1,721,046 research outputs found
A mixture fraction space model for counterflow diffusion flames with incident electric field
No full textApplying an external electric field is a well-known strategy to control combustion processes. However, the high computational complexity of the numerical approaches formulated so far often forestalls the study of configurations of practical and scientific interest. This work proposes a reduced order model for predicting the behaviour of diffusion flames impinged by an external electric field that is based on the classic mixture fraction space formulation. The model takes into account differential diffusion effects as wells as the electric drift of ions in order to predict the species distribution in the mixing layer. The results obtained with the proposed model are in good agreement with those of the two-dimensional calculations presented by Di Renzo et al. (2018) for the case of a methane/air laminar counterflow diffusion flames impinged by sub-breakdown DC electric fields. The reduction of the computational cost associated with the prediction of each flame by a factor one million has allowed the authors to perform a preliminary exploration of the diffusion flame phase-space, in particular defining the variation of the ion-current produced by the reacting layer along its S-curve
Turbulent thermal convection over grooved plates
No full textDirect numerical simulations of thermal convection over grooved plates are presented and discussed, in comparison with the standard flat-plate case, in order to gain a better understanding of the altered near-wall dynamics and of the enhancement of the heat transfer. The simulations are performed in a cylindrical cell of aspect-ratio (diameter over cell height) Γ = 1/2 at fixed Prandtl number Pr = 0.7 with the Rayleigh number Ra ranging from 2 × 10^6 to 2 × 10^11. The results show an increase of heat transfer, or in non-dimensional form the Nusselt number Nu when the mean thermal boundary-layer thickness becomes smaller than the groove height, in agreement with earlier experimental investigations available from the literature. The present increase, however, results in a steeper power law of the Nu vs. Ra law rather than a simple upward shift of the Nu law of the flat plate. This finding agrees with some studies, but it is at variance with others. Possible causes for this difference are discussed with the help of an electrical analogy
Impact of fundamental molecular kinetics on macroscopic properties of high-enthalpy flows: The case of hypersonic atmospheric entry
Full text linkThermochemical nonequilibrium is one of the most challenging issues when dealing with hypersonic flows experienced by objects (space vehicles, meteoroids, space debris) at atmospheric entry. The case of a hypersonic flow past a sphere is considered as a test model for systems in strong chemical and thermal nonequilibrium conditions, mimicking the extreme environment experienced by objects entering a planetary atmosphere. The problem has been studied using the state-to-state approach, calculating directly the distribution of vibrational levels of O2 and N2, together with the flow field, including also viscous effects. Nonequilibrium distributions are observed and the results have been compared with macroscopic experimental data, showing that the state-to-state model is able to provide better capabilities for predicting experimental results than the traditional multitemperature approach. The use of graphics processing units allowed us to obtain these results in a two-dimensional configuration, opening additional perspectives in the investigation of reacting flows
A review of vorticity conditions in the numerical solution of the ζ–ψ equations
No full textIn this review the conditions to be imposed on the vorticity in the calculation of two-dimensional incompressible viscous flows are discussed. Existing boundary vorticity formulas, commonly regarded as a surrogate Dirichlet boundary condition for the vorticity, are more properly interpreted as the discrete counterpart of the Neumann boundary condition for the stream function. This viewpoint helps to elucidate the algebraic equivalence of coupled numerical methods with uncoupled methods based on conditions of integral type for the vorticity. A unified understanding of several available treatments for determining correct vorticity boundary values is achieved by including in the present analysis spatial discretizations by finite differences and finite elements, coupled and uncoupled formulations of the problem as well as steady and unsteady equations. Results of some test calculations are presented to illustrate the numerical consequences of the analysis
Fuzzy logic controller applied to a variable geometry turbine turbocharger
No full textThis paper provides an adaptive technique for the control of a variable geometry turbine (VGT) in a turbocharged compression ignition engine. The adaptive control is based on a fuzzy logic control scheme and a least-squares parameter estimator algorithm. In order to test the performance of the proposed control technique, a numerical model of the engine has been used, which employs a thermodynamic (zero-dimensional) approach. The paper will show that the fuzzy logic control technique is able to take into account the non-linearity of the controlled system and to reject white noise affecting the measurement chain
Accurate and efficient solutions of unsteady viscous flows
No full textThis paper describes two accurate and efficient numerical methods for computing unsteady viscous flows. The first one solves the incompressible Navier-Stokes equations in their vorticity-velocity formulation, using a staggered-grid finite-volume spatial discretization to provide second-order accuracy on arbitrary grids, and combines effectively and alternating direction implicit scheme for the vorticity transport equation and a multigrid line-Gauss-Seidel relaxation for the velocity equations. The second method solves the compressible Reynolds-averaged Navier-Stokes equation in strong conservation form, with k-omega turbulence closure model. The equations are discretized in time using an implicit three-time-level scheme, combined with a dual time stepping approach, so that the residual at every physical time step is annihilated using an efficient multigrid Runge-Kutta iteration with variable time stepping and implicit residual smoothing. The space discretization uses a Roe's flux difference unsteady cascade flow is used to demonstrate the accuracy and efficiency of the method. The authors are currently working towards extending the two approaches described in this paper to three space dimensions
VGT Turbocharger Controlled by an Adaptive Technique
No full textThis paper provides an adaptive technique for the control of the variable geometry turbine in a turbocharged compression ignition engine. The adaptive control is based on a one-step-ahead (OSA) technique and a least-square parameter estimator algorithm. In order to test the performance of the proposed control technique, a numerical model of the engine has been developed, which employs a thermodynamic (zero-dimensional) approach. The paper will show that the OSA technique is able to improve dramatically the control performance with respect to that provided by a commonly applied proportional integral derivative control technique
Effect of finite-rate catalysis on wall heat flux prediction in hypersonic flow
No full textThis work deals with the simulation of hypersonic flows past a copper sphere by using a finite-rate catalysis model coupled with either a state-to-state (StS) or a multitemperature thermochemical nonequilibrium approach. Without detailed state-specific data for copper catalytic recombination, molecules formed on the surface are described with three different statistics: one reproducing the incoming distribution, one considering a uniform distribution, and the third populating only the highest vibrational level. Following recent experimental and theoretical results, very high enthalpy and very low pressure conditions have been considered. The finite-rate partial catalysis model provides results that are closer to the experimental ones than those obtained by a fully catalytic approach. The multitemperature model shows better agreement with experiments, whereas among the StS catalytic approaches the outcomes have shown that the surface recombination on only the highest energy level gives more accurate results
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