232 research outputs found
Navier-stokes simulations of hypersonic flows with coupled graphite ablation
No full textA study of graphite ablation in reentry flows is carried out by a surface ablation approach integrated with a two- dimensional axisymmetric Navier-Stokes solver. The gas-surface interaction model is based on chemical equilibrium ablation with surface mass and energy balances fully coupled with the numerical solver and can account for both surface oxidation and sublimation. The surface temperature is obtained from the steady-state ablation approximation. This numerical procedure can predict aerothermal heating, chemical species concentrations, and carbon material ablation rate over the heat-shield surface of reentry vehicles. Two-dimensional axisymmetric simulations have been performed to numerically reproduce the ablation of a graphite sphere cone that has been tested in the Interaction Heating Facility at the NASA Ames Research Center. The freestream conditions of the selected test case are typical for Earth reentry from a planetary mission. The predicted ablation rate and surface temperature assuming frozen chemistry in the flow show a good agreement with the available experimental data. The agreement is further improved freezing the nitrogen recombination reaction at the surface to be more consistent with experimental observation, which has shown nitrogen atom recombination not to occur at the graphite surface. Copyright © 2010 by D. Bianchi, F. Nasuti, and E. Martelli
On the onset of heat transfer deterioration in supercritical coolant flow channels
No full textThe deterioration of forced convection heat transfer can affect channel flows of supercritical fluids, and therefore has to be taken in consideration when dealing with regenerative cooling of liquid rocket engines. A threshold value of the ratio between the heat flux and the specific mass flow rate is identified as the main parameter controlling the heat transfer deterioration onset. The threshold parameter depends on the specific thermodynamic conditions of the coolant and in particular on its pressure level. In the present study, a parametric numerical analysis has been carried out on the flow of supercritical methane in heated channels, for an assigned inlet temperature level and varying the inlet pressure. A correlation for the threshold parameter as a function of pressure is then proposed on the basis of the obtained results. © 2012 by A. Urbano and F. Nasuti
Parametric analysis of cooling properties of candidate expander cycle fuels
No full textFlow evolution and heat transfer capability in the cooling system of liquid rocket engines heavily depend on propellant thermophysical properties. Coolant thermophysical property analysis and modeling is therefore important to study the possibility to rely on regenerative cooling system, whose performance is crucial to determine feasibility and convenience of pump-fed liquid rocket cycles of the expander type. Aim of the present study is to compare the behavior of different light hydrocarbons and their mixtures, which could be used as fuels for expander cycle engines. A parametric analysis is carried out by a validated numerical solver comparing temperature increase, pressure loss and heat transfer evolution of the different fuels along the same straight tube and subjected to assigned heat loads. © 2012 by A. Urbano and F. Nasuti
Thermochemical erosion analysis for graphite/carbon-carbon rocket nozzles
No full textA 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, and heat conduction in the nozzle material.Two different ablation models are considered and compared: a surface equilibrium approach and a finite-rate model. 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 two models 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. 2010. Copyright © 2010 by D. Bianchi, F. Nasuti, M. Onofri, and E. Martelli
Parametric analysis of heat transfer to supercritical pressure methane
No full textMethane is an interesting propellant to be used together with oxygen in a liquid rocket engine and has the required features to be used as a coolant in a regenerative cooling sys- tem also in the case of an expander cycle engine: in this context its behavior as a coolant flow must be controlled and this is complicated by the fact that methane could be in a near critical condition in the cooling channels and thus be subjected to high thermophys- ical properties variations. The present study investigates the heat transfer to methane looking at the in fluence of the thermodynamic and transport properties variations on the methane heat transfer capabilities. In particular the heat transfer deterioration that could affect methane is deeply investigated. To carry out the necessary parametric studies a parabolized Navier Stokes solver developed by the authors has been used together with accurate equation of state and transport properties models able to describe methane in all the thermodynamic conditions of interest. © 2011 by A. Urbano and F. Nasuti
Numerical analysis of nozzle material thermochemical erosion in hybrid rocket engines
No full textAblative materials are commonly used to protect the nozzle metallic housing and to provide the internal contour to expand the exhaust gases in both solid rocket motors (SRM) and hybrid rocket engines (HRE). Due to the extremely harsh environment in which these materials operate, they are chemically eroded during motor firing with a resulting nominal performance reduction. The objective of the present work is to study the erosion behavior of graphite nozzles in HRE at different operating conditions and comparing results with those obtained for SRM. A mean distinctive feature of HRE operating conditions is, in fact, a greater concentration of oxygen-containing combustion products than SRM. The adopted approach relies on a validated full Navier-Stokes flow solver coupled with a thermochemical ablation model which takes into account heterogeneous chemical reactions at the nozzle surface, rate of diffusion of the species through the boundary-layer, ablation species injection in the boundary layer, heat conduction inside the nozzle material, and variable multispecies thermophysical properties. The parametric analysis performed in this study allows to assess the impact of various parameters that affect the nozzle erosion rate, taking into account various combinations of fuels and oxidizers operating at different conditions. © 2012 by D. Bianchi and F. Nasuti
PREVENTIVE STRATEGIES TO COUNTERBALANCE FOOD PESTICIDE EFFECT ON EPIGENOME AND GUT MICROBIOTA
No full textFood pesticide residues have been identified in food as well as in people’s urine [1]. Studies on early life exposure to the food pesticide permethrin demonstrate its ability to modulate gene expression, epigenome and gut microbiota leading to long term effects later in life [2-4].
In particular, permethrin pesticide, binding to sodium channels, induces neuron depolarization which modulate DNMTs activities responsible for DNA methylation [5].
Previously, we demonstrated changes in the DNMTs in striatum of animals exposed to permethrin during brain development [2]. Besides, we observed that permethrin is able to decrease global DNA methylation and dopamine level in mothers exposed to food pesticide during early life as well as in their untreated offspring, underlining the intergenerational effect of the food pesticide [6]. Of particular interest is the evidence that permethrin exposure can also promote significantly changes in gut microbiota.
With the aim to propose strategies for prevention, bioactive supplements have been studied and positive nutrigenomic effects able to counterbalance the brain and gut microbiota alterations will be discussed.
References
1. Tang W, Wang D, Wang J, Wu Z, Li L, Huang M, Xu S, Yan D. Pyrethroid pesticide residues in the global environment: An overview. Chemosphere. 2018;191:990-1007.
2. Fedeli D, Montani M, Bordoni L, Galeazzi R, Nasuti C, Correia-Sá L, Domingues VF, Jayant M, Brahmachari V, Massaccesi L, Laudadio E, Gabbianelli R. In vivo and in silico studies to identify mechanisms associated with Nurr1 modulation following early life exposure to permethrin in rats. Neuroscience. 2017 Jan 6;340:411-423.
3. Nasuti C, Brunori G, Eusepi P, Marinelli L, Ciccocioppo R, Gabbianelli R. Early life exposure to permethrin: a progressive animal model of Parkinson's disease.
J Pharmacol Toxicol Methods. 2017;83:80-86.
4. Nasuti C, Coman MM, Olek RA, Fiorini D, Verdenelli MC, Cecchini C, Silvi S, Fedeli D, Gabbianelli R. Changes on fecal microbiota in rats exposed to permethrin during postnatal development.
Environ Sci Pollut Res Int. 2016; 23(11):10930-7.
5. Sharma RP, Tun N, Grayson DR (2008) Depolarization induces downregulation of DNMT1 and DNMT3a in primary cortical cultures. Epigenetics 3:74–80
6. Bordoni L, Nasuti C, Mirto M, Caradonna F, Gabbianelli R. Intergenerational Effect of Early Life Exposure to Permethrin: Changes in Global DNA Methylation and in Nurr1 Gene Expression.
Toxics. 2015;3(4):451-46
Numerical analysis of film cooling in advanced rocket nozzles
No full textThe 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. Copyright © 2009 by E. Martelli, F. Nasuti, and M. Onofri
Numerical study of heat transfer in film cooled thrust chambers
No full textFilm cooling as a thermal protection for the walls of liquid rocket engines is studied numerically for hydrogen and methane thrust chamber tests. The aim is to verify the capability of the Reynolds Average Navier-Stokes model to capture the basic characteristics of film-cooled thrust chambers, considering a simplified approach, named pseudo-injector approach, which does not model propellant injection and combustion. This assumption allows a great saving in computational time, in particular when considering 3D simulations. The present study takes its origin from the European Community In-Space Propulsion 1 (ISP-1) program where, among various projects, an experimental campaign has been designed to study the film cooling technique in an oxygen/methane thrust chamber and to provide a database for computational fluid dynamics validation. The results show that the present approach gives good results in terms of heat flux characterization, in particular when dealing with test cases of high chamber pressure. © 2012 by B. Betti, E. Martelli, F. Nasuti, M. Onofri
Secondary Gas Injection Effect on Dual-Bell Nozzle Flow Transition
No full textDual bell nozzles represent a possible solution to improve the performance of large liquid rocket engines for launcher first stages. 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 and where side loads may occur. At high altitude the flow attaches to the nozzle wall and the full area ratio is used. The flow behavior during the transition between the two operating modes is analyzed by time-accurate simulations carried out on an axi-symmetric geometry and considering two configurations: one with a wall film cooling and the other without film. Aim of this study is to analyze the eect of the secondary gas injection on the dynamics of the separation point and on the transition time. Results show that film injection may substantially change the characteristic transition time and evolution
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