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Assessment of Unsteadiness Modelling for Transient Natural Convection

[+] Author Affiliations
M. Fadl, L. He

University of Oxford, Oxford, UK

P. Stein, G. Marinescu

GE Power, Baden, Switzerland

Paper No. GT2017-63592, pp. V008T29A016; 12 pages
doi:10.1115/GT2017-63592
From:
  • ASME Turbo Expo 2017: Turbomachinery Technical Conference and Exposition
  • Volume 8: Microturbines, Turbochargers and Small Turbomachines; Steam Turbines
  • Charlotte, North Carolina, USA, June 26–30, 2017
  • Conference Sponsors: International Gas Turbine Institute
  • ISBN: 978-0-7918-5095-4
  • Copyright © 2017 by ASME

abstract

Flexible operations of steam turbines with faster startups and shutdowns are required to accommodate emerging renewable power generations, needing more advanced prediction tools for transient thermal design and analysis. A major challenge is the time scale disparity. For a natural cooling, the physical process is typically in hours or tens of hours, but on the other hand, the time step sizes typically usable tend to be very small (in seconds or sub-seconds) due to the numerical stability requirement for natural convection as often observed. A general issue to be addressed is what time step sizes can be and should be used in terms of stability as well as accuracy.

In the present work, the impact of the temporal gradient in unsteady flow and its modelling is examined in relation to numerical stability and modelling accuracy for natural convectio n. A source term based dual timing for mulation is adopted and implemented in a commercial code, which is shown to be numerically stable for very large time steps for natural convection analysis. Furthermore, a loosely coupled partitioned procedure is developed to combine this enhanced flow solver together with a solid conduction solver for solving transient conjugate heat transfer problems for natural convection. This allows very large computational time steps to be used without any stability issues, and thus enables to assess the impact of using different time step sizes entirely in terms of the temporal accuracy requirement. Computational case studies demonstrate that the present method is more stable at a markedly shortened computational time than the baseline solver. The method is also shown to be more accurate than the commonly adopted quasi-steady methods when unsteady effects are non-negligible.

Copyright © 2017 by ASME

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