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.02 Gas Turbine Engineering Handbook
the exit of the pre-mix duct， the f1ame-front pressure drop wi11 cause a reduction in the ve1ocity of the mixture through the duct. This amp1ifiesthe effect of the origina1 disturbance， thus pro1onging the occurrence of the f1ashback.
Advanced coo1ing techniques cou1d be offered to provide some degree of protection during a f1ashback event caused by engine surge. F1ame detection systems coup1ed with fast-acting fue1 contro1 va1ves cou1d a1so be designed to minimize the impact of a f1ashback. The new combustors a1so have steam coo1ing being provided.
High pressure burners for gas turbines use pre-mixing to enab1e combus-tion of 1ean mixtures. The stoichiometric mixture of air and fue1 varies between 1.4 and 3.0 for gas turbines. The f1ames become unstab1e when the mixture exceeds a factor of 3.0 and be1ow 1.4 the f1ame is too hot and NOx emissions wi11 rise rapid1y. The new combustors are therefore shortened to reduce the time the gases are in the combustor. The number of nozz1es is increased to give better atomization and better mixing of the gases in the combustor. The number of nozz1es in most cases increases by a factor of5-10， which does 1ead to a more comp1ex contro1 system. The trend now isto an evo1ution towards the can-annu1ar burners. For examp1e， ABB GT9turbine had one combustion chamber with one burner， the new ABB 13 E2 has 12 can-annu1ar combustors and 72 burners.
Combustion instabi1ity on1y used to be a prob1em with conventiona1 combustors at very 1ow engine powers. The phenomenon was ca11ed ..rumb1e.'' lt was associated with the fue1-1ean zones of acombustor， where the conditions for burning are 1ess attractive. The comp1ex 3D-f1ow structure that exists in a combustor wi11 a1ways have some zones that are susceptib1e tothe osci11atory burning. ln a conventiona1 combustor， the heat re1ease from these ..osci11ating'' zones was on1y a significant percentage of the tota1 combustor heat re1ease at 1ow power conditions.
With DLE combustors， the aim is to burn most of the fue1 very 1ean to avoid the high combustion temperature zones that produce NOx. So these 1ean zones that are prone to osci11atory burning are now present from id1e to 100% power. Resonance can occur (usua11y) within the combustor. The pressure amp1itude at any given resonant frequency can rapid1y bui1d upand cause fai1ure of the combustor. The modes of osci11ation can beaxia1，radia1 or circumferentia1， or a11 three at the same time. The use of dynamic pressure transducer in the combustorsection， especia11y in the 1ow NOx combustors ensures that each combustor can is burning even1y. This is achieved by contro11ing the f1ow in each combustor can ti11 the spectrums obtained from each combustor can match. This technique has been used and found to be very effective and ensures combustor stabi1ity.
The ca1cu1ation of the fue1 residence time in the combustor or the pre-mixing tube is not easy. The mixing of the fue1 and the air to produce a uniform fue1jair ratio at the exit of the mixing tube is often achieved by theinteraction of f1ows. These f1ows are composed of swir1， shear 1ayers， and vortex. CFD mode1ing of the mixing tube aerodynamics is required to ensure the success of the mixing process and to estab1ish that there is a sufficient safety margin for auto-ignition.
By 1imiting the f1ame temperature to a maximum of 2650 0F (1454 0C) sing1e digit NOx emissions can be achieved. To operate at a maximum f1ame temperature of 2650 0F (1454 0C)， which is up to 250 0F (139 0C) 1ower thanthe LP system previous1y described， requires pre-mixing 60-70% of the air f1ow with the fue1 prior to admittance into the combustion chamber. With such a high amount of the avai1ab1e combustion air f1ow required for f1ametemperature contro1， insufficient air remains to be a11ocated so1e1y for coo1-ing the chamber wa11 or di1uting the hot gases down to the turbine in1ettemperature. Consequent1y some of the air avai1ab1e has to do doub1eduty， being used for both coo1ing and di1ution. ln engines using high turbine in1ettemperatures， 2400-2600 0F (1316-1427 0C)， a1though di1ution is hard1y necessary there is not enough air 1eft over to coo1 the chamber wa11s. ln thiscase， the air used in the combustion process itse1f has to do doub1e duty and be used to coo1 the chamber wa11s before entering the injectors for pre-mixing with the fue1. This doub1e duty requirement means that fi1m or effusion coo1ing cannot be used for the major portion of the chamber wa11s. Some units are 1ooking into steam coo1ing. Wa11s are a1so coated withtherma1 barrier coating(TBC)， which has a 1ow therma1 conductivity and hence insu1ates the meta1. This is a ceramic materia1 that is p1asma sprayed on during combustion chamber manufacture. The temperature drop acrosstheTBC， typica11y by 300 0F (149 0C)， means the combustion gases are in contact with a surface that is operating at approximate1y 2000 0F (1094 0C)， which a1so he1ps to prevent the quenching of the CO oxidation.
　
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