The software and CFD

The wetcooling software package does not use multidimensional CFD, instead it is a one-dimentional analytical model. The multidimentional effects are addressed in the semi-empirical equations of the transfer and pressure drop coefficients. These semi-empirical equations are derived from numerical and experimental studies. The wetcooling software is therefore very cost effective when compared to CFD.

The big question is how do the accuracy of the wetcooling software compare to the accuracy of CFD. Experience has shown that the analytical based models (like the wetcooling software) give results of the same order of magnitude than full-blown CFD simulations. The results of the wetcooling software are, however, obtained at a fraction of the time and cost of the CFD simulations.

Why are CFD models of cooling towers not necessarily more accurate than analytical models? The geometry of a cooling tower is very complex. The water distribution system with the support structure, pipes and nozzles are difficult to accurately model in CFD. Add the water droplets, fill, fill support structure and fan to the CFD model and you can appreciate the increase in complexity of the model. CFD models are therefore simplified by many simplifying assumptions. Some transfer and loss coefficients are obtained from experimental research. These are the same semi-empirical models that are employed in the analytical models. Therefore the same models are used in both CFD and the analytical models and the end results are therefore not too different.

CFD has its place, but...
Having said the above, CFD is still a very powerful and useful tool to analyze cooling tower performance. It can run many scenarios that will be impossible to do with analytical tools. When you consider the cost and time to do CFD analyses, analytical models like the wetcooling software, will always be in demand.

CFD reference
If you are interested in the application of CFD in the analysis of cooling towers I can highly recommend the PhD thesis of N.J.Williamson as a starting point. His thesis is entitled Numerical Modelling of Heat and Mass Transfer and Optimisation of a Natural Draft Wet Cooling Tower, The University of Sydney, Australia, 2009.

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