CROSS REFERENCE TO RELATED APPLICATIONS
FIELD OF INVENTION
This application is the US National Stage of International Application No. PCT/EP2012/065121 filed Aug. 2, 2012, and claims the benefit thereof. The International Application claims the benefit of European Application No. EP11179513 filed Aug 31, 2011. All of the applications are incorporated by reference herein in their entirety.
- BACKGROUND OF INVENTION
The invention relates to a mixing unit for mixing a flow medium with a cooling medium, having a pipe conduit section, to which a mixing section is coupled fluidically, the mixing section having a plurality of Laval nozzles through which the flow medium can flow, there being formed in the Laval nozzles injection ducts through which the cooling medium flows in such a way that mixing of the flow medium with the cooling medium takes place.
In steam power plants, steam is generated in a steam generator which converts the thermal energy of the steam into rotational energy in a turbo set coupled fluidically to the steam generator. The rotational energy is finally converted into electrical energy. As long as the steam power plant operates continuously and the load on the electrical generator is comparatively constant, the thermal dynamic conditions are comparatively constant over time.
There are situations, however, in which the steam power plant has to be adapted to rapidly changing load situations. It may be, for example, that an incident occurs and the generator suddenly has to be separated from the network. It may also happen that the steam power plant has to change over from full load to part load unpredictably. Such load changes are a challenge to the technology for regulating the overall steam power plant. One possibility for following or counteracting rapidly changing load situations is to route the steam generated by the steam generator, and flowing directly to the high-pressure subturbine during continuous operation or full-load operation, directly to the condenser via a bypass station. In this bypass station, devices are provided, which mix the highly heated steam with water, in order thereby to change the thermodynamic conditions of the steam. This water is injected into the steam. According to the prior art, this takes place in a bypass station in which is arranged a Laval nozzle having injection ducts through which water is sprayed into the steam.
It has been shown, however, that, because of this, the noise emission is comparatively high. Moreover, it has been shown that the temperature distribution is not sufficiently homogeneous, thus leading to a non-optimal operating behavior under part load.
- SUMMARY OF INVENTION
Bypass stations used nowadays are composed essentially of a bypass valve and of the bypass steam infeed. The bypass steam infeed comprises a diaphragm, a water injection device and a mixing pipe. When the steam power plant is started up or after a trip, steam occurring in the steam turbines is cooled via the bypass station by the injection of water and is introduced directly into the condenser.
An object herein is to allow better mixing of the water with the steam during operation and at the same time to reduce noise emission.
This object is achieved by a mixing unit for mixing a flow medium with a cooling medium, comprising a pipe conduit section, to which a mixing section is coupled fluidically, the mixing section comprising a plurality of Laval nozzles, through which the flow medium can flow, there being formed in the Laval nozzles injection ducts through which the cooling medium flows in such a way that mixing of the flow medium with the cooling medium takes place, Laval nozzles adjacent to one another being arranged so as to be offset in relation to one another in the direction of flow of the flow medium.
An aspect of the invention thus pursues the path of using a plurality of diaphragm orifices, contrary to the existing concept in which the steam flows through only a single diaphragm orifice. The disadvantage arising from the use of a single diaphragm orifice is that mixing is not optimal, particularly at the margins of the mixing section. Better mixing and reduced noise emission are achieved, using a plurality of diaphragm orifices.
An aspect of the invention is that two Laval nozzles adjacent to one another are arranged so as to be offset in relation to one another in the direction of flow. To cool the steam, in bypass mode water is injected into the bypass steam line. In order to achieve good atomization of the water and therefore effective cooling, the steam, before being admixed, is routed through a Laval nozzle or through a perforated diaphragm, with the result that the flow velocity rises sharply. The high relative velocity between the steam and the water drops leads to good atomization, but has the disadvantage that the water drops do not reach into the core of the steam stream and therefore the inner part or the inner core of the steam stream is not sufficiently cooled. The Laval nozzles are in this case displaced axially in the direction of flow of the flow medium. When a flow passes through a Laval nozzle, a sound wave is generated. The sound wave arises behind a Laval nozzle. If Laval nozzles are additionally displaced axially with respect to one another by a length, so that sound wave peaks and sound wave troughs of different Laval nozzles adjacent to one another cancel each other out, the overall sound emission is markedly reduced.
A feature here, therefore, is that the individual Laval nozzles which are arranged adjacently to one another are mutually displaced axially.
The Laval nozzles are coupled to a displacement device, displacement of the Laval nozzles being possible during operation. It is thus proposed to provide active displacement which can take place electrically or hydraulically or by other means, so that the Laval nozzle planes can be displaced with respect to one another in such a way that different frequency bands can be influenced during operation. Noise emission can thus be actively reduced in different operating states.
In a first advantageous development, the Laval nozzles are designed identically to one another. This leads to better computability of the sound wave troughs and sound wave peaks, and it can thus be predetermined more effectively by computations from the sound emission how far axial displacement has to take place.
BRIEF DESCRIPTION OF THE DRAWINGS
In a further advantageous development, the Laval nozzles are coupled to a displacement device, displacement of the Laval nozzles being possible during operation. It is thus proposed to provide active displacement which can take place electrically or hydraulically or by other means, so that the Laval nozzle planes can be displaced with respect to one another in such a way that different frequency bands can be influenced during operation. Noise emission can thus be actively reduced in different operating states.
The invention, then, is explained in more detail by means of an exemplary embodiment. In the drawings:
FIG. 1 shows a cross-sectional view of a conventional mixing unit;
FIG. 2 shows a cross-sectional view of a mixing unit according to the invention;
DETAILED DESCRIPTION OF INVENTION
FIG. 3 shows a cross-sectional view of part of the mixing unit.
FIG. 1 shows a mixing unit 1 according to the prior art. Such a mixing unit 1 is characterized by a pipe conduit section 2 in which a flow medium 3 flows in the direction of a mixing section 4. In this mixing section 4, a Laval nozzle 5 is arranged, in which the flow medium is accelerated. Arranged in the Laval nozzle 5 are injection ducts 6 through which a cooling medium, such as water, flows. The cooling medium is mixed with the flow medium 3 in a pipe section 7 which is connected fluidically to the mixing unit 4.
FIG. 2 shows an illustration according to the invention of the mixing unit 1. The difference from the mixing unit 1 according to FIG. 1 is that, in the mixing section 4, a plurality of Laval nozzles 5 a, 5 b, 5 c are arranged, through which the flow medium 3 flows and in each of which is formed an injection duct 6, by means of which water is mixed with the flow medium. Furthermore, the difference between the mixing unit 1 of FIG. 1 and that of FIG. 2 is that the Laval nozzles 5 a, 5 b, 5 c are displaced with respect to one another in the flow medium direction 8, which may also be designated as the axial direction. As a result of this displacement, the sound wave troughs, which coincide with sound wave peaks of the adjacent Laval nozzles, are cancelled. An overall reduction in sound emission is thereby achieved. Finally, the pipe section 7 is connected to a condenser, not illustrated in any more detail.
The axial displacement of the Laval nozzles 5 a, 5 b, 5 c with respect to one another may take place by active displacement by means of electrical or hydraulic forces. This may take place during operation where different operating states arise. Different frequency bands can thereby be influenced, thus reducing noise emission, overall, even during operation.
The frequency band can be measured during operation and the diaphragms can then be displaced with respect to one another such that noise emission becomes minimal. The most favorable axial positions can be determined beforehand for each load point during the commissioning of the plant, and these can then simply be input during operation, without the frequency spectrum having to be measured actively.
FIG. 3 shows by way of example an illustration of the displacement of the Laval nozzles 5 a and 5 b. The Laval nozzle 5 a is displaced with respect to the Laval nozzle 5 b by the length L. With a sound frequency of 1000 Hz, this would give a sound velocity of approximately 500 m/s, thus resulting in the required length of 0.5 m. This length may be set statically in the first approximation or, as described further above, may be obtained, even during operation, by active displacement.