The refrigerant charge reduction in HVAC\R systems is an important issue because it falls within environmental policies regarding refrigerants contributions to the greenhouse effect. A way to move toward charge reduction is to increase the compactness of heat exchangers, which means more complex designs of the evaporators. Nevertheless, while a large amount of studies have has been published on the thermal and hydraulic analysis of flow boiling of refrigerants in horizontal tubes, very little attention has been given to flow boiling in geometries different from straight tubes. This PhD thesis aims at studying the flow boiling characteristics of third generation (HFC) and fourth generation fluids such as HFO-1234yf in geometries which modify the fluid dynamics and two-phase flow with respect to horizontal straight tubes. To achieve this goal, an experimental test facility was specifically designed and built to conduct refrigerant evaporation experiments. This test facility allowed to perform flow regimes visualizations and pressure drop measurements in singularities (such as sudden contractions and return bends). First, two-phase flow regimes visualizations have been carried out using HFO-1234yf, R-134a and R-410A either in straight tubes or in singularities. A qualitative analysis of the flow behavior and also several comparisons to flow pattern prediction methods from the literature were conducted. The second objective of this work was to characterize the flow disturbances caused by singularities such as sudden contractions and return bends, and to study their effects on the hydrodynamic performance (e.g. pressure drop) of refrigerants.Lastly, pressure drop databases for third and fourth generation refrigerants are presented.

No description available.

Group psychotherapy with borderline patients is an activity that is fraught with ominous apprehension and it is perhaps for this reason that little research has been done in the field. The aim of the present studies was to map out the area and to test some basic hypotheses about patient and therapist behaviour during the therapy process. Two therapy groups of carefully diagnosed clinical borderline patients were studied by means of video-technique during a therapy process of 20 months. Special technical arrangements and ethical agreements were elaborated for the research. Outcome data was collected concerning the patients’ self-image, symptom-level and personality structure. A follow-up was done 2-3 years after therapy on functional level, capacity for work, medication and need of further treatment. As expected a high drop-out rate occurred. Within a year 40% of the patients had left, and within 20 months 60%. Drop-out was not found possible to predict before the period of therapy commenced but early drop-outs were generally younger and in a more acute state of distress than late drop-outs. In order to study the therapy process two major perspectives were elaborated and operationalized: 1) classification of focus and depth of therapists’ interventions and 2) relational capacity of the patient (Borderline Relatedness). In contrast to remainers, late drop-outs were found most sensitive to disturbances in the frame of therapy (group instability) and were considered prone to interact malignantly with the therapists, eventually leading to drop-out. As to outcome and follow-up, late drop-outs were characterized by the poorest outcome all over, while early drop-outs and remainers were generally more stabilized although the early drop-outs seemed more dependent on medication for their functioning. By means of a principal components analysis the large amount of outcome data was condensed to two important factors: 1) symptom and functional level and 2) direction of anger. Thus, it was found that an important feature of a positive outcome was the ability to function at work and in social life and to direct anger outwards, not just towards oneself, thus protecting and idealizing important others, as was often the case with these patients before treatment. Taken together, the results point to the need for more precise and sensitive diagnostic methods and outcome criteria both to predict and to evaluate therapeutic outcome. To minimize drop-out and negative therapeutic reaction the research speaks in favor of further studying the therapist-patient interaction during the group therapy process and to evaluate the relative importance of therapist interventions and non-specific curative factors. / <p>Diss. (sammanfattning) Umeå : Umeå universitet, 1989, härtill 4 uppsatser.</p> / digitalisering@umu.se

The objective of this thesis was to develop applications of monolithic fiber devices and actively controlled fibers. A special twin-core fiber known as a ‘Gemini’ fiber was used to construct equal arm-length fiber interferometers, impervious to temperature and mechanical perturbations. A broadband add/drop multiplexer was constructed by inscribing fiber Bragg gratings in the arms of a Gemini Mach-Zehnder interferometer. A broadband interferometric nanosecond switch was constructed from a micro-structured Gemini fiber with incorporated metal electrodes. Additionally, a Michelson fiber interferometer was built from an asymmetric twin-core fiber and used as a high-temperature sensor. While the device could be readily used to measure temperatures below 300 °C, an annealing process was required to extend the range up to 700 °C. The work included development, construction and evaluation of the components along with numerical simulations to estimate their behaviors and to understand the underlying processes. The thesis also explored the use of electrically controlled fibers for filtering in the microwave domain. An ultra-narrow phase-shifted fiber Bragg grating inscribed in a fiber with internal electrodes was used as a scanning filter to measure modulation frequencies applied to an optical carrier. A similar grating was used inside a dual-wavelength fiber laser cavity, to generated tunable microwave beat frequencies. The studied monolithic fiber interferometers and actively controlled fibers provide excellent building blocks in such varied field as in microwave photonics, telecommunications, sensors, and high-speed switching, and will allow for further applications in the future. / Syftet med denna avhandling var att utveckla tillämpningar av monolitiska fiber komponenter samt aktivt kontrollerbara fiber. En speciell tvillingkärnefiber, även kallad ’Geminifiber’ användes för att konstruera fiber interferometrar med identisk armlängd som ej påverkas av termiska och mekaniska variationer. En bredbanding utbytarmultiplexor konstruerades genom att skriva in fiber Bragg gitter inuti grenarna på en Gemini Mach-Zehnder interferometer. Geminifibrer med interna metallelektroder användes för att konstruera en bredbandig nanosekundsnabb interferometrisk fiberomkopplare. Därtill användes en tvillingkärnefiber som en hög-temperatursensor. Även om komponenten direkt kan användas upp till 300 °C, måste den värmebehandlas för att kunna användas upp till 700 °C. Arbetet har innefattat utveckling, konstruktion och utvärdering av komponenterna parallellt med numeriska simuleringar för att analysera deras beteenden samt få insikt om de underliggande fysikaliska processerna. Avhandlingen behandlar även tillämpningar av en elektriskt styrbar fiber för att filtrera radiofrekvenser. Ett ultrasmalt fasskiftat fiber Bragg gitter skrevs in i en fiber med interna elektroder och användes som ett svepande filter för att mäta modulationsfrekvensen på en optisk bärfrekvens. Ett liknande gitter användes inuti en laserkavitet för att generera två olika våglängder samtidigt. Dessa två våglängder användes sedan för att generera en svävningsfrekvens i mikrovågsbandet. De undersökta monolitiska fiberinterferometrarna och de aktivt styrbara fibrerna erbjuder en utmärkt byggsten inom så pass skiljda områden som Mikrovågsfotonik, Telekommunikation, Sensorer samt Höghastighets-omkopplare och bör kunna användas inom många olika tillämpningar i framtiden. / <p>QC 20130226</p>

Drop-on-demand (DOD) inkjet print-head has a major share of the market due to simplicity and feasibility of miniature system. The efficiency of droplet generation from DOD print-head is a result of several factors, include viscosity, surface tension, nozzle size, density, driving waveform (wave shape, frequency, and amplitude), etc. Key roles in the formation and behavior of liquid jets and drops combine three dimensionless groups: Reynolds number, Weber number and Ohnesorge number. These dimensionless groups provide some bounds to the “printability” of the liquid. Adequate understanding of these parameters is essential to improve the quality of droplets and provide guidelines for the process optimization. This thesis research describes the application of computational fluid dynamics (CFD) to simulate the creation and evolution process of droplet generation and transport of a highly viscous Newtonian fluid. The flow field is governed by unsteady Navier-Stokes equations. Volume of Fluid (VOF) model is used to solve this multi-phase (liquid-gas) problem.

Two-phase heat transfer in mini/micro-channels is capable of meeting the high cooling demands of modern high heat flux applications. The phase change process ensures better temperature uniformity and control for local hot spots. Furthermore, these compact channels could be helpful in reducing the required charge and material inventories.Environmental concerns—mainly ozone depletion and global warming—have instigated a search for new alternatives in refrigeration industry. While new compounds are being developed to address stringent legislative demands, natural alternatives are also coming into prominence. A limited number of investigators have reported on thermal performance of such alternatives. The current study is therefore focused on saturated flow boiling heat transfer, pressure drop and dryout characteristics for three low global warming potential (GWP) refrigerants (R152a, R600a and R1234yf) in a vertical mini-channel.In this study experiments were carried out by uniformly heating a test section (stainless steel tube with 1.60 mm inside diameter and 245 mm heated length) at 27 and 32 oC saturation temperature with 50-500 kg/m2s mass velocities. The effects of various parameters of interest (like heat flux, mass flux, system pressure, vapor quality, operating media) on flow boiling heat transfer, frictional pressure drop and dryout characteristics were recorded. R134a, which has been widely used in several applications, is utilized as a reference case for comparison of thermal performance in this study.Experimental results for saturated boiling heat transfer showed strong influence of heat flux and system pressure with insignificant contributions from mass flux and vapor quality. Two phase frictional pressure drop increased with mass flux, vapor quality and with reduced operating pressure. The dryout heat flux remained unaffected with variation in saturation temperature, critical vapor quality in most cases was about 85%. The experimental results (boiling heat transfer, two-phase pressure drop and dryout heat flux) were compared with well-known macro and micro-scale correlations from the literature. / <p>QC 20141124</p>

No description available.

In recent years carbon dioxide (CO2, R-744)has moved to the foreground as an environmentally friendly alternative to commonly used CFCs and HFCs, which are being phased out due to its high ozone depleting and global warming potentials. R-744 is not only environmentally friendly but due to its unique properties, it is also ideally suited for the use in heat pump water heaters. High cycle efficiencies are achievable even at high hot water temperatures. The high cycle efficiency not only leads to energy and cost savings but also ties in with the drive for implementation of energy saving measures in South Africa. It is therefore paramount to continue development and implementation of R-744 in heat pump water heaters. Optimizing the cycle efficiency is only possible if detailed component simulation models, taking these unique properties of R-744 into account, are available. The purpose of this study therefore was to develop a detail simulation model of a concentric tube-in-tube water-to-refrigerant evaporator, as well as a fin-and-tube air-to-refrigerant evaporator model. Data from the North-West University R-744 heat pump test bench were used to verify the tube-in-tube evaporator simulation model. The discrepancies in the cooling capacity between the simulation and test bench can be attributed to the presence of lubricant in the system.The fin-and-tube model was verified by testing it against the NIST program EVAP-COND (NIST 2010). Overall there was good agreement between the results of the two programs, with EVAP-COND predicting a lower cooling capacity(6% to 14%) and and a higher pressure refrigerant pressure drop (30% to 50%). It was found that both the heat transfer correlation of Jung et al. (1989) and the pressure drop correlation of Choi et al. (1999) are able to predict the experimental values accurately and are valid for use in both the evaporator models developed. To demonstrate the use of the detail evaporator fin-and-tube model, an evaluation of the different tube geometries, commercially available in South Africa, for use with R-744 fin-and-tube evaporators was done. For a fin-and-tube evaporator it was found that the most cost effective option is to use ⅜" (10.05 mm)copper tubes and the least effective is " (12.6 mm) stainless steel tubes. / Thesis (MIng (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2013.

In recent years carbon dioxide (CO2, R-744)has moved to the foreground as an environmentally friendly alternative to commonly used CFCs and HFCs, which are being phased out due to its high ozone depleting and global warming potentials. R-744 is not only environmentally friendly but due to its unique properties, it is also ideally suited for the use in heat pump water heaters. High cycle efficiencies are achievable even at high hot water temperatures. The high cycle efficiency not only leads to energy and cost savings but also ties in with the drive for implementation of energy saving measures in South Africa. It is therefore paramount to continue development and implementation of R-744 in heat pump water heaters. Optimizing the cycle efficiency is only possible if detailed component simulation models, taking these unique properties of R-744 into account, are available. The purpose of this study therefore was to develop a detail simulation model of a concentric tube-in-tube water-to-refrigerant evaporator, as well as a fin-and-tube air-to-refrigerant evaporator model. Data from the North-West University R-744 heat pump test bench were used to verify the tube-in-tube evaporator simulation model. The discrepancies in the cooling capacity between the simulation and test bench can be attributed to the presence of lubricant in the system.The fin-and-tube model was verified by testing it against the NIST program EVAP-COND (NIST 2010). Overall there was good agreement between the results of the two programs, with EVAP-COND predicting a lower cooling capacity(6% to 14%) and and a higher pressure refrigerant pressure drop (30% to 50%). It was found that both the heat transfer correlation of Jung et al. (1989) and the pressure drop correlation of Choi et al. (1999) are able to predict the experimental values accurately and are valid for use in both the evaporator models developed. To demonstrate the use of the detail evaporator fin-and-tube model, an evaluation of the different tube geometries, commercially available in South Africa, for use with R-744 fin-and-tube evaporators was done. For a fin-and-tube evaporator it was found that the most cost effective option is to use ⅜" (10.05 mm)copper tubes and the least effective is " (12.6 mm) stainless steel tubes. / Thesis (MIng (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2013.

No description available.