| Author | Sahoo, Goloka Behari |
| Call Number | AIT Diss. no. WM-02-02 |
| Subject(s) | Reservoirs--Destratification
|
| Note | A dissertation submitted in partial fulfillment of the requirement for the degree of Doctor
of Engineering, School of Engineering and Technology |
| Publisher | Asian Institute of Technology |
| Series Statement | Dissertation ; no. WM-02-02 |
| Abstract | Many reservoirs around the world are eutrophic or are subjected to other water quality
problems, which are due to or exacerbated by thermal stratification within the reservoir.
In such cases, artificial mixing or destratification has been commonly used to counter
these effects. Of the various artificial mixing methods used, air bubbler diffusers are one
of the more widespread methods. Air bubblers are often operated at sub-optimal
mechanical efficiencies due to a lack of appropriate design (for example see McDougall,
1978; Asaeda and Imberger, 1993; Schladow, 1993; and Little and McGinnis, 1998). This
translates into increased costs for reservoir managers.
A one-dimensional numerical bubble plume model has been developed based on the
model of Schladow (1992) but also incorporates the features of gas transfer as described
by Wi.iest et al. (1992) and McGinnis and Little (1998). Two important bubbler
performance criteria, the mechanical efficiency 1'Jmech and the destratification time r are
analyzed as functions of two dimensionless parameters G, the strength of stratification,
and M, the source strength. General equations to estimate optimum bubbler design
conditions i.e. airflow rate Q0 (via M) and corresponding mechanical efficiency 1'Jmech and
destratification time per unit area r (s m"2
) for a known linear or an equivalent linear
stratification G in a reservoir having any shape have been derived. In cases where the
airflow rate may be adjusted (i.e. some form of real time control) as the reservoir
destratifies, it is recommended that the bubbler be designed to operate at the first peak
mechanical efficiency 1'Jmech (%) if reducing destratification time is the plimary objective.
Due to difficulties in accurately determining the actual G, it is demonstrated that an
appropriate practice is to reduce the design G value by around 10%. It is shown that the
equivalent linear stratification method may lead to sub-optimal design for stratification
profiles that deviate substantially from a linear profile. Rather, a bubble plume model
should be applied.
The gas transfer rate from a bubbler to the water column is primalily a function of the
airflow rate, bubble size, reservoir depth and bubble slip velocity. The stratification only
has a weak effect on the gas transfer. Considering all these variables affecting the gas
transfer, relationships have been derived to predict the gas transferred to the ambient
water from air bubbles; gas transferred per unit volume entrained and gas transferred per
unit energy consumed for a known or equivalent linear stratification G. The equations are
verified using a lD bubbler model and it is shown that the oxygen transfer to the water
column can be significant if small bubbles are used. The mechanical destratification
efficiency 1lmech (% ), destratification time per unit smface area r (s m"2
), oxygen
dissolution efficiency Q (%) and oxygen transferred per unit input energy are examined
as function of bubble size. It is concluded that an average bubble radius size of 1 mm
should be considered for design purposes. However, if oxygen transfer from the bubbler
is not considered important, then a bubble size of up to 4 mm is quite acceptable for
destratification purposes.
A one-dimensional reservoir model has been developed to examine climatic impact on
lake/reservoir stratification and mixing. The combined reservoir and bubbler model is
applied to a case study (the Upper Peirce Reservoir, Singapore) from which some
generalizations are made about the response of tropical reservoirs to bubbler operation.
For the tropical reservoir case with low wind velocity (time averaged wind speed "" 1.0
m s· 1
) it was shown that bubbler operation dominates oxygen transfer to the reservoir
water when compared to all other sources, including surface re-aeration. In particular,
higher dissolved oxygen levels were obtained by increasing the airflow rate above that
associated with optimal mechanical efficiency. Dete1mining an appropriate airflow rate is
shown to be a trade-off between increased dissolved oxygen levels and increased
operating costs as airflow increases. If improving water quality is the primary objective,
then such a trade-off needs to be examined in order to select a suitable airflow rate. |
| Year | 2002 |
| Corresponding Series Added Entry | Asian Institute of Technology. Dissertation ; no. WM-02-02 |
| Type | Dissertation |
| School | School of Engineering and Technology |
| Department | Department of Civil and Infrastucture Engineering (DCIE) |
| Academic Program/FoS | Water Engineering and Management (WM) |
| Chairperson(s) | Luketina, David Andrew; |
| Examination Committee(s) | Gupta, Ashim Das;Suphat Vongvisessomjai;Huynh Ngoc Phien;Schladow, S. Geoffrey; |
| Scholarship Donor(s) | France Government; |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology, 2002 |