The thermal transmission coefficient for a micro-ribbed tube has been determined using theoretical relationships and the outcomes of such calculations have been compared with experiments conducted using a R1234yf refr...The thermal transmission coefficient for a micro-ribbed tube has been determined using theoretical relationships and the outcomes of such calculations have been compared with experiments conducted using a R1234yf refrigerant undergoing condensation.In particular four theoretical single-phase flow and three multi-phase flow models have been used in this regard.The experimental results show that:the Oliver et al.criterion equation overestimates the experimental results as its accuracy is significantly affected by the specific conditions realized inside micro-fin tubes;the Miyara et al.criterion equation prediction error is less than 15%;the Cavallini et al.approach gives the highest prediction accuracy;the Goto et al.model overestimates the test data.Such results are critically discussed and some indications for the improvement of such models are provided.展开更多
The article analyzes a shell and tube type condenser’s thermal performance using concepts of efficiency and effectiveness. Freon 134a is used as a coolant flowing through the shell. Water or water-based aluminum oxid...The article analyzes a shell and tube type condenser’s thermal performance using concepts of efficiency and effectiveness. Freon 134a is used as a coolant flowing through the shell. Water or water-based aluminum oxide nanoparticles are at relatively low saturation pressure in the tube. The condenser consists of 36 tubes divided into three central regions for analysis: superheated steam, saturated steam, and subcooled liquid. The three regions contain four tubes with three steps each, that is, 12 tubes. Region I, superheated steam, includes three horizontal baffles. Profiles of temperature, efficiency, and effectiveness are presented graphically for the three regions, with fixed refrigerant flow equal to 0.20 kg/s and fluid flow rate in the tube ranging from 0.05 kg/s to 0.40 kg/s. The experimental result for vapor pressure equal to 1.2 MPa and water flow equal to 0.41 kg/s was used as one of the references for the model’s physical compatibility.展开更多
The work’s objective is to analyze the influence of the saturation temperature of the R134a refrigerant on the thermal performance of a shell and tube type condenser, with water and aluminum oxide (Al<sub>2<...The work’s objective is to analyze the influence of the saturation temperature of the R134a refrigerant on the thermal performance of a shell and tube type condenser, with water and aluminum oxide (Al<sub>2</sub>O<sub>3</sub>) nanoparticles flowing into the tube. For analysis, the heat exchanger is subdivided into three regions: subcooled liquid, saturated steam, and superheated steam. The shell and tube heat exchanger assumed as the basis for the study has 36 tubes, with rows of 4 tubes in line and three passes into the tube in each region. The parameters used to analyze the performance are efficiency and effectiveness, through variations of quantities such as saturation temperature, the nanofluid’s mass flow rate, fraction in the nanoparticles’ volume, and the number of passes in the tube in each region of the heat exchanger. The obtained results demonstrate that the efficiency is relatively high in all the analyzed situations. In each saturation temperature, the effectiveness can be increased by introducing fractions of nanoparticles in the water or increasing the number of passes in the tube.展开更多
In loss-of-coolant accidents,a passive containment heat removal system protects the integrity of the containment by condensing steam.As a large amount of air exists in the containment,the steam condensation heat trans...In loss-of-coolant accidents,a passive containment heat removal system protects the integrity of the containment by condensing steam.As a large amount of air exists in the containment,the steam condensation heat transfer can be significantly reduced.Based on previous research,traditional methods for enhancing pure steam condensation may not be applicable to steam–air condensation.In the present study,new methods of enhancing condensation heat transfer were adopted and several potentially enhanced heat transfer tubes,including corrugated tubes,spiral fin tubes,and ring fin tubes were designed.STAR-CCM+was used to determine the effect of enhanced heat transfer tubes on the steam condensation heat transfer.According to the calculations,the gas pressure ranged from 0.2 to 1.6 MPa,and air mass fraction ranged from 0.1 to 0.9.The effective perturbation of the high-concentration air layer was identified as the key factor for enhancing steam–air condensation heat transfer.Further,the designed corrugated tube performed well at atmospheric pressure,with a maximum enhancement of 27.4%,and performed poorly at high pressures.In the design of spiral fin tubes,special attention should be paid to the locations that may accumulate high-concentration air.Nonetheless,the ring-fin tubes generally displayed good performance under all conditions of interest,with a maximum enhancement of 24.2%.展开更多
Numerical simulation on R245fa condensation inside an inner diameter of 8 mm horizontal tube is researched in this paper. The effect of variation in velocity, condensation temperature and superheat of inlet steam and ...Numerical simulation on R245fa condensation inside an inner diameter of 8 mm horizontal tube is researched in this paper. The effect of variation in velocity, condensation temperature and superheat of inlet steam and variation in cooling water temperature on heat transfer coefficient are investigated as a parametric study. Condensation process of steam has been successfully modeled by applying a user defined function (UDF) added to the commercial computational fluid dynamics (CFD) package. By analyzing the corresponding condensate contours and the curves of local heat transfer coefficient, the relationships between condensation heat transfer coefficient and various parameters of R245fa inside horizontal tube are obtained. It shows that the heat transfer coefficient increases by the increase in velocity, condensation temperature and superheat of inlet steam and the decrease in cooling water temperature. The errors between the heat transfer coefficient of simulation result and model of Wang and Shah are within ±30%. The parametric study will provide the basis for designing efficient heat exchangers of R245fa.展开更多
<span style="font-family:Verdana;">Accord</span><span style="font-family:Verdana;">ing</span><span style="font-family:Verdana;"> to </span><span sty...<span style="font-family:Verdana;">Accord</span><span style="font-family:Verdana;">ing</span><span style="font-family:Verdana;"> to </span><span style="font-family:Verdana;">the </span><span style="font-family:Verdana;">study of basic Rankin thermal cycle, the steam exh</span><span style="font-family:Verdana;">aust pressure of a typical steam turbine toward </span><span style="font-family:Verdana;">the </span><span style="font-family:Verdana;">condenser, plays a great rol</span><span style="font-family:Verdana;">e</span><span style="font-family:Verdana;"> in the efficiency and the net output power of </span><span style="font-family:Verdana;">the </span><span style="font-family:;" "=""><span style="font-family:Verdana;">steam turbine, so most surface conden</span><span style="font-family:Verdana;">sers </span></span><span style="font-family:Verdana;">that</span><span style="font-family:Verdana;"> are working in thermal power plants are kept at va</span><span style="font-family:;" "=""><span style="font-family:Verdana;">cuum condition so that the maximum power of thermal cycle can be achieved. The </span><span style="font-family:Verdana;">vacuu</span><span style="font-family:Verdana;">m pressure at condenser leads to </span></span><span style="font-family:Verdana;">the </span><span style="font-family:Verdana;">entering of air </span><span style="font-family:Verdana;">and Non-</span><span style="font-family:Verdana;">condensable gases from turbine gland seals to condenser so that the special air ejection equipment is being used to take apart air from steam and vent it to out of condenser.</span><span style="font-family:;" "=""> </span><span style="font-family:Verdana;">In this study</span><span style="font-family:Verdana;">,</span><span style="font-family:Verdana;"> a special steam and air separator mechanism in </span><span style="font-family:Verdana;">an </span><span style="font-family:Verdana;">evacuating system called </span><span style="font-family:Verdana;">“</span><span style="font-family:Verdana;">Aircooler</span><span style="font-family:Verdana;">”</span><span style="font-family:Verdana;"> at a 16</span><span style="font-family:;" "=""> </span><span style="font-family:Verdana;">MW steam turbine condenser is being studied and the Fluent CFD software is utilized to analyze the behavior of steam plus air in a typical aircooler system of 16</span><span style="font-family:;" "=""> </span><span style="font-family:;" "=""><span style="font-family:Verdana;">MW steam turbine condenser of Neka power plant to find a way to reduce the risk of cooling tube rupture in aircooler ducts. The critical condition which tube rupture happens is determined and it is demonstrated that in hot seasons of year, by increasing the seawater cooling temperature and increasing in turbine steam exhaust pressure and temperature, the risk of tube rupture due to more mixture velocity at the first row of aircooler cooling tubes increases and also</span><span style="font-family:Verdana;"> the effect of tube plugged condition on the performance of </span><span style="font-family:Verdana;">aircooler shows that the risk of other tubes rupture will increase and thus the efficiency of aircooler decreases due to more aircooler exhaust temperature. Finally</span></span><span style="font-family:Verdana;">,</span><span style="font-family:Verdana;"> two modified plans at aircooler system design will be studied and simulated via Fluent CFD software which leads to reduce the risk of tube rupture. The results show that by modification of aircooler ducts and holes, the mixture air and steam flow velocity to first aircooler cooling tube row decreases significantly and causes the risk of tube rupture </span><span style="font-family:Verdana;">to </span><span style="font-family:Verdana;">decrease remarkably and also the exhaust temperature of aircooler decreases and causes the higher ejector performance.</span>展开更多
In this paper,homogeneous condensation induced by unsteady rarefaction waves and reflected rarefaction waves in vapor-gas mixture was investigated experimentally.It is shown that the temperature of condensation onset ...In this paper,homogeneous condensation induced by unsteady rarefaction waves and reflected rarefaction waves in vapor-gas mixture was investigated experimentally.It is shown that the temperature of condensation onset during very fast unsteady expansion in vapor-gas mixture is much lower than that during equilibrium process in the atmosphere. It is of interest to indicate that the size of droplets approximates a constant,but the number density and the mass density of droplets change rapidly in the region of static flow.展开更多
基金supported by the National Natural Science Foundation of China(No.41877251)Major Science and Technology Projects of Xinxiang City(No.21ZD012).
文摘The thermal transmission coefficient for a micro-ribbed tube has been determined using theoretical relationships and the outcomes of such calculations have been compared with experiments conducted using a R1234yf refrigerant undergoing condensation.In particular four theoretical single-phase flow and three multi-phase flow models have been used in this regard.The experimental results show that:the Oliver et al.criterion equation overestimates the experimental results as its accuracy is significantly affected by the specific conditions realized inside micro-fin tubes;the Miyara et al.criterion equation prediction error is less than 15%;the Cavallini et al.approach gives the highest prediction accuracy;the Goto et al.model overestimates the test data.Such results are critically discussed and some indications for the improvement of such models are provided.
文摘The article analyzes a shell and tube type condenser’s thermal performance using concepts of efficiency and effectiveness. Freon 134a is used as a coolant flowing through the shell. Water or water-based aluminum oxide nanoparticles are at relatively low saturation pressure in the tube. The condenser consists of 36 tubes divided into three central regions for analysis: superheated steam, saturated steam, and subcooled liquid. The three regions contain four tubes with three steps each, that is, 12 tubes. Region I, superheated steam, includes three horizontal baffles. Profiles of temperature, efficiency, and effectiveness are presented graphically for the three regions, with fixed refrigerant flow equal to 0.20 kg/s and fluid flow rate in the tube ranging from 0.05 kg/s to 0.40 kg/s. The experimental result for vapor pressure equal to 1.2 MPa and water flow equal to 0.41 kg/s was used as one of the references for the model’s physical compatibility.
文摘The work’s objective is to analyze the influence of the saturation temperature of the R134a refrigerant on the thermal performance of a shell and tube type condenser, with water and aluminum oxide (Al<sub>2</sub>O<sub>3</sub>) nanoparticles flowing into the tube. For analysis, the heat exchanger is subdivided into three regions: subcooled liquid, saturated steam, and superheated steam. The shell and tube heat exchanger assumed as the basis for the study has 36 tubes, with rows of 4 tubes in line and three passes into the tube in each region. The parameters used to analyze the performance are efficiency and effectiveness, through variations of quantities such as saturation temperature, the nanofluid’s mass flow rate, fraction in the nanoparticles’ volume, and the number of passes in the tube in each region of the heat exchanger. The obtained results demonstrate that the efficiency is relatively high in all the analyzed situations. In each saturation temperature, the effectiveness can be increased by introducing fractions of nanoparticles in the water or increasing the number of passes in the tube.
基金supported by the National Key R&D Program of China(No. 2020YFB1901405)
文摘In loss-of-coolant accidents,a passive containment heat removal system protects the integrity of the containment by condensing steam.As a large amount of air exists in the containment,the steam condensation heat transfer can be significantly reduced.Based on previous research,traditional methods for enhancing pure steam condensation may not be applicable to steam–air condensation.In the present study,new methods of enhancing condensation heat transfer were adopted and several potentially enhanced heat transfer tubes,including corrugated tubes,spiral fin tubes,and ring fin tubes were designed.STAR-CCM+was used to determine the effect of enhanced heat transfer tubes on the steam condensation heat transfer.According to the calculations,the gas pressure ranged from 0.2 to 1.6 MPa,and air mass fraction ranged from 0.1 to 0.9.The effective perturbation of the high-concentration air layer was identified as the key factor for enhancing steam–air condensation heat transfer.Further,the designed corrugated tube performed well at atmospheric pressure,with a maximum enhancement of 27.4%,and performed poorly at high pressures.In the design of spiral fin tubes,special attention should be paid to the locations that may accumulate high-concentration air.Nonetheless,the ring-fin tubes generally displayed good performance under all conditions of interest,with a maximum enhancement of 24.2%.
文摘Numerical simulation on R245fa condensation inside an inner diameter of 8 mm horizontal tube is researched in this paper. The effect of variation in velocity, condensation temperature and superheat of inlet steam and variation in cooling water temperature on heat transfer coefficient are investigated as a parametric study. Condensation process of steam has been successfully modeled by applying a user defined function (UDF) added to the commercial computational fluid dynamics (CFD) package. By analyzing the corresponding condensate contours and the curves of local heat transfer coefficient, the relationships between condensation heat transfer coefficient and various parameters of R245fa inside horizontal tube are obtained. It shows that the heat transfer coefficient increases by the increase in velocity, condensation temperature and superheat of inlet steam and the decrease in cooling water temperature. The errors between the heat transfer coefficient of simulation result and model of Wang and Shah are within ±30%. The parametric study will provide the basis for designing efficient heat exchangers of R245fa.
文摘<span style="font-family:Verdana;">Accord</span><span style="font-family:Verdana;">ing</span><span style="font-family:Verdana;"> to </span><span style="font-family:Verdana;">the </span><span style="font-family:Verdana;">study of basic Rankin thermal cycle, the steam exh</span><span style="font-family:Verdana;">aust pressure of a typical steam turbine toward </span><span style="font-family:Verdana;">the </span><span style="font-family:Verdana;">condenser, plays a great rol</span><span style="font-family:Verdana;">e</span><span style="font-family:Verdana;"> in the efficiency and the net output power of </span><span style="font-family:Verdana;">the </span><span style="font-family:;" "=""><span style="font-family:Verdana;">steam turbine, so most surface conden</span><span style="font-family:Verdana;">sers </span></span><span style="font-family:Verdana;">that</span><span style="font-family:Verdana;"> are working in thermal power plants are kept at va</span><span style="font-family:;" "=""><span style="font-family:Verdana;">cuum condition so that the maximum power of thermal cycle can be achieved. The </span><span style="font-family:Verdana;">vacuu</span><span style="font-family:Verdana;">m pressure at condenser leads to </span></span><span style="font-family:Verdana;">the </span><span style="font-family:Verdana;">entering of air </span><span style="font-family:Verdana;">and Non-</span><span style="font-family:Verdana;">condensable gases from turbine gland seals to condenser so that the special air ejection equipment is being used to take apart air from steam and vent it to out of condenser.</span><span style="font-family:;" "=""> </span><span style="font-family:Verdana;">In this study</span><span style="font-family:Verdana;">,</span><span style="font-family:Verdana;"> a special steam and air separator mechanism in </span><span style="font-family:Verdana;">an </span><span style="font-family:Verdana;">evacuating system called </span><span style="font-family:Verdana;">“</span><span style="font-family:Verdana;">Aircooler</span><span style="font-family:Verdana;">”</span><span style="font-family:Verdana;"> at a 16</span><span style="font-family:;" "=""> </span><span style="font-family:Verdana;">MW steam turbine condenser is being studied and the Fluent CFD software is utilized to analyze the behavior of steam plus air in a typical aircooler system of 16</span><span style="font-family:;" "=""> </span><span style="font-family:;" "=""><span style="font-family:Verdana;">MW steam turbine condenser of Neka power plant to find a way to reduce the risk of cooling tube rupture in aircooler ducts. The critical condition which tube rupture happens is determined and it is demonstrated that in hot seasons of year, by increasing the seawater cooling temperature and increasing in turbine steam exhaust pressure and temperature, the risk of tube rupture due to more mixture velocity at the first row of aircooler cooling tubes increases and also</span><span style="font-family:Verdana;"> the effect of tube plugged condition on the performance of </span><span style="font-family:Verdana;">aircooler shows that the risk of other tubes rupture will increase and thus the efficiency of aircooler decreases due to more aircooler exhaust temperature. Finally</span></span><span style="font-family:Verdana;">,</span><span style="font-family:Verdana;"> two modified plans at aircooler system design will be studied and simulated via Fluent CFD software which leads to reduce the risk of tube rupture. The results show that by modification of aircooler ducts and holes, the mixture air and steam flow velocity to first aircooler cooling tube row decreases significantly and causes the risk of tube rupture </span><span style="font-family:Verdana;">to </span><span style="font-family:Verdana;">decrease remarkably and also the exhaust temperature of aircooler decreases and causes the higher ejector performance.</span>
文摘In this paper,homogeneous condensation induced by unsteady rarefaction waves and reflected rarefaction waves in vapor-gas mixture was investigated experimentally.It is shown that the temperature of condensation onset during very fast unsteady expansion in vapor-gas mixture is much lower than that during equilibrium process in the atmosphere. It is of interest to indicate that the size of droplets approximates a constant,but the number density and the mass density of droplets change rapidly in the region of static flow.