Mechanical Engineering Dictionary-Gas Turbines

Important Terms and their Meaning of Mechanical Engineering- Gas Turbines:

ADIABATIC PROCESS – Thermodynamic process in which no heat is transferred to or from the system during the process. A reversible adiabatic process is called ISENTROPIC PROCESS.

AIRFOIL – A streamlined form bounded principally by two flattened curves and whose length and width are very large in comparison with thickness. The airfoil may be a symmetrical airfoil or a non-symmetrical airfoil.

AIRFOIL DRAG FORCE – Force acting on the airfoil in the direction of motion, represents frictional forces.

AIRFOIL LIFT FORCE – Force acting on the airfoil in the direction perpendicular to the direction of motion. Basic force causing the aeroplane to maintain its lift.

AIR RATE – Kilograms per second of airflow required per net horse power developed. Also called FLOW RATE.

ANGLE OF ATTACK – The angle of inclination of the non-symmetrical airfoil with the direction of the undisturbed flow.

ANGLE OF DEVIATION – The difference between the fluid angle at outlet and the blade angle at outlet. This may be positive or negative. Sometimes called DEVIATION.

ANGLE OF INCDIENCE – The difference between the fluid angle at inlet and the blade angle at inlet. This may be positive or negative. Sometimes called INCIDENCE.

ANNULAR TYPE COMBUSTION CHAMBER – The combustion chamber which is made up of four concentric surfaces surrounding the axis of the rotor, forming three chambers on either side of the rotor, the middle casing acts as a flame tube and the inner and outer casings act as air casings, with a series of burners at the front end.

ASPECT RATIO OF BLADE – Ratio of blade height to blade chord.

AXIAL FLOW COMPRESSOR – A type of compressor in which the fluid flow is almost parallel to the axis of the compressor and the flow is decelerating or diffusing and pressure rises are obtained by causing the fluid to pass through a number of expanding spaces with consequent reduction in velocity.

BLADES or BUCKETS – The parts that form the rotor flow passages and serve to change the direction, and hence the momentum, of the fluid received from the stationary nozzles.

BLADE SPEED RATIO – Ratio of mean blade speed to the absolute velocity of the fluid stream at the blade inlet.

BOUNDARY LAYER – A thin layer of fluid adhering to a surface, when the fluid flows along the surface, in which there is a steep velocity gradient due to viscous friction, the velocity dropping to zero at the boundary surface.

BRAYTON CYCLE – Basic cycle for gas turbines. The cycle in which air is compressed isentropically, heated at constant pressure and expanded isentropically thus delivers work until the low pressure is reached and then heat is rejected. Also called JOULE CYCLE.

CAN TYPE COMBUSTION CHAMBER – Combustion chamber in which the air leaving the compressor is split into several streams and each stream is supplied to a separate cylindrical combustion chamber.

CARRY OVER LOSS – Kinetic energy discarded in the exhaust. Axial exit of the fluid from the turbine blades reduces this loss.

CASING – Turbine enclosure to which the nozzles and guides are fixed. Also called a SHELL or CYLINDER.

CENTRIFUGAL COMPRESSOR – A type of compressor in which air is sucked into the impeller eye, whirled around at high speed by the vanes on the impeller disc and flung out by centrifugal force.

CLOSED CYCLE TURBINE – Turbine in which the working fluid does not come in contact with the atmospheric air and the heat to the working fluid is provided in the heater by burning the fuel externally.

COMBINATION PLANT – A gas turbine plant that utilizes reheat, intercooling and regeneration.

COMBUSTION CHAMBER – The unit in which the chemical combination of oxygen in the air supplied by the compressor takes place with the carbon and hydrogen components of the fuel in such a manner that a steady stream of the gases at uniform temperature is produced and delivered to the turbine.

COMBUSTION EFFICIENCY – The ratio of the actual heat realised by the combustion of fuel to the ideal value i.e., calorific value.

COMBUSTION INTENSITY – Ratio of the rate of heat supply by fuel to the product of volume of combustion chamber and inlet pressure in atmospheres.

COMPRESSOR EFFICIENCY – Ratio of work required for isentropic compression to the actual work input. Also called ISENTROPIC COMPRESSION EFFICIENCY.

COMPRESSOR MECHANICAL EFFICIENCY – Ratio of rotor horse power to shaft horse power supplied to the compressor.

CONSTANT PRESSURE TURBINE – Turbine in which the fuel is burnt at constant pressure. Combustion is a continuous process.

CONSTANT VOLUME TURBINE – Turbine in which the combustion takes place at constant volume. Also called EXPLOSION TYPE TURBINE.

CONVERGENT DIVERGENT DIFFUSER – A type of diffuser which can build up pressure when velocities are reduced from supersonic to subsonic values.

COOLING OF TURBINE BLADES – Turbine blades are cooled by water or air. This enables the temperature of the blade metal to be several hundred degrees lower than the gas temperature and permits employment of correspondingly higher turbine inlet temperatures, with the metals available at present, resulting in higher turbine efficiency.

COUNTERFLOW HEAT EXCHANGER – A heat exchanger in which compressed air and hot gases let out by the turbine, flow in opposite directions.

CROSS COMPOUNDED UNIT – The system in which the low pressure compressor is driven by the high pressure turbine and the high pressure compressor by the low pressure turbine.

CROSS FLOW HEAT EXCHANGER – A heat exchanger in which the compressed air and the hot gases let out by the turbine flow normal to one another.

CYCLE PRESSURE RATIO – Ratio of the pressure at inlet to the gas turbine to that at inlet to the compressor.

CYCLE WITH INTERCOOLED COMPRESSION – Gas turbine cycle in which the compression of the working fluid is cut off at some intermediate pressure and the fluid is cooled by passing it through a heat exchanger supplied with coolant from some external source before being compressed in the second compressor to the required pressure ratio.

DEFLECTION ANGLE – Total fluid turning angle i.e., difference between the fluid angle at inlet and the fluid angle at outlet.

DEGREE OF REACTION (compressor) – The ratio of the static temperature rise in the rotor to that in the whole stage.

DEGREE OF REACTION (turbine) – Ratio of enthalpy drop in rotor blades to enthalpy drop in the stage.

DIAPHRAGM – The component fixed to the cylinder or casing and contains the nozzles and serves to confine the fluid flow to the nozzle passages.

DIFFUSER – Unit connected to the outlet of the centrifugal compressor, which provides a gradually increasing area to convert velocity energy into pressure energy.

DIFFUSION – Process in which the energy of a moving stream of fluid is transformed in such manner, then an increase in pressure occurs.

DISC or WHEEL – The component to which the moving blades are attached directly and it is keyed or shrunk on the shaft.

DISC FRICTION – When a disc rotates in free air, a certain amount of pumping action would take place, imparting motion to the surrounding air, and this relative motion between the disc and air causes friction, called disc friction.

DOUBLE SIDED IMPELLER – Impeller of a centrifugal compressor in which suction takes place from both sides. Here, two similar impellers are placed back to back.

DYNAMIC HEAD – The difference between the total head pressure and the static pressure.

EFFICIENCY OF IMPULSE BLADING – Ratio of the delivered power or energy, to the power or energy supplied in kinetic form to an impulse blade.

ERICSON CYCLE – The gas turbine cycle which incorporates multistage compression with intercooling, and multistage expansion with reheating.

FLAME STABILIZATION – Making the flame to be more or less stable at a particular location in the combustion chamber by a system whereby part of the high temperature products of combustion can be caused to recirculate in order to ignite fresh reactants.

FLOW COEFFICIENT – Ratio of axial components of absolute velocity of fluid stream to the blade velocity at that location.

FLOW LOSSES – Pressure loss due to friction and turbulence. This consists of combustion chamber loss, heat exchanger loss (air side), heat exchanger loss (gas side), intercooler loss (air side) and duct losses occurring between components and at intake and exhaust.

FLUID ANGLES – Angles at which the fluid enters and leaves a blade. These are seldom the same as the blade angles.

FOIL NOZZLE – A nozzle formed by curved airfoil sections or facsimiles of airfoils and is characterised by its high efficiency.

FREE VORTEX FLOW – In a compressor, the condition when the whirl velocity of a flowing fluid varies inversely as the radius.

FUEL RATIO – The weight of fuel used to heat unit weight of compressed air to the turbine inlet temperature.

FULL ADMISSION – Admission of gas stream over the full blade entry, and this becomes possible when the nozzles subtend the whole blade circumference (annulus area).

GAGING – Ratio of the net area of gas flow to the total free annular area in the blade ring.

GAS TURBINE – A rotary machine, which consists of a compressor, combustion chamber and a turbine. Air is compressed in the compressor, passed into the combustion chamber where fuel is burnt, products of combustion impinge over rings of turbine blades with high velocity and work is done.

GUIDE BLADES – Row of blades interposed between the blade rows comprising rotor passages, so as to reverse the direction of the fluid leaving the preceding moving blade row and make the general direction of the fluid entering all the moving blade rows to be similar.

HEAT EXCHANGER – Device which uses some of the heat in the turbine exhaust gas to preheat the air entering the combustion chamber. This reduces the fuel supply for a given required temperature increase. Also called REGENERATOR.

HEAT EXCHANGER THERMAL RATIO – Ratio of the actual heat picked up by the compressed air in the heat exchanger to the maximum possible heat that could be absorbed. Also called EFFECTIVENESS OF HEAT EXCHANGER.

INTER COOLING – Cooling of air in between the stages of compression so as to reduce the work of compression. The device used for cooling is intercooler.

INTERNAL EFFICIENCY OF A TURBINE – Ratio of work (power) delivered to the rotor of the turbine by the gaseous medium compared with the ideal energy available for work (power) from the medium in expanding through the same pressure range.

ISENTROPIC EFFICIENCY – Ratio of work to compress isentropically to the actual work to compress. Also the ratio of isentropic temperature rise to the actual temperature rise.

JET PROPULSION – The unit in which the gas turbine is designed to produce just sufficient power to drive the compressor and the exhaust gases from the turbine are then expanded to atmospheric pressure in a propelling nozzle to produce a high velocity jet.

LACING WIRES – Wires called lacing wires or lashing wires are used to keep long blades in alignment and to add stiffness.

LEAKAGE LOSSES – Energy loss due to leakage of the working fluid in turbines, between stages, past the shaft and around the balance piston.

MACH NUMBER-M – Ratio of the stream velocity to the local acoustic velocity.

MULTI SHAFT GAS TURBINE – Turbine unit in which two or more compressor combinations or turbines are carried on independent shafts. In each turbine compressor combination, the machines are coupled to each other in a series arrangement.

NET JET THRUST – That part of the thrust of a turbojet engine which is available for climb and acceleration.

NOZZLE – A flow passage specially shaped to produce kinetic energy at the expense of other forms of energy (available thermal energy).

NOZZLE EFFICIENCY – The ratio of the actual kinetic energy produced on discharge (or between any two points in a nozzle) to that obtainable by assuming an isentropic expansion in the nozzle.

ONE DIMENSIONAL FLOW – The fluid flow in which the variables are constant over any cross-section of the flow.

OPEN CYCLE TURBINE – Turbine in which the heat is transferred by direct combustion and after doing work in the turbine, the gases are exhausted into the atmosphere.

OVERALL EFFICIENCY OF JET PROPULSION – Product of the propulsion efficiency of the jet unit and the thermal efficiency of the jet unit.

OVERALL EFFICIENCY OF PROPELLER UNIT – Product of the propeller (propulsion) efficiency, thermal efficiency of the engine (power turbine) and the transmission efficiency from prime mover to propeller shaft.

OVERALL TURBINE EFFICIENCY – Ratio of the delivered shaft work (power) to the ideal energy available from the medium.

PARALLEL FLOW HEAT EXCHANGER – A heat exchanger in which both compressed air and gases let out by the turbine flow in the same longitudinal direction. Also called UNI DIRECTIONAL or COCURRENT HEAT EXCHANGER.

PARTIAL ADMISSION – Admission of gas stream over only a part of the blade circumference, since the nozzles are covering only a fraction of the inlet blade circumferential annulus. This is the case in impulse turbines.

PLANE SHOCK WAVE – Shock wave in which the variables of flow ego pressure, temperature and velocity are constant along the wave front.

POLYTROPIC EFFICIENCY – The isentropic efficiency of an elemental stage of the compression which is constant throughout the process. Also called the SMALL STAGE EFFICIENCY.

POSITIVE DISPLACEMENT COMPRESSOR – Type of compressor in which a fixed amount of working fluid is being positively contained during its passage through the machine.

POWER INPUT FACTOR – For a compressor this is the ratio of the actual work to the theoretical work of compression. Represents an increase in the work input the whole of which is absorbed in overcoming frictional loss and which is therefore degraded into heat energy.

POWER RATIO – Ratio of useful or net horse power of the cycle compared with the power developed by the turbine of the system. Also called WORK RATIO.

PRESSURE COMPOUNDED IMPULSE TURBINE – The turbine in which the pressure range available for expansion is broken into a series of steps or stages. Each stage consists of a nozzle or bank of nozzles (which increase the kinetic energy) followed by a row of turbine blades or buckets (which absorb the kinetic energy).

PRESSURE RATIO – Ratio of pressure of air at the end of compression to the pressure of air at the begining of compression.

PREWHIRL – Whirl velocity (tangential component of the absolute velocity at intake), imparted to the air that enters the centrifugal compressor impeller, by allowing the air to be drawn into the impeller eye over curved inlet guide vanes attached to the impeller casing.

PRIMARY AIR – Part of the air which flows through the core of the combustion chamber, in just sufficient quantity for combustion.

PRIMARY ZONE – Portion of the combustion chamber wherein about 15 to 20% of the air is introduced around the jet of fuel and the burning of this rich mixture provides the high temperature necessary to prepare the mixture for further reaction and burn it almost completely in a very short time.

PROPELLER JET ENGINE – The unit in which the gas turbine develops power in excess of that required to drive the compressor and employs this excess power to drive a propeller through reduction gearing and the leaving jet also contributes to the thrust power.

PROPULSION EFFICIENCY – Ratio of thrust power to the jet power.

RAM EFFECT – The effect which causes an increase of temperature and pressure of the air that enters the compressor of an aircraft gas turbine unit due to aircraft speed. Sometimes called RAM.

RAM EFFICIENCY – Actual pressure rise realized in a diffuser compared with the pressure rise possible under reversible conditions. Also called INTAKE EFFICIENCY.

REACTION TURBINE – The turbine in which the nozzles and moving blades are each made in the same general form, with the cross-section in the direction of the gas flow reduced so that both the fixed and moving blades act as expanding nozzles.

REGENERATIVE PLANT – The plant that utilizes a heat exchanger to recover heat from the turbine exhaust gases and thereby decreases the heat required to be added in the combustor.

REHEAT CYCLE – The unit in which the expansion of the hot gases is carried out in two stages, and reheating of the working fluid to the upper limit of temperature takes place between the stages of expansion.

ROCKET – A self propelled unit in which the fuel and the oxidant are contained within the shell.

SECONDARY AIR – Air quantity which is about five times the minimum air for combustion, that flows around the annular space of the combustion chamber and cools the products of combustion.

SECONDARY ZONE – Portion of the combustion chamber wherein about 30% of air is added at the right points in the combustion process so as to complete the combustion of fuel.

SHAFT, ROTOR, SPINDLE – The rotating assembly of the turbine which carries the blades.

SHROUD – A band placed around the periphery of the blade tips in order to stiffen the blades and prevent spillage of the fluid over the blade tips. Shrouds may be continuous or in segments integral with one or more blades.

SINGLE SHAFT GAS TURBINE – Turbine unit in which all compressors and turbines in the plant are mounted on one shaft and are coupled in series arrangement.

SLIP – The failure of the whirl velocity of air at the outlet of the centrifugal impeller becoming equal to the impeller tip speed.

SLIP FACTOR – Ratio of whirl velocity of air at the outlet of the compressor (centrifugal type) to the impeller tip speed. This factor limits the work capacity of the compressor even under isentropic conditions.

SOLIDITY OF A BLADE – The ratio of blade chord to pitch, and ranges from 1.0 to 2.0.

SONIC VELOCITY – Speed of sound in a gas. This is the velocity at which a pressure wave is propagated throughout the gas and this velocity depends upon the pressure and density of the gas.

SPECIFIC POWER – Net horse power developed per kg per second of air flow.

STAGGER ANGLE – The angle made by the axial direction and the chord line, a parameter describing the setting of a row of blades of given form and spacing.

STAGE OF A TURBINE – Unit which consists of the fixed nozzle row (or fixed blade row) and the moving row of blades which receives the gases.

STAGNATION ENTHALPY – Enthalpy of a moving gas when brought to rest isentropically. Also called TOTAL HEAD.

STAGNATION TEMPERATURE – The hypothetical temperature which would result if all the kinetic energy of a flowing gas were to be converted into heat under conditions of no gain or loss of heat i.e., adiabatic conditions. Also called TOTAL HEAD TEMPERATURE. This is the sum of static temperature and dynamic temperature.

STALLING – The phenomenon of reduction in the lift force (acting on an air foil) at higher angles of incidence. Also called LIMIT OF STABILITY.

STATIC HEAD EFFICIENCY – Ratio of the temperature equivalent of the work output and leaving energy to the isentropic temperature drop from the total head inlet to the static outlet pressure.

STEADY FLOW – Flow of fluid in which the quantities such as velocity, pressure, temperature etc., may change from point to point but they do not vary with time at any particular point.

STRAIGHT COMPOUNDED UNIT – The system in which the low pressure compressor is driven by the low pressure turbine and the high pressure compressor by the high pressure turbine. Power is taken from the low pressure turbine shaft.

SUBSONIC DIFFUSER – A diffuser having a diverging cross-section in the direction of flow.

SUPERSONIC DIFFUSER – A diffuser having a converging cross-section in the direction of flow.

SYMMETRIC STAGE AXIAL FLOW COMPRESSOR – An axial flow compressor which has symmetric blade arrangement so that the pressure rise in the moving row and the pressure rise in the fixed row are equal.

TAPERED BLADES – The turbine blades taper (decrease in depth) from base to tip, so as to diminish the centrifugal stress at the various sections, at the base or hub sections of the blade.

TERTIARY ZONE – Portion of the combustion chamber wherein the left out 50% of air is mixed with the burnt gases so as to cool them down to the temperature suitable to turbine materials.

THRUST OF A JET – Sum total of the pressure thrust and that due to change of momentum.

TOTAL HEAD EFFICIENCY – Ratio of the actual work output to the maximum possible work output that could be obtained with the existing leaving energy.

TOTAL HEAD PRESSURE – Pressure of the moving fluid corresponding to the stagnation or total head temperature.

TURBINE MECHANICAL EFFICIENCY – Ratio of shaft horse power to rotor (internal) horse power.

TURBINE STAGE EFFICIENCY – Ratio of the work delivered to the rotor per unit of gas flowing divided by the isentropic drop available in each unit of gas flowing.

TURBO PROP UNIT – The unit in which the gas turbine drives a propeller and the gases after leaving the turbine are exhausted as a jet to augment the thrust of the propeller.

TWISTED BLADES – Long turbine blades are usually twisted from hub to tip so as to compensate for blade velocity variations and in some cases as well as to satisfy radial pressure equilibrium conditions.

VELOCITY COMPOUNDED IMPULSE TURBINE – The turbine in which the kinetic energy created by the expansion of gas in nozzles is absorbed in two or more rows of moving blades so as to reduce the speed of the turbine rotor.

VOLUMETRIC EFFICIENCY – Ratio of the equivalent volume of free atmospheric air finally delivered by a compressor to the volume of free atmospheric air entering the suction pipe of the compressor.

WINDAGE LOSSES – When moving blades come in contact with inactive fluid, some kind of kinetic energy is imparted to the fluid at the expense of the kinetic energy of the blades. There is also frictional effect. This loss is known as windage losses.

WORK RATIO – The ratio of the actual work output (in heat units) to the isentropic heat drop from the total head inlet to the static outlet conditions.

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