GRANULAR FLOW SIMULATIONS OF LIMITING REGIMES OF PARTICLES–WALL INTERACTION RELEVANT TO SLAGGING COAL GASIFIERS

In pilot entrained-flow slagging coal gasifiers, high conversion efficiency and low pollutant emission levels have been observed, but the mechanism leading to this behaviour is not fully understood. Recent literature proposes several different mechanisms as playing an important role, ranging from the sticking properties of both particles and slag-covered walls to the thermal and chemical history along the trajectory of the particles in the entire gasifier. Nonetheless, very few attention has been devoted to the role of particle–particle interactions, even if it has been shown that this mechanism can lead to new regimes likely to occur in slagging gasifiers and to promote the rise in the coal conversion efficiency. This study presents the results of a simplified configuration that allows to highlight the role of the four different interactions that can be envisaged when considering particles and confining walls as either sticky or non sticky. Particles are subjected to a body force that mimics the action of the drag exerted by a swirling flow field in a cylindrical vessel. Particle–particle collisions are modelled with an Hertzian approach that includes torque and cohesion effects. Results clearly indicate the different structure of the layer of particles establishing on the wall surface in the different interaction regimes. They confirm the importance to adequately take into account particle–particle interactions for a correct prevision of the fate of coal particles in slagging gasifiers

Using symbolic computation software packages in production of multidimensional finite volume-based large eddy simulation codes

The numerical simulation of turbulence is one of the most challenging tasks in the field of the modern computational science. At present, the most advanced approach is the large eddy simulation (LES) technique wherein a formal separation between resolved (large) and unresolved (small) scales of the motion is in effect by means of a filtering operation applied onto the governing equations. However, LES requires very sophisticated numerical discretizations in terms of both accuracy and efficiency. Often, the modelling of the unresolved subgrid scale terms adds further computational complexities. This paper illustrates the suitability in using software packages for symbolic computation (in the present case, Maple© for helping in the production of subroutines for a new multidimensional, high-order accurate finite volume-based LES code. Specifically, it will be detailed how producing, rapidly and efficiently, the routines for computing convective, diffusive as well as subgrid scale modelling fluxes. It is particularly detailed how exploiting the package for differential calculus and linear algebra for the analytical integration of the flux polynomials over the finite volume faces. The structure of the LES code is illustrated, and an accuracy analysis of the local truncation errors is performed comparing the third-order accurate multidimensional upwind and the classical second-order centred reconstruction in the wavenumbers space. Then, some numerical results for the turbulent plane channel and some brief points concerning the parallelization issue are addressed. Copyright © 2012 John Wiley & Sons, Ltd

On the control of the mass errors in Finite Volume-based approximate projection methods for large eddy simulations

Filtering in Large Eddy Simulation (LES) is often only a formalism since practically discretization of both the domain and operators is used as implicit grid-filtering to the variables. In the present study, the LES equations are written in the integral form around a Finite Volume (FV) Ώ rather than in the differential form as is more usual in Finite Differences (FD) and Spectral Methods (SM). Grid-filtering is therefore associated to the use of an explicit local volume average, by the way of surface flux integrals, and specific LES equations are here described. Moreover, since the filtered pressure characterizes itself only as a Lagrange multiplier used to satisfy the continuity constraint, projection methods are used for obtaining a divergence-free velocity. The choice of the non-staggered collocation is often preferable since is easily extendable on general geometries. However, the price to be paid in the so-called Approximate Projection Methods, is that the discrete continuity equation is satisfied only up to the magnitude of the local truncation error. Thus, the effects of such source errors are analyzed in FD and FV-based LES of turbulent channel flow. It will be shown that the FV formulation is much more efficient than FD in controlling the errors

FREE TOPOLOGY GENERATION OF THERMAL PROTECTION SYSTEM FOR REUSABLE SPACE VEHICLES USING INTEGRAL SOFT OBJECTS, XXIV Congresso AIMETA 2019

The present paper deals with a modelling procedure developed to design the thermal protection system of a Reusable Space Vehicle. Clusters of Skeleton-based Integral Soft Objects are created to assign independent distributions of thickness according to an arbitrary boolean map which represents two different insulating materials. The procedure is morphologically independent, and allows a powerful and local control of thickness and material distribution. The effectiveness of the modelling procedure is shown with applications to Reusable Space Vehicle concepts

Low speed longitudinal aerodynamics of a blended wing-body re-entry vehicle

This paper deals with the evaluation of low-speed longitudinal aerodynamic performances of a vehicle concept with an unconventional blended wing-body aeroshape. The spacecraft is intended as a multipurpose vehicle for future International Space Station payload and/or crew servicing and support, able to perform a lifting re-entry from Low Earth Orbit and to land on a conventional runway. The aeroshape features a high-sweep near double delta-shaped configuration, equipped with two functionally independent body flaps, which can be used for both longitudinal control (i.e., elevon mode) and lateral-directional control (i.e., aileron mode). Longitudinal aerodynamic force and moment coefficients are investigated with Computational Fluid Dynamics simulations carried out at Mach number equal to 0.3, typical of landing conditions. A comparison of low-speed performances for clean and flapped configurations is performed considering several vehicle attitudes and elevon deflection angles. High-lift performances of the aeroshape are discussed and related to the onset conditions of vorticity field at various angles of attack. Additionally, comparison of aerodynamic coefficients with classical delta-wing theory is also discussed, addressing the promising capabilities of the selected design to perform a glided horizontal landing. Finally, a description of vortex break-down phenomena occurring on the aeroshape at landing incidence is discussed, accounting for the aerodynamic coefficients in post-stall condition, also providing an overall picture of the longitudinal static stability of the vehicle