• 한국가시화정보학회:학술대회논문집
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• 한국가시화정보학회 2006년도 추계학술대회 논문집
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• pp.135-140
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• 2006

The stabilization mechanism of turbulent, lifted jet flames in a non-premixed condition has been studied experimentally. The objectives are to explain the phenomenon of a liftoff height decreasing as increasing fuel velocity and to reveal the mechanisms of flame stability Hydrogen was varied from 100 to 300 m/s and a coaxial air was fixed at 16 m/s with a coflow air less than 0.1 m/s. The technique of PIV and OH PLIF was used simultaneously with CCD and ICCD cameras. It was found that the liftoff height of the jet decreased with an increased fuel jet exit velocity. The leading edge at the flame base was moving along the stoichiometric line. Finally we confirmed that the stabilization of lifted hydrogen diffusion flames is related with a turbulent intensity, which means combustion is occurred where the local flow velocity is equal to the turbulent flame propagation velocity.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2002년도 학술대회지
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• pp.525-528
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• 2002

The objective of this study is to investigate characteristics of flow by the Rod shape and the choice of the turbulent intensity enhancement section. The Rod was setup vertically to the way of a nozzle exit flow and nozzle diameter is 17mm. Rod height is 5mm and its shapes are square, triangle, and circle. Characteristics of fluid such as velocity vector distribution, kinetic energy, turbulent intensity, and etc. were visualized, observed, and considered at 3 kinds of Re No. such as 2000, 3000, and 4000. The characteristics of flow field were investigated in each case of the distance rate from the nozzle exit to impinging plate(H/B=8, 10). The temperature of water is $20^{\circ}E$ and the measurement region divided by 3 sections(I, II, III). The nozzle diameter is 17mm. As the experimental result by PIV measurement, scale of the vector profile showed a tendency to an unbalance parabola distribution as increasing of the Re No. When the impinging plates such as square, triangle, and circle shape are installed respectively in front of the flow accelerated, rod shape of the highest velocity vector is circle shape and rod shape of the highest turbulent Intensity is square shape.

• Nuclear Engineering and Technology
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• 제53권1호
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• pp.79-92
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• 2021

Venturi tube is based on turbulent flow, whereby the microbubbles can be generated by the turbulent fragmentation. This phenomenon is common in several venturi bubblers used by the nuclear, aerospace and chemical industries. The first objective of this paper is to study the liquid-phase velocity field experimentally and develop correlations for the turbulent quantities. The second objective is to research velocity field characteristics theoretically. Stereoscopic PIV measurements for the velocity field have been analyzed and utilized to develop the turbulent kinetic energy in the venturi tube. The tracking properties of the tracer particles have been verified enough for us to analyze the turbulence field. The turbulence kinetic energy has a bimodal distribution trend. Also, the results of turbulence intensity along the horizontal direction is gradually uniform along the downstream. Both the mean velocity and the fluctuation velocity are proportional to the Reynolds number. Besides, the distribution trend of the mean velocity and the velocity fluctuation can be determined by the geometric parameters of the venturi tube. An analytical function model for the flow field has been developed to obtain the approximate analytical solutions. Good agreement is observed between the model predictions and experimental data.

• 대한기계학회논문집B
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• 제33권7호
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• pp.477-485
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• 2009

The study of nitrogen dilution effect on the flame stability was experimentally investigated in a non-premixed turbulent lifted hydrogen jet with coaxial air. Hydrogen gas was used as a fuel and coaxial air was used to make flame liftoff. Each of hydrogen and air were injected through axisymetric inner and outer nozzles ($d_F=3.65\;mm$ and $d_A=14.1\;mm$). And both fuel jet and coaxial air velocity were fixed as $u_F=200\;m/s$ and $u_A=16\;m/s$, while the mole fraction of nitrogen diluents gas was varied from 0.0 to 0.2 with 0.1 step. For the analysis of flame structure and the flame stabilization mechanism, the simultaneous measurement of PIV/OH PLIF laser diagnostics had been performed. The stabilization point was selected in the most upstream region of the flame base and defined as the point where the turbulent flame propagation velocity was equal to the axial component of local flow velocity. We found that the turbulent flame propagation velocity increased with the decrease of nitrogen mole fraction. We concluded that the turbulent flame propagation velocity was expressed as a function of turbulent intensity and axial strain rate, even though nitrogen diluents mole fraction was changed.

• 한국가시화정보학회지
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• 제15권3호
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• pp.47-51
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• 2017

In this study, we investigate the wake characteristics in laminar inflow and two different turbulent inflow cases. To solve the flow with wind turbines and its wake, we use large eddy simulation (LES) technique with actuator line method (ALM) and turbulent inflow of Turbsim. We perform the quantitative analysis of velocity deficit and turbulent intensity in laminar inflow case and turbulent inflow case with different turbulent intensity. In turbulent inflow, unsteady strong wake recovery which is highly fluctuated in time. Normalized power in turbulent inflow case is also highly fluctuated with unsteady wake recovery, while that in laminar inflow has quasi steady characteristic in power generation.

• 설비공학논문집
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• 제3권3호
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• pp.186-196
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• 1991

Numerical analysis has been performed to investigate the effects of the turbulence intensity and Prandtl number on the local heat transfer around a circular cylinder in crossflow. The governing equations were reformulated in a non-orthogonal coordinate system with Cartesian velocity components and discretised by the finite volume method with a non-staggered variable arrangement. For laminar flow, the calculations were performed for the Reynolds numbers 26 and 200. The results showed good agreement with the experimental results. For turbulent flow of the Reynolds number $1{\times}10^5$ and $2{\times}10^6$, the results showed that with an increase in the turbulent intensity in the main stream, the local Nusselt number increases in the front region of the circular cylinder. But the effect of turbulent intensity on the local Nusselt number diminishes in the wake region. The influence of Prandtl numbers show similar trend to that of turbulent intensity.

• 대한기계학회논문집B
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• 제33권9호
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• pp.699-708
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• 2009

To understand hydrogen jet liftoff height, the stabilization mechanism of turbulent lifted jet flames under non-premixed conditions was studied. The objectives were to determine flame stability mechanisms, to analyze coexistence of two different flame structure, and to characterize the lifted jet at the flame stabilization point. Hydrogen flow velocity varied from 100 to 300 m/s. Coaxial air velocity was changed from 12 to 20 m/s. Simultaneous velocity field and reaction zone measurements used, PIV/OH PLIF techniques with Nd:YAG lasers and CCD/ICCD cameras. Liftoff height decreased with the increase of fuel velocity. The flame stabilized in a lower velocity region next to the faster fuel jet due to the mixing effects of the coaxial air flow. The flame stabilization was related to turbulent intensity and strain rate assuming that combustion occurs where local flow velocity and turbulent flame propagation velocity are balanced. At the flame base, two different flame structures were found that was the partial premixed flames and premixed flame.

• 한국연소학회지
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• 제6권2호
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• pp.1-7
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• 2001

In order to elucidate the turbulent burning velocity of the two-component fuel mixtures, the lean and rich two-component fuel mixtures, where methane, propane and hydrogen were used as fuels, were prepared keeping the laminar burning velocity nearly the same value. Clear difference in the measured turbulent burning velocity at the same turbulence intensity can be seen among the two-component fuel mixtures with different addition rate of fuel, even under nearly the same laminar burning velocity. The burning velocities of lean mixtures change almost monotonously as changing addition rate, those of rich mixtures, however, do not show such a monotony. These phenomena can be explained qualitatively from the local burning velocities, estimated by considering the preferential diffusion effect for each fuel component. In addition, a prediction expression of turbulent burning velocity proposed for the one-component fuel mixtures can be applied to the two-component fuel mixtures by using the estimated local burning velocity of each fuel mixture.

• 한국입자에어로졸학회지
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• 제9권2호
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• pp.79-87
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• 2013

In this study, the characteristics of turbulent flow and collection efficiency for an one-stage electrostatic precipitator(ESP) with slit type collecting electrode for urban railway underground tunnels were obtained using computational fluid dynamics(CFD) commercial code FLUENT 6.3 and lab-scale experiments. The electrostatic precipitator was operated under high gas velocity(3~12m/s). Five different designs of collecting electrode, flat plate-type and a slit-type of 3mm, 5mm, 7mm and 10mm slit width and four various gas velocity(3, 6, 9, and 12m/s) were used and applied. A standard k-${\varepsilon}$ model in CFD commercial code FLUENT 6.3 was used for flow simulation. The flow simulation results showed that the turbulent intensity of flat plate-type was higher than slit-type under all gas velocity conditions and also the turbulent intensity of flat plate-type was increased continuously, but in case of slit-type was maintained at constant range. And, the turbulent intensity was decreased according to increasing of slit width. The experimental results showed that the collection efficiency of slit-type was higher than flat plate-type under all gas velocity conditions. And, over 6m/s gas velocity condition, the collection efficiency of 5mm and 7mm was highest, when compared to 3mm and 10mm.

• 대한기계학회논문집B
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• 제22권5호
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• pp.708-715
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• 1998

The shelter effect of a porous wind fence on a triangular prism was experimentally investigated in a circulating water channel. A porous fence of porosity .epsilon.=38.5% was installed in front of the prism model. The fence and prism model were embedded in a turbulent boundary layer. The instantaneous velocity fields around the fence and prism model were measured by using the instantaneous velocity fields around the fence and prism model were measured by using the 2-frame PTV(Particle Tracking Velocimetry) system. By installing the fence in front of the prism, the recirculation flow region decreases compared with that of no fence case. The porous fence also decreases the mean velocity, turbulent intensity and turbulent kinetic energy around the prism. Especially, at the top of the prism, the turbulent kinetic energy is about half of that without the fence.