Page 27 - Advanced Thermodynamics for Engineers, Second Edition
P. 27

1.5 INTERACTIONS BETWEEN SYSTEMS AND SURROUNDINGS                 11




                  Again, the value of c p can be obtained from either the continuous or discontinuous scales of
               temperature. The units of c p are the same as those of c v , namely J/kg K, or more commonly in
               engineering kJ/kg K.

               1.5.7.1.1 Constant temperature processes
               Constant temperature processes are referred to as isothermal processes. There are a number of
               isothermal processes encountered in common usage.
                  •  Boiling and condensing processes occur at constant temperature and are accompanied by a
                     change of phase of the working fluid.
                  •  Slow expansion and compression processes in equilibrium with constant temperature
                     surroundings will be isothermal.
                  •  Isothermal processes require heat or work transfer to or from the surroundings. They are not
                     adiabatic.

               1.5.7.1.2 Constant internal energy processes
               A constant internal energy process occurs when dU ¼ 0. This can occur if the following circumstances
               exist:
                  •  Heat and work transfers are both equal to zero, i.e. dQ ¼ 0; dW ¼ 0.
                  •  Heat and work transfers are equal and opposite, so that dQ   dW ¼ 0.

                  An example of a constant internal energy system is the expansion of an insulated system against a
               vacuum.


               1.5.8 APPLICATION OF THE FIRST LAW OF THERMODYNAMICS TO OPEN SYSTEMS
               The First Law of Thermodynamics for Closed Systems can be extended to open systems. At this stage it
               is necessary to reconsider the definitions of closed and open systems.
                     A closed system is a quantity of matter contained within prescribed boundaries, i.e. it has a fixed
                     mass.
                     An open system is defined by its boundaries, or control surface, i.e. it has variable mass.

               1.5.8.1 Unsteady Flow Energy Equation (USFEE)


                                     2    !                  2
                                   V                        V i                      V 2 e
                  dQ   dW ¼ d  U þ    þ gZ    dm i u i þ p i v i þ  þ gz i þ dm e u e þ p e v e þ  þ gz e
                                    2                        2                        2
                                            cv
                                                                                            (1.27)
                  Giving further consideration to the flow energy term p i v i , it can be seen that this is the specific
               enthalpy, h, of a substance. The enthalpy, H, of a system is given by

                                                     H ¼ mh                                 (1.28)
               where m ¼ mass of the system.
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