EXPERIMENTAL INVESTIGATION ON SOOTY FLAMES AT ELAVATED PRESSURES

1. EXPERIMENTAL INVESTIGATION ON SOOTY FLAMES AT ELAVATED PRESSURES A first year PhD progress report presented by: Hamidreza Gohari Darabkhani Supervisor : Dr. Yang Zhang School of Mechanical, Aerospace and Civil Engineering The University of Manchester 20.09.2007

2. Introduction • Increasing efficiency and decreasing size in modern turbines and internal combustion engines requires higher operating pressures. • At high pressure flames, the carbon (soot) particles are so dense that the flame is opaque to electromagnetic waves in the visible or near infrared. • In the optical diagnostic methods, measuring the exact amount of combustion products is difficult but temperature and concentration of soot can be measured with high accuracy. • The effect of pressure on thermo physical properties of laminar coflow diffusion flames (LCDF) was studied by optical diagnostic methods (Two-Colour Pyrometry) over the pressure range of 1 to 18 bar in ethylene-air and methane-air and 1 to 7 bar in propane–air LCDF.

3. Objectives of this Research • Investigation on physical sooty flame’s properties at elevated pressures (The changes in the height, diameter and shape of the flames…). • Applying emission imaging techniques (two-colour pyrometry) by using just one CCD digital camera and two narrow bands filters (CH and C2). • Applying the IR infratherm pyrometry in order to measure soot temperature profiles in high pressure sooty flames for comparison with two-colour results. • Thermo acoustic or thermo diffusivity flame’s instability observation due to elevated pressures. • Chemilominescent emission measurement by using optical fibre system. • Terahertz time domain spectroscopy (THz-TDS) to study the combustion reactions in regimes inaccessible to optical diagnostics (if equipments will be ready). • Cross-correlating the obtained data in order to gain unique physical insights into the flame’s properties under high pressure and sooty conditions.

4. Burner Types Three types of laminar diffusion flame burners are commonly used by soot researchers: • Coflow : Axisymmetric 2-D flames with demonstrated stability at high pressures. • Counterflow (opposed jet); 1-D flames with instability problems at elevated pressures flames and critical location of the stagnation point. • Wolfhard–Parker; 1 and 2-D Flames with instability problems at elevated pressures

5. Summary of non-intrusive optical based combustion diagnostic techniques

6. , Standard • Increase in soot formed proportional to where defined pressure exponent ( • The primary particle diameters can be described by a logarithm of normal size distribution with a mean standard deviation of , increases with increasing carbon/oxygen ratio (C/O)and with pressure. At 70 bars can reach in strongly sooting flames. The final particle diameter, , for otherwise fixed conditions decreases with pressure, so that at 70 bar small diameters of result. Therefore the mean final soot surface, , becomes very large, with values up to observed ( and temperatures High Pressure Soot Diagnostics (pressure dependence of soot)

7. Schematic diagram of apparatus used for Line of Sight measuring the temperature of soot by Flower (1989)

8. Optical Layout of Spectral Soot Emission (SSE) Diagnostic Optical Layout of the Line-of-Sight Attenuation (LOSA) diagnostic

9. Two-Colour Soot Temperature Measurement Theory (refractive index ) m=n-ik n,k from Lee and Tien(1981) report

10. High Pressure Chamber Narrow Band Filter Optical Windows Sooty Flame Infratherm Pyrometer CCD Camera Test Setup Schematic

11. Experiment Arrangement Olympus E-100RS Visible Windows Infrathem Pyrometer Filters (C2 or CH) High Pressure Chamber

12. Narrow band filters and IR Pyrometer CH filter (430 ± 5 nm) C2 filter (516 ± 2.5 nm) Two-colour IR Pyrometer

13. Camera calibration setup with tungsten ribbon lamp 12V Battery Tungsten Lamp CH and C2 Filters Rheostat Digital Voltmeter Digital Camera

14. Test Results Ethylene (100 ml/min)-Air (20 l/min) coflow diffusion flame 1 bar 2 bar 4bar 6 bar 8 bar 10 bar 14 bar 16 bar 18 bar

15. Flame Heights in Methane and Propane Flames

16. The cross-sectional area of the Propane flame As the pressure was increased, axial flame diameters decreased, giving an overall stretched appearance to the flame. In This Study: The cross-sectional area of the flame (Acs)  inverse dependence on pressure to the power of 0.6±0.1 Glassman (1998): Acs inverse dependence on pressure to the power of 0.5 Thomson et al. (2006), Bento et al.(2006) and McCrain and Roberts(2005):Acsinverse dependence on pressure(1/P).

17. Flame Visualisation: • The height of the flames increases gradually as pressure increases and then decrease with further increases in pressure. • The shape of the flame changes dramatically with increasing pressure. At atmospheric pressure, the flame has a bulbous appearance and is wider than the exit diameter of the burner nozzle. By increasing the pressure the flame changes in shape from wide and convex to slender and concave. • As the pressure was increased, axial flame diameters decreased, giving an overall stretched appearance to the flame. The cross-sectional area of the propane flame was observed to decrease with pressure as AcsP-n , where , n=0.6±0.1 • The flame can be considered as a laminar axisymmetrical coflow diffusion flame.

18. Soot temperatures along the centre line at different pressures

19. Soot Temperature Measurement • In atmospheric pressure most centre parts of flames was blue and the presence of soot is limited to the region near the tip of the flame. • By increasing the chamber pressure the overall soot temperature was decreased. • Temperature increment by increasing the flame height from nozzle tip and temperature drop after a certain height was observed for all flames. • The temperature trend line plots show steep radial temperature gradients across the soot annulus and a general axial increase in temperature. • It is found that at lower pressures the temperature of soot annuals are more than centreline soot temperature and after a critical pressure this trend will be changed. • It is shown that rate of temperature dropping are more in lower pressures in compare with higher pressures.

20. Two-Colour Soot Temperature Results (Ethylene-Air)

21. Two-Colour Soot Temperature Results (propane-Air)

22. Soot Concentration Results by Applying Two-Colour Method(Ethylene-Air)

23. Soot volume fraction Results by Applying Two-Colour Method(Propane-Air)

24. Soot Formation • It is found that the methane-air diffusion flame is less sooty in compare with ethylene and propane diffusion flames. • It was observed that by increasing the pressure the soot concentration and proportionally soot volume fraction dramatically increased. • Soot formation at lower pressures was occurred mainly at the tip of the flame and in an annular band near the burner rim, as the pressure was increased, the luminous carbon zone moved downward, filling an increasingly large portion of the flame. • It is found that the more sooty flame the less soot temperature. In our measurements for example in 4 bar the maximum temperature which was recorded for methane-air diffusion flame is 1552 ºC, however for same pressure in ethylene-air diffusion flame the maximum recorded soot temperature is 1444 ºC and for propane-air diffusion flame 1400 ºC.

25. Two-Colour Pyrometry • The values of soot volume fraction measured using two-colour method are coupled to the measured soot temperatures, any errors in measured temperatures will lead to errors in soot volume fractions. • The main problem in applying two-colour method in our experiments was spectrum region of two selected narrow band filters. • Also in separate pictures of flame by CH and C2 filters it is inherently difficult to find exactly the same points in two pictures of flame, for intensity evaluation. • The sensitivity of the main equation was tested on all the parameters and was found that F1 and F2 have the less effect; however the I1 and I2 show maximum change. • calibration of two colour optical setup (camera with filters)was performed on a certified tungsten ribbon lamp. • The maximum average error that is recorded in our temperature measurement in two-colour calculation was about 10%.

26. Instability Observations • Pressure is very influence on stability behaviour of different gaseous flame. • The ethylene flames with fuel flow rates of 100ml/min and 115ml/min exhibited good, long term stability at all pressures up to 16 bars. • In Methane flame from 8 bar flame dramatically changed to an unstable flame. • It was observed that propane presented more stable flame in comparison with ethylene and methane. • Fuel and air flow rates play an important role in instability behaviour of gaseous flames. Methane (120 ml/min)-Air (20 l/min) at 18 bar Ethylene(300 ml/min)–Air(20 l/min) at P=16 bar)

27. Filters (Two-colours and natural density) Anti-Heat Filter Optical Windows Roof Prism Quartz Plate Infratherm Pyrometer CCD Camera High Pressure Chamber Tungsten Ribbon Lamp Future Works (1) Applying Modified and Specified Two-Colour Pryometry

28. Future Works (2) Chemilominescent Emission Measurement (evaluation of flame dynamic) Digital CCD Camera Infrathem Pyrometer Collection Lens 3D-Traverse gear Fibre Optic Cable Maas Flow Meters

29. Future Works (3) Terahertz Time-Domain Spectroscopy of Sooty Flames at High-Pressure • Removing the effects of ambient water vapour • Improving dynamic range and measurable bandwidth • Reducing thermal lensing effects to increase THz transmission

30. Future Works (4) Thermo-Acoustic Instability Simulation at Elevated Pressures • Combustion instability is the main problem in developing new low emission combustors and burners. • In our experiments instability was observed for ethylene flame in higher flow rates and in methane flame after 8 bar flame started flicking and in higher pressures it became totally unstable. • Feasibility study of thermo aquatic simulation in this high pressure burner by using Gambit and Fluent software. • It means in addition with Chemilominescent emission measurement, we can predict and discussed the effect of pressure on flame instabilities, flame buoyancy and Reynolds number

31. Thank you for your attention