Method for Parameters Selection and Assessment of Aggregate Three-Stage Axial Turbines at the Initial Stage of the Design Process

Аuthors

Matveev V. N.^*, Shcherban' A. I.^**, Popov G. M.^***, Baturin O. V.^****, Nguen T. B.^*****

Samara National Research University named after Academician S.P. Korolev, Moskovskoe shosse, 34, Samara, Russia

*e-mail: matveev.vn@ssau.ru
**e-mail: korneeva.ai@ssau.ru
***e-mail: popov@ssau.ru
****e-mail: oleg_baturin@ssau.ru✉
*****e-mail: atskyfall81027010@gmail.com

Abstract

Aggregate turbines are being characterized by the small power outputs (from several tens to hundreds of kilowatts) and, consequently, by relatively small flow rates of the working fluids. These turbines have the following features.
Reduced blade heights due to low mass flow rates of the working fluid. The latter sometimes stipulates partiality introduction of gas supplying through the nozzle blades and/or significant reduction of axial flow velocities and, hence, flow angles in the flow path. 
To avoid significant leakage through the relatively large radial gaps, the degree of reaction of such turbine stages is assumed to be close to zero. 
Due to the diametric overall dimensions limiting and specified rotation speed, which is based on the needs of the driven units, the turbines operate generally at higher loads on the stage. This is being accompanied by lower values of the ratio of the circumferential speed of the rotor at mid-diameter to the turbine isentropic speed Uavg/Cst. 
The above-listed features of the aggregate turbines and, mainly, small flow rates of the working body should be accounted for when selecting their parameters and assessing their energy efficiency in the process of design computation.
It should be noted that the requirement for the design simplicity leads to the single-stage and two-stage aggregate turbines implementing. The parameter selecting methods of such turbines at the initial design stage are now well known. At the same time, it is reasonable to consider the possibility of the three-stage axial turbine application at lower values of the Uavg/Cst ratio in the range of 0.05–0.15. However, the method of parameters selection and energy efficiency assessment of three-stage aggregate turbines for the initial stage of design has not been developed so far. Thus, this article proposes such a method.
It allows selecting a rational value of the Uavg/Cst ratio with account for the diametric dimensions limitation. The method provides the possibility to iterative determining the optimum values of the degree of the NGV partiality, relative blade height and flow angle at the outlet of the nozzle blades, as well as to estimate the values of the performance parameter and power efficiency of a three-stage aggregate axial turbine.
When creating the method, expressions for finding the partial efficiencies that account for the effect of both the degree of partiality and the relative height of the nozzle blades were obtained, which allows accounting for these parameters impact on the three-stage turbine efficiency.
At the developed method approbation, in the process of parameters selection of a three-stage aggregate turbine with the power of 450 kW at a rotation speed of 40000rpm the power efficiency was equal to 52%. The optimum value Uavg/Cst = 0,11was accepted herewith at the degree of partiality of 0.44, the nozzle blades height of 2.9 mm and the flow angle of 15.7°at the nozzle outlet.

Keywords:

aggregate three-stage axial turbine; optimal degree of partiality, blades height, flow angles; turbine parameters assessment, power efficiency

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