Francesco Nasuti

Material-dependent catalytic recombination modeling for hypersonic flows

A new model to predict catalytic recombination rates of O and N atoms over silica re-entry thermal protection system is reported. The model follows the general approach of Halpern and Rosner, but adds estimates of some key physical mechanism parameters based on realistic surface potentials. This novel feature can therefore produce rate expressions for any surface for which structure is known. Testing the model for N over W, and N and O over SiO2 produces recombination probabilities in good agreement with published measurements at high surface temperature. In the case of N and O over SiO2, the model accounts for surface NO production due to O and N cross recombination.

Analysis of unsteady supersonic viscous flows by a shock-fitting technique

An extension of Morettis classical shock-fitting technique is proposed to solve complicated unsteady viscous flows. This version allows the automatic treatment of flow structures featuring triple points and shock interactions. A fitting of contact discontinuities has also been introduced for a number of problems. The fitting procedure is used with a Navier-Stokes solver based on the λ scheme. Validation tests for selected cases are presented.

Numerical Analysis of Deterioration in Heat Transfer to Near-Critical Rocket Propellants

Liquid propellants, which are typically used for regenerative cooling of rocket thrust chambers, can flow in channels at supercritical pressures and in the neighborhood of pseudocritical temperature (near-critical fluid). This could be for instance the case for the envisioned liquid-oxygen/liquid-methane engines with chamber pressures larger than about 50 bar. When the fluid is in such a near-critical condition, deterioration in heat transfer can occur if the heat transfer level is higher than a threshold value. Aiming to improve flow prediction capabilities for the design of such systems, the present study is devoted to numerical simulations of near-critical fluids flowing in uniformly heated straight tubes. After code validation against experimental data of near-critical-hydrogen flow, numerical simulations of near-critical-methane flow in heated tubes are carried out, each characterized by a different wall heat flux. Results are discussed in detail and the near-critical-methane flow condition that exhibits the heat transfer deterioration is identified and emphasized.

Numerical analysis of three-dimensional flow of supercritical fluid in asymmetrically heated channels

The knowledge of the flow behavior inside asymmetrically heated channels is of great importance to improve design and performance of regeneratively cooled rocket engines. The modeling of the coolant flow is a challenging task because of its particular features, such as the high wall temperature gradient, the high Reynolds number, the three-dimensional geometry of the passages, and the possible supercritical conditions of the fluid. In the present work, a numerical approach to study the turbulent flow of supercritical fluids is presented and validated by comparison with experimental data. Solutions of the supercritical nitrogen flowfield in an asymmetrically heated three-dimensional channel with a high-aspect ratio (channel height-to-width ratio) are presented and discussed. Emphasis is given to the analysis of the peculiar behavior and cooling performance of the supercritical fluid as compared with perfect gas. In particular, a long channel is considered, such that entrance effects are negligible, to analyze in detail wall heat-flux evolution throughout the channel.

Thermochemical erosion analysis for graphite/carbon-carbon rocket nozzles

A study is conducted to predict graphite/carbon–carbon nozzle erosion behavior in solid rocket motors for wide variations of propellant formulations. The numerical model considers the solution of Reynolds-averaged Navier– Stokes equations in the nozzle, heterogeneous chemical reactions at the nozzle surface, variable transport and thermodynamic properties, andheat conduction in the nozzlematerial. Twodifferent ablationmodels are considered and compared: a surface equilibriumapproach and afinite-ratemodel. Results show that the erosion rate is diffusion limited for metallized propellants, ensuring sufficiently high wall temperatures, and it is kinetic limited for nonmetallized propellants. For low surface temperatures, the twomodels are consistent with each other and predict the same erosion rate, while the surface equilibrium model overpredicts the recession at low surface temperatures. The calculated results show an excellent agreement with the experimental data from the ballistic test and evaluation system motor firings, and the finite-rate model actually improves the predictions when the kinetic-limited regime is approached.

Analysis of In-Flight Behavior of Truncated Plug Nozzles

Plug nozzles are usually designed to achieve altitude adaptation in a wide range of chamber/ambient pressure ratios.In actual in-e ight operationsofplug nozzles, this phenomenon musttakeplacein a e owing airstream,which can affect the performance. To understand the e ow behaviorin such conditionsand to evaluate the departure from the ideal nozzle performance, an investigation is carried out with a validated numerical tool based on the solution of turbulent Navier ‐Stokes equations and on shock e tting. A sample rocket plug nozzle is analyzed parametrically to evaluate the effect of varying Mach number at constant pressure and the effect of varying pressure ratio at constant Mach number. The results indicate that the interaction with a e owing airstream reduces the pressure of the external air, as seen from the nozzle exhaust jet, yielding a reduction of the performance. In particular, a dramaticdecrease of nozzleperformance may takeplacein the transonicregion iftheslipstream effect is neglected in thedesign. Theresults also provide useful indications on how the Mach numberand the shroud shapecan affect the value of ambient pressure where the transition from open to closed wake takes place.

Coupled Analysis of Flow and Surface Ablation in Carbon-Carbon Rocket Nozzles

A study is conducted to predict C/C nozzle recession behavior in solid rocket motors for broad variations of propellant formulations and motor operating conditions. The numerical model considers the turbulent flow in the nozzle, heterogeneous chemical reactions at the nozzle surface, variable transport and thermodynamic properties, and heat conduction in the nozzle material. Results show that the recession rate is largely determined by the diffusion of the major oxidizing species (H2O, CO2, OH) to the nozzle surface. Both the concentration of the major oxidizing species -affected by the aluminum content of the propellant- and the chamber pressure exert a strong influence on the recession rate. The erosion rate increases almost linearly with chamber pressure and decreases with propellants with higher aluminum content. The calculated results show a very good agreement with the experimental data from the BATES motor firings.

Role of Wall Shape on the Transition in Axisymmetric Dual-Bell Nozzles

Dual-bell nozzles represent a possible solution to improve the performance of large liquid rocket engines for launcher first stages. The present paper studies the role of the second bell shape on the side loads that can occur during the transition between the two main operating modes of the nozzle. In particular, the design of the second bell profile is critically discussed on the basis of results obtained from suitable test cases. The analysis of performance and behavior during the transition is carried out by a validated turbulent Navier-Stokes solver. Geometries were generated by the method of characteristics. Results show that slightly different geometries designed by the method of characteristics yield modest performance changes but severe differences in behavior during the transition phase.

Coupled wall heat conduction and coolant flow analysis for liquid rocket engines

Coolant flow modeling in regeneratively cooled rocket engines fed with turbo machinery is a challenging task because of the high wall temperature gradient, the high Reynolds number, the high aspect ratio of the channel cross section, and the heat transfer coupling with the hot-gas flow and the solid material. In this study the effect of wall heat conduction on the coolant flow is analyzed by means of coupled computations between a validated Reynolds-averaged Navier–Stokes equations solver for the coolant flowfield and a Fourier’s equation solver for the thermal conduction in the solid material. Computations of supercritical-hydrogen flow in a straight channel with and without coupling with the solid material are performed and compared to understand the role played by the coupling on the coolant flow evolution. Finally, the whole cooling circuit of the space shuttle main engine main combustion chamber is analyzed in detail and discussed for the sake of comparison of results obtained with the present couple...

Analysis of curved-cooling-channel flow and heat transfer in rocket engines

DOI: 10.2514/1.B34163 Coolant-flow modeling in regeneratively cooled rocket engines fed with turbomachinery is a challenging task because of the high wall-temperature gradient, the high Reynolds number, the high aspect ratio of the channel cross section,andthecurvedgeometry.Inthepresentstudy,tobettercomprehendtheroleofthethrust-chambershapeof a rocket engine on the heat exchange, computations of supercritical hydrogen flow in single- and double-curvature channels are carried out. In particular, a parametric numerical analysis of the flow in an asymmetrically heated rectangular channel with a high aspect ratio and various radii of curvature is performed by means of a Reynoldsaveraged Navier–Stokes solver for real fluids, which is validated against experimental data of heated and curvedchannel flow taken from open literature. Results permit the effect of curvature on global heat transfer coefficient, pressure loss, and bulk temperature increase to be quantified.