自然通风湿式冷却塔加装挡风板优化设计

以湿冷机组自然通风冷却塔相关理论为基础,借助于CFD模拟软件,建立了火电机组湿式冷却塔的传热传质模型,主要的换热区域如填料、雨区和喷淋区采用离散相模型。由于冬季气温较低和塔内的换热不均,在冷却塔的填料下面、进风口处、基环面容易结冰,提出了在进风口处加装挡风板的方案,数值模拟分析结果显示,该方案改善了塔内温度场,有效的防止了塔内结冰。     

模型饱和器冷态喷雾场的实验研究

针对HAT循环关键部件增湿饱和器内典型的传热，传质过程现象，为了最优化该过程和为今后数值分析提供有效的数据，设计了模拟饱和顺内传热，传质过程中的开放式冷态实验系统。使用相位多普勒分析仪DualPDA（Phase Doppler Analyzer）对冷态模型饱和器的喷雾场进行了详细的实验研究，测量了不同水压，不同气流速度下的喷雾场，得到喷雾场内液滴的三维平均速度分布，脉动速度分布，平均粒径大小分布等，分析了喷雾水压和鼓风气流速度对喷雾场的影响，分析结果表明，改良设计的离心喷嘴具用良好的喷雾性能和轴对称性，水压增大可以增大喷雾场中粒子速度，通量及降低平均直径，喷嘴喷雾长度随气流速度减小，气流速度可以改变喷雾场的分布，有利于液滴蒸发和液滴破裂。     

Numerical model of evaporative cooling processes in a new type of cooling tower

A numerical model for studying the evaporative cooling processes that take place in a new type of cooling tower has been developed. In contrast to conventional cooling towers, this new device called Hydrosolar Roof presents lower droplet fall and uses renewable energy instead of fans to generate the air mass flow within the tower. The numerical model developed to analyse its performance is based on computational flow dynamics for the two-phase flow of humid air and water droplets. The Eulerian approach is used for the gas flow phase and the Lagrangian approach for the water droplet flow phase, with two-way coupling between both phases. Experimental results from a full-scale prototype in real conditions have been used for validation. The main results of this study show the strong influence of the average water drop size on efficiency of the system and reveal the effect of other variables like wet bulb temperature, water mass flow to air mass flow ratio and temperature gap between water inlet temperature and wet bulb temperature. Nondimensional numerical correlation of efficiency as a function of these significant parameters has been calculated.     

NUMERICAL MODELING FOR INTERACTION OF A WATER SPRAY WITH SMOKE LAYER

Interaction of a water spray with a smoke layer was studied numerically. A one-dimensional model was developed by taking smoke and air as two quasi-steady layers. W ater droplets of the spray were divided into several typical classes based on the droplet distribution function. Spray characteristics such as droplet diameter and velocity, shape of the spray envelope, gas temperature, and flow rate inside the spray envelope were calculated with heat and mass transfer to the induced air flow. The simulated results agreed with the experiments on full cone water spray in a normal atmospheric environment, and indicated some physical principles for using water spray to minimize smoke damage.     

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.     

Numerical simulation of counter-flow spray saturator for humid air turbine cycle

The numerical simulations of simultaneous heat and mass transfer process in the counter-flow spray saturator and humid air turbine cycle are carried out in this work, according to the experimental conditions and actual size of a prototype saturator. This humidifying process involves two-phase flow of air and water droplets, also including interaction, breakup and collision of water droplets. Eulerian approach is used for gas phase flow, Lagrangian approach is used for liquid phase flow, and the two-way coupling is used between two phases. The simulations agree well with the experimental measurements. The simulations show the flow is with high turbulence intensity, the relative humidity and temperature of humid air increase along with the height of saturator, some water droplets carried by air escape from the saturator, and the humid air is mainly humidified at the lower part of saturator and is simultaneously humidified and heated at the upper part.     

Evaporative cooling of water in a mechanical draft cooling tower

A new mathematical model of a mechanical draft cooling tower performance has been developed. The model represents a boundary-value problem for a system of ordinary differential equations, describing a change in the droplets velocity, its radii and temperature, and also a change in the temperature and density of the water vapor in a mist air in a cooling tower. The model describes available experimental data with an accuracy of about 3%. For the first time, our mathematical model takes into account the radii distribution function of water droplets.Simulation based on our model allows one to calculate contributions of various physical parameters on the processes of heat and mass transfer between water droplets and damp air, to take into account the cooling tower design parameters and the influence of atmospheric conditions on the thermal efficiency of the tower. The explanation of the influence of atmospheric pressure on the cooling tower performance has been obtained for the first time.It was shown that the average cube of the droplet radius practically determines thermal efficiency. The relative accuracy of well-defined monodisperse approximation is about several percent of heat efficiency of the cooling tower. A mathematical model of a control system of the mechanical draft cooling tower is suggested and numerically investigated. This control system permits one to optimize the performance of the mechanical draft cooling tower under changing atmospheric conditions.     

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.     

Numerical simulation of a closed wet cooling tower with novel design

A closed wet cooling tower with novel design was proposed and numerically investigated. The studied cooling tower consists of two main parts: one heat and mass transfer unit (HMTU) and one heat transfer unit (HTU). In the HMTU, copper tubes are arranged as heat transfer tubes while plastic tubes are collocated to enlarge the mass transfer area between the spray water and the airflow. In the HTU, only copper tubes are adopted as heat transfer tubes. Heat and mass transfer process takes place among the process water, airflow and spray water in the HMTU, while in the HTU only heat transfer between the process water and the spray water is observed. A transient one dimensional distributed-parameter model was adopted to evaluate the cooling tower performance under different operating conditions. Determination of heat and mass transfer coefficients, as well as the influence of Lewis number on the cooling tower performance, was presented.     

蛋白粉尾气余热回收喷淋塔的热工特性研究

大豆蛋白粉干燥工艺过程产生的高温尾气中蕴含着大量显热和潜热,而无填料、垂直逆流喷淋塔可以深度回收尾气中的余热。对此建立了喷淋塔的数值模型,并通过实验验证其准确性;应用所建模型,分析喷淋高度、入口水温、喷淋密度、尾气流速和尾气入口湿球温度对喷淋塔热回收性能的影响规律,进而获得了喷淋塔在实验工况范围内的换热效率曲线及经验关联式,为尾气喷淋热回收塔的优化设计与工程应用提供了分析工具。     

Investigation of heat and mass transfer between the two phases of an evaporating droplet stream using laser-induced fluorescence techniques: Comparison with modeling

Heat and mass exchanges between the two phases of a spray is a key point for the understanding of physical phenomena occurring during spray evaporation in a combustion chamber. Development and validation of physical models and computational tools dealing with spray evaporation requires experimental databases on both liquid and gas phases. This paper reports an experimental study of evaporating acetone droplets streaming linearly at moderate ambient temperatures up to 75 °C. Two-color laser-induced fluorescence is used to characterize the temporal evolution of droplet mean temperature. Simultaneously, fuel vapor distribution in the gas phase surrounding the droplet stream is investigated using acetone planar laser-induced fluorescence.Temperature measurements are compared to simplified heat and mass transfer model taking into account variable physical properties, droplet-to-droplet interactions and internal fluid circulation within the droplets. The droplet surface temperature, calculated with the model, is used to initiate the numerical simulation of fuel vapor diffusion and transport in the gas phase, assuming thermodynamic equilibrium at the droplet surface. Influence of droplet diameter and droplet spacing on the fuel vapor concentration field is investigated and numerical results are compared with experiments.     

Thermodynamic performance experiment and cooling number calculation of a counter-flow spray humidifier in the HAT cycle

An experimental investigation of the thermodynamic performance of a counter-flow spray humidifier was conducted on the basis of theoretical analysis of the heat and mass transfer mechanism inside the humidifier. Critical parameters such as the temperature and relative humidity of air and the temperature of water at the inlet and outlet were measured. The influence of every measured parameter on the thermal performance of the humidifier was obtained under different experimental conditions. The cooling number, whose variation was also obtained, was calculated according to the measured data. The experimental results show that both the temperature and the temperature increment of outlet humid air and the temperature of outlet water increase with an increase of the water-gas ratio, whereas the cooling number decreases. Under all experimental conditions, the outlet humid air reaches or is close to the saturation level. The lower cooling number is favorable for the system, but it has an optimal value for a certain humidifier.     

Heat and mass transfer analysis of a wavy fin-and-tube heat exchanger under fully and partially wet surface conditions

In this study a mathematical model of heat and mass transfer performance of a wavy fin-and-tube heat exchanger under wet surface condition is presented. The heat exchanger is a counterflow heat exchanger in which humid air and liquid are flowing in opposite direction. A water film that causes evaporative cooling of the humid air is circulated on the humid air side. The heat and mass transfer equations are first derived for fully wet heat exchanger and then by defining a wettability parameter, these equations are obtained for partially wet heat exchanger. In modeling, values of Lewis number and wettability parameter are not necessarily specified as unity. The temperature distributions of humid air, liquid and water film, and relative humidity distribution of humid air are obtained numerically. The theoretical results are found to be in good agreement with the available experimental measurements.     

Characterisation of the spray cooling heat transfer involved in a high pressure die casting process

A systematic experimental study was conducted to examine the heat transfer characteristics from the hot die surface to the water spray involved in high pressure die casting processes. Temperature and heat flux measurements were made locally in the spray field using a heater made from die material H-13 steel and with a surface diameter of 10 mm. The spray cooling curve was determined in the nucleate boiling, critical heat flux, as well as the transition boiling regimes. The hydrodynamic parameters of the spray such as droplet diameters, droplet velocities, and volumetric spray flux were also measured at the position in the spray field identical to that of the test piece. Droplet size and velocity distribution were measured using a PDA system. A new empirical correlation was developed to relate the spray cooling heat flux to the spray hydrodynamic parameters such as liquid volumetric flux, droplet size, and droplet velocity in all heat transfer regimes. The agreement between experimental data and predicted results is satisfactorily good.     

Mass and heat transfer model of Tubular Solar Still

In this paper, a new mass and heat transfer model of a Tubular Solar Still (TSS) was proposed incorporating various mass and heat transfer coefficients taking account of the humid air properties inside the still. The heat balance of the humid air and the mass balance of the water vapor in the humid air were formulized for the first time. As a result, the proposed model enabled to calculate the diurnal variations of the temperature, water vapor density and relative humidity of the humid air, and to predict the hourly condensation flux besides the temperatures of the water, cover and trough, and the hourly evaporation flux. The validity of the proposed model was verified using the field experimental results carried out in Fukui, Japan and Muscat, Oman in 2008. The diurnal variations of the calculated temperatures and water vapor densities had a good agreement with the observed ones. Furthermore, the proposed model can predict the daily and hourly production flux precisely.     

An exergy analysis on the performance of a counterflow wet cooling tower

Cooling towers are used to extract waste heat from water to atmospheric air. An energy analysis is usually used to investigate the performance characteristics of cooling tower. However, the energy concept alone is insufficient to describe some important viewpoints on energy utilization. In this study, an exergy analysis is used to indicate exergy and exergy destruction of water and air flowing through the cooling tower. Mathematical model based on heat and mass transfer principle is developed to find the properties of water and air, which will be further used in exergy analysis. The model is validated against experimental data. It is noted from the results that the amount of exergy supplied by water is larger than that absorbed by air, because the system produces entropy. To depict the utilizable exergy between water and air, exergy of each working fluid along the tower are presented. The results show that water exergy decreases continuously from top to bottom. On the other hand, air exergy is expressed in terms of convective and evaporative heat transfer. Exergy of air via convective heat transfer initially loses at inlet and slightly recovers along the flow before leaving the tower. However, exergy of air via evaporative heat transfer is generally high and able to consume exergy supplied by water. Exergy destruction is defined as the difference between water exergy change and air exergy change. It reveals that the cooling processes due to thermodynamics irreversibility perform poorly at bottom and gradually improve along the height of the tower. The results show that the lowest exergy destruction is located at the top of the tower.     

AP1000核电机组巨型冷却塔型体优化数值计算

根据气水两相间热质传递原理,利用Fluent数值计算软件建立了APl000核电机组用20000m。淋水面积巨型冷却塔的三维计算模型,对双曲线塔筒的母线方程中的特征值及喉部半径的最优取值进行了计算验证,综合分析了特征值及喉部半径对塔内流场、进塔风量、蒸发水量和平均出塔水温的影响,得到了210m高巨型冷却塔的最优型体结构参数．计算结果表明：喉部半径与塔底半径的比值r0／r2=0．6--0．7、塔筒母线方程中的曲率特征值a=0．17～0．18时,冷却塔运行最优．     

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.     

Transient two-dimensional model of frost formation on a fin-and-tube heat exchanger

In the paper, numerical and experimental analyses of heat and mass transfer during frost formation on a fin-and-tube heat exchanger have been presented. Modelling of the frost formation on cold surfaces placed in a humid air stream, requires a complex mathematical approach. A transient two-dimensional mathematical model of frost formation has been developed. The applied mathematical model has been defined using governing equations for the boundary layer that include air and frost sub-domains as well as a boundary condition on the air–frost interface. The mathematical model with initial and boundary conditions has been discretised according to the finite volume method and solved numerically using the SIMPLER algorithm for the velocity–pressure coupling. Results have shown that the frost layer formation significantly influences the heat transfer between air and fins. As a result of numerical calculations, time-wise frost thickness variations for different air humidities, temperatures and velocities have been presented. Using the developed mathematical model, the algorithm and the computer code, which have been experimentally validated, it is possible to predict a decrease of exchanged heat flux in the heat exchanger under frost growth conditions.