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The laminar burning velocities of biogas-hydrogen-air mixture at different fuel compositions and equivalence ratios were determined and studied using the spherical flame method. The combined effects of H2 and CO2 on the laminar burning velocity were investigated quantitatively based on the kinetic effects and the thermal effects. The results show that the laminar burning velocities of the BG40, BG50 and BG60 are increased almost linearly with the H2 addition owing to the improved fuel kinetics and the increased adiabatic flame temperature. The dropping trend of laminar burning velocity from the BG60-hydrogen to the BG40-hydrogen is primarily attributed to the decreased adiabatic flame temperature (thermal effects). The GRI 3.0 mechanism can predict the laminar burning velocity of biogas-hydrogen mixture better than the San Diego mechanism in this study. Whereas, the GRI mechanism still needs to be modified properly for the hydrogen-enriched biogas as the CO2 proportion exceeds 50% in the biogas at the fuel-rich condition. The increased CO2 exerts the stronger suppression on the net reaction rate of H + O2=OH + O than that of H + CH3(+M) = CH4(+M), which contributes to that the rich-shift of peak laminar burning velocity of biogas-hydrogen mixture requires higher H2 addition as the CO2 content is enhanced. For the biogas-hydrogen fuel, the H2 addition decreases the flame stability of biogas fuel effectively due to the increased diffusive-thermal instability and hydrodynamic instability. The improved flame stability of biogas-hydrogen fuel with the increased CO2 content is resulted from the combined effects of diffusive-thermal instability and hydrodynamic instability.

A study on the effect of diluents (N2/CO2) on laminar flame speed and flame instability of H2/CO/CH4/air premixed flame was conducted using outwardly propagating spherical flame and CHEMKIN package. Experiments and numerical computation were conducted at an initial temperature of 303 ± 3 K, pressures of 0.1–0.5 MPa, equivalence ratios of 0.6–1.5 with N2 or CO2 dilution ratios from 0% to 45%, at an interval of 15%. The measured laminar flame speed was compared with values predicted using the Li mechanism. The thermal and chemical kinetic analyses were also presented. The results show that the Li mechanism can give a good predication on laminar flame speed of H2/CO/CH4/air premixed flames. But with N2/CO2 dilution, the Li mechanism gives slight underestimations at fuel-rich conditions. Laminar flame speed decreases with the increase of N2/CO2 dilution ratios. Results indicate that CO2 dilution has a stronger dilution effect, thermal effect, and chemical effect than those of N2 dilution, and thereby it can substantially decrease the laminar flame speed of H2/CO/CH4/air mixture. On the other hand, flame instability is slightly promoted with N2/CO2 dilution at atmospheric pressure, while at elevated pressure, it is suppressed with N2/CO2 dilution due to the significant decrease of thermal expansion ratio and increase of flame thickness. Compared with N2 dilution, CO2 dilution has a stronger inhibiting effect on hydrodynamic instability.

Biomass-derived syngas composition varies considerably depending on different feedstocks and processing techniques and thereby complicates the combustion control. A study on the effects of variations in the fuel composition and initial pressure on the characteristics of premixed H2/CO/CH4 flames was conducted using the spherical expanding flame method and CHEMIKIN package. Experimental measurements and numerical simulations were performed at an initial temperature of 303 K, equivalence ratios of 0.6–1.5 and pressures of 0.1–0.5 MPa with a wide range of H2/CO/CH4 compositions. The thermal and chemical kinetic analyses are also presented. The measured laminar flame speed was compared with simulations using the Li mechanism. The experimental data show a reasonable agreement with the calculated values, especially at fuel-lean and low pressure conditions. With the increase of H2 fraction in the fuel, the laminar flame speed increases significantly, but for the CH4 enrichment flame, the behavior is quite the contrary that it has the lowest laminar flame speed. With the increase of CO fraction in the fuel, the laminar flame speed does not change much. The thermal and chemical kinetic analyses indicate that the CO addition has more effect on the adiabatic flame temperature but only plays a small role in the chemical effect compared to that of the H2 addition. On the other hand, at elevated pressures, the Li mechanism gives slight overestimations for lean mixtures but underestimations for rich mixtures. The laminar flame speed decreases with the increase of initial pressure under tested equivalence ratio which is mainly due to the increasing unburned mixture density and decreasing H, OH radicals concentrations.

This paper presents an experimental study of free flame patterns, impinging flame structures and flame impingement heat transfer characteristics of a partially premixed flame. The partially premixed flame is produced on a double-concentric-pipe burner, with both flow strength and mixedness of its inner and outer jets controlled by digital flowmeters. The aim of the study is, through examination of the effects of inner and outer jet Reynolds numbers (Rei, Reo), equivalence ratios (Φi, Φo) and nozzle-to-plate distance (H), to understand the heat transfer behaviors of such partially premixed flame and further identify the best impinging flame to achieve higher energy usage efficiency. A total of twelve different free flame patterns are observed by varying Rei-Reo-Φi-Φo, and W-shaped flame is the most representative flame under the conditions tested. The partially premixed dual flame is higher in stability than any single component of it, i.e. Bunsen flame or annulus flame. Further, leaner flammability limit is achieved by the dual flame due to mutual interaction which effectively supports leaner inner flame. Both inner and outer flames influence the radial heat flux distribution of W flame, while the contribution of inner flame becomes significant only when the difference in two flame heights is large. The outer flame globally dominates the total heat transfer rate, leading to general decay with increasing H. Comparison of W flame with single Bunsen and annulus flame shows that W flame has the favorable structure for higher energy efficiency due to double-pipe configuration and mutual flame interaction.

This work presents an experimental study the effects of DC electric field on a laminar premixed flame which impinges on a plate. Positive DC and ground levels of voltage are applied to the plate and burner, respectively. A reverse application of the electric field is also tested. Detailed measurements are undertaken to understand the ionic wind effect on flame impingement heat transfer. Testing shows a favorable effect of positive field on flame stability, as the flame stands leaner combustion. When a sufficiently high voltage is applied, discharge occurs which breaks down the electric field. The results of thermal field testing reveal that the temperature change observed is due to flame shape distortion caused by ionic wind. It is found that ionic wind is majorly physical in nature, inducing the changes in flame stability and temperature through a force. Further, it is mainly the post-combustion part of the flame, instead of reaction zone, bears such force. Under positive field, the burner nozzle is rapidly heated by the flame. However, flame impingement heat transfer has no significant change under either positive or reverse field. It is because heat loss from the post-combustion flame to plate is less altered by DC fields. To further study the ionic wind effect, a co-flow is introduced to the flame, and the standard deviation data of flame temperature indicate that ionic wind insignificantly changes the mixing between flame and ambient air. It confirms that ionic wind is mainly a force effect, with negligible chemical effect in the scope of this work.

Turbulence induced by actuators has been widely utilized as a effective means to enhance jet impingement heat transfer (JIHT). Flame impingement heat transfer (FIHT) has parital similarity to JIHT, and thus it is of interest to investigate the role of an actuator-induced turbulence to flame combustion as well as its FIHT. On a Bunsen burner that is incorporable with a ring-type turbulence actuator, a detailed experimental comparison of the flame appearance, reaction zone structure, thermal field and heat transfer of both laminar and turbulent flames under identical bulk flow conditions and equivalence ratios were conducted. Results show that the turbulent flame has a narrower stable-operation range than the laminar couterpart, and thus turbulence reduces the flame stability. A major difference between two flames exists at the reaction zone, which is identified to be a ‘brush’ structure in the turbulent case, instead of being a perfect cone in the laminar case. The flame brush changes the heat release behavior such that the temperature field of the turbulent flame shows lowered peak temperature while enlarged high-temperature zone. The changed heat release or temperature field reults in distinct local heat fluxes in the impinging flames at the stagnation region and near wall jet region. Consequently, the overall heat transfer rate of the turbulent flame becomes higher than the laminar flame. Accordingly, this study proposes that the actuator turbulence can be utilized as a useful technique to promote FIHT.

Experimental measurements and numerical simulations were both conducted to study effects of unburned gases velocity on the formations and overall emissions of CO and NOx of the laminar premixed biogas-hydrogen impinging flame. Emission indexes (EI) of CO and NOx and NO2/NOx ratio were obtained based on the measured data, while four major routes to produce the NO were isolated and calculated in the simulation. The results show that EICO is decreased with Re at small H, while it is increased with Re at large H. The thermal NO is improved with Re as the flame front is affected strongly by the cold plate, while it is suppressed at larger H due to the reduced residence time and relatively insufficient oxidizer. The N2O route is decreased with Re, while the increased NO amounts of prompt route and NNH route are dominated by the higher flame temperature and more available active radicals. The contributions of thermal route and N2O route are decreased with Re while that of prompt route and NNH route are increased. With the increased Re, EINO2 and NO2/NOx ratio are decreased at small H but increased at large H.

An experimental study has been conducted to investigate stoichiometric biogas–hydrogen mixed fuel flames and the combustion characteristic and heating performance of three biogases enriched with hydrogen were examined. Both similar flame behavior and new finding are reported with reference to the literature. The dependence of flame stability on the fraction of hydrogen and the CO2 concentration was analyzed in the aspects of fuel property alteration and flow aerodynamics variation. The experiments showed a favorable effect of hydrogen and detrimental effect of CO2 on flame stability. Examination of flame cone height showed that under constant Re and Ф, the cone height is longer at either higher hydrogen fraction or higher CO2 concentration. In addition, at higher fraction of hydrogen in the mixed fuel, flame flammability strengthens, flame temperature enhances and CO emission reduces. Further, polyhedral reaction cone was detected for stoichiometric flames under high hydrogen concentration conditions. Great care was paid to the heat transfer behavior of stoichiometric biogas–hydrogen flames. First, when the cone tip touches the copper plate, the stagnation point heat flux is not the maximum peak when compared to heat flux peaks at other nozzle-to-plate distance. Comparison between two flames of identical cone height indicated that higher flame temperature is a dominant factor influencing heat transfer. Further, longer flame height tends favor heat transfer only at large nozzle-to-plate distances.

The overall emissions of CO, NOx and NO2/NOx ratio of laminar premixed biogas-hydrogen flames undergoing the flame-wall interaction (FWI) were measured experimentally while the emission indexes (EI) were calculated. The computational model was employed to calculate the combustion process of the biogas-hydrogen flame, and four major routes to produce the NO were isolated and calculated in the simulations. The effects of H2 addition on the formations and emissions of CO/NO2/NOx of BG75-hydrogen impinging flames were analyzed quantitatively. The results show that, with the H2 addition, the EICO of BG75-hydrogen flame is enhanced steadily at either small separated distance (H) or large H thanks to that the high reactivity of H2 suppresses the CO oxidization efficiently by competing for the OH radical. The H2 addition can improve the NO production via four routes and increase the EINOx of the BG75-hydrogen flame. Although the prompt NO is improved by the H2 addition, its rising trend is decreased considerably as hydrogen addition is larger than 20% owing to the reduced methane proportion and improved premixed combustion. The contribution of prompt NO is decreased with the H2 addition, while that of other routes are increased steadily. The NNH route dominates the increased EINOx at small H while the thermal route dominates the rising trend at large H. EINO2 is enhanced steadily with the H2 addition due to the improved HO2 production in the air mixing region, while the NO2 conversion ratio is improved and suppressed with H2 addition at small and large H, respectively.

Safety issues for production, transportation, storage, and usage of bio-syngas persist due to the variability of bio-syngas composition. Therefore, this study focuses on the effects of fuel composition variation (H2/CO/CH4) and diluents (N2/CO2) on the explosion characteristics of bio-syngas/air mixtures in a constant volume combustion vessel. The experimentally determined explosion parameters were obtained at an initial temperature of 303 K, equivalence ratios of 0.6–1.5 and atmospheric pressure with a wide range of H2/CO/CH4 compositions and dilution ratios. The results show that the concentration of H2 or CH4 in the fuel mixture has a great influence on the explosion characteristics of bio-syngas/air mixtures, while the concentration of CO in the fuel mixture plays a mild role in affecting the explosion behaviors. Moreover, it is observed that the addition of N2/CO2 to the fuel mixture can significantly reduce the potential of explosion hazards of bio-syngas/air mixtures, and CO2 dilution has stronger suppression effect on the explosion characteristics due to its larger negative influence on the thermal effect and chemical effect in comparison to that of N2 dilution. The corresponding correlations of Pmax/P0 = f(Zdilution), tc = f(Zdilution), and (dP/dt)max = f(Zdilution) were found for predicting the explosion behaviors of N2/CO2 diluted bio-syngas/air mixture at various dilution ratios.

The heat transfer behaviors of an annular jet flame impinging vertically normal to a flat surface were investigated experimentally. The relationship between the flow/flame structure of the annular jet flame and the local heat transfer behavior along the surface was analyzed. Further, the effects of Reynolds number (Re), equivalence ratio (Ф) and nozzle-to-plate distance (H) on the local and overall heat flux distributions were examined. The flame has two stable patterns, the ‘V’ shaped flame under lean conditions and the ‘M’ shaped flame under rich conditions. The two stable flames have similar heat flux distribution characteristics: (1) local heat flux in the region close to the jet axis is low; (2) peak heat flux occurs at a radial position at small H while at large H it coincides with the jet axis. Comparison of impinging annular jet and round jet flames under identical fuel/air flow rates shows that at low H, the annular jet flame has higher heat fluxes in the region near to the jet axis because a cool core is present in the round jet flame. Compared to the round jet flame, the annular jet flame achieves the highest heat transfer rate at smaller H and the maximum value is slightly lower, indicating that the annular jet flame is favorable for uniform heating.

Experiments were conducted to investigate the heat transfer and CO/NOX emissions of a premixed LPG/air circular flame jet impinging upwards normally to a flat rectangular plate. Temperatures of the impingement plate were controlled by cooling water at 38 °C, 58 °C and 78 °C which was circulating at its back in order to create different plate temperatures. Under each plate temperature, the effects of Reynolds number (Re), equivalence ratio (Ф) and nozzle-to-plate distance (H) on the heat transfer and CO/NOX emissions were examined. The Re was selected to be 500, 1000 and 1500 to ensure laminar flame jets. The values of Ф were chosen to cover fuel-lean, stoichiometric and fuel-rich conditions. The H varied from 3d to 7d with an interval of 1d. The flame-side temperature of the impingement plate is enhanced when the cooling water temperature increases, but the temperature difference across the impingement plate is reduced. Heat transfer from the flame to the plate is suppressed at higher cooling water temperature. The heat transfer rate is the highest when the cooling water temperature is at 38 °C and the lowest heat flux is obtained at 78 °C. At the highest cooling water temperature of 78 °C, the CO emission is reduced whereas the NOX emission is enhanced. However, this trend is reversed at the lowest cooling water temperature of 38 °C.

The heat transfer characteristics of a turbulent and swirling inverse diffusion flame (IDF) impinging vertically normal to a flat surface were investigated experimentally. The heat flux was measured by a heat flux sensor, Vatell HFM-6D/H. The effects of Reynolds number, overall equivalence ratio, nozzle-to-surface distance H and swirl number on the heat flux distributions were examined. The comparison of heat transfer of impinging IDFs with and without swirl was also conducted. The experimental results showed that the swirling effect influences the local heat flux in three ways. (1) The heat transfer at the stagnation point is severely suppressed. (2) The peak of local heat flux dwells at a radial distance from the stagnation point. (3) The radial position of peak local heat flux shifts farther away from the stagnation point with increasing H . There exists an optimum value of H at which the heat transfer to the target surface is the maximum and the optimum H increases with increasing Ф while the Reynolds number and the swirl number are unchanged. The comparison of IDFs revealed that the swirling IDF has more complete combustion and thus it is accompanied by higher heat transfer rates at small H at which there exists a cool core in the case of the non-swirling IDF. The IDF, however, has worse heat transfer at higher H where the non-swirling IDF achieves complete combustion while the swirling IDF has been cooled by the entrained ambient air. Upon comparing the swirling and non-swirling IDFs at the same Re and Ф , their respective optimum H showed an unfavorable effect of swirl on the overall heat transfer rate which has a reduction of up to 25% in the swirling IDF compared with the non-swirling IDF.

An array of three identical premixed butane–air-fired impinging circular flames with induced-swirl operating at low-pressure and low-Reynolds-number was developed. A swirling motion was imparted successfully to the flame by forcing the butane/air mixture through a specially designed burner assembly before ignition. The burner assembly consisting of a conical base and a nozzle tube into which a cylindrical bar fabricated with three spiral channels was inserted. Its thermal performance was compared with that of a similar impinging flame jet system without induced-swirl. Effects of varying the Reynolds number and the equivalence ratio of the butane/air mixture and the nozzle-to-plate distance on the thermal performance of each of these two impinging flame jet systems were studied. Experiments were conducted with different combinations of Reynolds number, equivalence ratio and nozzle-to-plate distance. In the present investigation, the Reynolds number ranged from 500 to 2500, the equivalence ratio ranged from 1.0 to 1.8 and the nozzle-to-plate distance ranged from 20 mm to 30 mm. To facilitate comparison, flame shapes of both impinging flame jet systems were also visualized by a high speed digital camera system. The comparison showed that the array of three small-scale, low-pressure and low-Reynolds-number premixed butane–air-fired impinging circular flame jets could enhance its thermal performance, with respect to heat transfer characteristics and blow-out limits, by incorporating an induced-swirl. The performance enhancement increased with increasing Reynolds number or equivalence ratio, but decreased with increasing nozzle-to-plate distance.

Dilute chromic acid and the organosilane bis-1,2-(triethoxysilyl)ethane (BTSE) were compared by their abilities to act as post-treatment rinses for samples of 2024-T3 aluminum alloy, which had previously been coated by zinc phosphate. The first approach is conventionally used to improve corrosion protection, but alternatives are needed because of environmental concerns with Cr(VI). Variations in conditions for application of the final rinse, especially through changes in solution concentration, were compared by measuring polarization curves, which provide an indication of corrosion stability. These comparisons were related to coating morphology, which was studied using scanning electron microscopy (SEM), and to studies of chemical composition with X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray (EDX) spectroscopy. Immersion tests in NaCl solution were applied to post-treated zinc phosphate coatings that had been formed on mirror-polished alloy surfaces. Overall, the post-treatment in 4% BTSE at 25 °C appears to provide a comparable corrosion resistance to that found when the final rinse is done in a conventional CrO3 solution (concentration 0.5 g/l) at 70 °C. At the microstructural level, initial observations were also made for comparing the effects of the post-treatments at protecting alloy matrix areas adjacent to Al–Cu–Mg second-phase particles.

We report a new super self-aligned graded SiGe base transistor that uses high energy implantation, rather than epitaxial growth, to form the sub-collector region. This new inexpensive process yields a device with f/sub T/ of 52 GHz and f/sub max/ of 70 GHz with the addition of only 4 lithography levels over our 0.25 /spl mu/m CMOS technology without any changes to the existing process steps. Also, we demonstrate 4:1 multiplexer and 1:4 demultiplexer circuits using this technology that show excellent performance at 10 Gbit/s.

This paper presents a study of the impurity concentration profile of crystals grown by floating crucible technique with the impurity introduced in the outer crucible. When the distribution coefficient of the impurity is 0·1 or less, the concentration is found to increase linearly with the length of the crystal. Further, the concentration at the seed end of the crystal can be designed to start from any percentage of the final concentration. Crystals prepared by this method offer advantages for material or device properties studies in which concentration variation is an important factor. By introducing impurity of the other type in the floating crucible at the same time, compensated crystals can be grown with one impurity concentration varying as described while the other impurity concentration is maintained constant. Calculations of this variable concentration profile are made; families of curves are plotted and the results are discussed.