• 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.

• Journal of applied mathematics & informatics
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• v.41 no.1
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• pp.51-70
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• 2023

The flow of an incompressible Ellis fluid in an inclined asymmetric channel, driven by peristaltic waves was studied under low Reynolds number and long wavelength assumptions. The wave on each side of the channel are assumed to be an infinite train of sinusoidal waves, both having the same constant wave speed and wavelength however, they vary in wave amplitude, channel half width and phase angle. We derived expressions for the axial and transverse velocities, volume flow rate, pressure rise per unit wavelength and streamlines. The effects of varying the wave amplitudes, the phase angle, the channel width, the angle of inclination of the channel as well as the fluid parameters on the flow were analyzed. Trapping conditions were determined and the presence of reflux highlighted using the streamlines for the necessary channel and fluid conditions. By varying the fluid parameters, changes in the fluid that deviated from the Newtonian case resulted in a reduction in the axial velocity in the neighborhood of the center of the channel and a simultaneous increase in the velocity at the periphery of the channel. A nonlinear relation was observed with the pressure rise and the volume flow rate. This nonlinear relation is more pronounced with an increase in the absolute value of the volume flow rate. For Newtonian fluids a linear relation exists between these two variables. The fluid parameters had little effects on the streamlines. However, variations of the wave amplitudes, volume flow, channel width and phase angle had greater effects on the streamlines and hence the trapped region.

• Journal of Korean Tunnelling and Underground Space Association
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• v.12 no.2
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• pp.145-155
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• 2010

Recently interest in subsea tunnels is increasing nationwide and the construction of a subsea tunnel is taking place. For the stability of such a subsea tunnel, grouting is necessary for the water barrier and reinforcement of the tunnel. In this study, therefore, it was investigated how the grouting reinforcement had an effect on the stability of a subsea tunnel located in a great depth. To this end, Hydro-mechanical coupled analyses were performed for a sensitivity analysis in terms of different grouting range, rock class, shotcrete thickness, coefficient of lateral earth pressure, grouting thickness, and pumping existence for the rock classes I, III, and V. FLAC-2D ver. 5.0 was used for the numerical analyses. It was came to the conclusion that the effect of the increased water pressure due to the water barrier of the grouting should be considered as well as the strength improved effect in designing grouting reinforcement of subsea tunnels.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.22 no.3
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• pp.415-421
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• 2021

This study analyzed the effect on the cooling performance of the channel shape of a heat sink for an insulated gate bipolar transistor (IGBT). A serpentine channel was used for this analysis, and the parameter for the analysis was the number of curves. The analysis was conducted using computational fluid dynamics with the commercial software ANSYS fluent. One curve in the channel improved the heat dissipation performance of the heat sink by up to 8% compared to a straight-channel heat sink. However, two curves in the channel could not improve the heat discharge performance further. Instead, the two curves caused a higher pressure drop, which induces parasitic loss for the pumping of coolant. The pressure drop of the two-curve channel case was 2.48-2.55 times larger than that of a one-curve channel. This higher pressure drop decreased the heat discharge efficiency of the heat sink with two curves. The discharge heat per unit pressure drop was calculated, and the result of the straight heat sink was highest among the analyzed cases. This means that the heat discharge efficiency of the straight heat sink is the highest.

• Journal of the Korean Society of Propulsion Engineers
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• v.18 no.1
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• pp.33-41
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• 2014

In a supersonic turbine, A rotor overlap technique reduced the chance of chocking in the rotor passage, and made the design pressure ratio satisfied. However, the technique also made additional losses, like a pumping loss, expansion loss, etc. Therefore, an approximate optimization technique was appled to find the optimal shape of overlap which maximizes the improvement of the turbine performance. The design variables were shape factors of a rotor overlap. An optimal design for rotor overlap reduces leakage mass flow rate at tip clearance by about 50% and increases about 4% of total-static efficiency compared with the base model. It was found that the most effective design variable is the tip overlap and that the hub overlap size is the lowest.

• 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.

• Atmosphere
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• v.28 no.1
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• pp.53-67
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• 2018

This study examined the lake effect of the Yellow Sea which was induced by the Siberian High pressure system moving over the open waters. The development mechanism of the convective cells over the ocean was studied in detail using the Weather Research and Forecasting model. Numerical experiments consist of the control experiment (CTL) and an experiment changing the yellow sea to dry land (EXP). The CTL simulation result showed distinct high area of relative vorticity, convergence and low-level atmospheric instability than that of the EXP. The result indicates that large surface vorticity and convergence induced vertical motion and low level instability over the ocean when the arctic Siberian air mass moved south over the Yellow Sea. The sensible heat flux at the sea surface gradually decreased while latent heat flux gradually increased. At the beginning stage of air mass modification, sensible heat was the main energy source for convective cell generation. However, in the later stage, latent heat became the main energy source for the development of convective cells. In conclusion, the mechanism of the west coast heavy snowfall caused by modification of the Siberian air mass over the Yellow Sea can be explained by air-sea interaction instability in the following order: (a) cyclonic vorticity caused by diabatic heating induce Ekman pumping and convergence at the surface, (b) sensible heat at the sea surface produce convection, and (c) this leads to latent heat release, and the development of convective cells. The overall process is a manifestation of air-sea interaction and enhancement of convection from positive feedback mechanism.

• Journal of Advanced Marine Engineering and Technology
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• v.37 no.1
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• pp.40-46
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• 2013

Turbulent flow and heat transfer in a channel with a detached rib array have been simulated. The computations are based on the finite volume method with the SIMPLE algorithm. The forms of ribs considered in this study were rib with rectangular cross section, rib with groove, broken rib, and V-shaped rib. The ribs were deployed transverse or aligned $60^{\circ}$ to the main direction of the flow. Local heat transfer coefficients were obtained at various Reynolds numbers within the turbulent flow regime. Area-averaged data were calculated in order to compare the overall performance of the tested ribbed surfaces and to evaluate the degree of heat transfer enhancement induced by the ribs with respect to the smooth channel. The highest heat transfer occurred for the rib with groove which was aligned $60^{\circ}$ to the main flow direction. Performance factor was decreased with the increase of velocity, and it was found that the best performance factor was obtained in the low velocity region.

• Tribology and Lubricants
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• v.25 no.6
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• pp.387-398
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• 2009

The operation condition of recently designed pistons for high power and high speed diesel engine become more severe due to the increment of combustion pressure and temperature. So, in order to overcome high temperature, the application of the mono-metal cast aluminum alloy piston featuring an enclosed cast-in open cooling gallery has increased. In this research, it is developed a PCJ (piston cooling jet) rig tester, described the test procedure and validated the performance of sample piston cooling gallery design. Then the test rig will be used for developing the design technology of piston cooling gallery. The test rig is composed with oil reservoir and pumping system, oil jet system, piston fixing and moving system, collecting oil measuring system, and data measuring and recording system. It will be measured collecting efficiencies under conditions of a few piston positions, oil jet pressures and oil viscosities for a piston cooling gallery. Furthermore, the PCJ rig tester will be used for the optimum design of the oil cooling gallery which being applied to increase the cooling efficiency of pistons in diesel engines satisfying the EURO V emission regulation and the more.

• International Journal of Fluid Machinery and Systems
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• v.5 no.3
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• pp.109-116
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• 2012

Water hammer pumps can effectively use the water hammer phenomenon for water pumping. They are capable of providing an effective fluid transport method in regions without a well-developed social infrastructure. The results of experiments examining the effect of the geometric form of water hammer pumps by considering their major dimensions have been reported. However, these conventional studies have not fully evaluated pump performance in terms of pump head and flow rate, common measures of pump performance. The authors have focused on the effects on the pump performance of various geometric form factors in water hammer pumps. The previous study examined how the hydrodynamic characteristics was affected by the inner diameter ratio of the drive and lift pipes and the angle of the drive pipe, basic form factors of water hammer pumps. The previous papers also showed that the behavior of water hammer pump operation could be divided into four characteristic phases. The behavior of temporal changes in valve chamber and air chamber pressures according to the air volume in the air chamber located downstream of the lift valve was also clarified in connection with changes in water hammer pump performance. In addition, the effects on water hammer pump performance of the length of the spring attached to the drain valve and the drain pipe angle, form factors around the drain valve, were examined experimentally. This study focuses on the form of the lift valve, a major component of water hammer pumps, and examines the effects of the size of the lift valve opening area on water hammer pump performance. It also clarifies the behavior of flow in the valve chamber during water hammer pump operation.