J. Lepers, K. Kusterer, B&B-AGEMA GmbH, D. Bohn, J. Ren,
Institute of Steam and Gas Turbines, Aachen University of Technology, Germany

Meeting the world’s demand for electricity at low costs and protecting the environment are key objectives of modern power plant design.

B&B-AGEMA GmbH is an engineering service company with worldwide activities, developing technological improvements for power plant components such as gas turbines, steam turbines, valves, and condenser systems in co-operation with manufacturers. Another focus of company activities is consulting power plant operating companies.

Cooling towers provide the cold end to combined gas and steam turbine (STAG) and stand alone steam turbine power plants. The objective of cooling tower and condenser operation is to condense steam exiting from the low pressure turbines at a pressure which should be as low as possible in order to maximize the power output from the plant.

The condenser pressure is thermodynamically linked to the cooling water temperature offered by the cooling tower. The closer it is to ambient
temperature, the better it is for cycle performance. High efficiencies are typically achieved with wet cooling towers. In this design, cooling water heated in the condenser is sprayed on a fill where it is cooled by convection and evaporation in contact to ambient air. Most efficient, although comparatively expensive designs are based on buoyancy driven air flow.

Natural draft cooling towers are very sensitive to side wind effects. The incoming air is unevenly distributed to the fill and the buoyancy driven air flow in the tower is affected by cold air entering the tower from the top, see Fig. 1. The consequence of both influences is a distorted heat transfer distribution across the cooling tower cross section, leading to an overall reduction of efficiency. Since the strong competition in the power market requires the plants to yield optimum performance at any kind of ambient conditions, counter-measures have been developed. For this, advanced computational technologies developed by CD-adapco were applied.

At B&B-AGEMA, STAR-CD was extended with user-routines and used to model cooling tower flow, mass transfer, and heat transfer. The computational model was successfully validated for a well-documented cooling tower operating point. The quality of the model was assessed on the basis of the cold water temperature at the cooling tower outlet. This is an integral value and therefore a good basis for judging the overall performance of the model. At design point operation, a deviation of 0.21 Kelvin occurred. This corresponds to an error of 0.9% and is much better than the 0.5 Kelvin accuracy originally requested. Subsequently, parameter studies were carried out investigating side wind effects. A substantial improvement could be obtained by adding an inner rim to the outlet of the cooling tower. By this means, a smaller outlet area and a more stable flow close to the cooling tower wall was achieved. With this design, stable operating conditions could be achieved, even at side wind velocities beyond 10 m/s. Flow conditions at side winds of 2m/s, 4m/s, 8m/s are shown in Figs 2, 3 and 4, respectively.

Based on these CFD studies with STAR-CD, we were able to demonstrate how design improvements could be made to cooling towers that would significantly increase their efficiency.