M.Sc Thesis


M.Sc StudentVan Der Lee Joel
SubjectImpact of the Injection Temperature of Liquid Fuel on the
Blowout Limit in a Cavity-Based Scramjet Combustor
DepartmentDepartment of Aerospace Engineering
Supervisors DR. Dan Michaels
DR. Joseph Lefkowitz
Full Thesis textFull thesis text - English Version


Abstract

This is an experimental study on combustion characteristics of a liquid fueled scramjet. Liquid hydrocarbons are the preferred fuel for Mach 5-8 cruise flight due to higher energy density as compared to hydrogen or lighter hydrocarbons, as well as their endothermic properties when used as a heat sink. Because the feasibility of liquid hydrocarbon powered scramjets is based on cooling with the fuel, it should be able to operate with varying fuel temperatures. The mixing behaviour and fuel entrainment into the cavity fundamentally changes when the injection temperature is increased. This has strong implications for local fuel concentration and therefore flame stability. The impact of fuel heating on flame stabilization, as would happen in cooling channels, is not fully understood.

This study investigates the effect of pre-heating a liquid hydrocarbon (n-dodecane, C12H26) on the mixing behaviour and consequently on the flame-stability in a cavity-stabilized scramjet combustion model. The air conditions were kept constant throughout the entire study. The Mach 2.2 crossflow had a flowrate of 365±5 g/s and stagnation conditions of 12±0.5 atm and 1250±25 K. Ignition and stabilization are assisted by a spark plug and hydrogen pilot flame in the cavity. The liquid n-dodecane was injected in room temperature (cold) and heated conditions ranging between 200 °C and 250 °C. The fuel was injected through a single-orifice with a diameter of 0.6 mm located 33.3 mm upstream of the cavity. Thereafter, the injection location was moved further upstream, at a distance of 60.3 mm from the cavity. The change in mixing behaviour was studied using shadowgraph imaging of a non-reacting jet. Observations were related to penetration depth, vaporization length and mixing phenomena. Information on flame development is obtained using pressure measurements along the combustion length and CH* chemiluminescence.

For the near cavity injection location, using the cold n-dodecane resulted in a stable flame over the full range of flowrates. Shadowgraph and chemiluminescence results suggests that the core of the jet passes over the flame holder. As the majority of the fuel passes over the cavity, the fuel entrainment into the cavity is limited thus resulting in an absence of the rich limit. Heating the fuel has a restraining effect on the rich blowout limit. Rapid vaporization improved the mixing, which appears to increases fuel entrainment into the cavity and leads to a fuel rich limit at lower fuel flow rates. With increased distance from the cavity, fuel injection becomes less sensitive to injection temperature. The increased distance to the flame holder allows for more complete vaporization and mixing. The rich blowout limit for cold n-dodecane shifts to a similar ER as for heated fuel. For heated n-dodecane, the injection location has little impact on the flame stability. The gained understanding from this fundamental study contributes to the design of injection strategies in liquid hydrocarbon fueled scramjets.