Numerical Simulation of Methane-Water Combustion at Transcritical Conditions
Combustion at elevated pressure brings advantages in terms of efficiency and emissions. In many different devices a liquid reactant is injected into a high pressure chamber where the reaction occurs. Common applications are diesel, jet, and rocket engines. Frequently, the thermodynamic conditions of the ambient fluid in the combustion chamber are supercritical for both fuel and oxidizer. However, the injected fluid is usually at subcritical temperature. Consequently, two phases exist until the temperature of the liquid rises to its critical value by means of heat transfer. This process is known as transcritical vaporization. Important and challenging problems associated with this scenario have been studied with increasing interest during the past years. The main aspects of study include transcritical single-droplet and spray vaporization, mixing, combustion, phase equilibrium, and transport.
Despite an extensive and growing list of publications, there are still unanswered questions. For example, the presence of sharp gradients in density or composition are not well understood under conditions that one would expect to be supercritical. The possible existence of a heat of vaporization and surface tension in the supercritical regime has been highlighted by some researchers. Understanding the physics governing these phenomena is crucial to control combustion chambers, as well as to provide an opportunity for improving their design.
Other relevant applications for transcritical combustion are the burning of gas hydrates and direct water injection. It is well known that large quantities of methane are stored in form of gas hydrates in the ocean at a depth of the order of 1 km. The possibility of burning these materials locally to extract energy has been suggested. The conditions in which combustion would occur with so much water in the environment and at high pressure are also not well understood.
A one-dimensional counterflow diffusion flame model is used in this dissertation together with real-fluid thermodynamics and laws of phase equilibrium. The presented solutions provide insights that are relevant to the scenarios posed above.