Study On The Design Of Radial Turbine Volute And Volute’s Influence On Turbine Performance

As a key component of modern internal combustion engines,turbocharger has been widely used in ships,vehicles,and many other types of machinery to increase output power,enhance efficiency and reduce emissions of the engines.Radial turbine is a main component of radial turbochargers,commonly found in small and medium-sized internal combustion engines.In addition,radial turbine is often used for micro gas turbines and various expanders.A radial turbine is mainly composed of a rotor and a volute which can be nozzled or nozzless.The main function of the nozzless volute is to accelerate the flow and distribute it circumferentially to downstream components evenly.The volute therefore influences turbine performance.However,up to now,the design methods and the understanding of these volutes are still inadequate.Therefore,this thesis reports the work by the author in this Ph D study on the design method and related theories of the nozzleless volutes of a radial turbine.Firstly,a two-dimensional design method for radial turbine volutes was presented,and a new volute was designed for a commercial radial turbine.With the same mass flow rates,the new volute improves the efficiency of the turbine and reduces the excitation forces on the rotor blades.However,since the method is two-dimensional,it is necessary to specify the volute cross-section shape to a certain degree(the three-dimensional method can reduce such manual intervention,but it is not yet mature).In order to improve the design method,it is necessary to find the optimal manual control.For this purpose,the flow fields in nozzless volute and the relationship between volute geometry and turbine performance were studied,and from which the two-dimensional volute design method was further improved.These include the following four aspects:(1)The flow mechanisms of the secondary flow in turbine volutes and the factors affecting this flow were studied.It was found that the secondary flow is not only controlled by centrifugal and viscous forces,but also the radial pressure gradient in the volutes.The strength of secondary flow in the volutes is mainly affected by the cross-sectional shape,A/R distribution and turbine operating conditions among others.The numerical results show for the first time that although the secondary flow loss itself is very small,account for only about3%of the total loss in volutes,it can increase the dominate boundary-layer friction loss by squeezing the main flow to the wall.Because of this,the cross-sectional shape of volutes may have little effect on turbine performance when the radial dimension,symmetry and A/R distributions of volutes are the same.This finding provides more choices for the design of volutes.(2)The effects of the circumferential distribution of volute A/R on the radial turbine performance were studied,and it was found that the slope of A/R is directly related to the circumferential distribution of the volute outlet mass flow rate and flow angle.It was also found that this slope affects the secondary flow in the volutes.The efficiency is higher when the slope of A/R is constant(A/R is linearly distributed),while the blade excitation force is smaller when the slope is convexly variable(A/R curve is S-shaped).Based on this,the A/R distribution of the commercial turbine volute were optimized,and the volutes with higher efficiency and smaller blade excitation forces were designed.(3)The effects of the distance between the tongue and rotor and the thickness of volute tongue on turbine performance were analyzed.When modifying the radius of the tongue,it is necessary to ensure that the(A/R)θ-(A/R)_t where(A/R)_t is the A/R value at volute tongue,of different volutes are the same at different azimuth angles,so the volutes will have the same flow capacity.The numerical results show that the change in tongue gap has little effect on the efficiency of the turbine,but a smaller gap leads to a significant increase in blade excitation force.Conversely,a larger gap can reduce the excitation force.The traditional view of tongue gap affecting the turbine efficiency is due to the change in volute(A/R)θ-(A/R)_tdistribution of the turbine when this gap was modifie in earlier studies,leading to the change of volute exit condition and turbine mass flow rate.It was also found that reducing the thickness of the volute tongue in a small azimuth range can have significant effects:the efficiency of the turbine is improved,the pressure and velocity fields near the tongue tip are more uniform,and the excitation force on the blade surface is significantly reduced.This finding paves the way for pratical applications of thin tongue concept.(4)On the basis of the above research,a non-uniform exit boundary condition for two-dimensional volute design methods was proposed to improve these methods.By adopting a non-uniform volute outlet boundary condition to count the effect of volute tongue on exit flow uniformness,the variations of the volute outlet flow parameters near the tongue were reduced with almost no loss of efficiency,and the excitation force on the rotor blade surfaces,especially the higher-order harmonics of the force,were significantly reduced.Finally,a harmonic response analysis was used to calculate the maximum resonant stresses on the rotor blades of the commercial radial turbine,the result shows the reductions of the stress by about30%compared with the volute designed from a uniform exit condition,thus verifying the effectiveness of the proposed non-uniform boundary condition method.