Liquid heaters and vaporizers – Circulation – Having restrictors and orifices
Reexamination Certificate
1999-11-01
2001-04-17
Ferensic, Denise L. (Department: 3749)
Liquid heaters and vaporizers
Circulation
Having restrictors and orifices
C138S037000, C138S039000
Reexamination Certificate
active
06216644
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a heat generator, into which heat generator a medium flows through a flow duct during operation, the flow duct having at least one discontinuous cross-sectional expansion in the direction of a main flow in such a way that at least one wall bounding the flow duct has a step extending substantially transversely to the main flow direction.
2. Discussion of Background
In combustion technology, it is frequently necessary to operate with widely varying flow velocities. Whereas, for reasons of flame stability, the flow velocity in the heat generators themselves is limited to quite low values, various reasons often make it necessary to provide the inlet flow to the heat generators with high velocities. Because of the demands made on the installation size, it is usually impossible to decelerate the inlet flow to a heat generator in a continuous manner. In consequence, sudden-expansion diffusers with discontinuous cross-sectional expansions are very frequently employed. Although these cause substantial losses in total pressure, they provide a very compact installation. In addition, reverse flows generated in sudden-expansion diffusers are quite desirable, particularly for flame stabilization in heat generators.
However, the vortex structures which occur in sudden-expansion diffusers can also involve extremely damaging consequences under certain circumstances, particularly where the sudden-expansion diffuser is designed simply as a discontinuous cross-sectional expansion of a flow duct. In this case, a step extending substantially transversely to the main flow exists in the flow duct and this step acts as a separation edge for the flow. In the case of a sufficiently large velocity of the incident flow to this edge, periodic separation vortices form which extend parallel to this edge. The coherent vortex structures thus occurring can propagate substantially undamped in the flow direction. Should these periodic vortex structures reach the heat supply location—generally the flame—the periodic pressure fluctuations by which the vortices are manifested are amplified because of the resulting large increase in volume. As a result, thermo-acoustic vibrations of high amplitude occur and these concentrate a high level of vibration energy within a narrow frequency band and have potential for permanently damaging the structure of a heat generator.
It is precisely in modern gas turbine technology—where high flow velocities, high heat release rates and high pressures are present locally—that these thermo-acoustic vibrations play a decisive roll with respect to the reliable operation of the combustion chambers. Mastering them is therefore an essential precondition for the manufacture of gas turbine power stations and combined power stations.
SUMMARY OF THE INVENTION
Accordingly, one object of the invention is to prevent the occurrence of high pressure fluctuations in a narrow frequency range, as discussed above, in a heat generator, into which heat generator a medium flows through a flow duct during operation, the flow duct having at least one discontinuous cross-sectional expansion in the direction of a main flow in such a way that at least one wall bounding the flow duct has a step extending substantially transversely to the main flow direction.
In accordance with the invention, this is achieved by an arrangement wherein a number of vortex-generating elements are arranged upstream of the step, the vortex-generating elements being arranged on a line extending transversely to the main flow direction at a distance from one another with a lateral pitch dimension, and wherein, in order to interfere with coherent periodic separation vortices whose separation frequency is located below a limiting frequency, the lateral pitch dimension is smaller than half the wavelength which is associated with the limiting frequency in the main flow downstream of the step, so that the following condition is satisfied
t
≤
u
c
2
f
G
in which relationship t represents the lateral pitch dimension of the arrangement of the vortex-generating elements, u
c
represents the velocity of the main flow downstream of the step and f
G
represents the limiting frequency. Because of the perturbations which these elements introduce into the incident flow, there is no homogeneous flow field at the step so that, at the step, no more separation vortices which have a constant phase position over the whole of the transverse extent of the step can appear. In consequence, gradients in the flow field are induced transversely to the main flow direction so that, on the one hand, the separation vortex is dissipated substantially more rapidly; in addition, in-phase separation vortices no longer reach the flame so that the occurrence of the damaging thermo-acoustic vibrations described at the beginning is effectively prevented.
In addition, it is advantageous for the vortex-generating elements to be arranged no further than 20% of the lateral pitch dimension upstream of the step so that these vortices are not themselves dissipated before reaching the step.
In addition, the height of the vortex-generating elements should not be more than 20% of the pitch dimension so that no excessive pressure losses are caused; the introduction of vortices into the boundary layer is itself sufficient to achieve the desired effect.
It is also advantageous to offset the vortex-generating elements relative to one another by a small distance in the flow direction in order to displace the phase of the vortices relative to one another and further improve the damping.
A preferred geometry of the vortex generators is described in EP 0 745 809 A1, this publication representing a constituent part which is integrated into the present description.
REFERENCES:
patent: 3974646 (1976-08-01), Markowski et al.
patent: 4662818 (1987-05-01), Hopfensperger et al.
patent: 5133519 (1992-07-01), Falco
patent: 5402964 (1995-04-01), Wygnanski
patent: 5803602 (1998-09-01), Eroglu et al.
patent: 3328973A1 (1985-02-01), None
patent: 0321379A2 (1989-06-01), None
patent: 0410924A2 (1991-01-01), None
patent: 0745809A1 (1996-12-01), None
patent: 1370370 (1988-01-01), None
“Susceptibility of Turbulent Separating Flow Downstream of a Step to Acoustic Perturbations”, Bardakhanov, et al., Fluid Mechanics—Soviet Research, vol. 15, No. 4, Jul.-Aug. 1986, pp. 9-14.
Eroglu Adnan
Joos Franz
Keller Jakob J.
Keller Vera
Paikert Bettina
ABB Alstrom Power (Schweiz) AG
Burns Doane Swecker & Mathis L.L.P.
Ferensic Denise L.
Keller Vera
Wilson Gregory A.
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