Gravity-flow tank, model

The solutions show two important aspects of this model as written The concentrations come to a constant and equal value and the volume continues to rise indefinitely. The reason for this is that we took the outlet concentration to be a constant. Although this may have made for a simple model to solve, it is also one that is not very realistic. The tank would be overflowing. A more realisitc model would be one in which the exit flowrate was either set to match the inlet flow rate, which would make zero and the volume a constant at its initial level, or we could assume the flow rate out was gravity driven and would respond to the level in the tank, that is, Torricelli s Law. 

A typical example in process control is a tank that holds a liquid and whose exiting flow rate may be due to gravity or because of a pump. This kind of system is modeled based on a mass balance in unsteady state ... 

In estimating limiting flows to a subchannel for in-assembly calculations, RELAP5 is used to determine the net loop coolant flow to the plenum as a function of moderator tank-level coupled to a modified RELAP5 nodal model of the upper plenum coolant level (Reference 23). The upper end fitting of the fuel assembly, which consists of a vertical three-slot sleeve and a multiple orificed cylinder in series, is resolved into a single slot gravity weir, which correlates reasonably well with plenum coolant level and assembly flows (independent of air flows). 

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