A control volume may involve one or more forms of work at the same time. If the boundary of the control volume is stationary, the moving boundary work is zero, and the work terms involved are shaft work and electric work. Another work form with the fluid is flow work. | ||
Flow Work
(Flow Energy)
|
||
Work
is needed to push the fluid into or out of the boundaries of a control
volume if mass flow is involved. This work is called the flow work
(flow energy). Flow work is necessary for maintaining
a continuous flow through a control volume. Consider a fluid element of volume V, pressure P, and cross-sectional area A as shown left. The flow immediately upstream will force this fluid element to enter the control volume, and it can be regarded as an imaginary piston. The force applied on the fluid element by the imaginary piston is: F = PA The work done due to pushing the entire fluid element across the boundary into the control volume is Wflow = FL = PAL = PV For unit mass, wflow = Pv The work done due to pushing the fluid element out of the control volume is the same as the work needed to push the fluid element into the control volume. |
||
Total Energy of a Flowing Fluid
|
||
The total energy of a simple compressible system consists of three parts:
internal, kinetic, and potential energy. E = U + KE + PE For unit mass, e = u + ke + pe = u + v2/2 + gz where e = total energy u = internal energy v = velocity of the system z = the elevation of the fluid The fluid entering or leaving a control volume possess an additional energy, the flow work (Pv). Hence, the total energy of a flowing fluid becomes θ = Pv + u + v2/2 + gz where θ = methalpy, the total energy of a flowing fluid The definition of enthalpy gives h = Pv + u Replacing Pv + u by h yields θ = h + v2/2 + gz By using the enthalpy instead of internal energy, flow work is not a concern. |
||
The Steady-flow Process
| ||
Steady flow process is a process where: the fluid properties
can change from point to point in the control volume but remains the
same at any fixed point during the whole process. A steady-flow process
is characterized by the following:
|
||
Mass and Energy Balance for Steady-flow Process
| ||
The conservation of mass principle, which has been previously introduced,
in rate format, is: During a steady-flow process, the total amount of mass contained within a control volume does not change with time. That is, dmsystem/dt = 0 Hence the conservation of mass principle gives the total amount of mass entering a control volume equal to the total amount of mass leaving it. In an equation format, it is |
||
(Total mass entering the control volume per unit time)
= (Total mass leaving the control volume per unit time)
or,
where
Also, the energy balance for a process, which has been previously
introduced, in rate format, is:i = inlet e = exit For a steady-flow process, the total energy content of a control volume remains constant. That is, dEsystem/dt = 0 Thus, the amount of energy entering a control volume in all forms (heat, work, mass transfer) must be equal to the amount of energy leaving it for a steady-flow process. In an equation format, it is
(Rate of net energy transfer in by heat, work and mass)
= (Rate of net energy transfer out by heat, work and mass)
Or,
For a general steady-flow process, the energy balance
can be written as
If the sign introduced previously for heat and
work is used, the energy balance for a general steady-flow process can
be rewritten as:
|
Diajukan untuk memenuhi tugas mata kuliah termodinamika dengan dosen pengampu Apit Fathurrahman,S.Pd.,M.Si.
Rabu, 11 Maret 2015
THERMODYNAMICS - THEORY
Langganan:
Posting Komentar (Atom)
Tidak ada komentar:
Posting Komentar