# Accumulation of Mass

Open
System

msys(t)

The diagram below shows a

#### uniform-flow process

in which mass flows into and out of a system.

Consider the process for an interval of time from t1 to t2, where t2 > t1.

At t1 the mass in the system is: msys(t1).
At t2 the mass in the system is: msys(t2).
If the inlet mass flow rate, , is greater than the outlet mass flow rate, , then mass accumulates within the system: msys(t2) > msys(t1).
If the outlet mass flow rate, , is greater than the inlet mass flow rate, , then the mass within the system decreases: msys(t2) > msys(t1).
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### Ch 5, Lesson D, Page 2 - Accumulation of Mass

• Let’s start with the SISO system shown in the diagram, below.
• Consider a process that begins at time t1 and ends at a later time, t2.
• In a uniform flow process, m-dot-in and m-dot-out are constant.
• So, it only makes sense that if m-dot-in is greater than mdot,out, then the mass inside the tank increases during our process.  We are adding mass faster than we are taking it out.
• Similarly, if mass is leaving the tank faster than it is entering, then we are draining the tank and the mass inside the tank decreases during the process.
• No tricks, right ?
• Now let’s look at the differential mass balance equation.