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Issue 2, Volume 2, April 2007
Print ISSN: 1790-5044
E-ISSN: 2224-3461







Title of the Paper: Density Wave Oscillation in the Horizontal Parallel Tube Paths of the Evaporator of a Natural Circulation Heat Recovery Steam Generator A Theoretical Investigation


Authors: Heimo Walter

Abstract: The paper presents the results of a theoretical stability analysis on a vertical type natural circulation heat recovery steam generator. The evaporator of the boiler consists of four parallel horizontal tube paths, which are connected at both ends with headers. For different operation pressures and geometries the dynamic instability and in particular the density wave oscillation (DWO) were analysed. The investigations show that the stability of the boiler can be improved by increasing the system pressure. A faster decay of the DWO can be achieved by the implementation of an orifice at the tube inlet of the evaporator (single phase flow), whereas the installation of an orifice at the tube outlet (two phase flow) results in a more unstable behaviour.

Keywords: Density wave oscillation, Dynamic instability, Natural circulation, Horizontal parallel tubes, Heat recovery steam generator

Title of the Paper: Prediction of Turbulent Thermal Boundary Layers Using Scalar Turbulence Modeling Techniques


Authors: M.D. El Hayek

Abstract: The present investigation deals with the development of advanced scalar turbulence modeling approaches and their application to the calculation of non-isothermal wall-bounded flow phenomena. A new scalar modeling technique based on scalar turbulent scales is proposed and implemented at a second-order modeling approach. Instead of the classical analogy concept between the mechanical and the scalar transport mechanisms, a scalar time scale, defined as the ratio of the temperature variance and its rate of dissipation or scalar dissipation rate, is used to tackle the scalar turbulence closure problem. At the level of second-order modeling technique, the scalar scales are directly used to derive relationships for the different unknown correlations like the turbulent diffusion, the pressure-temperature gradient correlation, etc. The result is a very simple scalar Reynolds stress model capable of predicting various heat transport problems. Such a new modeling approach is compared with its standard mechanical counterpart as well as first-order models like the k-ε model and the g-χ model, a scalar version of the standard k-ε. Validation against experimental data is performed and shows clearly the benefits of adopting the right scale for the right phenomenon: mechanical scales for momentum transport and specific scalar scales for scalar transport.

Keywords: Turbulence Scales, Turbulence Models, Wall Function, Convective Heat Transfer, Thermal Boundary Layer, Turbulent Prandtl Number.

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