Impinging jet spray formation using non-Newtonian liquids

Over the past two decades there has been a heightened interest in implementing gelled propellants for rocket propulsion, especially for hypergolic bi-propellants such as monomethylhydrazine (MMH) and nitrogen tetroxide oxidizer (NTO). Due to the very high level of toxicity of hypergolic liquid rocket propellants, increasing safety is an important area of need for continued space exploration and defense operations. Gelled propellants provide an attractive solution to meeting the requirements for safety, while also potentially improving performance. A gelling agent can be added to liquid propellants exhibiting Newtonian behavior to transform the liquid into a non-Newtonian fluid with some solid-like behavior, i.e. a gel. Non-Newtonian jet impingement is very different from its Newtonian counterpart in terms of fluid flow, atomization, and combustion. This is due to the added agents changing physical properties such as the bulk rheology (viscosity) and interfacial rheology (surface tension). Spray characterization of jet impingement with Newtonian liquids has been studied extensively in existing literature. However, there is a scarcity in literature of studies that consider the spray characterization of jet impingement with gelled propellants. This is a rather critical void since a major tradeoff of utilizing gelled propellants is the difficulty with atomization due to the increased effective viscosity. However, this difficulty can be overcome by using gels that exhibit shear-thinning behavior—viscosity decreases with increasing strain rate. Shear-thinning fluids are ideal because they have the distinct advantage of only flowing easily upon pressure. Thereby, greatly reducing the amount of propellant that could be accidentally leaked during both critical functions such as liftoff or engagement in the battlefield and regular tasks like refilling propellant tanks. This experimental work seeks to help resolve the scarcity in existing literature by providing drop size and drop velocity mean values and distribution of several non-Newtonian liquids using a like-on-like impinging jet doublet. The drop size and drop velocity are important areas of study because of the effect on mass transfer and mass dispersal. Phase Doppler Anemometry (PDA) is used to measure the drop diameter and drop velocity. The drop diameter is measured by finding a phase difference between two signals. The drop velocity is measured using Laser Doppler Anemometry (LDA), which is based on the Doppler shift. Parametric studies are conducted based on dimensionless groups, impinging jet geometry, and spatial position. The investigated non-Newtonian liquids collapse onto a single mean diameter versus Reynolds number curve. However, this behavior is not observed for the gels due to differences in surface tension and molecular structure. In general, increasing the inertial force results in smaller drops and greater drop velocities. The different geometric parameters are observed to have varying degrees of influence, based on the propellant simulant considered. Larger drops with lower axial velocities are generally observed with increasing transverse distances from the centerline of the impinging jet spray