Reservoir

The Reservoir junction type allows you to connect up to twenty-five pipes. One connecting pipe is required.

The Reservoir Properties window follows the second of the two basic Properties window formats. A table on the Loss Coefficients tab displays the connecting pipe information. This table grows to accommodate up to twenty-five pipes. After you add a fifth connecting pipe, a scroll bar appears, allowing you to review and enter loss factors for all pipes in the table. The pipe table displays the reference positive flow direction of each connecting pipe. To enter loss factors, select the cell in the table and edit the value. Each pipe can use a different loss model or custom value.

Tank Model

There are two possible Tank Models available in the Reservoir junction, Infinite Reservoir and Finite Open Tank.

• Infinite Reservoir - Infinite reservoirs refer to a massive body of fluid whose surface level does not change appreciably as a result of liquid inflow or outflow during the time frame of the simulation. An example is a large lake or the ocean. The Infinite Reservoir model is convenient for specifying a fixed pressure in your system. This tank model applies a defined pressure at the junction location in the model. When solving a pipe flow system, an Infinite Reservoir causes the rest of the system to distribute the flow in a manner consistent with the defined pressure.

• Finite Open Tank - Finite reservoirs (also called finite tanks) refer to a body of fluid which is small enough that its surface level changes significantly during the time frame of the simulation as a result of liquid inflow or outflow. An example is a tank which drains as the simulation progresses. Finite open reservoirs are open to the atmosphere or some other fixed surface pressure. Finite reservoirs require specification of tank geometry. Typically, the liquid level and surface pressure are defined initially, and then calculated over time based on the tank geometry.

For both models the Liquid Surface Elevation, Liquid Surface Pressure, and Pipe Depth/Elevation will be required to be defined by default.

If the Finite Open Tank model is chosen, additional information for the Cross-Sectional Area, Tank Height, and Tank Bottom Elevation will be required. The cross-sectional area can be defined as a constant area if it is a standard cylinder, or can be modeled as variable. If the variable option is chosen, an additional Tank Geometry input will be required on the Pipe Depth & Loss Coefficients tab to enter the Accumulated Volume in the Tank vs the Liquid Height.

Known Parameters Initially

For the Finite Open Tank option, the options become active to specify whether some parameters are defined by the user for the steady state. In some cases, the initial liquid level or pressure may not be known, and it is desired to have this calculated based on a mass balance in the system. This can be selected using the check box options provided. Impulse will only allow one of these parameters to be left unknown at a time.

Specifying Pipe Connectivity

If there is a single pipe connected to the junction, its elevation can be directly specified on the Reservoir Model tab. The elevation can be specified either as an absolute Elevation, or as a depth below the liquid surface. This means there are two ways to define the same pipe elevation.

If multiple pipes are connected, they can each have a unique elevation. This is displayed on the Pipe Depth and Loss Coefficients tab. In this table, all elevations must be specified as either depth or elevation. Additionally, losses such as re-entrant losses can be added to the pipe connections in this table. Loss factors can optionally be specified for flow into and flow out of the pipe.

Tip:  Since transient models allow the liquid level to change, and the user may desire to run different scenarios with different initial liquid levels, it is better to specify reservoir pipe connections based on elevation and not depth.

Pipes Connected Above Liquid Surface

A pipe elevation can be specified higher than that of the liquid surface. Pipes that empty into the Reservoir above the liquid surface are assumed to have liquid free fall to the liquid surface. AFT Impulse applies the proper boundary condition for above the liquid surface.

If the pipe is above the liquid surface, the only appropriate condition is for the fluid to be flowing from the pipe into the reservoir. Fluid cannot flow from the reservoir into the pipe as the pipe is above the liquid surface. However, AFT Impulse will assume the fluid flowing into the pipe is the same as the reservoir fluid, solve the system, then issue a warning to the user in the output that this is unrealistic.

Finite Tank Overflow/Drain

When a finite tank liquid level reaches the top, it is assumed that the liquid spills over the top. Hence the liquid height is maintained at the top of the tank. Note that this causes a loss of mass from the system model.

When a tank drains to the bottom, it cannot supply any more liquid to the connected pipes. The system behavior after a tank has drained cannot be accurately modeled by AFT Impulse. In a real system, and in the absence of a valve to stop the flow, a drained tank which continues to flow would result in the connected pipes themselves draining and a gas/liquid interface moving down the pipes. AFT Impulse assumes all pipes are liquid full, and cannot model draining pipes. A warning will be displayed in the output tab if this occurs.

Since it is not desirable in most applications to have the pipes drain, this limitation is not a significant issue. The user can, for example, use a valve in the pipe which closes when the tank drains. This would use an event transient.

Transient Data

On the transient data tab the surface elevation can be changed with time. For more information on transient data, including event transients, see Junction Transient Data.

Graphing Reservoir Data

You can graph various parameters such as tank liquid height and total mass or volume inflow by selecting to save the transient output for these junctions in the Output Junctions panel of the Pipe Sectioning and Output group. These parameters can be graphed or reviewed in the Output window.