• Nuclear Engineering and Technology
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• v.36 no.5
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• pp.403-414
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• 2004

An experimental study of heat transfer characteristics near the critical pressure has been performed with an internally-heated vertical annular channel cooled by R-134a fluid. Two series of tests have been completed: (a) steady-state critical heat flux (CHF) tests, and (b) heat transfer tests for pressure reduction transients through the critical pressure. In the present experimental range, the steady-state CHF decreases with increase of the system pressure for fixed inlet mass flux and subcooling. The CHF falls sharply at about 3.8 MPa and shows a trend towards converging to zero as the pressure approaches the critical point of 4.059 MPa. The CHF phenomenon near the critical pressure does not lead to an abrupt temperature rise of the heated wall, because the CHF occurs at remarkably low power levels. In the pressure reduction transients, as soon as the pressure passes below the critical pressure from the supercritical pressure, the wall temperatures rise rapidly up to very high values due to the departure from nucleate boiling. The wall temperature reaches a maximum at the saturation point of the outlet temperature, and then tends to decrease gradually.

• Nuclear Engineering and Technology
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• v.42 no.6
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• pp.608-619
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• 2010

A multi-scale analysis of water-cooled reactor thermalhydraulics can be used to take advantage of increased computer power and improved simulation tools, including Direct Numerical Simulation (DNS), Computational Fluid Dynamics (CFD) (in both open and porous mediums), and system thermalhydraulic codes. This paper presents a general strategy for this procedure for various thermalhydraulic scales. A short state of the art is given for each scale, and the role of the scale in the overall multi-scale analysis process is defined. System thermalhydraulic codes will remain a privileged tool for many investigations related to safety. CFD in porous medium is already being frequently used for core thermalhydraulics, either in 3D modules of system codes or in component codes. CFD in open medium allows zooming on some reactor components in specific situations, and may be coupled to the system and component scales. Various modeling approaches exist in the domain from DNS to CFD which may be used to improve the understanding of flow processes, and as a basis for developing more physically based models for macroscopic tools. A few examples are given to illustrate the multi-scale approach. Perspectives for the future are drawn from the present state of the art and directions for future research and development are given.

• Nuclear Engineering and Technology
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• v.52 no.5
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• pp.918-924
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• 2020

Film boiling is of great importance in nuclear safety as it directly influences the integrity of nuclear fuel in case of accidents involving loss of coolant. Recently, nuclear power plant safety under earthquake conditions has received much attention. However, to the best of our knowledge, there are no existing studies reporting film boiling in an oscillating system. Most previous studies for film boiling were performed on stationary systems. In this study, numerical simulations were performed for saturated film boiling of water on a horizontal surface under low frequencies to investigate the effect of system oscillation on film boiling heat transfer. A coupled level-set and volume-of-fluid method was used to track the interface between the vapor and liquid phases. With a fixed oscillation amplitude, overall, heat transfer decreases with oscillation frequency. However, there is a frequency region in which heat transfer remains nearly constant. This lock-on phenomenon occurs when the oscillation frequency is near the natural bubble release frequency. With a fixed oscillation frequency, heat transfer decreases with oscillation amplitude. With a fixed maximum amplitude of the additional gravity, heat transfer is affected little by the combination of oscillation amplitude and frequency.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.18 no.11
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• pp.874-880
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• 2006

The power density and heat dissipation rate per unit area of the telecommunication equipment have been increased with the technology development in the footprint of telecommunication hardware. A proper heat dissipation method from the PCB module is very important to allow reliable operation of its electronic component. In this study, a hybrid air-conditioner for the telecommunication equipment room was designed to save energy and obtain system reliability. For high outdoor temperatures, the hybrid system operates in the vapor compression cycle, while, for low outdoor temperatures, the hybrid system works in the secondary fluid cooling cycle with no operation of the compressor. The performance of the hybrid air-conditioner was measured by varying outdoor and indoor temperatures. The hybrid air-conditioner yielded 50% energy saving compared with the conventional refrigeration system when the mode switch temperature was $8.3^{\circ}C$.

• Advances in materials Research
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• v.2 no.4
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• pp.181-193
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• 2013

Description and theory of spin coating technique has been elaborately outlined and a spin coating machine designed and fabricated using affordable components. The system was easily built with interdisciplinary knowledge of mechanics, fluid mechanics and electronics. This equipment employs majorly three basic components and two circuit units in its operation. These include a high speed dc motor, a proximity sensor mounted at a distance of about 15 mm from a reflective metal attached to the spindle of the motor to detect every passage of the reflective metal at its front and generate pulses. The pulses are transmitted to a micro-controller which process them into rotational speed (revolution per minute) and displays it on a lead crystal display (LCD) which is also a component of the micro-controller. The circuit units are a dc power supply unit and a PWM motor speed controlling unit. The various components and circuit units of this equipment are housed in a metal casing made of an 18 gauge black metal sheet designed with a total area of 1, $529.2cm^2$. To illustrate the use of the spin-coating system, ZnO sol-gel films were prepared and characterized using SEM, XRD, UV-vis, FT-IR and RBS and the result agrees well with that obtained from standard equipment and a speed of up to 9000 RPM has been achieved.

• Nuclear Engineering and Technology
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• v.53 no.9
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• pp.3112-3121
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• 2021

One of the major factors affecting the life span of a Reactor Pressure Vessel (RPV) is the Pressurised Thermal Shock (PTS). PTS is a thermo-mechanical load on the RPV wall due to steep temperature gradients and structural load created by internal pressure of the fluid within the RPV. Safe operating life of a nuclear power plant is ensured by carrying out fracture analysis of the RPV against thermal shock. Carrying out fracture tests on RPV/large scale components is not always feasible. Hence, studies on laboratory level specimens are necessary to validate and supplement the prototype results. This paper aims to study the fracture behaviour of standard Compact Tension [C(T)] specimens, made of RPV steel 20MnMoNi55, subjected to thermal shock through experimental and numerical investigations. Fracture tests have been carried out on the C(T) specimens subjected to thermal transient load and tensile load to quantify the effect of thermal shock. Crack resistance curves are obtained from the fracture tests as per ASTM E1820 and compared with those obtained numerically using XFEM and a good agreement was found. A quantitative study on the crack tip plastic zone, computed using cohesive segment approach, from the numerical analyses justified the experimental crack initiation toughness.

• Nuclear Engineering and Technology
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• v.53 no.7
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• pp.2162-2173
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• 2021

In this paper, the coupling of fine-mesh computational fluid dynamics (CFD) thermal-hydraulics (TH) code and neutronics code is achieved using the Ansys Fluent User Defined Function (UDF) for code development, including parallel meshing mapping, data computation, and data transfer. Also, some CFD schemes are designed for mesh mapping and data transfer to guarantee physical conservation in the coupling computation. Because there is no rigorous research that gives robust guidance on the various CFD schemes that must be obtained before the fine-mesh coupling computation, this work presents a quantitative analysis of the CFD meshing and mapping schemes to improve the accuracy of the value and location of key physical prediction. Furthermore, the effect of the sub-pin scale coupling computation is also studied. It is observed that even the pin-resolved coupling computation can also create a large deviation in the maximum value and spatial locations, which also proves the significance of the research on mesh mapping and data transfer for CFD code in a coupling computation.

• Journal of Drive and Control
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• v.16 no.3
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• pp.23-32
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• 2019

This study proposed a fault tolerant control algorithm for electro-hydraulic brake systems where permanent magnet synchronous motor (PMSM) drive is adopted to boost the braking pressure. To cope with motor position sensor faults in the PMSM drive, a braking pressure controller based on an open-loop speed control method for the PMSM was proposed. The magnitude of the current vector was determined from the target braking pressure, and motor rotational speed was derived from the pressure control error to build up the braking pressure. The position offset of the pump piston resulting from a leak in the hydraulic system is also compensated for using the open-loop speed control by moving the piston backward until it is blocked at the end of stroke position. The performance and stability of the proposed controller were experimentally verified. According to the results, the control algorithm can be utilized as an effective means of degraded control for electro-hydraulic brake systems in the case that a motor position sensor fault occurs.

• Journal of Drive and Control
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• v.16 no.3
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• pp.59-66
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• 2019

Hydraulic systems are essential for most of the construction equipments due to their various advantages, such as very powerful, quick response speed, precision control and remote control. Moreover, they are necessary to apply the electro hydraulic systems for precise and remote controls. Operating the small electronic joystick of the remote controller for the control of a multipurpose work machine with remote control technology increases the possibility of a sudden operation compared to the use of a conventional hydraulic joystick. When a joystick is suddenly operated, the peak pressure is generated in the system due to the quick response of the system. Then a vibration is generated due to the peak pressure, which causes instability to the operation of the construction equipment. Therefore, in this study, we confirmed the level of reduction of peak pressure occurring in the electro hydraulic system by using AMESim, when the output signal of the step shape generated by the sudden operation of the electronic joystick was changed by using the convolution operation.

• Journal of Drive and Control
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• v.16 no.3
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• pp.33-41
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• 2019

This study presents the development of a lifetime test bench for the strain wave reducer which is a precision gear reducer of the robot to realize fault diagnosis and failure prognostics. To this end, the lifetime test bench was designed to detect the vertical forward/reverse direction rotation load. Through the lifetime test bench, it is possible to apply the same load spectrum from robot working scenarios. We developed a data integration gateway for fault data collection. Through the development of dedicated software for fault diagnosis and failure prognostics, these data from vibration, noise and temperature sensors were collected and analyzed along with the operation of the lifetime evaluation.