Marcello Onofri

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.

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.

Numerical Parametric Analysis of Dual-Bell Nozzle Flows

Dual-bell nozzles permit two different operating modes, which provide higher performance than conventional bell nozzles when applied to rocket engines operating from sea level. During the low-altitude mode, the flow is separated and the separation line is located near the inflection of the nozzle wall, in a region characterized by a negative value of the wall pressure gradient, as in conventional nozzles, and in which side loads may occur. The characteristics of this region in hot full-scale applications are addressed by means of numerical simulations, and scaling laws are sought for cold subscale side-load experiments. Moreover, the flow behavior during the transition between the two operating modes is analyzed by time-accurate simulations.

Methodology to Solve Flowfields of Plug Nozzles for Future Launchers

In the development of future launchers, in particular Single Stage to Orbit (SSTO), the use of plug nozzles looks promising because of the possible improvement of performance, and therefore a capability to achieve accurate evaluations of the relevant flowfields is required. Nevertheless, due to the typical flow features, analyses performed by the common Computational Fluid Dynamics (CFD) methods lead to uncertain and time consuming results. To circumvent some of these problems, an approach based on a simple simulation model that replaces the mixing layer between nozzle jet and external flow by a contact discontinuity, was recently proposed by the authors. Following this approach, the complex calculation of the turbulent mixing of flows having different thermodynamic characteristics can be avoided, while correctly accounting for the main features of the flowfield. In this paper, this simple model has been implemented to compute linear plug nozzle flowfields at different ambient conditions and for different geometries of the plug. The expected flow behavior has been well reproduced and good agreement with the theoretical nozzle performance has been achieved, thus allowing to carry out a performance analysis for some of the main characteristic parameters.

Chemical Erosion of Carbon-Phenolic Rocket Nozzles with Finite-Rate Surface Chemistry

Ablative materials are commonly used to protect the nozzle metallic housing and to provide the internal contour to expand the exhaust gases in solid rocket motors. Because of the extremely harsh environment in which these materials operate, they are eroded during motor firing with a resulting nominal performance reduction. The objective of the present work is to study the thermochemical erosion behavior of carbon-phenolic material in solid rocket motor nozzles. The adopted approach relies on a validated full Navier–Stokes flow solver coupled with a thermochemical ablation model, which takes into account finite-rate heterogeneous chemical reactions at the nozzle surface, rate of diffusion of the species through the boundary layer, pyrolysis gas and char-oxidation product species injection in the boundary layer, heat conduction inside the nozzle material, and variable multispecies thermophysical properties. The results obtained with the proposed approach are compared with two sets of experimental data: subs...

Numerical Analysis of Film Cooling in Advanced Rocket Nozzles

The key demand on future space transportation systems is the concurrent reduction of Earth-to-orbit launch costs and increase of launcher reliability and operational efficiency. A common way of slightly improving performance of gas-generator open-cycle engines is the injection of the turbine exhaust gas into the nozzle divergent section, which is also used for wall film cooling. The present study focuses on a numerical parametric analysis of the film-cooling efficiency in dual-bell nozzles. The secondary gas injection is made in the first bell, and it is found that the expansion fan originating from the inflection helps the film to better protect the wall. The results of fully-attached-flow simulations are also used to study the influence of film cooling on the expected behavior of nozzle side loads during operation with separated flow in the second bell.