Fundamentals of High Pressure Combustion

The objective of the following chapter is to present an overview of the fundamentals of combustion processes in high pressure environments. Developing such an understanding is important to the further development of modern combustion devices. Many such devices operate under supercritical pressure conditions. The critical pressure (or “critical locus” for a mixture) of the majority of hydrocarbon fuels is in the range of approximately ∼ 15 < PC < 30atm [1]. In comparison, aircraft gas turbine engines operate at quasi-steady pressures ∼ 30atm [2]. Furthermore, gas turbine combustion pressures have been increasing at a near linear rate for more than 50 years and are expected to continue to do so [2]. Diesel engines obtain pressures as large as ∼ 60atm after ignition [3, 4, 5, 6, 7]. Figure 1 presents a schematic of diesel and gas turbine engine combustion in thermodynamic state space. The mixture saturation curves of both diesel fuel and JET-A fuel are both illustrated. For both systems, the fuel is injected at very large pressures into the combustion chamber. For diesel engines the chamber is at approximately 25atm at injection which then rapidly increases with ignition. Rocket engines utilizing hydrogen and oxygen represent another example. The critical pressures for hydrogen and oxygen are approximately 13atm and 50atm, respectively [1]. In comparison, rocket combustion chambers achieve pressures well in excess of 100atm [8]. Sutton states that the largest combustion chamber pressure yet achieved in the U.S. is approximately 225atm measured in the space shuttle. The fundamentals of such supercritical pressure combustion processes are the focus of this chapter. At supercritical conditions the state of the species is often referred to simply as “fluid. ” This is because changes in temperature at fixed supercritical pressure do not result in phase change. The fluid simply becomes less dense with increasing temperature. Figure 2 illustrates this effect by presenting the variation of density (and compressibility) for both nitrogen (as a model for air) and dodecane which is a primary component species in most hydrocarbon fuel blends. The pressure is fixed at 50atm and the properties are predicted by the cubic Peng Robinson equation of state (see below). The hydrocarbon in particular experiences large variations in the fluid density from “liquid-like” at low temperatures to ∼ 100kg/m3 at larger temperatures; however, no discontinuity exists. Note in addition, that for the purposes of the following discussion, a fluid (mixture) is defined as being in the supercritical state if either the pressure or temperature are above the critical values. In either case no phase change is possible. Related experimental studies typically address fuel jets or atomizers issuing into high pressure chambers, and are primarily based on qualitative visualizations or relatively simple quantitative measurements [9, 10, 11, 12, 13, 14, 15, 16, 17]. This is because it is difficult to make intrusive measurements within pressurized combustion chambers [18]. Most of these studies do not include combustion due to further difficulties associated with heat release and associated pressure increases. Exceptions include the work of Candel et al. [13] and Mayer et al. [17]; both of whom conducted experiments of oxygen-hydrogen flames at pressures as large as 1MPa and 6MPa, respectively. The results of these experimental studies show that the behavior of supercritical jets is dramatically altered from