Research On Test Methods And Evaluation Models Of Three-Dimensional Thermal Performance Of Natural Draft Counterflow Wet Cooling Tower

Cooling tower is a kind of commonly used heat exchanging equipment in industry, and its thermal performance greatly affects the efficiency of energy usage of thermodynamic system. Thereinto, natural draft wet cooling towers （NDWCT） is the most widely used type. However the cooling efficiency of NDWCT is found not very high in practice, which is only 50%～60%. Part of the reason can be owed to the lack of maintenance, but fundamentally, a very important reason is that the environmental crosswind effect is not taken into consideration in design process. Actually, because of crosswind effect on the aerodynamic field of NDWCT, the situation of heat and mass transfer has been obviously changed in the tower; hence it is not sufficient to reflect the reality by the thermal performance calculation and evaluation without considering crosswind effect. Especially as the NDWCT is getting bigger and bigger,this divergence will be more and more prominent。Heat transfer of NDWCT is coupled to its aerodynamic field. Crosswind destroys the centrosymmetry of airflow field and makes the water-air heat transfer distribution possess three-dimensional features in the vast space of cooling tower. What is more,the effect of variable working condition factors such as water drenching density,inlet water temperature and inlet air temperature adds the complexity of the changing law of cooling tower thermal performance. But at present, researches concerning this point are still very limited. For the huge volume and complex structure of NDWCT, little test data can be found in literature; the facilities of model experiment are generally very simple which makes it difficult to deepen the research contents; the evaluation model of three-dimensional thermal performance has not been reported so far; though numerical simulations have provided some pridictions and analysis in form, the application process is very cumbersome, the cost of calculation is very high, the model itself and the conclusions still need to be proved by test data of real tower.In view of this, the thesis here has improved previous methods of experiment and field test, studied the changing law of thermal performance under crosswind conditions, put forward both new performance optimization measures and guiding principles of the existing optimization measures and laid theoretical foundations of three-dimensional performance evaluation by means of filed test, thermal state model experiment, theoretical analysis and modeling and advanced test technique development. The main efforts of this thesis are as follows.1. Field test methods were put forward about three-dimensional thermal performance of NDWCT. The crosswind effect on inlet airflow was studied by field test and tracer test. The changes of aerodynamic field and temperature field by varying crosswind speed were acquired by measuring air velocities and temperatures inside tower. And by introducing the newly defined indices, the uniformities of air velocity and temperature distributiones were evaluated respectively. Based on the above researches, the mechanism of crosswind effect on thermal performance was analyzed. The main conclusions are as follows.（1）When the wind is calm, inlet airflow inclines downward and flows smoothly into cooling tower; under small crosswind conditions, it still can be found that the air around tower inlet is accelerated from far to near; but if crosswind speed is big, inlet airflow will be seriously deteriorated, especially at the rear side of 135°where the situation is worse in central region than in bottom area. On the whole, with the increase of crosswind speed, the circumferential air intake uniformity of NDWCT decreases continually.（2） Crosswind reduces the windward updraft and increases the leeward updraft, which makes the aerodynamic field more and more deviate from windless state and results in the decrease of circumferential ventilation uniformity. However on the other hand, it also enhances the penetration of crossflow and improves the radial ventilation uniformity. But the overall ventilation uniformity is still getting worse and worse.（3）The air temperature changes most at the outer ring of 0-1/3R, so it is the sensitive area of crosswind influences, where more attention should be paid to optimize the water-air heat transfer in engineering.（4）When vc/vo=1.0 goes up to vc/vo=4.9, the loss coefficient of vertical ventilation increases almost 57% and the tower cooling efficiency decreases from 63% to 46%. But the cooling effect of cross ventilation is remarkable at higher crosswind velocities.2. Based on working conditions of real tower, mathematical model was built to predict the tower performance of windless state, and then the predicted results were applied to compare with that of field test to achieve the specific effect of crosswind on tower performance. By defining different boundary conditions, the dynamic characteristics of cooling tower performance were analyzed. At last, an evaluation method of ventilation diversion measures was put forward by an instance of engineering. The main conclusions are as follows.（1）The windless model was validated by design conditions of 4 NDWCTs with the drenching areas of 3500m2～12000m2 and proved to be accurate.（2）Crosswind effect on thermal performance of NDWCT is very significant. To improve the influences of crosswind can make the cooling tower release large energy saving potential.（3）When the crosswind speed varies, outlet water temperature changes rapidly to reach a steady level, but inlet water temperature needs to go through a relatively slow system balancing process to stabilize. As crosswind speed gets reduced, airflow rate and Merkel number are increased first and then decreased.The steady-state level is higher than that before the change of wind speed. In addition, analysis of Merkel number shows that the crosswind effect is related to the changing working conditions, i.e. ambient air condition, inlet water temperature and circulating water flow rate and so on.（4）The performance of cooling tower has been greatly improved after the modification of ventilation diversion. The proposed method is generally suitable for evaluating the efficiency of ventilation diversion measures in view of crosswind effect.3.Equipments of the thermal state model test system were reformed and redesigned. The tower shape was transformed into the type of hyperbolic curve which was consistent with real tower; the water distribution system was designed into the form of four sectors, each of which was made up of one main pipe and some movable branch pipes and had independent water supply. By the newly designed system, both the uniformity and flexibility of water distribution were greatly increased. Moreover, fill materials that easy to cut out and of good thermodynamic performance were chosen and applied in the thermal state model.Results of pilot test denote that the experimental system can accurately simulate three dimensional properties of real tower performance. Based on this, plus rational arrangement of measuring points and improved test techniques, three dimensional thermal performance of NDWCT can be tested and analyzed by the system.4. Outlet airflow of NDWCT under crosswind conditions was studied by the thermal state model test system and the preventive method that installed wire mesh over outlet was put forward. The results are shown as follows.（1）Outlet airflow is invaded, extruded, sealed and sucked successively as crosswind velocity increases. The ratio of cold air invasion arrives at maximum at the crosswind velocity of 0.2m/s （corresponds to 2m/s in reality）. Higher inlet water temperature and larger water flow rate are helpful to reduce the ratio of cold air invasion. The crosswind effect on outlet airflow makes cooling ranges generally decreased by 2%-3% and even 4% at most.（2） For crosswind effect on outlet airflow, the loss coefficients of NDWCT increase first and then decrease and with the maximum added values of 25%-35% as the wind speed increases.（3）The wire mesh over tower outlet can both hinder cold air invasion and lighten the skew degree of outlet airflow, but its porosity should be properly determined, the value of which here in the experiment is 0.857.5. By the newly defined indices that used to describe the uniformity of air and water temperatures in NDWCT, the water-air heat transfer uniformity and its changing rules under crosswind conditions were analyzed and discussed. And on this basis, the mechanism of non-uniform layout fillings effect on thermal performance was analyzed and guiding principles of engineering were put forward. In addition, a new optimization measure namely extended air deflectors was introduced to enhance the water-air heat transfer uniformity in a larger space of NDWCT. The results are shown as follows.（1）The annular region of 0.707R-0.837R is a transition zone between the high temperature zone and the low temperature zone. The circular area of 0.707R～0.949R that with the transition zone included is the most severe temperature changing region under crosswind conditions. And at the same time, the pool water temperature at the circle of 0.707R is influenced most by crosswind.（2）The radial air temperature uniformity coefficient Ψθr increases, while the circumferential air temperature uniformity coefficient Ψθc decreases first and then increases with the crosswind speed. The circumferential water-air heat transfer uniformity is more affected by crosswind than the radial one.（3）Both higher inlet water 1;emperature and bigger water drenching density are beneficial to the water-air heat transfer uniformity. However marginal effect of the former is decreasing and that of the latter is increasing. What is more, higher ambient1;emperature is bad for the water-air heat transfer uniformity under crosswind conditions,（4）At a constant total volume, non-uniform layout fillings can make air temperature field inside the tower more uniform and improve the cooling performance.When the filling is divided into three regions, increasing the range of intermediate regions is conducive to the circumferential uniformity of air temperature but is bad for the radial uniformity of air temperatiwe. The reasonable value is 0.71R for the outer edge of intermediate region.（5）Extended air deflectors can effectively improve the circumferential uniformity of air temperature in a larger space, simultaneously raise the temperature of central zone and increase the airflow rate under crosswind conditions, which make the total optimization effect on thermal performance better than the common air deflectors.6. Theoretical models of vertical ventilation and cross ventilation were presented and the coolmg effect of NDWCT under crosswind conditions was correspondingly divided into natural draft cooling effect due to vertical ventilation and force draft cooing effect due to vertical ventilation. Furthermore,the calculation model of Merkel number under crosswind conditions was built and crosswind sensitive coefficients a3、a4 were defined. Through thermal state model experiment, the changing rules of a3、a4 affected by working conditions factor including water drenching density,inlet water temperature,ambient air temperature and the airflow velocity inside NDWCT were studied and the fimetions al relationships between a3、a4 and the constant numbers K₃、K₄ were achieved.At last the universal prediction model of three-dimensional thermal performance of NDWCT under variable working conditions was bulit and the predictions methode were also given. Conclusions obtained in the process are as follows.（1）Under crosswind conditions,vertical ventilation were thested at the throat of tower model by tfie equal area method,and the results were shown to be consistent wkh the theoretical model, which indicated the validity of the model.（2）The crosswind sensitive coefficients a3>、a4 are bigger under the working conditions of smaller water drenching density, lower inlet water temperature, higher ambient air temperature and airflow velocity inside the tower.（3）The test data used to recognize the constant number K₃、K₄ shoudle be more than 4 groups, at least 5 groups.（4）By the sophisticated numerical models developed by our group,the above3500m² NDWCT was simulated and the results were compared with the predicted values by the model herein. It was shown that the theoretical prediction model can accurately reflect the changing law of Merkel number under crosswind conditions.7. Based on ZigBee kchniques,wireless data acquisition system was designed for the field test of three-dimensional thermal performance of NDWCT. Taking a 9000m² NDWCT for example, the layout and in stallation scheme of measuring points were presented. By using the system, the three-dimensional distributions of thermodynamic parameters can be collected simultaneously to realize the integral analysis and evaluation of the three-dimensional thermal performance; in addition,the efficiency of data collection is greatly improved to make long term acquisition of a large number of data feasible; And moreover,by using PC software developed by the author,the collected data can be displayed visually and orderly in the form of pictures and animations,which make to convenient both to monitor the state of measuring points and to realize the preservation,inquiry, management and analysis of historical data.Through the above research,the thesis has established theoretical models to evaluate three-dimensional thermal performance of NDWCT under cross wind conditions,developed the experimental methods,test methods a打d evaluations methods and proposed optimization measures to improve the three-dimensional thermal performance of NDWCT. The conclusions have a highly tiieoretical significance and engineering application value.