On Monday 23rd June 2000 (MNM Week 26) we reported the Annual Seminar and Summer Management Meeting of the EuroFlam consortium at Florence Italy on Wednesday 21st and Thursday 22nd June. Twelve Graduates/Visiting Investigators who presented their completed reports or progress reports attended the meeting.

Last week (MNM Week 28) we commenced the publication of abstracts of the presentations made. This week, we are publishing the second series of abstracts.

Erratum: Last week, we have incorrectly mentioned the presentation by Heinz-Jörg Tomczak - ENEL as the best presentation award. We regret this error. The best presentation award was given to Christian Skaug - ENEL on the subject:

Large Eddy Simulation using G­equations
by Christian Skaug - ENEL

Best Presentation Award, was given by the Italian Committee for Flame Research - CI.

Abstract
Solving the time dependent three-dimensional Navier­Stokes equations with turbulent combustion is a formidable numerical task. Despite the steadily increasing power of microprocessors, direct numerical simulation of a realistic industrial burner is still beyond the capacity of the computers found at the engineer's desk. Unless this situation is coped with somehow, design of power stations will probably continue for a long time largely with out a profound theoretical understanding of the processes taking place in the burner. To circumvent the computational barrier, the Large Eddy Simulation (LES) technique has been developed. This approach calls for a turbulence model to capture the effects on the scale below the computational grid. Recently, the use of so-called G­equations has been suggested, and promising results have been obtained for simple test cases. At ENEL, a large and complex computational code for LES has been developed over several years. But this code uses another turbulence model. However, it is believed that the code could be reused and modified as to take into account the G­equation as a sub­gridscale model. The work that has taken place under the EuroFlam programme was carried out in order to modify the LES code to include the G-equation.

Combustion Studies of a Jet Flame using CFD Analysis
by Ulla Steil - UWC

Abstract
Greenhouse effects, ozone problems, smog and acid rain are terms used when people talk about human influence on nature. 90% of world - wide energy is produced by fossil energy through combustion reactions. This is an important reason to work with combustion systems.

In the last years there has been a growing interest in mathematical modelling of combustion processes in many industrial sectors. This is more or less traditionally related to the development of Computational Fluid Dynamics (CFD). The main reason for this growing interest are the increasing reliability and the decreasing costs (in term of computational resources and required know-how) of these tools and the range of possibility for analysis of existing systems and evaluation, design and optimisation of the new ones.

The energy industry is facing the same problems since more than 20 years, due to the need for costs control and a tightening of pollutant law limits.

In this work, a mathematical simulation has been done to study the combustion parameter at a free turbulent jet flame. The numerical approach uses the commercial CFD-code FLUENT and it is divided in two parts: the numerical grid (pre-processing with Gambit) and the solver (calculation with FLUENT 5).

The aim of the work is to develop and set up a reliable 3D-combustion model with a commercial code. It includes the geometry, mesh and the calculation for different cases. Using this numerical model it should be possible to analyse and study the existing combustion systems to understand and solve process problems and give a laws for existing systems improvement and new design. A major feature in high temperature combustion is heat transfer by radiation, the model will have to be carefully evaluated, including different models, which describe and approximate the complicated process of methane combustion.

A numerical modelling of a combustion process is a complex venture, which depends on various matters like turbulence, reaction, heat transfer and material properties.

To simplify these matters the calculation has been done with two fixed models. One was the k-e turbulence model and the other was the DTRM radiation model.

This works shows that there is quite a lot of influence by different reaction model, the chemical kinetic and the material data.

The aim is to get first a converged solution with the current settings and then to implement a more complex reaction model. It should be possible, demonstrated by earlier works, to implement more reactions. The next step is to work also with various turbulence models.

Results of the High Temperature Air Combustion Experiment using COAL as fuel (HTAC 99)
by Stefano Costantini - IFRF

Abstract
The HTAC 99 trial is the final part of the 'High Performance Industrial Furnace Development" project. The project offers industrial furnace designs which reduce CO₂ emissions, realize low NO_x and reduce substantially the equipment size. The most effective way of achieving these goals is increasing the thermal efficiency of industrial furnaces by using high preheated combustion air up to 1300 °C, offering great potential in fuel savings. In conventional burner design such a high-preheated level of the combustion air determines far too high NO_x emissions. The excess enthalpy combustion technique allows achieving low NO_x emissions with high temperature of the combustion air.

In July 1999 the IFRF Research Station carried out a trial applying this techniques for coal combustion. A Nippon Furnace Kogyo Co. burner has been tested, the burner consists of a central injector trough with hot air is injected at high velocity and two coal gun injectors. The two coal guns supply the fuel at large distance from the central air.

In IFRF Research Station furnace No. 1 a regenerative burner was simulated using a precombustor to reproduce the combustion air at high temperature. The flue gas of the precombustor were split in two steams, one is injected into the furnace after oxygen addition to keep the O₂ level at 20.9% vol. wet producing the called 'vitiated combustion air" and the second one is vented off after cooling it by two heat exchangers. During the two-weeks trial the flow field and the mixing have been characterized performing detailed in-flame and input-output measurements.

Performed measurements were:

In-flame Measurements

• Gas composition;
• Temperature;
• Velocity;
• Total radiative heat flux;
• Total radiance (narrow angle radiometer);
• Videos and photos.

Input-Output Measurements

• Gas composition;
• Temperature;
• Solids.

The flow field shows a strong central air jet creating a large recirculation zone. The COAL jets are immersing into hot combustion products and they entrain a large amount of combustion products before mixing with the air. The combustion is not intense and it is taken place in large volume in the furnace. The furnace was operating under conditions similar to a well-stirred reactor. The O₂ and temperature fields were uniform all over the furnace. The measured O₂ concentration is practically in all the furnace volume around 2-3%. The temperature shows a very flat profile. The peak temperature is substantially suppressed. Despite the high combustion air temperature the in flame measured temperature were as low as 1500 °C. That means that the furnace temperature is 200 °C higher than the preheated air temperature. The resulting very flat temperature profile allows reaching a substantial decrease in the NO_x emissions. At this temperature level the thermal NO_x formation is almost suppressed. The resulting measured heat flux shows a very flat profile along the furnace.

It was concluded that this technique represents a very interesting tool for the design of new furnaces matching increase in the efficiency with low NO_x and CO emissions.

 

International Meetings Update 07/00: Contributed by Lois Brett Annual EuroFlam Seminar 2000 - Abstracts, Vol 2: Contributed by Aristide Mbiock and Peter Roberts World Energy News: Contributed by Aristide Mbiock

 

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Edited: Peter Roberts
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