2017 11 th International High Energy Materials Conference & Exhibits, HEMCE-2017, Nov.23 – 25, 2017, Pune, INDIA Investigating the Effect of Injector Type on the Regression Rate of the Hybrid Rocket Pragya Department of Space Engineering and Rocketry Birla Institute of Technology, Mesra Ranchi, India pragyaberwal16@gmail.com Shelly Biswas Department of Space Engineering and Rocketry Birla Institute of Technology, Mesra Ranchi, India shellybiswas@bitmesra.ac.in Abstract— Injection pattern of the oxidizer into the combustion chamber is the deciding factor of combustion efficiency and regression rate in a hybrid rocket. In the hybrid rocket combustion process, oxidizer flows over the solid fuel grain surface leading to a turbulent diffusion boundary layer formation and the flame is established inside the boundary layer. This boundary layer formation acts as a hindrance in the interaction between the oxidizer particles and the burning surface of fuel grain and reduces the fuel regression rate. An attempt has been made in this paper to study the effect of the injector design pattern in the enhancement of the fuel regression rate of the hybrid rocket. The fuels used were Paraffin wax and Polyvinyl chloride (PVC) with gaseous oxygen as oxidizer. Two injector configurations namely shower head and multi-angle diverging injectors were used to study the flow behavior of oxidizer in the combustion chamber of the hybrid motor. The effects of formation of recirculation zone and flow velocity were studied numerically by a cold flow simulation using ANSYS- Fluent software. It has been observed that direct impingement of the multi-angle diverging injector produces velocity in three directions leading to disruption of the boundary layer. An increase of 9.4% in the average fuel regression rate for PVC fuel grain and 35% for paraffin wax fuel were observed as compared to the shower head injector for the same oxidizer mass flow rate. Reduction in sliver and uniform fuel consumption is also observed using novel multi-angle diverging injector. Keywords- Hybrid rocket, Injection pattern, Regression rate, Paraffin wax, Polyvinyl chloride I. INTRODUCTION A hybrid rocket is a type of chemical rocket in which fuel and oxidizer are present in different phases. Generally, fuel is present in combustion chamber in solid phase and the liquid or gaseous oxidizer is injected into the combustion chamber through injector. The ignition is achieved with the help of an external heat source. The solid fuel surface is subjected to pyrolysis and hence it is decomposed and gasified and these vapors further mix with the oxidizer particles to undergo combustion which further expand the combustion products through the nozzle to develop the thrust [1]. The hybrid rockets are known to provide several distinct advantages over the solid and liquid propellant rockets. Simplicity, safety, lower cost and thrust tailoring are among the most attractive features of a hybrid rocket. The solid fuel regression rate is the key parameter for the characterization of the hybrid rocket’s internal ballistics [2]. It was widely demonstrated, both theoretically and experimentally, that in the absence of radiation, the regression rate of the solid fuel depends on the convective heat transfer from the flame to the fuel surface. Marxman and Gilbert [3] developed a turbulent boundary layer regression rate model yielding that mass flux is the fundamental factor governing the rate of fuel consumption (ṙ α G 0.8 ). Apart from various advantages, hybrid rocket motors face some technical challenges such as low regression rate, changing O/F ratio, reduced combustion efficiency, combustion instability and sliver loss [4]. Several techniques have been proposed to increase the regression rate of solid fuel in hybrid rocket which includes use of protrusion [5], use of multi-port grain, use of paraffin-wax fuels, high mixture ratio values, swirling the flow inside chamber, particle additives in solid fuel, energetic compounds like plasticizers and binders with solid fuel [6], and manufacturing of grains with inner helical shape [7]. The combustion process in a hybrid rocket is achieved by flow of oxidizer over the solid fuel grain leading to the formation of a turbulent diffusion boundary layer and the flame is established inside the boundary layer. This boundary layer formation acts as a hindrance in the interaction between the oxidizer particles and the burning surface of fuel grain and hence reduces the heat feedback to the fuel surface. The two different injection patterns that can be used to reduce this hindrance due to boundary layer is either by creating recirculation zones in the chamber or by direct impingement of the oxidizer flow onto the fuel grain surface. Several researches have been conducted on various injector designs such as converging nozzle injector, shower head injector, swirl injector and radial injector to enhance the fuel regression rate and reduce the combustion instability in the hybrid rockets. Pucci (2002) [8] studied the flame holding instability in the hybrid rocket using axial, radial and swirl injectors. He concluded that swirl injector with super-critical swirl flow does produce stable combustion possibly due to the establishment of a central toroidal recirculation zone (CTRZ) and also an increase in regression rate by 182% in comparison to the axial injector. He also concluded that swirl injector with