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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.
31

Boiling heat transfer

Bailey, Nigel Anthony January 1968 (has links)
No description available.
32

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.
33

Measurement of scale formation in an experimental heat exchanger circuit

05 September 2012 (has links)
M.Ing. / Deposition of scale on heating surfaces is a major problem in industry as well as households. The scale that forms on the heating surfaces acts as an insulator and results in decreased heat transfer effectiveness . These are two main approaches to prevent or reduce scaling. Although these approaches are claimed to be efficient, there is a need to evaluate or verify their efficacy. This calls for a method which should preferably enable quantitative and rapid evaluation of these techniques in the laboratory. A reliable, rapid and quantitative measurement method which was comprised of stripping the scale from heat exchanger pipes with a 10% acetic acid solution and measuring the Ca concentration in the acid after stripping, was developed. A total of 11 tests, 7 to test the reproducibility, 2 to test a physical water treatment device and 2 to test the effect of zinc, were conducted. The reproducibility amongst different pipes, and amongst different experiments could not be achieved. Attempts to explain the inconsistency through statistical analysis of the data showed that, the inconsistency in the results could partly attributed to chemical differences, particularly changes in calcium concentration and TDS. Metal contamination, particularly zinc could also be responsible for a part of the inconsistency.
34

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.
35

Numerical investigation of heat transfer in one-dimensional longitudinal fins

Rusagara, Innocent 07 May 2015 (has links)
A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, 2014. / In this thesis we will establish effective numerical schemes appropriate for the solution of a non-linear partial differential equation modelling heat transfer in one dimensional longitudinal fins. We will consider the problem as it stands without any physical simplification. The main methodology is based on balancing the non-linear source term as well as the application of numerical relaxation techniques. In either approach we will incorporate the no-flux condition for singular fins. By doing so, we obtain appropriate numerical schemes which improve results found in literature. To generalize, we will provide a relaxed numerical scheme that is applicable for both integer and fractional order non-linear heat transfer equations for one dimensional longitudinal fins.
36

The determination of parameters concerned with heat flow into underground excavations

Gould, Michael John 22 January 2015 (has links)
No description available.
37

Analysis of heat transfer in a hot body with non-constant internal heat generation and thermal conductivity

Lourenco, Marcio Alexandre 19 September 2016 (has links)
A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of requirements for the degree of Master of Science. May 27, 2016. / Heat transfer in a wall with temperature dependent thermal conductivity and internal heat generation is considered. We rst focus on the steady state models followed by the transient heat transfer models. It turns out that the models considered are non-linear. We deliberately omit the group-classi cation of the arbitrary functions appearing in the models, but rather select forms of physical importance. In one case, thermal conductivity and internal heat generation are both given by the exponential function and in the other case they are given by the power law. We employ the classical Lie point symmetry analysis to determine the exact solutions, while also determining the optimal system for each case. The exact solutions for the transient models are di cult to construct. However, we rst use the obtained exact solution for the steady state case as a benchmark for the 1D Di erential Transform Method (DTM). Since con dence in DTM is established, we construct steady state approximate series solutions. We apply the 2D DTM to the transient problem. Lastly we determine the conservation laws using the direct method and the associated Lie point symmetries for the transient problem / MT2016
38

Boiling heat transfer phenomena during rapid decompression

Kung, Shin-Ping January 2011 (has links)
Typescript. / Digitized by Kansas Correctional Industries
39

Surveys on harmonic map heat flows.

January 1996 (has links)
by Wu Fung Leung. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1996. / Includes bibliographical references (leaves 92-95). / Acknowledgements --- p.i / Notations --- p.ii / Introduction --- p.1 / Chapter 1 --- Preliminaries --- p.8 / Chapter 1.1 --- Formulations of Harmonic Maps --- p.8 / Chapter 1.2 --- Function Spaces --- p.11 / Chapter 1.3 --- Penalized Equations --- p.13 / Chapter 2 --- Main Lemmas --- p.15 / Chapter 2.1 --- Short Time Existence --- p.16 / Chapter 2.2 --- Energy Inequalities --- p.18 / Chapter 2.3 --- The Monotonicity Inequalities --- p.23 / Chapter 2.4 --- e - Regularity Theorem --- p.30 / Chapter 3 --- The Compact Case --- p.39 / Chapter 3.1 --- Existence and Regularity for dim M = 2 --- p.39 / Chapter 3.2 --- Existence and Regularity for dim M ≥ 2 --- p.49 / Chapter 3.3 --- Blow-up Results --- p.61 / Chapter 3.4 --- Existence of Harmonic maps --- p.69 / Chapter 4 --- The Noncompact Case --- p.74 / Chapter 4.1 --- Heat-flows from Rm --- p.75 / Chapter 4.2 --- Basic Lemmas --- p.77 / Chapter 4.3 --- Nonpositive Curvature Target Manifolds --- p.83 / Chapter 4.4 --- Dirichlet Problem at Infinity --- p.88 / Bibliography --- p.92
40

Transient heat transfer

Roth, Eric 01 January 1991 (has links)
With the advent of the new high Tc superconductors, liquid nitrogen will be one of the preferred cryogens used to cool these materials. Consequently, a more thorough understanding of the heat transfer characteristics of liquid nitrogen is required. In our investigations we examine the transient heating characteristics of liquid nitrogen to states of nucleate and film boiling under different liquid flow conditions. Using a platinum hot wire technique, it is verified that there is a premature transition to film boiling in the transient case at power levels significantly lower than under steady state nucleate boiling conditions. It is also shown that the premature transition can be reduced or eliminated depending on the flow velocity.

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