3D direct pore level simulations of radiant porous burners

Inside porous burners, chemical combustion reactions coincide with complex interaction between thermo-physical transport processes that occur within solid and gaseous phase and across phase boundary. Fluid flow, heat release and resulting heat flows influence each other. The numerical model used in this work considers gaseous and solid phases, includes fluid flow, enthalpy transport, conjugate heat transfer, and radiative heat transfer between solid surfaces, as well as combustion kinetics according to a skeletal chemical reaction mechanism, fully resolved on the pore scale in three-dimensional space (Direct Pore Level Simulation, DPLS). The calculations are performed based on the finite volume method using standard applications implemented in the OpenFOAM library. The present study presents simulations of three different structures, each at four settings of specific thermal power. Results indicate that specific surface area of the porous structure is a major influencing parameter for increasing radiation efficiency, whereas no correlation of the orientation of an anisotropic structure on radiation efficiency was observed

Challenges and progress on the modelling of entropy generation in porous media: a review

Depending upon the ultimate design, the use of porous media in thermal and chemical systems can provide significant operational advantages, including helping to maintain a uniform temperature distribution, increasing the heat transfer rate, controlling reaction rates, and improving heat flux absorption. For this reason, numerous experimental and numerical investigations have been performed on thermal and chemical systems that utilize various types of porous materials. Recently, previous thermal analyses of porous materials embedded in channels or cavities have been re-evaluated using a local thermal non-equilibrium (LTNE) modelling technique. Consequently, the second law analyses of these systems using the LTNE method have been a point of focus in a number of more recent investigations. This has resulted in a series of investigations in various porous systems, and comparisons of the results obtained from traditional local thermal equilibrium (LTE) and the more recent LTNE modelling approach. Moreover, the rapid development and deployment of micro-manufacturing techniques have resulted in an increase in manufacturing flexibility that has made the use of these materials much easier for many micro-thermal and chemical system applications, including emerging energy-related fields such as micro-reactors, micro-combustors, solar thermal collectors and many others. The result is a renewed interest in the thermal performance and the exergetic analysis of these porous thermochemical systems. This current investigation reviews the recent developments of the second law investigations and analyses in thermal and chemical problems in porous media. The effects of various parameters on the entropy generation in these systems are discussed, with particular attention given to the influence of local thermodynamic equilibrium and non-equilibrium upon the second law performance of these systems. This discussion is then followed by a review of the mathematical methods that have been used for simulations. Finally, conclusions and recommendations regarding the unexplored systems and the areas in the greatest need of further investigations are summarized

Performance Characterization Of Micro Porous Media Burner For Heat Or Power Generation

The threat of fossil fuel depletion affects the nation’s economy. Consequently, attempts are made to improve the use of fuels by developing highly efficient burners. With this intention, present work was focused to develop premixed butane based micro porous media burner. The burner was designed to undergo surface and submerged flames by varying equivalence ratio. Two types of reaction layer were tested; foam and ball type porous media (PM), while porcelain foam in preheat zone. Thickness of reaction and preheat layer was varied suitably to get optimum burner performance. Thus 90% thermal efficiency was noted by using 15 mm alumina foam along with 10 mm porcelain foam. Values of NOx and CO at optimum equivalence ratio was less than 15 and 60 ppm respectively. Further, 4% improvement in the thermal efficiency was achieved by adding 80 μL of vegetable oil droplets over reaction layer. In addition, electric power of 2.018 W was generated from the surface flame using TE cells. These TE cells are integrated to a hybrid configuration, it includes circuit fan powered from solar panels. Moreover, height between reaction layer and TE cells was optimized (69 mm) using design of experiments to further increase electric power by 8%. Finally, three dimensional numerical study was performed to compare experimental data for both temperature and emissions (NOx and CO) at a critical equivalence ratio (ER=0.7

Numerical and experimental evaluation of preheated premixed flames at lean and rich conditions

Preheating a combustible mixture enhances the laminar burning flux characteristic mad with high reaction firing rates. As a result, the flammable zone as defined by inlet conditions of equivalence ratio and temperature is expanded beyond that available at standard ambient conditions; however, fundamental questions in these combustion regimes have not been addressed. In this thesis, preheated lean and rich combustion of methane/air mixtures is studied numerically and experimentally to catalog and confirm expected trends in these regimes. Numerical simulations were completed using both GRI-Mech 3.0 and San Diego mechanisms in the combustion code Cantera. An adiabatic simulation data set is obtained over a vast range of equivalence ratios (φ = 0.15-3.5) and inlet temperatures Tin = 200-1000 K), while further study is completed at lean (φ \u3c 0.89) and rich conditions (φ \u3e 1.3). Detailed analyses of flame structure and reaction pathway analysis, sensitivity, and heat release are completed at a total of ten reference cases, five lean and five rich, selected along contours of constant equivalence ratio φ = 0.7, 1.6 and mass flux mad = 0.2190 kg/m2-s. A regression analysis of each regime links adiabatic flame propagation to a characteristic temperature T★, shown to be primarily a function of m, while φ and Tin are shown to play a subordinate role. Analyses together reveal causal kinetic phenomena contributing to differences in lean and rich combustion. Experiments connect the adiabatic findings to the simplest non-adiabatic application, where stand-off distances of a flat flame burner are used as a metric for flame behavior. Viable flames are established at ultra-lean and rich conditions, but results show mechanism uncertainty at preheated conditions in addition to unmodeled heat transfer phenomena. Further study of flat flame behavior is performed in the computational fluid dynamics code Fluent 12.0, where a two dimensional axisymmetric flame is stabilized for three mass fluxes at a reference case of φ = 0.7, Tin = 300 K. The model does not attempt to replicate the exact conditions seen experimentally, rather it seeks to evaluate boundary effects and other two dimensional flame structures resulting from exceeding the laminar burning flux

An experimental and numerical study of evaporation enhancement and combustion in porous media.

The results showed that the pressure drop across the porous media increased as the coflow air velocity, temperature, and linear pore density of the medium were increased. The measured and predicted surface temperatures of evaporation and combustion porous media showed that the temperature distribution was uniform within +/- 25 K and 50 K, respectively. The droplet Sauter mean diameter data revealed that the spray core region contained droplets with lower diameter, and the droplet diameter increased radially outward. A heat feedback rate to the evaporation porous medium section of about 1% of the average heat release in the combustion section was needed to completely vaporize the kerosene fuel. The vapor concentration level downstream of evaporation porous medium with 1% combustion heat release feedback was 63% higher than that with no heat feedback.Our results also suggest that the use of porous media in combustors allows operation at a lower coflow air temperature or with a shorter evaporation section. The porous-medium-burner concepts developed in this dissertation can be employed in many practical liquid combustion systems such as gas turbine combustors, air-heating systems, industrial burners, porous chemical reactors, heat recovery systems, and hybrid burners for bio-fuels.Stable spray flames were established both inside (referred to as interior flames) and on the downstream exit surface (surface flames) of the combustion porous medium. The equivalence ratio at flame extinction in each mode was determined. The extinction equivalence ratio decreased with a decrease in coflow air velocity. A nominal value of Damkohler number of 5.0 was required to initiate the interior combustion mode. As Damkohler number was increased, the extinction equivalence ratio decreased (i.e., extending the fuel lean operation). The axial temperature profiles in evaporation and combustion porous media were measured. Also measured were the radiative heat release from porous medium downstream exit surface, and pollutant emissions of carbon monoxide and nitric oxide. The results demonstrate the benefits of porous medium in making NO emission somewhat insensitive to operating parameters such as equivalence ratio and location of injector.Blocks of open-cell, silicon carbide coated, carbon-carbon ceramic foam of bulk cross section 4 x 4 cm and thickness of 2.5 cm were used as porous medium sections for liquid evaporation and subsequent combustion. Liquid fuel (kerosene, n-heptane, and methanol) was sprayed into a co-flowing, preheated (350--490 K) air environment using an air-blast atomizer, and the spray subsequently entered the porous medium. In controlled evaporation studies, combustion heat feedback to evaporation porous medium was simulated with a resistive heating mechanism. The minimum heat feedback rate required for complete vaporization of liquid and the vapor concentration profiles downstream of evaporation porous medium were measured. The stable operating regimes of spray flames in the combustion porous medium were determined and a general understanding of flame extinction in porous media was developed using a Damkohler number analysis.A two-energy equation model was developed to study the evaporation enhancement of liquid spray in the porous media. Combustion in the porous media was simulated by using a uniform volumetric heat source in the porous region. The solid and gas phase equations were coupled using a volumetric heat transfer coefficient. The computer simulations were performed with a commercial code, Fluent(TM) 6.0.Combustion of gaseous fuels in porous media improves combustion performance and reduces pollutant emissions by transferring combustion heat upstream via conduction and radiation to preheat reactants. Such heat feedback may be beneficially exploited to enhance vaporization of a liquid sprayed upstream of the porous medium, in addition to improving combustion performance. This dissertation presents an experimental and computational study of evaporation enhancement and combustion of liquid spray aided by porous media

Design and Development of Heterogenous Combustion Systems for Lean Burn Applications

Combustion with a high surface area continuous solid immersed within the flame, referred to as combustion in porous media, is an innovative approach to combustion as the solid within the flame acts as an internal regenerator distributing heat from the combustion byproducts to the upstream reactants. By including the solid structure, radiative energy extraction becomes viable, while the solid enables a vast extension of flammability limits compared to conventional flames, while offering dramatically reduced emissions of NOx and CO, and dramatically increased burning velocities. Efforts documented within are used for the development of a streamlined set of design principles, and characterization of the flame\u27s behavior when operating under such conditions, to aid in the development of future combustors for lean burn applications in open flow systems. Principles described herein were developed from a combination of experimental work and reactor network modeling using CHEMKIN-PRO. Experimental work consisted of a parametric analysis of operating conditions pertaining to reactant flow, combustion chamber geometric considerations and the viability of liquid fuel applications. Experimental behavior observed, when utilizing gaseous fuels, was then used to validate model outputs through comparing thermal outputs of both systems. Specific details pertaining to a streamlined chemical mechanism to be used in simulations, included within the appendix, and characterization of surface area of the porous solid are also discussed. Beyond modeling the experimental system, considerations are also undertaken to examine the applicability of exhaust gas recirculation and staged combustion as a means of controlling the thermal and environmental output of porous combustion systems. This work was supported by ACS PRF 51768-ND10 and NSF IIP 1343454

The combustion mitigation of methane as a non-CO2 greenhouse gas

These research results have received funding from the EU H2020 Programme (No. 689772) and from MCTI/RNP-Brazil under the HPC4E Project, grant agreement no 689772

Modelado CFD de la combustión en calderas de biomasa – Revisión del estado del arte

Combustion is the main method of converting biomass to energy, either by direct heating systems or by boilers. By means of CFD models, it is possible to optimize the behavior of those systems and improve significantly its performance, without incurring the economic and environmental cost of experimental studies. However, modelling of biomass combustion is a complex process that requires a large number of sub-models and computational resources for a detailed description, therefore, different approaches have been developed which depend on the system and simulation objective. In this work, a review of the state of art of modelling of solid biomass combustion in the last years is presented, including classification, description and analysis of several of the main models about the subject.La combustión es el principal método de transformación de biomasa en energía, ya sea en sistemas de calefacción directa o en calderas. Por medio de los modelos CFD se puede optimizar el funcionamiento de estos sistemas y lograr mejoras significativas en su desempeño, sin incurrir en los costos económicos y ambientales que los estudios experimentales acarrean. No obstante, el modelado de la combustión de biomasa sólida es un proceso complejo que requiere de gran cantidad de sub-modelos y recursos computacionales para una descripción detallada, por lo que se han desarrollado diversos enfoques que dependen del sistema a modelar y del objetivo de la simulación. En el presente trabajo se realiza una revisión del estado del arte sobre el modelado de la combustión de biomasa sólida en los últimos años, incluyendo la clasificación, descripción y análisis de varios de los principales modelos desarrollados sobre el tema