• Nuclear Engineering and Technology
• /
• v.38 no.8
• /
• pp.809-818
• /
• 2006

The existing experimental data related to the turbulent mixing factor in rod arrays is examined and a new definition of the turbulent mixing factor is introduced to take into account the turbulent mixing of fluids with various Prandtl numbers. The new definition of the mixing factor is based on the eddy diffusivity of energy. With this definition of the mixing factor, it was found that the geometrical parameter, ${\delta}_{ij}/D_h$ correlates the turbulent mixing data better than Sid, which has been used frequently in existing correlations. Based on the experimental data for a highly turbulent condition in square rod arrays, a correlation describing turbulent mixing dependent on the parameter ${\delta}_{ij}/D_h$ has been developed. The correlation is insensitive to the Re number and it takes into account the effect of the turbulent Prandtl number. The proposed correlation predicts a reasonable mixing even at a lower S/d ratio.

• 한국연소학회:학술대회논문집
• /
• 2004.06a
• /
• pp.133-137
• /
• 2004

The present study is focused on the small scale turbulent mixing processes in the scalar field. In order to deal with molecular mixing in turbulent flow, the linear eddy model is addressed. In each realization, the molecular mixing term is implemented deterministically, and turbulent stirring is represented by a sequence of instantaneous, statistically independent rearrangement event called by triplet map. The LEM approach is applied with relatively simple conditions. The characteristics of scalar mixing and PDF profiles are addressed in detail.

• Proceedings of the Korean Society of Propulsion Engineers Conference
• /
• 2008.03a
• /
• pp.391-398
• /
• 2008

LES analysis was conducted with in-house CFD code to investigate the turbulence evolution and interaction due to turbulence ring and splash plate in the gas generator. Though chemical reaction was not accounted for, the results can be useful in determining the turbulence characteristics generated by ring and plate. The calculation results show that the installation of turbulence ring can introduce additional turbulences and improve turbulent mixing in the downstream flow. However, the addition of splash plate in the downstream of TR brings totally different shape of perturbation energy and enstrophy distribution for turbulent mixing. This enhancement can be done by the formation of the intensively strong vorticity production and mixing behind the plate. Pressure drop was found to be a reasonable level of about 1% or less of initial pressure in all calculation cases. Also, calculation results revealed that the variation of TR shape and intrusion length did not change the characteristics of turbulent mixing in the chamber. Even though the effect of installation location of splash plate on the turbulent mixing is not investigated yet, calculation results conclude the addition of splash plate leads to the increase in turbulent mixing with an acceptable pressure drop.

• Journal of the Korean Society of Combustion
• /
• v.2 no.1
• /
• pp.39-51
• /
• 1997

A review of probability density function(PDF) methodology and direct numerical simulation for the purpose of modeling turbulent combustion are presented in this study where particular attention is focused on the modeling problem of turbulent molecular mixing term appearing in PDF transport equation. Existing mixing models results were compared to those evaluated by direct numerical simulation in a turbulent premixed medium with finite rate chemistry in which the initial scalar field is composed of pockets of partially burnt gases to simulate autoignition. Two traditional mixing models, the least mean square estimations(LMSE) and Curl#s model are examined to see their prediction capability as well as their modeling approach. Test calculations report that the stochastically based Curl#s approach, though qualitatively demonstrates some unphysical behaviors, predicts scalar evolutions which are found to be in good agreement with statistical data of direct numerical simulation.

• Nuclear Engineering and Technology
• /
• v.31 no.1
• /
• pp.104-115
• /
• 1999

The turbulent mixing rate is a very important variable in the thermal-hydraulic design of nuclear reactors. In this study, the turbulent mixing rate the fluid flows through rod bundles is estimated with the scale analysis on the flow pulsation phenomenon. Based upon the assumption that the turbulent mixing is composed of molecular motion, isotropic turbulent motion (turbulent motion without the flow pulsation), and How pulsation, the scale relation for the mixing is derived as a function of P/D, Re, and Pr. The derived scale relation is compared with published experimental results and shows good agreements. Since the scale relation is applicable to various Prandtl number fluid flows, it is expected to be useful for the thermal-hydraulic analysis of liquid metal coolant reactors as well as of moderate Prandtl number coolant reactors.

• Journal of Power System Engineering
• /
• v.15 no.3
• /
• pp.38-45
• /
• 2011

The computational flow numerical analysis was introduced to predict thc turbulent characteristics in the mixing flow structure of $45^{\circ}$ impinging round jet. This analysis has been carried out through the commercial fluent software. Realizable(RLZ) k-${\varepsilon}$ was used as a turbulent model. It can be known that mean velocities analysed through RLZ k-${\varepsilon}$ turbulent model comparatively predict well the experiments and show well the elliptic shape of mixing flow structure in the Y-Z plane, but analysed turbulent kinetic energies show somewhat differently from the experiments in certain regions.

• Journal of ILASS-Korea
• /
• v.11 no.1
• /
• pp.1-6
• /
• 2006

The present study is focused on the small scale turbulent mixing processes in the scalar Held. In order to deal with molecular mixing in turbulent flow, the linear eddy model is addressed. In each realization, the molecular mixing term is implemented deterministically, and turbulent stirring is represented by a sequence of instantaneous, statistically independent rearrangement event called by triplet map. The LEM approach is applied with relatively simple conditions. The characteristics of scalar mixing and PDF profiles are addressed in detail.

• Journal of the Korean Wood Science and Technology
• /
• v.26 no.2
• /
• pp.38-44
• /
• 1998

A new device that uses turbulent air for mixing wood fibers with thermoplastic fibers was designed and its mixing effectiveness was evaluated in wood fiber and polypropylene fiber composites. Composites made by the turbulent air mixing (TAM) process performed better than composites made by the conventional Rando-Webber forming or nonwoven web process with an additional needling step. Thus, the TAM process proved to be a simple and efficient method in mixing wood fibers with short thermoplastic fibers for the production of wood fiber and thermoplastic fiber composites.

• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.36 no.12
• /
• pp.1171-1179
• /
• 2008

LES analysis was conducted with in-house CFD code to investigate the turbulence evolution and interaction due to turbulence ring and splash plate in the gas generator. The calculation results show that the installation of turbulence ring can introduce additional turbulences and significantly improve turbulent mixing in the downstream flow. However, the addition of splash plate in the downstream of TR(Turbulence Ring) brings totally different shape of perturbation energy and enstrophy distribution into turbulent mixing. This enhancement can be done by the formation of the intensively strong vorticity and mixing behind the plate. Pressure drop was found to be a reasonable level of about 1% or less of initial pressure in all calculation cases. Also, calculation results revealed that the variation of shape and intrusion length of TR did not greatly affect the characteristics of turbulent mixing in the chamber. Even though the effect of installation location of splash plate on the turbulent mixing was not investigated yet, calculation results conclude the addition of splash plate leads to the increase in turbulent mixing with an acceptable pressure drop.

• Journal of the Korean Society of Propulsion Engineers
• /
• v.9 no.2
• /
• pp.97-104
• /
• 2005

A simple but efficient grid generation technique by using the modified compressible form of stream function has been formulated. Transformation of a physical plane to a streamline plane, the Von Mises Transformation, has been widely used to solve the differential equations governing flow phenomena, however, limitation arises in low velocity region of boundary layer, mixing layer and wake region where the relatively large grid spacing is inevitable. Modified Von Mises Transformation with simple mathematical adjustment for the stream function is suggested and applied to solve the confined coaxial turbulent jet mixing with simple $\kappa-\epsilon$ turbulence model. Comparison with several experimental data of axial mean velocity, turbulent kinetic energy, and Reynolds shear stress distribution shows quite good agreement in the mixing layer except in the centerline where the turbulent kinetic energy distributions were somewhat under estimated. This formulation is strongly suggested to be utilized specially for free turbulent mixing layers in axisymmetric flow conditions such as the investigation of mixing behavior, jet noise production and reduction for Turbofan engines.