The System

The basic conservation laws, as well as the transport models, are applied to a "system" (sometimes called a "control volume''). The system is not actually the volume itself but the material within a defined region. For flow problems, there may be one or more streams entering and/or leaving the system, each of which carries the conserved quantity (e.g., Q) into and out of the system at a defined rate (Fig. 1-2). Q may also be transported into or out of the system through the system boundaries by other means in addition to being carried by the in and out streams. Thus, the conservation law for a flow problem with respect to any conserved quantity Q can be written as follows:

Rate of Q Rate of Q Rate of accumulation of Q

into the system out of the system within the system

If Q can be produced or consumed within the system (e.g., through chemical or nuclear reaction, speeds approaching the speed of light, etc.), then a "rate of generation" term may be included on the left of Eq. (1-10). However, these effects will not be present in the systems with which we are concerned. For example, the system in Fig. 1-1 is the material contained between the two plates. There are no streams entering or leaving this system, but the conserved quantity is transported into the system by microscopic (molecular) interactions through the upper boundary of the system (these and related concepts will be expanded upon in Chapter 5 and succeeding chapters).

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