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Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1995.Includes bibliographical references (leaf 71).by Jin Mo Kim.M.Eng

Control Method Of Circulating Refrigerant Amount For Heat Pump System

A heat pump system requires proper refrigerant charge amount. Once refrigerant is charged into a heat pump system, its charge amount is fixed. For this reason, prediction of optimal refrigerant charge amount is very important in order to yield best performance. Too low charge amount degrades capacity of heat pump. On the other hand, excessive charge amount decreases coefficient of performance (COP). The optimal value of refrigerant charge amount highly depends on secondary fluid temperature conditions. Consequently, fixed charge amount of refrigerant in heat pump shows the best performance only at certain temperature condition. Several ideas have revealed to change charge amount of the heat pump system. One is to have an additional reservoir to store or release refrigerant which is attached to a heat pump system. This method may seem simple but to measure exact amount of refrigerant in reservoir, additional pressure transducer, temperature measurement device, level sensor and other apparatus are required that increase the cost of heat pump. Another idea is to have reservoir between condenser outlet and expansion device. Rajapaksha and Suen (2004) showed that existence of reservoir at this point helps improve capacity while reducing the system COP. In this study, a new method for refrigerant charge amount control technique is presented. It has very simple control logic and requires only a few additional cost factors; several valves and additional tubes are only required. This method is based on different refrigerant phase distribution at each point of inlet and outlet of components in heat pump system. In a simple cycle heat pump system, refrigerant at condenser outlet (before expansion device) is in a subcooled liquid state at high pressure, while refrigerant is in a superheated vapor state at evaporator outlet (before compressor inlet) at low pressure. This technique regulates refrigerant charge by holding some volume of refrigerant in the connecting tube of considerable volume installed between the condenser outlet and the evaporator outlet. Using several solenoid valves (on/off) desired amount of refrigerant can be stored into a volume provided by a connecting tube. This connected volume is referred as ‘stagnation volume’ (Vstag). When one of this installed valve is closed and the rest of the valves are open, certain amount of refrigerant is stored in the stagnation volume (Vstag) while operating heat pump system. If closed valve is adjacent to condenser outlet, charge amount to the heat pump system increases while the charge is reduced when the valve adjacent to evaporator outlet is closed. This method is numerically verified and there are very little variation of COP. Therefore, heat pump can be operated at optimized circulating amount of refrigerant in spite of the secondary fluid temperature variation during heating or cooling operation

PREVENTING SUBSTANCE ABUSE AMONG ADOLESCENTS: EVALUATION OF AN INTEGRATED MODEL COMBINING LIFE SKILLS TRANING AND PARENT TRANING

This study examines the effectiveness of an integrated model combining Life Skills Training (LST) for students and Love and Logic Training (LLT) for parents. 310 middle school students and 49 parents were participated in the study. A paired sample t-test was computed to compare pre and post-test scores for each participant. An independent sample t-test was used to determine if statistical differences exist between the treatment and control groups. Alcohol use and resistance to alcohol use among both treatment and control groups were significantly changed after the intervention in a positive way. No significant change in substance use was found in both groups. When it comes to comparison between treatment and control groups, a significant difference was found only in resistance to alcohol use, meaning participants in treatment group showed significantly higher level of resistance to alcohol use than control group after the intervention. Implications of these results for practice and training were discussed

Reliability Analysis on Flexural Behavior of FRP Bridge Decks

Design codes for the design of FRP bridge decks shall be established to promote the use of such innovative materials. For the purpose of preparing code provisions, reliability analyses were conducted to evaluate proper levels of safety and serviceability. Based on the results, several guidelines on design codes are suggested

Extraction of Prosodic Information from Spoken Word for Synthesizing Korean Speech

This paper describes a study on prosodic information extracted from spoken Korean words, as a preliminary study to synthesize natural Korean speech. We investigated prosodic features-duration, amplitude and pitch-of some 500 words, of which selection was based on appearing frequency in Korean elementary school textbooks. Some results on word-based prosodic information are deduce

Oxygen Partial Pressure during Pulsed Laser Deposition: Deterministic Role on Thermodynamic Stability of Atomic Termination Sequence at SrRuO3/BaTiO3 Interface

With recent trends on miniaturizing oxide-based devices, the need for atomic-scale control of surface/interface structures by pulsed laser deposition (PLD) has increased. In particular, realizing uniform atomic termination at the surface/interface is highly desirable. However, a lack of understanding on the surface formation mechanism in PLD has limited a deliberate control of surface/interface atomic stacking sequences. Here, taking the prototypical SrRuO3/BaTiO3/SrRuO3 (SRO/BTO/SRO) heterostructure as a model system, we investigated the formation of different interfacial termination sequences (BaO-RuO2 or TiO2-SrO) with oxygen partial pressure (PO2) during PLD. We found that a uniform SrO-TiO2 termination sequence at the SRO/BTO interface can be achieved by lowering the PO2 to 5 mTorr, regardless of the total background gas pressure (Ptotal), growth mode, or growth rate. Our results indicate that the thermodynamic stability of the BTO surface at the low-energy kinetics stage of PLD can play an important role in surface/interface termination formation. This work paves the way for realizing termination engineering in functional oxide heterostructures.Comment: 27 pages, 6 figures, Supporting Informatio

Sources of Value Gains in Minority Equity Investments by Private Equity Funds: Evidence from Block Share Acquisitions

Using a large sample of block share acquisitions made by private equity (PE) funds over the 1990 to 2006 period, we examine the sources of value gains in PE minority equity investments. We find that compared to non-PE acquirers, PE acquirers are more likely to place representatives with finance experience on a target’s board, particularly when the target performs poorly or when it has more pronounced agency problems. PE acquirers are also more likely to place representatives with experience in the target’s industry on the target’s board when the target has more complex operations (e.g., multiple segments or higher R&D intensity). The targets in PE acquisitions, particularly those whose boards have representatives from PE acquirers, realize both higher abnormal announcement returns and better post-acquisition operating performance than do targets in other types of acquisitions. Target announcement abnormal returns and post-acquisition operating performance are also higher when PE-appointed directors have expertise in the target’s industry, when they sit on the boards of poorly performing targets, or when they sit on the boards of targets with higher R&D intensity. Thes