The influence of the angle of attack on the efficiency supersonic
microturbines working wheels
Andrey Yu. Fershalov1, a, Timofey V. Sazonov 1,b , Sergey P. Solov`ev 1,c, and
Viktor V. Porshkevich1,d
1
Far Eastern Federal University, 8, Suhanova St., Vladivostok, 690950, Russia
a
[email protected], [email protected], [email protected], [email protected],
([email protected])
Keywords: Turbine, Microturbine, turbine stage, angle of attack, Rotor wheel, nozzle box.
Abstract. The article presents the results of a study of rotor wheel microturbines. Design features
considered rotor wheel is a high angle of rotation of the running working fluid. The results of
research on the effectiveness of rotor wheels, depending on the angle of attack are shown. The
analysis of the physical processes occurring in the flow of the rotor wheels. Optimization
calculations performed to determine the angle of attack corresponding to the maximum possible
efficiency rotor wheel. The recommendations for the selection of the angle of attack for impellers
microturbines are given.
Intorduction
Transport energy, which includes autonomous underwater vehicles, air and space technology
requires a high aggregate capacity at small dimensions of the motor. One of the possible types of
engine providing these requirements is the axial microturbine. It has a small radial dimension and
the possibility of increasing the number of stages with less structural complexity than the radial
turbine. Ensuring high power density microturbines possible at supersonic flow velocity in the flow
part of which is provided at high pressure ratio at the turbine inlet and outlet from it, as well as a
large rotation angle of flow in the impeller to provide a more stable torque characteristics. In this
case nozzle assembly should be performed with a small design angle output [1]. Non-optimal
combination of design parameters for a given stage microturbines operating modes can lead to a
significant deterioration of the efficiency of microturbines. One of the most important
characteristics of turbine stage is the angle of attack (i) at the inlet of the rotor wheel [2]. It is
defined as the difference between constructive entry angle of the blades of the rotor wheel (β1k) and
the angle of the leakage flow nozzle apparatus (β1). The purpose of this work was to study the
effect of the angle of attack on the efficiency of the rotor wheel and the determination of optimal
values of the angle of attack in the range of research.
Experimental Researches
Despite substantial progress in mathematics [3, 4], experimental studies have not lost their
relevance, especially in the study of supersonic microturbines with small dimensions of the flow.
The work carried out experimental research stages microturbines with nozzle apparatus having
the exit angle of 5 ° ... 9 ° [5]. Working wheels carried out with a large angle of rotation of the flow
part and large relative pitch (Fig. 1). Characteristics studied rotor wheels are presented in Table 1.
All rotor wheels had 26 blades.
Based on the results of experimental research was developed regression mathematical model of
efficiency of rotor wheel [5], which includes the following features: angle upstream into rotor
wheel; constructive entrance angle of the rotor wheel's blades and the Mach number at the outlet
from the rotor wheel taking from theoretical parameters. It allows to perform evaluation and
optimization calculations in the range of the research.
Fig. 1 Rotor wheels
а1 – the width of the channel at the entrance; а2 – the width of the channel in the middle section; а3 –
the width of the channel at the outlet; β1к (β2к) – constructive entry (outlet) angle of the blades of the
rotor wheel.
Table 1 Geometric characteristics of rotor wheels
Geometrical characteristics
Angle of entry [°]
Exit angle [°]
The width of the channel in the middle section [mm]
The width of the channel at the entrance [mm]
The width of the channel at the outlet [mm]
Rotor wheels
RW-1
RW-2
RW-3
8.1
11.2
14.1
8.4
12.1
15.3
2.24
3.34
4.19
2.40
3.48
4.50
2.63
3.82
4.93
Results of research
The study revealed that there exists an optimal value of the angle of attack, deviation from which
leads to increase of losses in the rotor wheel.
In the turbine stage at angles leakage flow on the blade that are significantly different from the
optimum, there is a the flow separation of the boundary layer, which increases the profile losses.
This phenomenon occurs when a large uneven distribution of pressure along the contour of the
profile, in which in some areas of the profile of an intense diffuser sections with a significant
thickening of the boundary layer flow due to inhibition of reverse flow at the surface of the blade.
Most probable conditions for flow separation occur in the boundary layer flow around the blade at
the back portion of greatest curvature, and also at the front edge (Fig. 2) [6].
At angles of attack less than optimal on the back of the blade is formed zone of increased
velocity and the diffuser portion that when a significant deviation angle of the entrance leads to the
separation of the flow near inlet edge. When entrance angles greater than optimal, high velocity
zone is similar to the diffuser followed by a convex portion formed on the side surface of the inlet
edge.
Fig. 2 Place of boundary layer separation
- negative angles of attack
- positive angles of attack
At small the angle of attack Mw2t little effect on the speed ratio of rotor wheel, but with
increasing influence Mw2t β1k increases. This is due to the fact that the subsonic flow value
deviation from the optimum angle of attack does not lead to a significant increase of the profile
energy's loss. Increasing β1k at large Mw2t accompanied by a reduction speed ratio of rotor wheel
(Fig. 3), and due to the increase of wave losses caused by the deviation of the angle of attack from
optimal.
Fig. 3 Speed ratio of rotor wheels (Mw2t=2.82)
The increase in profile loss is particularly noticeable at positive angles of attack. This is due to
the fact that flow separation at the entrance region of the convex surface causes loss stronger than
the separation of the flow at the inlet portion of the concave surface of the blade. This is due to an
increase in pressure in the site with the greatest curvature of the channel when rotor wheel flow
separation on the convex side of the blade. This can cause a "locking" of the channel of rotor wheel,
thereby reducing the efficiency of the turbine stage.
As a result of optimization calculations were set the maximum possible value of rotor wheel's
speed ratio (Table 2).
From Table 2 that the highest possible efficiency is obtained from the RW-1. In RW- speed ratio
is less than 2, and RW-3 - least. In general, the maximum efficiency of model RW is at high Mach
numbers, due to narrowing-expanding form of the rotor wheel's flow channel, designed to operate at
high supersonic speeds.
The results of optimization calculations angle of attack investigated working wheels are shown
(Table 3).
From Table. 3 follows that at low angles of attack, the energy losses in the flow part of rotor
wheel reduced. It was found that at constructive input rotor wheel's low angles the angle of attack
should be as negative. With increasing constructive input angle of rotor wheel negative angle of
attack should be less.
Table 2 Results of the optimization model rotor wheels
Values
Rotor wheels
β1
Mw2t
RW -1
RW -2
RW -3
18.68
18.76
19.73
2.82
2.82
2.42
ψ
0.92
0.87
0.79
Table 3 Optimal angles of attack for the investigated rotor wheels
Values
Rotor wheels
RW-1
RW-2
RW-3
i
ψ
-10.55
-7.56
-5.63
0.92
0.87
0.79
This phenomenon can be explained by possible separation in the area of greatest curvature of the
channel, which will arise in rotor wheel channels with the largest angle of rotation. To exclude the
separation in the flow optimum the angle of attack for him with the greater curvature channels
should be maximum negative, with a decrease in the curvature of the channel - less negative.
Выводы
The study revealed that there exists an optimal value of the angle of attack, deviation from which
leads to increase of losses in the rotor wheel.
It is established that at low Mw2t the angle of attack significantly affects the speed ratio of rotor
wheel, but with increasing influence Mw2t β1k increases. This is due to the fact that the subsonic
flow value deviation from the optimum angle of attack does not lead to a significant increase in the
profile loss of energy.
Was established that the increase in profile loss is particularly noticeable when positive angles of
attack. This is due to the fact that flow separation at the entrance region of the convex surface
causes loss stronger than the separation of the flow at the inlet portion of the concave surface of the
blade.
It was found that rotor wheels with a large angle of rotation necessary to design with flow part at
negative angles of attack, and the greater the rotor wheel's angle of rotation, the smaller should be
the angle of attack.
In [7, 8] revealed the effect of changing the operating mode of the turbine on the degree of
reactivity, which influences the gas-dynamic characteristics of the microturbine stage. This should
be considered in the design of microturbine.
To improve the efficiency of microturbines is planned to conduct experimental research on the
stand for a single nozzle [9, 10], as well as involve in research methods used in other fields of
science, in particular - medicine [11].
For transfer of research results to other dimensions of rotor wheel under different conditions,
consider the possibility of transferring the test results on the full-scale model [12].
This work was supported by the Far Eastern Federal University, project № 14-08-02-23_и.
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