STEAM NOZZLES CLASS NOTES FOR MECHANICAL ENGNEERING

STEAM NOZZLES CLASS NOTES FOR

MECHANICAL ENGINEERING

 Turbines and steam engines use

steam. Turbines produce

electricity in thermal power plants.

Turbines use nozzles to produce a high

velocity jet. This jet runs the steam turbine.

NOZZLES

A nozzle is a passage of variable cross section. A nozzle produces high velocity jet. It converts
  (i) Pressure energy into kinetic energy
  (ii) Enthalpy into kinetic energy
  (iii) Solar energy into electrical energy
These jets may be

(a) Sub-sonic

(b) Sonic 

(c) Super-sonic
These jets are used to produce thrust and hence power.

TYPES OF NOZZLES

Convergent Nozzles

Area reduces from inlet to outlet. Throat has the minimum area at the exit of the convergent portion. Angle of convergence  is up to 30^0. Length of convergence is less. These nozzles produce subsonic or sonic jets. At the most, there is sonic velocity at the throat. Only convergent nozzles are used as spray nozzles.
Divergent Nozzles———area increases from inlet to outlet—-Rarely used as nozzles. These are used as diffusers—to increase pressure by reducing the velocity.
Convergent-divergent nozzles (C-D nozzles)

It is a combination of convergent and divergent portion. Maximum angle of divergence is 12⁰. Smaller angle of divergence is to avoid SEPARATION OF FLOW AND EDDY FORMATION. Hence length of divergence is much more than the convergent length. THESE NOZZLES ARE IN MAXIMUM IN USE  to get super-sonic jets. Therefore only convergent -divergent nozzles are used in steam turbines to produce power.

PRACTICAL APPLICATIONS OF NOZZLES

The high velocity jets are used to run
  (i) Steam turbines
  (ii) Gas Turbines
  (iii)  Rockets, jets produce required thrust for supersonic flight
  (iv) Spray nozzles used in industry

EFFICIENCY OF A NOZZLE 

Actual enthalpy drop/Isentropic enthalpy drop

 NOZZLES

These are convergent divergent nozzles only. In steam turbines, only super-sonic jets are required to run the high speed turbines.
V = √2gh

The velocity at inlet of nozzle is much smaller than velocity at outlet. Hence inlet velocity is neglected.

 NOZZLES WITHOUT FRICTION

V = 44.72√h           m/s   where h is enthalpy drop in k J /kg

 NOZZLES WITH FRICTION

V = 44.72√k h                       m/s
k = nozzle friction coefficient or nozzle efficiency, it reduces the enthalpy drop by 10 to 15 %

Therefore value of k = 0.85 to 0.90
h = isentropic enthalpy drop (k J/kg) in the nozzle

ANALYSIS OF NOZZLES

Velocity ‘v’ in terms of pressure and volume
v = √2(n/(n-1))p1V1 [1 –(p2/p1) ^{(n – 1)/n}
Where p1 and p2 are the pressures at the inlet and outlet of the convergent portion
v is the velocity of steam at the throat
V1 is the specific volume at the inlet of the convergent portion

CONDITION FOR MAXIMUM DISCHARGE THROUGH THE NOZZLE

p₂/p₁ = [2/(n+1)] ^n/(n-1)
FOR SATURATED STEAM, n= 1.135
p₂/p₁= 0.58
FOR SUPERHEATED STEAM, n = 1.3
p₂/p₁= 0.546

MAXIMUM FLOW RATE

m^. _max = A_t [n(p₁/v₁)[2/(n+1)] ^(n+1)/(n-1)]^0.5
where A_t is the area at throat
v _max = √(2(n/(n+1))p₁V₁)
v _max maximum velocity at the throat

RELATION BETWEEN AREA, VELOCITY AND PRESSURE IN FLOW THROUGH THE NOZZLE

dA/A = (1/n)(dp/p)[(1 – M2)/M2]
In flow through the nozzle, dp/p is negative—decrease of pressure with increase of length

TWO DIFFERENT CASES ARISE

CASE 1—ACCELERATED FLOW—–REQUIRED IN STEAM TURBINES
Two different possibilities arise
(i) If v < c, M < 1, where c is the velocity of sound

∴dA/A should be negative i.e. area should decrease i.e. CONVERGENT

∴ Convergent portion produces sonic velocity.

(ii) If v > c, M > 1, ∴dA/A should be positive i.e. area should increase i.e. DIVERGENT
∴ Divergent portion produces super sonic velocity.

CASE 2

DEACCELERATED FLOW

IT IS NOT APPLICABLE TO STEAM NOZZLES

Decrease of velocity throughout
It will occur in the diffuser
dp/p will be positive.
V > c , M > 1
dA/A has to be positive—-divergent portion
kinetic energy is converted into pressure energy.

CASE 2 IS NOT APPLICABLE TO NOZZLES USED IN STEAM TURBINES SINCE ONLY ACCELERATED FLOW IS REQUIRED.

https://mesubjects.net/wp-admin/post.php?post=14522&action=edit        MCQ St Turbines 

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