I recommend you to use follow the best practices video on how to simulate heat transfer of fluid through the pipe and also attached the user guide link on how to simulate the heat transfer in mixing elbow. This procedure will make your simulation run faster and convergence easier for the 500m length tube. If it still takes time please reduce and alter the under relaxation factor and check it will lead to faster convergence. Or you can use symmetry about y axis and give periodic condition and model half of the geometry .
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