The Application of a Two-Dimensional Zone Model to the Design and Control of a Continuously Operated, Gas-Fired Furnace

This paper describes the development of a two-dimensional zone model to predict the throughput and thermal performance of a continuously operated gas-fired furnace heating steel bars to a nominal discharge temperature of 1250°C. Ultimately the model is intended to be a tool which can be used for the design and control of industrial furnaces. Consequently relatively short computing times are necessary and this was achieved by employing an isothermal computational fluid dynamics simulation to estimate the relative mass flows, and hence enthalpy flows to or from adjacent volume zones in the overall model. This simplified approach, which utilises a single “once off” isothermal computation of the flows, was considered to be adequate since isothermal flow models have been used successfully in the past to study the flow related behaviour of combustion systems. The coupling of a multi-zone model with a single “once off” isothermal computation of the flows enables a wide range of furnace design modifications to be studied quickly and easily. To illustrate the potential use of the model in a furnace design application, it was then used to investigate the effects of inclining the burners downwards towards the load as well as those associated with increasing the length of the furnace.

Numerical Simulation of Natural Gas Combustion Process Using a One-Step and a Two-Step Reaction

The work evaluates the combustion of natural gas in a cylindrical furnace. The Generalized Finite Rate Reaction Model was selected for predicting the reactions. Two situations were considered. In the first case the combustion of the fuel was predicted by a single global reaction, and in the second case a two-step reaction was considered for predicting the combustion process. The conservation equations of mass, momentum, energy and chemical species were solved by the finite volume procedure, with the commercial software FLUENT. The turbulent flow was modeled by employing the two differential equation κ–ε model. The solutions obtained with the two reaction models, for the temperature and species concentration fields, were compared among them and against experimental data available in the literature. It was observed that the two-step reaction model represents better the physical phenomena, showing a better agreement with the experimental data.

Mixing and Combustion Improvement Using Jet Injection Into Swirling Flowfields

The aerodynamics benefits of lateral jet injection into swirling crossflow have long been recognized and used by combustion engineers. Studies are reported here on experimental and theoretical research on lateral jet injection into typical combustor flowfields for low-speed turbulent swirling flow conditions in the absence of combustion. The main flow is air in a round cross-sectioned plexiglass tube. The degree of swirl can be varied by varying the angles of the blades of an annular swirler, located upstream of the test section. Lateral jet injection is normal to the main airflow, from round-sectioned nozzles. Either a single lateral jet or two diametrically opposed jets are used for this secondary injection of air into the main airflow. The principal aim is to investigate the trajectory, penetration and mixing efficiency of the lateral injection. Flow visualization with helium-filled soap bubbles and multi-spark ionized path techniques, five-hole pitot probe time-mean velocity measurements, and single-wire time-mean velocity and turbulence data (normal and shear stress) have been obtained in the experimental research program. A fully three-dimensional computer code with two-equation turbulence model has been developed and used in the theoretical research program.

Analysis of Two-Phase Air/Water Bubbly Flow Experiment

Two different techniques, the Particle Image Velocimetry (PIV) and the Shadow-Image Velocimetry (SIV) techniques have been used to capture detailed two-phase bubbly flow experimental data. The PIV has provided a two-dimensional velocity field of the liquid phase for analysis of the continuous phase. The SIV has utilized to reconstruct the bubble shape and velocity of the dispersed phase in three-dimensions.

Mixing Field Analysis of a Gas Turbine Burner

An analytical and numerical study has been carried out with the view on the understanding of the physical mechanisms of the mixing process in a gas turbine burner. To this end, three methods at various levels of approximation have been used: At the simplest level an analytical model of the burner flow and the mixing process has been developed. It is demonstrated how this approach can be used to understand basic issues of the fuel-air mixing and how it can be applied as a design tool which guides the optimisation of a fuel injector device. At an intermediate level of approximation, steady-state CFD simulations, based on the k–ε- and RSM-turbulence models are used to describe the mixing process. All steady simulations fail to either predict the recirculation zone or the turbulence level correctly, and can therefore not be expected to capture the mixing correctly. At the most involved level of modelling time-accurate CFD based on unsteady RSM and LES-turbulence models are performed. The simulations show good agreement with experiments (and in the case of LES excellent agreement) for both, velocity and turbulence fields. Mixing predictions close to the fuel injectors suffer from a simplification used in the numerical setup, but the mixing field is predicted very well towards the exit of the burner. The contribution of the asymmetric coherent flow structure (which is associated with the internal recirculation zone) to the mixing process is quantified through a triple decomposition technique.

On the Mathematical Modeling of Heat Transfer Characteristics of Turbulent Flames in Industrial Furnaces

The recent advances in numerical methods and the vast development of computers had directed the designers to better development and modifications to air flow pattern and heat transfer in combustion chambers. Extensive efforts are exerted to adequately predict the air velocity and turbulence intensity distributions in the combustor zones and to reduce the emitted pollution and noise abatement to ultimately produce quite and energy efficient combustor systems. The present work fosters mathematical modeling techniques to primarily predict what happens in three-dimensional combustion chambers simulating boiler furnaces, areo engines in terms of flow regimes and interactions. The present work also demonstrates the effect of chamber design and operational parameters on performance, wall heat transfer under various operating parameters. The governing equations of mass, momentum and energy are commonly expressed in a preset form with source terms to represent pressure gradients, turbulence and viscous action. The physical and chemical characteristics of the air and fuel are obtained from tabulated data in the literature. The flow regimes and heat transfer play an important role in the efficiency and utilization of energy. The results are obtained in this work with the aid of the three-dimensional program 3DCOMB; applied to axisymmetrical and three-dimensional complex geometry with and without swirl with liquid or gaseous fuels. The present numerical grid arrangements cover the combustion chamber in the X, R or Y and Z coordinates directions. The numerical residual in the governing equations is typically less than 0.001%. The obtained results include velocity vectors, turbulence intensities and wall heat transfer distributions in combusors. Examples of large industrial furnaces are shown and are in good agreement with available measurements in the open literature. One may conclude that flow patterns, turbulence and heat transfer in combustors are strongly affected by the inlet swirl, inlet momentum ratios, combustor geometry. Both micro and macro mixing levels are influential. The present modeling capabilities can adequately predict the local flow pattern and heat transfer characteristics in Complex combustors. Proper representation of the heat transfer and radiation flux is important in adequate predictions of large furnace performance.

Numerical Simulation of Combustion Air Systems for Large Industrial Boilers

Combustion air systems are an important aspect of the performance of industrial boilers and are increasingly being used as an integral means of reducing emissions. With the wide range of unique air system designs presently deployed, traditional methods of analyzing performance, based on experience, have become more difficult and less reliable. Numerical simulations are currently being used as a tool to analyze the operation and design of these air systems and how they relate to boiler performance and emission control. A proprietary CFD combustion code has been applied to a wide range of industrial boilers. Recent results from three selected studies are presented. These case studies demonstrate the benefits of using numerical simulation to: 1) analyze and optimize an existing air system, 2) validate an upgrade design, and 3) design and develop a new air system concept.

Schlieren and Shadowgraph Images of Transient Expanding Spherical Thin Flames

Experimental facilities have been built to visualize transient expanding spherical flames. Facilities include a cylindrical chamber with two end glasses for optical observation. Shadowgraph and Schlieren pictures of flame propagation have been taken using a high speed Charged Coupled Device (CCD) camera. In this paper the optical behavior of spherical flames has been investigated using both Schlieren and Shadowgraph methods. A mathematical model has been developed to predict the intensity of refracted light beams interacting with a transient expanding thin flame. Experimental results are in very good agreement with theoretical model. Schlieren and Shadowgraph techniques have also been used to view smooth, cracked and cellular flames, which are useful in determining the stability of propagating flame.

Development of a Monitoring System for Near Burner Slag Formation

A series of experiments on two different coals at a range of burner conditions have been conducted to investigate the behaviour of pf coal combustion on a 150kW pulverised fuel (pf) coal burner with a simulated eyebrow (a growth of slag in the near burner region). The simulation of a burner eyebrow was achieved by inserting an annulus of refractory material immediately in front of the face of the original burner quarl. Results obtained from monitoring the infrared (IR) radiation and sound emitted by the flame were processed into a number of features which were then used to train and test a self organising map neural network. Results obtained from the neural network demonstrated a classification success, never lower than 99.3%, indicate that it is not only possible to detect the presence of an eyebrow by monitoring the flame, but it is also possible to give an indication as to its size, over a reasonably large range of conditions.

Effect of Particle Size on Pipeline Flow of Solid-Liquid Slurry With Fixed Particle Size Distribution

The transport properties of solid-liquid slurries having the same well-defined particle size distribution but different median particle sizes have been studied in a 22-mm I.D. horizontal pipeline flow loop. The solid-liquid slurries were glass beads-water mixtures. The particle size distribution of solids was Rosin-Rammler with median diameters of 50 mm and 250 mm. The relationship between the pressure drop in the straight horizontal sections of the flow loop and the mean slurry velocity was determined for different solids volume concentrations varying from 4.5 to 25% and mean slurry velocity ranging from 0.5 to 2.5 m/s. Critical deposition velocity was measured from visual observations. An existing empirical model of Wasp et al. that predicts the pressure gradient for a single-species slurry flow in a horizontal pipeline was used to describe the pressure drop data. The Oroskar-Turian correlation for critical velocity was used for comparison with the measured critical velocities.