Lecture 9- 1st law of thermodynamics for
open systems
Conservation of mass and the control volume
•Volume flow rate =Av (v: average velocity
throughout the cross section)
•Mass flow rate =ρAv
For steady state condition, LHS of mass conservation equation =0
Inlet
conditions
Exit
conditions
Air is flowing in a 0.2 m diameter pipe at a uniform velocity of 0.1 m/sec. The temperature
is 25oC and the pressure is 150 kPa. Determine the mass flow rate. Gas constant of air is
0.287 kJ/kgK.
Mass flow rate-0.0055 kg/sec
The first law of thermodynamics for a control volume
For closed system or control mass
For open system or control volume
Inlet & exit mass flow rate : 𝑚𝑖 𝑎𝑛𝑑 𝑚𝑒
Inlet & exit internal energy : ei and ee
h: enthalpy
For multiple inlet
and outlet points
Steady state : LHS=0 ; SFEE: Steady flow energy equation
Nozzle and Diffuser
Nozzle is a device which increases velocity or KE of a fluid at the
expense of its pressure drop. Diffuser slows down the velocity by
increasing the pressure of the fluid.
The cross-sectional area of a nozzle decreases in the
flow direction for subsonic flows and increases for supersonic flows.
The reverse is true for diffusers.
Steady state
Insulated walls, no work
transfer as well as negligible
change in PE.
The same principle will hold for diffuser.