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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
71

Condensing coefficients of the refrigerant mixture R-22/R-142b in smooth tubes and during enhanced heat transfer configurations

22 January 2009 (has links)
D.Ing. / The heating of water with hot-water heat pumps is extremely energy-efficient. With the refrigerant R-22 hot water temperatures of 60° C to 65° C are possible. However, these temperatures are low in comparison with the temperatures obtained from other methods of water heating, for instance electrical geysers. Should higher water temperatures be obtained, the applications of hot-water heat pumps will increase. This is possible by using a zeotropic refrigerant mixture as working fluid. A R-22 and R-142b zeotropic refrigerant mixture shows exceptional potential in achieving hot water temperatures. The condensing coefficients need to be predicted correctly to optimize the condenser design. Unfortunately, there is a lack of detailed literature available on condensing coefficients for the recommended mass fractions of R-22 with R-142b at condensing temperatures of 60° C or more. Micro-fin tubes perform outstanding in enhancing heat transfer and are widely used to save energy. Unfortunately, there is also a lack of detailed literature on condensing coefficient at the recommended mass fractions of R-22/R-142b refrigerant mixtures condensing in micro-fins, twisted tapes and high fins at temperatures of 60° C or more. In this study condensing coefficients of R-22 and the zeotropic refrigerant mixture R-22 with R-142b were obtained in smooth tubes at mass fractions of 90%/10%, 80%/20%, 70%/30%, 60%/40%, 50%/50%. The experimental data were used to evaluate some of the methods that are commonly used to predict condensing coefficients. Experiments were also conducted at the same zeotropic mass fractions, to compare three different methods of heat transfer enhancement to that of the smooth tubes namely: micro-fins, twisted tapes and high fins. All measurements were conducted at an isobaric inlet pressure of 2.43 MPa. The test sections consisted of a series of eight tubes with lengths of 1 603 mm. The smooth tubes had an inner diameter of 8.11 mm. With the R-22/R-142b zeotropic refrigerant mixture condensing in smooth tubes, it was observed in the sight glasses that a predominantly stratified wavy flow regime exists at low mass fluxes, from 40 kg/m2s to 350 kg/m2s. The refrigerant mass fraction decreased the condensing coefficient by up to a third on average from 100% R-22 to a 50%/50% mixture of R-22 with R142b. A predominantly annular flow regime was observed at mass fluxes of 350 kg/m2s and more. At this flow regime the condensing coefficients were not strongly influenced by the refrigerant mass fraction, decreasing only by 7% as the refrigerant mass fraction changed from 100% R-22 to a 50%/50% mixture of R-22 with R142b. When the experimental data were compared with three methods that are commonly used to predict condensing coefficients it was found that the flow pattern correlation of Dobson and Chato (1998) gave the best predictions for R-22. The Silver (1964) and Bell and Ghaly (1964) method gave the best predictions for the R-22/R-142b mixtures. When the three heat transfer enhancement methods were compared with smooth tubes it was found that micro-fins were more suitable as an enhancement method than twisted tubes or high fins. It was also found that the condensing coefficients and pressure drops decrease as the mass fractions of R-142b increases.
72

Heat transfer performance during in-tube condensation in horizontal smooth, micro-fin and herringbone tubes

27 November 2008 (has links)
M.Ing. / An experimental investigation was conducted into the heat transfer characteristics of horizontal smooth, micro-fin and herringbone tubes during in-tube condensation. The study focused on the heat transfer coefficients of refrigerants R-22, R-134a and R-407C inside the three tubes. The herringbone tube results were compared to the smooth and micro-fin tube results. The average increase in the heat transfer coefficient when compared to the smooth tube was found to be as high as 322% with maximum values reaching 336%. When compared to the micro-fin tube, the average increase in heat transfer coefficient was found to be as high as 196% with maximum values reaching 215%. A new unified correlation was also developed to predict the heat transfer coefficients in a herringbone and micro-fin tube. The correlation predicted the semi-local heat transfer coefficients accurately with 96% and 89% of the data points falling in the ± 20% region for the herringbone and micro-fin tube respectively. The average heat transfer coefficients were also accurately predicted with all the data points for the herringbone tube and 83% of the data points for the micro-fin tube falling in the ± 20% region. The trend of the new correlation also fitted the data accurately and the conclusion was made that the correlation is accurate and could be used successfully in practice.
73

The mechanism of dropwise condensation of steam

Fang, Chung-Chih January 1949 (has links)
The present investigation can be divided into two parts: (a) experiments made to examine the mechanism of dropwise condensation of steam with particular reference to the stability of drop promoting surfaces as affected by the material of cooled surface, the drop promoter, the surface finish, the rate of heat transmission, and the presence of non-condensable gas. and (b) a theoretical analysis of the beat transmission through individual droplets, the transient heat transfer through exposed areas, the statistical study of drop size distribution, and the estimation of steam side coefficient. An apparatus was developed to examine qualitatively,the behaviour of drop promoting surfaces on a small scale. It is considered that sufficient evidence was found to show that steam in contact with a cooled surtace condenses as a thin liquid film which later breaks into droplets. surfaces treated to give dropwise condensation deteriorate into mixed condensation in due time, and the duration tor which a treated surface maintains dropwise condensation varies between a few hours to several days, depending on many factors among which· the presence of non-condensable gas must not be overlooked. An approximation to the heat transmission through individual droplets has been worked out with assumed heat flow lines. The result, checked by the relaxation method. is correct within . + 10%. An analysis Of the transient heat transfer through exposed areas was made neglecting the increasing resistance of any accumulating liquid. The drop size distribution was analyzed tor one drop promoting surface at three different heat transmission rates. Based on this drop size distribution, the heat transmission through the drops was estimated by assuming they were held at rest on a cooled surface conducting heat under a steady state. . The estimated coefficient comes within the range or experimental results of many investigators.
74

Coherent heteronuclear spin dynamics in an ultracold spinor mixture / 超冷旋量混合物中的異核間相干自旋動力學 / CUHK electronic theses & dissertations collection / Coherent heteronuclear spin dynamics in an ultracold spinor mixture / Chao leng xuan liang hun he wu zhong de yi he jian xiang gan zi xuan dong li xue

January 2015 (has links)
Li, Xiaoke = 超冷旋量混合物中的異核間相干自旋動力學 / 李小科. / Thesis Ph.D. Chinese University of Hong Kong 2015. / Includes bibliographical references (leaves 126-142). / Abstracts also in Chinese. / Title from PDF title page (viewed on 25, October, 2016). / Li, Xiaoke = Chao leng xuan liang hun he wu zhong de yi he jian xiang gan zi xuan dong li xue / Li Xiaoke.
75

A classical field treatment of colliding Bose-Einstein condensates

Norrie, Adam Anson, n/a January 2005 (has links)
In this thesis we develop a fully quantum-mechanical treatment of collisions between distinct atomic Bose-Einstein condensate wavepackets, with particular emphasis on the incoherently scattered atoms that form s-wave haloes around the condensate wavepackets. Previous theoretical treatments of these systems have been unable to account simultaneously for both the evolution of the halo and the depletion of the condensates, and were therefore restricted to the small scattering limit. Our approach uses the truncated Wigner method, a particular example of the classical field methods familiar from quantum optics. The atomic field is restricted to a low-energy subspace of single-particle states, and the method is applicable even to highly-scattered systems. We present a comprehensive derivation of the truncated Wigner method for ultracold bosonic fields, and discuss in detail the validity regime of the Wigner truncation for inhomogeneous multimode systems. The method gives rise to a set of coupled stochastic differential equations that describe the evolution of a single realisation of the atomic field, and have a form similar to that of the well known Gross-Pitaevskii equation, but with the important difference that the stochastic differential equations include well prescribed quantum fluctuations. To propagate our systems we develop algorithms that allow for highly efficient numerical evolution of realistic experimental collisions. By investigating individual trajectories of the colliding system, we find that the scattering halo is composed of many distinct highly-populated phase grains separated by large numbers of vortices, a behaviour we label quantum turbulence. We develop a spatial averaging method for approximately calculating quantum correlation functions from a single trajectory, and calculate various properties of the halo. Based on these results, we propose a mechanism to explain the observed features of scattering halo formation. We find by using an appropriately extended truncated Wigner approach that three-body recombination events have negligible effect on the collisions. Using an ensemble of trajectories we calculate correlation functions of a particular collisional system to give a rigorous characterisation of the quantum statistics of the field, and obtain results that are remarkably similar to those obtained using single trajectory spatial averaging. For global field quantities, such as the total coherent population, we find that accurate estimates can be achieved using just two trajectories, a result we use to efficiently explore the dependence of the system on key physical parameters. Finally, we apply the truncated Wigner method to collisions between condensates in differing hyperfine states, whose (single-trajectory and ensemble) behaviour we find is qualitatively similar to that of single-component collisions.
76

Condensation and Phase Transformation of (Ni,Ti)O2 vs. (Ni,Co)O Nanoparticles and Sublimination-Condensation of Sintered (Co,Mg)O Polycrystals

Huang, Chang-Ning 24 June 2007 (has links)
Abstract This research is focused on the condensation and phase transformation of NiO-TiO2, Co1-XO-NiO, and Co1-XO-MgO solid solution via dynamic laser ablation condensation and high temperature annealing. TiO2 rutile nanospheres with enhanced solid solution of NiO were synthesized via very energetic pulse laser ablation on clamped Ni/Ti target in oxygen for a very rapid heating/cooling, and hence pressure effect. Upon electron irradiation, the NiO-dissolved rutile (r) were partially transformed into 2(01 ) commensurate superstructure and Ni2(1+x)Ti1-xO4 spinel (s) following the crystallographic relationship [111]r//[012]s; (10 )r//(200)s. Alternatively, random NiTiO3 nanodomains were formed from amorphous regions in such a rapid decomposition process. In addition, the dense fluorite-type (f) derived TiO2 condensates dissolved up to 5 at% Ni2+ of the cations were synthesized via the same route. The nanocondensates less than 20 nm in size are nearly cubo-octahedral in shape and tended to transform martensitically to monoclinic (m) baddeleyite-type following the crystallographic relationship (100)f//(110)m; [001]f//[001]m. The condensates twice larger in size, with considerable matrix constraint, are nearly spherical in shape and consist of mosaic m-twin variants following complicated crystallographic relationships with each other and with the relic f-phase: (010)f//( 20)m; [001]f//[001]m. The charge and volume compensating oxygen vacancies due to NiO dissolution in the dense TiO2 condensates could facilitate the relaxation and amorphization process. Further more, pulse laser irradiation of Ni, Co, and Co-Ni (or Ni-Co) targets in an oxygen background gas produced nanocondensates with rock-salt type structure. Analytical electron microscopic observations indicated that such nanocrystals are cubic in shape for NiO and cubo-octahedral for Co1-xO and Ni1-xCoxO. The nanocrystals coalesced predominantly with {100} or {111} facets to form nano chain aggregates or closer packed manner. The Co1-xO was more or less oxidized as Co3-
77

High quality SGOI (SiGe-On-Insulator) substrate preparation using Ge-Condensation technology

Chen, Pain-Chin 18 July 2003 (has links)
In our thesis, we develop a modified fabrication method based on Ge condensation mechanism to fabricate SGOI (SiGe-on-insulator) Wafer. The advantages of this technique are as follows; (1) Low fabrication temperature. (2) Smooth SiGe/SiO2 interface without using CMP and good crystal quality. (3) Better gate dielectric layer quality by dry oxidation. In our experiment, we use silicon wafer rather than the SOI wafer to avoid cost because of the high price of the SOI wafer. First, a 700Å Si0.85Ge0.15 layer was grown on a thin SOI layer. The Ge atoms were rejected from the oxidized layer and pushed into the remaining SiGe layer by using dry oxidation at 925¢J. Since it has been confirmed that the total amount of the Ge atom in the SGOI layer is conserved, the Ge fraction can be varied from 15% to 35%. During the fabrication procedure, we use semiconductor measurement instruments like AFM /SEM /Raman spectroscopy to verify the SiGe layer quality and built complete parameters database. Then we make two different structure Si/SiGe heterojunction MOS capacitors on this wafer to verify the necessity of the Si cap layer to SGOI substrate. According to the experiment results, we can find the device with Si cap layer has better performences than the one without Si cap about 10% ~ 20% in electric characteristics. Based on the experiment results, it is proved that a high quality SGOI wafer on the SOI wafer can be fabricated.
78

Condensation heat transfer in microchannel /

Siu, Billy Chin Pang. January 2004 (has links)
Thesis (M. Phil.)--Hong Kong University of Science and Technology, 2004. / Includes bibliographical references (leaves 43-46). Also available in electronic version. Access restricted to campus users.
79

Nonlinear dynamics of Bose-Einstein condensates : semiclassical and quantum /

Salmond, Grant Leonard. January 2002 (has links) (PDF)
Thesis (M. Sc.)--University of Queensland, 2002. / Includes bibliographical references.
80

Instability in a cold atom interferometer

Pulido, Daniel. January 2003 (has links)
Thesis (M.S.)--Worcester Polytechnic Institute. / Keywords: interferometry; bose-einstein condensation. Includes bibliographical references (p. 49-50).

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