Using FloEFD as an Engineering Tool: Part III2018-07-18 Karl Du Plessis
At the start of Part I of this series, the question: “What do you do when faced with analysing a shell and tube heat exchanger as in the model shown in Figure 1” was raised? The answer is simple, you use FloEFD. Part I and Part II focused on the capability of FloEFD to provide accurate engineering results for heat transfer in internal as well as external flow applications. Both of these cases were considered in isolation though, however, in this discussion, the revelations made during those investigations are combined and finally applied to the full heat exchanger example.
Figure 1: Shell and tube heat exchanger.
Part III: Full heat exchanger
The reason for all of this was basically to establish just how coarse of a mesh one could dare to use when having to analyze large or complex heat exchangers like this. Since, you might find yourself in the same position as many engineers in South Africa, usually required to make do with limited computer resources. Therefore, it would be very beneficial if you can use CFD software that can double-up as an engineering tool to solve large problems on your standard issue laptop or desktop computer. And this is exactly where FloEFD starts to make a lot of sense.
In order to analyse the heat exchanger in question, the only limiting factor would be the computer memory (RAM), as the memory effectively limits the size of the model in terms of the number of cells that can be used. Due to the sheer length of piping care needed to be taken in order to obtain a mesh that can fit into the 32GB memory limit. Therefore based on the knowledge gained, the ‘four cells per diameter’ value were used as a gauge to generate a reasonable mesh that could still provide a high level of confidence in the ‘engineering’ answer. Setting up the mesh was as simple as inserting a few control planes in the base mesh settings that “box” the tube bank and specifying the number of cells between the set of planes. Thereafter, based on the base mesh, local mesh refinement with level 3 was applied to the tube bank part/component to ensure all of the tubes met the characteristic ‘four cells per diameter’ requirement. Within the limit of 32GB memory, some stretching of the cells still had to be applied to save a little on the memory requirements. Stretching the cells away from the bends resulted in a mesh size of approximately 5.7 million cells in total. The mesh setup and generation only took a few minutes and FloEFD did not have any problems in generating the mesh, almost like magic it just happens. For this size mesh, the memory usage peaked at 29GB from time to time during solving. A portion of the resulting mesh is shown in Figure 2.
Figure 2: Mesh resolution around the tubes
Onto the question of solving full heat exchanger. First of all, it must be stated that the solution was very stable and convergence simply just happened. Quite astonishing really, considering the type of problem. Regarding the required calculation time, this particular model solved in a very respectable 15 hours on a mere quad-core CPU, with the respective outlet temperatures already converged after 1.5 travels (flow freezing enabled). The resulting outlet temperatures obtained were, Tair,out = 51.6°C and Twater,out = 24.6°C. See the tube internal and shell-side temperatures in Figure 3 and Figure 4 respectively. Based on the merits of the previous discussions, this result would already be very useful to base decisions on, especially when doing comparative studies of various baffle plate designs for example.
Figure 3: Tube-side – Water temperature.
Figure 4: Shell-side – Air temperature.
I have long since realized the value of FloEFD whenever it comes to solving heat transfer problems. However, it has only now also become evident that FloEFD has made it possible for engineers to solve large problems like shell and tube heat exchangers with the minimum amount of effort and resources required, compared to ‘old school’ CFD programs, thanks to the underlying SmartCells™ technology and the ever-so-fantastic thin boundary layer model. The only demand being placed on computer resources is on the memory which limits the mesh size of these models. It is simply astonishing how easy it is and how little effort is required by the user to set up such a model, including the meshing. It goes without saying that all of this would be useless were it not for the remarkable accuracy, stability, and robustness of the solver. From an ‘Engineering in South Africa’ perspective, i.e. to be as resourceful as possible, FloEFD really resonates well with our kind of thinking.
FloEFD, the only CFD software that can be used as an Engineering tool.