• Journal of the Korean Society of Propulsion Engineers
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• v.16 no.5
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• pp.1-9
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• 2012

The gas jet from a coaxial porous injector for two-phase flows is discharged from the porous surface, which encloses the center liquid jet, and the gas and liquid jet interact with each other physically. The wall injected gas jet transfers the radial momentum effectively while the radial gas jet develops to axial jet, and the performance of atomizing and mixing can be improved. In this study, the Weber number and the ratio of momentum flux were controlled by changing the gas injection area and the mass flow rate of the gas jet, and a study on the spray characteristics at the cold-flow test using water and air simulant was performed. It is concluded that the radial momentum transfer concept of a coaxial porous injector gives a positive effect on the atomization and mixing of the two-phase spray.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.49 no.8
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• pp.699-708
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• 2021

In this paper, the aerodynamic and aeroacoustic characteristics that vary depending on the rotation axial distance between the upper and lower rotor, which is one of the design parameters of the coaxial rotor, is analyzed in the hovering condition using the computational fluid dynamics. Aerodynamic analysis using the Reynolds Averaged Navier Stokes equation and the aeroacoustic analysis using the Ffowcs Williams ans Hawkings equation is performed and the results were compared. The upper and lower rotor of the coaxial rotor have different phase angle which changes periodically by rotation and have unsteady characteristics. As the distance between the upper and lower rotors increased, the aerodynamic efficiency of the thrust and the torque was increased as the flow interaction decreased. In the aeroacoustic viewpoint, the noise characteristics radiated in the direction of the rotational plane showed little effect by axis spacing. In the vertical downward direction of the axis increased, the SPL maintains its size as the frequency increases, which affects the increase in the OASPL. As the axial distance of the coaxial rotor increased, the noise characteristics of a coaxial rotor were similar with the single rotor and the SPL decreased significantly.

• Transactions of the Korean Society of Mechanical Engineers
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• v.11 no.1
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• pp.145-153
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• 1987

Stability limits of a double concentric jets flame and the structure of recirculation zone formed behind a thick burner rim were investigated. To control the flame stability, swirled secondary air flow ranging 0.13-0.71 of swirl number, and air, fuel, and mixture gas injection from an injection coaxial slit set on burner rim were examined. Flame stability limits, flame shapes, lengths of recirculation zone, temperature distributions, residence times, air ratios in the recirculation zone were measured. The following results were obtained. (1) Lean limits were considerably widened by a strong swirl because the recirculation zone was enlarged. (2) At fuel injection as well as mixture injection, lean limits were also extended. But, air injection had no effect on stability limits. (3) Injected gas seems to diffuse into the recirculation zone through its outer boundary surrounded the secondary air. Therefore, chemical structure in the recirculation zone with air injection coincides with that without injection. (4) Injection position had no effect on flame stability limits.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.29 no.9A
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• pp.1083-1090
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• 2004

In this paper, a meander-type microstrip patch antenna for application in 5GHz-hand is designed and fabricated To obtain enough bandwidth in VSWR≶2, the foam is inserted between substrate and ground plane, the coaxial probe source 1S used Antenna is simulated varing the length and width of meander line, the position of probe feeding and the thick of airgap. Later antenna is fabricated with optimizated antenna parameter. The measured result of Fabricated antenna obtained IGHz(l7.5%) bandwidth in VSWR≶2, the gain of 7.3-9.5dBi, Unidirectional pattern.

• Transactions of the Korean Society of Mechanical Engineers
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• v.14 no.5
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• pp.1234-1243
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• 1990

Double coaxial are jets(annular and coaxial air jets) between which propane gas is fed was selected to study the structure of diffusion flames in turbulent shear flow. Schlieren and direct photographs are taken to visualize the flame structure. Mean and fluctuating temperatures and ion currents were measured to investigate the macroscopic and the instantaneous flame structure. The objective of this study is to understand the interaction between combustion and mixing process especially in the transition region of turbulent shear flow. The investigation reported in this paper focuses on the macroscopic and the instantaneous structures of three flames obtained. The increased mixing effect resulting from increase of Reynolds number of central air jet makes the flame bluish and short. When the velocity of surrounding air stream is higher than that of central air jet, the instantaneous flame structure is composed of coherent structure. It is considered that the flame structure of transitional region of mixing layer depends on the structure of mixing layer of non-reacting conditions.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.30 no.4A
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• pp.312-319
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• 2005

In this paper, a meander-type microstrip patch antenna for application in 5GHz-band is designed and fabricated. To obtain enough bandwidth in VSWR<2, the foam is inserted between substrate and ground plane, the coaxial probe source is used. Antenna is simulated varing the length and width of meander line, the position of probe feeding and the thick of airgap. Later anterlna is fabricated with optimizated antenna parameter. The measured result of Fabricated antenna obtained $1GHz(17.5\%)$ bandwidth in VSWR<2$, the gain of $.3\sim9.5$dBi, Unidirectional pattern.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.35 no.4
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• pp.431-438
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• 2011

Flame lift-off conditions determine the operating conditions of burners. It is known that a flame can be lifted when the Schmidt number (Sc), which is the ratio of the dynamic viscosity to the mass diffusivity, is greater than unity. In this study, the flame lift-off characteristics of non-premixed flames of propane (Sc > 1) and methane (Sc < 1) in a coaxial outer air tube were experimentally compared. The experimental results indicated that stable lifted flames could be obtained even when Sc < 1 in a confined air tube. On the basis of the results of a simple numerical analysis, it was confirmed that a new flame stabilization mechanism exists in the tube. A velocity field is preferentially developed upstream of the flame, and it results in a new stabilization condition. This result can be very useful in explaining the stabilization of the flames of ordinary burners in which a flame is produced in a confined space.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.7
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• pp.2386-2396
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• 1996

A numerical simulation has been performed for isothermal and reacting flows in an exisymmetric, bluff-body research combustor. The present formulation is based on the density-weighted averaged Navier-Stokes equations together with a k-epsilon. turbulence model and a modified eddy-breakup combustion model. The PISO algorithm is employed for solution of thel Navier-Stokes system. Comparison between measurements and predictions are made for a centerline axial velocities, location of stagnation points, strength of recirculation zone, and temperature profile. Even though the numerical simulation gives acceptable agreement with experimental data in many respects, the present model is defictient in predicting the recoveryt rate of a central near-wake region, the non-isotropic turbulence effects, and variation of turbulent Schmidt number. Several possible explanations for these discrepancies have been discussed.

• 한국연소학회:학술대회논문집
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• 2014.11a
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• pp.211-213
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• 2014

Simultaneous measurements of planar laser-induced fluorescence (PLIF) of OH radicals and particle image velocimetry (PIV) were used to investigate the strain rates and OH structure characteristics of turbulent syngas non-premixed jet flames close to blowoff. Mean values of the maximum principal strain rate on OH layer decreases with the axial distance, and its standard deviation is significantly large upstream. Strain rate on stabilization region of the stable flame is only about a half of that of the flame near blowoff.

• Journal of the Korean Society of Combustion
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• v.10 no.1
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• pp.13-19
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• 2005

Measurements of flame length and NOx emissions have been conducted to investigate the effects of acoustic excitation on flame structure in turbulent hydrogen diffusion flames with coaxial air. When the acoustic excitation of a specific frequency is applied to coaxial air stream, flame length is dramatically reduced, resulting in reduction of flame residence time. Consequently, EINOx could decrease up to 35 % and this shows that acoustic excitation is effective in reducing NOx emissions. Mie scattering technique has been used to visualize the vortex structure induced by acoustic excitation and vortex formation, development and destruction were observed quantitatively. As a result, vortex entrains coflow air into fuel stream and mixing rate between fuel and air is significantly enhanced, which may contribute to reduction of NOx emissions.