Modern Engineering Thermodynamics

4.4 Energy Transport Mechanisms 105
System
boundary
System
boundary
−W
+W
+Q
−Q
E T
−W
+W
+Q
−Q
+E
−E
Mass
flow
E T
(a) Closed system
(b) Open system
FIGURE 4.3
Energy transport mechanisms.
WHAT ARE HEAT AND WORK ANYWAY?
Heat is usually defined as energy transport to or from a system due to a temperature difference between the system and its
surroundings. This can occur by only three modes: conduction, convection, and radiation.
Work is more difficult to define. It is often defined as a force moving through a distance, but this is only one type of work;
there are many other work modes as well. Since the only energy transport modes for moving energy across a system’s
boundary are heat, mass flow, and work, the simplest definition of work is that it is any energy transport mode that is
neither heat nor mass flow. 5
5 Work can also be defined using the concept of a “generalized” force moving through a “generalized” displacement, see Table 4.2 later in this chapter.
of a system is “lost” (i.e., negative), while work produced by a system (such as an engine) should be assigned a positive
value.
By definition, a closed system has no mass crossing its system boundary, so it can experience only work and
heat transport mechanisms. Also, since the gain, transport, and production terms in the balance equation are
defined to be net values (see Eq. (2.10)), we define
1. The net heat transport of energy into a system = ∑ i Q i = Q and the net heat transport rate of energy into a
system = ∑ i
_Q i = _Q :
2. The net work transport of energy out of a system = ∑ i W i = W and the net work transport rate of energy out of
a system = ∑ i
_W i = _W :
3. The net mass transport of energy into the system = ∑ i E i = ∑E mass flow and the net mass transport rate of
energy into the system = ∑ i
_E i = ∑ _E mass flow :
Thus, for a closed system, the total energy transport becomes
and the total energy transport rate is
For open systems, the same quantities are
E T = Q − W (4.11)
_E T = _Q − _W (4.12)
and
E T = Q − W +∑Emass
flow
(4.13)
_E T = _Q − _W +∑ _Emass
flow
(4.14)
In Eqs. (4.13) and (4.14), note that we write the summation signs on the net mass transport of energy terms,
but for simplicity, we do not write the summation signs ontheworkorheattransportterms.Thisisbecause
you often have open systems with more than one mass flow stream, but seldom do you have more than one