You are currently reviewing an older revision of this page.
Real water distribution tanks cannot be exactly described as plug flow or completely mixed but these are reasonable approximations to fluid behavior in tanks. WaterGEMS V8i and WaterCAD V8i support four types of tank mixing models. The tank attribute Tank Mixing Model allows you to select from the following options.
The Completely Mixed model assumes that all water that enters a tank is instantaneously and completely mixed with the water already in the tank. It applies well to a large number of facilities that operate in fill-and-draw fashion with the exception of tall standpipes.
The 2-Compartment Mixing model divides the available storage volume in a tank into two compartments, both of which are assumed completely mixed. The inlet/outlet pipes of the tank are assumed to be located in the first compartment. New water that enters the tank mixes with the water in the first compartment. If this compartment is full, then it sends its overflow to the second compartment where it completely mixes with the water already stored there. When water leaves the tank, it exits from the first compartment, which if full, receives an equivalent amount of water from the second compartment to make up the difference. The first compartment is capable of simulating short-circuiting between inflow and outflow while the second compartment can represent dead zones. The user must supply a single parameter, which is the fraction of the total tank volume devoted to the first compartment. This value can be determined during calibration if this model is selected.
The FIFO Plug Flow model assumes that there is no mixing of water at all during its residence time in a tank. Water parcels move through the tank in a segregated fashion where the first parcel to enter is also the first to leave. Physically speaking, this model is most appropriate for baffled tanks that operate with simultaneous inflow and outflow such as ideal clear wells at water treatment plants. There are no additional parameters needed to describe this mixing model.
The LIFO Plug Flow model also assumes that there is no mixing between parcels of water that enter a tank. However in contrast to FIFO Plug Flow, the water parcels stack up one on top of another, where water enters and leaves the tank on the bottom. This type of model might apply to a tall, narrow standpipe with an inlet/outlet pipe at the bottom and a low momentum inflow. It requires no additional parameters be provided.
Book: Advanced Water Distribution Modeling and Management page 53.
Wiki: Does the Top Fill Tank property factor into mixing?
Help: Tanks