Tank heat loss model
 

Hi all. How can i approach modeling the heat loss from the tank to estimate the time it takes for the temperature to drop to a particular point? Setting up the transient simulation isn't the problem for me. I can predict from my calculation that it will take about 3 days to drop from initial to final temperature. However, using a very small timestep like 0.1s to run this would mean i would even need much more than 3 days (physical time) to get the solution.
Is there any method in ansys, that can use the lumped capacitance assumption to estimate this time, without having to simulate for that long. I just need to estimate the time and not really interested in the temperature trends.