S T. NO U.10, AUER 30512

Transcription

1 31y,1111l 11l111,1,11131,111 CoA Memo 21 February, 1964 S T. NO U.10, AUER SOME EXPERIMENTAL TECHNIQUES IN THE STUDY OF FLAME STABILIZATION by E.R. NORSTER and A.H. LEFEBVRE L L (Paper prepared for presentation at 73rd meeting of the G.T.C.C. Combustion Sub-Committee. February 1964). Propulsion Department College of Aeronautics Cranfield.

2 Contents Page Symbols Introduction and Summary Low pressure simulation by nitrogen dilution Theoretical background Experimental techniques Experimental verification of relationship between 5 quantity of added nitrogen and equivalent reduction in pressure 5.0 An application of the local injection method in the 7 investigation of air entrainment in stabilized flames References 8

3 List of Symbols B Co Cf - Fraction of fuel burned - Concentration of oxygen Concentration of fuel Activation energy cats K K t Ratio of nitrogen/propane by weight Local ratio of nitrogen/propane by weight - Mass flow rate lb/sec. P - Pressure (atmospheres) Universal Gas constant (1.987 oal/m31, 0K) TR Reaction temperature OK Reaction volume ft3. Base of natural logarithms f k 9fR VID - fuel/air ratio by weight - 2nd order reaction velocity constant Equivalence ratio, reaction zone. - Equivalence ratio preheating

4 Introduction and Summary The purpose of this note is to describe one or two techniques which are currently being used at Cranfield in various investigations into the effects of flameholder geometry and flow parameters on the process of flame stabilization. Much of the report is concerned with a description of the nitrogen dilution technique as a means of simulating low combustion pressures, and a comparison is made between this and the now established water injection technique. It is shown that many aspects of flame stability can be effectively studied by local injection of nitrogen directly into the combustion zone, with appreciable economies in the amounts of nitrogen required. A further application of this local injection method is also described which has proved useful in an investigation of the factors governing the amount of air entrained in the recirculation zone. 1.0 Low pressure simulation by nitrogen dilution A number of techniques for simulating low combustion pressure have appeared in the literature in recent years (1) (2) (3). One of these techniques in particular has proved to be cheap and effective. Basically, this method depends upon decreases in both temperature and partial pressure of the reactants involved in the combustion process, which are achieved by the addition of water to the combustible mixture. The theory and application of this method have been fully described in an earlier paper presented to the G.T.C.C. Combustion Sub-Committee (4). The use of water as a diluent has the advantage of a cheap working fluid, but there are certain drawbacks mainly in terms of experimental complication and pressure loss. An essential item for the water dilution method is an air preheater to vaporise the injected water and cause it to remain in a vapour state during, its passage through the combustion zone. Although the pressure loss of a preheat chamber is normally quite small it can be very significant if a fan is used as the source of air. In any case its associated fuel pump, fuel system and

5

6 - 3 - than water. It should be added that if very large quantities of nitrogen are contemplated, liquid installation is considerably cheaper than bulk storage of gas 2.0 Theoretical background The principles of simulating low combustion pressure by the dilution technique are confined to the use of a reaction rate criterion. This criterion is the equality of the reaction rate at the simulated and true pressure condition. One may formulate this equality for second order homogeneous gaseous reactions by the use of well-known looading parameter, _ 1_, in the following way:- VP2 [ M k VP2 SIM k VP2 TRUE ] where k is the unknown 2nd order reaction velocity constant. (1) The derivation of the loading parameter has been described many times and, therefore, only the bare essentials need be given here. It can be shown that for second order reactions the equation which relates the loading to other indepenednt variables is:- k VP2 ' Co. Cf. p2 fbr2 -EATa e T R 3 /2 (2) The effect of adding an inert gas on the reaction rate becomes apparent by considering equation (2). By the addition of a diluent gas the concentration of oxygen, Co and of fuel, Cf, will decrease and the reaction rate will vary as the product of these decreased values. In addition, the presence of the inert gas will decrease the reaction temperature, TR, due to its capacity for absorbing heat. Expressions can be developed from equation (2) which relate the loading parameter to the weight ratio of diluent to fuel, K, and the following expression is that for propane/air mixtures and nitrogen dilution with factors included for preheating with kerosine:- M. -2L (1/ ) (1/4. _ 0 4: TR... ( 1.- 1) k VP2 3/2 B TR (1 + B Op K) 2 71i Since calculations of reaction temaerature rate at eauivalence ratios near

7

8

9

10

11 respectively, in conjunction with fifteen conical stabilizers each of 300 included angle. Values of stabilizer blockage ranged from 11 to 44%, the actual stabilizer dimension being carefully chosen in order to permit stabilizer size and blockage to be investigated independently. The experimental results obtained show that the amount of air entrained in the recirculation zone behind a bluff body stabilizer depends entirely on the velocity and temperature of the airstream and on the stabilizer blockage. References Stewart, D.G. Way, S. "Scaling of Gas Turbine Combustion Systems" AGARD. Selected Combustion Problems II, p Butterworths, London "Combustion in Turbojet Engines" AGARD. Selected Combustion Problems II, p.296. Butterworths, London Lefebvre, A.H. "The Simulation of Low Combustion Pressures by and Halls, G.A. Water Injection". Seventh Symposium on Combustion Butterworths, London Lefebvre, A.H. Rolls-Royce Report preparod for the G.T.C.C. and Halls, G.A. Combustion Sub-Committee 15th May, Coward, H.F. and Jones, G.W. J.Amer.Chem.Soc. Vol Odgers, J. "Recent Studies in a Spherical Combustor" Burnley Report No. B 48,124. Paper submitted to G.T.C.C. Sub-Committee, May 1962, 7. Lewis, B. & Von Elbe,G. "Combustion Flames and Explosions of Gases" Page 412, Academic Press.

12 FIG. I C T R K 1900 OR = 0.9 MIXTURE TEMP K 1800 NITROGEN/PROPANE RATIO K = I FRACT I ON OF FUEL BURNED -B EFFECT OF NITROGEN DILUTION ON REACTION TEMPERATURE.

13 FIG. 2 :4/1I11T1JRE T EMPERAT K 10 4 PRE SURE ("A MOS) I.0 r a r I L -8 FRACTION OF F-'1 ipl BURNED EFFECT OF PRESSURE ON REACTION PATE

14 FIG. 3 lo s SPECIFIC REACT ION RATE [ M X 10 1 P I VP2k E-4 TURE: _ PERATURE, T k' H 1! F OGENI/PROPANE RATIO 9 a 7 K 5 4 a I S.7 B FRACTION 07 Cn JEL L!R NED EFFECT OF NITRO''''' 9 DILUTION ON keaciion RATE.

15 4 MIXTURE TEMPERATURE 798 K - FRAC- ON OF Rim,. `q!..1rned (1)R = r K NITROGEN/PROPANE RATIO (0t) F")::LATIONSHIP BETWEEN NITROGEN Dtwrom AND EFc''7CTIVE DECREASE IN PRESSURE.

16 GAS I NJECTORS FLAME STABILISER -METERED AIR SUPPLY. PRE EATER COMBUSTION CHAMBER ulg AS TEMPERATURE THERMOCOUPLE. GAS FLOW METERS COMPRESSED NITROGEN `P. SUPPLY. P RO PA NE GAS SUPPLY FLAME STABILISATION RIG USING NITROGEN INJECTION TECHNIQUE. P) to

17 FIG I-1 GAS VELOCITY 30o, (ft/sec) K 3 4 S NITROGEN /PROPANE RATIO (wt). EFFECT OF GAS VELOCITY ON STABILITY LIMITS.

18 'EFFECT OF M " rufze TEMPERATURE C N STA SL' ry L IIVI 0 rs. 4. SYMBOL -1-0 El A MIXTURE TEMP 25 0C 200 C C 5000C I I I; K NITROGEN PROPA RATIO (Mt),

19 GAS INJECTOR FLAME STAB USER METERED MR SUPPLY PREHEAT ER GAS TEMPERATURE THERMOCOUPLE PROPANE GAS S 1PPLY M TR GEN SUPPLY A E S?ABILSATION RIG US NG NI ROC EN INJECT ION TECHNI!'" E. (LOCAL INJECTION OF NITROGEN)

20 GAS VELOC"-Y 5C5 /sec) O f, K1 LOCAL N.-?r_::`,GEN/PROPANE: RATIO ( ) EFFEC OF G.aS VELOC ( ON STABIL rry LIMITS. (LOCAL INJECTION OF NITROGEN&

21

22