Heat transfer performance during condensation inside spiralled micro-fin tubes
- Authors: Bukasa, Jean-Pierre Muenja
- Date: 2011-11-21
- Subjects: Heat transmission , Heat exchangers , Condensation
- Type: Thesis
- Identifier: uj:1713 , http://hdl.handle.net/10210/4056
- Description: D.Ing. , Many studies have been conducted in order to establish the respective influence of geometric parameters such as fins number, fin shape (apex angle), spiral angle, fin height, fin pitch etc. on the condensation heat transfer performance of the spiralled micro-fin tubes. However, the effect of the spiral angle could not be clearly established in those investigations, because other geometric parameters affecting the heat transfer performance such as fin height, fin thickness, apex angle were also varied. The influence of the spiral angle on the heat transfer performance during condensation inside spiralled micro-fin tubes having all other geometric parameters the same was experimentally investigated in this study. A new experimental-based predictive correlation was developed for practical design of this specific class of micro-fin tubes. Tests were conducted for condensation of R22, R134a and R407c inside a smooth and three micro-fin tubes having spiral angles of 10, 18 and 37 degrees. Experimental results indicated a heat transfer augmentation due to heat transfer area increase. As the spiral angle was increased, the heat transfer area increased causing a substantial heat transfer augmentation. Condensation inside the 10° spiralled micro-fins produced a heat transfer augmentation of about 170% for a heat transfer area increase of 1.87 when compared to condensation in ~he correspondent smooth tube while the 18° spiralled " micro-fins produced an augmentation of 180% for a heat transfer area increase of 1.94. The 37° spiralled micro-fins produced the highest enhancement of 220% for a heat transfer area increase of 2.13. Additional heat transfer augmentation was produced by: (a) the turbulence in the condensate film due to the presence of spiralled micro-fins (stronger effect at lower mass velocities and vapor qualities) and (b) the effect of surface tension forces (at higher vapor qualities). The proposed new correlation predicted the majority of experimental results of the present study within a deviation zone of ± 20 percent.
- Full Text:
- Authors: Bukasa, Jean-Pierre Muenja
- Date: 2011-11-21
- Subjects: Heat transmission , Heat exchangers , Condensation
- Type: Thesis
- Identifier: uj:1713 , http://hdl.handle.net/10210/4056
- Description: D.Ing. , Many studies have been conducted in order to establish the respective influence of geometric parameters such as fins number, fin shape (apex angle), spiral angle, fin height, fin pitch etc. on the condensation heat transfer performance of the spiralled micro-fin tubes. However, the effect of the spiral angle could not be clearly established in those investigations, because other geometric parameters affecting the heat transfer performance such as fin height, fin thickness, apex angle were also varied. The influence of the spiral angle on the heat transfer performance during condensation inside spiralled micro-fin tubes having all other geometric parameters the same was experimentally investigated in this study. A new experimental-based predictive correlation was developed for practical design of this specific class of micro-fin tubes. Tests were conducted for condensation of R22, R134a and R407c inside a smooth and three micro-fin tubes having spiral angles of 10, 18 and 37 degrees. Experimental results indicated a heat transfer augmentation due to heat transfer area increase. As the spiral angle was increased, the heat transfer area increased causing a substantial heat transfer augmentation. Condensation inside the 10° spiralled micro-fins produced a heat transfer augmentation of about 170% for a heat transfer area increase of 1.87 when compared to condensation in ~he correspondent smooth tube while the 18° spiralled " micro-fins produced an augmentation of 180% for a heat transfer area increase of 1.94. The 37° spiralled micro-fins produced the highest enhancement of 220% for a heat transfer area increase of 2.13. Additional heat transfer augmentation was produced by: (a) the turbulence in the condensate film due to the presence of spiralled micro-fins (stronger effect at lower mass velocities and vapor qualities) and (b) the effect of surface tension forces (at higher vapor qualities). The proposed new correlation predicted the majority of experimental results of the present study within a deviation zone of ± 20 percent.
- Full Text:
Measurement of scale formation in an experimental heat exchanger circuit
- Authors: Canbulat, Semsa Gulsum
- Date: 2012-09-05
- Subjects: Heat transmission , Incrustations , Descaling
- Type: Thesis
- Identifier: uj:9577 , http://hdl.handle.net/10210/7000
- Description: 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.
- Full Text:
- Authors: Canbulat, Semsa Gulsum
- Date: 2012-09-05
- Subjects: Heat transmission , Incrustations , Descaling
- Type: Thesis
- Identifier: uj:9577 , http://hdl.handle.net/10210/7000
- Description: 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.
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Uncertainty analysis of heat exchangers
- Authors: Coblentz, Liora Chana
- Date: 2009-02-26T12:17:05Z
- Subjects: Heat exchangers , Heat transmission , Uncertainty analysis
- Type: Thesis
- Identifier: uj:8149 , http://hdl.handle.net/10210/2156
- Description: M.Ing. , Experiments are being conducted with regard to heat exchange systems. However, there are errors and uncertainties attached to each system. Journals, which publish articles concerning heat transfer experiments, require an estimate of this uncertainty. These uncertainties must be calculated in order to determine how valid a set of results is. The uncertainty describes to what level one may rely on a set of experimental results and conclusions. The uncertainty was calculated by the formulation of an uncertainty equation with the use of various statistical methods. Adjustments or modifications had to be made to the present uncertainty equations in order to calculate the uncertainty in heat transfer systems. Uncertainty based on a general uncertainty equation by Schultz and Cole (1979) enabled the derivation of the equations to calculate the necessary uncertainty factor for heat transfer systems. Implementation of the equations in various experimental set-ups was achieved. The uncertainty equations yielded results that seemed consistent with the subjective view of the experimenter. Therefore, the equations were considered valid.
- Full Text:
- Authors: Coblentz, Liora Chana
- Date: 2009-02-26T12:17:05Z
- Subjects: Heat exchangers , Heat transmission , Uncertainty analysis
- Type: Thesis
- Identifier: uj:8149 , http://hdl.handle.net/10210/2156
- Description: M.Ing. , Experiments are being conducted with regard to heat exchange systems. However, there are errors and uncertainties attached to each system. Journals, which publish articles concerning heat transfer experiments, require an estimate of this uncertainty. These uncertainties must be calculated in order to determine how valid a set of results is. The uncertainty describes to what level one may rely on a set of experimental results and conclusions. The uncertainty was calculated by the formulation of an uncertainty equation with the use of various statistical methods. Adjustments or modifications had to be made to the present uncertainty equations in order to calculate the uncertainty in heat transfer systems. Uncertainty based on a general uncertainty equation by Schultz and Cole (1979) enabled the derivation of the equations to calculate the necessary uncertainty factor for heat transfer systems. Implementation of the equations in various experimental set-ups was achieved. The uncertainty equations yielded results that seemed consistent with the subjective view of the experimenter. Therefore, the equations were considered valid.
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Heat transfer and pressure drop characteristics of angled spiralling tape inserts in a heat exchanger annulus
- Authors: Coetzee, Hennie
- Date: 2011-11-21
- Subjects: Heat exchangers , Heat transmission
- Type: Thesis
- Identifier: uj:1716 , http://hdl.handle.net/10210/4059
- Description: M.Ing. , There are different types of heat transfer enhancement techniques that can be used, but some of these techniques are expensive and cannot be afforded by small manufacturing firms. An easy and affordable technique considered in this paper has an angled spiralling tape inserted into the annulus of a tube-in-tube heat exchanger. The purpose of this paper was to determine the single phase heat transfer and pressure drop characteristics in the annulus. Experimental measurements were taken on four set-ups; a normal tube-in-tube heat exchanger used as a reference and three heat exchangers with different angled spiralling tape inserts. From the results correlations were developed that can be used to predict the heat transfer and pressure drop characteristics. It was concluded that the angled spiralling tape inserts resulted in an increase in the heat transfer and pressure drop characteristics as can be expected.
- Full Text:
- Authors: Coetzee, Hennie
- Date: 2011-11-21
- Subjects: Heat exchangers , Heat transmission
- Type: Thesis
- Identifier: uj:1716 , http://hdl.handle.net/10210/4059
- Description: M.Ing. , There are different types of heat transfer enhancement techniques that can be used, but some of these techniques are expensive and cannot be afforded by small manufacturing firms. An easy and affordable technique considered in this paper has an angled spiralling tape inserted into the annulus of a tube-in-tube heat exchanger. The purpose of this paper was to determine the single phase heat transfer and pressure drop characteristics in the annulus. Experimental measurements were taken on four set-ups; a normal tube-in-tube heat exchanger used as a reference and three heat exchangers with different angled spiralling tape inserts. From the results correlations were developed that can be used to predict the heat transfer and pressure drop characteristics. It was concluded that the angled spiralling tape inserts resulted in an increase in the heat transfer and pressure drop characteristics as can be expected.
- Full Text:
The development of an experimental heat transfer enhancement in tube-in-tube heat exchangers
- Authors: Coetzee, S.
- Date: 2011-11-21
- Subjects: Heat exchangers , Heat transmission
- Type: Thesis
- Identifier: uj:1702 , http://hdl.handle.net/10210/4046
- Description: M.Ing. , Heat transfer enhancement is currently a very popular field of research in heat transfer engineering. Before an investigation of this nature can be performed a suitable experimental set-up is needed to perform the experimental work. The object of this study was to develop such an experimental set-up to be used in determining the heat transfer and pressure drop characteristics of the refrigerant condensing in the inner tube of a tube-in-tube heat exchanger, with water flowing in the annulus. The experimental set-up was built and experiments were conducted with water flowing in the inner tube and annulus. From these readings heat transfer correlations were derived for the water flowing in the inner tube as well as the annulus by using the modified Wilson plot technique.
- Full Text:
- Authors: Coetzee, S.
- Date: 2011-11-21
- Subjects: Heat exchangers , Heat transmission
- Type: Thesis
- Identifier: uj:1702 , http://hdl.handle.net/10210/4046
- Description: M.Ing. , Heat transfer enhancement is currently a very popular field of research in heat transfer engineering. Before an investigation of this nature can be performed a suitable experimental set-up is needed to perform the experimental work. The object of this study was to develop such an experimental set-up to be used in determining the heat transfer and pressure drop characteristics of the refrigerant condensing in the inner tube of a tube-in-tube heat exchanger, with water flowing in the annulus. The experimental set-up was built and experiments were conducted with water flowing in the inner tube and annulus. From these readings heat transfer correlations were derived for the water flowing in the inner tube as well as the annulus by using the modified Wilson plot technique.
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Characteristics of a semicircular heat exchanger used in a water heated condenser pump
- Authors: Da Veiga, Willem Richter
- Date: 2009-02-26T12:17:42Z
- Subjects: Heat exchangers , Heat pumps , Heat transmission , Nusselt number
- Type: Thesis
- Identifier: uj:8151 , http://hdl.handle.net/10210/2158
- Description: D.Ing. , According to literature 6% of South Africa’s primary energy consumption could be saved if heat pumps were used to their full technical potential. Although there is world-wide interest in the use of heat pumps and considerable effort has been expended on heat-pump research, heat pumps are not commonly used in South Africa. The objective of this thesis is to determine the possibility of a combined evaporator or condenser with a normal pump. This will reduce cost and space of a normal heat pump and make heat pumps economically more competitive against resistance element geysers. In order to investigate this combination research is done on semicircular heat exchangers, since this is the primary geometry of the heating channels in the condenser pump. Analyses is done experimentally on a standard 28.58 mm hard drawn copper tube, cut trough the middle, with a 1.6 mm copper plate in between to obtain a semicircular heat exchanger. Turbulent flow is investigated with the flat side of the semicircular heat exchanger being horizontal or vertical, a spiralled and a s-shape semicircular heat exchanger. In each case the heat transfer coefficient is determined with the use of the Wilson plot technique. It is found that there is a significant increase in Nusselt number for semicircular heat exchangers above a normal tube-in-tube heat exchanger but the pressure loss coefficient increase with an equal amount.
- Full Text:
- Authors: Da Veiga, Willem Richter
- Date: 2009-02-26T12:17:42Z
- Subjects: Heat exchangers , Heat pumps , Heat transmission , Nusselt number
- Type: Thesis
- Identifier: uj:8151 , http://hdl.handle.net/10210/2158
- Description: D.Ing. , According to literature 6% of South Africa’s primary energy consumption could be saved if heat pumps were used to their full technical potential. Although there is world-wide interest in the use of heat pumps and considerable effort has been expended on heat-pump research, heat pumps are not commonly used in South Africa. The objective of this thesis is to determine the possibility of a combined evaporator or condenser with a normal pump. This will reduce cost and space of a normal heat pump and make heat pumps economically more competitive against resistance element geysers. In order to investigate this combination research is done on semicircular heat exchangers, since this is the primary geometry of the heating channels in the condenser pump. Analyses is done experimentally on a standard 28.58 mm hard drawn copper tube, cut trough the middle, with a 1.6 mm copper plate in between to obtain a semicircular heat exchanger. Turbulent flow is investigated with the flat side of the semicircular heat exchanger being horizontal or vertical, a spiralled and a s-shape semicircular heat exchanger. In each case the heat transfer coefficient is determined with the use of the Wilson plot technique. It is found that there is a significant increase in Nusselt number for semicircular heat exchangers above a normal tube-in-tube heat exchanger but the pressure loss coefficient increase with an equal amount.
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Heat-extraction from solid-state electronics by embedded solids with application to integrated power electronic
- Authors: Dirker, Jaco
- Date: 2008-11-19T05:28:30Z
- Subjects: Solid-state electronics , Power electronics , Heat conduction , Heat transmission
- Type: Thesis
- Identifier: uj:14733 , http://hdl.handle.net/10210/1739
- Description: D. Ing. , Power electronics refers to electronic processing of electrical power. In this process the electrical power is controlled by switching of power semiconductor devices as well as electromagnetically stored in electrical and magnetic fields while the energy flow is directed through selective conduction paths. The losses/power-efficiency of all these actions is of paramount importance in the processing. The lack for standardisation and the absence of a modular approach is a barrier to the development of more compact systems. Recently more research resources have been invested in development of integrated power electronic modules as an attempt to solve this problem. By integrating power electronic components, an increase in the power density is achieved, which unfortunately also leads to higher internal heat-generation and higher operating temperatures. This has an unfavourable effect on electronic behaviour and the reliability of the structures. In order to maintain the advances made in volume reduction of integrated power electronics, efficient and cost effective methods for removing heat is of essence. In this investigation the performance of rectangular cross-section embedded solid-state heatextraction inserts to increase thermal heat spreading and the reduction of steady-state peak temperatures was evaluated theoretically and experimentally. Theoretically, the cross sectional aspect ratio of such inserts was thermally optimised for a wide range of dimensional, thermal, and material property conditions. Possible materials investigated for use as heat extractors in power electronics include aluminium nitride, beryllium oxide, and synthetic diamond. The presence of interfacial thermal resistance was theoretically found to have a significant detrimental influence on the thermal performance of an integrated heat-extraction system and should be minimised as far as possible. For conditions commonly found in integrated power passives, continuous embedded heatextraction layers are proposed. Theoretically it is shown that such inserts can aid in the increase of power density by reducing the temperature increase per unit volume of heat-generation. Experimental test results corresponded closely with the theoretically expected allowed increase in heat-generation that could be accommodated due to the heat-extraction action of the inserts. As an experimental system, insertion of aluminium nitride into ferrite in an integrated electromagnetic power passive module was investigated. An increase of 187% in the effective power density could be achieved due to the presence of aluminium nitride heat-extraction layers embedded into ferrite. Preliminary magnetic flux density optimisation, in terms of the volume fraction occupied by a parallel-layered heat-extraction system, was performed for a wide range of heat-extraction materials, and interfacial resistance values.
- Full Text:
- Authors: Dirker, Jaco
- Date: 2008-11-19T05:28:30Z
- Subjects: Solid-state electronics , Power electronics , Heat conduction , Heat transmission
- Type: Thesis
- Identifier: uj:14733 , http://hdl.handle.net/10210/1739
- Description: D. Ing. , Power electronics refers to electronic processing of electrical power. In this process the electrical power is controlled by switching of power semiconductor devices as well as electromagnetically stored in electrical and magnetic fields while the energy flow is directed through selective conduction paths. The losses/power-efficiency of all these actions is of paramount importance in the processing. The lack for standardisation and the absence of a modular approach is a barrier to the development of more compact systems. Recently more research resources have been invested in development of integrated power electronic modules as an attempt to solve this problem. By integrating power electronic components, an increase in the power density is achieved, which unfortunately also leads to higher internal heat-generation and higher operating temperatures. This has an unfavourable effect on electronic behaviour and the reliability of the structures. In order to maintain the advances made in volume reduction of integrated power electronics, efficient and cost effective methods for removing heat is of essence. In this investigation the performance of rectangular cross-section embedded solid-state heatextraction inserts to increase thermal heat spreading and the reduction of steady-state peak temperatures was evaluated theoretically and experimentally. Theoretically, the cross sectional aspect ratio of such inserts was thermally optimised for a wide range of dimensional, thermal, and material property conditions. Possible materials investigated for use as heat extractors in power electronics include aluminium nitride, beryllium oxide, and synthetic diamond. The presence of interfacial thermal resistance was theoretically found to have a significant detrimental influence on the thermal performance of an integrated heat-extraction system and should be minimised as far as possible. For conditions commonly found in integrated power passives, continuous embedded heatextraction layers are proposed. Theoretically it is shown that such inserts can aid in the increase of power density by reducing the temperature increase per unit volume of heat-generation. Experimental test results corresponded closely with the theoretically expected allowed increase in heat-generation that could be accommodated due to the heat-extraction action of the inserts. As an experimental system, insertion of aluminium nitride into ferrite in an integrated electromagnetic power passive module was investigated. An increase of 187% in the effective power density could be achieved due to the presence of aluminium nitride heat-extraction layers embedded into ferrite. Preliminary magnetic flux density optimisation, in terms of the volume fraction occupied by a parallel-layered heat-extraction system, was performed for a wide range of heat-extraction materials, and interfacial resistance values.
- Full Text:
Heat transfer enhancement during condensation in smooth tubes with helical wire inserts
- Authors: Ji, Tianfu
- Date: 2008-07-17T10:44:00Z
- Subjects: Heat transmission , Condensation
- Type: Thesis
- Identifier: uj:7311 , http://hdl.handle.net/10210/804
- Description: D.Ing. (Mechanical Engineering) , In the past two decades the refrigeration, air-conditioning and heat pump industries began the conversion from chlorofluorocarbon (CFC) refrigerants to hydrochlorofluorocarbons (HCFCs) and to natural refrigerants. This changeover not only involves redesigning, re-optimizing and re-testing all new original equipment but also involves retrofitting many large existing systems. Combining this process with the goal of developing more accurate design methods and more energy-efficient cycles, heat transfer and, specifically, heat transfer enhancement, has become a very active research field and will probably continue to boom in the next decades as the HCFCs are also phased out of use. The most prominent alternative refrigerants are R134a and R407C to replace the present market dominating refrigerant R22. Many heat transfer enhanced techniques have simultaneously been developed for the improvement of energy consumption, material saving, size reduction and pumping power reduction. Helical wire inserts in tubes are a typical technique that offers a higher heat transfer increase and, at the same time, only a mild pressure drop penalty. This study investigates the heat transfer characteristics of a horizontal tube-in-tube heat exchanger with a helical wire inserted in the inner tube. The influence of the pitch (or helix angle) of such geometry on the heat transfer performance and pressure drop during condensation (having all other geometric parameters the same) was investigated experimentally. Firstly, three refrigerants were tested in three helical wire-inserted tubes with different pitches of 5, 7.77 and 11 mm. The local and average heat transfer coefficients, and semi-local and average pressure drops were studied systematically. The experimental results were compared not only with the referenced experimental data of the smooth tubes, but also with the results of micro-fin tubes. The heat transfer enhancement factors, pressure drop loss penalty factors and overall efficiencies of the tested condensers with helical wire-inserted geometry were calculated. The tube with a helical wire pitch of 5 mm inserts was found to have the highest enhancement factor and overall efficiency. Secondly, the heat transfer enhancement mechanism was studied and explained. It was found that the extension of the annular flow regime contributed mainly to this enhancement. The transitional qualities from annular flow to intermittent flow were derived and incorporated in a flow regime map. Thirdly, heat transfer coefficient and pressure drop correlations for this special heat transfer enhancement geometry were deduced, and they predicted the experiment data to within 80% and 78% respectively, within a deviation of 20%. Finally, the water flowing through helical wire-inserted tubes (glass) was demonstrated, providing a visual understanding of the heat transfer enhancement mechanism. , Prof. J.P. Meyer Prof. L. Liebenberg
- Full Text:
- Authors: Ji, Tianfu
- Date: 2008-07-17T10:44:00Z
- Subjects: Heat transmission , Condensation
- Type: Thesis
- Identifier: uj:7311 , http://hdl.handle.net/10210/804
- Description: D.Ing. (Mechanical Engineering) , In the past two decades the refrigeration, air-conditioning and heat pump industries began the conversion from chlorofluorocarbon (CFC) refrigerants to hydrochlorofluorocarbons (HCFCs) and to natural refrigerants. This changeover not only involves redesigning, re-optimizing and re-testing all new original equipment but also involves retrofitting many large existing systems. Combining this process with the goal of developing more accurate design methods and more energy-efficient cycles, heat transfer and, specifically, heat transfer enhancement, has become a very active research field and will probably continue to boom in the next decades as the HCFCs are also phased out of use. The most prominent alternative refrigerants are R134a and R407C to replace the present market dominating refrigerant R22. Many heat transfer enhanced techniques have simultaneously been developed for the improvement of energy consumption, material saving, size reduction and pumping power reduction. Helical wire inserts in tubes are a typical technique that offers a higher heat transfer increase and, at the same time, only a mild pressure drop penalty. This study investigates the heat transfer characteristics of a horizontal tube-in-tube heat exchanger with a helical wire inserted in the inner tube. The influence of the pitch (or helix angle) of such geometry on the heat transfer performance and pressure drop during condensation (having all other geometric parameters the same) was investigated experimentally. Firstly, three refrigerants were tested in three helical wire-inserted tubes with different pitches of 5, 7.77 and 11 mm. The local and average heat transfer coefficients, and semi-local and average pressure drops were studied systematically. The experimental results were compared not only with the referenced experimental data of the smooth tubes, but also with the results of micro-fin tubes. The heat transfer enhancement factors, pressure drop loss penalty factors and overall efficiencies of the tested condensers with helical wire-inserted geometry were calculated. The tube with a helical wire pitch of 5 mm inserts was found to have the highest enhancement factor and overall efficiency. Secondly, the heat transfer enhancement mechanism was studied and explained. It was found that the extension of the annular flow regime contributed mainly to this enhancement. The transitional qualities from annular flow to intermittent flow were derived and incorporated in a flow regime map. Thirdly, heat transfer coefficient and pressure drop correlations for this special heat transfer enhancement geometry were deduced, and they predicted the experiment data to within 80% and 78% respectively, within a deviation of 20%. Finally, the water flowing through helical wire-inserted tubes (glass) was demonstrated, providing a visual understanding of the heat transfer enhancement mechanism. , Prof. J.P. Meyer Prof. L. Liebenberg
- Full Text:
Heat transfer performance during in-tube condensation in horizontal smooth, micro-fin and herringbone tubes
- Authors: Lambrechts, Adriaan
- Date: 2008-11-27T07:26:46Z
- Subjects: Heat transmission , Condensation , Refrigerants
- Type: Thesis
- Identifier: uj:14738 , http://hdl.handle.net/10210/1753
- Description: 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.
- Full Text:
- Authors: Lambrechts, Adriaan
- Date: 2008-11-27T07:26:46Z
- Subjects: Heat transmission , Condensation , Refrigerants
- Type: Thesis
- Identifier: uj:14738 , http://hdl.handle.net/10210/1753
- Description: 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.
- Full Text:
A unified prediction method for smooth and micro-fin tube condensation performance
- Authors: Liebenberg, Leon
- Date: 2009-01-22T05:36:54Z
- Subjects: Heat transmission , Heat exchangers , Condensation , Refrigerants
- Type: Thesis
- Identifier: uj:14813 , http://hdl.handle.net/10210/1939
- Description: D.Ing.
- Full Text:
- Authors: Liebenberg, Leon
- Date: 2009-01-22T05:36:54Z
- Subjects: Heat transmission , Heat exchangers , Condensation , Refrigerants
- Type: Thesis
- Identifier: uj:14813 , http://hdl.handle.net/10210/1939
- Description: D.Ing.
- Full Text:
Flow patterns during refrigerant condensation in smooth and enhanced tubes
- Authors: Owaga, Denis
- Date: 2009-01-20T07:05:17Z
- Subjects: Refrigerants , Condensation , Heat transmission , Heat exchangers
- Type: Thesis
- Identifier: uj:14806 , http://hdl.handle.net/10210/1931
- Description: M.Ing. , The Montreal Protocol led to the phasing-out of ozone layer depleting refrigerants and replacing them with more environmentally friendly refrigerants, which in many cases caused heat transfer degradation in heat exchanger equipment. To make up for the heat transfer degradation, there was a need for the application of heat transfer enhancement techniques. One such technique is the use of micro-fin tubes as opposed to traditional smooth tubes. The purpose of this study is to develop a flow regime map for the condensation of R-22, R-407C and R-134a in a herringbone micro-fin tube. It was perceived that with the knowledge of flow patterns inside the tube and especially the annular-to-intermittent transition, it is possible to perform improved analyses of the heat transfer and pressure drop characteristics. Experimental and analytical work was performed to investigate the flow regimes during condensation of the refrigerants in smooth, helical micro-fin and herringbone micro-fin tubes at an average saturation temperature of 40oC, with mass fluxes ranging from 300 to 800 kg/m2s. Condensation occurred in tube-in-tube type condensers with cooling water flowing in the annulus and the refrigerant in the inner tubes. The condensers consisted of eight sub-sections to allow for the acquisition of sectional heat transfer and pressure data. Various criteria were considered in order to generate flow regime maps. The Thome flow regime transition criterion was used and complemented with visually-observed and photographic imaging, as well as the objective power spectral density distributions of the pressure signals of the condensing refrigerants. The observed flow regimes were mainly annular flow and intermittent flow. Stratified-wavy flow was observed at low mass fluxes and low vapour qualities. There were notable similarities in the flow pattern between the smooth and micro-fin tubes. However, the experimental results show that the transition from annular to intermittent flow regimes occurred at average vapour quality values of 0.26, 0.29 and 0.48 for the herringbone micro-fin, the helical micro-fin and smooth tubes respectively. The combined analyses assisted in adapting the helical micro-fin tube condensing flow pattern map, to ensure its application in accurately predicting herringbone micro-fin tube condensation. The new transition criterion effectively predicts the delay in transition from annular to intermittent flow for all three refrigerants, condensing in the herringbone micro-fin tube.
- Full Text:
- Authors: Owaga, Denis
- Date: 2009-01-20T07:05:17Z
- Subjects: Refrigerants , Condensation , Heat transmission , Heat exchangers
- Type: Thesis
- Identifier: uj:14806 , http://hdl.handle.net/10210/1931
- Description: M.Ing. , The Montreal Protocol led to the phasing-out of ozone layer depleting refrigerants and replacing them with more environmentally friendly refrigerants, which in many cases caused heat transfer degradation in heat exchanger equipment. To make up for the heat transfer degradation, there was a need for the application of heat transfer enhancement techniques. One such technique is the use of micro-fin tubes as opposed to traditional smooth tubes. The purpose of this study is to develop a flow regime map for the condensation of R-22, R-407C and R-134a in a herringbone micro-fin tube. It was perceived that with the knowledge of flow patterns inside the tube and especially the annular-to-intermittent transition, it is possible to perform improved analyses of the heat transfer and pressure drop characteristics. Experimental and analytical work was performed to investigate the flow regimes during condensation of the refrigerants in smooth, helical micro-fin and herringbone micro-fin tubes at an average saturation temperature of 40oC, with mass fluxes ranging from 300 to 800 kg/m2s. Condensation occurred in tube-in-tube type condensers with cooling water flowing in the annulus and the refrigerant in the inner tubes. The condensers consisted of eight sub-sections to allow for the acquisition of sectional heat transfer and pressure data. Various criteria were considered in order to generate flow regime maps. The Thome flow regime transition criterion was used and complemented with visually-observed and photographic imaging, as well as the objective power spectral density distributions of the pressure signals of the condensing refrigerants. The observed flow regimes were mainly annular flow and intermittent flow. Stratified-wavy flow was observed at low mass fluxes and low vapour qualities. There were notable similarities in the flow pattern between the smooth and micro-fin tubes. However, the experimental results show that the transition from annular to intermittent flow regimes occurred at average vapour quality values of 0.26, 0.29 and 0.48 for the herringbone micro-fin, the helical micro-fin and smooth tubes respectively. The combined analyses assisted in adapting the helical micro-fin tube condensing flow pattern map, to ensure its application in accurately predicting herringbone micro-fin tube condensation. The new transition criterion effectively predicts the delay in transition from annular to intermittent flow for all three refrigerants, condensing in the herringbone micro-fin tube.
- Full Text:
Condensing coefficients of the refrigerant mixture R-22/R-142b in smooth tubes and during enhanced heat transfer configurations
- Authors: Smit, Floris Jakobus
- Date: 2009-01-22T05:36:38Z
- Subjects: Refrigerants , Condensation , Heat transmission
- Type: Thesis
- Identifier: uj:14812 , http://hdl.handle.net/10210/1938
- Description: 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.
- Full Text:
- Authors: Smit, Floris Jakobus
- Date: 2009-01-22T05:36:38Z
- Subjects: Refrigerants , Condensation , Heat transmission
- Type: Thesis
- Identifier: uj:14812 , http://hdl.handle.net/10210/1938
- Description: 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.
- Full Text:
Heat transfer characteristics of a fractal heat exchanger
- Authors: Van der Vyver, Hilde
- Date: 2009-01-22T05:37:25Z
- Subjects: Heat exchangers , Heat transmission , Nusselt number , Fluid dynamics , Fractals
- Type: Thesis
- Identifier: uj:14816 , http://hdl.handle.net/10210/1941
- Description: D.Ing.
- Full Text:
- Authors: Van der Vyver, Hilde
- Date: 2009-01-22T05:37:25Z
- Subjects: Heat exchangers , Heat transmission , Nusselt number , Fluid dynamics , Fractals
- Type: Thesis
- Identifier: uj:14816 , http://hdl.handle.net/10210/1941
- Description: D.Ing.
- Full Text:
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