The gas-dynamic calcualation of the axial turbine stage

The general law of circulation change across blade height. Determination of the axial turbine stages geometrical dimensions. Turbine stage calculation on the middle radius. Cinematic parameters determination on different turbine stage radiuses.
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MINISTRY OF EDUCATION AND SCIENCE OF UKRAINE

National Aviation University

The gas-dynamic calculation of THE axial turbine stage

Methodical guide for performing the course paper for students specialty 8.100106 “ Manufacturing, maintenance and repair of aircraft and engines”

Compiled by I.I. Gvozdetsky , V.V. Kharyton, S.I.Tkachenko

KYIV 2007

Contents

Introduction

The general law of circulation change across blade height

Determination of the turbine stage geometrical dimensions

Turbine stage calculation on the middle radius

Cinematic parameters determination on different turbine stage radiuses

Appendix 1 The example of gas-dynamic calculation of the axial turbine stage

Introduction

The turbine serves to provide the power to drive the compressor and accessories. in a case of turboprop or turboshaft engine the turbine, in addition, provides the power to rotate propeller or rotor. It does this by extracting energy from the hot gases released from the combustion system and expanding them to a lower pressure and temperature. These processes take place when hot gases flow along specially shaped passages created by two rows of airfoils: stator vanes and rotor blades. These two rows of airfoils form a turbine stage. To produce the driving torque , the turbine unit may consist of one or several stages. The useful torque, created by turbine is transmitted to compressor by turbine shaft. Three stage turbine unite assembly is shown in fig.1.

Structurally this turbine unit can be divided into two main parts (fig.2): all rotating components (three bladed disks joined with shaft) are named turbine rotor, and all unmovable components (three turbine nozzle diaphragms and turbine casing) create the turbine stator.

The main objectives of turbine stage gas-dynamic calculation are determination of stage geometrical dimensions, gas cinematic parameters and speed plans construction. In course paper cinematic parameters are determined in three sections: sleeve, middle and peripheral.



Stage scheme, sections designation and diametrical dimensions are shown in figure 3.

Fig. 3. Main geometrical dimensions of the turbine stage

The initial data for turbine stage calculation are taken from gas-dynamic calculation of the designed engine. They are:

· full gas pressure and stagnated gas temperature at the entry to the turbine stage;

· mass gas flow rate ;

· turbine stage work ;

· circumferential velocity on the middle radius of the working wheel ;

· jet velocity of gas at the exit from the nozzle diaphragm ;

· reduced velocity at the nozzle diaphragm exit ;

· angle of the stream output from the nozzle diaphragm ;

· pressure recovery coefficient in the nozzle diaphragm ;

· external, middle and sleeve diameters at the entry to the working wheel ; ; .

All of these parameters are chosen for the first turbine stage of the designed engine.

T

The general law of circulation change across blade height

The gas work, the reactivity rate, the gas velocity, Mach numbers, efficiency, blade incidence angles and other parameters depend on law of circulation change across stage working wheel radius. Different laws of circulation change across radius are expressed by general equation

, (1)

where ; m - index rate.

If m=1 law of circulation constancy is implemented. This law of profiling is used for comparatively short blades (), because in this case reactivity rate across blade height is changed very essentially. And using long blades the reactivity rate can be negative near sleeve.

For longer blades profiling with index rate m<1 is applied. Particularly, for law of profiling with constant angle of the stream output from nozzle diaphragm is realized.

To obtain small m angle is increased. It causes increase of the axial gas velocity, which can reach local sonic speed at exit from working wheel. It will mean “choking” of the turbine stage. As a result, it is no point in increasing of angle more then on 20-25 at first stages. At these values negative reactivity rate can occur near blade root, especially at high values of loading coefficient.

As a result of this, profiling on the base of equation (1) is common, because it gives possibility to avoid negative values of the reactivity rate near the blade root by matching of rate index m at the all values.

Determination of the turbine stage geometrical dimensions

Geometrical dimensions at the entry to the working wheel are determined in the gas-dynamic calculation of the designed engine. At first area at the exit from the nozzle diaphragm is calculated

,

where and are stagnated temperature and full pressure of the flow at the exit from the nozzle diaphragm; ; ; mg - constant magnitude, which can be computed by the formula

for kg=1,33 and Rg=288 J/(kgK) we will have mg=0,0396 (kgK)/J.

Relative density can be determined from tables of gas-dynamic functions using value of the reduced velocity or by the formula

.

At he given working wheel middle diameter other geometrical dimensions are computed by the following formulas:

; ; .

At he given relative sleeve diameter geometrical dimensions in the considered section are computed by the formulas:

Relative sleeve diameter for first stages is within the limits of , and for last stages .

Calculating first turbine stages the nozzle diaphragm is profiled to provide turbine blending with combustion chamber. In this case meridional profile of the nozzle diaphragm can be of arbitrary shape with the obligatory observance of sections areas.

To calculate section area at the exit from the stage (behind working wheel) it is necessary to compute gas parameters and behind calculated stage.

Stagnated gas temperature is determined from the energy equation:

Full gas pressure behind stage is calculated by the formula

where - stage efficiency.

Axial component of the jet velocity at the exit from the working wheel is assumed on 20-80 m/s more then gas velocity at the entry to the working wheel, i.e.

; m/s,

where .

Section area at the exit from the working wheel is determined from following expression:

,

where is computed by the value of

.

Further for chosen profiling law determine main dimensions at the exit from turbine stage in a similar manner as have been done turbine stage entrance.

On the base of computed diameters values draw turbine stage scheme.

Turbine stage calculation on the middle radius

At given circumferential velocity value on the middle radius of the inlet edge calculate circumferential velocity behind working wheel from the relation

Performing approximate calculations it is possible to suppose.

Stage loading coefficient on the middle radius is determined by the formula

For the first turbine stage .

Gas jet velocity at the exit from the nozzle diaphragm is determined from the equation

and reduced velocity - by the formula

Amount of must not exceed 1,25. If it is possible to decrease it by increasing the circumferential velocity , decreasing of the angle of the stream output from nozzle diaphragm , increasing of the stage work, applying of airtwist at the exit from the working wheel in the opposite direction of rotation. If first three methods can not be used, can be obtained from the Euler's equation, have assigned:

,

where .

Value of must not exceed, otherwise it is necessary to decrease to meet this requirement.

As long as all of calculations are carried out for middle radius in what follows we will withdraw subscript “md”. Following formulas flow parameters calculation are legible for every blade section.

Circumferential components of the jet velocity at the entry to the working wheel and behind turbine stage, and parameters , and are calculated by the formulas:

; ; ;

.

Axial component of the jet velocity and parameter at the entry to the working wheel are calculated by the formulas:

; .

Axial component of the relative velocity and the relative velocity at the entry to the working wheel are calculated by the formulas:

; .