Effects of Damköhler Number on Methane/Oxygen Tubular Combustion Diluted by N2 and CO2

2016 ◽  
Vol 139 (1) ◽  
Author(s):  
Baolu Shi ◽  
Qingzhao Chu ◽  
Run Chen
Keyword(s):  
Reaction Time ◽  
Fuel Injection ◽  
Swirling Flow ◽  
Mixing Time ◽  
Velocity Ratio ◽  
Inert Gases ◽  
Injection Velocity ◽  
Damkohler Number ◽  
Helical Structures ◽  
Damköhler Number

To fundamentally elucidate the mixing and its effects on the characteristics of methane/oxygen flame in a rapidly mixed tubular flame burner, experiments were conducted under various oxygen mole fractions and flow rates. Two inert gases of nitrogen and carbon dioxide were used, respectively. The inert gas was added to both the oxidizer and fuel slits to maintain the oxidizer/fuel injection velocity ratio near unity. Based on flow visualization, the mixing process around injection slits and that in the axial downstream were discussed. The Damköhler number (Da1), defined as the ratio of molecular mixing time to reaction time, was selected as a parameter to quantitatively examine the criterion for the establishment of tubular flame from low to ultrahigh oxygen mole fractions (0.21–0.86). The mixing around slit exit determined the tubular flame establishment. Due to a flow time between two neighboring injection slits of fuel and oxidizer, part of the fuel was mixed in the downstream swirling flow, resulting in luminous helical structures. Hence, the Damköhler number (Da2), defined as the flow to the reaction time ratio, was examined. Detailed observations indicated that when Da2 was smaller than unity, the flame was uniform in luminosity, whereas the flame was nonuniform when Da2 ≥ 1. The value of Da2 was about 1.5 times as Da1; however, they correspond to different mixing zones and Da2 can be more easily calculated. The differences in flame stability between N2 and CO2 diluted combustion were also studied.

Effects of Fuel Overpenetration and Overmixing During Ignition-Delay Period on Hydrocarbon Emissions From a Small Open-Chamber Diesel Engine

1988 ◽  
Vol 110 (3) ◽  
pp. 453-461 ◽  
Author(s):  
T.-W. Kuo ◽  
K.-J. Wu ◽  
S. Henningsen
Keyword(s):  
Diesel Engine ◽  
Ignition Delay ◽  
Fuel Injection ◽  
Swirling Flow ◽  
Delay Period ◽  
Injection Velocity ◽  
Spray Penetration ◽  
Hc Emissions ◽  
Start Of Combustion ◽  
Ignition Delay Period

A quasi-steady gas-jet model was applied to examine the spray trajectory in swirling flow during the ignition-delay period in an open-chamber diesel engine timed to start combustion at top dead center. Spray penetration, deflection, and the fractions of too-lean-mixed, burnable, and overpenetrated fuel at the start of combustion were calculated by employing the measured ignition delay and mean fuel-injection velocity. The calculated parameters were applied to correlate the measured exhaust hydrocarbon (HC) emissions. The engine parameters examined were bowl geometry, compression ratio, overall air-fuel ratio, and speed. Both the ignition delay and the relative spray-penetration parameter, defined as the ratio of the spray-penetration distances at the moments of start of combustion and wall impingement, gave good correlations for some of the engine parameters examined but could not explain all the measured trends. However, good correlation of the measured exhaust HC emissions was obtained by using the calculated too-lean-mixed and overpenetrated fuel fractions at the start of combustion. Correlation of the overpenetrated fuel with the measured HC indicated that approximately 2 percent of the fuel mass that overpenetrated before start of combustion emitted from the engine as unburned HC. This could account for 0 to 65 percent of the total HC emission from this engine. Additionally, it was found that the too-lean-mixed fuel could contribute 10 to 30 percent of the total HC emission, as found in a previous study on a somewhat similar engine. The remaining HC emission is caused by other sources such as bulk quenching.

Numerical Simulations of Highly Preheated Air Combustion in an Industrial Furnace

1998 ◽  
Vol 120 (4) ◽  
pp. 276-284 ◽  
Author(s):  
T. Ishii ◽  
C. Zhang ◽  
S. Sugiyama
Keyword(s):  
Numerical Simulations ◽  
Production Rate ◽  
Fuel Injection ◽  
Velocity Ratio ◽  
Combustion Process ◽  
Moment Closure ◽  
Combustion Model ◽  
Injection Velocity ◽  
Heating Efficiency ◽  
Reheat Furnace

The numerical simulations of reactive turbulent flows and heat transfer in an industrial slab reheat furnace in which the combustion air is highly preheated have been carried out. The influence of the ratio of the air and fuel injection velocities on the NOx production rate in the furnace has also been studied numerically. A moment closure method with the assumed β probability density function (PDF) for mixture fraction was used in the present work to model the turbulent non-premixed combustion process in the furnace. The combustion model was based on the assumption of instantaneous full chemical equilibrium. The turbulence was modeled by the standard k-ε model with a wall function. The numerical simulations have provided complete information on the flow, heat, and mass transfer in the furnace. The results also indicate that a low NOx emission and high heating efficiency can be achieved in the slab reheat furnace by using low NOx regenerative burners. It is found that the air/fuel injection velocity ratio has a strong influence on the NOx production rate in the furnace.

scholarly journals Homogeneous and Inhomogeneous Mixing in Cumulus Clouds: Dependence on Local Turbulence Structure

2009 ◽  
Vol 66 (12) ◽  
pp. 3641-3659 ◽  
Author(s):  
Katrin Lehmann ◽  
Holger Siebert ◽  
Raymond A. Shaw
Keyword(s):  
Reaction Time ◽  
Time Scale ◽  
Inertial Subrange ◽  
Observation System ◽  
Length Scale ◽  
Damkohler Number ◽  
Mixing Process ◽  
Cumulus Clouds ◽  
Damköhler Number ◽  
Transition Length

Abstract The helicopter-borne instrument payload known as the Airborne Cloud Turbulence Observation System (ACTOS) was used to study the entrainment and mixing processes in shallow warm cumulus clouds. The characteristics of the mixing process are determined by the Damköhler number, defined as the ratio of the mixing and a thermodynamic reaction time scale. The definition of the reaction time scale is refined by investigating the relationship between the droplet evaporation time and the phase relaxation time. Following arguments of classical turbulence theory, it is concluded that the description of the mixing process through a single Damköhler number is not sufficient and instead the concept of a transition length scale is introduced. The transition length scale separates the inertial subrange into a range of length scales for which mixing between ambient dry and cloudy air is inhomogeneous, and a range for which the mixing is homogeneous. The new concept is tested on the ACTOS dataset. The effect of entrained subsaturated air on the droplet number size distribution is analyzed using mixing diagrams correlating droplet number concentration and droplet size. The data suggest that homogeneous mixing is more likely to occur in the vicinity of the cloud core, whereas inhomogeneous mixing dominates in more diluted cloud regions. Paluch diagrams are used to support this hypothesis. The observations suggest that homogeneous mixing is favored when the transition length scale exceeds approximately 10 cm. Evidence was found that suggests that under certain conditions mixing can lead to enhanced droplet growth such that the largest droplets are found in the most diluted cloud regions.

Effects of the Air-Fuel Ejection Velocity Ratio on the Combustion Characteristics and the Unburned Gas Compositions of the Propane–Air Rapidly Mixed Tubular Flame Combustion

2011 ◽  
Author(s):  
Y. Wang ◽  
K. Kimura ◽  
N. Gokita ◽  
D. Shimokuri ◽  
S. Ishizuka
Keyword(s):  
Equivalence Ratio ◽  
Fuel Injection ◽  
Velocity Ratio ◽  
Gas Analysis ◽  
Air Injection ◽  
Range Shifts ◽  
Injection Velocity ◽  
Stable Combustion ◽  
Rich Mixture ◽  
The Difference

In this paper, effects of the difference between the air injection velocity and the fuel injection velocity on the rapidly mixed tubular flame have been investigated. A parameter of αst which is the ratio of the air injection velocity to the fuel injection velocity at stoichiometric condition has been introduced, and five tubular flame burners with different αst, 0.6, 1.2, 2.4, 6.0 and 11.9 were examined. Stability limits of the propane-air flame and the local fuel concentrations of unburned mixture have been determined. Results show that, with αst = 0.6 and αst = 1.2, in burner a stable tubular flame can be established in the range of Φ = 0.45 to 2.1 and Φ = 0.48 to 2.15. When αst is increased to 2.4, in which the air injection velocity is almost two times higher than that of the fuel at stoichiometric condition, the stable combustion range shifts to the relatively fuel rich side of Φ = 0.55 to 2.35. With further increase in the αst to 6.0 and 11.9, stable combustion range shifts to richer side of Φ = 0.6 to 2.45, and Φ = 0.7 to 2.9, respectively. Results of gas analysis have revealed that, for αst = 0.6 and αst = 1.2, although the total equivalence ratio of supplied air and fuel were stoichiometric, a fuel rich mixture gas of Φ = 1.13 and Φ = 1.17 was formed locally at the center of the burner. Increasing in the αst leads to a decrease in the local equivalence ratio, such as Φ = 0.95, 0.42, and 0.19 for αst = 2.4, 6.0 and 11.9, respectively. These results indicate that the mixing process of air and fuel in the rapidly mixed tubular flame is greatly affected by the injection velocity ratio, suggesting the possibility of the flame front structure control by the injection velocity ratio.

Dual-Location Fuel Injection Effects on Emissions and NO*/OH* Chemiluminescence in a High Intensity Combustor

2016 ◽  
Vol 138 (4) ◽  
Author(s):  
Ahmed O. Said ◽  
Ahmed E. E. Khalil ◽  
Ashwani K. Gupta
Keyword(s):  
Fuel Injection ◽  
Single Injection ◽  
Mixing Time ◽  
Fuel Mixture ◽  
Combustion Noise ◽  
Fuel Distribution ◽  
Mixture Preparation ◽  
Pollutants Emission ◽  
Fuel Mixing ◽  
Colorless Distributed Combustion

Colorless distributed combustion (CDC) has shown to provide ultra-low emissions of NO, CO, unburned hydrocarbons, and soot, with stable combustion without using any flame stabilizer. The benefits of CDC also include uniform thermal field in the entire combustion space and low combustion noise. One of the critical aspects in distributed combustion is fuel mixture preparation prior to mixture ignition. In an effort to improve fuel mixing and distribution, several schemes have been explored that includes premixed, nonpremixed, and partially premixed. In this paper, the effect of dual-location fuel injection is examined as opposed to single fuel injection into the combustor. Fuel distribution between different injection points was varied with the focus on reaction distribution and pollutants emission. The investigations were performed at different equivalence ratios (0.6–0.8), and the fuel distribution in each case was varied while maintaining constant overall thermal load. The results obtained with multi-injection of fuel using a model combustor showed lower emissions as compared to single injection of fuel using methane as the fuel under favorable fuel distribution condition. The NO emission from double injection as compared to single injection showed a reduction of 28%, 24%, and 13% at equivalence ratio of 0.6, 0.7, and 0.8, respectively. This is attributed to enhanced mixture preparation prior to the mixture ignition. OH* chemiluminescence intensity distribution within the combustor showed that under favorable fuel injection condition, the reaction zone shifted downstream, allowing for longer fuel mixing time prior to ignition. This longer mixing time resulted in better mixture preparation and lower emissions. The OH* chemiluminescence signals also revealed enhanced OH* distribution with fuel introduced through two injectors.

Diesel engine performance mapping using a parametrized mixing time model

2017 ◽  
Vol 19 (2) ◽  
pp. 202-213 ◽  
Author(s):  
Michal Pasternak ◽  
Fabian Mauss ◽  
Christian Klauer ◽  
Andrea Matrisciano
Keyword(s):  
Diesel Engine ◽  
Fuel Injection ◽  
Mixing Time ◽  
Combustion Process ◽  
Engine Performance ◽  
Injection Timing ◽  
Engine Control ◽  
Reactor Model ◽  
Time Model ◽  
Tabulated Chemistry

A numerical platform is presented for diesel engine performance mapping. The platform employs a zero-dimensional stochastic reactor model for the simulation of engine in-cylinder processes. n-Heptane is used as diesel surrogate for the modeling of fuel oxidation and emission formation. The overall simulation process is carried out in an automated manner using a genetic algorithm. The probability density function formulation of the stochastic reactor model enables an insight into the locality of turbulence–chemistry interactions that characterize the combustion process in diesel engines. The interactions are accounted for by the modeling of representative mixing time. The mixing time is parametrized with known engine operating parameters such as load, speed and fuel injection strategy. The detailed chemistry consideration and mixing time parametrization enable the extrapolation of engine performance parameters beyond the operating points used for model training. The results show that the model responds correctly to the changes of engine control parameters such as fuel injection timing and exhaust gas recirculation rate. It is demonstrated that the method developed can be applied to the prediction of engine load–speed maps for exhaust NOx, indicated mean effective pressure and fuel consumption. The maps can be derived from the limited experimental data available for model calibration. Significant speedup of the simulations process can be achieved using tabulated chemistry. Overall, the method presented can be considered as a bridge between the experimental works and the development of mean value engine models for engine control applications.

Rheology Innovation in the Study of Mixing Conditions of Polymer Blends during Chemical Reaction

2005 ◽  
Vol 15 (5) ◽  
pp. 314-325 ◽  
Author(s):  
C. Lacoste ◽  
L. Choplin ◽  
P. Cassagnau ◽  
A. Michel
Keyword(s):  
Reaction Time ◽  
Polymer Blends ◽  
Rheological Properties ◽  
Chemical Reaction ◽  
Mixing Time ◽  
Reactive Systems ◽  
Diffusion Time ◽  
Mixing Conditions ◽  
Stirred Tanks ◽  
Polymer Blending

Abstract Polymer melts can be mixed with many monomers, plasticizers, antistatics or foaming additives. Properties of such mixtures can change during blending because of chemical reactions like polymerization or crosslinking. The process may be carried out either in stirred tanks, extruders or in motionless mixers. In this paper we focused on the mixing time and the diffusion time of reagent, plasticizer and polymer thanks to rheological tools, and on the way how rheological properties can be studied during chemical reaction in polymer blending. The concept of rheoreactor and Couette analogy were introduced since we have a reactor on our disposal that can mix solution and measure rheological properties without taking sample. This apparatus appears to be an appreciable tool in complement of internal mixers that are specific to polymer blending. For example, we show the importance of the competition between mixing time and reaction time for reactive systems.

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