Research On The Performance Improvement Of Natural Draft Counter-flow Wet Cooling Towers With Cross Wall In The Rain Zone

The thermal performance of cooling tower greatly affects the safety and economy of the thermal power plant. Natural draft counter-flow wet cooling tower (NDCWT) is the most widely used form in China, and its performance is greatly influenced by environmental conditions, especially the crosswinds which may destroy the axisymmetrical aerodynamic field, reduce the ventilation and cooling efficiency. It has being taken more attentions and different wind-control schemes are supposed, put cross-wall in the rain zone is one of the schemes. This article is aimed to the performance optimization of cooling tower with cross wall in the rain zone.Heat and mass transfer and thermodynamic calculation of cooling towers has been introduced, and analyses the lack of cooling tower's performance evaluation. Accroding to the thermodynamic calculation and the nature that cooling tower is a heat exchanger, believes:the cooling numbe calculated by measured parameters can more accurately reflect the actual operation performance of the cooling tower, while ventilation can be used as evaluation of the cooling tower's resistance.Basing the choosing of evaluation and the CFD software, three-dimensional numerical computation model for a 600MW power plant unit's cooling tower with cross-wall in the rain zone is established. And the performance of cooling tower under crosswinds has been analysed, finding:(1) Ventilation and cooling number reduces, then increases with the cross-wind velocity rising, and less than the value under windless conditions. At the same time, the value of ventilation and cooling number are related to installation method of the cross-wall:in the case of the wall straight upwind, both the ventilation and cooling number are least and can be decreased by about 30% compared with the value under windless conditions when the cross-wind velocity is equal to 6.0m/s; but the cross-wind velocity is equal to 8.0m/s when the cross wall's installation angle is 45°, reduced nearly 50%. Under wind conditions, the value of ventilation and cooling number when the cross wall installation angle is 45°are less than that when the wall straight upwind.(2) Air velocity distribution of characteristic section, air humidity and temperature distribution of lower part of fill and temperature distribution of pool surface show that: parameters of each section have a certain correspondence. The parameters of each section are uniform along the circumferential distribution when there is no crosswind. Along the tower radius to the center, air velocity decreases, humidity and temperature, and temperature of pool surface water increases. Uniformity of the parameters is ruined by crosswind, air velocity on windward side increases, humidity and temperature is lower, as well as water temperature, while the air velocity on leeward decreases, but air humidity and temperature is higher, as well as water temperature.Based on the findings, different wind-control schemes has given:put air-intaker with different angles around the air inlet; put air-intaker in the vortex area in the rain zone and the two schemes couples. Calculation results show that:put air-intaker around inlet can increase ventilation, improve air velocity distribution, extend the gas-water contact time, enhance the effect of heat transfer, but the large kinetic energy of air on windward may not be fully utilized; put air-intaker in the rain zone can guide the windward air to the center of tower, and weaken the original vortex, but will have a new whirlpool and the effect is worse than put air-intaker around inlet; when they couple, the air-intaker around inlet will make the air rotational accelerate, while the air-intaker in rain zone leads it to the tower center possibly, so the ventilation and cooling number is larger than that of putting air-intaker around the air inlet, and the cooling performance is further improvedFinally, the thermal state model test bed about DNWCT is improved based on the original model, and test the performance under variable circulating water flow. Results show that:no matter what reform means, and the results are consistent with the numerical calculation. Meanwhile, the variation regularity of ventilation and cooling number with wind speed at different flow rates air similar illustrate the conclusions of generality.