There is an increasing interest in high-pressure liquid-fueled combustion systems for higher power outputs and efficiencies. Initially observed in rocket propulsion systems, recent technological developments are also gearing diesel and gas turbine engines toward higher-pressure ratios, with combustor pressures exceeding the critical pressures of fuel and air. At low or subcritical pressures, interaction between aerodynamic shear forces and surface tension promotes atomization processes; at supercritical pressures, surface-tension vanishes, and diffusive mixing becomes the dominant disintegration mechanism. Two recent developments in the field are concerned with unexpected phenomena: First, experimental observations indicate that hydrocarbon fuels injected into supercritical air atmospheres may exhibit both atomization and diffusive mixing, challenging the predictive capabilities of state-of-the-art models. Second, the ‘common knowledge’ of a homogeneous supercritical phase has to be replaced with a phase transition like phenomenon – pseudoboiling - across an extension of the coexistence line.
The aim of this tutorial is to discuss the thermodynamics of supercritical fluids and its implications for high-pressure liquid-fueled combustion systems, expose the state-of-the-art of physics modeling, and finally, motivate the need to develop thermodynamically consistent models based on the equations of fluid motion to further investigate transcritical phenomena. Lastly, challenges and open questions that may have to be addressed on this topic will be discussed.
Daniel Banuti is currently a Postdoctoral Fellow of the Center for Turbulence Research at Stanford University. He received his MSc in Mechanical Engineering from RWTH Aachen University, and his PhD in Aerospace Engineering from Stuttgart University, Germany. Before joining CTR, Daniel was a Research Associate at the German Aerospace Center (DLR) in Göttingen, Germany. His research focus is thermodynamics and modeling of high pressure injection, mixing, and combustion.
Lluis Jofre is currently a postdoctoral researcher in the Center for Turbulence Research at Stanford University. He graduated from Polytechnic University of Catalonia - BarcelonaTech (Spain) in conjunction with KTH - Royal Institute of Technology (Sweden), and obtained a PhD with honors in mechanical engineering from the same University. His main research interests are modeling and computational studies of turbulence and multiphase flow, advanced numerical methods for multiphysics applications, and high-performance computing.