A proposal for modeling the effects of system rotation on the turbulent scalar fluxes is presented. It is based on extension to rotating frames of an explicit algebraic model derived using tensor-representation theory. The model is formulated to allow for the turbulent scalar fluxes to depend on the details of the turbulence field and on the gradients of both the mean-velocity and the scalar. Such dependence, which is absent from conventional models, is required by the exact equations governing the transport of the scalar fluxes. The model’s performance is assessed, both a priori and by actual computations, by comparisons with results from recent direct numerical simulations (DNS) of flows in heated channels rotated about their streamwise, spanwise, and wall-normal axes. To place the new model’s performance in context, additional comparisons are made with predictions obtained from three alternative models, namely, the conventional gradient-transport model, a model that is implicit in the scalar fluxes derived by simplification of the modeled transport equations for the scalar fluxes, and a differential scalar-flux transport model. The results show that the present model yields predictions that are substantially in better agreement with the DNS results than the algebraic models, and which are indistinguishable from those obtained with the more complex differential model. However, important differences remain and reasons for these are discussed.
Laminar separation bubbles form on the back surfaces of aero-engine LP turbine blades. In recent years significant weight and cost reductions and performance improvements have been achieved through a better understanding of the behavior of such separation bubbles. A project is underway at the Universita¨t Stuttgart to study a possible technique to suppress laminar separation bubbles using actuated transition. This paper reports on DNS results with and without actuation for different frequencies, amplitudes and Reynolds numbers, revealing the nature of the transitional process. Early results from an experimental simulation are included. In addition numerical simulations of fluidic oscillators which are capable to provide the required frequencies at a size which would fit into an LP turbine are presented.
Power output and compressor efficiency of the gas turbine decrease over time due to compressor fouling. A major part of this power and compressor efficiency loss can be recovered by compressor online and offline washing. Nevertheless, with an enhanced filtration in the EPA class, it is possible to reduce the deployment of degradation and the necessity of washings to a minimum. After providing an overview of published research work from the past, this paper presents a thorough investigation and quantification of the effect of different air filter classes on degradation based on fleet wide analysis. The operating data of a total of 12 gas turbines (Alstom’s GT13E2, GT24 and GT26) from 6 power stations in 5 different countries are analyzed, giving a sum of 34 filter cycles for evaluation (1 cycle represents the time period between two filter exchanges or compressor offline washings). The filter houses of the assessed plants are equipped with 2- or 3-stage filtration systems with filter classes ranging from G4 to E11 and various combinations thereof. The relevant data for power output and compressor efficiency together with the exchange history of the air filters is used to determine the degradation as a function of the last filter stage class, which allows for a quantification of the degradation and reveals a clear correlation. As it is shown that not only the last stage filter with the highest filter class determines the degradation, but the filter system with all stages as a combination, this paper additionally evaluates the effect of the second-last filter class on the degradation. Due to more and more challenging market environments for the plant owners, the decision to opt for highly efficient EPA filters must be well-considered: more efficient filters generally have a higher pressure loss with a negative influence on power output and are generally more expensive. Nevertheless, EPA filtration can be a strong business case when all factors such as prices for electricity and gas, operating regime of the plant, pressure loss and exchange scenarios of the filters, necessity for compressor washing etc. are considered. An exemplary profitability analysis considering all the above mentioned factors is presented in this paper. The insights of this paper shall be used as a basis for decision making when it comes to the question of how to lay out the project specific GT air inlet filtration system, as well as of how to modify the existing GT air inlet filtration for improved performance and economics.
We propose a new approach to modeling the effects of system rotation on the turbulent scalar fluxes. The approach is based on extension to rotating frames of an algebraic model derived using tensor representation theory. The model is formulated to allow for the turbulent scalar fluxes to depend on the details of the turbulence field (via the Reynolds stresses), and on the gradients of both the mean velocity and temperature. Such dependence, which is absent from conventional models, is required by the exact equations governing the transport of the heat fluxes. The model’s performance is assessed by comparisons with results from recent Direct Numerical Simulations of flows in channels rotated about their streamwise, spanwise and wall-normal axes. The results show that the model yields results that are in good correspondence with the DNS results.
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