Investigation of Atomization Behaviors of Liquid Monopropellants in Pintle Injectors
Authors: Gamertsfelder Jacob , Khare Prashant , Bravo Luis 
Published: January 2021

Abstract
This research effort focuses on the atomization physics of liquid monopropellents emanating from a pintle-type injector at high-pressure conditions. These injectors are used extensively in liquid-fueled propulsion systems, such as rockets and diesel engines, and undersea vehicles and munitions. While extensive research has been conducted in the past on bipropellant fuel injection and atomization, limited literature exists on the understanding of atomization processes of monopropellant fuels in a pintle injector configuration for viscous fluids at elevated pressures. Therefore, in the current work, injection and subsequent atomization processes of a liquid monopropellant fuel are investigated as it is injected through a pintle injector in a stagnant environment using direct numerical simulations. The pintle injector consists of an annulus with an outer diameter twice the size of the inner diameter, and center pintle that throttles the fuel out of the injector. The theoretical and mathematical formulation to investigate these two-phase problems is based on the three-dimensional incompressible Navier-Stokes equations with surface tension. A critical issue is the treatment of multi-scale liquid-liquid and gas-liquid interfaces, therefore, a state-of-the-art, high resolution, volume-of-fluid (VOF) interface capturing method is adopted to resolve the interfacial evolution. Surface tension is accommodated as a Dirac delta distribution function on the interface. The theoretical formulation outlined above is solved numerically using a finite volume method augmented by an adaptive mesh refinement (AMR) technique, based on an octree spatial discretization to improve the solution accuracy and efficiency. As a first step, for model validation, we simulate water injection in the aforementioned geometry at a flowrate of 44.4 g/s in a stagnant chamber at 1 atm and room temperature conditions. Comparison of our results with experimentally measured sauter mean diameter and spray angle shows excellent agreement — both quantities are within 4.2% of the measured quantities. Next, Otto fuel II injection and atomization are studied to elucidate the atomization characteristics of the fuel in a pintle injector. The dynamic viscosity of the representative liquid monopropellant, Otto fuel II is 0.0044 Pa-s, density is 1232 kg/m3, and the surface tension at the gas-liquid interface is 0.03445 N/m. The operating conditions consist of p = 106.2 bar, T = 300 K, and inlet velocity u = 3.34 m/s, corresponding to a density ratio of 10, dynamic viscosity ratio of 212 and Weber number of 20 (based on gas density). Results indicate that a radially growing hollow cone spray film attached at the injector exit is formed. Instability waves are formed on the outer and inner surfaces of the cone that facilitates breakup. Once droplets are completely separated from the cone, they are convected to recirculation zones, thus interacting with the hollow cone and amplifying the instability waves to cause further breakup. Droplet size distributions and their time evolution are also calculated during the research effort to quantitatively characterize the atomization behaviors.