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Atmospheric Plume Modeling with Applications to Cooling Towers
- Author / Creator
- Li, Shuo
Visible plumes discharged by cooling towers have gained much attention due to their negative aesthetic and environmental impact. To achieve plume abatement, many previous designs mix warm, dry air with the hot, humid air rising from the cooling tower fill in the plenum chamber and thereby generate an air mixture that is unsaturated upon discharge. Here, we allow only partial mixing within the plenum chamber and the mixing continues above the cooling tower in the form of a wet plume core shielded by a dry plume envelope, i.e. a so-called coaxial plume. This coaxial plume structure is modeled via a three-way entrainment formulation between the inner and outer plumes and the ambient. Theoretical results predict that the inner plume rises quickly but shrinks until it disappears at some height. As the dry air mixing fraction (DAMF) increases, there is less likelihood of fog formation and/or recirculation. To further validate our theory and to inform the quantification of entrainment coefficients whose values cannot be obtained analytically, planar laser-induced fluorescence experiments have been performed. A pixel-by-pixel comparison of the scalar concentration images generated respectively by theory and experiment is conducted in order to determine the optimal entrainment coefficients. Experiments consider a still ambient, but a wind may be included in a similar fashion to the theoretical model, assuming, somewhat optimistically, that the axial symmetry is not broken. To this end, and in a windy environment, the inner plume is more rapidly cut off by the outer plume with increasing wind speed. For fixed DAMFs, the visible plume length varies nonmonotonically with the wind speed. Moreover, nontrivial differences in the visible plume length are predicted using two different entrainment formulations.
In contrast to a single plume, multiple plumes tend to merge and thereby the dilution characteristics are modified. Another major objective of this thesis is to explore the effect of plume merger under different cooling tower configurations. Previous theoretical descriptions of plume merger often consider nearly idealized plume sources, which are probably inappropriate in the case of the large area source plumes discharged by e.g. cooling towers. For two adjacent area source plumes, a theoretical model is proposed whereby (i) the boundaries of the merging plumes are prescribed and (ii) the entrainment coefficient varies exclusively with the plume boundary curvature. Theoretical results (full merger height, the total volume flux, etc.) are in good agreement with previous theoretical and experimental data.
Finally, we investigate the merging of long rows of plumes, which include (i) dual rows of plumes in a quiescent environment and (ii) a single row of plumes in a crosswind. With a moderate to large vertical to horizontal spacing ratio, the theory of the dual row case predicts an intermediate line plume scaling before approaching the far-field line plume limit. The theory describing a single row of plumes in a crosswind predicts the correct near- and far-field similarity limits. Model results of plume trajectories agree satisfactorily with previous theoretical and experimental data.
- Graduation date
- Fall 2020
- Type of Item
- Doctor of Philosophy
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