• 대한기계학회:학술대회논문집
• /
• 대한기계학회 2001년도 춘계학술대회논문집D
• /
• pp.636-641
• /
• 2001

In this paper, we constructed four valve type pulse tube refrigerator and found the characteristic of orifice (needle valve) opening for using phase shifter and optimum cycle time - The valve timing was controlled by the digital timers. The experimental results showed the optimum frequence and cycle time at operating conditions. The results showed that the optimum process time existed and the rate was same at each operating frequence. The no- load temperature of the refrigerator was 85K.

• Journal of Advanced Marine Engineering and Technology
• /
• 제31권6호
• /
• pp.723-728
• /
• 2007

In order to evaluate the performance of carbon dioxide cycle with a sub-cooling loop. a simulation system was developed to predict the steady state of $CO_2$ trans-critical cycle. Mathematical models are derived to describe the relationships between the system's coefficient of performance and other operating parameters The mathematical models are based entirely on the basic mass and energy conservation law and thermodynamic and transport properties of carbon dioxide A parametric study has been conducted in order to investigate the effect of sub-cooling loop and various operating conditions on the cycle performance. An optimal mass fraction of a refrigerant flowing through sub-cooling cycle existed for the given evaporating temperature, high pressure and air inlet temperature through gas cooler.

• 한국철도학회:학술대회논문집
• /
• 한국철도학회 2009년도 춘계학술대회 논문집
• /
• pp.636-642
• /
• 2009

The application of LCC-techniques is being introduced at the on-going procurement programs of various techniques. LCC-techniques have the common characteristic that all are designed to motivate contractors to design, manufacture and deliver equipment with lower life cycle costs. You may believe that savings may be in the acquisition cost component of life cycle costs. However, primary emphasis is generally on reducing and controlling operating and support costs by transferring more responsibility to the contractor for equipment operating and support cost performance. It has been found that life cycle cost procurement provisions must be individually tailored to each program. In this study, the currently identified LCC procurement techniques including a variety of LCC incentive provisions are introduced. Moreover, verification method, a procedural issue and incentive to application of LCC-techniques are examined.

• 한국군사과학기술학회지
• /
• 제6권1호
• /
• pp.30-38
• /
• 2003

The measurement on the working gas flow in the closed cycle diesel engine is insufficient to Identify its operating condition because of lack of sensors available. For this reason, the determination of optimum operating condition and controller parameters requires heavy experimental efforts. A simulation program is developed to minimize the experimental efforts. For the development of the simulation program, mathematical models are derived based on the physical laws and linear regression. The validity of the simulation program is demonstrated using the measured data of the closed cycle diesel engine.

• International Journal of Air-Conditioning and Refrigeration
• /
• 제17권3호
• /
• pp.100-106
• /
• 2009

As an effort to prevent environmental problems caused by ozone depletion and global warming, alternative refrigerants are being developed, and one of the candidates is carbon dioxide. To overcome slightly low efficiency of $CO_2$ refrigeration system, air-conditioning cycle using an ejector was suggested. Ejector compensates throttling loss in an expansion device by reducing compression work. In this study, the ejector refrigeration cycle using $CO_2$ as a refrigerant is investigated to understand the effect of the mixing section diameter and refrigerant charge amount on the performance. If mixing section diameter is too large or too small, either cases show low performance. The optimum refrigerant charge amount which gives the best performance is found for standard operating conditions. The air-conditioning cycle was analyzed for several operating conditions.

• 한국항해항만학회지
• /
• 제39권3호
• /
• pp.261-266
• /
• 2015

기업의 운전자본관리는 단기재무전략관리 차원에서 접근이 가능하며, 이는 기업의 유동성 및 수익성 등에 영향관계가 있음이 실증되고 있다. 따라서 효율적인 운전자본관리는 기업의 수익성을 증대시킬 수 있을 뿐만 아니라, 유동성 또한 개선시킬 수 있게 된다. 해운기업의 경우 글로벌금융위기와 맞물려 수익성과 유동성에 많은 어려움을 겪었음에도 불구, 운전자본관리와 수익성에 대한 연구는 전무한 상태이다. 이에 본 연구에서는 2004년 이후 46개 해운기업을 대상으로 운전자본관리와 수익성과의 관계를 규명함으로써 단기재무전략 수립에 유용한 정보를 제공하고자 하였다. 연구의 결과 기존연구들과 같이 매입채무기간, 재고기간, 현금주기, 영업주기가 해운기업의 수익성 즉, 매출액 총이익률과 매출액영업이익률을 개선할 수 있는 중요한 요인인 것으로 실증되었다. 특히 매입채무신용기간과 영업주기의 증가는 수익성의 증가를 가져오는 것으로 분석된 반면, 재고기간과 현금주기는 부(-)의 관계를 보여 이들이 줄어들 경우 수익성이 증가하는 것으로 나타났다. 매출채권기간은 일부 모형에서 유의하였으나, 기존 연구들과는 달리 해운기업의 경우 수익성과의 관계를 설명하기에는 한계가 있는 것으로 나타났다.

• International Journal of Air-Conditioning and Refrigeration
• /
• 제11권4호
• /
• pp.209-217
• /
• 2003

The term “energy saving is economical” is appropriate for the national view point and for design and assessment of one system, but not appropriate when choosing the system by comparing alternative systems in the early design step. Sometimes, non-energy saving system is more economical than energy saving system because of the price of electricity, gas or oil, which are used for operating the air conditioning system. Therefore, when designing a system, we should consider the efficient alternatives through economic assessment of energy saving method. However, research on non-operating number control of the system is not sufficient because it is more common to use operating number control of the system for most economic assessment of air conditioning systems. For this reason, this research can provide the economic operating number control method as basic design data. The data obtained through analysis of life cycle cost based on amount of yearly energy use, are produced by system simulation of HASP/ACLD/8501 and HASP/ACSS/8502 for six alternative heating$.$cooling systems based on seasonal air conditioning system, which is widely used for medium and large size office buildings in Busan.

• 전력전자학회:학술대회논문집
• /
• 전력전자학회 2019년도 전력전자학술대회
• /
• pp.317-319
• /
• 2019

In this paper, a three level duty cycle controlled half bridge LLC converter for EV charger application is presented. The topology and operating regions of the converter are discussed. The equations of the converter are derived in time domain. A small signal model of the converter is developed by perturbation and linearization of the steady state model about their operating point using Extended Describing function.

• 대한기계학회논문집
• /
• 제16권1호
• /
• pp.77-86
• /
• 1992

본 연구에서는 사이클은 터빈 출구로 부터 배출되는 폐열을 최대한 회수하여 얻은 증기를 연소기내에 분사시킴으로써 부가적인 압축기 및 비출력의 상향을 기할수 있다.아울러 폐열이용 암모니아 흡수기 냉동기를 구동하여 압축기 입구 온도를 낮 춤에 의해 열효율 및 비출력의 증대는 물론 대기온도 변화에 따른 기관 성능의 변동을 감소시킬 수 있다.

• 설비공학논문집
• /
• 제10권4호
• /
• pp.495-507
• /
• 1998

The transient characteristics of a 4.0㎾ inverter driven heat pump was investigated by theoretical and experimental studies. The heat pump used in this study consists of a high side scroll compressor and $\Phi$7 compact heat exchangers with two capillary tubes. A series of tests was peformed to examine the transient characteristics of heat pump in heating and cooling mode when the operating speed was varied from 30Hz to 102Hz. One of the major issues that has not been addressed so far is transient characteristics during speed modulation. A cycle simulation model has been developed to predict the cycle performance under frequency rise-up conditions, and the results of theoretical study were compared with the results of experimental study. The theoretical model was driven from mass conservation and energy conservation equations to predict the operation points of refrigerant cycle and the performances at various operating speeds. For transient conditions, the simulated results are in good agreement with the experimental results within 10%. The transient cycle migration of the liquid state refrigerant causes a significant dynamic change in system. Thus, the migration of refrigerant is the most important factor whenever An experimental analysis is performed or A simulation model is developed.