• Journal of Mechanical Science and Technology
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• v.18 no.8
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• pp.1435-1450
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• 2004

Experimental data are presented which describe the effects of a combustor-level high free-stream turbulence on the near-wall flow structure and heat/mass transfer on the endwall of a linear high-turning turbine rotor cascade. The end wall flow structure is visualized by employing the partial- and total-coverage oil-film technique, and heat/mass transfer rate is measured by the naphthalene sublimation method. A turbulence generator is designed to provide a highly-turbulent flow which has free-stream turbulence intensity and integral length scale of 14.7% and 80mm, respectively, at the cascade entrance. The surface flow visualizations show that the high free-stream turbulence has little effect on the attachment line, but alters the separation line noticeably. Under high free-stream turbulence, the incoming near-wall flow upstream of the adjacent separation lines collides more obliquely with the suction surface. A weaker lift-up force arising from this more oblique collision results in the narrower suction-side corner vortex area in the high turbulence case. The high free-stream turbulence enhances the heat/mass transfer in the central area of the turbine passage, but only a slight augmentation is found in the end wall regions adjacent to the leading and trailing edges. Therefore, the high free-stream turbulence makes the end wall heat load more uniform. It is also observed that the heat/mass transfers along the locus of the pressure-side leg of the leading-edge horseshoe vortex and along the suction-side corner are influenced most strongly by the high free-stream turbulence. In this study, the end wall surface is classified into seven different regions based on the local heat/mass transfer distribution, and the effects of the high free-stream turbulence on the local heat/mass transfer in each region are discussed in detail.

• Journal of Mechanical Science and Technology
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• v.14 no.12
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• pp.1365-1375
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• 2000

Two phase flows have been numerically calculated to analyze plume characteristics and liquid circulation in gas injection through a porous plug. The Eulerian approach has been for formulation of both the continuous and dispersed phases. The turbulence in the liquid phase has been modeled using the standard $textsc{k}$-$\varepsilon$ turbulence model. The interphase friction coefficient has been calculated using correlations available in the literature. The turbulent dispersion of the phase has been modeled by the "dispersion Prand시 number". The predicted mean flows is compared well with the experimental data. The plume region area and the axial velocities are increased with the gas flow rate and with the decrease in the inlet area. The turbulent intensity also shows the same trend. Also, the space-averaged turbulent kinetic energy for various gas flow rates and inlet areas has been obtained. The results are of interest in the design and operation of a wide variety of materials and chemical processing operations.

• Journal of ILASS-Korea
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• v.1 no.1
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• pp.76-84
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• 1996

The nozzle geometry and up-stream inject ion condition affect the characteristics of flow inside the nozzle. such as turbulence and cavitation bubbles. Flow details in fuel nozzle orifice with sudden contraction of cross sectional area have been investigated both experimentally and numerically. The measurements of velocities of internal flow in a scaled-up nozzle with different length to diameter rat io(L/d) were made by laser Doppler velocimetry in order to clarify the effect of internal flow on the characteristics of fuel spray. Mean and fluctuating velocities and discharge coefficients were obtained at various Reynolds numbers. The turbulent intensity and turbulence kinetic energy in a sharp inlet nozzle were higher than that in a round inlet nozzle. Calculations were also performed for the same nozzles as scaled-up experimental nozzles using the SIMPLE algorithm. External spray behavior under different nozzle geometry and up-stream flow conditions using Doppler technique and visualization technique were also observed.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.32 no.3
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• pp.207-215
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• 2008

The heat/mass transfer characteristics on the plane tip surface of a high-turning first-stage turbine rotor blade has been investigated by employing the naphthalene sublimation technique. At the Reynolds number of $2.09{\times}10^5$, heat/mass transfer coefficients are measured for the tip gap height-to-chord ratio, h/c, of 2.0% at turbulence levels of Tu = 0.3 and 14.7%. A tip-surface flow visualization is also performed for h/c = 2.0% at Tu = 0.3%. The results show that there exists a strong flow separation/re-attachment process, which results in severe local thermal load along the pressure-side corner, and a pair of vortices named "tip gap vortices" in this study is identified along the pressure and suction-side tip corners near the leading edge. The loci and subsequent development of the pressure- and suction-side tip gap vortices are discussed in detail. The combustor-level high inlet turbulence, which increases the tip-surface heat/mass transfer, provides more uniform thermal-load distribution.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.6
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• pp.1971-1982
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• 1996

The breakup of liquid jet is the result of competing, unstable hydrodynamic forces acting on the liquid jet as it exit the nozzle. The nozzle geometry and up-stream injection conditions affect the characteristics of flow inside the nozzle, such as turbulence and cavitation bubbles. A set of calculation of the internal flow in a single hole type nozzle were performed using a two dimensional flow simulation under different nozzle geometry and up-stream flow conditions. The calculation showed that the turbulent intensity and discharge coefficient are related to needle position. The diesel nozzle with sharp inlet under actual engine condition has possibility of cavitation, but round inlet nozzle has no possibility of cavitation.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 1996.11a
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• pp.107-116
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• 1996

Numerical simulations have been conducted to characterize unsteady flow structures in an axisymmetric supersonic inlet diffuser with sinusoidal pressure oscillations at the diffuser exit. The formulation is based on the unsteady Navier-Stokes equations and turbulence closure is achieved using a two-layer model with a too-Reynolds-number scheme for the near-wall treatment. The governing equations are formulated in an integral form, and are discretized by the four-stage Runge-Kutta scheme for temporal terms and the Harten-Yee upwind TVD scheme for convective terms. Results indicated that the inlet shock characteristics are significantly modified by acoustic oscillations originating from the combustor. The characteristics of shock/boundarv-layer interactions (such as the size of separation bubble, terminal shock shape, and vorticity intensity) are also greatly iufluenced by the shock oscillation due to acoustic waves.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.5
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• pp.1661-1670
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• 1996

Incompressible flow over a backward-facing step is computed by low Reynolds number turbulence models in order to compare with direct simulation results. In this study, selected low Reynolds number 1st and 2nd (Algebraic Stress Model : ASM) moment closure turbulence models are adopted and compared with each other. Each turbulence model predicts different flow characteristics, different re-attachment point, velocity profiles and Reynolds stress distribution etc. Results by .kappa.-.epsilon. turbulence models indicate that predicted re-attachment lengths are shorter than those by standard model. Turbulent intensity and eddy viscosity by low Reynolds number .kappa.-.epsilon. models are still greater than DNS results. The results by algebraic stress model (ASM) are more reasonable than those by .kappa.-.epsilon. models. The convective scheme is QUICK (Quadratic Upstream Interpolation for Convective Kinematics) and SIMPLE algorithm is adopted. Reynolds number based on step height and inlet free stream velocity is 5100.

• Wind and Structures
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• v.19 no.2
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• pp.219-232
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• 2014

The characteristics of turbulent boundary layers over hilly terrain depend strongly on the hill slope and upstream condition, especially inflow turbulence. Numerical simulations are carried out to investigate the neutrally stratified turbulent boundary layer over two-dimensional hills. Two kinds of hill shape, a steep one with stable separation and a low one without stable separation, two kinds of inflow condition, laminar turbulent, are considered. An auxiliary simulation, based on the local differential quadrature method and recycling technique, is performed to simulate the inflow turbulence be imposed at inlet boundary of the turbulent inflow, which preserves very well in the computational domain. A large separation bubble is established on the leeside of the steep hill with laminar inflow, while reattachment point moves upstream under turbulent inflow condition. There is stable separation on the side of low hill with laminar inflow, whilw not turbulent inflow. Besides increase of turbulence intensity, inflow can efficiently enhance the speedup around hills. So in practice, it is unreasonable to study wind flow over hilly terrain without considering inflow turbulence.

• Transactions of the Korean Society of Automotive Engineers
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• v.2 no.1
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• pp.38-50
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• 1994

To improve the efficiency of internal combustion engines, it is necessary to understand mixed air-fuel in-cylinder flow processes accurately at intake and compression strokes. There is experimental and numerical methods to analyse in-cylinder flow process. In numerical method, standard $k-{\varepsilon}$ model with wall function was mostly adopted in in-cylinder flow process. But this type model was not efficiently predicted in the near wall region. Therefore in the present study, low Reynolds number $k-{\varepsilon}$ model was adopted near the cylinder wall and standard $k-{\varepsilon}$ model in other region. Also QUICK scheme was used for convective difference scheme. This study takes axisymmetric reciprocating model engine motored at 200rpm with a centrally located valve, incorporated 60 degree seat angie, and flat piston surface excluding inlet port. Because in-cylinder flow processes are undergoing unsteady and compressible, averaged cylinder pressure and inlet velocity at arbitrary crank angle are determined from thermodynamic analytic method and incylinder states at that crank angle are iteratively determined from the numerical analytic method.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.21 no.12
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• pp.1635-1646
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• 1997

The nozzle length to diameter ratio of real diesel nozzles is about 2-8 which is not long enough for a fully developed and stabilized flow. The characteristics of the flow such as turbulence at the nozzle exit which affect the development of the spray can be enhanced by impinging the flow inside nozzle. The flow details inside the impinging nozzles have been investigated both experimentally and numerically. The mean velocities, the fluctuating velocities, and discharge coefficients in the impinging inlet nozzles, round inlet nozzle, and sharp inlet nozzle were obtained at various Reynolds number. The developing feature of the external spray were photographed by still camera and the droplet sizes and velocities were also measured by laser Doppler technique. The spray angle was greater and the droplet sizes near the spray axis were smaller with the impinging flow inside nozzle.