| Indirect dry cooling technology essentially uses ambient air to cool the waste steam of the power generating unit,based on the natural draft mechanism.Due to the excellent advantages of water saving,indirect dry cooling system has been widely applied in the units of northern China.However it's easily affected by environmental factors,and the problems of insufficient performance in summer and freezing risks of the radiators in winter arise.In present study,an indirect dry cooling system in the super critical 600MW unit is taken as an example,and system characteristics and optimization theories under various ambient conditions are researched.The main contents and conclusions are listed below:1.A numerical model of the dry cooling tower which couples flow and heat exchange processes of the circulating water and ambient air,is proposed.A mathematical model for water flow distribution of looped network is established,and water mass flow rates of the delta-type radiators and cooling sectors are calculated.Then,the porous media model and heat volume source term method are adopted to describe air pressure loss through the radiator and air heating process respectively.Based on the distributed-parameter method,heat exchanged between water and air in each radiator is calculated.Furthermore,thermal calculation of the condenser is inserted into the procedure of the numerical computation,so back pressures of the indirect dry cooling system under various ambient conditions are obtained.2.Based on the analysis of air inflow modes of the radiators,the relation between air inflow velocity and air inflow deviation angle is established,as for the two cooling columns of each radiator.The influences of air inflow mode,deviation angle and velocity on thermal performances of the radiators under various ambient conditions are analyzed.The variations of thermal performances of the two cooling columns of each radiator with circumferential angle are obtained.The results show that as circumferential angle increases from 0°to 90°,or decreases from 180°to 90°,air inflow deviation angle increases gradually,thermal performance of cooling Column A deteriorates sustainably,and thermal performance of cooling Column B slightly improves at first and then declines.When wind speed increases,air inflow deviation angle and thermal performances of the cooling columns vary more sharply with circumferential angle,however ambient temperature almost has no impact on the distributions of air inflow velocity and heat rejection of the radiators.3.Based on the proposed dimensionless relations between heat rejection of the dry cooling tower and ambient conditions(air temperature and wind speed),a novel theoretical model is developed to predicate thermal performance of the tower under off-design ambient conditions.When evaluating the effect of ambient temperature,heat rejection of the tower is converted to the form that is proportional to the power function of initial temperature difference of the radiators.Heat rejection variation with wind speed is simply calculated,according to the variation of outlet air velocity obtained by an algorithm.Example calculations validate the accuracy of the proposed theoretical model,which is also suitable to performance predication of other similar dry cooling towers.4.Under windy conditions,the effects of apex angle of the radiator on thermal performances of the radiators and velocity distribution of the rising air inside the tower,are revealed.With the increase of apex angle,air inflow conditions of the radiators,especially the windward radiators improve,so heat rejection of each single radiator increases.However,air inflow rate and heat rejection unbalances among the radiators enlarge with the apex angle,which leads to the more uneven velocity distribution of the internal rising airflows.Increasing the exponent in the resistance expression of the radiators is helpful to reduce heat rejection unevenness among the radiators.5.To address thermal performance degradations of local radiators caused by backflows behind the windbreak walls,the rotating walls with rotational angle equal to air inflow deviation angle of the nearby radiator,are introduced,and confirmed to be useful to restrain the formation of the backflow and improve air inflow conditions of the radiators.Besides,the novel windbreak walls can reduce performance deviations among the radiators and regain the lost cooling capacity under crosswind.6.To cope with the excess cooling capacity of the dry cooling tower caused by air leakage of radiator's louver under low ambient temperatures,the partial through-flow tower shell is introduced.The distributions of air leakage volumes of the radiators'louvers and internal flow fields under various wind speeds are obtained,and the related limiting ambient temperature is determined for safe operation.After that,the impacts of the inhaled cold air through the partial through-flow tower shell on air leakage volumes and heat rejections of the radiators are analyzed.The results show that the windward through-flow tower shell effectively reduces air leakage volumes,and increases the backpressure and outlet water temperature.7.An integrated numerical model with a coupling procedure is developed to combine the dynamics of spray injections and natural draft of the dry cooling tower.Evaporation characteristics of droplets under the real air inflow conditions of the tower are studied,and spray cooling effects on tower performance are directly quantified.The interaction is clearly revealed that pre-cooling slightly reduces the buoyancy and ventilation of the tower while enhancing heat transfer,but velocity decrease of inflow air in turn promotes droplet evaporation and enhances spray cooling effects.According to streamlines,effective air inflow area of the tower gradually shrinks,which impedes droplet evaporation and vapor diffusion,and promotes the formations of interferences among the injections and impacts the spray cooling result.8.In the integrated dry-cooling tower and stack system,the influences of air flow characteristics in and around the tower on gas diffusion are revealed.It's indicated that both the swirl and downwind deflection of internal rising airflows contribute to the chaotic diffusion of flue gas inside the tower,and increase the contact area between flue gas containing high-concentration sulfur dioxide and inner tower shell.The uneven velocity distribution of warm plume on tower top,and plume downwash above tower top would lead to the increase of contact area between flue gas and the leeward part of outer tower shell.9.To reduce the potential deposition and corrosion of flue gas on tower shells,two internal flow reconstruction strategies are proposed.One works in the stage of inflow air mixing,and the other works in the following stage of airflow rising.Numerical results show that both the two optimization strategies effectively reduce the chaos of internal rising airflows and plume downwash,so that the contact areas between flue gas and inner and outer tower shells are reduced respectively,and the downstream diffusion passage of flue gas in the atmosphere is lifted as well. |
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