Combustion characteristics of a heat-recirculating ceramic burner using a low-calorific-fuel

A new type of the heat recirculating burner was constructed and its combustion characteristics during steady low-calorific-fuel/air combustion were investigated. Flammability limits of the burner were measured by experiments, and longitudinal temperature distributions of air and burned gas flowing in the passes of the burner were determined by means of both experimental measurements and numerical simulations. Using the heat recirculation rate and the thermal efficiency as criteria for the performance of the burner, the optimal design of the burner was examined in terms of a chemical parameter (the equivalence ratio), a fluid-mechanical parameter (the Reynolds number) and a geometrical parameter (the number of passes).     

Formation characteristics of nitric oxide in a three-staged air/LPG flame

Experimental and numerical studies have been done to examine the effects of excess air ratio and tertiary air swirl number on the formation characteristics of NO in a pilot scale combustor adopting a multi-air staged burner. In numerical calculation the mathematical models for turbulence, radiation and nitric oxide chemistry were taken into account. The radiative transfer equation was solved using the discrete ordinates method with the weighted sum of gray gases model. In the NO chemistry model, the chemical reaction rates for thermal and prompt NO were statistically averaged using a probability density function. The results were validated by comparison with measurements. For the experiment, a 0.2 MW pilot multi-staged air burner has been designed and fabricated. Using the numerical simulation developed here, a variation of thermal and prompt NO formation was predicted by changing the excess air ratio and tertiary air swirl number. As the excess air ratio increased up to 1.9, the formation of the total as well as thermal NO at exit increased while the prompt NO decreased. The formation of thermal NO was more affected by concentration of O₂ and N₂ than gas temperature. When the tertiary air swirl number increased, the formation of the total as well as the prompt NO slightly decreased because of enhanced mixing of fuel and oxygen in the upstream reaction zone and reduced gas temperature at exit.     

An experimental and numerical study of stagnation point heat transfer for methane/air laminar flame impinging on a flat surface

A combined experimental and numerical study has been conducted to determine the stagnation point heat transfer for laminar methane/air flame impinging on a flat surface. Effects of Reynolds number, equivalence ratio and burner diameter on stagnation point heat flux were examined experimentally at different separation heights. Maximum stagnation point heat flux was obtained when the flat surface was closest to the tip of the inner premixed reaction zone. Heat flux decreased along the axial direction when the separation distance was further increased from the tip of inner reaction zone. There was a secondary rise in heat flux at the stagnation point at larger separation distances. Correlations were developed for stagnation point Nusselt number. Numerical simulations were carried out using a commercial CFD code (FLUENT) for laminar methane/air flame impinging on a flat surface for various separation distances. Results were compared with those found experimentally. The reason for conducting the simulations was to (a) gain more insight into how the presence of the plate affects the flame and the flow and temperature fields and (b) to explain the reason for high heat flux when the tip of the inner reaction zone was very close to the stagnation point.     

Evolution of soot size distribution in premixed ethylene/air and ethylene/benzene/air flames: Experimental and modeling study

The effect of benzene concentration in the initial fuel on the evolution of soot size distribution in ethylene/air and ethylene/benzene/air flat flames was characterized by experimental measurements and model predictions of size and number concentration within the flames. Experimentally, a scanning mobility particle sizer was used to allow spatially resolved and online measurements of particle concentration and sizes in the nanometer-size range. The model couples a detailed kinetic scheme with a discrete-sectional approach to follow the transition from gas-phase to nascent particles and their coagulation to larger soot particles. The evolution of soot size distribution (experimental and modeled) in pure ethylene and ethylene flames doped with benzene showed a typical nucleation-sized (since particles do not actually nucleate in the classical sense particle inception is often used in place of nucleation) mode close to the burner surface, and a bimodal behavior at greater height above burner (HAB). However, major features were distinguished between the data sets. The growth of nucleation and agglomeration-sized particles was faster for ethylene/benzene/air flames, evidenced by the earlier presence of bimodality in these flames. The most significant changes in size distribution were attributed to an increase in benzene concentration in the initial fuel. However, these changes were more evident for high temperature flames. In agreement with the experimental data, the model also predicted the decrease of nucleation-sized particles in the postflame region for ethylene flames doped with benzene. This behavior was associated with the decrease of soot precursors after the main oxidation zone of the flames.     

Study on nonpremixed methane/air combustion from flame structure and NOX emission aspect for different burner head structures

In the present study, the air turbulator, which is a part of a nonpremixed burner, is investigated numerically in terms of its effects on the diffusion methane flame structure and NO_X emissions. A computational fluid dynamics (CFD) code was used for the numerical analysis. At first, four experiments were conducted using natural gas fuel. In the experimental studies, the excess air ratio was taken constant as 1.2, while the fuel consumption rate was changed between 22 and 51 Nm³/h. After the experimental studies, the CFD studies were carried out. Pure methane was taken as fuel for the simulations. The nonpremixed combustion model with the steady laminar flamelet model (SFM) approach was used in the combustion analyses. Methane‐air extinction mechanism with 17 species and 58 reactions was used for the simulations. The results obtained from the CFD studies were confronted with the measurements of the flue gas emissions in the experimental studies. Then, a modified burner head was analysed numerically for the different air turbulator blade numbers and angles. The CFD results show that increasing the air turbulator blade number and angle causes the thermal NO emissions to be reduced in the flue gas by making the flame in the combustion chamber more uniform than the original case. This new flame structure provides better mixing of the fuel and combustion air. Thus, the diffusion flame structure in the combustion chamber takes the form of the partially premixed flame structure. The maximum reduction in the thermal NO emissions in the flue gas is achieved at 38% according to the original case.     

高原燃油暖风机的研制

介绍了高原型燃油暖风机的构造特点及工作原理。针对高原地区低温、缺氧、气压低的客观现实，通过对轻油燃烧器结构的改进，解决了燃烧器点火困难、火焰不稳定及燃烧不完全问题；通过优选并改制风机，采用变频控制技术，进行平原、高原冷态和热态的一系列实验，在此基础上优化设计燃烧换热器，最终较好地解决了高海拔换热困难，整机噪声过大，送风温度不稳定等问题．     

Combustion and heat release characteristics of hydrogen/air diffusion flame on a micro-jet array burner

Hydrogen (H₂) is considered as a carbon-free alternative fuel. The heat release characteristics of H₂ flame as a key parameter in its combustion process are unclear. In this study, the combustion and heat release characteristics of H₂/air diffusion flame on a micro-jet array burner were experimentally and numerically investigated. It is shown that the OH distribution and flame length based on Bilger mechanism are in good agreement with the experimental results. Furthermore, the intensity and distribution of OH and heat release rate can be adjusted by the thermal power and equivalence ratio. A uniform flame with intensive heat release rate can be achieved at a thermal power of 0.1 kW. R41: H + O₂ = OH + O and R43: H + O₂ + M = HO₂ + M are the main reactions with oxidizer consumption to form reactive radicals. R40: OH + H₂ = H₂O + H and R47: OH + OH = O + H₂O with OH consumption are the main heat release reactions at the upstream and downstream of the flame.     

Thermal performance of an n-pass solar air heater

A mathematical model is developed for the prediction of the thermal performance for an n-pass solar air collector. The thermal efficiency and the temperature rise in teh collector are increased by preheating the air within the collector before it comes into contact with the absorber plate. The thermal losses to the ambient, through the bottom plate, are minimized due to the lower bottom plate temperature. The analysis describes how the governing equations are manipulated with the aid of the boundary conditions to obtain a set of equations which is solved numerically. In order to demonstrate the capability of the model, the analysis is applied to investigate the effect of increasing the number of air passes within the collector on the thermal efficiency and the temperature rise for a range of air mass flow rates. The results indicate that the thermal efficiency of the collector increases rapidly and approaches the optical efficiency of the collector as the mass flow rate is increased. In general, the model can be used to analyze the effect of various design parameters on the thermal performance.     

Experimental investigation and modeling of a direct-coupled PV/T air collector

Photovoltaic/thermal (PV/T) systems refer to the integration of photovoltaic and solar thermal technologies into one single system, in that both useful heat energy and electricity are produced. The impetus of this paper is to model a direct-coupled PV/T air collector which is designed, built, and tested at a geographic location of Kerman, Iran. In this system, a thin aluminum sheet suspended at the middle of air channel is used to increase the heat exchange surface and consequently improve heat extraction from PV panels. This PV/T system is tested in natural convection and forced convection (with two, four and eight fans operating) and its unsteady results are presented in with and without glass cover cases. A theoretical model is developed and validated against experimental data, where good agreement between the measured values and those calculated by the simulation model were achieved. Comparisons are made between electrical performance of the different mode of operations and it is concluded that there is an optimum number of fans for achieving maximum electrical efficiency. Also, results show that setting glass cover on photovoltaic panels leads to an increase in thermal efficiency and decrease in electrical efficiency of the system.     

Impinging premixed butane/air circular laminar flame jet--influence of impingement plate on heat transfer characteristics

Experimental studies were performed to study the heat transfer characteristics of an impingement flame jet system consisting of a premixed butane/air circular flame jet impinging vertically upward upon a horizontal rectangular plate at laminar flow condition. There were two impingement plates manufactured with brass and stainless steel respectively used in the present study. The integrated effects of Reynolds number and equivalence ratio of the air/fuel jet, and distance between the nozzle and the plate (i.e. nozzle-to-plate distance) on heat transfer characteristics of the flame jet system had been investigated. The influence in using impingement plate with different thermal conductivities, surface emissivities and roughnesses on heat flux received by the plate was examined via comparison, which had not been reported in previous literatures. A higher resistance to heat transfer had been encountered when the stainless steel impingement plate of lower thermal conductivity was used, which led to a significantly lower heat flux at the stagnation region. However, the heat flux distribution in the wall-jet region of the plate was only slightly affected by using different impingement plates. Because of the significantly lower heat transfer, more fuel was not required to consume and existed at the stagnation region of the stainless steel impingement plate, which would be burned latter in the wall-jet region to release its chemical energy and enhance the local heat flux there.     

Hetero-/homogeneous combustion of hydrogen/air mixtures over platinum: Fuel-lean versus fuel-rich combustion modes

The catalytic combustion of hydrogen/air mixtures in platinum-coated channels was investigated by means of two-dimensional numerical simulations with detailed heterogeneous (catalytic) and homogeneous (gas-phase) chemical reaction schemes, detailed species transport, and heat transfer mechanisms in the solid wall. Three different combustion modes relevant to power generation were analyzed and their ranges of applicability were delineated. The investigated modes included the fuel-lean catalytically stabilized thermal combustion (CST), the inverse catalytically stabilized thermal combustion (i-CST), and the fuel-rich catalytic/gaseous-lean combustion. In the fuel-lean CST approach, the less-than-unity Lewis number of hydrogen led to superadiabatic surface temperatures. Even though the presence of homogeneous combustion moderated the wall temperature superadiabaticity, reactor and catalyst thermal stability considerations limited the CST mode to equivalence ratios less than 0.2. In the i-CST mode, the maximum attained wall temperatures strongly depended on the location of homogeneous ignition, as it directly affected the magnitude of the wall temperature superadiabaticity induced by heat recirculation inside the solid walls. The i-CST had advantages over CST in terms of reactor thermal management and extended the range of applicable equivalence ratios to 0.3. In the fuel-rich catalytic combustion mode, the larger-than-unity Lewis number of the deficient oxygen reactant and the cooling of the catalytic channel by the resulting bypass air led to surface temperatures below the adiabatic equilibrium value, even for those cases where homogeneous combustion was established in the channel. This mode facilitated safe reactor operation for overall equivalence ratios up to 0.5 and inlet temperatures up to 800 K, an operational range of particular interest for gas turbines.     

烧嘴参数对耐火砖热变形的影响分析

热变形是引起耐火砖脱落的重要因素之一,烧嘴参数对热变形的影响很大。针对烧结套筒式双旋流烧嘴的结构特点,采用热流固耦合方法建立了烧嘴的有限元模型,首先利用FLUENT分析了温度场和流场,结合测量数据验证了模型的正确性;然后对耐火砖通道进行热结构分析,获得了通道的热变形分布;最后采用响应面方法设计了81组试验进行模拟计算,获得了烧嘴结构参数和操作参数对耐火砖通道表面温度和热应变的影响规律。结果表明:燃烧器负荷增加、外侧空气预热温度增加会显著地增加耐火砖的热变形,比如燃烧器负荷增加1%,耐火砖应变增加0.75%;增加外侧空气流速、增加内侧空气预热温度有助于缓解耐火砖的热变形,比如外侧空气流速增加1.0%,耐火砖应变减少0.30%;增加外侧空气旋流片角度既有助于强化混合燃烧,又能降低耐火砖的热变形量。     

Effect of elliptic burner geometry and air equivalence ratio on the nitric oxide emissions from turbulent hydrogen flames

An experimental study on turbulent hydrogen flames from circular and elliptic burners with varying degrees of premixedness (diffusion, fuel-rich, stoichiometric, and fuel-lean) is presented. Flame stability, visible flame height, flame radiation, global nitric oxide (NO) concentration, and inflame temperature and NO concentration profiles were measured. We found that the elliptic burner flames had lower liftoff velocity, were shorter, and radiated less heat to the surrounding as compared to circular burner flames. Global NO concentration decreased with an increase in air equivalence ratio for both circular and elliptic burner flames. Peak in-flame NO concentration along the flame centerline increased with a decrease in air equivalence ratio. Elliptic burner flames produced higher peak in-flame temperatures. Overall, the elliptic burner flames produced less peak NO as compared to circular burner flames at all air equivalence ratios except zero (diffusion flames) in accordance with the global emission measurements.     

Two-color, two-photon laser-induced polarization spectroscopy (LIPS) measurements of atomic hydrogen in near-adiabatic, atmospheric pressure hydrogen/air flames

Two-color, two-photon laser-induced polarization spectroscopy (LIPS) of atomic hydrogen has been demonstrated and applied in atmospheric pressure hydrogen/air flames. Fundamental and frequency-doubled beams from a single 486-nm dye laser were used in the experiments. The 243-nm pump beam in the measurements was tuned to the two-photon n=1→n=2 resonance of the hydrogen atom. The 486-nm probe beam was tuned to the single-photon n=2→n=4 resonance of the hydrogen atom. Measurements were performed in an atmospheric pressure H₂/air flame stabilized on a near-adiabatic, flat-flame calibration burner (the Hencken burner). For the range of pump beam intensities used, the LIPS signal was found to be nearly proportional to the square of the pump beam intensity over a wide range of flame equivalence ratios. Spectral lineshapes were recorded at flame equivalence ratios ranging from 0.85 to 2.10. Vertical H-atom number density distribution profiles were measured in the Hencken burner. The vertical H-atom number density profiles measured along the burner centerline for various flame equivalence ratios were compared with the results of a numerical flame calculation using the UNICORN (Unsteady Ignition and Combustion with Reactions) code. Good agreement between theory and experiment was obtained for stoichiometric and rich flame conditions. For flames with equivalence ratios greater than 1.5, the H-atom concentration was substantially above the adiabatic equilibrium value, even at 50 mm above the burner surface. The slow approach to the adiabatic equilibrium H-atom concentration value can be explained by assuming partial equilibrium in the postflame gases; the H-atom concentration is proportional to the O₂ concentration which requires significant residence time to decrease to its very low equilibrium concentration. These results suggest that the use of the Hencken burner as a radical measurement technique calibration source may be of questionable value for equivalence ratios greater than 1.5 and less than 0.8.     

多孔燃烧器中氨/空气燃烧特性数值研究

氨具有氢密度高、生产成本低、基础设施完善等优点,作为一种潜在的可再生替代燃料受到了广泛的关注.目前,仅有少数研究关注氨气燃烧喷嘴的研究,针对氨气稳定燃烧喷嘴的研究尤其不足.为实现氨燃料的稳定燃烧和低污染物排放,本研究提出了一种氨用多孔介质燃烧器.对氨用多孔介质燃烧器建立了二维数值模型,并对预混氨/空气在多孔介质燃烧器中...     

Thermal efficiency improvement of an LPG gas cooker by a swirling central flame

The present study proposes a swirling central flame technique to improve the thermal efficiency of a conventional open‐flame atmospheric gas cooker which is now widely used as a domestic appliance. More extensive studies were done in an effort to improve the thermal efficiency of the cooker by reducing thermal inertia of the pan support and using the proposed porous medium technology to recover heat from flame radiation to preheat the secondary air entrained from the bottom of the burner. The experimental results showed that the thermal efficiency of the swirling central flame burner with conventional support is approximately 15 per cent higher than that of the conventional radial flow burner. This can be attributed to the higher heat transfer coefficient between hot flue gas and vessel surface of the swirl burner than that of the conventional one. By replacing the conventional support of the developed swirl burner with a lighter one, whose mass was reduced by a factor of 3.7, the thermal efficiency could be increased by about 3 per cent. By using the proposed preheating secondary air support instead of the light support, the thermal efficiency could be further improved by 3 per cent. The predicted thermal efficiency obtained from the proposed model showed good agreement with the experiment. Copyright © 2001 John Wiley & Sons, Ltd.     

Direct numerical simulations of the ignition of lean primary reference fuel/air mixtures with temperature inhomogeneities

The effects of fuel composition, thermal stratification, and turbulence on the ignition of lean homogeneous primary reference fuel (PRF)/air mixtures under the conditions of constant volume and elevated pressure are investigated by direct numerical simulations (DNSs) with a new 116-species reduced kinetic mechanism. Two-dimensional DNSs were performed in a fixed volume with a two-dimensional isotropic velocity spectrum and temperature fluctuations superimposed on the initial scalar fields with different fuel compositions to elucidate the influence of variations in the initial temperature fluctuation and turbulence intensity on the ignition of three different lean PRF/air mixtures. In general, it was found that the mean heat release rate increases slowly and the overall combustion occurs fast with increasing thermal stratification regardless of the fuel composition under elevated pressure and temperature conditions. In addition, the effect of the fuel composition on the ignition characteristics of PRF/air mixtures was found to vanish with increasing thermal stratification. Chemical explosive mode (CEM), displacement speed, and Damköhler number analyses revealed that the high degree of thermal stratification induces deflagration rather than spontaneous ignition at the reaction fronts, rendering the mean heat release rate more distributed over time subsequent to thermal runaway occurring at the highest temperature regions in the domain. These analyses also revealed that the vanishing of the fuel effect under the high degree of thermal stratification is caused by the nearly identical propagation characteristics of deflagrations of different PRF/air mixtures. It was also found that high intensity and short-timescale turbulence can effectively homogenize mixtures such that the overall ignition is apt to occur by spontaneous ignition. These results suggest that large thermal stratification leads to smooth operation of homogeneous charge compression-ignition (HCCI) engines regardless of the PRF composition.     

Maximising the energy output of a PVT air system

Simultaneously generating both electricity and low grade heat, photovoltaic thermal (PVT) systems maximise the solar energy extracted per unit of collector area and have the added benefit of increasing the photovoltaic (PV) electrical output by reducing the PV operating temperature. A graphical representation of the temperature rise and rate of heat output as a function of the number of transfer units NTUs illustrates the influence of fundamental parameter values on the thermal performance of the PVT collector. With the aim of maximising the electrical and thermal energy outputs, a whole of system approach was used to design an experimental, unglazed, single pass, open loop PVT air system in Sydney. The PVT collector is oriented towards the north with a tilt angle of 34°, and used six 110 Wp frameless PV modules. A unique result was achieved whereby the additional electrical PV output was in excess of the fan energy requirement for air mass flow rates in the range of 0.03–0.05 kg/s m². This was made possible through energy efficient hydraulic design using large ducts to minimise the pressure loss and selection of a fan that produces high air mass flow rates (0.02–0.1 kg/s m²) at a low input power (4–85 W). The experimental PVT air system demonstrated increasing thermal and electrical PV efficiencies with increasing air mass flow rate, with thermal efficiencies in the range of 28–55% and electrical PV efficiencies between 10.6% and 12.2% at midday.     

Bituminous coal combustion in a full-scale start-up ignition burner: Influence of the excess air ratio

A start-up ignition burner has been proposed to reduce oil fuel consumption during the firing-up process and partial-load operation. To investigate the influence of different excess air ratios on bituminous coal combustion in the start-up ignition burner, full-scale reacting-flow experiments were performed for an experiment setup. The ignition burner was identical to that normally used in an 800 MWe utility boiler. Gas temperature distributions in the burner were obtained for excess air ratios of 0.56, 0.75, 0.98 and 1.14 (corresponding to primary air velocities of 17, 23, 30 and 35 m/s). Coal burnout and the release of C and H were observed at the exit of the burner nozzle. Gases such as O₂ and CO were measured at the center of the burner. A change in resistance was obtained within the burner.     

Improved PV/T solar collectors with heat extraction by forced or natural air circulation

The photovoltaic (PV) cells suffer efficiency drop as their operating temperature increases especially under high insolation levels and cooling is beneficial. Air-cooling, either by forced or natural flow, presents a non-expensive and simple method of PV cooling and the solar preheated air could be utilized in built, industrial and agricultural sectors. However, systems with heat extraction by air circulation are limited in their thermal performance due to the low density, the small volumetric heat capacity and the small thermal conductivity of air and measures for heat transfer augmentation is necessary. This paper presents the use of a suspended thin flat metallic sheet at the middle or fins at the back wall of an air duct as heat transfer augmentations in an air-cooled photovoltaic/thermal (PV/T) solar collector to improve its overall performance. The steady-state thermal efficiencies of the modified systems are compared with those of typical PV/T air system. Daily temperature profiles of the outlet air, the PV rear surface and channel back wall are presented confirming the contribution of the modifications in increasing system electrical and thermal outputs. These techniques are anticipated to contribute towards wider applications of PV systems due to the increased overall efficiency.