| Natural draft dry cooling system(NDDCS)has been increasingly developed in thermal power plants,owing to its advantageous water conservation,small footprint,low noise and so on.As the cooling air flows through the air-cooled heat exchanger,the air density difference inside and outside the cooling tower is formed which generates the draft force of NDDCS,therefore the dry-cooling tower type and structure parameters may have influences on the flow fields of cooling air inside the tower shell,thus affecting the heat performances of the air-cooled heat exchanger.So it is of benefit to investigate dry-cooling tower type and optimize its structure parameters.In this research,4 tower types namely the traditional hyperbolic tower shell and three proposed ones as the conical/cylindrical,hyperbolic/cylindrical,and cylindrical tower shells are numerically developed.Besides,the heat-exchanger model is adopted to these drycooling systems so that the flow and heat performances of the finned tube bundles can be applied to the air-cooled heat exchanger.In both absence and presence of winds,the thermo-flow performances of NDDCS with the four dry-cooling tower shells have been thoroughly investigated,and then the air variable fields,outlet temperature of air-cooled heat exchanger,and turbine back pressure are analyzed and compared.As a result,the optimized dry-cooling tower shell is obtained for the natural draft dry cooling system.In the absence of winds,the heat transfer characteristics of NDDCSs with the hyperbolic,conical/cylindrical,and hyperbolic/cylindrical shells vary fairly small,whereas the cylindrical tower shell is a little inferior to the others.At the small wind speeds,the three proposed tower types have slightly worse cooling performances than the traditional one,with the cylindrical tower shell showing the worst.While for NDDCS with the hyperbolic/cylindrical tower shell at the high wind speeds,the air flow vortices are much smaller than other tower types,besides the high pressure area is expanded near the frontal air-cooled sector that the cooling efficiency is increased.For NDDCS with the hyperbolic/cylindrical tower shell,the effects of height ratio of cylindrical section and hyperbolic section on the heat transfer performances of cooling air are studied.In addition,three height ratios as are adopted considering the practical engineering.The results show that the heat transfer characteristics are almost same for the three height ratios in the absence of winds,since the driving force varies little inside the tower shell.At small wind speeds,the air flow resistance inside the tower shell with height ratio of 2:1 is higher than the other two cases,what's worse,the air flow acceleration potential shows also smaller.In such a case tower 1 has inferior heat performances than tower 2 and tower 3.Furthermore,tower 3 possesses higher heat rejection than tower 2.At the high wind speeds,the big wind dynamic pressure head plays a dominant role on the heat performances of cooling air compared with the tower shell structures.Additionally,for tower 1 with the higher cylindrical section,not only the air flow resistance can be decreased but also the flow vortices are relatively smaller which expands the effective flow area that more cooling air can flow through the tower shell.Therefore tower 1 has the supreme thermo-flow performances compared with others.It is also noticed that,tower 2 shows better heat performances than tower 3. |
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