Changjin Yoon

Numerical Modeling of Injection of Shear-Thinning Gel Propellants Through Plain-Orifice Atomizer

A series of axisymmetric Navier–Stokes simulations were performed to study the mean and unsteady characteristics of gel propellant orifice flows at conditions representative of rocket injectors. The rheology of the gel was simulated assuming a shear-thinning fluid behaving in accordance with the Carreau–Yasuda model. The effects of Reynolds number (flow velocity), orifice L=D, fluid rheology, and orifice inlet chamfering were studied in a series of 200 independent simulations. Unsteady conditions were observed in most cases, due to the high injection velocities typical of rocket injection conditions. The magnitude and frequency of pulsations were characterized in these cases. In general, steady flows were obtained for lower Reynolds numbers and smoother inlet (i.e., greater chamfering) conditions. Mean discharge coefficients were computed for all cases to support design studies and engineering analyses.

A Parametric Study of Combustion Dynamics in a Single-Element Lean Direct Injection Gas Turbine Combustor: Part II: Experimental Investigation

The current paper is an account of the experimental investigation of a model high pressure lean direct injection (LDI) combustor that exhibits self-excited combustion dynamics. The combustor uses a single nozzle that injects liquid fuel into a subsonic venturi section to rapidly atomize and distribute fuel, and a modular design that allows discrete changes in geometry between tests so that a range of unstable frequencies and amplitudes can be measured. A target frequency of a few hundred Hz and target amplitudes of about 2% of mean pressure was obtained with a configuration comprising a 3/8-wave air plenum and 1/2-wave combustor geometry. In this paper, results from a broader parametric investigation of the combustor with varying geometry, inlet air temperature and equivalence ratio is presented, along with a more focused study of the configuration presenting the target characteristics. In the latter study, higher equivalence ratios (> 0.45) show higher pressure fluctuation amplitudes with a dominant 4L mode in the combustor whereas, at lower equivalence ratios, the instability amplitudes are lower with energy spread across the first few modes in the combustor. In a concurrent study, the experimental data are compared with high fidelity simulations, which showed excellent agreement with mode shapes and reasonable agreement with mode amplitude. The simulation indicated presence of complex interactions between the combustor acoustic modes, heat release modes, and a hydrodynamic precessing vortex core (PVC) mode similar to what has been observed in lowpressure, premixed gas systems, indicating the LDI concept is successful in its goal of rapidly distributing the liquid fuel, and providing direction for future study.

Computational Investigation of Combustion Dynamics in a Lean-Direct Injection Gas Turbine Combustor

Abstract : Combustion dynamics is investigated using an integrated computational/experimental approach for a laboratory-scale, single-element lean direct injection model combustor in which self-excited pressure oscillations are produced. The present study focuses on physics based computational simulations that fully describe the turbulence, spray, combustion and acoustics phenomena in the combustion chamber. Baseline three-dimensional results at an equivalence ratio = 0.47 confirm the self-excitation of acoustic modes in the chamber and also indicate the presence of precessing vortex core instabilities. Preliminary comparisons of the pressure oscillations with experimental measurements are also presented. Further, the effects of multi-dimensionality, equivalence ratio and secondary atomization are computationally investigated. In contrast to the 3D simulations, two-dimensional models capture the pressure oscillations with reasonably similar amplitudes, but show inherent limitations in describing the vortex breakdown process. Pressure oscillations are also shown to be intensified when the equivalence ratio is increased and damped when the secondary atomization effects are included.

Experimental Velocity Profiles in the Cap Shock Pattern of a Thrust Optimized Rocket Nozzle

The cap shock pattern is observed in the plumes of over-expanded thrust optimized rocket nozzles at the low pressure ratios that typically occur during start-up and shut-down transients. This shock pattern is related to restricted shock separation, and associated flow transitions can result in damaging side loads. Instantaneous 2D planar velocity measurements are obtained here with hydroxyl tagging velocimetry, in which molecular tags are written into the flow by laser photo-dissociation and tracked by laser-induced fluorescence. Measured velocity profiles are compared with numerical simulation and show promise as a detailed validation tool.

Injector Flow Characteristics for Gel Propellants

Computational and experimental efforts to investigate the injector flow characteristics for gel propellants are presented. Two different types of injectors are considered: a tapered tube and plain-orifice type. Paraffin/Thixatrol and Water/HPC Gels are used as working fluids and are modeled as a shear-thinning fluid. Flows are characterized by a time-averaged discharge coefficient, viscosity at the exit, and velocity and viscosity profiles at the exit for various injector geometries. Factors in a parametric study include convergence angle in a tapered tube injector, orifice length-to-diameter ratio, and chamfer length in a plain-orifice injector. According to a parametric study, hydraulic resistance increases by increasing the convergence angle in a tapered tube injector and orifice L/D ratio in a plain-orifice injector. It is found that the discharge coefficient and viscosity at the exit are reduced under these conditions. The effect of chamfer length on steady injector flows are negligible in a plainorifice injector.

Simulations of Plain-Orifice Injection of Gelled Propellants Under Manifold Crossflow Conditions

Unsteady Navier–Stokes simulations are used to study the static and dynamic discharge characteristics of shear-thinning fluids passing through plain-orifice atomizers fed by a crossflow. The crossf...

Simulation of Injection of Shear-Thinning Gel Propellants Through Plain-Orifice Atomizer

AR Aspect ratio D Orifice diameter, m(in), Binary diffusion coefficient E Flux vector L Orifice length, m(in) Re Reynolds number X Chamfer length, m(in) Q Solution vector T Temperature, K Yk Mass fraction of the k-th species a A Carreau-Yasuda parameter c A pseudo speed of sound, m/s h Enthalpy, J n A Carreau-Yasuda parameter ∆p Pressure drop, Pa t Time, sec u Velocity, m/s V Mean flow speed, m/s x Variable α Volume fraction Γ Preconditioning matrix γ Shear rate, 1/s λ Thermal conductivity, Acoustic eigenvalues, or A Carreau-Yasuda parameter μ Molecular viscosity η Viscosity, Pa ∗ s ρ Density, kg/m Subscript L Left R Right b Back g Gas i Dimensional index j Dimensional index l Liquid p Primitive variable v Viscous part ∞ Infinity of a shear rate

Plain-Orifice Gelled Propellant Flow Characteristics with Rheological Hysteresis

Unsteady, three-dimensional Navier-Stokes simulations of orifice flows for the gelled propellants were presented. The thixotropic model was developed in order to describe the rheological hysteresis. According to the simulation results, thinning proceeds near the wall surface and thickening arises around the centerline. In addition, the thixotropic liquid flow is compared with the Newtonian and shear-thinning liquid flows. It is concluded that thixotropic liquid flow has the intermediate flow characteristics between the Newtonian and shear-thinning liquids and the level of thinning and thickening seem to depend on the rheological response rate of the material.

Investigation of the Cap-Shock Pattern in a Thrust-Optimized Rocket Nozzle

The cap-shock pattern from both experiments and simulations are presented. By axisymmetrical calculations, the cap-shock pattern at a nozzle pressure ratio = 20 is simulated as exhibited by an experimental visualization using hydroxyl tagging velocimetry. It is confirmed that the steady shock structures are driven by an Mach reflection with an internal shock. Furthurmore, the evolution of a cap-shock pattern was investigated experimentally and numerically using shadowgraphic images for a throttling process.