Characteristics of Buoyancy Driven Natural Ventilation through Horizontal Openings: PhD Thesis definded public at Aalborg University (101106)

Zhigang Li

    Research output: Book/ReportPh.D. thesis

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    Abstract

    Air flow through horizontal openings is an important issue of mass and energy transfer between different zones in buildings. This kind of mass and energy transfer have important implications regarding energy saving, thermal comfort, control of contaminants, micro-organisms and spread of fire and smoke. Air flow through vertical openings has been widely investigated but little is known about the flow in the horizontal openings, especially when they are driven by buoyancy. A literature survey shows that the brine-water system and the scale model are normally used forthe research work of air flow through horizontal openings.

    Two cases of full-scale measurements of buoyancy driven natural ventilation through horizontal openings are performed: one horizontal opening and one horizontal opening combined with one vertical opening. For the case of one horizontal opening, the measurements are made for opening ratios L/D range from 0.027 to 4.455. The basic nature of air flow through the openings, including air flow rate, air velocity, temperature difference between the rooms and the dimensions of the horizontal openings, are measured. Smoke visualizations show that the air flow patterns are highly transient, unstable and complex, and the air flow rates oscillate with time. Correlations between the Froude number Fr and the opening ratio L/D are obtained, which is reasonable agreement with Epstein's formula derived from brine-water measurements, but the obtained Fr values show considerable deviations for a range of L/D ratios. Thus, the developed formulas are established. Meanwhile, the correlation  ratios. Thus, the developed formulas are established. Meanwhile, the correlation between the Archimedes number Ar and the opening ratio L / A are also determined.

    For the case of one horizontal opening combined one vertical opening, the measurements are made for opening ratios AT/AB in the range from 0.11 to 25. The smoke visualizations show that three flow modes can be identified depending on the different AT/AB value: bidirectional flow through the bottom opening, unidirectional flow through the two openings and bidirectional flow through the top opening. The bidirectional flow through the horizontal opening shows that the flow patterns are highly transient and unstable. A new empirical model for calculation of the air flow rate is developed by introducing a new opening area ratio factor.

    Computational fluid dynamics (CFD) are used to study these two air flow cases. The air flow rate and air flow pattern are predicted and compared with the full-scale measurements. The measurement data are used to compare two CFD models: standard k- ε model and large eddy simulation (LES) model. The cases calculated by the LES model and large eddy simulation (LES) model. The cases calculated by the LESmodel agree well with the measured data, however, the cases simulated by the k-εmodel are inaccurate compared to the measured data. For the highly transient and unstable flow, the LES model is a suitable tool to predict detailed and accurate air flow.

    This work is important and quite useful in relation to natural ventilation systems. The measurement results can be used in both simple calculation tools to give a rough estimate of the capacity for design of a ventilation system, but also be implemented in more detailed models, especially multi-zone models, for simulation of the performance of natural ventilation systems.

    Original languageEnglish
    Place of PublicationAalborg
    PublisherDepartment of Civil Engineering, Aalborg University
    Number of pages123
    Publication statusPublished - 2007
    SeriesDCE Thesis
    Number8
    ISSN1901-7294

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    Keywords

    • Natural Ventilation
    • Buoyancy-Induced Ventilation
    • Wind Ventilation
    • CFD
    • Air Flow
    • Turbulence
    • Smoke Visualization

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