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:
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
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Condensation heat transfer and pressure drop coefficients of R22/R142b in a water cooled helicaly coiled tube-in-tube heat exchanger.
- Authors: Kebonte, Shiko A.
- Date: 2012-08-20
- Subjects: Heat exchangers - Fluid dynamics , Heat - Transmission , Refrigerants , Condensation
- Type: Thesis
- Identifier: uj:2741 , http://hdl.handle.net/10210/6181
- Description: M.Ing. , Heat transfer and pressure drop characteristics during in-tube condensation of nonazeotropic mixtures of R22/R142b in a smooth helically coiled copper tube with an inside diameter of 8.11 mm are investigated. The experimental results are compared with prediction from correlation. The coefficient of performance of.the heat pump built and used for experiments has been studied. The mass flux of the refrigerant was varied during the course of the experiments. At similar mass flow rate of fluids, the average heat transfer coefficients for mixtures were lower than those for pure refrigerant R22 used as reference for comparison. Also, the heat transfer coefficients of all the refrigerants increased with increasing mass flux.
- Full Text:
- Authors: Kebonte, Shiko A.
- Date: 2012-08-20
- Subjects: Heat exchangers - Fluid dynamics , Heat - Transmission , Refrigerants , Condensation
- Type: Thesis
- Identifier: uj:2741 , http://hdl.handle.net/10210/6181
- Description: M.Ing. , Heat transfer and pressure drop characteristics during in-tube condensation of nonazeotropic mixtures of R22/R142b in a smooth helically coiled copper tube with an inside diameter of 8.11 mm are investigated. The experimental results are compared with prediction from correlation. The coefficient of performance of.the heat pump built and used for experiments has been studied. The mass flux of the refrigerant was varied during the course of the experiments. At similar mass flow rate of fluids, the average heat transfer coefficients for mixtures were lower than those for pure refrigerant R22 used as reference for comparison. Also, the heat transfer coefficients of all the refrigerants increased with increasing mass flux.
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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:
A critical discussion of volatile organic compounds recovery techniques
- Authors: Muzenda, Edison
- Date: 2013
- Subjects: Abatement , Absorption , Condensation , Membrane separation , Volatile organic compounds - Recovery
- Type: Article
- Identifier: uj:4879 , ISSN 2277 – 4394 , http://hdl.handle.net/10210/12592
- Description: This paper discusses 4 volatile organic compounds recovery technologies. The study focusses on the principle of operation as well as the strengths and weaknesses of each abatement technique. The four techniques discussed are absorption, adsorption, condensation and membrane separation.
- Full Text:
- Authors: Muzenda, Edison
- Date: 2013
- Subjects: Abatement , Absorption , Condensation , Membrane separation , Volatile organic compounds - Recovery
- Type: Article
- Identifier: uj:4879 , ISSN 2277 – 4394 , http://hdl.handle.net/10210/12592
- Description: This paper discusses 4 volatile organic compounds recovery technologies. The study focusses on the principle of operation as well as the strengths and weaknesses of each abatement technique. The four techniques discussed are absorption, adsorption, condensation and membrane separation.
- Full Text:
Pressure drop during condensation inside smooth, helical micro-fin, and herringbone micro-fin tubest
- Authors: Olivier, Jonathan Albert
- Date: 2012-08-08
- Subjects: Heat -Transmission , Heat pumps , Condensation , Refrigerants
- Type: Thesis
- Identifier: http://ujcontent.uj.ac.za8080/10210/370199 , uj:8937 , http://hdl.handle.net/10210/5410
- Description: M.Ing. , Since the promulgation of the Montreal Protocol many refrigerants needed to be phased out. R-22, which is a widely used refrigerant in refrigeration systems, was one of these. Many replacements have been found throughout the years but very few have the same refrigeration capacity without being penalised by an increase in pressure drop. R-407C is one of the refrigerants having the potential to replace R-22 as it has the same theoretical coefficient of performance and has a lower global warming potential. However, due to its zeotropic characteristics there is a degradation in heat transfer during evaporation and condensation attributed to mass transfer resistance. Thus, augmentation techniques are needed not only to increase the heat capacity, but also to achieve an increase without incurring an excessive pressure drop. One approach to cope with this problem is to make use of the recently developed herringbone micro-fin tubes. Unfortunately very little data exists for refrigerants undergoing condensation inside herringbone micro-fin tubes. There is also little pressure drop information available for this type of tube. An experimental set-up was designed to determine the characteristics of this type of tube due to the scarcity of information. With the aid of current literature, various techniques were used to determine the pressure drops inside the herringbone micro-fin tube. One of these techniques was the use of the Kattan-Thome-Favrat flow regime map which helped to identify the flow patterns inside the tube. Knowledge of the type of flow occurring inside the tube helped to clarify the behaviour of the pressure drop relationships. The type of refrigerant being used also affected the behaviour of the pressure drop curves. A low-pressure refrigerant had a higher pressure drop due to the high vapour velocities achieved. Another cause for excessive pressure drop is the friction created by the high velocity vapour and condensate inside the tube. Many relationships for the friction factor exist and these are used to analyse the experimental data.The experimental facility comprised of a vapour compression loop and a water loop. The vapour compression loop consisted of a hermetically sealed compressor with a cooling capacity of 9.6 kW, a manually operated expansion valve and an evaporator. Three condensers were tested, namely a smooth tube, a helical micro-fin tube, and a herringbone micro-fin tube. The condensers were of the tube-in-tube type with the refrigerant flowing in the inner tube and the water in counter flow in the annulus. The hot water loop was used as a source for the evaporator and a cold loop as a heat sink for the condenser. Three refrigerants were tested, namely R-22, R-134a, and R-407C, all operating at a nominal saturation temperature of 40°C and at mass fluxes between 300 and 800 kg/m 2s. Accurate sensors and transducers were used to measure the temperatures, pressures, and mass flows at predefined points. Video cameras were attached to sight glasses to aid in the identification of the type of flow regime. Data were captured using a computerised data acquisition programme designed specifically for use with the experimental study. The experimental results showed that transition between the annular and intermittent flow regimes occurred at around 25% vapour quality for the herringbone micro-fin tube, as opposed to 30% for the helical micro-fin tube and 50% for the smooth tube. Pressure drops for the herringbone micro-fin tube were higher than those for the smooth tube but slightly lower than those for the helical micro-fin tube when using refrigerants R-22 and R-134a. The correlation of Liebenberg was modified for the pressure drops inside the herringbone micro-fin tube and gave a mean deviation of 12%. The efficiency ratio for the herringbone tube using R-22 was 1.85 and 1.69 when compared with the helical micro-fin and smooth tube respectively. For R-134 the efficiency ratio was 2.02 and 2.13 when compared with the helical micro-fin and smooth tube respectively, while for R-407C it was 1.58 and 1.26 for the two respectively. It was also concluded that R-407C could be used as a replacement refrigerant for R-22when used with a herringbone micro-fin tube.
- Full Text:
Pressure drop during condensation inside smooth, helical micro-fin, and herringbone micro-fin tubest
- Authors: Olivier, Jonathan Albert
- Date: 2012-08-08
- Subjects: Heat -Transmission , Heat pumps , Condensation , Refrigerants
- Type: Thesis
- Identifier: http://ujcontent.uj.ac.za8080/10210/370199 , uj:8937 , http://hdl.handle.net/10210/5410
- Description: M.Ing. , Since the promulgation of the Montreal Protocol many refrigerants needed to be phased out. R-22, which is a widely used refrigerant in refrigeration systems, was one of these. Many replacements have been found throughout the years but very few have the same refrigeration capacity without being penalised by an increase in pressure drop. R-407C is one of the refrigerants having the potential to replace R-22 as it has the same theoretical coefficient of performance and has a lower global warming potential. However, due to its zeotropic characteristics there is a degradation in heat transfer during evaporation and condensation attributed to mass transfer resistance. Thus, augmentation techniques are needed not only to increase the heat capacity, but also to achieve an increase without incurring an excessive pressure drop. One approach to cope with this problem is to make use of the recently developed herringbone micro-fin tubes. Unfortunately very little data exists for refrigerants undergoing condensation inside herringbone micro-fin tubes. There is also little pressure drop information available for this type of tube. An experimental set-up was designed to determine the characteristics of this type of tube due to the scarcity of information. With the aid of current literature, various techniques were used to determine the pressure drops inside the herringbone micro-fin tube. One of these techniques was the use of the Kattan-Thome-Favrat flow regime map which helped to identify the flow patterns inside the tube. Knowledge of the type of flow occurring inside the tube helped to clarify the behaviour of the pressure drop relationships. The type of refrigerant being used also affected the behaviour of the pressure drop curves. A low-pressure refrigerant had a higher pressure drop due to the high vapour velocities achieved. Another cause for excessive pressure drop is the friction created by the high velocity vapour and condensate inside the tube. Many relationships for the friction factor exist and these are used to analyse the experimental data.The experimental facility comprised of a vapour compression loop and a water loop. The vapour compression loop consisted of a hermetically sealed compressor with a cooling capacity of 9.6 kW, a manually operated expansion valve and an evaporator. Three condensers were tested, namely a smooth tube, a helical micro-fin tube, and a herringbone micro-fin tube. The condensers were of the tube-in-tube type with the refrigerant flowing in the inner tube and the water in counter flow in the annulus. The hot water loop was used as a source for the evaporator and a cold loop as a heat sink for the condenser. Three refrigerants were tested, namely R-22, R-134a, and R-407C, all operating at a nominal saturation temperature of 40°C and at mass fluxes between 300 and 800 kg/m 2s. Accurate sensors and transducers were used to measure the temperatures, pressures, and mass flows at predefined points. Video cameras were attached to sight glasses to aid in the identification of the type of flow regime. Data were captured using a computerised data acquisition programme designed specifically for use with the experimental study. The experimental results showed that transition between the annular and intermittent flow regimes occurred at around 25% vapour quality for the herringbone micro-fin tube, as opposed to 30% for the helical micro-fin tube and 50% for the smooth tube. Pressure drops for the herringbone micro-fin tube were higher than those for the smooth tube but slightly lower than those for the helical micro-fin tube when using refrigerants R-22 and R-134a. The correlation of Liebenberg was modified for the pressure drops inside the herringbone micro-fin tube and gave a mean deviation of 12%. The efficiency ratio for the herringbone tube using R-22 was 1.85 and 1.69 when compared with the helical micro-fin and smooth tube respectively. For R-134 the efficiency ratio was 2.02 and 2.13 when compared with the helical micro-fin and smooth tube respectively, while for R-407C it was 1.58 and 1.26 for the two respectively. It was also concluded that R-407C could be used as a replacement refrigerant for R-22when used with a herringbone micro-fin tube.
- 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.
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