横流式冷却塔换热模型与影响因素研究

根据Merkel的冷却塔传热传质理论,推导了适应于横流式冷却塔的换热模型,通过理论模型正交试验和实测数据因子相关性分析,研究了横流式冷却塔换热性能的影响因素。实测数据因子相关性分析结果表明,风量对横流式冷却塔换热性能影响程度最小,与理论模型正交实验结果存在一定的差异。运行时应保证横流式冷却塔进水流量的分布均匀,才能更有效的利用换热面积,提高横流式冷却塔换热效率。     

Simplification of analytical models and incorporation with CFD for the performance predication of closed‐wet cooling towers

Simplified analytical models are developed for evaluating the thermal performance of closed‐wet cooling towers (CWCTs) for use with chilled ceilings in cooling of buildings. Two methods of simplification are used with regard to the temperature of spray water inside the tower. The results obtained from these models for a prototype cooling tower are very close to experimental measurements. The thermal performance of the cooling tower is evaluated under nominal conditions. The results show that the maximum difference in the calculated cooling water heat or air sensible heat between the two simplified methods and a general computational model is less than 3%. The analytical model distribution of the sensible heat along the tower is then incorporated with computational fluid dynamics (CFD) to assess the thermal performance of the tower. It is found that CFD results agree well with the analytical results when the air flow is simulated with air supply from the bottom of the tower, which represents a uniform air flow. CFD shows the importance of the uniform distribution of air and spray water to achieve optimum design. Copyright © 2002 John Wiley & Sons, Ltd.     

A comprehensive approach to cooling tower design

In this paper, a mathematical model for a counterflow wet cooling tower is derived, which is based on one-dimensional heat and mass balance equations using the measured heat transfer coefficient. The balance equations are solved numerically to predict the temperature change of air and water, as well as the humidity as a function of the cooling tower high. Experimental measurements on two pilot-scale cooling towers were carried out in order to analyze the performance of different cooling tower filling materials. Also, the performance of other cooling tower elements, such as droplet separators and water spray nozzles, was investigated in the pilot experiments. The flow distribution, i.e. the velocity field, upstream to the filling material was predicted using the three-dimensional version of the computational fluid dynamics (CFD) code Fluent/uns, version 4.2. The calculated flow fields are presented for different distances between the inlet of the air and the filling material. In addition, the two-dimensional version of the CFD code Fluent/uns, version 4.2, was applied to predict the external airflow around the cooling tower and the backflow in different weather conditions in summer and winter. The research project was carried out in connection to an industrial cooling tower installation.     

间接空冷机组空冷塔塔群内空气流动及传热性能研究

由于具有巨大的节水优势,间接空冷机组在我国富煤少水区域得到广泛应用。研究环境风对间接空冷系统的影响机理对指导电厂运行具有重要意义。以某电厂间接空冷机组为基础,构建水平布置散热器的空冷塔群物理和数学模型,通过数值模拟方法分析环境风对塔内空气流场及空冷散热器换热性能的影响。结果表明:环境风对空冷系统塔内空气流场影响较大,进而影响空冷散热器的散热性能。随着风速的增加,空冷塔的换热性能不断恶化。在临界风速时额定负荷下,下游空冷塔换热量比上游空冷塔减少2.5%。     

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.     

Solution of heat and mass transfer in counterflow wet-cooling tower fills

Model of heat and mass transfer in wet cooling tower fills is presented. The model consist of a set of four 1D ODEs describing the mass and energy conservation and kinetics with boundary conditions prescribed on opposite sides of the computational domain. Shooting technique with self adaptive Runge–Kutta step control is applied to solve the resulting model equations. The developed model is designed to be included in a large scale CFD calculations of a natural draft cooling tower where the fill is treated as a porous medium with prescribed distributions of mass and heat sources. Thus, the technique yields the spatial distributions of all flow parameters, specifically the heat and mass sources. Such distributions are not directly available in standard techniques such as Merkel, Poppe and e-NTU models of the fill where the temperature of the water is used as an independent variable. The method is validated against benchmark data available in the literature.     

Three dimensional numerical modeling of simultaneous heat and moisture transfer in a moist object subjected to convective drying

The drying behavior of a moist object subjected to convective drying is analyzed numerically by solving heat and moisture transfer equations. A 3-D numerical model is developed for the prediction of transient temperature and moisture distribution in a rectangular shaped moist object during the convective drying process. The heat transfer coefficients at the surfaces of the moist object are calculated with an in-house computational fluid dynamics (CFD) code. The mass transfer coefficients are then obtained from the analogy between the thermal and concentration boundary layer. Both these transfer coefficients are used for the convective boundary conditions while solving the simultaneous heat and mass transfer governing equations for the moist object. The finite volume method (FVM) with fully implicit scheme is used for discretization of the transient heat and moisture transfer governing equations. The coupling between the CFD and simultaneous heat and moisture transfer model is assumed to be one way. The effect of velocity and temperature of the drying air on the moist object are analyzed. The optimized drying time is predicted for different air inlet velocity, temperature and moisture content. The drying rate can be increased by increasing the air flow velocity. Approximately, 40% of drying time is saved while increasing the air temperature from 313 to 353 K. The importance of the inclusion of variable surface transfer coefficients with the heat and mass transfer model is justified.     

Parametric study of heat/mass transfer performance of seawater–air two-phase counterflow in packing in seawater cooling towers

A large amount of waste heat generated in industrial production needs to be discharged by circulating cooling water systems. To save freshwater resources, freshwater cooling towers have been widely replaced by seawater cooling towers in coastal areas, but research on the thermal performance of seawater cooling towers is still relatively less. In this study, a detailed calculation model based on the heat/mass transfer process of seawater–air two-phase counterflow was established, and the reliability of the proposed model was verified. The computer program developed under the VC++ framework was used for the numerical solution of the model. The effects of five inlet parameters on the cooling efficiency and heat dissipation were studied. The simulation results showed that with the increase of salinity, the cooling performance was reduced. When the salinity increased by 10 g/kg, the outlet water temperature rose by about 0.13°C. The wet-bulb temperature increased by 1°C and the cooling efficiency increased by about 0.77%, while total heat dissipation was reduced by about 36.37 kW. When the air–water ratio increased, the cooling performance was improved, but the maximum cooling efficiency was affected by heat load. The change of dry-bulb temperature had little effect on the cooling performance. With the increase of water temperature, the cooling efficiency and heat dissipation increased. The calculation model and simulation results can provide practical guidance for the operation of seawater cooling towers.     

A critical investigation into the heat and mass transfer analysis of counterflow wet-cooling towers

This study gives a detailed derivation of the heat and mass transfer equations of evaporative cooling in wet-cooling towers. The governing equations of the rigorous Poppe method of analysis are derived from first principles. The method of Poppe is well suited for the analysis of hybrid cooling towers as the state of the outlet air is accurately predicted. The governing equations of the Merkel method of analysis are subsequently derived after some simplifying assumptions are made. The equations of the effectiveness-NTU method applied to wet-cooling towers are also presented. The governing equations of the Poppe method are extended to give a more detailed representation of the Merkel number. The differences in the heat and mass transfer analyses and solution techniques of the Merkel and Poppe methods are described with the aid of enthalpy diagrams and psychrometric charts. The psychrometric chart is extended to accommodate air in the supersaturated state.     

Performance analysis and improvement of the use of wind tower in hot dry climate

Wind towers for passive evaporative cooling offer real opportunity for improving the ambient comfort conditions in building whilst reducing the energy consumption of air-conditioning systems.This study aims at assessing the thermal performance of a bioclimatic housing using wind towers realized in a hot dry region of Algeria. Performance monitoring and site measurement of the system provide data which assist model validation. The analysis and site measurement are encouraging, and they confirm the advantage of the application of this passive cooling strategies in hot dry climate.A mathematical model is developed using heat and mass transfer balances. For a more effective evaporative cooling, a number of improvements on wind tower configurations are proposed.     

基于CFD的活塞振荡冷却的流动与传热仿真研究

由于柴油机热负荷不断提高,必须要对活塞进行有效冷却。应用计算流体动力学(CFD)方法对活塞的振荡冷却瞬态传热进行了仿真分析,得到了不同转速下冷却油腔的机油填充率以及壁面换热系数等随曲轴转角的变化规律。研究结果表明,随着发动机转速的提高,冷却油腔内的机油填充率下降,但是壁面换热系数却有所提高;机油的振荡冲击对冷却油腔顶部和底部的强化换热明显高于侧壁。此结果可为活塞有限元分析提供热边界条件,在活塞概念设计阶段为活塞的优化设计提供依据。     

CFD-based 3-D optimization of the mutual coil configuration for the effective cooling of an electrical transformer

An optimal mutual configuration of coils and cooling ducts for the effective cooling of a dry-type transformer is presented in this paper based on the method developed by the author. In the optimization procedure, a computational fluid dynamics (CFD) and a genetic algorithm are combined to optimize the diameters of both the ducts and the coils. The method was applied to cool a special dry-type unit to minimize the hot-spot temperature of the windings. These simulations were performed using various sets of optimized shape parameters and copper mass constraints in a real 3-D transformer geometry. The objective function value is computed using a CFD model that accounts for all three heat transfer modes. In the proposed model, the thermal properties of the coils and core are treated as anisotropic and temperature-dependent quantities, and the power losses are treated as heat sources and are computed based on the coupled CFD-electromagnetic (EMAG) model. Due to a shape change, both coil properties and power losses vary with each generated coil configuration. The results show that the nonuniform positioning of the wires and air ducts and an optimal splitting of high- and low-voltage coils can significantly lower the hot-spot temperature and improve the heat dissipation in comparison with current transformer designs.     

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.     

CFD Two-Scale Model of a Wet Natural Draft Cooling Tower

A 2-D axisymmetric model of multiphase heat, mass, and momentum transfer phenomena in natural draft cooling tower is developed using a CFD code Fluent. The fill of the tower is modeled as a porous medium. The energy and mass sources in this zone are evaluated solving a separate 1-D model of mass and heat exchange. The spatial dependence of the sources is accounted for by dividing the fill into a set of vertical channels. The CFD solver produces boundary conditions for each channel, while the model of the channel exports the heat and mass sources to the CFD solver. To accelerate the calculations, an original technique known as the proper orthogonal decomposition (POD) is applied. This approach produces a reduced dimensionality model resulting in significant time economy and accuracy loss lower than 2%. The Euler-Euler multiphase model is used in the rain zone. The simulation results have been validated against experimental data coming from field measurements of a large industrial installation.     

Heat and mass transfer and fluid flow in cryo-adsorptive hydrogen storage system

Compared to room temperature adsorption, cryo-adsorptive hydrogen storage capacity has been greatly improved, and has become the central issue of the hydrogen storage research. Accurate simulation and optimization for cryo-adsorptive hydrogen storage has important guidance and application value to the experimental research, and the finite element software Comsol Multiphysics™ and system analysis software Matlab/Simulink™ can be used to simulate the cryo-adsorptive hydrogen storage. However, the computational fluid dynamics (CFD) software Fluent™ can provide more information on the heat and mass transfer and the fluid flow than above softwares. Based on the mass, momentum and energy conservation equations, this paper uses the modified Dubinin–Astakhov (D–A) adsorption isotherm model, linear driving force (LDF) model and dynamic thermal boundary condition which are implemented by means of CFD software Fluent to simulate the hydrogen adsorption processes of charging and dormancy in the case of liquid nitrogen cooling. We study the variations of temperature and pressure during the processes of charging and dormancy. The results show that the experimental data is in good agreement with the simulation results. We also analyze the effect of variable specific heat and anisotropic thermal conductivity on the heat and mass transfer and the fluid flow in cryo-adsorptive hydrogen storage system.     

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.     

间接空冷系统空冷散热器运行特性的数值模拟

以某6×1000 MW间接空冷电厂主要建筑物和空冷塔平面布局为例,通过CFD模拟,得到了冷却空气流场、温度场,分析了机组热负荷、环境气温、风速、风向对空冷散热器进口空气流速的影响.结果表明:处于环境风上游的空冷散热器单元,其迎面风速最大,空气温度最低,冷却效果最好;而处于侧面的空冷散热器单元,迎面风速最小,空气温度最高,冷却效果最差.随机组热负荷增加,空冷散热器冷却空气流量增加,随环境气温、风速增加,空冷散热器冷却空气流量减小.风向的改变也会影响散热器的运行特性.     

Integrated analysis of cooling water systems: Modeling and experimental validation

Cooling towers are widely used in many industrial and utility plants as a cooling medium, whose thermal performance is of vital importance. Despite the wide interest in cooling tower design, rating and its importance in energy conservation, there are few investigations concerning the integrated analysis of cooling systems. This work presents an approach for the systemic performance analysis of a cooling water system. The approach combines experimental design with mathematical modeling. An experimental investigation was carried out to characterize the mass transfer in the packing of the cooling tower as a function of the liquid and gas flow rates, whose results were within the range of the measurement accuracy. Then, an integrated model was developed that relies on the mass and heat transfer of the cooling tower, as well as on the hydraulic and thermal interactions with a heat exchanger network. The integrated model for the cooling water system was simulated and the temperature results agree with the experimental data of the real operation of the pilot plant. A case study illustrates the interaction in the system and the need for a systemic analysis of cooling water system. The proposed mathematical and experimental analysis should be useful for performance analysis of real-world cooling water systems.     

Investigations of filling mass with the dependence of heat transfer during fast filling of hydrogen cylinders

Much attention has been paid to study the state of charge (SOC) during fast filling process. However, investigation on identifying the foremost factors and contribution of them to the filling mass is still an open issue. In essence, the contributing factors are multiple, of which, the most important factors can be found out by thermodynamical analysis. Based on the thermodynamical analysis and mass flow rate balance, some equations calculating filling mass and heat transfer are outstanding. The mass filling rate, the initial pressure in the cylinder and the inlet temperature of hydrogen are confirmed to be the utmost important factors influencing the filling mass. A computational fluid dynamics (CFD) model is established. The simulation results show the liner or inverse proportional relationship between filling mass and the three factors, hence, a formula for the final mass with diverse filling conditions is figured out. By means of studying the state of charge obtained by adiabatic and diathermal filling processes, a formula to investigate the heat transfer is proposed. Both the filling mass and heat transfer are coupled with the three factors. Therefore, the effect of heat transfer on filling mass has been investigated. It seems that the filling mass is dependent on the total heat transfer during fast filling.