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Date Submitted: 04/25/2014 01:08 AM
The First Law of Thermodynamics: Control Volumes
Here we will extend the conservation of energy to systems that involve mass flow across their boundaries, control volumes. Any arbitrary region in space can be selected as control volume. There are no concrete rules for the selection of control volumes. The boundary of control volume is called a control surface.
Conservation of Mass
Like energy, mass is a conserved property, and it cannot be created or destroyed. Mass and energy can be converted to each other according to Einstein’s formula: E = mc2, where c is the speed of light. However, except for nuclear reactions, the conservation of mass principle holds for all processes. For a control volume undergoing a process, the conservation of mass can be stated as: total mass entering CV – total mass leaving CV =
mCV
net change in mass within CV
m m
i
e
m°i
Control volume
m°o Fig. 1: Conservation of mass principle for a CV. The conservation of mass can also be expressed in the rate form:
m
i
m e dmCV / dt
The amount of mass flowing through a cross section per unit time is called the mass flow rate and is denoted by m°. The mass flow rate through a differential area dA is: dm°= ρVn dA where Vn is the velocity component normal to dA. Thus, the mass flow rate for the entire cross‐section is obtained by:
M. Bahrami ENSC 388 (F09) 1
st
Law of Thermodynamics: Control Volumes 1
m Vn dA
A
(kg/s)
Assuming one‐dimensional flow, a uniform (averaged or bulk) velocity can be defined: m°= ρ V A (kg/s) where V (m/s) is the fluid velocity normal to the cross sectional area. The volume of the fluid flowing through a cross‐section per unit time is called the volumetric flow, V°:
V Vn dA VA
A
(m 3 /s) ...