Juan A. Ramírez

Bio: Juan A. Ramírez is an academic researcher from University of Zaragoza. The author has contributed to research in topics: Reynolds stress & Reynolds number. The author has an hindex of 3, co-authored 3 publications receiving 32 citations.

Comparison of Different URANS Schemes for the Simulation of Complex Swirling Flows

Abstract: We perform numerical calculations of the unsteady, Reynolds-averaged Navier-Stokes (URANS) equations to simulate isothermal single-phase flow in the geometry of a pulverized-solids burner, with double air intake and swirl, at large Reynolds numbers. Two simulations are run with different turbulence closures, viz., the standard k–ϵ and Reynolds stresses models. Computations are validated concerning grid density and placement of boundaries. Results describe an almost periodic flow that exhibits very convincing time-dependent, coherent structures. We analyze it, as well as the differences arising from the nature of the turbulence model, which is an important issue given the cost involved.

Numerical characterization of the aerodynamics in fixed-grate biomass burners

Abstract: This paper investigates the numerical simulation of the aerodynamics of biomass burners operating in small-scale, fixed-grate technologies. The efficiency of these boilers is largely determined by the fluid patterns originated in the combustion chamber, as a consequence of the interaction of primary and secondary inlets. A set of CFD computations have been carried out for a case-study burner, seeking the comparison for the isothermal-flow solutions given by Reynolds Averaged Navier–Stokes equations (RANS) and by Unsteady RANS equations (URANS). The influence of both spatial and temporal discretization is discussed, using the Grid Convergence Index (GCI) based on Richardson extrapolation. The results indicate that RANS solutions are slightly more sensitive to grid parameters, while URANS solutions show a better convergence behavior. Validation has been reasonably achieved by comparing the URANS velocity profiles against experimental measurements. As a consequence, a mathematical tool is now available to support design modifications of the biomass burner, combining simplicity, reliability and economy.

Prediction of Flow Instabilities in an Atmospheric Low Swirl Burner Using URANS Models

Abstract: Swirl-induced phenomena are used in gas turbine burners as a mechanism to stabilize the flame. The formation of coherent structures under turbulent swirling conditions plays a fundamental role in the stabilization and needs to be completely understood also in the absence of combustion. In this work, numerical calculations of the unsteady, Reynolds-averaged Navier-Stokes (URANS) equations for isothermal flow in an unconfined annular low swirl burner (50 kW) are reported. The standard k-ϵ and Reynolds stress models are used to run computational cases at a Reynolds number of 12,000 and two swirl numbers (S L = 0.57 and S H = 0.64). The numerical method is validated with the experiments reported by Legrand et al. [27]. Numerical results agree well with experiments for mean flow, temporal pressure measurements, and transient coherent structures. 2-D proper orthogonal decomposition (POD), 3-D iso-surfaces and advanced, vortex-related visualization methods are used to document the latter.

Coal flame characterization by means of digital image processing in a semi-industrial scale PF swirl burner

Abstract: The potential of a new procedure of image processing for the characterization of a given combustion state through flame visualization is here presented and discussed. Experimental tests were carried out in a swirl-stabilized, semi-industrial scale burner of 500 kWth. Using an advanced vision based system, flame images have been recorded and subsequently processed, obtaining both luminous and spectral parameters from the grey values registered by each individual pixel. The acquisition system is based on a CCD (charge-coupled device) camera of high-speed frame rate. The innovative nature of the analysis lies in the 2D distribution of statistical and oscillatory parameters which can be interpreted as a “fingerprint” of the flame condition. By this method, flame spatial characterization was achieved allowing the identification of areas with different luminous and oscillating patterns. Their evolution regarding primary air-to-fuel ratio was also studied. First results suggest changes on flame symmetry and oscillation regimen. Additionally, quantitative flame analysis through global values of selected parameters and regression studies were conducted in order to analyse their usefulness for the development of monitoring and control algorithms in the combustion facility.

Combustion processes in a biomass fuel bed – Experimental results of the influence of airflow and of particle size and density

Abstract: Combustion processes in a biomass fuel bed – Experimental results of the influence of airflow and of particle size and density

Effect of inlet conditions on swirling turbulent reacting flows in a solid fuel ramjet engine

Abstract: This paper presents experimental and numerical investigation of turbulent reacting flows in a solid fuel ramjet engine with different inlet conditions. In simulations, three main parameters were varied independently, which are the swirl intensity, mass flow rate, and air inlet temperature to study these parameters influence on the regression rate and combustion phenomena. Firstly, a numerical model has been developed to solve axisymmetric unsteady Reynolds-averaged Navier-Stokes equations of the turbulent swirling compressible flow field with chemical reactions. Secondly, experiments have been performed on the solid fuel ramjet without swirl to validate the developed code. Thirdly, in order to assess the accuracy and robustness of the code three test cases are adopted. Finally, a series of unsteady simulations are carried out for swirling reacting turbulent flows in a solid fuel ramjet using high-density Polyethylene (HDPE) solid fuel. The main results obtained from this study show that swirl flow enhances the regression rate and the turbulent mixing throughout the ramjet. In addition, the results revealed that an increase of swirl number, mass flow rate, and air inlet temperature increases the heat and mass transport at the solid fuel surface and hence enhances the local regression rate. Three relations have been proposed to correlate the average regression rate.

Review on modelling approaches based on computational fluid dynamics for biomass combustion systems

Abstract: Biomass combustion is an important pathway of energy generation from renewable resources. Even though biomass combustion is an established conversion process, there is a high potential for further optimization of the used technologies. In particular, the advantage of numerical methods for the improvement of biomass combustion systems is not used to its full extent, because the simulation of these complex systems requires various sub-models for the thermo-chemical conversion of the biomass and sufficient computational resources for the combustion simulation. However, simulations are a valuable tool to enhance the design and operating conditions of biomass combustion systems with regard to high efficiency, low emissions and high flexibility. In addition, comparison of experimental and numerical results leads to a better understanding of the processes involved in biomass combustion. The present study gives a comprehensive overview of simulations of biomass combustion systems based on computational fluid dynamics that are available in the literature. It focusses on systems with fixed bed and covers various technologies (moving bed, pellet boilers, wood log stoves) as well as a wide range of sizes from laboratory reactors to industry scale. Besides woody biomass, also alternative fuels such as straw or municipal solid waste are considered. All relevant sub-models for the thermo-chemical conversion of the fuel on the one hand and for the gas-phase combustion on the other hand are discussed in detail. The recent advances in the concerned research fields are described.

Numerical Investigation of Transient Soot Evolution Processes in an Aero-Engine Model Combustor

Abstract: This article presents unsteady Reynolds averaged Navier–Stokes simulations (URANS) of a well-characterized aero-engine model combustor with finite-rate chemistry (FRC). The simulations give insight into the complex formation and destruction processes of soot at technically relevant conditions. It will be shown that a recently developed PAH (polycyclic aromatic hydrocarbons) and soot model is able to predict soot under complex combustion conditions at elevated pressure. Finite-rate chemistry is employed for the gas phase, a sectional approach for PAHs and a two-equation model for soot. Thus, feedback effects, such as the consumption of gaseous soot precursors by growth of soot and PAHs, are inherently captured accurately. In agreement with the experiment a precessing vortex core (PVC) is observed in the ethylene fueled combustor. This requires that the computational grid covers swirlers. The PVC intensifies mixing of fuel, primary air, and hot burned gas from the inner recirculation zone, thereby supportin...