D. Trimis

Combustion in a porous medium-advances and applications

Abstract In this paper research and development work of the authors is described, which led to the design of a new combustor-heat exchanger system based on the combustion in porous media. Combustion in inert porous media is possible if the Peclet-number is high enough ( > 65), so that quenching of the flame inside the pores is prohibited. The heat transfer from the combustion zone, to the porous medium itself is very effective because of the very large surface between them. The combustion temperature can be controlled through the porous medium temperature. Prompt and thermal NOx formation, which is temperature dependent, can be controlled by appropriate cooling of the combustion zone. The heat transfer mechanisms in the combustor are discussed and new designs of porous materials are proposed, which allow a high power density and a better control of the temperature level in the combustion zone. The possible application field and the expected benefits of this combustion technique are discussed.

Numerical and experimental investigation of matrix-stabilized methane/air combustion in porous inert media

Abstract Porous media combustion offers exceptional advantages compared with techniques involving free flame burners. Porous medium burners are characterized by higher burning rates, increased flame stability, and lower combustion zone temperatures, which lead to a reduction in NO x formation. In addition, they show a very high turndown ratio, low emissions of CO, and are of very small size. In order to optimize the combustion process further and to adapt the burner design, as well as to obtain a tool that allows fast adaptation to new industrial applications, a numerical code utilizing a pseudohomogeneous heat transfer and flow model for the porous material was applied. It considers conservation equations for 20 species, two momentum equations, and one energy equation. This model enabled a numerical parametric study to be made for a porous medium burner with a rectangular cross-section geometry. The calculated 2D temperature fields and species concentrations, with premixed methane/air combustion, are compared with data obtained from experiments with the same burner geometry. It is shown that there is good agreement between the numerical solutions and the experimental data and it is concluded that the developed numerical program is an excellent tool to investigate combustion in porous media.

Computational efficiency improvements of the radiative transfer problems with or without conduction--a comparison of the collapsed dimension method and the discrete transfer method

Abstract This paper deals with the performance evaluation of the collapsed dimension method (CDM) and the discrete transfer method (DTM) in terms of computational time and their abilities to provide accurate results in solving radiation and/or conduction mode problems in a 2-D rectangular enclosure containing an absorbing, emitting and scattering medium. For some pure radiation cases, studies were made for two representative benchmark problems dealing with radiative equilibrium and non-radiative equilibrium. For the combined mode, the transient conduction and radiation problem was solved. The alternating direction implicit scheme was used for the solution of the finite difference part of the energy equation. For the three types of problems considered, tests were performed for a wide range of aspect ratio, extinction coefficient, scattering albedo, conduction–radiation parameter and boundary emissivity. For pure radiation problems, results from the two methods were validated against the results from the Monte Carlo method. For the combined mode, some steady-state results were compared with results available in the literature. For the transient situations, results from the two methods were validated against each other. While both the methods were found to give the same results, the CDM was found to be much more economical than the DTM.

Heat transfer characteristics of a porous radiant burner under the influence of a 2-D radiation field

This paper deals with the heat transfer analysis of a 2-D rectangular porous radiant burner. Combustion in the porous medium is modelled as a spatially dependent heat generation zone. The gas and the solid phases are considered in non-local thermal equilibrium, and separate energy equations are used for the two phases. The solid phase is assumed to be absorbing, emitting and scattering, while the gas phase is considered transparent to radiation. The radiative part of the energy equation is solved using the collapsed dimension method. The alternating direction implicit scheme is used to solve the transient 2-D energy equations. Effects of various parameters on the performance of the burner are studied.

Measurement of instantaneous flow rates in periodically operating injection systems

Research and development work carried out to provide a method to measure accurately instantaneous flow rates in periodically operating injection systems is summarized. The instantaneous flow rate is reconstructed from axial velocity time series measured by a laser Doppler anemometer on the center-line of a capilary pipe flow. The theoretical background, on which the evaluation of the instantaneous flow rate is based is provided. It is shown that the axial mean velocity is sufficient to reconstruct the periodically varying flow rate and the pressure gradient. The application of the proposed method is described and an instrument is suggested that can be employed in many fields where fast, periodically varying flow rates occur and instantaneous information is needed.

Improved-performance knitted fibre mats as supports for pre-mixed natural gas catalytic combustion

Abstract As an improvement of previously developed catalytic burners based on LaMnO3 catalysts deposited on a FeCrAl alloy fibre mat (NIT100s by ACOTECH BV), new catalytic burners have been developed showing improved catalytic activity and sulphur resistance, a prevalent issue to be addressed owing to the presence of odorants (e.g. THT) in the natural gas distributed in the network. Such properties are entailed by adoption of new catalysts (LaMnO3+17MgO) developed on purpose, where the presence of MgO acts both as a structural promoter (reducing the average grain size of the perovskite catalyst by nearly one order of magnitude) and a sulphur-poisoning preventing agent (by selectively reacting with SO2 originated from the combustion of the odorant). The performance of the new catalytic burner is compared in this paper to those of the non-catalytic burner and the previously developed LaMnO3-catalysed burner in both fresh and aged status as a function of the prevalent parameters affecting the combustion regime (specific power input Q, excess of air Ea). Particularly promising results are enlightened at low Q and Ea values ( A preliminary mono-dimensional modelling of the combustion in the porous burners including heat, mass and momentum balances in the combustion chamber and a pseudo-homogeneous approach to the description of the behaviour of the porous burner is presented. Model calculations obtained through ad hoc developed code (FASTEST) show just encouraging results that pave the way towards further improvements of the modelling tool.

Determination of velocity profiles in oscillating pipe-flows by using laser Doppler velocimetry and ultrasonic measuring devices

Abstract The velocity profiles measured with a recently built ultrasonic velocity profile (UVP) sensor based on ultrasound (US) pulsed echography are compared with laser Doppler velocimetry (LDV) measurements. The results from the experiments show that both techniques are well suited to measurements in oscillating, piston-driven pipe-flows. LDV has a higher spatial resolution in the region of high-velocity gradients, but requires considerably greater effort to obtain a complete velocity profile than the UVP sensor, which can obtain the complete profile information simultaneously. However, in the near-wall regions, reflections of the US beam degrade the quality of the UVP measurements. This problem may be reduced by matching the wall material and the fluid so as to minimize the difference in the propagation speed of acoustic waves in both media.

Flame Stabilization of Highly Diffusive Gas Mixtures in Porous Inert Media

Flame propagation in porous inert media depends on structure and physical properties of the solid matrix and on the properties of the combustible gas. The resulting flame propagation modes can be classified into different regimes, of which some important parameters are given in Table 1.

Combustion by Free Flames Versus Combustion Reactors

In its efforts to optimize combustion processes, the Institute of Fluid Dynamics in Erlangen (LSTM-Erlangen) has succeeded in developing the technology of combustion in porous media. This technology does not operate with free flames. The entire combustion takes place in a porous matrix, which builds a combustion reactor. This technique stands out for the combination of advantages that no other modern burner technology can show so far, including an extremely high variable power dynamic range (1 : 20) combined with minimum waste gas emissions (NOx < 25 mg/kWh, CO < 3 mg/kWh) and a very compact size (15 times smaller than present compact burners). The concept of porous burner technology is briefly described in the present article. Starting with the general principles, the basic design as well as the structures and the properties of materials that are suitable for the combustion in porous media are described. Additionally, some important fields of application for this novel technology are outlined.

EFFECT OF ANGULAR QUADRATURE SCHEMES ON THE COMPUTATIONAL EFFICIENCY OF THE DISCRETE TRANSFER METHOD FOR SOLVING RADIATIVE TRANSPORT PROBLEMS WITH PARTICIPATING MEDIUM

This article deals with the effects of angular quadrature schemes on the computational efficiency of the discrete transfer method (DTM) used for solving radiative transport problems with a participating medium. Four different quadrature schemes are considered. The first scheme uses the ray directions and the associated weights as per the original formulation of the DTM. In the second and third schemes, the ray arrangements are different but the quadrature schemes are the same as that used in the collapsed dimension method. The fourth scheme uses the ray directions and the corresponding weights according to the imporved quadrature scheme employed in the discrete ordinates method (DOM). The effects of the four quadrature schemes on the computational efficiency are tested for four benchmark radiative transport problems. Results are compared with the exact methods and with those available in the literature. It has been found that if the ray directions and the weights are chosen as per the DOM, the computational efficiency of the DTM is the best.