Unsteady Interactions Between Axial Turbine and Nonaxisymmetric Exhaust Hood Under Different Operational Conditions

Journal of Engineering for Gas Turbines and Power

2013

This paper summarizes the findings from research studies carried out over the last 30 years, to better understand the flows in steam turbine low pressure exhaust hoods and diffusers. The work aims to highlight the areas where further study is still required. A detailed description of the flow structure is outlined and the influence of the last turbine stage and the hood geometry on loss coefficient is explored. At present, the key challenge faced is numerically modeling the three-dimensional, unsteady, transonic, wet steam exhaust hood flow given the impractically high computational power requirement. Multiple calculation simplifications to reduce the computational demand have been successfully verified with experimental data, but at present there is no 'best-practice' approach to reduce the computational time for routine design exercises. This paper highlights the importance of coupling the exhaust hood to the last stage steam turbine blades to capture the interaction; ensuring the total pressure and swirl angle profiles, along with the tip leakage jet are accurately applied to the diffuser inlet. The nonaxial symmetry of the exhaust hood means it is also important to model the full blade annulus. More studies have emerged modeling the wet steam and unsteady flow effects, but more work is required in this area to fully understand the impact on the flow structure.

Section: Numerical and Experimental Methodologiessupporting

confidence: 91%

Section: Numerical and Experimental Methodologiesmentioning

confidence: 99%

A Literature Review of Low Pressure Steam Turbine Exhaust Hood and Diffuser Studies

Burton¹,

Ingram²,

Journal of Engineering for Gas Turbines and Power

2013

“…(1)), which in ideal conditions would be C p ¼ 1 meaning all of the kinetic energy exiting the LSB is recovered into usable power. However, in reality the performance of LP exhaust designs typically have C p values ranging from À0.25 to þ0.5 [1,2].…”

Section: Introductionmentioning

confidence: 99%

“…This is typically done by modeling the full turbine annulus in either a steady frozen rotor calculation or a full unsteady sliding mesh approach [2,5]. The frozen rotor approach has become the most widely adopted method [6][7][8] in recent years for calculating exhaust hood flows.…”

Section: Introductionmentioning

confidence: 99%

Efficient Methods for Predicting Low Pressure Steam Turbine Exhaust Hood and Diffuser Flows at Design and Off-Design Conditions

Burton

Ingram

Journal of Engineering for Gas Turbines and Power

2015

The exhaust hood of a steam turbine is an important area of turbomachinery research as its performance strongly influences the power output of the last stage blades (LSB). This paper compares results from 3D simulations using a novel application of the nonlinear harmonic (NLH) method with more computationally demanding predictions obtained using frozen rotor techniques. Accurate simulation of exhausts is only achieved when simulations of LSB are coupled to the exhaust hood to capture the strong interaction. One such method is the NLH method. In this paper, the NLH approach is compared against the current standard for capturing the inlet circumferential asymmetry, the frozen rotor approach. The NLH method is shown to predict a similar exhaust hood static pressure recovery and flow asymmetry compared with the frozen rotor approach using less than half the memory requirement of a full annulus calculation. A second option for reducing the computational demand of the full annulus frozen rotor method is explored where a single stator passage is modeled coupled to the full annulus rotor by a mixing plane. Provided the stage is choked, this was shown to produce very similar results to the full annulus frozen rotor approach but with a computational demand similar to that of the NLH method. In terms of industrial practice, the results show that for a typical well designed exhaust hood at nominal load conditions, the pressure recovery predicted by all methods (including those which do not account for circumferential uniformities) is similar. However, this is not the case at off-design conditions where more complex interfacing methods are required to capture circumferential asymmetry.

“…In the majority of cases this value is low, ranging from -0.25 to +0.5 [1,2] which at the higher end of this range gives a lower turbine exit pressure than that in the condenser and subsequently a higher power output.…”

Section: Introductionmentioning

confidence: 99%

The influence of condenser pressure variation and tip leakage on low pressure steam turbine exhaust hood flows

Burton

Ingram

Proceedings of the Institution of Mechanical Engineers, Part A:

2014

Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. This paper aims to highlight the importance of the accurate computational modelling of both the inlet and outlet exhaust hood boundary conditions. The computations presented are calculated using the public domain LP exhaust diffuser test case proposed by Burton in 2012. The original test case did not include the effect of tip leakage on diffuser flows, this paper describes the inclusion of tip leakage and the results are shown to be in-line with the outputs produced by other authors. The key advance in this paper is that calculations were conducted with a representative condenser pressure gradient caused by the temperature variation inside the condenser tube nest. It is shown that accurately modelling the exit boundary calculation has a large influence on the flow structure and a smaller influence on the pressure recovery inside the exhaust diffuser. This influence is smaller than that seen by other authors when including unsteady effects or accounting for the circumferential non-uniformity of the turbine exit flow but will need to be included in design calculations as diffuser design advances. Nomenclature C p = P 2 −P 1 P T 1 −P 1 Static Pressure Recovery c p