Ashvin Hosangadi

Numerical Study of Cavitation in Cryogenic Fluids

Numerical simulations of cavitation in liquid nitrogen and liquid hydrogen are presented; they represent a broader class of problems where the fluid is operating close to its critical temperature and thermal effects of cavitation are important. A compressible, multiphase formulation that accounts for the energy balance and variable thermodynamic properties of the fluid is described. Fundamental changes in the physical characteristics of the cavity when thermal effects become significant are identified

UPWIND UNSTRUCTURED SCHEME FOR THREE-DIMENSIONAL COMBUSTING FLOWS

As an interim step towards the development of a hybrid upwind structured/unstructured solver for combusting/multiphase flowfields, the TRI3D unstructured code of Barth has been extended to analyze multicomponent combusting flows. The extensions mimic the Roe/total variation diminishing (TVD) based thermochemical extensions in the structured solver, CRAFT, and entail a strong coupling of chemical species equations and complete linearization of the chemical source term, treated hi a fully implicit manner. Issues regarding the quality of solutions obtained using locally one-dimensional Riemann flux procedures and TVD limiters are more pronounced in unstructured formulations and are dealt with in depth in this article. Comparative studies of structured and unstructured analyses of laminar premixed flames, ducted shock-induced combustion, and blunt-body shock-induced combustion serve to delineate these issues and the need for solution adaptive gridding and unproved flux limiters to capture flame zones properly.

DYNAMIC UNSTRUCTURED GRID METHODOLOGY WITH APPLICATION TO AERO/PROPULSIVE FLOWFIELDS

An edge-based, three-dimensional upwind unstructured flow solver with fully coupled finite-rate chemistry is presented for transient aerodynamic and propulsive flowfields with moving and/or deforming boundaries. This viscous flow solver features higher order Roe/TVD numerics where the grid dynamics is integrated with the flux computation. Three different grid methodologies have been investigated for specific classes of problems; a spring analogy method, a solid-body elasticity formulation, and a cell layering technique. The solid-body elasticity method is shown to provide superior results over the spring analogy formulation for problems where boundary surfaces are shearing relative to each other. A significant contribution of this work is the development of a cell layering technique for handling large boundary motion. The cell layering technique embeds/deletes layers of tetrahedral cells as the flow domain expands/shrinks allowing for efficient and accurate flow computations without global remeshing. These methodologies have been demonstrated on practical problems such as multiple body aerodynamics and automotive in-cylinder combustion.

Three-Dimensional Hybrid RANS/LES Simulations of a Supercritical Liquid Nitrogen Jet

Simulations of an uni-element, shear coaxial injector configuration with liquid nitrogen in the inner tube mixing with a co-flowing warm nitrogen gas stream are presented; the chamber has a supercritical pressure of 4.95 MPa with the liquid nitrogen jet temperature mildly supercritical at 129 K. Unsteady solutions on a complete three-dimensional configuration were computed using a hybrid RANS/LES framework and compared with experimental data. Our results reveal a highly unsteady flow field with strong fluctuations in the post region between the inner jet and co-flowing outer jet. This unsteadiness is caused by strong thermodynamics gradients near the critical point, which cause the inner jet to expand out radially as it mixes with the warmer fluid from the outer jet. Analysis of the turbulence field indicates that nominal temperature fluctuations generate large amplitude density fluctuations which in turn increases the Reynolds stress in the liquid jet shear layer. A key finding of this study is that the unsteady mixing is dominated by three-dimensional helical instabilities on the interface of the liquid jet shear layer. In this important respect, a trans-critical/supercritical jet is very different from a gas jet where the jet nearfield is unstable only to axisymmetric instabilities. A comparison of the mean radial temperature distribution indicates reasonable comparison with experimental measurement; the dramatic mixing of the liquid shear layer is captured well but the mixing in the outer gas shear layer is underpredicted. A comparison of the jet mixing core length prediction along the centerline with the dark core length measurements from flow visualization indicates that the numerical results overpredict the mixing and this may be due to a combination of uncertainty in the temperature measurement values as well as numerical errors.

Simulations of Cavitating Flows in Turbopumps

Simulations of a cavitating inducer at design flow conditions are reported in this paper. The ability of a Navier-Stokes based, multiphase formulation in modeling the effects of large-scale extensive cavitation on the performance of inducers is demonstrated. The simulations were performed on the Simplex inducer geometry that has been extensively tested at NASA Marshall Space Flight Center. An acoustically accurate, compressible multiphase model that has been previously validated is utilized. The model is implemented within a multi-element unstructured framework that permits efficient grids with locally high resolution near the cavitating zones and in the tip gap region. The simulations were performed at a fixed flow rate with different inflow pressures or net suction specific speeds (Nss). The computational analysis indicates a strong correlation between performance loss and the extent of cavitation blockage and accurately identifies the critical N S S number where breakdown occurs. Predictions of head loss compare well with experimental data. Key insights are provided through a sequence of relevant simulations into the growth of cavitation zones, blockage in blade passages, and change in blade loading as a result of cavitation.

Impeller Design of a Centrifugal Fan with Blade Optimization

A method is presented for redesigning a centrifugal impeller and its inlet duct. The double-discharge volute casing is a structural constraint and is maintained for its shape. The redesign effort was geared towards meeting the design volute exit pressure while reducing the power required to operate the fan. Given the high performance of the baseline impeller, the redesign adopted a high-fidelity CFD-based computational approach capable of accounting for all aerodynamic losses. The present effort utilized a numerical optimization with experiential steering techniques to redesign the fan blades, inlet duct, and shroud of the impeller. The resulting flow path modifications not only met the pressure requirement, but also reduced the fan power by 8.8% over the baseline. A refined CFD assessment of the impeller/volute coupling and the gap between the stationary duct and the rotating shroud revealed a reduction in efficiency due to the volute and the gap. The calculations verified that the new impeller matches better with the original volute. Model-fan measured data was used to validate CFD predictions and impeller design goals. The CFD results further demonstrate a Reynolds-number effect between the model- and full-scale fans.

Analysis of thermal effects in cavitating liquid hydrogen inducers

Evaporative cooling effects due to cavitation can significantly improve the performance of liquid rocket turbomachinery that operate with cryogenic fluids, in which the fluid is operating close to its critical temperature and the thermal effects resulting from phase change become important. A detailed numerical analysis to quantify these thermal effects of cavitation and to better understand their impact on cavitation flow physics in liquid hydrogen inducers is presented. Simulations were performed on a helical flat-plate inducer that was extensively tested in both liquid hydrogen and cold water. Predictions of cavitating performance over the operational range of inlet pressures were conducted and compared with experimental data. Fundamental differences were observed in the cavity for liquid hydrogen compared with the cold-water inducer; the cavity in liquid hydrogen shows far less vapor content, although spreading out further in the spanwise direction, which may generate blade-to-blade interactions. Thermal effects result in a more gradual breakdown of the head in liquid hydrogen resulting in improved overall performance; the liquid hydrogen inducer performed to a suction specific speed (N SS number) of 75,000 and the corresponding value in water is 35,000. The temperature depressions due to evaporative cooling in the liquid hydrogen fluid are found to be only on the order of 0.5-1.0 K, highlighting the strong coupling of thermodynamic properties and the phase-change process at these flow conditions.

Multi-element unstructured methodology for analysis of turbomachinery systems

In recent years unstructured mesh techniques have become popular for computational e uid dynamics analysis of external aerodynamic-type problems. The main advantages of such an approach include mesh generation over complex domains, grid adaptation in localized areas, and accuracy in efe ciently identifying complexities in local e ow physics. A hybrid unstructured methodology is used to carry out simulations for predominantly internal e ow turbomachinery applications. Issues related to skewness and other constraints of tetrahedral meshes are addressed in the context of turbomachinery-based propulsive e ows that exhibit a rich variety of length scales and timescales, as well asinteresting e ow physics. The unstructured framework permits the generation of a contiguous grid without internal boundaries between different components of a turbomachinery system and provides good local resolution in regions where the e ow physics becomes important. The increased numerical stability resulting from these factors coupled with the parallel solution framework yields an efe cient solution procedure for complex turbomachinery e ows. Numerical results are presented and compared against experimental measurements for a transonic diffuser‐ volute cone guration and a high Reynolds number pump.

Transient Simulations of Valve Motion in Cryogenic Systems

Valve systems in rocket propulsion systems and testing facilities are constantly subject to dynamic events resulting from the timing of valve motion leading to unsteady fluctuations in pressure and mass flow. Such events can also be accompanied by cavitation, resonance, system vibration leading to catastrophic failure. High-fidelity dynamic computational simulations of valve operation can yield important information of valve response to varying flow conditions. Prediction of transient behavior related to valve motion can serve as guidelines for valve scheduling, which is of crucial importance in engine operation and testing. In this paper, we present simulations of valve motion utilizing a multi-element unstructured computational approach that permits the use of variable grid topologies, thereby permitting solution accuracy and resolving important flow physics in the seat region of the valve. The approach is based on a mesh library strategy in which generalized mesh movement is applied between a sequence of meshes and the solution is transferred between meshes at specific valve positions pre-defined in the library. We discuss important valve flow characteristics such as valve response to variable plug control speeds as part of our simulations. Furthermore, we present an alternative methodology that utilizes a single grid that deforms and adapts during valve motion.

Computational Analyses of Pressurization in Cryogenic Tanks

A comprehensive numerical framework utilizing multi-element unstructured CFD and rigorous real fluid property routines has been developed to carry out analyses of propellant tank and delivery systems at NASA SSC. Traditionally CFD modeling of pressurization and mixing in cryogenic tanks has been difficult primarily because the fluids in the tank co-exist in different sub-critical and supercritical states with largely varying properties that have to be accurately accounted for in order to predict the correct mixing and phase change between the ullage and the propellant. For example, during tank pressurization under some circumstances, rapid mixing of relatively warm pressurant gas with cryogenic propellant can lead to rapid densification of the gas and loss of pressure in the tank. This phenomenon can cause serious problems during testing because of the resulting decrease in propellant flow rate. With proper physical models implemented, CFD can model the coupling between the propellant and pressurant including heat transfer and phase change effects and accurately capture the complex physics in the evolving flowfields. This holds the promise of allowing the specification of operational conditions and procedures that could minimize the undesirable mixing and heat transfer inherent in propellant tank operation. In our modeling framework, we incorporated two different approaches to real fluids modeling: (a) the first approach is based on the HBMS model developed by Hirschfelder, Beuler, McGee and Sutton and (b) the second approach is based on a cubic equation of state developed by Soave, Redlich and Kwong (SRK). Both approaches cover fluid properties and property variation spanning sub-critical gas and liquid states as well as the supercritical states. Both models were rigorously tested and properties for common fluids such as oxygen, nitrogen, hydrogen etc were compared against NIST data in both the sub-critical as well as supercritical regimes.