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• Theoretical and experimental study of natural-draft wire-on-tube condensers. • Semi-empirical correlation to predict the air-side heat transfer coefficient. • Tube and wire pitch es play the most important role in the heat transfer rate. • Condenser optmization focused on reducing the material mass. This paper investigates the heat transfer performance of natural draft wire-on-tube condensers considering the geometric range of current designs adopted by the household refrigeration industry. Fifty-four samples with distinct geometries were manufactured and tested inside a standardized climate chamber that sets the air-side conditions, while a purpose-built apparatus was used to circulate hot brine through the coil with a strict control of the inlet temperature and mass flow rate. The results pointed out that the condenser height, and the wire and tube pitches are the key parameters affecting the heat transfer rate. An improved correlation was put forward to calculate the air-side heat transfer coefficient, being able to predict 90% of the experimental data within the ±10% error bounds. Also, it was noticed that the condenser design can be optimized by reducing the wires and tubes mass for a given heat duty. [ABSTRACT FROM AUTHOR]

The power consumption of axial flow fans may account for more than 1% of the rated power output of the power generating unit, so it is of benefit to the energy efficiency of the power generating unit to propose an operation adjustment approach to axial flow fans. On the basis of representative 2 × 600 MW direct dry cooling generating units, a computational model of air‐side flow and heat transfer of an air‐cooled condenser (ACC) combined with exhaust steam condensation is developed, by which the airflow rate, inlet air temperature of ACCs, the power consumption of axial flow fans, turbine backpressure, and net power output of power generating units at various wind speeds and in various wind directions are obtained. The results show that the net power output in the presence of winds always decreases when the rational speeds of the first upwind row axial flow fans increase from the rated speed of 79 rpm by 10% to 86.9 rpm. However, the net power output will increase in various wind directions if the rational speeds of all the fans except the upwind first row fans increase to 86.9 rpm. This can contribute to the optimal operation of the ACC by rotational speed adjustment of axial flow fans. [ABSTRACT FROM AUTHOR]

• A conjugate heat transfer model is applied to finned-tube steam condensers. • Both the incomplete/complete condensations are considered. • The in-tube heat transfer and film behaviors are quantitatively studied and compared. • The presented model allows for the improved design and operation of condensers. Considering both the incomplete and complete condensation, a conjugate-model-based approach is developed to characterize the distributed thermodynamic development inside finned tube condensers. Applied to an engineering oval finned tube, the approach verifies the in-tube flow pattern as annular and river in incomplete and complete condensation respectively; furthermore, quantifies and compares the varying characteristics of cooling wall temperature, condensation heat flux, condensation HTC, film thickness, wall shear in the two condensation. In incomplete condensation, the condensation HTC increases slowly with the condensation approaching, unaffected by the steam quality, or film Reynolds number. With regarding complete condensation, an immediate decline of cooling wall temperature, and condensation heat flux and HTC is observed at the end of condensation, one easy-freezing location is marked for the safety operation of condensers, and the backflow of air is found at the vapor outlet. Concerning the phase change section, due to a much lower cooling wall temperature, the complete condensation provides a much greater condensation heat flux, but a nearly equivalent condensation HTC, as comparing to the incomplete mode. The approach is beneficial in designing an enhanced finned tube and operating a field condenser efficiently. [ABSTRACT FROM AUTHOR]

Water spray cooling is an efficient way to enhance heat transfer of the air-cooled condenser (ACC) of power generating unit under high ambient temperatures. Taking the air-cooled 300 MW power generating unit as object, the flow and temperature fields of ACC with water spray cooling were analyzed by numerical simulation with experimental validation. Based on the air flow field inside ACC cell, an improved water spray nozzles arrangement was proposed to enhance the cooling capacity. The inlet air temperature and back pressure of turbine were compared with that under the original nozzles arrangement. The water consumptions including that of water droplets pre-cooling evaporation in air flow, liquid film evaporation on the finned tube bundles, and drainage of redundant water were obtained. The influences of water spray rate, spray direction and nozzles distance on cooling performance were investigated. The results showed that the improved nozzles arrangement is expected to provide efficient cooling performance and significant reduction in back pressure of the objective power generating unit. The improved nozzles arrangement with upward spray direction and nozzles distance of 0.8 m can be applied to the design of water spray system in ACC. In addition, the obtained redundant drainage water under various ambient conditions provided the evidence for spray water rate optimization. [ABSTRACT FROM AUTHOR]

Ambient winds may deteriorate the thermo-flow performances of air-cooled condenser (ACC) in power plants, so it is of benefit to the energy efficient operation of ACC to propose measures against adverse wind effects. In this work, the air deflectors installed under the fan platform of ACCs are proposed on the basis of a 2 × 300 MW direct dry cooling power plant. The flow and temperature fields of cooling air are obtained and analyzed for the air deflectors with various geometric parameters. The air mass flow rate and turbine back pressure are computed and compared with the original ACCs. The results show that the thermo-flow performances of ACCs are significantly influenced by the width, pitch, inclination angle and number of air deflectors, and get improved due to the restrained reverse flows in windward condenser cells. In most cases, the wide and more air deflectors are preferred, but the moderate pitch is recommended to improve the ACC performances. Moreover, the 45° inclination of air deflectors is superior to others in restraining the adverse wind effects. [ABSTRACT FROM AUTHOR]

For natural draft direct dry cooling system, the triangularly arranged air-cooled condenser may lead to the unexpected airflow deviations and unbalanced flow distributions although with an additional buoyancy force from the dry-cooling tower. In this work, an annularly arranged air-cooled condenser is proposed for natural draft direct dry cooling system. By numerical simulations, the flow and heat transfer performances of these two dry cooling systems are analyzed and compared at various wind speeds. The results show that for the annularly arranged air-cooled condenser, the air flow interactions between the neighboring cooling columns are totally avoided at small wind speeds, and moreover, the vortices at the inlets of the cooling columns may vanish for the middle sector at high wind speeds. The air inflow and outflow deviations through the cooling columns are clearly restrained, which improve the local thermo-flow performances, thus increase the heat rejection conspicuously and recover the cooling efficiency of natural draft direct dry cooling system compared with the triangularly arranged air-cooled condenser, especially at small wind speeds. The annular configuration of air-cooled condenser could be recommended for the potential engineering applications thanks to its more energy efficient performance. [ABSTRACT FROM AUTHOR]

Ambient winds are generally unfavorable to the thermo-flow performances of air-cooled condensers in power plants. More emphases are placed to weaken the negative effects of ambient winds, but no layout alternative of air-cooled condensers is considered. In this work, a novel vertical arrangement of air-cooled condensers is proposed on the basis of a 2 × 600 MW direct dry cooling power plant, which can weaken the adverse wind effects and utilize the wind power to improve the cooling capacity of air-cooled condensers. By means of the CFD simulation and experimental validation, the flow and temperature fields of cooling air for the vertically arranged air-cooled condensers at ambient winds are obtained. The mass flow rate, inlet air temperature and turbine back pressure are computed and compared with the traditional air-cooled condensers. The results show that the flow rate of the novel air-cooled condensers increases conspicuously compared with the current ones both in the absence and presence of winds. In the wind directions of 60° and 90°, the off-axis flow distortions of axial flow fans are greatly weakened and the average inlet air temperature of the novel air-cooled condensers is reduced and almost equals the ambient temperature. The thermo-flow performances of the air-cooled condensers are improved, thus the turbine back pressure is reduced by the novel layout of air-cooled condensers. [ABSTRACT FROM AUTHOR]

The purpose of this study is to determine the influence of face-air velocity distribution of finned tube bundles on the thermal-performance of power plant Air-Cooled Condensers (ACCs) by experimental method and to explore Favorable Face-Air Velocity Distributions (FFAVDs) of the ambient cooling air inside the air-cooled cells by Computational Fluid Dynamics (CFD) method. FFAVD is a face-air velocity distribution favorable for thermo-performance of power plant ACCs. Based on experiments on a specially-constructed wind tunnel test-bed located on-site of a 1000-MW ultra-supercritical direct air-cooled power plant, it was found that face-air velocity distribution of ambient cooling air of finned tube bundles used in ACCs notably affects their thermo-performance, and FFAVD s can be determined. One type of FFAVD tended to increase in the retrograde direction of tube-side turbine-exhaust-condensate flow. The face-air velocity distributions of ambient cooling air that increase in the direction of tube-side turbine-exhaust-condensate flow have an adverse effect on the thermo-performance of power plant ACCs. The existence of FFAVDs can be scientifically explained by the principle of field synergy. Experiments on an A-frame cell on the ACC employed in the 1000 MW ultra-supercritical direct air-cooled power plant showed that the face-air velocity distribution in an A-frame ACC cell has the opposite trend of an FFAVD. Therefore, one type of distributing net with the characteristics of an FFAVD is introduced. The favorable characteristics of the wind distributing net have been verified based on modelling and numerical calculation. [ABSTRACT FROM AUTHOR]

Abstract: Numerous factors, including the meteorological and the geographical conditions as well as the configurations of the fin tube bundles, can affect the performance of an air-cooled condenser (ACC) in a power generating unit. In this work a new model is presented to rapidly and accurately predict the flow and heat transfer characteristics of an ACC under the influence of natural wind. This reduced-order model is based on proper orthogonal decomposition (POD) of the three-dimensional velocity and temperature fields and encompasses natural wind directions ranging from 0° to 90°. In the process of operation in the present paper, a combined procedure involving spare and complementary POD is formulated to treat the issue of high nonlinearity associated with the large variable gradients which generate vortices around the ACC. A flux matching procedure is also introduced for estimation of the weight coefficients of the POD modes. The velocity and temperature fields of four typical test cases were estimated with maximum relative errors of 1.75% and 0.19%, respectively. At the same time, the degree of freedom was reduced by 105 compared to calculations made using computational fluid dynamics. The present research may provide a new and more reliable approach to real-time operational control of ACCs in power plants under polytropic natural wind conditions. [Copyright &y& Elsevier]

Numerical models of a standard organic Rankine cycle (ORC) system and the heat exchangers comprising the system are developed as a design tool platform for a flexible design. The objective is design of an efficient, cost-effective ORC power plant that can effectively exploit low-grade industrial waste heat or low to medium-temperature geothermal fluid. Typical heat exchanger configurations were modelled, including the circular finned-tube evaporator, air-cooled condenser, and flat-plate preheater. A published ORC configuration and process conditions from experiments are used for the thermodynamic cycle analysis in order to validate of the system model. Heat transfer correlations and friction factors are described for the modelling of the heat exchangers. The simulation results of the ORC system provide the design requirements for the heat exchangers. Geometric specifications and performance of the heat exchangers are determined by iterative simulations. [ABSTRACT FROM PUBLISHER]

Abstract: Ambient wind plays important roles in the thermo-flow performances of air-cooled condensers, but the wind effect mainly imposes on the upwind and bilateral condenser cells and will decay immediately. It is of benefit to the design and operation optimization of air-cooled condensers in a power plant to investigate the wind effect and its decay characteristics. On the basis of a representative 2 × 600 MW direct dry cooling power plant, the physical and mathematical models of the air-side fluid and heat flows for the air-cooled condensers at various ambient wind speeds and directions are set up. The velocity and temperature fields are presented and the volumetric flow rate, inlet air temperature and heat rejection for different condenser cells are obtained by using CFD simulation. The results show that the reversed flows happened in the upwind condenser cells lead to the high inlet air temperature, worsening the cooling capability of air at high wind speeds. Due to the combined behavior of hot plume discharge and ambient wind, the wind effect decays rapidly along the wind direction. At the wind direction of 90°, the deficiencies of the thermo-flow performances of air-cooled condensers are mainly resulted from the flow rate decrease of the upwind condenser cells, not the hot plume recirculation. The hot plume recirculation flows play adverse roles in deteriorating the performances of air-cooled condensers at the wind direction of 0°. [Copyright &y& Elsevier]

Abstract: The wave-finned flat tube heat exchangers are widely used in air-cooled condensers of a power plant. Due to the oblique arrangement of the wavy fins on the A-frame tube bundles of the air-cooled condenser, the finned tube is prone to fouling thus the thermo-flow performances of air-cooled condensers are likely to become deteriorated. A new configuration of wave-finned flat tube, in which the fin surface rotates to be perpendicular, is proposed in this paper to prevent the fouling from gathering on the finned tube. The physical and mathematical models for the fluid and heat flows of the cooling air through the present and proposed wave-finned flat tube bundles are setup, and the thermo-flow characteristics are compared. For the proposed finned tube, the influences of the fin pitch, height and the Reynolds number on the flow and heat transfer performances are investigated, and the correlations of the friction factor and Nusselt number are recommended. The results show that the buoyancy effect should be taken into account for the correlation of the friction factor at low Reynolds numbers, but this buoyancy force can be ignored for the Nusselt number. [Copyright &y& Elsevier]

Abstract: Wind-break wall is considered to be an effective way to weaken the inlet flow distortions and hot plume recirculation of air-cooled condensers in a power plant. It is of use to investigate the effects of wind-break wall configurations on the thermo-flow performances of air-cooled condensers. The physical and mathematical models of the air-side fluid and heat flows for the air-cooled condensers in a representative 2 × 600 MW direct dry cooling power plant are established with three different configurations of the wind-break wall. The volumetric flow rate and heat rejection of the air-cooled condensers are calculated and compared on the basis of the simulation results of air velocity and temperature fields at various ambient wind speeds and directions. The results show that the thermo-flow performances of the air-cooled condensers are improved by the extensions of the inner and outer walkways and elevation of the wind-break wall, especially at the wind directions ranging between 0° and 90°. The improvement thanks to the width increase of the inner or outer walkway is superior to that resulting from the elevated wind-break wall. [Copyright &y& Elsevier]

Abstract: Ambient winds may lead to poor fan performance, exhaust air recirculation and mal-distribution of the air across the tube bundles of the air-cooled condensers in a power plant. Investigations of the impacts of the ambient winds on the air-cooled condensers are key area of focus. Based on a representative 2×600MW direct dry cooling power plant, the physical and mathematical models of the air-side fluid and heat flow in the air-cooled condensers at various ambient wind speeds and directions are set up by introducing the radiator model to the fin-tube bundles. The volumetric flow rate, inlet air temperature and heat rejection for different air-cooled condensers as a whole, condenser cells and fin-tube bundles are obtained by using CFD simulation. The results show that the thermo-flow performances for the air-cooled condenser as a whole, condenser cells and heat exchanger bundles vary widely in space. The thermal performances of the air-cooled condensers, condenser cells and fin-tube bundles at the downstream are generally superior to those at the upwind. It is of use for the upwind fan regulations and the A-frame condenser cell geometric optimization to investigate the space characteristics of the thermal performance for the air-cooled condensers in a power plant. [Copyright &y& Elsevier]

Abstract: Mal-distribution of airflow is an important factor for the performance of air-cooled multi-coil air-cooled condensers. This study is an attempt to investigate the effects of different included angles between the coils of the condenser. It has been found in this study that it can be a mean to improve the performance of multi-coil condensers without using larger heat transfer surfaces. A commercially used four-coil condenser of an air-cooled water chiller was used as the base case in the tests and analysis. The results show that the variation of the included angle can increase the airflow rate by 7.85%, which corresponds to 5.29% increase in heat transfer. The improvements were found to be due to the reduction of the stagnant flow regions of the heat exchanger coils, and more even flow distribution through the coils. Test data were used to verify the computer model of the four-coil heat exchanger. The same tested fan performance characteristic was used in all of the analyses. The research results are important as air-cooled condensing units can be designed to better performance merely by changing the configuration of the coil arrangements. [Copyright &y& Elsevier]

• A combined RANS/URANS study on the natural draft and heat transfer of a thermal chimney consisting one row or two rows of horizontally-aligned cylinders in multiple columns is presented. • The effect of horizontal pitch on thermal chimney performance is balanced by two counteracting mechanisms: the chimney effect and the blockage effect. • At a fixed horizontal pitch of intermediate value, a minor modification of the vertical pitch could change the flow pattern substantially from a plume-dominant mode to a chimney-dominant one, leading to significant enhancement of natural draft strength and heat transfer for the thermal chimney system. To address the scarcity of natural convection research for the tube array with three or more columns of horizontally-aligned cylinders, this paper conducts RANS/URANS simulations for multiple cylinder columns in one or two rows, validated by particle-image velocimetry and heat transfer measurements. Parametric study with varying horizontal and vertical pitches is implemented by RANS with transition SST model. Key findings are then substantiated and examined by URANS. It is found that the horizontal pitch influences the balance between the chimney effect and the blockage effect, thus has an optimum with respect to the natural draft velocity. For all the vertical distances computed in the two-row thermal chimney model, the flow pattern stays in a plume-dominant mode at the large horizontal pitch and a chimney-dominant mode at the small horizontal pitch. Interestingly, at the intermediate horizontal pitch (pitch/diameter≈2.5), the flow pattern switches from plume-dominant to chimney-dominant over a minor increase of the vertical pitch (from 4.5 to 6 diameters), leading to significant augmentation in natural draft velocity and heat transfer on cylinder surfaces. This distinct thermo-flow behavior, identified for the first time to the authors' knowledge, may be harnessed to improve the performance of passive heat exchangers. [ABSTRACT FROM AUTHOR]

The operating conditions of axial flow fans are closely related to the thermo-flow characteristics of the mechanical draft direct dry cooling system. Moreover, the uneven distribution of cooling air driven by axial flow fans may lead to the deterioration of the heat transfer capacity of air-cooled condensers (ACCs). Therefore, developing energy-saving operating methods for axial flow fans is very meaningful. In this work, two kinds of adjustment strategies to make the flow field more uniform are proposed for a 2 × 300 MW direct dry cooling power-generating unit. The performance of ACCs in the prevailing wind direction is predicted with the help of the macro heat exchanger model. It is found that the inlet air temperatures of fans are significantly reduced by proposed strategies, especially at high wind speeds. Moreover, the minimum cooling air can meet the cooling demand of ACCs for the strategy which made the air flow rates of all fans consistent. Compared with the case without adjustment of fans, the total power consumption of the fan array was cut down effectively, up to 13.94% at the wind speed of 12 m/s. In conclusion, the energy efficiency of ACCs can be improved by the uniform flow field. [ABSTRACT FROM AUTHOR]

• Capacity of air-cooled condenser equivalent in counter-flow and co-current. • Capacity of counter-flow condenser reduces when tube inclination angle <20°. • Flooding observed for inclination angles of less than or equal to 5 degrees. Experimental results for counter-flow steam condensation in a 5.7 m-long air-cooled condenser tube with a flattened-tube cross section are presented. The tube is inclined at upward inclination angles from 0.5° to 40°, with both the vapor inlet and liquid outlet at the lower end of the tube. The effect of inclination angle on flow regimes, void fraction, capacity and heat transfer coefficient is presented. In addition, a water-cooled test section provides local steam-side heat transfer coefficient along the tube circumference at increased accuracy to that determined in the air-cooled test section. The range of condensation pressures tested is 88–120 kPa and all tests have a mass flux of less than 4.2 kg m−2 s−1. Stratified flow is found for nearly all conditions and locations in the condenser. Flooding of this stratified condensate layer is found for tube inclination angles of 5° and lower. This flooding is found to reduce the condenser capacity. The results are compared to those for co-current flow in the same condenser tube. Capacity is found to be unaffected by the flow configuration (counter-flow vs. co-current) when flooding does not occur. Steam-side heat transfer coefficient is also found to be unaffected by flow configuration for all conditions tested. [ABSTRACT FROM AUTHOR]

• Local heat transfer coefficient determined experimentally for steam condensation. • Effect of variable wall temperature and heat flux on heat transfer is analyzed. • Heat transfer coefficient increases with increased wall temperature glide. • Heat transfer coefficient is strongly affected by wall-steam temperature difference. Steam condensation heat transfer coefficient (HTC) in a large, flattened tube with non-uniform heat flux and wall temperature, and variable inclination angle is determined experimentally. The condenser tube is that typically used in an air-cooled condenser for power plants. The steel tube has an elongated-slot cross section with inner dimensions of 216 × 16 mm. Water vapor and liquid flow co-currently through a 5.7 m long air-cooled conditioning section, followed by a 0.12 m long water-cooled test section. The long conditioning section creates conditions in the test section that mimic the conditions in an operating condenser – allowing for the realistic development of flow regime and void fraction. HTC is then determined in the water-cooled section. The water-cooled section is designed as a crossflow heat exchanger to match the temperature and heat flux conditions of an air-cooled condenser. Visualization sections at the tube inlet and outlet allow determination of flow regime and void fraction. The flow is found to be stratified for all conditions. Tube inclination angle is varied from 0 to 38° downwards. Inlet quality in the water-cooled section ranges from 0 to 0.74. HTC is found to increase by more than 400% along the condenser height. In addition, inclination angle, wall-steam temperature difference, inlet water-steam temperature difference, water temperature glide and vapor quality are all found to affect the condensation HTC. [ABSTRACT FROM AUTHOR]