Combustion Behavior of Solid-Fuel Ramjets

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AN experimental investigation was conducted of the combustion behavior in solid fuel ramjets in order to determine the effects of configuration variables and operating conditions on combustion performance. Variables considered were fuel port flow rates, bypass dump momentum and geometry, and bypass ratio. Contents To be used in a tactical situation, the solid-fuel ramjet has to demonstrate combustion stability and high efficiency over the expected operating envelope of altitudes and Mach numbers. It must also show performance comparable to that of liquid-fuel ramjets and ducted rockets. Combustion studies on the solid-fuel ramjet have been underway at United Technologies—Chemical Systems Division since 1971.l The solid-fuel ramjet has two distinct combustion zones within the fuel grain: the recirculation zone behind the sudden expansion inlet, which provides flame stabiliation; and a diffusion flame in the developing boundary layer region after flow reattachment. Unburned gaseous fuel escapes from under the flame at the aft end of the fuel grain and results in decreased combustion efficiency. Aft mixing chambers and bypass air designs are being used to increase the efficiency. A schematic of the solid-fuel ramjet is shown in Fig. 1. The apparatus employed a polymethylmeth acrylate (PMM) fuel with a fuel port to dump inlet area ratio of 9.0 and a fuel port to nozzle throat area ratio of 4.0. The exit area of the grain was held fixed at the initial port area by using a thin orifice plate. The aft mixing chamber had a length to diameter ratio of 2.93. The aft mixing chamber consisted of three interchangeable sections such that the axial location and angular orientation of the bypass dumps could be varied. A summary of the test conditions is presented in Table 1. Figure 2 presents data obtained without bypass air. The regression rate expression indicates a weaker dependence on pressure than the earlier data of Boaz and Netzer.2 Figure 3 presents data obtained with bypass air. A slightly stronger dependence on pressure was shown while regression rate indicated very little or no dependence on air flux for the values tested. In the bypass situation, the mass flux through the grain is low but the pressure is maintained high, due to the total mass flux through the nozzle throat. These conditions minimize the convective heat flux to the fuel surface. For lower mass flux through the grain, the regression rate increases relative to the air flux; thus, more gas with radiative properties (fuel rich) is present. These results indicate that using bypass with PMM fuel changes the principal wall heat flux mechanism from convection to radiation.