DE7411399U - Axial turbine - Google Patents

Description

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BBC Aktiengesellschaft Brown, Boveri & Cie., Baden (Switzerland)

Axial turbine

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The invention relates to a turbine of the axial type with at least one consisting of a guide grille and a running grille Stage, for example a single-stage turbocharger turbine, through which a compressible working medium flows

Of eminent importance for the economic interpretation of a Turbine is the choice of the axial component and the optimal distribution of the tangential components of the flow velocity in the. Shovel grilles.

The axial component of the flow can be determined by taking into account an optimization method known in turbomachinery construction (Buoh von Dzung "Flow Research On Blading", Verlag Elsevier

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Publishing Company, Amsterdam, 1970, pp. 26-28), the so-called Mehldahl criterion M _T , which is a function of (k, -0, u) where

k takes into account the lattice geometry and the strength properties of the blade material,

Ό the ratio of the axial flow velocity to the peripheral speed is,

u is the ratio of the tangential change in speed 4c when passing through the blade grille to the circumferential speed u is.

u is referred to as the working number and is decisive for the Work output of a turbine stage.

If the turbine stage of an exhaust gas turbocharger is considered, then
it is assumed that the maximum circumferential speed of the impeller is given, among other things, by the reduced material strength due to the high exhaust gas temperatures. There are now two options for increasing the workload in one stage:

a) The tangential © inflow component to the impeller can be in
Direction of rotation can be enlarged.

b) The tangential outflow component from the impeller can

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can be increased in the direction opposite to the rotation.

The first measure increases the flow losses in particular in the diffuser, the second mainly increases the outlet losses from the turbine.

These exit losses are based in particular on only one partial usability of the velocity energy present at the exit of an impeller. Is at a multi-level Turbine the considered impeller is not the last, so the energy that is not converted into mechanical work becomes the greatest Part processed in the next stage.

However, if the impeller under consideration is the last C in the single-stage Turbocharger turbine is the ultimate impeller), then the energy present at its exit becomes at least that of the tan / jentialen speed component, only partly in a.ti diffuser exploited. Most of the time there is not enough space for one good diffuser, so that you often have to accept a complete loss of the exit energy. An additional increase in this Losses due to measure b) are therefore extremely unfavorable.

The invention is based on the object of utilizing the exit energy from the last running grate of a turbine.

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The aforementioned object is achieved according to the invention by that downstream from the last playpen and in front of the turbine outlet nozzle an additional axial vane grid is arranged.

With a certain available working gradient, this arrangement results in a better turbine efficiency achieved or given the opportunity to achieve a greater turnover while maintaining the same level of efficiency.

The additional blade grille is expediently a guide grille. This provides a means at hand for a given turbine of the advantageous characteristics mentioned above (Efficiency, workload) with only low constructive Achieve change.

In a preferred embodiment, the vane grille is a retardation grille i.e. a diffuser vaning, with which a reduction of the tangential component of the exit speed down to zero is made possible and an advantageous one Pressure increase at the turbine outlet can be achieved.

It is also advantageous if the blade grille is made of twisted Blades is formed, the cross-sections of which have different profile shapes over the length of the blade. This measure

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made possible by the precise adjustment of the inflow and outflow sections of the blades to the given flow conditions a low-loss crossing of the blades and a favorable one spatial velocity profile at the blading outlet.

Since you are fairly free in the design of the blade grille, it is particularly advantageous to have the last impeller with untwisted To provide shovels. Compared to the expensive production of the previous twisted last rotor blades requires one such a measure significantly lower manufacturing costs.

It is advisable to have the turbine with a larger work factor operate as 1.5. Turbine blading, which normally work at working figures of around 1, has a low Exit energy, which, however, increases as the work factor increases, so that the degrees of efficiency drop sharply.

Becomes the tangential component of the relative flow velocity at the exit from the playpen at least 20% larger than the corresponding component Turbines without an additional vane grille, this has the advantage that the inflow component is used to maintain the work rate to the playpen entrance can be reduced by the same amount, whereby the flow losses in the guide

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grid can be reduced.

An exemplary embodiment of the invention is shown in simplified form in the drawing.

The same elements are given the same reference numbers in all figures Mistake.

Show it:

1 shows a longitudinal section of an exhaust gas turbocharger,

Fig. 2 is a prior art blade plan as a development of a cylinder section through the Flow channel,

3 shows a vector diagram of the flow velocities corresponding to the blade diagram according to FIG. 2,

U, as an embodiment of the invention, shows a blade plan as a development of a cylinder section the flow channel according to FIG. 1,

FIG. 5 a corresponding to the blade plan according to FIG Vector diagram of flow velocities,

6 shows a state diagram of enthalpy-entropy,

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7 shows the relationship between the efficiency and the work factor.

1 shows schematically the upper half of an exhaust gas turbocharger consisting of a compressor and a turbine. 1 is the wave which is stored at 2 and 3. H is the turbine disk, which carries a blade ring, the running grille 7. 6 is the inlet housing the turbine, 5 the guide grille consisting of a blade ring, 8 the turbine outlet nozzle, 9 is that Compressor wheel, 10 the compressor housing provided with an outlet spiral. All of the items listed so far are Components of a known turbocharger, the mode of operation of which is briefly described:

Outside air that is sucked in is compressed in the compressor and supplied as combustion air to an internal combustion engine (not shown). The higher air density increases the performance of the engine; it also increases its efficiency and improves combustion. The combustion gases of the engine are after finished Work at high temperature and superatmospheric pressure is supplied to the exhaust gas turbine, which drives the compressor.

The turbine stage under consideration (5,7) is its axial flow velocity taking into account the end and game

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Losses as well as the strength properties of the blade material is determined, be for the very high work factor

u = 3 designed. The following tangential speeds c. prove to be optimal:

absolutely relative

Blade inlet + 2.6 'u 1.6' u Blade outlet - 0.4 'u -1.4 * u

Difference + 3.0 'u +3.0 * u

This difference Ac. corresponds to the tangential change in speed.

u is the circumferential speed, which, as already noted in relation to the prior art, cannot be increased without further ado unless one reaches for nobler, much more expensive and difficult to process materials.

In the present example, for the sake of simplicity, the flow through the blading is assumed to be purely axial, that is, the circumferential speed at the blade inlet and outlet is the same and is set to u - 1, which in the following consideration is the coefficient of performance or coefficient of performance

Ac _t

- · and the shovel work a -Ac * u have the same value

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accept.

For a better understanding of the invention, the following are known Connections briefly repeated

a) The loss ratio - = f C u), where i is the total loss * per unit of mass and a mean the shovel work, increases inevitably with increasing work factor. this means that turbine stages for high stage gradients cannot be made just as well by suitable shaping can as for deep step slopes,

b) The degree of reaction R - f (u) of an optimally designed single-stage turbine for the work factor u - 1 is 0.5 (the prerequisite is the same glide ratio for guide and rotor blades) and decreases with increasing work factor.

c) When studying the diffusers and outlet casings of turbines one comes across the problem, the spatial velocity profile at the outlet of the blading, i.e. the tangential and, if possible, also the axial outlet speed must be brought close to the optimal values. The shovels of the last playpen you will twist it, and thus optimize not only in the middle cut, but over the entire length of the shovel.

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d) For working numbers u greater than 1, the optimal exit speed is opposite to the rotation, i.e. a purely axial exit is not optimal because it does secures the lowest leakage losses, but causes larger grid losses.

After the above it is understandable that the blading Due to the given circumferential speed and the high step gradient, a very low response, in the present case Case even shows reaction = 0. This means, among other things, large deflection angles and therefore unfavorable Blade design, which leads to "spoiled" efficiencies.

Nevertheless, such single-stage machines are operated with high loads because there is often not enough space for a two-stage machine and the latter is also much more expensive and is more complicated.

In the blade plan according to FIG. 2, a cylindrical section is developed through blades belonging to the prior art. The mode of operation of the blades of the guide grille 5 with exit angle o (and the blades of the running grate 7 with exit angle β ₀ can be seen from the speed triangles in FIG. 3.

The entry triangle shows the speed oc> on the that

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Working fluid is brought by the pressure reduction in the guide grille at its exit and whose direction is determined by the angle 'JL . oc, is also the absolute speed of entry into the playpen. The relative inflow speed into the playpen ow is obtained by vectorial addition of ocT with the circumferential speed of the playpen u = w ^ c.

Since in the playpen 7 no further pressure decrease occurs in the outlet triangle, the relative exit velocity ow ₂ from the playpen, the direction by the angle β ~ ^be ~ true is ow due to the wall friction by only slightly smaller than the relative entry speed ,. The vectorial addition with the circumferential speed u = ^W ₂ ^C 2 provides the absolute exit velocity oc_, which for the reasons mentioned above is not purely axial and whose energy dissipates into the exit housing. The sections c-jC ₂ ^ents P ^r i ^{cnt de} ** tangential speed change 4c. and, according to the Euliii "equation, leads to the shovel work a = .4c." and

To determine the efficiency of the Ent is ^r> pannungsvorgang in the state diagram of FIG. 6 considered, which carries as the abscissa and as the ordinate the entropy s enthalpy h. p _E and p. are the static inlet and outlet pressures.

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The distance nb represents the speed energy of the working medium flowing into the guide grille 5. The relaxation bc from ρ «to p. is made entirely in the guide grille 5, whereupon the work is transferred to the playpen 7 in ce. The kinetic energy ef the absolute at the playpen outlet Exit speed (oc "in Fig. 3) is no longer used, swirls and finally comes to rest, which process is represented by fg. The vertical distance ng corresponds to that delivered to the rotor of the turbine Work per unit of mass, i.e. the shovel work a.

Since in the present case a direct use of the kinetic energy ef is not possible in a further stage, is the efficiency i ^ is defined in such a way that the blade work a it is compared with the greatest work that has to be done in adiabatic relaxation from state b to pressure p. at the given Inflow energy nb could be obtained at all. This greatest gain in work would be achievable if there was no friction there would be and, moreover, the work equipment with negligibly small kinetic energy, i.e. with ef = 0, flowed out of the playpen and is the isentropic enthalpy gradient nk

therefore η = ---
* ■ nk

According to the task, the exit energy ef

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can be used from the playpen 7 in such a way that the required shovel work a with a lower enthalpy gradient, thus can be generated with better efficiency η.

The blade grid 11 according to the invention in FIG. 1 is a guide grid attached in the blade carrier 12.

In FIG. 4, only this blade grille 11 is considered for the time being. It is a retardation grating that works as a diffuser with an exit angle of ^ ₃ .

In Fig. 5 its mode of operation can be seen. The absolute exit speed o "c _o is directed almost axially due to a deflection in the grating and is strongly delayed due to the diffuser effect.

The change in state from the inlet to the outlet pressure is shown in FIG. 6. The known diffuser mode of operation causes an expansion in the guide grille to a pressure below the static outlet pressure _{- p A.} From c 'to d ^{1 the work} is transferred to the playpen 7 and in d'e ¹ the compression in the blade grille 11, the increase in entropy from d' to e ^{1 being} due to friction losses in the latter. The kinetic energy e'f 'present at the outlet of the playpen

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• ·

the exit speed ° "c ~ is again taken into account as a loss, whereby the state point g 'is reached. By plotting the blade work a, the point n ^T corresponding to the required total enthalpy is reached 'is equal to nb, which gives the static inlet pressure pi, and that the expansion b'c ¹ takes place with the same increase in entropy as bc.

The comparison with the prior art blading now shows that with a means that is small in and for you - as such, it can be fixed and easy Shovel grids 11 to be arranged are designated -, a shovel work of the same size a with a smaller gradient n'k can be achieved, which of course is expressed in a better degree of efficiency:

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n'k

Another embodiment of the invention examined below is appropriate. Still maintaining the same turbine work a and despite the increase in loss mentioned at the beginning when transitioning to higher degrees of reaction if u is high, the reaction of the turbine stage (5,7) is increased i.e. part of the pressure gradient is transferred to the

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wheel acting impeller 7 displaced. This from deliberation out «that« now that the exit energy is made usable by the downstream delay grid, the absolute exit speed can be increased.

Since the tangential speed change ic _t remains the same and the inflow speed of the working medium to the guide grille 5 is purely axial as before, this results in a weaker deflection and thus lower losses in the guide grille 5.

In the blade plan of FIG. U, when compared with that of FIG. 2, the outflow angles *. £ are greater than "(. And / Ϊ) ¹ J less than / b". In order to make this clearer, the same profile shapes were selected in FIGS. 2 and U. In reality, of course, you will not simply use the same profiles, including angles of attack, but rather coordinate the blade shape of the guide and rotor blades with one another in accordance with the existing load and the deflection selected.

If the same exit speed o "c is sought,

This is due to the increased absolute exit speed from the playpenTo "^ 'a stronger diversion in the shovel grille 11. Particular care is required here, there

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one in the design of the scooping of the retardation grid has much less freedom than with the turbine stage, since the Currents are associated with an increase in pressure. Starting from the velocity triangle, the deflection properties of the sitter *, its losses and its flow behavior must be known.

The considerations made on the basis of the present example lead to the tangential component of the relative The exit speed from the last playpen 7 should be selected to be at least 20% greater than that of ow. of the Prior art example. The following tangential speeds c. are now considered optimal:

absolutely relative

Blade inlet + 2.2; u 1,2 u Blade exit - 0.8. u - 1.8 u Difference + 3.0.u + 3, Ou

The mode of operation is shown in the vector diagram of the speeds in FIG. o "c" is the exit velocity from the guide grille 5, which is reduced compared to öcT. Da a part of the relaxation is made in the playpen 7, is also the not shown axial component of o "c"

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smaller than that of oc .. This of course provided that the blade heights are the same (shown in Fig. 1). For the same reason, the relative exit ^{speed from the playpen "w 'f is} greater than the relative entry speed o" w "of the same. By vectorial addition of o" w "with the circumferential speed, one obtains the absolute exit speed from the playpen ο" cJj »which is delayed in the following blade grille 11 to O ⁷¹ OT.

In the state diagram, the expansion in the guide grille 5 to state c "is carried out with an increase in entropy that is less than in the previous examples. substantially lower pressure ρ 7. From d "to ^" there is an increase in pressure and a delay from "zrc ^ to o"c.; the kinetic energy e "f" resulting from the latter swirls to g ". With a the total inlet condition n "results and from this the static inlet pressure p" with n "b" = nb.

Result of the proposed measure: n "= - ^ - > n ¹ > η

n "k

With what has been stated so far, the purpose of the invention has been through

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Utilization of the kinetic energy at the playpen outlet To improve machine efficiency. Often the aim is not to increase efficiency, but rather to increase it Process expansion gradients without or with not too great a loss of efficiency.

This is the case, for example, when the compression is increased in a given internal combustion engine, be it to increase engine output, to save weight per unit of power, to improve efficiency or to combat emissions. The higher pressure ratio caused by the higher compression requires a higher circumferential speed of the compressor wheel in the exhaust gas turbocharger, which ^{can be done, for example, by increasing the wheel diameter (9 f} in FIG. 1). The increased drive power of the compressor wheel requires a higher expansion gradient of the engine exhaust gases in the turbine. With the arrangement according to the invention of the blade grille 11, what is required can be achieved. With the chosen definition of the degree of efficiency, the so-called isentropic degree of efficiency, a proof in the state diagram according to FIG. 6 is not easily possible, since the degree of efficiency changes its value on transition to other states (different enthalpy gradients and different powers).

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However, the relationship can be seen in the diagram in FIG. 7. The efficiency η is shown * as a function of the work factor u. 13 is the characteristic curve of the optimal single-stage turbine, 14 that of the so-called one-half-stage turbine according to the invention Turbine. If u = 2, it can be clearly seen that either with the same u the efficiency of the iy2-stage Turbine is better or that a greater gradient can be processed with the same efficiency.

The possibility of increasing the working gap offered in this way is particularly interesting in cases in which only individual exhaust gas turbochargers of a standard series require a higher compression, while the normal ones for the others 1-stage construction is sufficient. The adaptation is like Fig. 1 shows, quite simple and cheap, as both the external dimensions of the loader and most of the internal parts are preserved stay.

In what has been said so far, there has been general discussion of mean velocity values, which are those at the outlet cross-sections of the Considered grid replace the prevailing complicated velocity fields and usually relate to one refer to the mean reference diameter of the turbine. This consideration is only correct with weakly fanned blades, where the fanning is the ratio of the blade height

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applies to the reference diameter and the latter usually the Is the center circle of the annular channel through which the air flows. Various tangential velocity components arise along the blade height, which have a major influence on the outflow angle. At least with widely diversified blading the above considerations have to be repeated for different blade heights, what with regard to adaptation of the outlet conditions leads to twisted blades at the respective optimal values.

Since you have certain freedom in the design of the blade grille described, this is called a "made-to-measure grille" can shape, there is the possibility of the heavily twisted blades of the playpen 7 (especially with single-stage Machines) to twist less or even not at all.

This option can also be used, for example, with the large and heavily twisted end blades of steam turbines are made use of, since the invention is self-evident not limited to what is shown in the drawing, but also to the last multi-level stage Turbines can be used.

Of course, it can be in the case of the invention

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The shovel fence is also a playpen, its performance could for example be delivered via a counter-rotating shaft.

Claims (5)

S C h u t ζ a η s ρ r ü c h e 1.- Turbine of axial design with at least one stage consisting of a guide grille and a running grille, for example a single-stage turbocharger turbine through which a compressible working medium flows, characterized in that downstream from the last running grille C7) and an additional axial vane grille (11) is arranged in front of the turbine outlet nozzle (8). 2. Turbine according to claim 1, characterized in that the blade grille CIl) 'is a guide grille . 3. Turbine according to claim 1, characterized in that the blade grid (11) is a delay grid. f. Turbine according to one of Claims 1 to 3, characterized in that the blade grille (11) is made of twisted
There are blades, the cross sections of which have different profile shapes over the length of the blade. 5. Turbine according to claim 1, characterized in that when the blade grid (11) is arranged, the last impeller (7) has untwisted blades. 7411399 02/19/76