Tesis sobre el tema "Impinging flame"

Le phénomène de flamme de diffusion impactant une paroi est fréquent dans les scénarios d’incendie en milieu clos. Celui-ci peut entraîner à avoir des conséquences désastreuses en termes de vie humaine et de biens matériels. En effet, lorsqu'une flamme incidente se produit dans un compartiment, elle peut augmenter le risque de propagation du feu de celui-ci vers une autre pièce à travers une explosion de fumée représentant une menace pour les personnes pié-gées. Afin d’apporter des éléments de compréhension sur le comportement de ce type de flamme, de nombreuses études ont réalisé. Celles-ci se sont intéressées sur des flammes impac-tant un plafond en milieu ouvert ou semi-confiné. Cependant il y a peu, voire aucuns travaux qui se sont penchés sur l’étude du comportement d’une flamme incidente dans un compartiment confiné sous ventilé. Dans l’objectif d’apporter des éléments de compréhension en lien avec l’effet du confinement sur la dynamique d’une flamme impactant un plafond, une étude expé-rimentale et numérique est réalisée dans le cadre de cette thèse.L’ensemble des données a été obtenu à l’aide d’un dispositif expérimental représentant un appartement d’étudiant à échelle réduite.Le banc d'essai est un compartiment représentant une maquette d’appartement à petite échelle (1 :10). La conception et dimensionnement a été réalisée sur la base des lois de simili-tudes. Les niveaux de confinement ont été définis en fonction des ouvertures de l’enceinte et du débit calorifique potentielle. A partir de ces deux paramètres, le niveau de confinement peut être associé à la richesse de l’enceinte. Pour cela, huit débits caloriques différents ainsi que cinq possibilités d’ouvertures ont été proposés. À partir des expériences réalisées avec les huit débits calorifiques et les cinq configurations d’ouvertures, l'effet de confinement sur la dynamique d’une flamme impactant un plafond a été effectué en se basant sur les paramètres physico-chimiques, tels que l'extension de la flamme, l'oscillation de la flamme, la distribution de la température et l'analyse des gaz.De plus, grâce à la modélisation numérique de la flamme impactant le plafond à l’aide du code CFD : Fire Dynamics Simulator (FDS), il a été possible d’apporter des éléments supplé-mentaires dans l’analyse des écoulements réactifs associée à l’interaction flamme paroi en fonc-tion du niveau de confinement. Le choix des modèles numériques a été effectué à partir d’une étude préliminaire visant à justifier la fiabilité et la précision du modèle numérique à reproduire les données expérimentales ainsi que des évolutions obtenues à partir de corrélations empiriques obtenues dans les littératures.A partir des analyses réalisées dans cette étude, il est possible de fournir des éléments de décisions lors de la conception et la mise en place de détecteurs d'incendie au plafond dans un compartiment et également d’aider à une meilleure estimation de la probabilité de propagation du feu lors d'un incendie de compartiment par le biais d’une explosion de fumée riche en gaz imbrûlés
The phenomenon of diffusion impinging flame is common in industrials, leading to disas-trous consequences in terms of life and property. When impinging flame occurs in a compart-ment, it may enhance the risk of fire propagation and pose a greater threat to trapped people. Lots of studies dealt with flame impinging an unconfined or confined ceiling while little work focused on the impinging flame in a confined compartment. With the objective of providing understanding related to the confinement effect on the impinging flame in a compartment, both experimental and numerical studies carried out to build up the framework of this thesis. A compartment model representing a reduced scale (1:10) student compartment was uti-lized based on the scaling law such that a test bench with suitable instrumentations for carrying out measurements was developed. Configurations of five confinement levels were constructed by the condition of windows and door in the compartment and heat release rate (HRR) was var-ied between 0.5 kW and 18.6 kW. Through series of experiments, the confinement effect on the dynamics of flame impinging a ceiling was addressed with physicochemical parameters, such as flame extension, flame oscillation, temperature distribution and gas analysis. In addition, on account of the numerical modeling of flame impinging a ceiling using the CFD code: Fire Dynamics Simulator (FDS), it was possible to provide additional elements in the analysis of reactive flows associated with the flame-wall interaction as a function of the confinement level. The choice of numerical models was made on the basis of a preliminary study aimed at justifying the reliability and precision of the numerical modelling in reproducing the experimental data as well as the empirical correlations obtained in the literatures. From the analyzes in this study, it is possible to provide guidance for fire safety engineering in the field of fire risk assessment and fire protection design of buildings

This dissertation examines the use of electric fields as one mechanism for controlling combustion as flames are partially extinguished when impinging on nearby surfaces. Electrical aspects of flames, specifically, the production of chemi-ions in hydrocarbon flames and the use of convective flows driven by these ions, have been investigated in a wide range of applications in prior work but despite this fairly comprehensive effort to study electrical aspects of combustion, relatively little research has focused on electrical phenomena near flame extinguishment, nor for flames near impingement surfaces. Electrical impinging flames have complex properties under global influences of ion-driven winds and flow field disturbances from the impingement surface. Challenges of measurements when an electric field is applied in the system have limited an understanding of changes to the flame behavior and species concentrations caused by the field. This research initially characterizes the ability of high voltage power supplies to respond on sufficiently short time scales to permit real time electrical flame actuation. The study then characterizes the influence of an electric field on the impinging flame shape, ion current and flow field of the thermal plume associated with the flame. The more significant further examinations can be separated into two parts: 1) the potential for using electric fields to control the release of carbon monoxide (CO) from surface-impinging flames, and 2) an investigation of controlling electrically the heat transfer to a plate on which the flame impinges. Carbon monoxide (CO) results from the incomplete oxidation of hydrocarbon fuels and, while CO can be desirable in some syngas processes, it is usually a dangerous emission from forest fires, gas heaters, gas stoves, or furnaces where insufficient oxygen in the core reaction does not fully oxidize the fuel to carbon dioxide and water. Determining how carbon monoxide is released and how heat transfer from the flame to the plate can be controlled using the electric field are the two main goals of this research. Multiple diagnostic techniques are employed such as OH chemiluminescence to identify the reaction zone, OH PLIF to characterize the location of this radical species, CO released from the flame, IR imaging and OH PLIF thermometry to understand the surface and gas temperature distribution, respectively. The principal finding is that carbon monoxide release from an impinging diffusion flame results from the escape of carbon monoxide created on the fuel side of the flame along the boundary layer near the surface where it avoids oxidation by OH, which sits to the air side of the reaction sheet interface. In addition, the plate proximity to the flame has a stronger influence on the emission of toxic carbon monoxide than does the electric field strength. There is, however, a narrow region of burner to surface distance where the electric field is most effective. The results also show that heat transfer can be spatially concentrated effectively using an electric field driven ion wind, particularly at some burner to surface distances.

The experimental work involves calculation of radial distribution of heat transfer coefficient at the surface of a flat Aluminium plate being impinged by a turbulent flame jet. Heat transfer coefficient distribution at the surface is computed from the measured heat flux and temperature data using a reference method and a slope method. The heat transfer coefficient (h) has a nearly bell shaped radial distribution at the plate surface for H/d =3.3. The value of h drops by 37 % from r/d =0 to r/d= 2. Upon increasing the axial distance to H/d = 5, the stagnation point h decreased by 15%. Adiabatic surface temperature (AST) distribution at the plate surface was computed from the measured heat flux and temperature. AST values were found to be lower than the measured gas temperature values at the stagnation point. Radial distribution of gas temperature at the surface was estimated by least squares linear curve fitting through the convection dominated region of net heat flux data and was validated by experimental measurements with an aspirated thermocouple. For low axial distances (H/d =3.3), the gas temperature dropped by only 15 % from r/d = 0 to r/d = 2. Total heat flux distribution is separated into radiative and convective components with the use of calculated heat transfer coefficient and estimated gas temperatures. At H/d = 3.3, the radiation was found to be less than 25 % of the net heat flux for r/d ≤ 2.
Master of Science

Two series of experiments were performed to determine the flow characteristics and to quantify the heat transfer components from a propane diffusion flame impinging onto a ceiling. A 0.3 m square sand burner with propane as the fuel type provided a steady-state fire. In the first series of experiments, measurements of gas temperature and velocity were made at 76 mm vertical intervals above the burner up to the ceiling. Fire heat release rates (HRRs) of 50 kW and 90 kW with free flame length to ceiling height ratios, Lf/H, of 2, 1.5, 1, 0.8, 0.85 were used to determine their effects on the measured parameters. Gas temperatures within the continuous flaming region were relatively constant, and measured to be independent of ceiling height and HRR, while velocities increased with elevation and were independent of ceiling height yet weakly dependent on HRR. Within the intermittent region, gas temperature was weakly affected by the presence of the ceiling at various heights, while the effect on velocity was more pronounced. HRR had an effect on both temperature and velocity within the intermittent region of the fire plume. Comparisons with existing fire plume correlations showed that the unbounded correlations can be used to provide a good approximation of the gas temperature for the ceiling bounded case; while the correlations for the velocity can only be used for elevations up to approximately 60% of the ceiling height. Elevations above this cutoff were significantly affected by the presence of the ceiling. The second series of experiments investigated HRRs of 50 kW and 90 kW with free flame length to ceiling height ratios, Lf/H, of 2, 1.5, and 1. Heat flux and gas temperature at the stagnation point of the ceiling were measured using hybrid heat flux gauges and an aspirated Type K thermocouple. Four methods of calculating the convective heat transfer coefficient, h, were developed and adapted; two reference methods and two slope methods. The components of heat transfer at the impingement point were separated using these calculated h values. The reference method 2, and both slope methods only required the use of the non-cooled hybrid gauge measurements and were in overall good agreement with one another. The reference method 1 differed significantly, being up to 15.8 times lower than the others. The trends in the two groups were contradictory, with the h calculated using the reference method 1 increasing with ceiling height while the others showed no strong trend with ceiling height. The disagreements between the methods greatly affected the components of heat transfer, particularly at the lowest ceiling heights. Convection calculated using the h from reference method 1 contributed only 2-5% of the total exposure heat flux at the lowest ceiling heights, whereas with the other methods convection contributed 20-50% of the total exposure heat flux. The limitations of each method are discussed. Further investigation is required for all methods to determine their applicability within the flaming region of a fire.
Master of Science

Experimental diagnostic techniques have been utilised and developed to investigate the flame wall interaction for impinging flames of propane, methane, hydrogen and syngas. Thermal imaging has been used to evaluate the plate temperatures and radiation losses at steady state. A methodology has been developed for temperature dependent emissivity materials. Schlieren and direct imaging have been used to visualise flame shapes and flow structure. A methodology has been developed to quantify the relative effects of visual turbulent structures on the flame wall interaction. High speed schlieren has been used to assess the time dependent flame front propagation following ignition at various ignition locations. The combination of these techniques has allowed the flame wall interaction to be analysed for fuel composition, thermal loading, equivalence ratio, nozzle-to-plate distance, Reynolds number, geometry and fuel exit velocity. It has been found that fuel composition significantly affects the wall temperature profiles even at similar nozzle conditions. Mixing in different regions of the impingement configuration caused significant differences in the wall temperature profiles for the different fuels due to differences in diffusivity and laminar flame speed. Syngas premixed flames produce similar wall temperature profiles near the lift-off limit but at different equivalence ratios and Reynolds numbers, due to the similar turbulence shown in the schlieren images. Plate material and nozzle-to-plate distance significantly affected the wall temperature profiles. Radiation losses from the plate helped to explain the differences in heat transfer for the different conditions. Delays in the initial downwards propagation were observed for the hydrogen flames. The competing factors of the upstream propagation and heat production, causing decelerations and accelerations of the flame front respectively, differed significantly for different fuels and conditions. The propagation of the flame front immediately after ignition was observed to be very complex, changing significantly for relatively small changes in nozzle conditions.

碩士
元智大學
機械工程研究所
87
The structures of the jet impinging flame are observed under different impinging angles, Renolds number and fuel dilution. The impinging flame of methane fuel is therefore constructed at the stagnation plane. This simple mechanism may reduce the combustion length and the combustion efficiency. But there is a high temperature flashback in the root of flame. When the Renolds number increase, the temperature of impinging flame reduce at the stagnation point and the combustion efficiency increase. Along with the ration of volume nitrogen and volume methane, the combustion length is reduced and the blue proportion of flame is increased. The temperature distribution of flame is uniform, too. Along with the impinging angle is increased, the area of flashback in the root of flame obvious more and more.

碩士
元智大學
機械工程研究所
89
Experimental investigations on the pulsating jet-impinging diffusion flame were executed. A solenoid valve was aligned upstream of the jet orifice and the methane fuel and the outlet-field condition were controlled in open-closed cycles from 2 Hz to 17 Hz. By changing some parameters such as impinging angle, outlet fuel Renault Number and fuel supplying pulsation frequencies, to confer the changing of the temperature contours of a impinging jet diffusion flame. Results show that a solenoid which is the impinging jet flame source supplying a regular disturbing fountainhead to increase the turbulence flow strength can get better flame stability; the flame length between 13 Hz to 15 Hz was lower than that without pulsating, and the impinging angle at 54 degree with more obviously shorting phenomenon can improve the designed length of the burning room; observing temperature contour plane, a solenoid as a fuel supplying source can burn the fuel more completely because the reacting fuel molecules in the flame increase as a result of sucking more air by interrupted frequency. We can get a better burning efficiency when the operating frequency at about 13 Hz in flame temperature contours. Results show that the best operating frequency is at about 13 Hz, and if the frequency were below 10 Hz, it would result an uncontinuous burning and decrease the stability of the flame.

碩士
元智大學
機械工程研究所
88
The experiments invest major on the reverse jet-pumping diffusion flame of burning phenomenon by using equal-mass-fluid-rate to observe the methane fuel and forbid burn nitric of face-to-face-pump phenomenon. And observe the penetration flame configuration change each other with control fuel-fluid-rate. Because forbid burn nitric has the character of endothermic and forbid burn. So, in observing face-to-face-pump of the methane fuel and forbid burn nitric of isothermal pictures , we can be from the change of flames temperature finding out the penetrating depth of jet-impinging diffusion flame .By experiment and observation, the pulsating flame behaves more stable and efficient than the continuous impinging flame. From the frame type and the veins shadow , the flames are normally symmetry. But when the fuel fluid rate varies large, the total flame temperature is non-symmetry , So, we measure the total flame field to get the detail data. The experiment of fluid rate uses following three type sources, 250ml/min(Re=81), 350ml/min(Re=113) and 450ml/min(Re=145). And the experiment also separate two type to study ; The first type is methane fuel to methane fuel; The second type is methane fuel to forbid burn nitric. The experimental results have following three point: (1) If the fluid rate adjust large , the pulsating depth of flame isn’t large and the flame temperature isn’t rising. (2) If the Renold Number is equal to 113 and the fluid rate is 350ml/min , adds twice the pure methane fuel, the combustion effects isn’t increasing. (3) If the methane fuel fluid rate is equal to 350ml/min, the natural air is full, the combustion is well mixed, the pulsating point is on the upright pulsating line , and use equal fluid rate of forbid burn nitric pumping each other, we can obtain more large penetrating depth. Recording to observation and experiment, the reverse jet-pumping pulsating flame is more stable and efficient the reverse jet-pumping diffusion flame of burning phenomenon by using equal-mass-fluid-rate.

碩士
元智大學
機械工程研究所
88
Experimental investigations on the pulsating jet-impinging diffusion flame were executed. A solenoid valve was aligned upstream of the jet orifice and the methane fuel was controlled in open-closed cycles from 2 Hz to 17 Hz. Results show that the open-closed cycles indeed increase the fluctuations of the methane fuel obviously. The evolutions of pulsating flame therefore develop faster than the continuous impinging flame. The optimized pulsating frequencies are near 9 to 11 Hz from the Re = 97 to 161. The temperature differences between that under optimized pulsating rate and full open condition (no pulsation) are ranging from 100 to 150 degree. The pulsating effect is more significant at low Reynolds number. When Re=129, the tip of the impinging flame obviously crosses at Y/Din=23 above the impinging point. Because of the phenomenon of pulsation flame, the flame sheet or flame front may not be identified clearly in the averaged temperature contours. When Re=129, 11 Hz. Results show that the averaged end-contour of pulsation flame rears at Y/Din=21 above the impinging point. By observation and experiment, the pulsating flame behaves more stable and efficient than the continuous impinging flame.

碩士
國立臺灣大學
機械工程學研究所
105
In this work, the innovative burner with v-shaped circular impinging structure was proposed. An experimental study was conducted to investigate the influence of burner structure on the feature of premixed propane flame. The design applied in the burner may reduce the consumption of fuel and improve the performance simultaneously. There are two main sections in this thesis. Firstly, the superior performance was verified through the comparison between v-shaped circular impinging burner and circular planar burner. The v-shaped circular impinging structure was beneficial to the numerous characteristics of combustion. The experimental results exhibited that blue flame (ϕ = 1.0) was the most stable flame type in the four types identified and was less influenced by surrounding flow filed from the high-resolution pictures took. This mechanism of impinging flame extended the stable operating region and effectively reduced the limit of blow-out flame and lift-off flame happening because of the preheated effect. Based on distribution of the temperature field, the high temperature zone was more concentrated and around 50 ℃ higher than circular planar burner. The v-shaped impinging structure in the rectangular outlet region enhanced the speed of chemical reaction by high temperature and made the unburned gas effectively blend in the central region. Furthermore, the computed results of non-intrusive diagnostics (PIV) revealed that the velocity and vorticity of v-shaped circular impinging burner were dramatically strengthened double compared to circular planar burner due to thermal buoyancy and shear stress. The mixing of unburned gas was proved by computing the horizontal velocity. There were also numerous vortices observed in the pictures of visualization, they swirled the outside air to the combustion field and increased the mixing of fuels and oxidants. According to the theoretical analyses and the experimental results discussed above, the increased flame interaction caused by the impinging structure takes several advantages to flame feature relative to circular planar burner. Based on the results of the first section, the v-shaped circular impinging burner was redesigned and discussed in the second section. The overall size was minified to reduce the interval between flames, and the angle of impinging was changed to find a suitable angle used in v-shaped circular impinging structure. The results show that the highest temperature was measured when the impinging angle was 60 degree. However, the highest magnitudes of velocity and vorticity were measured when the impinging angle was 30 degree. It seems possible that these results are due to different angles of impinging and chemical reaction. Therefore, what the angle of impinging should be chosen is dependent on the objective of use. This research may be the significant reference for the industrial and domestic applications using v-shaped circular impinging structure.

碩士
元智大學
機械工程學系
93
A Study on Impinging diffusion Flame with Splash Plate Student：Meng-Yuan Tsai Advisors：Dr. Ay Su Institute of Mechanical Engineering Yuan-Ze University ABSTRACT The objective of this study is to enhance the combustion efficiency of the one-direction and reciprocal jet impinging flames by using different splash plates and regulating the locations of splash plates and nozzles at various momentum ratios and Reynolds numbers. To investigate the combustion efficiency, a splash plate was installed between two nozzles (72.5o included angle). Air and methane were supplied form two nozzles for adjusting operational conditions. A K-type thermal couple was employed to measure the temperature profiles of the flames. In this study, a plane and an incline splash plates were employed. The momentum ratios between methane and air were set to be 1 and 0.8, and the fuel of Reynolds numbers were set to be 225 and 290. The results show that the combustion efficiency increased by employing the incline splash plate with one-direction flames: Teff=7.1% at Refuel=225 and Teff=11.3% at Refuel=290. Splash plates also can enhance the flame at lower Reynolds numbers. The Teff= 10.7% by using the plant splash plate, and Teff=6.5% by using the incline splash plate at Refuel =225 than these at Refuel =290. At heterogenous jet impinging diffusion flame conditions, the Teff= 7.63% without the splash plate when the air/methane momentum ratios decreased from 1 to 0.8. The Teff=7.3% when the fuel of Reynolds number increased from 225 to 290 due to the enhancement of the velocity field. At Refuel =225 with an incline splash plate, the Teff= -8.9% when nozzles were fully-blocked than half-blocked at the same momentum ratio. In contrary, the Teff= 6.7% at Refuel =290. When the momentum ratios decreased from 1 to 0.8, the Refuel =6.7% when nozzles were half -blocked and Teff =8.3% when nozzles werefully-blocked. In conclusions, to enhance temperature-effective, employing the incline splash plate is better than the plane one. Higher Reynolds numbers and lower momentum ratios also can increase the Teff value.

碩士
中正理工學院
兵器系統工程研究所
88
The objective of the study is to observe phenomenon of impacted metal surface and fire suppression and fire extinguishment by droplet impingement. The first part is to observe phenomenon of impacted metal surface by droplet impingement. The objective of the present work is systematically to study the effects of flow property and surface characteristic on the fluid dynamic by employing the high-speed CCD camera techniques and experimental measurements. The experimental work is unique in that the droplet impact factor is systematically rated in different non-dimensional numbers and parameters such as Reynolds number, Weber number. The parameter study includes the effects of the Reynolds number, Weber number, surface material (thermal diffusivity) and temperature. The dynamic behavior of droplet impingement will be mainly characterized by the splash, spread, rebound. The second objective of the study is to investigate the phenomenon of fire suppression of a kerosene lamp by water droplet impinging on the flame. The macro- and micro-photographic was used in this experimental work to obtain quantitative data. The main part of the experimental setup is the CCD camera assembled with micro- or macro-photographic. A monodisperse water droplet generator was used to generate a droplet stream. The stream was used to extinguishing the flame. The concerned parameters were flow rate, droplet size and flame size etc. The results show that the suppression of a flame without touching the lamp core could be related to the droplet size, frequency and velocity, which could cool down the flame and interfere in the structure of the flame.

碩士
國立臺灣大學
機械工程學研究所
101
The mechanisms of lean combustion of propane with/without CO addition on a V-shaped burner have been investigated through the analysis of both flow and combustion fields. The flame patterns and flame temperatures under different mixing conditions were measured and recorded. Moreover, the measurement of chemiluminescence and the flow visualization were applied to obtain the distributions of C2* and CH* intensity, and flow velocity, respectively, during the combustion process. Also, the exhaust gas was analyzed in order to identify the participation of CO in the combustion taking place on the V-shaped burner. The flame patterns observed in this work could be divided into two groups, M-typed and hill-typed flames. The empirical results showed that the flame propagation speed increased with CO concentration in the fuel, and the addition of CO might cause a change of the flame pattern from hill-type to M-type. Besides, both the flame temperature and the intensity of chemiluminescence increased, and the lower flammability limit decreased when CO concentration in the fuel rose. By analyzing the chemiluminescence and the exhaust gas, the imping effect caused by the V-shaped burner was found improving the combustion of mixtures of Propane/CO due to the high temperature in the impinging area. In the M-typed flame, the collision of gas jets reduced heat loss, and hence the impinging flow structure was able to maintain a high temperature, so that the combustion of CO, whose activation energy is comparatively high, can still take place in the impinging area. The findings provide industries a good concept of burning CO, and it can be further extended to the combustion of syngas.

博士
國立臺灣大學
機械工程學研究所
106
Abstract Syngas, a clean and alternative fuel, has a great potential to replace hydrocarbon fuels in combustion applications. An impinging flow field has attracted interest in the investigation of its mixing characteristics of fuel and oxidant in many fuel-injection systems, but up to now the research on jet-impinging flames has been focused mainly on diffusion flames with hydrocarbon fuels. For a practical application, we therefore propose a concept of clean combustion through combining the advantages of syngas and an impinging burner. Furthermore, the varied proportions of H2 and CO are the crucial causing a variation in the fuel mixing and combustion reaction when using syngas as a principal fuel. We performed experimental measurements of particle image velocimetry (PIV), chemiluminescence of free radicals, flame temperature, and CO emission to examined how and why the varied proportions of H2 and CO affected the fuel mixing and combustion reaction of a syngas premixed impinging flame. For a C3H8 premixed impinging flame on the V-shaped burner, its flame propagation speed increased with the addition of H2 and CO into the fuel mixture, which expanded its lean flammability. The addition of H2 in the fuel mixture enhanced the reaction intensity of flame sheet, but, decreased the reaction intensity of flame tip, which shows that the reaction zone was dominated by strong mass diffusivity. The temperature of flame sheet hence increased, and the temperature of flame tip decreased with increasing H2 proportion. Although the mass diffusivity of reaction zone on the flame sheet became weaker when CO presented a large proportion of fuel, the fuel mixture conducted the second reaction within the impinging zone through the well preheating and deceleration. The reaction intensity of impinging zone hence increased, and the emission of CO decreased. We further examined the characteristics of fuel mixing and reaction of CH4/syngas/air impinging flame with H2/CO in varied proportions using a multi-way impinging burner. The results showed that a deceleration area in the main flow formed through the mutual impingement of two jet flows, which enhanced the mixing of fuel and air because of an increased momentum transfer. The deceleration area expanded with an increased CO proportion, which indicated that the mixing of fuel and air also increased with the increased CO proportion. CO provided in the syngas hence participated readily in the reaction of the CH4/syngas/air premixed impinging flames when the syngas contained CO in a large proportion. Our examination of the OH* chemiluminescence demonstrated that its intensity increased with increased CO proportion, which showed that the reaction between fuel and air accordingly increased. Finally, to enhance the reaction intensity, we introduce a central air jet injecting into a CH4/syngas/air impinging flame. For a fuel-rich CH4/syngas/air impinging flame, the added central air jet caused no acceleration of the fuel mixture flowing toward downstream when ratio UC/UF was less than 1.0. The fuel mixture obtained additional oxidant from the central air jet, which increased its reaction intensity; the CO emission hence decreased and the flame temperature increased when the UC/UF ratio was less than 1.0. When UC/UF exceeded 1.5, however, the central air jet caused the fuel mixture to accelerate in its escape downstream because of the increased upward momentum; the reaction intensity thus exhibited a decreasing trend and the CO emission greatly increased. The results shown in our work provide a significant reference and a prospective concept for the utilization of syngas, which improves the feasibility of fuel-injection systems using syngas as an alternative fuel.

碩士
國立成功大學
機械工程學系碩博士班
100
This investigation explores the flame transition process and impinging phenomenon of a burning dodecane drop in a convective high-temperature gaseous environment. In the experiment, the plate was inserted at three different positions in the high-temperature flow. And the temperature of the impinging plate was 720, 509 or 407 (oC). The drop diameter ranged from 400 to 1400 μm. Weber number ranged from 150 to 1800. A stream of burning dodecane drops moved with the high-temperature gaseous flow and impinged onto the plate. By changing the drop diameter, the Weber number was varied. The drop was ignited after a certain distance into the combustion chamber. When the forced convection term equaled the natural convection term, the flame became a spherical flame. As the drop approached the heated plate, the flame moved toward the trailing side of the drop because the drop was accelerated by gravity. The gas flow slowed down as the drop approached the plate because of the stagnation-point flow near the plate. The drop impinged the quartz plate with the flame completely leaving the drop. After impact, the drop experienced expansion and contraction to disintegrate. There were two different expansion types, namely crown film expansion and plate film expansion. And there were two disintegration types, namely uniform disintegration and non-uniform disintegration depending on the Weber number (We).

碩士
元智大學
機械工程學系
92
Abstract The research is to study the effect of jet momentum ratio to the combustion efficiency in unlike jet impinging diffusion flame. In the set-up, one of the methane tubes is substituted by the air, by adjusting the momentum ratio to observe the flame structure and measure the flame temperature to determine the efficiency. In addition, an infrared economizer is added to the methane tube to compare the difference. The impinging angle is fixed at 72.5 degrees. By changing the fuel supply rate, momentum ratio between air and methane and economizer, we visualize the flame structure and calculate the average temperature per unit area. The results of X-Y plane temperature profile and Schlieren photoghaphy indicate the optimum momentum ratio between methane and air is 1:1.25. The high temperature area is highly concentrated and the flame is stabilized. The average temperature per unit area reaches 978 degrees. The results with infrared economizer can increase efficiency up to 9.6% at momentum ratio 1:1. Keywords：impinging diffusion flame, un-like fuel

博士
國立成功大學
機械工程學系碩博士班
92
This investigation is aimed at studying the characteristics of laminar conical premixed flames in an impinging jet flow experimentally, numerically, and theoretically with emphasis on the effects of negative flame stretch from flame curvature, positive stretch from the flow field and preferential diffusion. 　　The results show that flame shapes exhibit double-solution characteristics in a certain range of methane concentration. Experimentally, by following different paths of adjusting methane concentration (decreasing from rich to lean or increasing from lean to rich), two different flame configurations (planar or conical flame) may exist at the same flow conditions: burner-to-plate distance, inlet velocity and methane concentration. At the higher (or lower) critical methane concentration, the transition from a flat flame to a conical flame (or from a conical flame to a flat flame) occurs. When the operating condition is in this region, the flame is unstable since an externally mechanical disturbance may transform the flame shape back and forth between conical flame and flat flame, i.e., flames in the double solution region are unstable. The double solution region is strongly influenced by the burner-to-plate separation distance (H/d) and inlet velocity. Lower H/d or lower inlet velocity decreases the double solution region; In other words, the flame is relatively stable at lower H/d or lower inlet velocity. Instability in the double solution region may strongly affect the flame shapes. Therefore, the design of low-Reynolds-number heating devices, such as domestic gas burners, should take the double solution region into consideration, especially for those used in lean premixed flame applications. 　　Stretch calculation along a conical flame in an impinging flow shows the conical flame still endures negative stretch. However, the effect of positive flow stretch due to the impinging flow reduces the extent of negative stretch. When the methane flame is outward open-tip or flat, it then receives positive stretch. 　　Theoretical analysis reveals that for a negatively-stretched conical flame in a positively-stretched flow, positive stretch is a positive effect to the lean methane flame (Le<1), but negative to rich methane flame (Le>1). The downstream heat loss is a negative effect to both rich and lean methane flame (Le>1 and Le<1). Experimental results agree well with the theoretical and numerical analysis qualitatively.

To model the combustion of long-chain hydrocarbon fuels, an accurate kinetics mechanism must first be developed for the oxidation of small hydrocarbons, such as methane, ethane, and ethylene. Even for methane, a generally accepted mechanism is still elusive due to a lack of kinetically independent experimental data. In this work, a combined experimental and modeling technique is developed to validate and further optimize these mechanisms. This technique relies on detailed measurements of strained flames in a jet-wall stagnation flow using simultaneous Particle Streak Velocimetry (PSV) and CH Planar Laser Induced Fluorescence (PLIF). Stagnation flames are simulated using an axisymmetric, one-dimensional model with accurate specification of the requisite boundary conditions. Direct comparisons between experiment and simulation allow for an assessment of the various models employed, with an emphasis on the chemistry model performance.

The flow field for a cold impinging laminar jet is found to be independent of the nozzle-to-plate separation distance if velocities are scaled by the Bernoulli velocity. The one-dimensional formulation is found to accurately model the stagnation flow if the velocity boundary conditions are appropriately specified. The boundary-layer-displacement-thickness corrected diameter is found to be an appropriate scale for axial distances and allows the identification of an empirical, analytical expression for the flow field of the impinging laminar jet.

Strained methane-air flame experiments confirm that the reacting flow is also independent of the nozzle-to-plate separation distance. Methane, ethane, and ethylene flames are studied as functions of the applied strain rate, mixture dilution, and mixture fraction. Mechanism performance is found to be relatively insensitive to both the mixture dilution and the imposed strain rate, while exhibiting a stronger dependence on the fuel type and flame stoichiometry. The approach and diagnostics presented here permit an assessment of the predictions of strained-hydrocarbon flames for several combustion chemistry mechanisms. The data presented in this thesis are made available to kineticists looking for optimization targets, with the goal of developing a predictive kinetics model for hydrocarbon fuels. The methodology described here can allow new optimization targets to be rapidly measured, reducing the experimental burden required to fully constrain the chemistry models.