Author(s): Bernhard H. Schmid
Linked Author(s):
Keywords: Buoyant jets; Cooling water; Multiport diffuser discharges; CORMIX
Abstract: In the wake of the construction of the River Danube hydropower plant Vienna-Freundenau the cooling water discharge of the Donaustadt thermal power plant was redesigned, with the associated mixing simulations performed by means of CORMIX, the CORnell MIXing zone expert system. Particular attention had to be paid to an ecologically sensitive zone in the receiving Freudenau impoundment reservoir, comprising the fish pass of the Freudenau hydropower plant, which was (and is) located on the same (left) bank of the River Danube as the Donaustadt thermal power station. To achieve a reliably high initial dilution of the cooling water, a unidirectional multiport diffuser was chosen to be installed, a decision which consequently resulted in the first construction of a cooling water diffuser in Austria. This diffuser now discharges up to 20 m³/s of cooling water into an impounded reach of the River Danube with a water depth of some 11 m. Injection is co-flowing in plan view and oriented 45° upwards. The diffuser comprises two feeder pipes with 16 nozzles each, giving a total of 32 discharge ports, equally spaced at a hydraulically effective distance of 2 m (with 62 m, therefore, between first and last discharge port centreline). CORMIX-based mixing simulations indicated that the Freudenau fish pass would not suffer heating in excess of 0. 7 °C even under adverse conditions (Danube River low flow of 830 m³/s combined with full operation of the Donaustadt cooling water system, i. e. maximum permitted discharge of 20 m³/s and a heating span of 10 °C). To evaluate the performance of the cooling water diffuser under operation conditions, a thermal monitoring programme was implemented. The recorded data sets were used to compare simulated and measured temperature values downstream of the discharge. The study reported here showed clearly that the cooling water diffuser performs very well indeed, even to a degree that made quantitative comparison of simulated and measured induced excess temperatures difficult, due to the very small temperature rises encountered during the monitoring period. Although induced excess temperatures kept close to the detection limit for much of the monitoring interval, it was possible to identify data suitable for CORMIX (module 2) validation, nevertheless. In that situation Danube River flow amounted to 1052 m³/s, which is between low and mean flow conditions. The Donaustadt plant discharged 17. 2 m³/s of cooling water at a temperature 10 °C above that of the ambient river water. At the inflow section of the fish pass (2. 03 km downstream of the diffuser) an induced temperature rise of 0. 2 °C was recorded, and the CORMIX 2 simulation of this buoyant mixing process indicated that a temperature increase by 0. 18 °C was to be expected. As the temperature sensor gave values with a resolution of one digit (0. 1 K) only, it can be concluded that recorded and simulated diffuser-induced excess temperatures matched very well.
Year: 2007