Numerical Study on the Performance of a Natural Draft Cooling Tower With Water-Cooled Collectors

ABSTRACT

The natural draft cooling tower with water-cooled collectors has advantages including pump lift saving, being a type of energy-saving tower, and being suitable for application in huge capacity nuclear power plants. The height of the rain zone is reduced due to the existence of water-cooled collectors under the fill, which leads to a change of the tower's performance. In this paper, a three-dimensional mathematical model of the thermal and aerodynamic field of the tower is established. The thermal and resistance characteristics of the tower have been studied by a numerical simulation method. The study results include: (1) Owing to the existence of water-cooled collectors, the resistance of the rain zone is decreased and the total resistance of the tower is also decreased; (2) as the height of the rain zone for the natural draft cooling tower with water-cooled collectors is lower, the air velocity distribution in the fill section is more uniform than that of the natural draft cooling tower; and (3) the cooling efficiency of the tower with water-cooled collectors is superior to that of the natural draft cooling tower.     

Solar enhanced natural draft dry cooling tower for geothermal power applications

This paper presents a new concept of hybrid cooling, named solar enhanced natural draft dry cooling tower (SENDDCT), in which solar collectors are added to traditional natural draft dry cooling towers to increase their performance. The purpose of using solar energy in this new cooling system is to increase the suction through the tower so that more air flow is achieved through the compact heat exchangers that cool condensers of a geothermal power plant. For the same size of the cooling tower, more air flow across the heat exchangers means more heat can be rejected by the system. The governing equations for the SENDDCT are similar to those of a conventional natural draft dry cooling tower except that solar heating is added after the heat exchanger bundles. Performance comparisons show that SENDDCT has substantial advantages over conventional natural draft dry cooling towers for geothermal power plants as well as standalone solar chimney power plants.     

Water Consumption Comparison Between a Natural Draft Wet Cooling Tower and a Natural Draft Hybrid Cooling Tower—An Annual Simulation for Luoyang Conditions

ABSTRACT

The Merkel method, the Poppe approach, and an empirical equation are employed in the developed computer program to predict the evaporation loss for the natural draft wet cooling tower (NDWCT). All the three models are validated with the experimental results. The calculation results in the NDWCT of a 300-MW power plant in Luoyang show that both the Merkel method and the empirical equation could not make the accurate evaporation loss prediction as the Poppe approach does. The annual water consumption and economic benefits for the power plant employed with the NDWCT and the natural draft hybrid cooling tower (NDHCT) are calculated by using the hourly weather condition. The results show that the power plant would save more than 95% of water but lose US$15,464,306 per year by replacing the NDWCT with the NDHCT.     

Optimization of Thermal-Flow Processes in a System of Conjugate Cooling Towers

Abstract

The paper presents the thermal-flow study of a closed cooling system with special emphasis on the working parameters of natural draft wet cooling towers. The authors analyze the possibility of the improvement of the overall cooling efficiency of a closed cooling system consisting of several cooling towers by the proper redistribution of cooling water between individual units. The problem of the optimal redistribution of circulating water between cooling towers is formulated as a mathematical issue involving finding the extrema of the multivariate function with constraints fixing the total mass flow rate of cooling water circulated in the hydraulic installation and the ranges of the hydraulic loads of individual cooling towers. The optimization process requires information about the individual characteristics of each cooling tower, which is achieved by experimental measurements done on real objects. The research done inside the cooling towers enables the identification of the heat and mass transfer processes across its radius. Next, these characteristics are used to calculate the optimal cooling water flow rates to the cooling towers, giving the highest possible mean cooling water temperature drop in the system.     

The Effect of Windbreak Walls on the Thermal Performance of Natural Draft Dry Cooling Towers

A three-dimensional study using the standard k-? turbulence model to simulate airflow in and around a natural draft dry cooling tower (NDDCT) has been conducted using a general-purpose CFD code. This investigation considered the location and the porosity of windbreak walls' structure on the NDDCT thermal performance. In addition, the effect of the windbreak walls on the thermal performance of two NDDCTs with different capacities has been investigated. Two parameters have been used to show the effect of the windbreak walls on the NDDCT thermal performance. At the reference heat exchanger temperature, the thermal effectiveness parameter has been employed. At the reference rejected heat from the NDDCT, the change in the cooling tower approach parameter has been employed. The results in this paper show an improvement in the NDDCT thermal performance due to the introduction of windbreak walls. Moreover, optimizing the location of the windbreak walls has been shown to have a more significant effect on the NDDCT thermal performance than the porosity of the walls. In addition, the effect of the windbreak walls on the thermal performance is similar for the two NDDCT with different capacities.     

湿式瘦高冷却塔淋水装置节能技术应用

主要叙述了一种基于FLUENT UDF方法设计湿式冷却塔淋水装置的节能技术,并介绍其应用于电厂冷却塔的情况。该节能技术的应用研究主要包括,环境侧风对超大型自然通风冷却塔热力性能的影响;塔群效应和塔群效应与环境侧风叠加等条件下,超大型逆流式自然通风冷却塔的热力计算方法;基于FLUENT UDF方法准确分析塔内每个区域的热力参数(喷头实际出流能力、填料实际淋水密度和对应区域的集水池平均水温)等工艺性能关键技术研究;冷却塔新材料(不同波形、不同片距)、新设备的研发制造;冷却塔环境保护关键技术研究等。     

Numerical Simulation of Influence of Arrangement of Flue Gas Discharge Devices on Performances of Indirect Dry Air Cooling Tower

ABSTRACT

For this paper, thre-dimensional (3D) numerical simulations have been carried out by means of one 3D model established using FLUENT software according to the indirect dry air cooling towers of one 2 × 660-MW power plant. The arrangement types of the flue gas discharge devices include the horizontal and the vertical arrangement types. The flow fields in the cooling towers have been studied. Meanwhile, the thermal and resistance performances have been analyzed. The study results include: (a) When the ambient wind is not considered, the arrangement types have little influence on the thermal and resistance performances of the indirect dry air cooling tower; and (b) when the wind is considered, the horizontal arrangement type plays a role of the cross-wall, which induces a mass flow increase compared with the case without the flue gas discharge devices.     

Effect of Inlet Window Deflectors on the Performance of a Natural-Draft Cooling Tower Subjected to Crosswinds

The effect of inlet window deflectors on the performance of a natural draft cooling tower subjected to a crosswind was studied numerically. Three patterns of the inlet window deflectors with different setting angles (62, 72, and 82 degrees) were tested under various crosswind velocities. The assumption of heat and mass transfer equivalence was adopted for simulation. A numerical model was developed and verified by comparison with experimental data. The simulations show that the 72-degree inlet window deflectors are efficient in a wide range of crosswind velocities. The effect of the number of deflectors was also investigated and the link between the velocity profile and the critical wind velocity was clarified. It was shown that the use of deflectors in the inlet window increases the thermal efficiency of a cooling tower up to 8.6% at wind velocities greater than 2 m/s and does not exhibit any effect at the wind velocities close to critical wind velocity. The investigation may be helpful in optimal design of natural draft and “hybrid” cooling towers.     

Theoretical Relation between the Number of Diffusion Units and Pressure Drop in Direct Contact Heat Transfer

Abstract

A simple theoretical expression has been developed to relate the number of diffusion units involved in thermal processes controlled by mass transfer with the pressure loss in associated convection or forced air flows. The expression allows the analytical evaluation of a previously defined thermo-fluid dynamic parameter as a function of the liquid/gas ratio in cooling towers. It has been found that experimental data, obtained from different commercial cooling tower fills, behave in a similar way to the calculated theoretical values. A thermo-fluid dynamic efficiency has also been derived that is useful for the quantitative qualification of cooling towers, cooling tower fills, and heat transfer processes in which the mass transfer prevails.     

Effect of Cross Wind on Performances of Natural Draft Counterflow Cooling Tower Outlet

ABSTRACT

The cooling efficiency of the counterflow natural draft cooling tower could be reduced by a cross wind. Therefore, it is necessary to study the cross-wind impact for optimizing the design. In this paper, a three-dimensional numerical model is used to study the influence of the cross wind on the pressure loss coefficient at the outlet of the counterflow natural draft cooling tower. The results show that an obvious additional draft is formed when the cross-wind velocity is more than four times of the mean velocity at the fill section. A formula to quantitatively analyze the influence of the cross wind is given in this paper.     

Theoretical Prediction With Numerical and Experimental Verification to Predict Crosswind Effects on the Performance of Cooling Towers

A simple theoretical model, validated against available numerical and experimental data in the literature, is presented to predict the effects of crosswind on the performance of natural draft dry cooling towers. The intersection of asymptote method, along with scale analysis, is used to find a closed-form solution for the airflow rate at the tower exit for given crosswind speeds. The total heat rejected under a windy condition is then calculated based on the air mass flow rate at the tower exit. This theoretical model allows for parametric studies and can generate accurate data. Interestingly, the model results, expected to be accurate within an order of magnitude, are more accurate than anticipated when compared to available experimental and numerical data in the literature. In fact, the maximum relative error is observed to be 15% when current theoretical predictions are compared to available experimental data. The results of this study will be useful for future work on the development of air-cooled condensers, especially for geothermal and solar thermal power plants in Australia.     

Three-dimensional numerical study on thermal performance of a super large natural draft cooling tower of 220m height

Based on the heat and mass transfer theory and the characteristics of general-purpose software FLUENT, a three-dimensional numerical simulation platform, composed of lots of user defined functions（UDF）, has been developed to simulate the thermal performance of natural draft wet cooling towers（NDWCTs）. After validation, this platform is used to analyse thermal performances of a 220m high super large cooling tower designed for inland nuclear plant under different operational conditions. Variations of outlet temperature of the cooling tower caused by changes of water flow rates, inlet water temperatures are investigated. Effects of optimization through non-uniform water distributions on outlet water temperature are discussed, and the influences on the flow field inside the cooling tower are analyzed in detail. It is found that the outlet water temperature will increase as the water flow rate increases, but the air flow rate will decrease. The outlet water temperature will decrease 0.095K and 0.205K, respectively, if two non-uniform water distribution approaches are applied.     

The thermal efficiency improvement of a steam Rankine cycle by innovative design of a hybrid cooling tower and a solar chimney concept

In the present work, a dry cooling tower and a solar chimney design are recombined in order to increase the thermal efficiency of a steam Rankine cycle. The rejected heat from the condenser into the dry cooling tower supplemented by the solar radiation gained through its transparent cover are the sources of wind energy generation that is captured by a wind turbine which is located at the beginning of the chimney. In this research a case study for a 250 MW steam power plant of Shahid Rajaee in Iran has been performed. A CFD finite volume code is developed to find the generated wind velocity at the turbine entrance for a 250 m dry cooling tower base diameter and a chimney height of 200 m. Calculations have been iterated for different ambient temperatures and solar irradiances, representing temperature gradient within day length. A range of 360 kW to 3 MW power is obtained for the change in the chimney diameter from 10 to 50 m. The results show a maximum of 0.37 percent increase in the thermal efficiency of a 250 MW fossil fuel power plant unit; which proves this design to be a significant improvement in efficiency of thermal power plants, by capturing the heat that is dissipated from dry cooling towers.     

Experimental study on the performance of a mechanical cooling tower fitted with different types of water distribution systems and drift eliminators

Water drift emitted from cooling towers is objectionable for several reasons, mainly due to human health hazards. Generation and control of drift depends mostly on the couple of elements water distribution system and drift eliminator. The configuration of these two components not only affects drift but also the cooling tower thermal performance. However, no references regarding the effect of the water distribution system on the cooling tower characteristic have been found in the reviewed bibliography. This paper presents an experimental investigation of the thermal performances of a forced draft counter-flow wet cooling tower fitted with a gravity type water distribution system (GWDS) for six drift eliminators and when no drift eliminator was fitted. The interaction between distribution system and drift eliminators is analyzed. Heat and mass transfer processes taken place in the cooling tower have found to be affected by the mass transfer coefficient and the exchange mass-heat area per unit of cooling tower volume. The comparison between the obtained results and those found in the literature indicates that the pressure water distribution systems type (PWDS) achieve better performances than the GWDS. Maximum averaged differences of 38.66% in terms of cooling tower performance have been obtained between the two water systems. The data registered in the experimental set-up were employed to obtain correlations of the tower characteristic. The outlet water temperature predicted by these correlations was compared with the experimentally registered values, obtaining a maximum averaged difference of ±1.61% for the water-to-air mass flow ratio correlation and ±0.95% for the water and air mass flow ratios.     

Reduction in Performance Due to Recirculation in Mechanical Draft Cooling Towers

Abstract

The influence of recirculating warm plume air on the performance of mechanical-draft cooling towers is investigated analytically, numerically, and experimentally. It is shown that the amount of recirculation that occurs is a function of the flow and the thermal and geometric characteristics of the tower. The presence of a wind wall tends to reduce the amount of recirculation. An equation is presented with which the performance effectiveness due to recirculation can be evaluated approximately for a mechanical-draft cooling tower.     

Contrastive analysis of cooling performance between a high-level water collecting cooling tower and a typical cooling tower

A three-dimensional (3D) numerical model is established and validated for cooling performance optimization between a high-level water collecting natural draft wet cooling tower (HNDWCT) and a usual natural draft wet cooling tower (UNDWCT) under the actual operation condition at Wanzhou power plant, Chongqing, China. User defined functions (UDFs) of source terms are composed and loaded into the spray, fill and rain zones. Considering the conditions of impact on three kinds of corrugated fills (Double-oblique wave, Two-way wave and S wave) and four kinds of fill height (1.25 m, 1.5 m, 1.75 m and 2 m), numerical simulation of cooling performance are analysed. The results demonstrate that the S wave has the highest cooling efficiency in three fills for both towers, indicating that fill characteristics are crucial to cooling performance. Moreover, the cooling performance of the HNDWCT is far superior to that of the UNDWCT with fill height increases of 1.75 m and above, because the air mass flow rate in the fill zone of the HNDWCT improves more than that in the UNDWCT, as a result of the rain zone resistance declining sharply for the HNDWCT. In addition, the mass and heat transfer capacity of the HNDWCT is better in the tower centre zone than in the outer zone near the tower wall under a uniform fill layout. This behaviour is inverted for the UNDWCT, perhaps because the high-level collection devices play the role of flow guiding in the inner zone. Therefore, when non-uniform fill layout optimization is applied to the HNDWCT, the inner zone increases in height from 1.75 m to 2 m, the outer zone reduces in height from 1.75 m to 1.5 m, and the outlet water temperature declines approximately 0.4 K compared to that of the uniform layout.     

Energy and Mass Transfer Phenomena in Natural Draft Cooling Towers

In this paper, the development of natural draft cooling towers diagnostics is presented. Diagnostic method is based on measurements of velocity and temperature fields of the airflow in the entire surface area of cooling tower and the raised phenomenological model of heat and mass transfer in a selected reference vertical segment of cooling tower. Velocity and temperature fields of the airflow were measured with the aid of a remote control mobile robot unit that was developed to enable measurements in an arbitrary measurement point above the spray zones over the entire cooling tower area. Topological structures of the humid air velocity profiles and temperature profiles above the spray zones were obtained at constant integral parameters of a power plant. Measurement results of temperature and mass flow characteristics of the air and water flows in a selected reference vertical segment of cooling tower are presented in the form of phenomenological dependence. Phenomenological dependence links local cooling tower efficiency, geometrical characteristics of spray elements, and air and water flow rates. In the concluding part, both methods are applied together on a selected segment of cooling tower, and local and integral cooling tower efficiency can be determined.     

New Developments in Reboilers and Evaporators

Abstract

A prototype cooling tower was used to explore the potential of using cooling towers compared with radiator cooling systems with 3 MW diesel engines. The working parameters were the water mass flow rate, water inlet temperature, air mass flow rate, and humidity ratio. The water mass flow rate was relatively the most effective. Three methods of calculation were used to evaluate performance—namely, heat and mass balance, psychrometric chart, and the heat and mass transfer method. The first was the best in comparison with experiments. The economic analysis of both the cooling tower and radiator systems showed that it would be more economical in the long run to use cooling towers for diesel engines.     

CFD Modeling of Legionella's Atmospheric Dispersion in the Explosive Outbreak in Murcia,Spain

ABSTRACT

Cooling towers, among other equipment, could have an important atmospheric impact, becoming a source of pollutants or biological agents. The most important, due to its frequency and importance of the outbreaks, is Legionella. Since its discovery in 1976 in Philadelphia, PA, several outbreaks have been reported causing tens of deaths. The most important one took place in the city of Murcia, Spain, in 2001, with more than 600 cases. In the present work, a validated numerical model using the computational fluid dynamics code ANSYS Fluent is employed to simulate the dispersion of the drift from the cooling tower causative of the outbreak in the urban environment of Murcia in the days of highest emissions. The results of the model are compared with the results of the epidemiological investigation carried out by the Epidemiology Service (Consejería de Sanidad, Murcia, Spain). The main objective of this model is to predict a cooling tower influence area, and what will help to reduce environmental and personal impact in case of an infection of its water or to improve the resources used to find the focus of infection after an outbreak has taken place. The model was previously validated using data from an experimental cooling tower installation.     

Experimental investigation of the hydraulic characteristics of a counter flow wet cooling tower

Thermal and nuclear electric power plants as well as several industrial processes invariably discharge considerable energy to their surrounding by heat transfer. Although water drawn from a nearby river or lake can be employed to carry away this energy, cooling towers offer an excellent alternative particularly in locations where sufficient cooling water cannot be easily obtained from natural sources or where concern for the environment imposes some limits on the temperature at which cooling water can be returned to the surrounding. This paper concerns an experimental investigation of the hydraulic characteristics of a counter flow wet cooling tower. The tower contains a “VGA.” (Vertical Grid Apparatus) type packing which is 0.42 m high and consists of four (04) galvanised sheets having a zigzag form, between which are disposed three (03) metallic vertical grids in parallel with a cross sectional test area of 0.15 m × 0.148 m. The present investigation is focused mainly on the effect of the air and water flow rates on the hydraulic characteristics of the cooling tower, for different inlet water temperatures. The two hydrodynamic operating regimes which were observed during the air/water contact operation within the tower, namely the Pellicular Regime (PR) and the Bubble and Dispersion Regime (BDR) have enabled to distinguish two different states of pressure drop characteristics. The first regime is characterized by low pressure drop values, while in the second regime, the pressure drop values are relatively much higher than those observed in the first one. The dependence between the pressure drop characteristics and the combined heat and mass transport (air–water) through the packing inside the cooling tower is also highlighted. The obtained results indicate that this type of tower possesses relatively good hydraulic characteristics. This leads to the saving of energy.