Analysis of Re Influence on MILD Combustion of Gas Turbine

The paper numerical studied the MILD(Moderate or Intense Low-oxygen Dilution) combustion mode and performances in the designed gas turbine chamber. The influence of air jet Re number on flue gas recycles ratio Kv and hereby on kerosene fuel MILD combustion were modeled. For fixed equivalence ratio, increasing the air jet Re number to the Kv value of 3.3 3.8, MILD combustion mode will be formed. It has MILD combustion performances of volume combustion, excellent outlet temperature field and very low pollutant emissions. Combustor confinement has little effects on MILD combustion. Calculating results agree with other’s similar experimental data.


Introduction
In the last decades of the 20th century, there are many researches focus on the high efficiency and low emission combustion.New types of combustion mode and theory such as LPP(Lean Premix Pre-vaporized Combustion) and RQL(Rich Burn-Quench-Lean Burn) had put forward.However, these combustion molds can hardly satisfy the requirements of high efficiency and low emissions simultaneously unless combined with the staged combustion or variable geometry combustor, and has not been applied to gas turbine successfully [1].The current MILD (Moderate ＆ Intense Low Oxygen Dilution) mode have more advantageous performances of high combustion efficiency and super low emissions [2].
MILD combustion is a burning mode under the condition of low oxygen diluted, which is also known as Flameless Combustion, Colourless Combustion or FLOX (Flameless Oxidation).MILD combustion mode has the characteristics of volume or dispersion combustion which eliminates the flame frontal surface under normal temperature air.Gas, liquid, solid and other low caloric fuel can be extensively used to reduce pollutants emission and combustion noise [2].From 1991 when Wunning [3] applied flameless combustion to industrial furnace by high speed jet entrain flue gas on, it is necessary to preheat the air above 1000K for MILD combustion mode at the beginning.For the current period, all kinds of fuel can be used for MILD combustion based on the air jet of normal temperature [4][5][6].The application scope of MILD combustion is expanded from various industrial furnaces to high-tech field includes gas turbine and aeroengine [7][8][9][10][11].Great differences of operating condition between industrial furnace and gas turbine lead to grand technical challenge [10].Nowadays, this research is on a stage of rapid development including mechanism analysis, experiment and numerical simulation.

Model and Computational Grid
Figure 1 shows the 1/12 symmetrical body of the chamber and the local computational mesh of crosssection near the head of chamber.Tubular combutor model combustor is adopped and the working pressure in chamber is 0.4 MPa with the combustion intensity of 25 MW/(m3•atm).The model chamber is composed of head section and flame tube.The air into the flame tube injected cooling air in the casing, which improve the air flow distribution.12 air-atomizer noozles and 12 dilution holes are installed circumferential distributed evenly.The The ructured mixing meshes for its complicated structures.Grid numbers of direct tube and shrinking outlet are 884,869-857,855 respectively.

The Governing Equatio
The basic governing equations for turbulent combustion reaction flow are expressed as r is chemical equivalence ratio.A, E and T stand for

Calculation Conditions
mpressor of gas turbine

Results and Discussion
Combustion than 1.13× 10   2) Fl of experiment and ca ue Gas Recirculation Rate K v Previous MILD combustion results lculation indicated flue gas recirculation rate K v was important for MILD combustion mode [2].The larger the combustion air jet momentum is, the greater of flue gas recirculation rate K v , and the lower oxygen concentration [11].High velocity air jet which induced stiring action of momentum, that accelerated by combustion heat release and viscous shearing force is main influence factor of K v .Numerical simulation is an effective means of researching on this complicated action.Local K v (x) value is defined as M rec is the mass flow of flue gas, kg/s.M air and M fuel are urning, K mass flow rate of air jet and kerosene, kg/s.M injection is the secondary mass flow rate of injected air from the casing.A(x) is local across section area of flame tube, m 2 .ρ rec , v rec is local density and velocity separately.
For ordinary combustion mode like bluff body b v is kept 0.3 -0.5, but Kv for MILD combustion is larger than 3 [2].The calculating results of Kv in the gas turbine this MILD chamber for kerosene fuel is showed in Figure 4, which can be verified by Craya-Curtet experimental formula [13].Levy Y. etc. had researched on th ode in the gas turbine chamber and indicate that MILD combustion mode may be built as Kv is larger than 3.0 [10].Figure 3 showed that when Rein ≥ 1.13 × 105, at the flame lift-off location x = 0.132m, its local Kv is 1.365, 1.191 and 1.182 respectively, are both larger than 0.3~0.5 of ordinary stable combustion mode such as bluff body combustion.The backflow entrainment of high temperature flue gas has effects of heating and ignition on lift off zone of combustible mixture and may induce MILD combustion mode.Furthermore, Kv at recirculation zone center is between 3.3 and 3.8 in the Figure 3, which represents the formed MILD mode and corresponds to the existing experiment results of fuels.The entrainment air from casing has a stabilized effect on the Kv, which is beneficial to MILD state.K v calculation results fit the Craya-Curtet formula in Figure 3.When Rein ≥ 1.13 × 105 e average combustion temperature is about 1530~ 1541 K and NOx emission is between 15 and 16.5ppm analogous to 18ppm of experiment result [14].

3) NOx Emossion MILD combustion isssion show in
4) Outlet temperature field quality The temperature distribution co scribed as OTDFmax, which is showed as T 4max is outlet temperature peak value, T 4 is the outlet circumferential average temperature, T 3 is the entrance average temperature.OTDF max calculation results are between 0.293 and 0.267, decrease with Rein increasing, but far less than 0.35 of ordinary combustor.The lager Rein, the smaller OTDFmax.Temperature field quality of outlet section is clearly uniform.
EI CO and EI HC are emission index of CO and HC, △  2, MILD combustion mode and performance influenced by Re number conform to the gas turbine flameless combustrion experiment results [14].Table 3 contains the numerical results comp the experimental of similar conditions [14].The calculation results are coincidence with experiment.
For the designed MI number on MILD combustion has been numerical simulated, which is concluded as follows: 1) Air jet Reynolds number has important high temperature flue gas recycling rate K v and MILD combustion mode.MILD combustion mode is formed when K v larger than 3.3 ~ 3.8.
2) The formed MILD combus stics of space reaction, high combustion efficient, very lower NOx and CO emissions, and good equality temperature field of outlet section.
3) MILD chamber c on MILD combustion mode.The calculation results are periments and laws, which have engineering reference value for MILD applications to gas turbine.
Figure 1.1/12 Model chamber and local section me the source term of the gas continuous phase, Ｓ p, is the interaction source term between the particles phase of kerosene and continuous phase, ρ stands for density of gas phase, which depends on the gas state equation.The governing equations are closed by Realizable kε turbulence model and standard wall function is adopted near the wall.C 12 H 23 represents aviation kerosene.A joint model with multistage finite-rate chemical reaction and EDC model is adopted to simulate the interaction between the turbulence and chemical reaction, revealed in formula(2).The reaction rate is controlled by the minor rate of EDC conceptual model (3) and finite-rate chemical reaction model rate(4) 2311 K,Taver = 1530 K,T wall = 977 K,T out = 1035 K (a) Re in = 1.13×10 5 T max = 2326 K, T aver = 1533 K, T wall = 994 K, T out = 1046 K (b) Re in =1.50×10 5 T max = 2351 K,T aver = 1541 K,T wall = 1016 K,T out = 1053 K (c) Re in = 1.88×10 5

Figure 2 .
Figure 2. Effect of Rein on temperature fie .ld inner diameter of the injector, m. ρflue is flu e MILD combustion m performance of pollutant of NOx em i.e.Kv is larger than 3.3, th efficient can be de in e gas density in the reference temperature, kg/m 3 .ρ in is the reactant density of inlet temperature, kg/m 3 .x 0 is the original injecting coordinate, m.
. H CO and △H C12H23 is the low heating value of CO and C 12 H 23 .In the Table

Table 1 . Calculation conditions. Re in Fuel g
[12]erature polynomial function is used for describing physicochemical properties of gas component and fuel.Thermal-gas-solid coupled boundary condition is advisable.CFD calculation is carried on the commercial software FLUENT6.3 designed by ANSYS[12].shows that, when Rein is larger 5, MILD combustion mode has formed volume flame of flame front surface disappears, local temperature difference is less than 50 K after flame lift off zone, T aver is about 1540 ~ 1541 K.
fu pre-exponential factor, activation energy and gas temperature respectively.Thermal NO and fast NO model are both used for NOx generation.P 1 Radiation model and radiative properties calculation of flue gas are used.Discrete phase of oil fuel particles are calculated by discrete random wander model, and the Lagrange equation describing fuel particles speed, mass and rate of temperature change is solved.The source termＳ p, in equation (1) is calculated by PSI-CELL model.The size of atomized particles conforms Rosin-Rammler distribu-tion, and the effect of gas turbulent fluctuation on particles speed is considered.

Table 2 .
The combustion efficiency ε in ow-NOx Combustor De within the Scope of the Engine 3E German National Research Program in a Cooperative Effort among Engine Manufacturer, University of Karlsruhe and DLR German Aerospace Research Center," Aerospace Science and Technology, Vol. 6, No. 7, 2002, pp.531-544.doi.org/10.1016/S1270-9638(02)01179-3