• International Journal of Fluid Machinery and Systems
• /
• v.10 no.2
• /
• pp.119-137
• /
• 2017

Although the stall stagnation phenomena have often been experienced in site and also analytically in numerical experiments in surges in systems of compressors and flow paths, the fundamental causes have not been identified yet. In order to clarify the situations, behaviours of infinitesimal disturbance waves superposed on a main flow were studied in a simplified one-dimensional flow model. A ratio of the amplifying rate of the system instability to the characteristic slope of the compressor element was surveyed as the instability enhancement factor. Numerical calculations have shown the following tendency of the factor. In the situation where both the sectional area ratio and the length ratio of the delivery flow-path to the suction duct are sufficiently large, the enhancement factors are greater in magnitude, which means occurrence of ordinary deep surges. However, in the situation where the area ratio and/or the length ratio is relatively smaller, the enhancement factor tends to lessen significantly, which situation tends to suppress deep surges for the same value of the characteristic slope. It could result in the stall stagnation condition. In the domain of area ratio vs. length ratio of the delivery duct to the suction duct, contour-lines of the enhancement factor behave qualitatively similar to those of the stall stagnation boundaries of a fan analytically obtained, suggesting that a certain range of the enhancement factor values could specify the stagnation occurrence. The significant decreases in the factors are observed to accompany appearances of phase lags and travelling waves in the wave motions, which macroscopically suggests breaking down of the complete surge actions of filling and emptying of the air in the delivery duct. The strength of the action is deeply related with acoustic interferences and is evaluated in terms of the volume-modified reduced resonance frequency proposed by the author. These observations have shown the fundamental cause and the sequence of the stall stagnation in principle.

• Journal of the Korean Institute of Navigation
• /
• v.23 no.2
• /
• pp.29-42
• /
• 1999

An accurate method of estimating ship maneuverability needs to be developed to evaluate precisely and improve the maneuverability of ships according to the water depth. In order to estimate maneuverability by a mathematical model. The hydrodynamic forces acting on a ship hull and the flow field around the ship in maneuvering motion need to be estimated. The ship speed new the berth is very low and the fluid flow around a ship hull is unsteady. So, the transient fluid motion should be considered to estimate the drag force acting on the ship hull. In the low speed and short time lateral motion, the vorticity is created by the body and grow up in the acceleration stage and the velocity induced by the vorticity affect to the body in deceleration stage. For this kind of problem, CFD is considered as a goof tool to understand the phenomena. In this paper, the 2D CFD code is used for basic consideration of the phenomena to solve the flow in the cross section of the ship considering the ship is slender and the water depth is large enough. The flow fields Added and hydrodynamic forces for the some prescribed motions are computed and compared with the preliminary experiment results. The comparison of the force with measurement is shown a fairly good agreement in tendency. The 3D Potential Calculation based on the Hess & Smith Theory is employed to predict the surge, sway added mass and yaw added moment of inertia of hydrodynamic coefficients for M/V ESSO OSAKA according to the water depth. The results are also compared with experimental data. Finally, the sway added mass of hydrodynamic coefficients for T/S HANNARA is suggested in each water depth.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.35 no.5
• /
• pp.487-494
• /
• 2011

The safety valve used in LNG/LNG-FPSO ships plays an important role in maintaining a fixed level of pressure by emitting LNG gas out of the pipes in the LNG piping system. The discharge coefficient is regarded as the most important factor in the valve performance. To satisfy the ship's classification, the discharge coefficient of the safety valve must usually be over 0.8. Despite the importance of understanding the flow phenomena inside the safety valve, the valve design is usually based on experience and experiments. We carried out a computational fluid dynamics (CFD) investigation using the ANSYS-CFX software. We observed the flow phenomena inside the valve and measured the discharge coefficients according to changes in the valve lift, which is the distance between the exit of the nozzle and the lower part of the disc plate. We verified our CFD results for the discharge coefficients using available experimental data.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.20 no.9
• /
• pp.2953-2964
• /
• 1996

Heat Transfer with periodic fluctuation of fluid temperature caused by oscillatory flow or compression expansion can be out of phase with balk fluid-wall temperature difference. Newton's law of convection is inadequate to describe this phenomenon. In order to solve this problem the concept of the complex Nusselt number has been introduced by severla researchers. The complex Nusselt number expresses out of phase excellently while the first harmonic is dominant in the variations of both fluid-wall temperature difference and heat flux. However, in the case of oscillatory flow with non-linear wall temperature distribution, the complex Nusselt number is not appropriate to predict the heat transfer phenomena since the higher order harmonic components appear in periodic temperature variation. Analytic solutions to the heat transfer with an sinusoidal well temperature distribution were obtained to investagate the effect of non-linear wall temperature distribution. A new formula considering the thermal boundary layer was suggested based on the solutions. A comparison was also made with the complex Nusselt number. It was verified that the new formula describes well the heat transfer of oscillating flow even if the first harmonic component is not dominant in the fluid-wall temperature difference.

• Journal of Advanced Marine Engineering and Technology
• /
• v.31 no.6
• /
• pp.687-693
• /
• 2007

It has been known that one of the main obstacles of improving the performance of positive displacement hydraulic turbine is pressure pulsation which occurs at the regions upstream and downstream of the turbine. In order to suppress the pressure pulsation. occurrence reason of the pressure pulsation should be understood in detail Therefore. this study aims to establish a CFD analysis method by which the phenomena of unsteady pressure pulsation can be examined with high accuracy. Internal flow field of the turbine is modeled simply to generalize the relation between the pressure pulsation and internal flow. The results show that the Present CFD method adopting unsteady calculation can be applied successfully to the analysis of the Phenomena of Pressure Pulsation. Occurrence of the Pressure pulsation is due to the difference of the rotational speed of turbine rotors When driving rotor rotates by uniform speed and fellowing rotor rotates by variable speed, very large Pressure pulsation occurs within the turbine periodically.

• Nuclear Engineering and Technology
• /
• v.49 no.3
• /
• pp.466-475
• /
• 2017

Coolant mixing under natural circulation flow regime constitutes a key parameter that may play a role in the course of an accidental transient in a nuclear pressurized water reactor. This issue has motivated some experimental investigations carried out within the OECD/NEA PKL projects. The aim was to assess the coolant mixing phenomenon in the reactor pressure vessel downcomer and the core lower plenum under several asymmetric steady and unsteady flow conditions, and to provide experimental data for code validations. Former studies addressed the mixing phenomenon using, on the one hand, one-dimensional computational approaches with cross flows that are not fully validated under transient conditions and, on the other hand, expensive computational fluid dynamic tools that are not always justified for large-scale macroscopic phenomena. In the current framework, an unsteady coolant mixing experiment carried out in the Rossendorf coolant mixing test facility is simulated using the three-dimensional porous media capabilities of the thermal-hydraulic system CATHARE code. The current study allows highlighting the current capabilities of these codes and their suitability for reproducing the main phenomena occurring during asymmetric transient natural circulation mixing conditions.

• 한국전산유체공학회:학술대회논문집
• /
• 2008.03a
• /
• pp.14-17
• /
• 2008

Recently, several advanced flow visualization techniques such as Particle Image Velocimetry (PIV) including stereo PIV, holographic PIV, and dynamic PIV have been developed. These advanced techniques have strong potential as the experimental technology which can be used for verifying numerical simulation. In addition, there would be indispensable in solving complicated thermo-fluid flow problems not only in the industrial fields such as automotive, space, electronics, aero- and hydro-dynamics, steel, and information engineering, but also in the basic research fields of medical science, bio-medical engineering, environmental and energy engineering etc. Especially, NT Nano Technology) and BT (Bio Technology) strongly demand these advanced measurement techniques, because it is difficult for conventional methods to observe most complicated nano- and bio-fluidic phenomena. In this paper, the basic principle of these advanced visualization techniques and their practical applications which cannot be resolved by conventional methods, such as flow in automotive HVAC system, ship and propeller wake, three-dimensional flow measurement in micro-conduits, and flow around a circulating cylinder will be introduced.

• 한국전산유체공학회:학술대회논문집
• /
• 2008.10a
• /
• pp.14-17
• /
• 2008

Recently, several advanced flow visualization techniques such as Particle Image Velocimetry (PIV) including stereo PIV, holographic PIV, and dynamic PIV have been developed. These advanced techniques have strong potential as the experimental technology which can be used for verifying numerical simulation. In addition, there would be indispensable in solving complicated thermo-fluid flow problems not only in the industrial fields such as automotive, space, electronics, aero- and hydro-dynamics, steel, and information engineering, but also in the basic research fields of medical science, bio-medical engineering, environmental and energy engineering etc. Especially, NT (Nano Technology) and BT (Bio Technology) strongly demand these advanced measurement techniques, because it is difficult for conventional methods to observe most complicated nano- and bio-fluidic phenomena. In this paper, the basic principle of these advanced visualization techniques and their practical applications which cannot be resolved by conventional methods, such as flow in automotive HVAC system, ship and propeller wake, three-dimensional flow measurement in micro-conduits, and flow around a circulating cylinder will be introduced.

• Journal of the Semiconductor & Display Technology
• /
• v.17 no.4
• /
• pp.91-96
• /
• 2018

In this study temperature distribution and gas flow inside a planetary type reactor in which a number of satellites on a spinning susceptor were rotating were analyzed using numerical simulation. Effects of flow rates on gas flow and temperature distribution were investigated in order to obtain design parameters. The commercial computational fluid dynamics software CFD-ACE+ was used in this study. The multiple-frame-of-reference was used to solve continuity, momentum and energy conservation equations which governed the transport phenomena inside the reactor. Kinetic theory was used to describe the physical properties of gas mixture. Effects of the rotation speed of the satellites was clearly seen when the inlet flow rate was small. Thickness of the boundary layer affected by the satellites rotation became very thin as the flow rate increased. The temperature field was little affected by the incoming flow rate of precursors.

• Journal of computational fluids engineering
• /
• v.13 no.2
• /
• pp.68-72
• /
• 2008

NUFLEX is a general purpose thermo/fluid flow analysis program which has various physical models including spray. In NUFLEX, spray models are composed of breakup and collision models of droplet. However, in case of diesel engine, interaction between wall-film and impingement model considering heat transfer is not coded in NUFLEX. In this study, Lee & Ryou impingement & wall-film model considering heat transfer is applied to NUFLEX. For the verification of this NUFLEX program, numerical results are compared with experimental data. Differences of film thickness and radius between numerical results and experimental data are within 10% error range. The results show that NUFLEX can be used for comprehensive analysis of spray phenomena.