Air-fuel Combustion Research Articles

Numerical analysis of the transient heating of steel billets and the combustion process under air-fired and oxygen enriched conditions

A three-dimensional analysis of the gas phase combustion and transient heating of steel billets in an industrial walking hearth furnace was done by CFD. An iterative solution procedure was used for the steady-state simulation of the reactive flow and the heat conduction in the billets for air-fired and oxygen enriched combustion with 25vol% O2 and 75vol% N2 in the oxidizer. Information about the turbulent flow, species concentrations, temperatures and heat fluxes in the furnace was obtained with low computational demand due to the used solution procedure. Modelling of the gas phase combustion considered 17 species and 25 reversible reactions including radical formation. According to operating conditions of the furnace, a gas saving of 8% was determined due to the oxygen enrichment. Although the fuel input was reduced with increasing oxygen concentration, the calculated heat fluxes to the billets were similar due to the oxygen enrichment. Furthermore, a higher heating rate was achieved in the heating zone compared to air-fuel combustion, as a consequence of the increased radiative heat transfer. The overall efficiency of the reheating process was raised from 57.6% (air-fuel) to 61.4% (oxygen enriched). Surface temperature and temperature uniformity of the billets were calculated using transient simulations and investigated for the two combustion cases. In air-fuel combustion, a faster heating of the billets was detected due to the higher convective heat flux in the pre-heating zone. This effect in the air-fuel case was compensated in the following heating zone in oxygen enriched combustion by radiation. This study shows that higher efficiency of reheating processes can be achieved by means of oxygen enrichment with a similar heating characteristic of the furnace load.

PERFORMANCE ANALYSIS OF A BIOGAS ENGINE

A single zone S.I. engine model based on the physical geometries (bore, stroke, crank radius, and connecting rod length), variation of specific heats of gas mixture (air-fuel mixture and combustion products) and chemical equilibrium has been developed in an in-house code to predict the pressure and temperature variation along crank angle, work done, mean effective pressure, power output and other engine parameters suitable for biogas. The intake model has also been incorporated in the developed model. Model has been validated using biogas (methane). Effect of spark timing in spark ignition model have been observed and optimized to obtain maximum work as well as brake mean effective pressure with biogas. It has been observed that at 27° BTDC spark timing give the best performance. The spark ignition model at optimized condition, the brake thermal efficiency is calculated as 24% and brake mean specific fuel consumption is 0.29 m³/kWh.

Effect of water on tin penetration in subsurface layer of glass

Since the float glass produced by oxy-fuel combustion technology contains higher structural water than that produced by air-fuel combustion, ‘bloom phenomenon’ is a serious issue. In order to study the effect of water in the glass on the tin penetration, glass with different water contents was lab-prepared and the glass forming process was simulated by floating glass melt on the surface of liquid tin. Water content in glass was determined by Fourier translation infrared. The penetrated amount of tin and its distribution in glass were analysed by X-ray fluorescence spectrometer and Electron Probe MicroAnalyer (wavelength Dispersive Spectrometer). The results showed that the water content in glass strongly depended on the moisture concentration in the melting atmosphere. Meanwhile, it was found that both the penetrated amount of tin and the penetration depth in glass were related to the water content in glass.

YSZ-Polyester Abradable Coatings Manufactured by APS

Abradable seals are used in aircraft engine compressor and turbine to decrease fuel consumption. Their role is to minimize the clearance between the rotating blades of an engine rotor and the casing to reduce air leakages (compressor) or air-fuel combustion product leakages (turbine). Operating temperatures in turbines (up to 1800 °C) can induce a thermal expansion of the blades and give rise to contacts providing damages to the blades or casing. Thus, in case of contact, the blade should remove the abradable seals applied to the casing without being damaged. Besides, the seal must be resistant to the turbine environment. Direct relationships have been observed between plasma spray operating parameters and abradable coating performances. The aim of this study is to determine those relationships for YSZ-Polyester abradable composite coatings. This study is conducted within the framework of the 7FP European project E-BREAK to reach the environmental objectives of the European Advisory Council for Aviation Research and innovation.

A comprehensive evaluation of different radiation models in a gas turbine combustor under conditions of oxy-fuel combustion with dry recycle

The oxy-fuel combustion is a promising CO2 capture technology from combustion systems. This process is characterized by much higher CO2 concentrations in the combustion system compared to that of the conventional air-fuel combustion. To accurately predict the enhanced thermal radiation in oxy-fuel combustion, it is essential to take into account the non-gray nature of gas radiation. In this study, radiation heat transfer in a 3D model gas turbine combustor under two test cases at 20atm total pressure was calculated by various non-gray gas radiation models, including the statistical narrow-band (SNB) model, the statistical narrow-band correlated-k (SNBCK) model, the wide-band correlated-k (WBCK) model, the full spectrum correlated-k (FSCK) model, and several weighted sum of gray gases (WSGG) models. Calculations of SNB, SNBCK, and FSCK were conducted using the updated EM2C SNB model parameters. Results of the SNB model are considered as the benchmark solution to evaluate the accuracy of the other models considered. Results of SNBCK and FSCK are in good agreement with the benchmark solution. The WBCK model is less accurate than SNBCK or FSCK. Considering the three formulations of the WBCK model, the multiple gases formulation is the best choice regarding the accuracy and computational cost. The WSGG model with the parameters of Bordbar et al. (2014) [20] is the most accurate of the three investigated WSGG models. Use of the gray WSSG formulation leads to significant deviations from the benchmark data and should not be applied to predict radiation heat transfer in oxy-fuel combustion systems. A best practice to incorporate the state-of-the-art gas radiation models for high accuracy of radiation heat transfer calculations at minimal increase in computational cost in CFD simulation of oxy-fuel combustion systems for pressure path lengths up to about 10barm is suggested.

Effects of Nozzle Diameter on the Spray Characteristics of Premix Injector in Burner System

An essential component of the injector nozzle geometry is to see the results spray atomization and mixture formation of the fuel-air combustion to improve performance, and reduce pollution from a burner. Studies involving the injectors in the combustion burner are still in a small proportion, particularly in the premix injector type. Thus, this study involves the efforts to determine the appropriate diameter of the premix injector where the injector spray characteristics is produced by using Computational Fluid Dynamics (CFD). Multiphase of the volume of fluid (VOF) cavitations flow in the nozzle is determined through steady simulation while Eulerian-Eulerian two-fluid approach is used for performing mixing of Jatropha oil and air. Further simulation is conducted using a spray with a discrete phase injection at the outflow hole injector nozzle. The investigation involves the modification of nozzle geometry on three different sizes of 0.8 mm, 1.0 mm and 1.5 mm with the analysis focused on nozzle flow characteristics of the injector. The results indicate that a small changes in injector gives high impact to the spray and combustion of a burner. This shows the importance of nozzle dimensions which influences the nozzle flow and affects the spray characteristics, hence influence the combustion and emission of the burner system.

Improvement in thermal efficiency of regenerator system by using oxy-fuel combustion

In this experimental study, a new system combining the oxy-fuel combustion with the concept of regeneration is developed. The performance of the regenerator system, which consists of a pair of oxy-fuel burners and regenerators, is demonstrated. To improve the thermal efficiency of the oxy-fuel combustion-based regenerator system, new methods are considered, such as exhaust gas bypass and CO2 recirculation. The regeneration characteristics obtained when using oxy-fuel combustion are investigated by applying each method to the oxy-fuel combustion-based regenerator system.The temperature variation in the gas passing through the regenerator is significantly affected by the different operating methods and design parameters such as the switching time, regenerator configuration, and regenerator weight. The reference method, which is the same as air-fuel combustion, shows a low heat recovery ratio in the regenerator, whereas the exhaust gas bypass method has the highest value of the heat recovery ratio, followed by the CO2 recirculation method, because of an improvement in the enthalpy imbalance between the exhaust gas and regeneration gas. From the study results, it is confirmed that the use of the exhaust gas bypass method or CO2 recirculation method should be effective in improving the thermal efficiency of the oxy-fuel combustion-based regenerator system.

Numerical simulations of the industrial circulating fluidized bed boiler under air- and oxy-fuel combustion

Measured and numerical results of air-fuel combustion process within large scale industrial circulating fluidized bed (CFB) boiler is presented in this paper. For numerical simulations the industrial compact CFB boiler was selected. Numerical simulations were carried out using three-dimensional model where the dense particulate transport phenomenon was simultaneously modelled with combustion process. The fluidization process was modelled using the hybrid Euler-Lagrange approach. The impact of the geometrical model simplification on predicted mass distribution and temperature profiles over CFB boiler combustion chamber two kinds of geometrical models were used, namely the complete model which consist of combustion chamber, solid separators, external solid super-heaters and simplified boiler geometry which was reduced to the combustion chamber. The evaluated temperature and pressure profiles during numerical simulations were compared against measured data collected during boiler air-fuel operation. Collected data was also used for validating numerical model of the oxy-fuel combustion model. Stability of the model and its sensitivity on changes of several input parameters were studied. The comparison of the pressure and temperature profiles for all considered cases gave comparable trends in contrary to measured data. Moreover, some additional test was carried out the check the influence of radiative heat transfer on predicted temperature profile within the CFB boiler.

An Integrated Model of Energy Transport in a Reciprocating, Lean Burn, Spark Ignition Engine

This paper presents a combined experimental and numerical method for analysing energy flows within a spark ignition engine. Engine dynamometer data is combined with physical models of in-cylinder convection and the engine's thermal impedances, allowing closure of the First Law of Thermodynamics over the entire engine system. In contrast to almost all previous works, the coolant and metal temperatures are not assumed constant, but rather are outputs from this approach. This method is therefore expected to be most useful for lean burn engines, whose temperatures should depart most from normal experience. As an example of this method, the effects of normalised air-fuel ratio (λ), compression ratio and combustion chamber geometry are examined using a hydrogen-fueled engine operating from λ = 1.5 to λ = 6. This shows large variations in the in-cylinder wall temperatures and heat transfer with respect to λ. In keeping with our other works, thermal efficiency also appears to be limited by in-cylinder heat transfer on the rich side of optimum λ, and diminishing combustion quality on the lean side. By comparing different compression ratios, this method confirms the widely-reported significance of in-cylinder heat transfer in determining the optimal compression ratio of hydrogen-fueled engines. As part of this, while increased in-cylinder pressures contribute to increased heat transfer at higher compression ratio, the accompanying increased piston surface area was also found to be significant. As a final contribution, potential improvements in piston design are therefore investigated numerically, and suggestions for high efficiency, hydrogen fuelled engine designs are made.

Design and Control of the Dry Methane Reforming Process

The much lower cost of natural gas and the desire to reduce carbon dioxide emissions have stimulated interest in the dry methane reforming process in which these two gases react to produce synthesis gas. The ratio of hydrogen to carbon monoxide in the resulting synthesis gas is close to unity, which makes it suitable for feeding to the Fischer–Tropsch process to produce liquid transportation fuel. The dry methane reforming reaction is favored by low pressure, but high pressure synthesis gas is required in the downstream process. Higher reactor pressures reduce compression costs of the synthesis gas. However, higher pressures also require a higher reactor temperature to achieve high methane conversion. When a maximum reactor temperature limitation is encountered, high methane conversion can be maintained by operating with a higher than stoichiometric CO2-to-CH4 ratio. But this means an excess of carbon dioxide must be fed to the reactor and subsequently recycled, which imposes additional compression costs. These competing effects produce interesting design trade-offs, which are explored in this paper. The dynamic control of the process is also studied. The dominant issue is the modeling of the reactor, which is a fired furnace with combustion of fuel providing the endothermic heat of the syngas reaction. Dynamic modeling is achieved by using a tubular process reactor in which the heat flux is determined by the heat generated in a fuel–air combustion reactor.

Characteristics of Oxyfuel and Air–Fuel Combustion in an Industrial Water Tube Boiler

The characteristics of oxyfuel combustion and air–fuel combustion in the furnace of a typical industrial water tube boiler using methane as the operating fuel are investigated. Two oxyfuel cases are considered. The analysis is conducted for two oxyfuel cases that correspond to 21% O2 and 29% O2 in the oxidizer mixture (O2 + CO2). A renormalized group (RNG) turbulence model and the eddy dissipation model are utilized in the present work to provide the turbulence characteristics and the production rate of species. The solution of the radiative transfer equation was obtained using the discrete ordinates radiation model. The set of governing equations and the boundary conditions are solved numerically using Fluent computational fluid dynamics code considering a single-step reaction kinetics model for methane–oxyfuel combustion. Comparison of both oxyfuel combustion and air–fuel combustion indicates that the temperature levels are reduced in oxyfuel combustion. The results show that the temperature levels are greatly reduced as the percentage of recirculated CO2 is increased. It is concluded that the flame propagation speed in the CO2 environment is lower than that in N2. It is found that the natural gas and oxygen consumption rates are slower in oxyfuel combustion relative to air–fuel combustion. Heat transfer from the burnt gases to the water jacket along the different surfaces of the furnace is calculated. It is shown that the energy absorbed is much lower in the case of oxyfuel combustion along all surfaces except for the end part of the furnace close to the furnace rear wall. However, the same performance of the methane-oxy-flames is expected by increasing the oxygen concentration slightly above 29%.

Heat Transfer, Fluid Mechanics, and Thermodynamics in Our Environment—HEFAT2012

This editorial provides an overview of a special issue dedicated to the 9th International Conference on Heat Transfer, Fluid Mechanics, and Thermodynamics—HEFAT2012—hosted on Malta. All papers for this conference were peer reviewed and 270 papers were accepted. Of these, 10 of the best papers were selected for this issue and peer reviewed for a second time according to journal standards. The 10 papers focus on the characteristics of oxyfuel and air-fuel combustion in an industrial water tube boiler, numerical and optical analysis of a weather-adaptable solar reactor, the mitigation of crystallization fouling using projectiles in tubular heat exchangers, the mitigation of crystallization fouling in a single heated tube using projectiles of different sizes and hardness, a framework for the analysis of thermal losses in reciprocating compressors and expanders, an annular impinging jet controlled by radial synthetic jets, multi-effect plants and ionic liquids for improved absorption chillers, the effect of climatic parameters on the heat transfer mechanisms in a solar distillation still, empirical correlations for slightly decaying grid turbulence, and pool boiling on modified surfaces using R-123. The current issue of Heat Transfer Engineering is the ninth special journal issue dedicated to selected papers from the HEFAT conferences.

Modeling of particle transport and combustion phenomena in a large-scale circulating fluidized bed boiler using a hybrid Euler–Lagrange approach

The constantly developing fluidized combustion technology has become competitive with a conventional pulverized coal (PC) combustion. Circulating fluidized bed (CFB) boilers can be a good alternative to PC boilers due to their robustness and lower sensitivity to the fuel quality. However, appropriate engineering tools that can be used to model and optimize the construction and operating parameters of a CFB boiler still require development. This paper presents the application of a relatively novel hybrid Euler–Lagrange approach to model the dense gas–solid flow combined with a combustion process in a large-scale industrial CFB boiler. In this work, this complex flow has been resolved by applying the ANSYS FLUENT 14.0 commercial computational fluid dynamics (CFD) code. To accurately resolve the multiphase flow, the original CFD code has been extended by additional user-defined functions. These functions were used to control the boiler mass load, particle recirculation process (simplified boiler geometry), and interphase hydrodynamic properties. This work was split into two parts. In the first part, which is referred to as pseudo combustion, the combustion process was not directly simulated. Instead, the effect of the chemical reactions was simulated by modifying the density of the continuous phase so that it corresponded to the mean temperature and composition of the flue gases. In this stage, the particle transport was simulated using the standard Euler–Euler and novel hybrid Euler–Lagrange approaches. The obtained results were compared against measured data, and both models were compared to each other. In the second part, the numerical model was enhanced by including the chemistry and physics of combustion. To the best of the authors’ knowledge, the use of the hybrid Euler–Lagrange approach to model combustion is a new engineering application of this model. In this work, the combustion process was modeled for air-fuel combustion. The simulation results were compared with experimental data. The performed numerical simulations showed the applicability of the hybrid dense discrete phase model approach to model the combustion process in large-scale industrial CFB boilers.

Evaluation analysis of scavenging process of two-stroke marine diesel engine by experiment and simulation

The performance of the two-stroke engine depends on the scavenging process, which affects air-fuel mixture and combustion. This paper presents results from experiments in which the timing of exhaust valve opening and closing was varied in two-stroke marine diesel engine during shop testing, and reveals the effect on performance and emission, as well as the corresponding concept analysis. A simulation model was then developed based on AVL BOOST and experimental results, its validity was confirmed by actual measurements. This simulation enables a more thorough investigation into engine performance features and the scavenging process. In particular it provides a detailed examination of how changes in EVO and EVC timing impact the scavenging process and ultimately engine performance. These results are summarized, and based on these results, optimal EVO and EVC settings are suggested for balancing the scavenging process, and also the engine performance and NOx emissions.

Hierarchical 1D/3D Approach for the Development of a Turbulent Combustion Model Applied to a VVA Turbocharged Engine. Part I: Turbulence Model

It is widely recognized that air-fuel mixing, combustion and pollutant formation inside internal combustion engines are strongly influenced by the spatial and temporal evolution of both marco- and micro- turbulent scales. Particularly, in spark ignited engines, the generation of a proper level of turbulence intensity for the correct development of the flame front is traditionally based on the onset, during the intake and compression strokes, of a tumbling macro-structure.Recently, in order to both reduce pumping losses due to throttling and develop advanced and flexible engine control strategies, fully variable valve actuation systems have been introduced, capable of simultaneously governing both valve phasing and lift. Despite the relevant advantages in terms of intake system efficiency, this technology introduces uncertainties on the capability of the intake port/valve assembly to generate, at low loads, sufficiently coherent and stable structures, able therefore to promote adequate turbulence levels towards the end of the compression, with relevant effects on the flame front development.It is a common knowledge that 3D-CFD codes are able to describe the evolution of the in-cylinder flow field and of the subsequent combustion process with good accuracy; however, they require too high computational time to analyze the engine performance for the whole operating domain. On the contrary, this task is easily accomplished by 1D codes, where, however, the combustion process is usually derived from experimental measurements of the in-cylinder pressure trace (Wiebe correlation). This approach is poorly predictive for the simulation of operating conditions relevantly different from the experimental ones. To overcome the above described issues, enhanced physical models for the description of in-cylinder turbulence evolution and combustion to be included in a 1D modeling environment are mandatory.In the present paper (part I), a 0D (i.e. homogeneous and isotropic) phenomenological (i.e. sensitive to the variation of operative parameters such as valve phasing, valve lift, intake and exhaust pressure levels, etc.) turbulence model belonging to the K-k model family is presented in detail. The model is validated against in-cylinder results provided by 3D-CFD analyses carried out

Review of the Investigation of Fuel-Air Premixing and Combustion Process Using Rapid Compression Machine and Direct Visualization System

s :This paper reviews of some applications of optical visualization system to compute the fuel-air mixing process during early stage of mixture formation and late injection in Diesel Combustion Engine. This review has shown that the mixture formation is controlled by the characteristics of the injection systems, the nature of the air swirl and turbulence in thecylinder, and spray characteristics. Few experimental works have been investigated and found that the effects of injection pressure and swirl ratio have a great effect on the mixture formation then affects to the flame development and combustion characteristics.This paper presents the significance of spray and combustion study with optical techniques access rapid compression machine that have been reported by previous researchers. Experimental results are presentedin order to provide in depth knowledge as assistance to readers interested in this research area. Analysis of flame motion and flame intensity in the combustion chamber was performed using high speed direct photographs and image analysis technique. The application of these methods to the investigation of diesel sprays highlights mechanisms which provide a better understanding of spray and combustion characteristics.

Refined Weighted Sum of Gray Gases Model for Air-Fuel Combustion and Its Impacts

Radiation is the principal mode of heat transfer in utility boiler furnaces. Models for radiative properties play a vital role in reliable simulations of utility boilers and simulation-based design and optimization. The weighted sum of gray gases model (WSGGM) is one of the most widely used models in computational fluid dynamics (CFD) simulation of air-fuel combustion processes. It represents a reasonable compromise between an oversimplified gray gas model and a comprehensive approach addressing high-resolution dependency of radiative properties and intensity upon wavelength. The WSGGM coefficients evaluated by Smith et al. for several partial pressures of CO2 and H2O vapor are often used for gas temperatures up to 2400 K, which is supplemented by the coefficient values presented by Coppalle and Vervisch for higher temperatures until 3000 K. This paper refines the air-fuel WSGGM in terms of accuracy, completeness, and implementation and demonstrates the use and impacts of the refined model in CFD simulati...

On choice of a swirler for the impulse combustion chamber

In this paper, some problems on implementation of an air-fuel mixture combustion process in the flame tube with a GTE impulse combustion chamber flow swirler are considered. When the peripheral swirler is placed in the flame tube head of this chamber, a number of 3-D reverse flow zones are formed to effectively mix up the air-fuel mixture providing its steady-state combustion. As a result, the thrust impulse is increased and a level of atmospheric emissions of NOx, CO, and some other detrimental compounds is significantly decreased.

Uncertainty quantification of reaction mechanisms accounting for correlations introduced by rate rules and fitted Arrhenius parameters

We study correlations among uncertain Arrhenius rate parameters in a chemical model for hydrocarbon fuel–air combustion. We consider correlations induced by the use of rate rules for modeling reaction rate constants, as well as those resulting from fitting rate expressions to empirical measurements arriving at a joint probability density for all Arrhenius parameters. We focus on homogeneous ignition in a fuel–air mixture at constant-pressure. We outline a general methodology for this analysis using polynomial chaos and Bayesian inference methods. We examine the uncertainties in both the Arrhenius parameters and in predicted ignition time, outlining the role of correlations, and considering both accuracy and computational efficiency.

Research on Match of Swirl-chamber and Conical Spray in Indirect Injection Engine

Swirl-chamber diesel engine has the advantages of lower NOx emission and noise, but the deficiency of comparatively higher fuel consumption rate constrains its further development. In order to improve the fuel economy of a swirl-chamber diesel engine, the engine was modified by applying a conical spray injector in the swirl-chamber to improve its fuel-air mixture formation and combustion processes. In addition, a series of swirl-chamber schemes were designed, and the match of the conical spray with these swirl-chamber schemes were investigated with the aid of a 3-D CFD software package. The measurement and simulation results indicate that with the application of the conical spray to the prototype engine, the fuel economy is improved. Meanwhile, one of the modification schemes on the swirl-chamber presents the potential to further improve the fuel economy of the engine thanks to its more homogeneous fuel-air mixture compared with that of the original swirl-chamber.