Yet Another Four Losses in Turbines - 2

P M V SubbaraoProfessor

Mechanical Engineering Department

A Set of Losses not Strictly due to Geometry of Blading….

New Losses due to Partial Admission

• PA improves part load efficiency due to decreased secondary losses.

• There are three main concerns regarding partial admission turbines.

• These are related to:

• Aerodynamics/Fluid Dynamics

• Thermodynamics

• Aeromechanics.

Special Fluid Dynamics due to Partial Admission

Firstly, there are special aerodynamic losses; pumping-, emptying- and filling losses attributed to the partial admission stage.

New Issues due to PA

• Secondly, in multistage turbines the downstream stages experience non-periodic flow around the periphery and substantial circumferential pressure gradients and flow angle variations that produce additional mixing losses.

• Thirdly, compared to full admission turbines, the forcing on downstream components is also circumferentially non-periodic with rapid load changes.

• This is very high for the rotor in the admission stage.

The Effect of Distribution of Given PA Level

aiVU

Circumferential Variation of Absolute Flow Angle

Variation of Stage Exit Total Temperature

Variation of DoR

Estimation of PA Losses

• Partial admission losses can be broken down into pumping loss, filling loss and emptying loss.

• The pumping loss refers to the pumping in the inactive blade channels rotating in a fluid-filled casing.

• The losses that originate from the filling and emptying of the rotor passages as the blades pass through the active sector are sometimes combined and referred to as sector loss.

The Pumping Power Loss

The Sector Losses

The sector loss, associated with the emptying and filling of rotor passages as the blades pass by the active stator arc, is found to be

where Ks is a loss coefficient representing the decrease of the momentum of the fluid passing through the rotor compared to the available energy of the fluid.

η efficiency of full-admission turbineη p efficiency of partial-admission turbineKw exit-to-inlet relative velocity ratio ( Vre/Vri)SR rotor blade pitch

Effect of PA on Number of Stages

Old Last Stage LP Blade

Adiabatic Expansion of Steam

• The liquid in the LP turbine expansion flow field is considered to progressively appear, with lowering pressure, in four forms, namely as:

• A fine mist (or fog) suspended in the steam;

• As a water stream running in rivulets along the casing (mainly OD);

• As a water film moving on the surface of the blades (mainly stator; not particularly evident on the rotor blades owing to centrifugal-flinging action);

• As larger droplets created when the water flowing along the surface of the blades reaches the trailing edge.

Notional Diagram of Path Break Down

Deposition of Part of Fog and Coarse water

Coarse water spray

Fog

Impact & Splashing of Coarse water

Re-entrainedCoarse water

Coarse water spray centrifuged from blade

Impact & Splashing of Coarse water

Centrifuging of deposited Fog and Coarse water

Notionally envisaged progressive process

• Formation of fog that continues to appear in the through-flow, some of which is deposited.

• Deposition of fog droplets on blade surfaces.

• Coarse water re-entrained in through-flow primarily from fixed blades.

• Impact of coarse water on the moving blades.

• Coarse water re-entrained in through-flow from moving blades

• Coarse water entering the next stage (fixed blades).

• Continued process, with more fog formation and some deposition, in the successive stages.

Deviation of Eater Droplets

Velocity triangles for coarse water droplets

Velocity triangles for steam

Transport Losses

• Impact of droplets on blade surfaces, with strong resulting momentum exchange.

• Slip of the droplets relative to the main steam flow, causing drag between the droplets and the dry steam.

• This is because of the high-density water droplets that cannot accelerate as fast as the dry steam under the same pressure gradient.

Wetness losses

• The level of allowable moisture in the last stages of the LP turbine has been a practical limit on the usable temperatures and pressures of steam since the earliest turbine designs.

• Severe erosion was found in LP blades of early turbine designs and lead to the imposition of a limitation of about 12% on exit wetness.

• A second, although less limiting effect, was characterized by Baumann as early as 1910: that the efficiency, of wet stages of the LP decreases approximately 1% for every 1% increase in wetness in the stage.

Losses Vs Wetness

Exhaust Diffuser For L P Turbine

Exhaust Hood

Path Lines in Exhaust Hood

Steam Turbine Exhaust Size Selection

• The steam leaving the last stage of a condensing steam turbine can carry considerably useful power to the condenser as kinetic energy.

• The turbine performance analysis needs to identify an exhaust area for a particular load that provides a balance between exhaust loss and capital investment in turbine equipment.

Turn-up loss

Total Exhaust Loss

Gross hood loss

Annulus restriction loss

Annulus Velocity (m/s)

Exh

aust

Los

s, k

J/kg

of

dry

flow

0 120 240 180 240 300 360

10

20

30

40

50Annulus velocity (m/s)

Condenser flow rate

Annulus area

Percentage of Moisture at the Expansion line end point

Typical exhaust loss curve showing distribution of component loss

SP.Volume

an

steamexan A

xvmV

3600

01.01.

Actual leaving loss

Optimal Design of Exhaust Hood

The

rmod

ynam

ic

Opt

imum

Economic Optimum

Total Exhaust Losses

Axial Leaving Losses