Bipropellant spray combustion modeling in small rocket engines

A CFD-based model for bipropellant spray combustion in small rocket engine combustors is formulated and developed into a predictive computational tool. This model is comprised of a two-component interacting liquid phase imbedded within a turbulent chemically reactive gas. A self-consistent mathematical description is formulated for the liquid and gas phases. The liquid phase is assumed to be composed of two immiscible collections of quasi-spherical material volumes call "globules". This allows a conservation equation similar to the well-known spray equation to be formulated for the globule number density distribution. A pair of unlike globules are allowed to interact through impingement collision which results in a collision induced oscillation of the globule surface. The amplitude of the oscillation can cause the globule surface to deform and eventually breakup into spray droplets. Thus, a simulation of the gross effects of atomization via impinging liquid streams is possible. Furthermore, secondary breakup due to aerodynamic forces are also included. Various other sub-models are developed for droplet combustion, and spray/wall collision. State-of-the-art sub-models are employed for globule acceleration, turbulence dispersion, and spray droplet coalescence. The conservation equations for a continuum gas with imbedded condensed liquid phase elements are derived via a formal averaging procedure. This leads to a consistent form for the interphase exchange rates between the liquid and gas phases. A standard k ∈ turbulence model along v\/ith a gas phase chemistry model are incorporated. The formulated model is integrated within the computational framework of the KIVA-II reactive flow code. An efficient coupling of the various sub-models with the ALE scheme is obtained. A brief outline of this procedure is provided. Applications to three problems related to small bipropellant spray rocket combustion are demonstrated. A simulation of the spray structure from a like-doublet impinging stream injector is presented. The computed spray structures showing the effects of impingement angle and injection velocity compare favorably with photographs taken by Heidmann. Atomized drop sizes are matched with the correlation of Dickerson. Unlike-doublet stream impingement is also presented for N2H4 and N204 in a confined chamber. The spray/wall impingement flow structure is appraised. Simulations of bipropellant spray combustion via an unlike-doublet impinging stream injector are investigated. Two bipropellant systems (N2H4/N204and N2H4/LOX)are studied. Vastly differing combustion characteristics are computed for the two bipropellant systems. Finally, an investigation of the combustion flow structure with a single-element coaxial pintle injector is presented.