• Title/Summary/Keyword: Cross Efficiency

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Effect of Nozzle Shape on the Performance and Internal Flow of a Cross-Flow Hydro Turbine (횡류수차 노즐형상이 성능과 내부유동에 미치는 영향)

  • Choi, Young-Do;Lim, Jae-Ik;Kim, You-Taek;Lee, Young-Ho
    • The KSFM Journal of Fluid Machinery
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    • v.11 no.4
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    • pp.45-51
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    • 2008
  • The purpose of this study is to examine the effect of nozzle shape on the performance and internal flow of a cross-flow hydro turbine. CFD analysis for three kinds of nozzle shape is conducted to simulate the effect of nozzle shape. The results reveal that relatively narrow nozzle width is effective to increase the turbine efficiency and output power. Almost output power is achieved at Stage 1. Therefore, optimum design of the nozzle shape is necessary to improve the turbine performance. Recirculation flow in the runner passage decreases the turbine efficiency and output power because the flow make hydraulic loss and collision loss in the region. Air should be put into the runner passage and the recirculating flow should be suppressed by the air layer in the runner.

Design of the recuperator for the gas turbine/fuel cell hybrid power generating system (가스터빈/연료전지 혼합발전 시스템의 열교환기 설계)

  • Kwak, Jae-Su;Yang, Soo-Seok;Lee, Dae-Sung
    • Proceedings of the KSME Conference
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    • 2004.04a
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    • pp.2105-2110
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    • 2004
  • Plate-fin type recuperators for the gas turbine/fuel cell hybrid power generating system were designed using commercial design software, MUSE. Heat transfer efficiency and total pressure drop in the recuperator were calculated to confirm required recuperator performance. Both counter flow and cross flow type plate-fin recuperators were designed. Results show that the counter flow type has higher efficiency and short core length, but the cross flow type is simpler to construct because the cross flow type does not need additional distributors. Two or three headers for the each recuperator core will be designed and tested to evaluate best header design. The designed recuperators and headers which will be designed later will be constructed, tested, and used in gas turbine/fuel cell hybrid power generating system.

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CFD Analysis on a Flow Channel of a Bipolar Plate with Varying Cross-sectional Area in a PEM Fuel Cell (PEM 연료전지용 Bipolar Plate의 변화단면 유로에 대한 CFD 해석)

  • Yang, Dong-Jin;Park, Woon-Jean
    • New & Renewable Energy
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    • v.3 no.3
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    • pp.14-19
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    • 2007
  • A flow channel model of a bipolar plate with varying cross-sectional area was newly designed for improving performance and efficiency of a PEM fuel cell stack. As a result, the varying cross-sectional area model showed poor uniformity in velocity distribution, however, maximum velocity in the flow path is about 30% faster than that of the uniform cross-sectional area model. The proposed varying cross-sectional area model is expected to diffuse operating fluids more easily into diffusion layer because it has relatively higher values in pressure distribution compared with other flow channel models. It is expected that the implementation of the varying cross-sectional area model can reduce not only the mass transport loss but also the activation loss in a PEM fuel cell, and open circuit voltage of a fuel cell can thus be increased slightly.

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Elimination of the effect of strain gradient from concrete compressive strength test results

  • Tabsh, Sami W.
    • Computers and Concrete
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    • v.3 no.6
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    • pp.375-388
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    • 2006
  • Poor strength test results are sometimes not an indication of low concrete quality, but rather inferior testing quality. In a compression test, the strain distribution over the ends of the specimen is a critical factor for the test results. Non-uniform straining of a concrete specimen leads to locally different compressive stresses on the cross-section, and eventual premature breaking of the specimen. Its effect on a specimen can be quantified by comparing the compressive strength results of two specimens, one subjected to uniform strain and another to a specified strain gradient. This can be done with the help of a function that relates two parameters, the strain ratio and the test efficiency. Such a function depends on the concrete strength and cross-sectional shape of the specimen. In this study, theoretical relationships between the strain ratio and test efficiency are developed using a concrete stress-strain model. The results show that for the same strain ratio, the test efficiency is larger for normal strength concrete than for high strength concrete. Further, the effect of the strain gradient on the test result depends on the cross-sectional shape of the specimen. Implementation of the results is demonstrated with the aid of two examples.

An Energy Efficient Data Delivery Scheme based on Cross-Layer Design in Wireless Sensor Networks (무선 센서네트워크에서 교차계층 설계 기반의 에너지 효율적인 데이터 전송 기법)

  • Shin, Jong-Whoi;Kim, Jae-Hyun;Kim, Seog-Gyu
    • Journal of the Korea Society of Computer and Information
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    • v.13 no.4
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    • pp.177-184
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    • 2008
  • The design goal of protocols in wireless sensor networks (WSN) is mainly energy efficiency because of their stringent resource and energy constraints. In this paper, we propose a simple cross-layered protocol for WSNs, so called EATD(Energy-Aware Tree based Delivery scheme). EATD is a tree-based energy aware data delivery algorithm by using a SYNC packet with link and node cost to maximize the network lifetime. Our simulation results show significant improvements compared with existing schemes in terms of energy efficiency and delay.

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Time-Series Forecasting Based on Multi-Layer Attention Architecture

  • Na Wang;Xianglian Zhao
    • KSII Transactions on Internet and Information Systems (TIIS)
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    • v.18 no.1
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    • pp.1-14
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    • 2024
  • Time-series forecasting is extensively used in the actual world. Recent research has shown that Transformers with a self-attention mechanism at their core exhibit better performance when dealing with such problems. However, most of the existing Transformer models used for time series prediction use the traditional encoder-decoder architecture, which is complex and leads to low model processing efficiency, thus limiting the ability to mine deep time dependencies by increasing model depth. Secondly, the secondary computational complexity of the self-attention mechanism also increases computational overhead and reduces processing efficiency. To address these issues, the paper designs an efficient multi-layer attention-based time-series forecasting model. This model has the following characteristics: (i) It abandons the traditional encoder-decoder based Transformer architecture and constructs a time series prediction model based on multi-layer attention mechanism, improving the model's ability to mine deep time dependencies. (ii) A cross attention module based on cross attention mechanism was designed to enhance information exchange between historical and predictive sequences. (iii) Applying a recently proposed sparse attention mechanism to our model reduces computational overhead and improves processing efficiency. Experiments on multiple datasets have shown that our model can significantly increase the performance of current advanced Transformer methods in time series forecasting, including LogTrans, Reformer, and Informer.

A Study on Selection of Cross-Docking Center based on Existing Logistics Network (기존 물류 네트워크 기반에서 크로스 - 도킹 거점선정에 관한 연구)

  • Lee, In-Chul;Lee, Myeong-Ho;Kim, Nae-Heon
    • IE interfaces
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    • v.19 no.1
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    • pp.26-33
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    • 2006
  • Many Firms consider the application of a cross-docking system to reduce inventory and lead-time. However, most studies mainly concentrate on the design of a cross-docking system. This study presents the method that selects the cross-docking center under the existing logistics network. Describing the operation environment to apply the cross-docking system, the selection criteria of the cross-docking center, and the main constraints of transportation planning under the environment of multi-level logistics network, we define the selection problem of the cross-docking center applied to a logistics field. We also define the simulation model that can analyze variously the cross-docking volume and develop the selection methodology of the cross-docking center. The simulation model presents the algorithm and influence factors of the cross-docking system, the decision criteria of the system, policy parameter, and input data. In addition, this study analyzes the effect of increasing the number of simultaneous receiving and shipping docks, and the efficiency of the overnight transportation and cross-docking by evaluating each scenario after simulating the scenarios with the practical data of the logistics field.

A Case Study on the Construction of Large Cross Section Tunnel for Underground Ventilation System (지하환기소 대단면 터널 시공 사례 연구)

  • Noh, Seung Hwan;Choi, Sung Wook;Noh, Sang Lim
    • Tunnel and Underground Space
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    • v.27 no.4
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    • pp.195-204
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    • 2017
  • This case study introduces the construction of large cross section tunnel for underground ventilation system in Sillim-Bongcheon Tunnel Project. In order to grant the safety and efficiency in connecting the ventilation shaft (7.8 m of width, and 6.6 m of height) to a tunnel for axial fan facility (20.8 m of width, and 12.3 m of height), gradual enlargement of tunnel cross section was employed between those and temporary support method was determined based on Q system. In addition, some original designs were revised during construction stage to improve the efficiency of excavation in large cross section tunnel. The advance length was optimized and top heading of the tunnel was excavated without partition in accordance with ground condition and numerical stability analysis results. It is believed that some experiences and considerations in this case study will be useful for the future design and construction in similar large cross section tunnel such as large underground ventilation system or road tunnel with four lanes.

High Efficiency and Small Area DC-DC Converter for Gate Driver using LTPS TFTs

  • Kim, Kyung-Rok;Kim, Hyun-Wook;Kwon, Oh-Kyong
    • 한국정보디스플레이학회:학술대회논문집
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    • 2007.08b
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    • pp.1085-1088
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    • 2007
  • A new DC-DC converter was designed for gate driver circuit using low temperature poly-Si TFT technology. To achieve high efficiency and small area, we proposed a cross-coupled type DC-DC converter which converts 5V of input voltage to 9V of output voltage and supplies 120$\mu$A of current to load. Its efficiency is 92.9% and the area is reduced as much as 19% compared to the previously reported latch type DC-DC converter.

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Performance Characteristics of Direct Methanol Fuel Cell with Methanol Concentration (메탄올 농도에 따른 직접 메탄올 연료전지의 성능 해석)

  • Cho, Chang-Hwan;Kim, Yong-Chan;Chang, Young-Soo
    • Korean Journal of Air-Conditioning and Refrigeration Engineering
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    • v.20 no.3
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    • pp.197-204
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    • 2008
  • DMFC(Direct Methanol Fuel Cell) is one of promising candidates for power sources of small mobile IT devices like notebook, cell phone, and so on. Efficient operation of fuel cell system is very important for long-sustained power supply because of limited fuel tank size. It is necessary to investigate operation characteristics of fuel cell stack for optimal control of DMFC system. The generated voltage was modeled according to various operating condition; methanol concentration, stack temperature, and load current. It is inevitable for methanol solution at anode to cross over to cathode through MEA(membrane electrode assembly), which reduces the system efficiency and increases fuel consumption. In this study, optimal operation conditions are proposed by analyzing stack performance model, cross-over phenomenon, and system efficiency.