Natural Draft Wet Cooling Tower Research Articles

Numerical study on heat and mass transfer performance of a natural draft wet cooling tower based on baffle optimization

In the natural draft wet cooling towers (NDWCTs), the resistance of the packing layer to air and the effect of the circular design of its cross-section, the air reaches the tower center area to overcome the maximum resistance. The air velocity in the center of the tower is low and the heat transfer is poor. To improve the performance of the cooling tower, the optimized design of the cooling tower distribution pipe surface with baffles is firstly investigated. Then, the three-dimensional numerical model of NDWCTs is developed and validated with power plant data. Finally, the effect of plates on cooling tower performance is analyzed through the influence of baffles on the cooling tower air field and combined with the circulating water temperature drop, evaporation, and ventilation. Results show that the air field uniformity inside the cooling tower is effectively improved by adding baffles. The cooling efficiency of the circulating water inside the cooling tower is improved. The air velocity in the packing zone is significantly increased by adding the a1a2 type baffle and the air temperature and circulating water in the central area of the tower decreases significantly and the a1a2 baffles performed best in terms of circulating water temperature drop in the packing zone and were able to significantly improve the cooling effect. The cooling tower with pro-wall baffles can cool the circulating water better than the conventional cooling tower. This study provides ideas for subsequent structural and energy efficiency studies.

Anti-Freezing Study of High-Level Water-Collecting Natural Draft Wet-Cooling Tower Based on Its Water Temperature Distribution Characteristics

Thermal power units play a crucial role in the deep peak regulation of power generation. During deep peak regulation, the load of the unit changes significantly, causing fluctuations in the inlet water temperature of the cooling tower and the water temperature in the filler. Therefore, in cold regions in winter, cooling towers have a high risk of freezing, which threatens the economic and safe operation of the unit. This paper establishes a three-dimensional numerical model based on constant heat dissipation and explores the average and minimum water temperatures at the bottom of filler under different water distribution methods, crosswind velocities, and ambient temperatures. The results show that the water distribution method has a significant impact on the water temperature at the filler bottom. Reducing the water distribution area can significantly increase the minimum water temperature at the filler bottom and reduce the risk of freezing. Although the presence of crosswind is not conducive to the cooling performance of the cooling tower, the higher the crosswind velocity, the higher the minimum outlet water temperature at the filler bottom and the lower the risk of freezing. The minimum water temperature at the filler bottom is approximately linearly related to the ambient temperature and is less affected by the unit load at the same temperature.

Influence of environmental parameters on the cold-end and thermal system of coal-fired power plant based on Ebsilon simulation

To analyze the influence of environmental parameters on cold-end systems, especially the natural draft wet cooling tower (NDWCT), and the thermal system, the 660 MW power plant was taken as the research subject. Based on Ebsilon, this paper researched numerically the cooling performance of NDWCT and thermal system under different environmental conditions. The simulation results showed that increasing dry bulb temperature and environmental humidity causes a decline of water temperature drop (△t), which deteriorates the cooling performance and leads to adverse effects on the thermal system. As dry bulb temperature rises from 20℃ to 30℃, total power generation and system efficiency drop by 6484.89 kW and 0.18 %, respectively, and drop by 9912.72 kW and 0.86 % when environmental humidity elevates from 35 % to 75 %. Meanwhile, simulation investigations demonstrated that crosswind affects seriously NDWCT and further affects the thermal system. When crosswind speed increases from 0.21 m/s to 4.75 m/s, △t and system efficiency drop by 1.6℃ and 0.17 %, respectively, and gross coal consumption rate rises by 1.14 g/(kW∙h), which leads to the reduction of thermal system economy. In all, influence of environmental parameters on cold-end system, especially NDWCT, and the entire thermal system were revealed, which provided a basis for dynamic analysis of coal-fired power plants.

Evaluating vaporization heat, thermal performance, and efficiency of a natural draft wet cooling tower

This study aimed to improve the accuracy of the thermal performance analysis of a natural draft wet cooling tower (NDWCT) and explore the effect factors of the decrease water and energy consumption. In this study, we determined the thermal performance and water loss of the NDWCT under dynamic conditions using mass-balance equations, and obtained water loss from 10.4 kg s−1 to 16.9 kg s−1. The analyzed vaporization heat accounted for 71.5%–86.9% of the total heat flow. Furthermore, the total amount of water discharged exceeded the average standard is 33.06 m3 has been spotted for the first time. Increasing the air-water ratio from 0.38 to 0.77, the cooling efficiency is increased by 44%. The change relationship between water temperature drop, water loss, air enthalpy, and air flow in the tower were discovered and established. The thermal performance analysis model based on water loss and air flow was used, compared with the total heat of the measured data, the maximum deviations were by 4.5% and 1.88%, respectively. The validity of the model was fully verified. These two models were combined with the heat balance equation analysis model based on three variables, which can provide support for optimal operation conditions that the energy savings and water reductions in NDWCTs, and providing convenience and a new reference for thermal performance evaluation, design, and research. Finally, we also proposed the suggestion for improving low efficiency of cooling tower is the most economical operation mode.

A sensitivity-coefficients method for predicting thermal performance of natural draft wet cooling towers under crosswinds

• A sensitivity-coefficients method for predicting performance of NDWCTs is proposed. • Both the environmental and operating parameters are considered in this method. • The relationships between the coefficients and the parameters are obtained. • This method can predict the performance of NDWCTs under crosswinds accurately. The thermal performance of natural draft wet cooling tower (NDWCT), which can be evaluated through the Merkel number and outlet water temperature, is greatly affected by the environmental crosswinds and has important impact on the economical and safe operation of power plant. Previous studies on the impact of crosswinds were mainly based on two methods include numerical simulation and experimental research, which caused high economical and time costs. This study proposes a sensitivity-coefficient method for predicting thermal performance of NDWCT under different conditions. This method consists of two processes. First, according to the principle of aerodynamic and thermal balance, the performance parameters are iteratively calculated under windless condition. Next, based on the above results, the prediction method for thermal performance under crosswinds, in which two sensitivity coefficients, namely α 3 and α 4 are introduced, is proposed by combining theoretical analysis and experimental research. Using the proposed method, the performance parameters of a practical NDWCT are predicted, and the error between the predicted values and the measured or numerical values meets the accuracy requirements. Therefore, this method can conveniently and accurately analyze the change in NDWCT thermal performance under variable working conditions.

Effect of cooling water salinity on the cooling performance of natural draft wet cooling tower

To clarify the effect of the cooling water salinity on the cooling performance of natural draft wet cooling tower (NDWCT) and relevant impact mechanisms, a three-dimensional (3D) numerical model of NDWCT was established with full consideration of the cooling water salinity, which was validated by comparing with the theoretical code method and the field test data of NDWCT with fresh water. The influences of the cooling water salinity and ambient crosswind on the key parameters, such as cooling tower outlet water temperature, ventilation rate and evaporation of circulating water, were all analyzed. The results show that the cooling performance of NDWCT with salinity circulating water is weaker than that with fresh water. The ambient crosswind had a great influence on the heat and mass transfer of the cooling tower. When the wind speed exceeded 6.6 m/s, the performance of the cooling tower is improved, this trend is less affected by salinity. The established 3D numerical model comprehensively considered the influence of salinity and ambient crosswind, the analyzed results are more valuable for practical guidance.

Evaluation of thermal performance for natural and forced draft wet cooling tower

This paper presents an experimental and numerical investigation of the thermal performance of natural draft wet cooling tower (NDWCT). The experimental investigation is carried out under natural draft condition and forced draft condition created by an axial fan. The operational parameters considered in this study are the thickness of the fill (10 and 20 cm), inlet water temperature (40, 45, and 50 °C) and inlet water volume flow rate (5.68, 7.75, and 9.46 L/min). The experimental results showed that the thermal performance is improved when the fans are used with the NDWCT. The temperature difference between inlet and outlet and effectiveness increase by 35% and 37.2%, respectively at fill thickness of 20 cm and water volume flow rate of 11.35 L/min. The temperature distribution of the air and the relative humidity were numerically simulated for both cases of natural and forced draft by employing the commercial CFD software ANSYS Fluent 15. The experimental and numerical results were validated with results from a previous work and showed a good agreement. The experimental results showed that the effectiveness increase by 22% and 30% for NDWCT and FDWCT respectively when in case of fill thickness 20 cm.

Numerical simulation of the influence of flue gas discharge patterns on a natural draft wet cooling tower with flue gas injection

This paper introduces a distributed arrangement of flue gas emissions and presents conclusions on the optimal placement and quantity of flue gas vents in a certain horizontal plane of a natural draft wet cooling tower (NDWCT). An NDWCT set up based on the arrangement of flue gas vents established herein will raise efficiency with the increase of radial distance and flue gas vents. The authors found a way to inject flue gas at a certain angle, which can make the flue gas and humid air in the tower swirl upward, thereby increasing the pumping force of the cooling tower and further improving the cooling tower efficiency. When the incident angle is parallel to the generatrix of the hyperboloid cooling tower, the cooling tower efficiency is maximized. The flue gas flowrate effect on the cold-water temperature was also investigated based on determining the optimal value of the radial distance, quantity, and angle of the flue gas vent reach. Moreover, the formula that can be generalize the result to other NDWCTs is obtained to reach the optimal emission mode of flue gas. Comparing with the case before improved, the flow rate of NDWCT increases by 822.05 kg/s (18.9%), and the cold-water temperature decreases by 1.12 k, under a 200% flue gas flowrate.

Field test study on thermal and ventilation performance for natural draft wet cooling tower after structural improvement

In order to improve the operating performance of the natural draft wet cooling towers (NDWCTs), some structural improvement measures, including adding air-deflectors, using non-uniform fillings, and adding air-ducts, were taken in one large-scale NDWCT in China. The effects of structural improvement on the ventilation and thermal performance were researched based on the field test method. Field test results showed that, in the crosswind velocity range of 1.15–3.2 m/s (test condition), the thermal and ventilation performance decrease gradually with the rising of crosswind velocity, but the structural improvement weakens the adverse influence of crosswind. Compared with before structural improvement, the intake air uniformity coefficient, the ratio factor of ventilation rate, the temperature uniformity coefficient, and the ratio factor of Merkel number increase by about 36.6%, 15.5%, 5.5%, and, 14.5%, respectively. It manifested that the structural improvement for the NDWCT can improve the thermal and ventilation performance to a great extent.

Development of Modified Incompressible Ideal Gas Model for Natural Draft Cooling Tower Flow Simulation

The article deals with the development of incompressible ideal gas like model, which can be used as a part of mathematical model describing natural draft wet-cooling tower flow, heat and mass transfer. It is shown, based on the results of a complex mathematical model of natural draft wet-cooling tower flow, that behaviour of pressure, temperature and density is very similar to the case of hydrostatics of moist air, where heat and mass transfer in the fill zone must be taken into account. The behaviour inside the cooling tower is documented using density, pressure and temperature distributions. The proposed equation for the density is based on the same idea like the incompressible ideal gas model, which is only dependent on temperature, specific humidity and in this case on elevation. It is shown that normalized density difference of the density based on proposed model and density based on the nonsimplified model is in the order of 10-4. The classical incompressible ideal gas model, Boussinesq model and generalised Boussinesq model are also tested. These models show deviation in percentages.

Numerical Simulation of Counter Flow Natural Draft Wet Cooling Tower with Flue Gas Injection

Numerical Simulation of Counter Flow Natural Draft Wet Cooling Tower with Flue Gas Injection

A Dimensioning Methodology for a Natural Draft Wet Cooling Tower

The paper proposes a methodology for the dimensioning of a natural draft wet cooling tower. The main geometrical dimensions depend on the packing type, the cooling and the weather conditions. The study is based on splitting the tower in three main zones: the spray and packing zone, the rain zone and the natural draft zone. The methodology is developed on modular bases, by using block-modules both for the three main zones of the cooling tower and for the inlet/outlet air properties. It is useful in explaining to the students the complex physical phenomena within the cooling tower but also for the development of a computer program to be used in engineering, management and education.

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

ABSTRACTThe 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.

Development and validation of a CFD model for numerical simulation of a large natural draft wet cooling tower

Électricité de France (EDF) has commissioned the development of a CFD model, based on ANSYS Fluent v.16, which includes the specific equations (Poppe and Merkel) describing the physical phenomena occurring in a natural draft wet cooling tower (NDWCT). Heat and mass transfers between water and air, as well as drag equations for the two-phase system, are computed in specific zones such as the cooling fill, spray and rain zones.This paper presents the CFD model created and the two-step methodology implemented for the validation process:•The model is first tested and set regarding thermal performance results obtained on EDF test cell Mistral located on the Bugey site. The Mistral facility includes a 7m×7m counter-flow section, equipped with a large number of temperature and pressure sensors, which represents an efficient and reliable mean for cooling fill performances evaluation.•Secondly, the model is fully validated thanks to acceptance tests results gathered from a large scale NDWCT refurbishment. To this end, advanced instrumentation was positioned in the tower to provide additional data for extensive validation of the model.

Comparative study on the performance of natural draft dry, pre-cooled and wet cooling towers

Most geothermal and concentrated solar thermal (CST) power plants are dry cooling due to the lack of water sources. Unfortunately, the performance of dry cooling is particularly reduced when the ambient air is hot, which is because dry cooling relies mainly on convective heat transfer to reject heat from the working fluid rather than water evaporation as is in wet cooling systems. This is even worse for natural draft cooling towers as their air flow is created by means of buoyancy effects due to the difference in air density between the inside and outside of the towers. The heat rejection capacity of natural draft dry cooling towers is significantly reduced at high ambient temperatures because the driving buoyancy force is small, resulting in small air flow through the cooling towers. To offset the low performance of dry cooling, pre-cooled cooling systems which combine dry and wet cooling are proposed. The wet cooling system only operates at high ambient temperatures to assist dry cooling. This paper is to investigate the variations in annual performance of three proposed natural draft cooling towers, i.e., a Natural Draft Dry Cooling Tower (NDDCT), a pre-cooled NDDCT and a Natural Draft Wet Cooling Tower (NDWCT). The water consumption of the pre-cooled NDDCT is compared with that of the NDWCT. The present study finds that: the performances of the NDDCT, the pre-cooled NDDCT and the NDWCT are significantly affected by the ambient conditions in operation; the NDWCT provides the highest heat rejection rate, followed by the pre-cooled NDDCT and the NDDCT; the pre-cooling enhancement can go up to 46% by increasing the NDDCT heat rejection rate from 93 MW to 136 MW in the hottest month. The use of the pre-cooling system is season-dependent (usually for hot summers); the pre-cooled NDDCT consumes 70% less water for every MW heat rejection than the NDWCT.

Mathematical Model of Two Phase Flow in Natural Draft Wet-Cooling Tower Including Flue Gas Injection

The previously developed model of natural draft wet-cooling tower flow, heat and mass transfer is extended to be able to take into account the flow of supersaturated moist air. The two phase flow model is based on void fraction of gas phase which is included in the governing equations. Homogeneous equilibrium model, where the two phases are well mixed and have the same velocity, is used. The effect of flue gas injection is included into the developed mathematical model by using source terms in governing equations and by using momentum flux coefficient and kinetic energy flux coefficient. Heat and mass transfer in the fill zone is described by the system of ordinary differential equations, where the mass transfer is represented by measured fill Merkel number and heat transfer is calculated using prescribed Lewis factor.

3D CFD modeling of natural draft wet-cooling tower with flue gas injection

The paper presents a study on numerical modeling of a natural draft wet-cooling tower with flue gas injection. A previously developed 3D CFD model of a natural draft wet-cooling tower is adopted to account for the flue gas injection. The model is validated against experimental data and then applied to verify the effect of flue gas injection on the cooling tower performance. The effect of wind speed on the rising plume and injected gas is also studied. It is concluded that the injected flue gas has insignificant influence on the cooled water temperature. The increasing wind speed is a reason of recirculation regions formation near the tower outlet which leads to flue gas flow near the tower shell increasing the risk of corrosion.

English

The flow field in the hyperbolic natural draft wet-cooling tower, which has great effects on the economy and security of power plant, was studied through numerical simulation. The mathematical model was established and analysed in order to optimize the cooling-tower and to evaluate its efficiency and a Numerical study on a channelchimney system was carried out in order to elucidate the behaviour of heat transfer and fluid flow. A 2D CFD model of natural draught Cooling tower and chimney like heat sink plate is developed the Euler multiphase model was used to simulate the heat and mass transfer in a natural draught cooling tower rain zone and the RNG turbulent model with the dispersed option is used for the turbulence modeling of multiphase flow. The results are presented in terms of local air temperature measurements inside the symmetrically heated channel and between the adiabatic extensions.

Quasi One-Dimensional Model of Natural Draft Wet-Cooling Tower Flow, Heat and Mass Transfer

The article deals with the development of CFD (Computational Fluid Dynamics) model of natural draft wet-cooling tower flow, heat and mass transfer. The moist air flow is described by the system of conservation laws along with additional equations. Moist air is assumed to be homogeneous mixture of dry air and water vapour. Liquid phase in the fill zone is described by the system of ordinary differential equations. Boundary value problem for the system of conservation laws is discretized in space using Kurganov-Tadmor central scheme and in time using strong stability preserving Runge-Kutta scheme. Initial value problems in the fill zone is solved by using standard fourth order Runge-Kutta scheme. The interaction between liquid water and moist air is done by source terms in governing equations.

Experimental study regarding the evolution of temperature profiles inside wet cooling tower under crosswind conditions

Based on similarity theory, this research details a thermal-state model experiment, concerning the evolution of the air/water temperature profiles inside a Natural Draft Wet Cooling Tower (NDWCT) under windless and crosswind conditions. Prior studies have shown that the air/water temperature distribution is fairly uniform and stable under windless (stagnant) conditions, but the uniformity is destroyed in the presence of windy conditions, and the air/water temperature of different points displays a large variation subject to the same crosswind velocity. Generally speaking, the highest air/water temperature values inside the whole tower lie on the windward and leeward direction, but the highest air temperature at the tower outlet appears near the leeward side zone, rather than exactly on the leeward side. Based on this research, the air/water temperature profiles regarding measurement of values can be obtained accurately under windless and crosswind conditions, a fact that can help confirm the specific location of vortex on the windward and leeward side. All of above findings can provide an important theoretical foundation concerning further research, specifically for energy-saving aspects NDWCTs.