Numerical Investigation Of Blade Tip Clearance Patterns Effect On Performance Of An Axial Flow Fan

The variable pitch axial flow fan can change the installation angle of blade to adjust flow rate and pressure head,with the regulation of high efficiency,and can adjust the scope of work in the fan area of high efficiency which has become a main ventilation equipment of major thermal power units.Therefore,the pursuit of fan of high performance and high efficiency have practical value for the economic operation of power plants.Appropriate changes of the blade tip clearance structure can reduce the loss of tip leakage and improve the efficiency of the fan.At the same time,the stability of the working range can be increased.The performance and dynamic characteristics of the axial fan of type OB-84 under casing treatment and sloped trench casing treatment and non-uniform tip clearance were simulated with Fluent.By analyzing the flow field distribution,the pressure field distribution,the entropy generation distribution and the impeller working ability of the top of the blade tip gap structure,the internal mechanism of its influence on the fan performance is investigated.The results reveal that an apparent impact of the different schemes of casing treatment and sloped trench casing treatment is observed;the total pressure rise and efficiency are enhanced at the design condition and large flow rate,while manifest the opposite variations at small flow rate after the casing treatment;the static pressure distribution in the suction and pressure surfaces is altered by casing treatment,resulting in the reduction of tip leakage flow and consequent improvement of the flow capacity in the middle and upper blade;a comparison of different casing treatments shows that the circular groove structure presents the most notable enhancement of fan performance.The structure with sloped trench casing treatment changes the distribution of leakage flow,which relieves the development of leakage flow and thus impairs the mixing of the leakage and passage flow;Compared with the original wall,the total pressure and efficiency of the fan are increased to some extent at design condition after sloped trench casing treatment.The case of possess the optimal aerodynamic performance with an increasing efficiency by 0.59 percent than original fan at rated flow rate.When average tip clearance keeps constant,the total pressure rise and efficiency of the fan with convergent tip clearance patterns are higher than those of uniform tip clearance,and the domain of higher efficiency is expanded.With the increase of convergent level,the improvement of performance tends to be appreciable.Convergent tip clearance changes the distribution of vorticity field,resulting in the leakage losses of blade tip are reduced and the promotion of work capability in the middle and upper part of blade.When the divergent tip clearance patterns are employed,the work capability of blade is declined,leading to slightly decreased fan performance compared to the uniform blade tip.The convergent tip clearance pattern is considered as a preferred mean to improve fan performance in practical applications.