https://www.jfnc.ir/ Fuel and Combustion en daily 1 Wed, 13 Dec 2017 00:00:00 +0330 Wed, 13 Dec 2017 00:00:00 +0330 https://www.jfnc.ir/article_53995.html Biodiesel due to its environmental benefits and similar properties with gasoil is considered as fossil fuels alternative. Rapeseed oil is used to produce biodiesel in presence of K2CO3/Al2O3. For catalyst preparation K2CO3 was loaded on the Al2O3 as support using impregnation. Taguchi experimental design was used in a completely randomized design with two replications for trans esterification reaction and investigating the effect of reaction time, temperature and catalyst amount on the biodiesel production efficiency and optimize the number of experiments. Also Molar ratio (alcohol: oil) 15 to 1, catalyst amount 0.5, 1, 1.5, 2, 3 and 5 wt.%, reaction temperature of 55, 65 and 75 °C, reaction time of 0.5, 1, 1.5, 2, 2.5 and 3 hr. and stirring rate of 600 rpm was used for trans esterification reaction. Biodiesel yield (99%) was obtained by trans esterification process at 65 °C using 2 wt. % of K2CO3/Al2O3 in 2 hr. Other researcher’s results comparison shows that the loading ratio of K2CO3 compared to KNO3 and Ca(NO3)2 can increase the biodiesel production efficiency in alumina base catalysts. Analysis of variance analysis with regard to catalyst, reaction time and temperature variables indicates that temperature changes have no significant effect on the efficiency of methyl ester, but the effect of time and amount of catalyst variations used on the conversion rate of the reaction is quite significant. https://www.jfnc.ir/article_242580.html In ramjet and scramjet engines, due to the very high velocity of the incoming flow into the combustion chamber, efficiency and effective fuel–air mixing face significant challenges. One efficient method to increase fuel residence time and create a suitable recirculation zone for stable combustion is the use of cavity flameholders. Despite the widespread application of this method, the effect of cavity geometry on combustion performance in both ramjet and scramjet engines, especially under heated conditions, still requires comprehensive investigation. The aim of this study is to numerically investigate the effect of cavity geometric parameters—including height, length, and ramp angle—on combustion efficiency in both ramjet and scramjet engines. For this purpose, simulations were performed using ANSYS Fluent with the SST k–ω turbulence model in a compressible, hydrogen-fueled combustion configuration. The results indicate that increasing the cavity height in both engines improves combustion efficiency. Reducing the cavity length can also enhance combustion efficiency in both engines. These findings can serve as a basis for optimizing flameholder geometry in future supersonic engines. https://www.jfnc.ir/article_242581.html In this study, an air intake was designed for the combustion chamber of a supersonic projectile operating at flight conditions of Mach 3.4 and an altitude of 15 km. Initially, a three-dimensional design methodology for the supersonic air intake is presented. Subsequently, the designed intake was integrated with the combustion chamber, and its performance was evaluated. To ensure the accuracy of the analyses, the simulation process was validated against the results of an existing combustion chamber. The results indicate that the air intake’s performance closely aligns with the calculated theoretical values. The maximum error observed was 6.25%, corresponding to the Mach number at the first section of the intake. The obtained total pressure recovery factor also showed a 2.43% difference compared to the calculated values. The performance of the combustion chamber, using the airflow supplied by the intake, was examined, yielding a combustion temperature of 1298 K and a combustion efficiency of 83.5%. Furthermore, the distance between the air inlet and the fuel injection point was reduced to investigate its effect on combustion. It was determined that at a distance-to-diameter ratio of 1, the combustion efficiency increased by 3.9%. https://www.jfnc.ir/article_243197.html Double swirl burners are an advanced type of burner designed to improve combustion and increase flame stability. The burner used consists of two separate swirling air streams. In this paper, various experiments were performed on the burner. Subsequently, the results obtained from the experiments were repeated with a combustion chamber installed around the burner to analyze and investigate the changes that occurred. Key variables include the equivalence ratio, inner swirl number, outer swirl number, and flow split ratio.Double swirl burners are a type of advanced burner designed to improve combustion and increase flame stability and efficiency. The burner used in current consists of two separate swirling air streams. Various experiments have been performed and the results obtained from the tests have been repeated in the case of installing a chamber on the burner to analyze and examine the changes that have occurred. The basic variables include the equivalence ratio, inner swirl number, outer swirl number and flow split ratio. Tests conducted in both cases without a chamber and with a chamber indicate that adding a chamber generally limits the stability of the burner and in some cases results in a longer flame length. In addition, the results show that adding a chamber in some places causes the lean blowout limit to extend to lower equivalence ratios. It is also shown that in certain conditions, increasing the outer flow split ratio leads to a shorter flame length and an increase in flame width; both effects were simultaneously observed in the flame area as well. In addition, at the points that were performed for both configuration (with and without the chamber), with a certain confinement ratio, the flame liftoff height did not change. https://www.jfnc.ir/article_245174.html The gas turbine combustion chamber, as one of the key components, has been extensively studied. The temperature distribution in the combustion chamber is of great importance because it can not only lead to changes in the composition of exhaust gases but also cause significant damage to the chamber walls or turbine blades. Among the methods for investigating combustion chambers, analytical methods, with appropriate accuracy and high simplicity and computational speed, can provide an effective assessment of combustion chamber performance. In this research, the distribution of inlet air to a laboratory-scale cylindrical combustion chamber was initially investigated and validated using a one-dimensional code developed in the MATLAB environment. Subsequently, combustion and temperature distributions along the chamber were simulated and validated using the CANTERA software through a Chemical Reactor Network (CRN). Following this, the sensitivity of control parameters such as the number of reactors, the initial reactor temperature, and the number of time steps in the chamber was investigated. The results showed that increasing the number of reactors up to 50, although providing a more detailed temperature distribution along the combustion chamber, did not improve solution accuracy and even led to computational errors due to an excessive reduction in reactor size. Varying the initial temperature up to 2300 K had no noticeable effect on solution accuracy. In addition, a 50fold increase in the number of time steps from 100 to 5000 showed no improvement in solution accuracy but resulted in an approximately twofold increase in computational time. https://www.jfnc.ir/article_246841.html MILD combustion has emerged as a promising technology for achieving uniform temperature distribution, reduced pollutant formation, and improved thermal efficiency in industrial reheating furnaces. However, the performance of different burner geometries under MILD conditions—particularly in full-scale steel‑heating scenarios—has not been adequately characterized. In this study, the thermal behavior of three burner configurations, namely a co‑flow axial burner, a swirl burner, and a perforated lining burner, was experimentally investigated in a laboratory-scale MILD furnace developed at Tarbiat Modares University. The primary objective was to Steel reheating furnaces are critical in metallurgical industries for preparing billets or slabs prior to hot forming. Conventional combustion systems in these furnaces often create large temperature gradients and emit high levels of nitrogen oxides (NOₓ). Flameless oxidation, also known as Moderate or Intense Low‑oxygen Dilution (MILD) combustion, has emerged as a promising alternative. By strongly diluting fuel and oxidizer with recirculated combustion products before ignition, MILD combustion suppresses visible flame formation, reduces peak temperatures, and promotes a more uniform thermal field within the furnace volume. Despite these advantages, the performance of MILD combustion systems is highly sensitive to burner geometry and the resulting mixing characteristics of fuel and oxidizer streams. Different burner configurations can produce distinct flow structures, internal recirculation rates, and temperature distributions. Therefore, systematic experimental evaluation of burner designs is essential for optimizing industrial reheating furnaces operating under MILD conditions. The present work addresses this need by experimentally comparing three burner types—co‑flow axial, swirl, and perforated lining—in a purpose‑built laboratory‑scale MILD furnace. The study focuses on both heating rate and flame stability, providing practical insights for burner selection in steel reheating applications. Methodology Laboratory‑Scale MILD Furnace Experiments were conducted in a laboratory‑scale flameless oxidation furnace designed and constructed at the National Combustion Laboratory of Tarbiat Modares University. The furnace was scaled down from a 7 MW industrial steel reheating furnace using the constant residence time (CRT) scaling method, resulting in a 10 kW laboratory model. Key dimensions of the scaled furnace are 919 mm (length) × 513 mm (width) × 293 mm (height). The furnace walls and roof were insulated with multi‑layer refractory materials (cordierite‑mullite, lightweight firebrick, and perlite) to maintain external surface temperatures below 80 °C during operation up to 1600 °C. Burner Configurations Three distinct burner geometries were fabricated and tested: · Co‑flow axial burner: A 1‑inch diameter burner with an axially adjustable fuel tube. The fuel nozzle could be positioned upstream or downstream of the air exit to control premixing and flame stability. Optimal stability was achieved with the fuel tube retracted by 2.5 cm relative to the air outlet. · Swirl burner: A 1‑inch burner featuring a helical swirler (4 cm pitch) along the fuel tube. Air flowing over this spiral acquires a tangential velocity component, inducing a swirling motion at approximately 45° relative to the burner axis. This enhances mixing, flame volume, and stability. · Perforated lining burner: A 1‑inch burner consisting of a perforated inner tube (½‑inch, 16 holes of 3 mm diameter) enclosed within a perforated outer casing (1‑inch, holes of ~1.5 mm, porosity ≈ 0.26). The design promotes distributed, multi‑point injection for uniform heating and increased power capacity (up to 30 kW). Instrumentation and Procedure Each burner was installed separately in the furnace, and tests were performed under comparable firing rates and air preheating conditions. Fuel (natural gas) and air flow rates were controlled via calibrated rotameters and pressure regulators. Temperatures were measured using S‑type thermocouples (for high‑temperature regions up to 1600 °C) and J‑type thermocouples (for stack and lower‑temperature zones). Thermocouples were arranged in a 9‑column, 3‑row grid on the sidewall to capture spatial temperature distributions. A CCD camera recorded flame behavior, and RGB image analysis was employed to assess flame uniformity and thermal structure. Results and Discussion Flame Structure and Stability The co‑flow axial burner produced a long, narrow flame (≈35 cm) with relatively low momentum. The flame exhibited sensitivity to hot gas recirculation from the furnace rear wall, leading to intermittent instabilities. Due to insufficient mixing at an equivalence ratio of ~1.1, unburned gases were observed in the furnace. In contrast, the swirl burner generated a voluminous, helical flame (≈40 cm) with significantly enhanced mixing and momentum. The swirling motion stabilized the flame, prevented lift‑off, and improved heat transfer to the furnace walls. However, increased turbulence led to moderate flame oscillations over time. The perforated lining burner, when operated at 18.4 kW, produced a uniform, continuous flame sheet across its surface. During initial startup, distinct flame plumes were visible, but as the burner body heated up, the plumes diminished, and combustion transitioned toward a distributed, nearly flameless regime—characteristic of high‑temperature air combustion (HiCOT). Unfortunately, prolonged exposure to high temperatures caused severe surface oxidation and eventual localized fracture of the burner tube, as shown in Figure 12 of the full manuscript. Thermal Performance and Heating Rate Figure 14 of the original study illustrates the temporal response of furnace roof temperature for each burner: · The co‑flow axial burner (10 kW) required approximately 450 minutes (7.5 hours) to reach 500 °C and exhibited no further temperature increase, indicating limited thermal capacity. · The perforated lining burner (18.4 kW) rapidly heated the furnace, reaching 550 °C in just 150 minutes (2 hours 33 minutes). However, the test was terminated prematurely due to burner structural failure. · The swirl burner (10 kW) achieved 550 °C after approximately 7 hours and surpassed 600 °C after 10 hours, with a continuing upward trend. This demonstrates reliable and sustained heating capability. Stack gas temperature measurements (Figure 15) corroborated these trends: the swirl burner achieved the highest exhaust temperature (~213 °C) with an ongoing rising trend, whereas the co‑flow burner plateaued at lower values. RGB Image Analysis RGB intensity scatter plots (Figure 13) provided quantitative insight into flame uniformity. The co‑flow burner displayed the most concentrated and consistent color distribution, suggesting a homogeneous thermal field at the captured moment, though long‑term stability remained problematic. The swirl burner showed increased scatter—particularly in the blue channel—indicating the presence of turbulent, multi‑temperature zones. The perforated lining burner exhibited the widest dispersion, reflecting significant local temperature fluctuations inherent to its multi‑jet design. Conclusions This experimental investigation compared the performance of co‑flow axial, swirl, and perforated lining burners in a laboratory‑scale MILD furnace for steel reheating. The following conclusions are drawn: · Co‑flow axial burner: Produces an elongated flame but suffers from low momentum and instability when exposed to recirculating hot gases. Its heating capacity is insufficient for reaching target temperatures within a reasonable timeframe. · Perforated lining burner: Offers the fastest heating rate, achieving 550 °C in 2 hours 33 minutes. However, severe oxidation and structural failure at elevated temperatures render it unsuitable for long‑term industrial use without expensive alloy upgrades. · Swirl burner: Provides the most stable and uniform heating performance. Although its heating rate is slower than that of the linear burner, it reliably attains and exceeds 600 °C without structural degradation. Scaling this design to larger diameters (e.g., 1.5–2 inches) could further improve heating rate while maintaining stability. Overall, the swirl burner emerges as the most practical and robust option for industrial MILD furnaces, balancing thermal uniformity, operational safety, and long‑term durability. The findings underscore the critical influence of burner geometry on furnace startup time and temperature distribution, offering valuable guidance for optimizing burner selection in steel reheating applications. Author Contributions Ali Ashouri: Conceptualization, Methodology, Investigation, Formal analysis, Writing – original draft, Writing – review & editing, Visualization, Project administration. Soroush Sarrafan Sadeghi: Investigation, Formal analysis, Data curation, Writing – review & editing, Visualization. Mohammad Zabetian Targhi: Supervision, Conceptualization, Resources, Writing – review & editing, Funding acquisition. Funding This research received no specific grant from any funding agency in the public, commercial, or not‑for‑profit sectors. The study was conducted entirely at Tarbiat Modares University, Tehran, Iran, using the facilities of the National Combustion Laboratory. Conflict of Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Steel reheating furnaces are critical in metallurgical industries for preparing billets or slabs prior to hot forming. Conventional combustion systems in these furnaces often create large temperature gradients and emit high levels of nitrogen oxides (NOₓ). Flameless oxidation, also known as Moderate or Intense Low‑oxygen Dilution (MILD) combustion, has emerged as a promising alternative. By strongly diluting fuel and oxidizer with recirculated combustion products before ignition, MILD combustion suppresses visible flame formation, reduces peak temperatures, and promotes a more uniform thermal field within the furnace volume. Despite these advantages, the performance of MILD combustion systems is highly sensitive to burner geometry and the resulting mixing characteristics of fuel and oxidizer streams. Different burner configurations can produce distinct flow structures, internal recirculation rates, and temperature distributions. Therefore, systematic experimental evaluation of burner designs is essential for optimizing industrial reheating furnaces operating under MILD conditions. The present work addresses this need by experimentally comparing three burner types—co‑flow axial, swirl, and perforated lining—in a purpose‑built laboratory‑scale MILD furnace. The study focuses on both heating rate and flame stability, providing practical insights for burner selection in steel reheating applications. Methodology Laboratory‑Scale MILD Furnace Experiments were conducted in a laboratory‑scale flameless oxidation furnace designed and constructed at the National Combustion Laboratory of Tarbiat Modares University. The furnace was scaled down from a 7 MW industrial steel reheating furnace using the constant residence time (CRT) scaling method, resulting in a 10 kW laboratory model. Key dimensions of the scaled furnace are 919 mm (length) × 513 mm (width) × 293 mm (height). The furnace walls and roof were insulated with multi‑layer refractory materials (cordierite‑mullite, lightweight firebrick, and perlite) to maintain external surface temperatures below 80 °C during operation up to 1600 °C. Burner Configurations Three distinct burner geometries were fabricated and tested: · Co‑flow axial burner: A 1‑inch diameter burner with an axially adjustable fuel tube. The fuel nozzle could be positioned upstream or downstream of the air exit to control premixing and flame stability. Optimal stability was achieved with the fuel tube retracted by 2.5 cm relative to the air outlet. · Swirl burner: A 1‑inch burner featuring a helical swirler (4 cm pitch) along the fuel tube. Air flowing over this spiral acquires a tangential velocity component, inducing a swirling motion at approximately 45° relative to the burner axis. This enhances mixing, flame volume, and stability. · Perforated lining burner: A 1‑inch burner consisting of a perforated inner tube (½‑inch, 16 holes of 3 mm diameter) enclosed within a perforated outer casing (1‑inch, holes of ~1.5 mm, porosity ≈ 0.26). The design promotes distributed, multi‑point injection for uniform heating and increased power capacity (up to 30 kW). Instrumentation and Procedure Each burner was installed separately in the furnace, and tests were performed under comparable firing rates and air preheating conditions. Fuel (natural gas) and air flow rates were controlled via calibrated rotameters and pressure regulators. Temperatures were measured using S‑type thermocouples (for high‑temperature regions up to 1600 °C) and J‑type thermocouples (for stack and lower‑temperature zones). Thermocouples were arranged in a 9‑column, 3‑row grid on the sidewall to capture spatial temperature distributions. A CCD camera recorded flame behavior, and RGB image analysis was employed to assess flame uniformity and thermal structure. Results and Discussion Flame Structure and Stability The co‑flow axial burner produced a long, narrow flame (≈35 cm) with relatively low momentum. The flame exhibited sensitivity to hot gas recirculation from the furnace rear wall, leading to intermittent instabilities. Due to insufficient mixing at an equivalence ratio of ~1.1, unburned gases were observed in the furnace. In contrast, the swirl burner generated a voluminous, helical flame (≈40 cm) with significantly enhanced mixing and momentum. The swirling motion stabilized the flame, prevented lift‑off, and improved heat transfer to the furnace walls. However, increased turbulence led to moderate flame oscillations over time. The perforated lining burner, when operated at 18.4 kW, produced a uniform, continuous flame sheet across its surface. During initial startup, distinct flame plumes were visible, but as the burner body heated up, the plumes diminished, and combustion transitioned toward a distributed, nearly flameless regime—characteristic of high‑temperature air combustion (HiCOT). Unfortunately, prolonged exposure to high temperatures caused severe surface oxidation and eventual localized fracture of the burner tube, as shown in Figure 12 of the full manuscript. Thermal Performance and Heating Rate Figure 14 of the original study illustrates the temporal response of furnace roof temperature for each burner: · The co‑flow axial burner (10 kW) required approximately 450 minutes (7.5 hours) to reach 500 °C and exhibited no further temperature increase, indicating limited thermal capacity. · The perforated lining burner (18.4 kW) rapidly heated the furnace, reaching 550 °C in just 150 minutes (2 hours 33 minutes). However, the test was terminated prematurely due to burner structural failure. · The swirl burner (10 kW) achieved 550 °C after approximately 7 hours and surpassed 600 °C after 10 hours, with a continuing upward trend. This demonstrates reliable and sustained heating capability. Stack gas temperature measurements (Figure 15) corroborated these trends: the swirl burner achieved the highest exhaust temperature (~213 °C) with an ongoing rising trend, whereas the co‑flow burner plateaued at lower values. RGB Image Analysis RGB intensity scatter plots (Figure 13) provided quantitative insight into flame uniformity. The co‑flow burner displayed the most concentrated and consistent color distribution, suggesting a homogeneous thermal field at the captured moment, though long‑term stability remained problematic. The swirl burner showed increased scatter—particularly in the blue channel—indicating the presence of turbulent, multi‑temperature zones. The perforated lining burner exhibited the widest dispersion, reflecting significant local temperature fluctuations inherent to its multi‑jet design. Conclusions This experimental investigation compared the performance of co‑flow axial, swirl, and perforated lining burners in a laboratory‑scale MILD furnace for steel reheating. The following conclusions are drawn: · Co‑flow axial burner: Produces an elongated flame but suffers from low momentum and instability when exposed to recirculating hot gases. Its heating capacity is insufficient for reaching target temperatures within a reasonable timeframe. · Perforated lining burner: Offers the fastest heating rate, achieving 550 °C in 2 hours 33 minutes. However, severe oxidation and structural failure at elevated temperatures render it unsuitable for long‑term industrial use without expensive alloy upgrades. · Swirl burner: Provides the most stable and uniform heating performance. Although its heating rate is slower than that of the linear burner, it reliably attains and exceeds 600 °C without structural degradation. Scaling this design to larger diameters (e.g., 1.5–2 inches) could further improve heating rate while maintaining stability. Overall, the swirl burner emerges as the most practical and robust option for industrial MILD furnaces, balancing thermal uniformity, operational safety, and long‑term durability. The findings underscore the critical influence of burner geometry on furnace startup time and temperature distribution, offering valuable guidance for optimizing burner selection in steel reheating applications. Author Contributions Ali Ashouri: Conceptualization, Methodology, Investigation, Formal analysis, Writing – original draft, Writing – review & editing, Visualization, Project administration. Soroush Sarrafan Sadeghi: Investigation, Formal analysis, Data curation, Writing – review & editing, Visualization. Mohammad Zabetian Targhi: Supervision, Conceptualization, Resources, Writing – review & editing, Funding acquisition. Funding This research received no specific grant from any funding agency in the public, commercial, or not‑for‑profit sectors. The study was conducted entirely at Tarbiat Modares University, Tehran, Iran, using the facilities of the National Combustion Laboratory. Conflict of Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.