• 한국연소학회:학술대회논문집
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• 2004.06a
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• pp.49-56
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• 2004

A two-dimensional twin-jet counterflow system has been designed, in which two streams from two double-slit nozzles form a counterflow. This flow system enables one to systematically investigate various effects on non-premixed flames, including the non-premixed flame interaction, the edge flame behavior and the effect of curvature. Non-premixed flame interaction in the twin-jet counterflow system has been investigated numerically for methane fuel diluted with nitrogen. Three types of non-premixed flame(conventional counterflow flame, crossed twin-jet flame and petal shaped flame) were simulated depending on the combination of fuel/oxidizer supply to each nozzle. The extinction characteristics of non premixed methane flame in the twin-jet counterflow have been investigated numerically. The boundary of the existence of petal-shaped flames was identified for the twin-jet counterflow flames. Due to the existence of the unique petal-shaped flames, the extinction boundary for the twin-jet counterflow can be extended significantly compared to that for the conventional counterflow non-premixed flames, through the interaction of two flames. Through the comparison of the crossed twin-jet flame and the conventional counterflow flame, structure of the crossed twin-jet counterflow flame is analysed. Through the comparison of the petal shaped flame and the conventional counterflow flame, the extension of the extinction boundary for the twin-jet counterflow is investigated.

• 한국연소학회:학술대회논문집
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• 2002.11a
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• pp.25-31
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• 2002

For the better understanding of the stability of turbulent combustion, more researches on extinction and re-ignition are needed. Flame interactions in non-premixed flame have also not been greatly researched. We made a hybrid twin jet flame, the combinations of diffusion flame and partially-premixed diffusion flame, in a twin jet counterflow configuration. The extinction limits of a crossed twin jet counterflow have been extended in comparison with those of a one-dimensional counterflow because of flame interactions through heat transfer and joint ownership of various radicals. Besides, we have obtain ignition $Damk\"{o}hler$ number by experimental method without external ignition source using the extinction characteristic in a crossed twin jet counterflow flame. From results, we can identify the hysteresis between extinction and ignition $Damk\"{o}hler$ number in S-curve.

• Journal of the Society of Disaster Information
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• v.15 no.1
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• pp.76-83
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• 2019

Purpose: The purpose of this study is to present data of density and speed through the experiment of the counterflow by firefighter in corridor. Method: Experimental setup including a corridor in building was prepared for measuring data with 1.5m and 2m width. Normal flow and counterflow were created for each. Data were measured using camera and acquired by video image analysis. Results: The counterflow in corridor resulted in increasing average density of about $0.55P/m^2$ and decreasing average movement speed of about 0.61 m/s. These data measured during the time when the counterflow occurred. Conclusion: It was found that counterflow by firefighter in corridor momentary increasing the density and decreasing walking speed of evacuee. Further experiments of the counterflow effect in the total evacuation time are needed.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2003.05a
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• pp.37-40
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• 2003

The thrust vector control using a fluidic counterflow concept is achieved by applying a vacuum to a slot adjacent to a primary jet which is shrouded by a suction collar. The vacuum produces a secondary reverse flowing stream near the primary. The shear layers between the two counterflowing streams mix and entrain mass from the surrounding fluid. The presence of the collar inhibits mass entrainment and the flow m the collar accelerates causing a drop in pressure on the collar. For the vacuum asymmetrically applied to one side of the nozzle, the jet will vector toward the low-pressure region. The present study is performed to investigate the effectiveness of thrust vector control using the fluidic counterflow concept. A computational work is carried out using the two-dimensional, compressible Navier-Stokes equations, with several kinds of turbulence models. The computational results are compared with the previous experimental ones. It is found that the present fluidic counterflow concept is a viable method to vector the thrust of a propulsion system.

• 한국연소학회:학술대회논문집
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• 2002.11a
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• pp.33-39
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• 2002

Unsteady behaviors of counterflow flame were studied experimentally in opposing jet counterflow burner using diluted methane. To generate the unsteadiness on the flame, the counterflow diffusion flame was perturbed by velocity changes made by the pistons installed on both sides of the air and fuel stream. The velocity changes were measured by Hot wire and Laser Doppler Velocimetry, and the flame behaviors were observed by High speed ICCD and ICCD. In this investigation, the spatial irregularity of the strain rate caused the flame to extinguish from the outside to the axis during the extinction, and we found the following unsteady phenomena. First, the extinction strain rates of unsteady cases are much larger than those of the steady ones. Second, the extinction strain rates become larger as the slope of the change of the strain rate increases. Third, the unsteady extinction strain rates become smaller with the increase of the initial strain rate.

• Progress in Superconductivity and Cryogenics
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• v.7 no.2
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• pp.1-6
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• 2005

The Particle Image Velocimetry (PIV) technique can be used to obtain a whole-field view of thermal counterflow velocity profile in He II. Using commercially available microspheres, we have been able to visualize the normal fluid velocity in He II thermal counterflow; however, the measured velocities are less than predicted from the two fluid model. None the less, the PIV is a useful tool for observing the counterflow field in He II flow, particularly where the flow is complex as occurs through channel constrictions or around bluff objects. The present paper shows recent results using PIV to observe He II counterflow. Two cases are discussed: 1D channel flow and turbulent flow around a circular cylinder.

• Proceedings of the KSME Conference
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• 2003.04a
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• pp.1948-1954
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• 2003

The thrust vector control using a fluidic counterflow concept is achieved by applying a vacuum to a slot adjacent to a primary jet which is shrouded by a suction collar. The vacuum produces a secondary reverse flowing stream near the primary jet. The shear layers between the two counterflowing streams mix and entrain mass from the surrounding fluid. The presence of the collar inhibits mass entrainment and the flow near the collar accelerates causing a drop in pressure on the collar. For the vacuum asymmetrically applied to one side of the nozzle, the jet will vector toward the low-pressure region. The present study is performed to investigate the effectiveness of thrust vector control using the fluidic counterflow concept. A computational work is carried out using the two-dimensional, compressible Navier-Stokes equations, with several kinds of turbulence models. The computational results are compared with the previous experimental ones. It is found that the present fluidic counterflow concept is a viable method to vector the thrust of a propulsion system.

• Transactions of the Korean hydrogen and new energy society
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• v.28 no.3
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• pp.315-321
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• 2017

A combustion chamber is designed and fabricated for studying the combustion characteristics of counterflow flames at elevated pressure and establishing the fundamental combustion database of counterflow flames. The combustion chamber design aims to allow the maximum operating pressure of 11 bar and be able to conduct flame visualization and the measurements of flame extinction limits, flame temperature and combustion emissions at elevated pressure. Preliminary tests for counterflow nonpremixed $CH_4-NH_3-N_2$/air flames at 1-3 bar have been conducted, and the results confirm the proper operation of the designed chamber.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.20 no.6
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• pp.82-91
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• 2021

We compared the heat transfer characteristics of the parallel and the counterflow flow in the concentric double tube of the Al₂O₃/water nanofluids using numerical methods. The high- and low-temperature fluids flow through the inner circular tube and the annular tube, respectively. The heat transfer characteristics according to the flow direction were compared by changing the volume flow rate and the volume concentration of the nanoparticles. The results showed that the heat transfer rate and overall heat transfer coefficient improved compared to those of basic fluid with increasing the volume and flow rate of nanoparticles. When the inflow rate was small, the heat transfer performance of the counterflow was about 22% better than the parallel flow. As the inflow rate was increased, the parallel flow and the counterflow had similar heat transfer rates. In addition, the effectiveness of the counterflow increased from 10% to 22% rather than the parallel flow. However, we verified that the increment in the friction factor of the counterflow is not large compared to the increment in the heat transfer rate.

• Journal of Mechanical Science and Technology
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• v.16 no.12
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• pp.1710-1718
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• 2002

The effect of fuel concentration gradient on the propagation characteristics of tribrachial (or triple) flames has been investigated experimentally in both two-dimensional and axisymmetric counterflows. The gradient at the stoichiometric location was controlled by the equivalence ratios at the two nozzles; one of which is maintained rich, while the other lean. Results show that the displacement speed of tribrachial flames in the two-dimensional counterflow decreases with fuel concentration gradient and has much larger speed than the maximum speed predicted previously in two-dimensional mixing layers. From an analogy with premixed flame propagation, this excessively large displacement speed can be attributed to the flame propagation with respect to burnt gas. Corresponding maximum speed in the limit of small mixture fraction gradient was estimated and the curvefit of the experimental data substantiates this limiting speed. As mixture fraction gradient approaches zero, a transition occurs, such that the propagation speed of tribrachial flame approaches stoichiometric laminar burning velocity with respect to burnt gas. Similar results have been obtained for tribrachial flames propagating in axisymmetric counterflow.