Optimization of Heat Transfer Process in a Walking Beam Reheat Furnace Using Computational Fluid Dynamics

In the steelmaking process, reheating furnaces are used to reheat steel slabs to a target rolling temperature. The bottom intermediate zone inside the reheating furnace plays a decisive role in controlling the slab temperature distribution before slabs enter the soaking zone. Efforts to maintain a uniform slab surface temperature and thus enhance product quality require a good understanding of the furnace’s operation. However, traditional physical experiments are costly and have high risks as well. In this study, a three-dimensional steady-state computational fluid dynamics (CFD) model was developed to investigate the flow field in the bottom intermediate zone of a full-scale reheating furnace. The commercial software ANSYS Fluent® was used to solve the transport equations to predict the flame length, heat transfer, and gas temperature near the slab. Total input mass flow rate, preheated air temperature, and air/fuel ratio were selected to investigate the comprehensive influence of the furnace’s performance, which can be evaluated from the flame length, flame angle, and average gas temperature near the slab. Importantly, an orthogonal experimental design was conducted to optimize the evaluation factors by considering the multi influencing factors simultaneously. The simulation results indicate that a higher mass flow rate produces a lower upwards flame angle, which can prevent the hot spot detected on the slab surface. A higher preheated air temperature leads to a higher average gas temperature in this furnace; meanwhile, the flame becomes shorter by enhancing the air-fuel ratio.

The Effect of Nitrogen on Flame Characteristics in Biogas External Premixed Combustion

Biogas contains more than 50% methane (CH4), is a renewable and eco-friendly fuel produced by bacterial action. Not only is biogas flammable but it also contains inhibitors like carbon dioxide and nitrogen, as well as small amounts of H2, O2, H2S and others. Several associated studies have been conducted in order to examine biogas combustion characteristics in external combustion and flame angle, flame height and dimensionless flame height are the important characteristics in external premixed combustion. This research’s aims were to discover the influences of N2 as it is the second most prevalent inhibitor in biogas by burning stoichiometric fuel mixtures (CH4 and N2 (0%-50% of fuel)) and oxygen in an experimental external premixed combustion burner whose nozzle tip diameter was 5 mm. The burner was connected to a hose from the oxygen tank and another hose from the fuel tank. Two regulators and flowmeters were placed on each tank to monitor the flow supplied to the mixer and burner. The valves were used to stop or open the fluid supply. The outcome flame propagation is then recorded by a high speed camera and then processed through a computer system. The results indicate that N2 influenced the flame angle, flame height and dimensionless flame height. The higher the N2 content inside the fuel, the shorter the flame height and the lower the dimensionless flame height. Moreover, increasing the N2 content created larger the flame angle.

Characteristics of diffusion flames with accelerated motion

The aim of this work is to present an experiment to study the characteristics of a laminar diffusion flame under acceleration. A Bunsen burner (nozzle diameter 8 mm), using liquefied petroleum gas as its fuel, was ignited under acceleration. The temperature field and the diffusion flame angle of inclination were visualised with the assistance of the visual display technology incorporated in MATLAB?. Results show that the 2-d temperature field under different accelerations matched the variation in average temperatures: they both experience three variations at different time and velocity stages. The greater acceleration has a faster change in average temperature with time, due to the accumulation of combustion heat: the smaller acceleration has a higher average temperature at the same speed. No matter what acceleration was used, in time, the flame angle of inclination increased, but the growth rate decreased until an angle of 90?: this could be explained by analysis of the force distribution within the flame. It is also found that, initially, the growth rate of angle with velocity under the greater acceleration was always smaller than that at lower accelerations; it was also different in flames with uniform velocity fire conditions.

Characterization of Forced Flame Response of Swirl-Stabilized Turbulent Lean-Premixed Flames in a Gas Turbine Combustor

Flame transfer function measurements of turbulent premixed flames are made in a model lean-premixed, swirl-stabilized, gas turbine combustor. OH∗, CH∗, and CO2∗ chemiluminescence emissions are measured to determine heat release oscillation from a whole flame, and the two-microphone technique is used to measure inlet velocity fluctuation. 2D CH∗ chemiluminescence imaging is used to characterize the flame shape: the flame length (LCH∗ max) and flame angle (α). Using H2-natural gas composite fuels, XH2=0.00–0.60, a very short flame is obtained and hydrogen enrichment of natural gas is found to have a significant impact on the flame structure and flame attachment points. For a pure natural gas flame, the flames exhibit a “V” structure, whereas H2-enriched natural gas flames have an “M” structure. Results show that the gain of M flames is much smaller than that of V flames. Similar to results of analytic and experimental investigations on the flame transfer function of laminar premixed flames, it shows that the dynamics of a turbulent premixed flame is governed by three relevant parameters: the Strouhal number (St=LCH∗ max/Lconv), the flame length (LCH∗ max), and the flame angle (α). Two flames with the same flame shape exhibit very similar forced responses, regardless of their inlet flow conditions. This is significant because the forced flame response of a highly turbulent, practical gas turbine combustor can be quantitatively generalized using the nondimensional parameters, which collapse all relevant input conditions into the flame shape and the Strouhal number.

Characterization of Forced Flame Response of Swirl-Stabilized Turbulent Lean-Premixed Flames in a Gas Turbine Combustor

Flame transfer function measurements of turbulent premixed flames were made in a model lean premixed, swirl-stabilized, gas turbine combustor. OH*, CH*, and CO2* chemiluminescence emissions were measured to determine heat release oscillation from a whole flame, and the two-microphone technique was used to measure inlet velocity fluctuation. 2-D CH* chemiluminescence imaging was used to characterize the flame shape: the flame length (LCH* max) and flame angle (α). Using H2-natural gas composite fuels, XH2 = 0.00 ∼ 0.60, very short flame was obtained and hydrogen enrichment of natural gas had a significant impact on the flame structure and flame attachment points. For a pure natural gas flame, the flames exhibit a “V” structure, whereas H2-enriched natural gas flames have an “M” structure. Results show that the gain of “M” flames is much smaller than that of “V” flames. Similar to results of analytic and experimental investigations on the flame transfer function of laminar premixed flames, it shows that the dynamics of a turbulent premixed flame is governed by three relevant parameters: the Strouhal number (St = LCH* max / Lconv), the flame length (LCH* max), and the flame angle (α). Two flames with the same flame shape exhibit very similar forced responses, regardless of their inlet flow conditions. This is significant because the forced flame response of a highly turbulent, practical gas turbine combustor can be quantitatively generalized using the non-dimensional parameters which collapse all relevant input conditions into the flame shape and the Strouhal number.

Flame characteristics, temperature - time curves, and rate of spread in fires propagating in a bed of Pinus pinaster needles

An extensive set of experiments was carried out in order to collect data to validate fire propagation models being developed in the context of an European research project. The experiments were performed in a dedicated burning tray (2.0 m × 0.70 m working section), where wind velocity, fuel moisture content and slope were varied to study fire propagation in beds of Pinus pinaster needles. All the runs were videotaped and, from the recordings, information on flame geometry (i.e. flame height, flame length and flame angle) and rate of spread was obtained. Temperature measurements were also carried out by a small tower of six thermocouples at different heights above the fuel bed. Results show that headfire rate of spread increases steeply with wind speed for wind-driven fires but does not depend on wind speed for backing fire spread rates. Rate of spread increases slightly with slope for up-hill propagation, and is not slope dependent for down-hill cases. Rate of spread decreases when fuel moisture content increases. Flame angle and flame height are also dependent on wind velocity, slope, and fuel moisture content. The importance of temperature for fire propagation is discussed, emphasizing the role of radiation heat transfer in the process. Correlations between temperature and other indicators of fire behaviour (namely the rate of spread) are presented. Results are discussed and compared. The results obtained provide a good database for the assessment of fire propagation models.

Effects of wind velocity and slope on flame properties

The combined effects of wind velocity and percent slope on flame length and angle were measured in an open-topped, tilting wind tunnel by burning fuel beds composed of vertical birch sticks and aspen excelsior. Mean flame length ranged from 0.08 to 1.69 m; 0.25 m was the maximum observed flame length for most backing fires. Flame angle ranged from −46° to 50°. Observed flame angle and length data were compared with predictions from several models applicable to fires on a horizontal surface. Two equations based on the Froude number underestimated flame angle for most wind and slope combinations; however, the data support theory that flame angle is a function of the square root of the Froude number. Discrepancies between data and predictions were attributed to measurement difficulties and slope effects. An equation based on Byram's convection number accounted for nearly half of the observed variation in flame angle (R2 = 0.46). Byram's original equation relating fireline intensity to flame length overestimated flame length. New parameter estimates were derived from the data. Testing of observed fire behavior under a wider range of conditions and at field scale is recommended.