An introduction to gas turbine (GT)

In the name of Allah who is the most beneficient and merciful       

The aim is to be able to identify and explain the function of gas turbine major engine component and discuss the mode of operation of simple and complex cycle gas turbine engines compared with diesel and steam plant.

Definition of a Gas Turine

A continuous cycle self contained heat engine using a gas as the working fluid.

Breakdown of definition

1.      Energy output. After providing the power to drive the compressor(self contained) the energy output may be in the form of :

·         Jet thrust

·         Shaft power

·         Compressed air from compressor

·         Heat

2.      Self contained. Once running it is not dependent on any other machine.

3.      Gas may be.

·         Air – Open cycle

·         Hydrogen, helium etc closed cycle . In this case heat is added by means of a heat exchanger.

4.      Energy source. This may be provided by

·         Oil fuel

·         Natural gas

·         Sewage gas

·         Waste gas

·         Waste heat (eg blast furnance)etc .

5.      Compressor. This is usually either axial or centrifugal, but free piston and reciprocating compressor have been tried, although the process then is no longer continuous.

6.      Continuous. Unlike a diesel which operates with gas in a similar way the GT uses a constant pressure process which is continuous.

Comparison of cycles

In the services we are concerned with three main forms of providing power (see below). These all have particular advantages and disadvantages, some of which can be readily appreciated by a quick comparision of the ideal cycle on a T-S chart.

The following factors show themselves:

1)      Steam Plant

• Operates across saturation envelope.
• Closed cycle – recovery process needed to condense and re-use steam, hence much additional machinery.
• Small work to compress fluid. Good work from expanding vapour- large exess.

2)      Diesel

·         Fairly good available work output after compressing gas.

·         Open cycle.

·         Non-continuous cycle means valves and moving parts. Stress problems.

3)      Gas turbines

• Very poor specific work output available after compressing gas. High powers only available if:
  
• Component efficiencies high
• High mass flow
• High mass flow possible because process is continuous.

Conclusions

From the above comparisons the factors which stand out about gas turbines are:

• Unlike The others, an overall power output is possible purely because the constant pressure lines diverge with increase in entropy.
• Its specific output is very small and dependent upon the efficient operation of the various components. Mass flows are high hence small increases in component efficiencies will give relatively large increases in output.

Marine gas turbine  (GT)

An aircraft can use the energy in the gas stream directly to provide the propulsive power eg in a jet. In the marine environment this is not feasible because of the very nature of the surroundings , the noise, the requirement to reverse and limitations imposed by the construction of the ship. The GT must be used to drive a propeller/water jet/generator. Because of this requirement we may consider the marine GT to be split into two main parts.

·         The gas generator- that providing the high energy gas stream.

·         The power producer- that converting the energy in the stream into a useful form of power eg shaft power(power turbine)

Hence it is possible to adapt an aircraft engine for  marine use by the addition of a power turbine eg Olympus SMIA.

In addition marine GTs are not nearly so restricted for space as the aero versions and therefore it is theoretically possible to attempt to improve the performance by using one or more of the following:

• Intercooling
• Reheat
• Heat exchanger
• Water injection
• Waste heat recovery

REASONS FOR THE ADOPTION OF GAS TURBINE

1. The aim to be able to explain the reasons why the royal navy and other navy is adopted gas turbines as main propulsion units in major surface warships, the typical problems associated with running them at sea and their solutions.

1. The resons for using gas turbine in warships:

·         High power/weight ratio.

·         Quick startup capability

·         Comparatively low development cost.(benefits from aero engine development)

·         Low onboard maintenance requirement.

·         Ease of upkeep by exchange of critical parts.

·         Reduced watchkeeping manpower.

·         Good SFC at high power.

·         Availability

·         Reduced underwater noise(fewe hull openings)

1. Typical problems and solutions

·         Distortion of combustion chambers- ongoing design effort, regular inspection.

·         Combustion of naval fuels-redesign of combustion system.

·         Compressor fouling-air filtration, regular washing.

·         Surge and rotating stall- air bleeds, variable geometry blades.

·         Turbine Disc failures – Design effort, defined service life.

·         Bearing failures- Uprated bearings, earlier detection of possible failure.

·         Fuel consumption at part load- multispool variable geometry engines, higher operating temperature.

·         Practical problem of a complex cycle- no solutions at present.

·         Poor life of aero types –rapid engine change capability-comprehensive repair/rebuild facilities, life continually being upgraded.

Written by Zeeshan Ahmed