• Journal of Magnetics
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• v.18 no.3
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• pp.321-325
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• 2013

The problem of mixed convection in a differentially heated lid-driven square cavity filled with Cu-water nanofluid under effect of a magnetic field is investigated numerically. The left and right walls of the cavity are kept at temperatures of $T_h$ and $T_c$ respectively while the horizontal walls are adiabatic. The top wall of the cavity moves in own plane from left to right. The effects of some pertinent parameters such as Richardson number (ranging from 0.1 to 10), the volume fraction of the nanoparticles (ranging 0 to 0.1) and the Hartmann number (ranging from 0 to 60) on the fluid flow and temperature fields and the rate of heat transfer in the cavity are investigated. It must be noted that in all calculations the Prandtl number of water as the pure fluid is kept at 6.8, while the Grashof number is considered fixed at 104. The obtained results show that the rate of heat transfer increases with an increase of the Reynolds number, while but it decreases with increase in the Hartmann number. Moreover it is found that based the Richardson and Hartmann numbers by increase in volume fraction of the nanoparticles the rate of heat transfer can be enhanced or deteriorated compared to the based fluid.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.17 no.6
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• pp.569-580
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• 2005

The present study investigates the convective heat/mass transfer characteristics in wavy ducts of a primary surface heat exchanger. The effects of duct aspect ratio and flow velocity on the heat/mass transfer are investigated. Local heat/mass transfer coefficients on the corrugated duct sidewall are determined using a naphthalene sublimation technique. The aspect ratios of the wavy duct are 7.3, 4.7 and 1.8 with the corrugation angle of $145\Omega$. The Reynolds numbers, based on the duct hydraulic diameter, vary from 300 to 3,000. The results show that at the low Re(Re $\leq$ 1000) the secondary vortices called Taylor-Gortler vortices perpendicular to the main flow direction are generated due to effect of duct curvature. By these secondary vortices, non-uniform heat/mass transfer coefficients distributions appear. As the aspect ratio decreases, the number of cells formed by secondary vortices are reduced and secondary vortices and comer vortices mix due to decreased aspect ratio at Re$\leq$1000. At Re >1000, the effects of corner vortices become stronger. The average Sh for the aspect ratio of 7.3 and 4.7 are almost same. But at the small aspect ratio of 1.8, the average Sh decreases due to decreased aspect ratio. More pumping power (pressure loss) is required for the larger aspect ratio due to the higher flow instability.

• The KSFM Journal of Fluid Machinery
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• v.9 no.1 s.34
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• pp.25-31
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• 2006

The present study investigates the effects of bleed flow on heat/mass transfer and pressure drop in a rotating channel with transverse rib turbulators. The hydraulic diameter $(D_h)$ of the square channel is 40.0mm. The bleed holes are located between the rib turbulators on leading surface and the hole diameter (d) is 4.5 mm. The square rib turbulators are installed on both leading and trailing surfaces. The rib-to-rib pitch is 10.0 times of the rib height(e) and the rib height-to-hydraulic diameter ratio $(e/D_h)$ is 0.055. The tests were conducted at various rotation numbers (0, 0.2, 0.4), while the Reynolds number and the rate of bleed flow to main flow (BR) were fixed at 10,000 and $10\%$, respectively. The results suggest that the heat/mass transfer characteristics in the internal cooling passage are influenced by rib turbulators, bleed flow and the Coriolis force induced by rotation. For the rotating ribbed passage with bleed flow, the heat/mass transfer on the leading surface is hardly affected by bleed flow, but that on the trailing surface decreases due to the diminution of main flow. The results also show that the friction factor decreases with the bleed flow.

• Journal of the Korean Society of Propulsion Engineers
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• v.20 no.2
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• pp.94-101
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• 2016

In this paper, aerodynamic effects on the flow field under the wing with varying aspect ratio were investigated by measuring pressures on the lower surface of wing and analysing velocity components using Particle Image Velocimetry at Reynolds numbers of $1.384{\times}10^5$ and $2.306{\times}10^5$. In case of aspect ratio 4.8 which keeps the wing tip at a distance of 80% chord length from the pylon, the vortex from the wing tip influenced the flow field under the wing by reducing static pressures on the lower surface and increasing the velocity in proximity of the wing tip. Throughout the results, it is observed that aerodynamic effects of wing tip on the flow field around pylon under wing become insignificant as the aspect ratio increases.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.7
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• pp.789-799
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• 2004

An experimental study is conducted to investigate the effects of duct corrugation angle on heat/mass transfer characteristics in wavy ducts of a primary surface heat exchanger application. Local heat/mass transfer coefficients on the wavy duct sidewalls are determined by using a naphthalene sublimation technique. The corrugation angles(${\alpha}$) of the wavy ducts are 145$^{\circ}$, 130$^{\circ}$, 115$^{\circ}$ and 100$^{\circ}$. And the Reynolds numbers based on the duct hydraulic diameter vary from 300 to 3,000. The results show that at the low Re(Re $\leq$1000), the secondary vortices called Taylor-Gortler vortices perpendicular to the main flow direction are generated due to effect of duct curvature. By these secondary vortices, high heat/mass transfer regions are formed on both pressure-side and suction-side walls. At the high Re(Re $\geq$ 1000), these secondary flows are vanished with helping flow transition to turbulent flow and the regions which show high heat/mass coefficients by flow reattachment are formed on suction side. As corrugation angle decreases, the local peak Sh induced by Taylor-Gortler vortices increase at Re $\leq$1000. At high Re(Re $\geq$ 1000), by the existence of different kind of secondary flows called Dean vortices, non-uniform Sh distribution appears along spanwise direction at the narrow corrugation angle (${\alpha}$=100$^{\circ}$). Average Sh also increase by the enhanced effect of secondary vortices and flow reattachment. More pumping power (pressure loss) is required with the smaller corrugation angle due to the enhancement of flow instability.

• Proceedings of the KSME Conference
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• 2007.05b
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• pp.2402-2407
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• 2007

Heat transfer distributions and friction factors in square channels (3.5 ${\times}$ 3.5 cm) with twisted tape inserts and with twisted tape inserts plus interrupted ribs are respectively investigated. Tests are performed for Reynolds numbers ranging from 8,900 to 29,000. The rib height-to-channel hydraulic diameter, e/Dh, is kept at 0.057 and test section length-to-hydraulic diameter, L/Dh is 30. The twisted tape is 0.1 mm thick carbon steel sheet with diameter of 3.3cm, length of 90cm, and 2.5 turns. The square ribs are arranged to follow the trace of the twisted tape and along the flow direction defined as axial interrupted ribs. Each wall of the square channel is composed of isolated aluminum sections. The following conclusions from the experimental study were drawn as: 1) In the 4 heating wall channel with twisted tape inserts, Nusselt number based on bottom wall temperature is enhanced by 1.2 - 1.6 times if adding the axial interrupted ribs on the bottom wall only. 2) The twisted tape with interrupted ribs under the two-sided heating condition produces the highest heat transfer performance. 3) Friction factor data obtained for the square channel with twisted tape inserts plus axial interrupted ribs are less than those in the past publications for circular tubes with axial interrupted ribs and twisted tape inserts.

• Journal of Power System Engineering
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• v.21 no.5
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• pp.35-40
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• 2017

The effects of the different rib geometries such as V-shaped continuous (case A), parallel broken (case B), and V-shaped broken (case C) ribs on the heat transfer and pressure drops in a divergent channel with $45^{\circ}$ inclined ribs on one wall or two walls are checked out. The top and bottom walls are adiabatic; two side walls are uniformly heated in the divergent rectangular channel. The tested Reynolds numbers are ranged from 22,000 to 75,000. The channel with two opposite walls inclined only has the length of test section of 1 m and the channel divergence ratio of $D_{ho}/D_{hi}=1.49$, corresponding to $1.43^{\circ}$ inclined walls. The results show in the identical pumping power that the V-shaped continuous rib (case A) with two ribbed walls is the greatest, but the parallel broken rib (case B) with one ribbed wall is the worst in the thermal performance.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.21 no.1
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• pp.165-173
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• 1997

The present study investigates heat/mass transfer for flow through perforated plates for application to combustor wall and turbine blade film cooling. The experiments are conducted for hole length to diameter ratios of 0.68 to 1.5, for hole pitch-to-diameter ratios of 1.5 and 3.0, for gap distance between two parallel perforated plates of 1 to 3 hole diameters, and for Reynolds numbers of 60 to 13, 700. Local heat/mass transfer coefficients near and inside the cooling holes are obtained using a naphthalene sublimation technique. Detailed knowledge of the local transfer coefficients is essential to analyze thermal stress in turbine components. The results indicate that the heat/mass transfer coefficients inside the hole surface vary significantly due to flow separation and reattachment. The transfer coefficient near the reattachment point is about four and half times that for a fully developed circular tube flow. The heat/mass transfer coefficient on the leeward surface has the same order as that on the windward surface because of a strong recirculation flow between neighboring jets from the array of holes. For flow through two perforated plate layers, the transfer coefficients on the target surface (windward surface of the second wall) affected by the gap spacing are approximately three to four times higher than that with a single layer.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.34 no.11
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• pp.965-973
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• 2010

Numerical analysis has been carried out to investigate the heat transfer characteristics of a plate heat exchanger. The multi-cell models with inlet part and outlet part are used for performing numerical simulation. The plate heat exchanger is characterized by chevron angle of $15^{\circ}$, corrugation pitch of 24mm and corrugation height 6~12mm. The length of the inlet-part considered in the analysis ranges from 24.8 to 124mm and Reynolds numbers range from 1,000 to 10,000. The correlations such as friction factor and Colburn factor are compared with previous experimental data. The results can be utilized for designing the plate heat exchanger.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.34 no.5
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• pp.1-11
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• 2006

The effects of continuous blowing on flow control and stall suppression for flows over a NACA 0015 airfoil at low Reynolds numbers were numerically investigated through its parameter variation on unstructured meshes. The aerodynamic force and moment variations due to flow control were examined, along with the stall angle-of-attack change for stall suppression. The results showed that blowing with relatively strong jet increases lift at the cost of drag increment below stall angle. Continuous blowing delays flow stall when it is implemented near the leading edge. When the blowing jet was aligned along the flow direction on the airfoil, the favorable flow control effect was most significant below the stall angle of attack.