and liquid Reynolds numbers on the hydrodynamics and heat transfer in a channel is investigated. Results of the simulation indicate that the inlet flow parameters such as gas and liquid Reynolds numbers on have a significant influence on heat transfer. Parametric analysis is employed to interpret the mechanism of wave dynamics under the influence of gas Reynolds number. The main parameters considered in this research are liquid Reynolds numbers (300≤ReL≤650), gas Reynolds numbers (10000≤ReG≤70000), and wall heat flux at 20 W/cm^2. The flow characteristics and film thickness in two-phase flow are significantly affected by increasing gas velocity. The heat transfer rate is enhanced due to influence of wavy flow with increasing gas and liquid Reynolds numbers. Ultimately, this numerical study helps to explain unsteady two-phase flow behavior under heat load.