NON FLOW AND FLOW WORK

NON FLOW AND FLOW WORK

CLASS NOTES FOR MECHANICAL

ENGINEERING


NON FLOW WORK

(i)                 Work input to a compressor during compression process ( from the start of compression to end of compression)

(ii)               Work obtained from a turbine during expansion only (During start of expansion to the end of expansion)

Mathematical form of non-flow work = area under the curve= w =∫pdv= (p2v2  — p1v1)/(γ – 1) kJ/kg

w= work input kj/kg

v1 = specific volume at point 1

v2 = specific volume at point 2

 Total energy entering a system=Internal energy + Flow energy + Kinetic energy +Potential energy + heat supplied= m(ui +Pivi +Ci2/2 + gzi)+Q

Total energy leaving a system=Internal energy + Flow energy + Kinetic energy +Potential energy + Work obtained= m(u0 +P0v0 +C02/2 + gz0)+W

FLOW WORK

It includes the work during

(i)                 Suction

(ii)               Compression

(iii)             Discharge

It is applicable to a compressor,  turbine,  pump and a  nozzle.

Mathematical form of Flow Work = ∫ (–vdp) = γ (p2v2  — p1v1)/(γ – 1)      kJ/kg

This flow work is γ times the non flow work.

As per the law of conservation of energy,  input = output gives the steady flow energy equation(SFEE).

m(ui +Pivi +Ci2/2 + gzi)+Q = m(u0 +P0v0 +C02/2 + gz0)+W

therefore, this equation is applicable for an ideal or real fluid, liquids, gases , compressible and in-compressible.

Now using  mi = mo and h =u+Pv

we get   (hi +Ci2/2 + gzi)+q =  (h0 +C02/2 + gz0)+w

q =Q/m and w =W/m

Steady flow energy equation can also be written on per unit time basis as given below:

(hi +Ci2/2 + gzi)+q. (h0 +C02/2 + gz0)+w.

q. — w. (h0 +C02/2 + gz0) — (hi +Ci2/2 + gzi)