HEAT TRANSFER PROBLEMS IN GAS TURBINE COMBUSTION CHAMBERS AND TURBINE BLADES.

INTRODUCTION

The overall efficiency and thermal efficiency of the gas turbine engine increases by increase in compressor pressure ratio combustion chamber and turbine inlet temperature.

This increase in turbine and combustion chamber inlet temperature causes severe thermal stresses and spoils the life of the structural components.

To solve this problem and to achieve more efficiency we have two options .

1.using high temperature materials 2.adapting cooling methods.

PROBLEMS IN USING HTM’S :

Most of the high temperature withstanding materials are poor in possessing mechanical strength example ceramics.

High temperature materials needs strengthening mechanisms to get good mechanical properties this in turn creates manufacturing difficulties and expensive.

Some times TBC (thermal barrier coating) are used this will cause of adding extra materials and weight is increased this is again undesirable and affecting performance parameters.

ADAPTING COOLING METHODS:

Even though high temperature materials are having some disadvantages still this is been used in gas turbine applications.

Providing cooling to the surfaces of the combustion chamber liner and turbine blade surface is the another option to increase engine performance.

Cooling method involves heat transfer problems , because flow over the turbine blades and cooling needed depends time factor how fast the turbine blade is to be cooled is the matter of cooling.

METHODS OF COOLING :

Convection cooling Impingement cooling Film cooling Transpiration cooling Water/Steam cooling

CONVECTION COOLING:

This form of cooling is achieved by designing the cooling air to flow inside the turbine blade or vane, and remove heat through the walls. Usually, the airflow is radial, making multiple passes through a serpentine passage from the hub to the blade tip. Convection cooling is the most widely used cooling concept in present-day gas turbines.

IMPINGEMENT COOLING:

In this high-intensity form of convection cooling, the cooling air is blasted on the inner surface of the airfoil by high-velocity air jets, permitting an increased amount of heat to be transferred to the cooling air from the metal surface. This cooling method can be restricted to desired sections of the airfoil to maintain even temperatures over the entire surface. For instance, the leading edge of a blade needs to be cooled more than the mid chord section or trailing edge, so the gas is impinged.

FILM COOLING:

This type of cooling is achieved by allowing the working air to form an insulating layer between the hot gas stream and the walls of the blade. This film of cooling air protects an airfoil in the same way combustor liners are protected from hot gases at very high temperatures.

TRANSPIRATION COOLING:

Cooling by this method requires the coolant flow to pass through the porous wall of the blade material. The heat transfer is directly between the coolant and the hot gas. Transpiration cooling is effective at very high temperatures, since it covers the entire blade with coolant flow.

WATER/STEAM COOLING Water is passed through a number of tubes

embedded in the blade. The water is emitted from the blade tips as steam to provide excellent cooling. This method keeps blade metal temperatures below (537.8 ◦C).Steam is passed through a number of tubes embedded in the nozzle or blades of the turbine. In many cases, the steam is bled from the HP Steam Turbine of a combined cycle power plant and returned after cooling the gas turbine blades , where the steam gets heated in the process to the IP steam turbine. This is a very effective cooling scheme and keeps the blade metal temperature below (649 ◦C).

VARIOUS SUGGESTED COOLING SCHEMES :

TURBINE BLADE COOLING CONCEPTS:

TEMPERATURE AND STRAIN DISTRIBUTION OF THE TURBINE BLADE:

TURBINE COOLING:

TURBINE COOLING:

TURBINE NOZZLE COOLING:

COOLING SYSTEM:

COOLING SYSTEM:

COMBUSTION CHAMBER COOLING:

COMBUSTION CHAMBER COOLING:

COMBUSTION CHAMBER COOLING:

REFERENCES:

Gas Turbine Engineering Handbook by Meherwan P. Boyce

© Rolls-Royce plc 1986 the jet engine Project report submitted by Maja Munktell

Dept. of Energy Sciences, Faculty of Engineering

Lund University, Box 118, 22100 Lund, Sweden

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