Stability and Combustion Efficiency of a Meso-Scale Combustor Burning Different Hydrocarbon Fuels

Transcription

1 Stability and Combustion Efficiency of a Meso-Scale Combustor Burning Different Hydrocarbon Fuels F. Cozzi, A. Coghe, A. Olivani, M. Rogora Dipartimento di Energetica Politecnico di Milano, Milan ITALY 1. Introduction In the last decade the interest in the development of meso-scale combustors (i.e. combustor having a characteristic dimension of the order of the quenching diameter) has grown due the interest in the development of portable power generation systems and small scale propulsion systems. Micro systems based on combustion are appealing because energy density of liquid hydrocarbons fuels is much higher than those of currently available top chemical batteries (~40-45 MJ/kg vs. ~1.2 MJ/kg) [1]. Moreover macro systems based on combustion show high reliability and efficiency, thus their miniaturization offers the potential to realize very efficient micro systems. Nevertheless when dealing with combustion at such small scale, problems related to heat losses, residence time and mixing became increasingly important and they will significantly affect the combustor performances. A review of scaling issues involved in micro-devices can be found in several papers [1-7]. The meso-combustor under investigation is based on the same concept found in many macro scale combustors: to use a swirl flow to achieve high efficiency, high power density and wide stability limits. Macro scale combustor usually operate at high swirl level to induce the vortex breakdown and the appearance of a central recirculation zone. The latter is the main responsible for the high performances evidenced by such combustors. Even if the concept of a small scale whirl combustor has been experimentally validated by researchers at Penn State University [8-9], at the present time there is no experimental evidence that a recirculation zone actually exist in such small system. Stability, chemical and thermal efficiencies of a meso-scale whirl combustor burning H 2 and CH 4 have been experimentally investigated in previous works by Cozzi et al. [10-11]. The experimental results showed moderate chemical efficiency and narrow stability limits when burning 100% CH 4. The meso scale combustor used in this study is identical to that used in Refs. [10-11] with the exception that the exit port is now located in the radial direction. This modification is expected to affect the internal flowfield in such a way to improve chemical efficiency and stability. To be competitive over classical electrochemical batteries, small scale power systems, besides other requirements, needs to operate with liquid hydrocarbon fuels. Propane and butane could be two fuels candidate, nevertheless few data are available on efficiency and stability of the meso scale combustor when burning such fuels. The objective of the present work is to experimentally investigate stability limits, efficiencies and pollutant emissions of a meso scale swirl combustor when burning different hydrocarbon fuels (methane, propane and butane). 2. Experimental Set-Up and Procedure The meso-scale whirl combustor is shown in Fig. 1, the cylindrical combustion chamber is 6 mm in diameter and 9 mm height and it has a volume of about mm 3. The meso-scale combustor is made of Inconel and it is fabricated with conventional machining. The combustor operates in a non-premixed configuration, tangential air injection allows to generate a whirl flow, while gaseous fuel is injected in the radial direction at 90 respected to 1

2 30th Meeting on Combustion the air flow. Air and fuel injection ports have a diameter of 1 mm each. The exit port is 2 mm in diameter, and differently from the previous configuration [10, 11] it is now located in the tangential direction, Fig. 2. Fuel ignition is achieved by means of an electrical spark located inside the combustor and close to the gas exit. Ignition wire is introduced into the chamber through the exit port, and it is removed as soon as combustion starts. Fig. 1: Photo of the meso-scale combustor combustor Exhaust fuel combustor fuel air air Exhaust Fig. 2: Sketch of the meso-scale combustor (front view), location of injection and exhaust ports. Left: old configuration [10-11], Right: current configuration. Fig. 3: Sketch of the experimental setup. To allow the visual observation of the flame, the front and the back ends of the cylindrical combustion chamber can be closed by quartz window. An insulated K type thermocouple ( mm in diameter) is inserted along the longitudinal axis and it is located in the center of the combustor to measure the temperature of combustion gases, T g. Another insulated K type thermocouple ( mm in diameter) is located at the exit port to measure the exhaust gas temperature, T e. A bare wire K type thermocouple (1 mm in diameter ) is inserted into a blind hole (2 mm depth) on the later side of the combustor to measure the temperature of the combustor body, T s. All the measured temperature values are not corrected for radiative or conductive losses. Thermocouples signals are amplified, sampled and digitized by an ADC board with 16 bit resolution at 5000 Hz sampling frequency; the mean value of 100 instantaneous temperature data is recorded at a rate of about 20 Hz. Steady state conditions are usually reached after about 20 min of continuous operation. Four different fuels have been used: H 2 (99.995%), CH 4 (99.5%), C 3 H 8 (9%) and C 4 H 10 (99.0%), while dry air is used as oxidizer. Fuels were supplied by compressed gas cylinder, 2

3 Italian Section of the Combustion Institute and all mass flow rates are metered and regulated independently by thermal mass flow meters and controllers. Thermocouple signal acquisition and mass flow rates control are handled by a PC through a LabView program. A sketch of the experimental apparatus is shown in Fig. 3. Burned gases are sampled for pollutant emissions analysis. A chemiluminescence analyzer is used to measure the concentrations of NO and NO x, a non-dispersive infrared (NDIR) analyzer is used to measure CO and CO 2 and paramagnetic technique is used for O 2 concentrations. All measurements are on a dry basis. The chemical efficiency, η c, is defined as the ratio between the total heat released by the combustion and the input thermal power, P in. The latter is defined as the product of the low heating value of the fuel, LHV, by the fuel mass flow rate, m& f. The chemical efficiency is estimated from the measured molar fraction of CO, CO 2, O 2 as shown in Eq.(1):! = Y ( Y + Y ) (1) c CO2 CO2 CO Where Y CO2, Y CO are respectively the measured fractions of CO 2 and CO in the exhaust gas. Actually Eq. (1) overestimates the chemical efficiency when the fraction of unburned hydrocarbon is not negligible. To check the results from Eq. (1), chemical efficiency is also computed as the sum of the thermal power in the exhaust, P e, and the thermal losses from the combustor, P l, divided by the input thermal power, P in :! = ( P + P ) P = ( P + P ) ( m& LHV ) (2) c l e in l e f Thermal power in the exhaust is estimated from exhaust gas composition and temperature, while thermal losses P l are evaluated as the sum of the convective and radiative heat losses at the external surface of the combustor: P = Ah T # T + A T # T (3) 4 4 l ( s amb )! " ( s amb ) Where A is the area of external surface, h is the convective heat transfer coefficient, σ= W m 2 K -4 is the Stefan-Boltzmann constant, and ε is the total hemispherical emissivity of oxidized Inconel. The values of the above parameters are A= m 2, ε = T s (deduced from data of Ref. [12]) and h=23 W m -2 K -1 (average value of data taken from Ref. [11]). Thermal efficiency, η t, is defined as the ratio of the thermal power in the exhaust, P e, divided by the fraction of the chemical energy effectively released by combustion: Pe Pe Pe! t = P! = m& LHV! = P + P in c f c e l (4) 3. Experimental results All the experimental results shown here refers to tests performed at ambient pressure. A stable gas phase combustion has been observed for all the gaseous fuels tested, nevertheless successful ignition occurs only using pure hydrogen. Thus flame ignition takes place with pure hydrogen, then the hydrocarbon fuel flow rate is increased up to reach the desired value, while hydrogen flow rate is brought down to zero. Stability limits are investigated at constant total (i.e. air+fuel) volumetric flow rate, by slowly changing the equivalence ratio, φ, up to reach blow out. Lean and reach stability limits are defined here as respectively the minimum and the maximum equivalence ratio before blow out. 3

4 30th Meeting on Combustion 3.1. Stability limits Rich and lean stability limits have been observed for all the investigated fuels, Fig. 4. The full scale of hydrogen flow meter set a maximum in the allowable value of equivalence ratio, see the solid red line in Fig. 4. Thus rich blow-out limit have been estimated only up to a total (air+h 2 ) flow rate of about 600 Nml/min. When burning H 2 stable combustion is observed for equivalence ratio between about 14 and Being fuel and air injected separately, the equivalence ratio φ has to be interpreted as a global value and likely inside the combustor the fuel-air mixture is locally non uniform. This could justify the fact that the rich stability limit is higher than the rich flammability limit of H 2 in air, φ rich =7.14 [13]. Lean limit decreased from 0.29 to 0.14 when the total volumetric flow rate is increased from 500 to 3000 Nml/min. Propane and butane evidenced very similar stability limits, while much narrows limits are observed when burning methane, Fig. 4. At small total flow rates and for all the hydrocarbon fuels, the rich and the lean stability limits collapsed to a single value. This behavior is likely due to the increased relevance of thermal losses, P l. As a matter of fact the ratio between P l and P in increased when the total volumetric flow rate decreased, Fig. 5. BLOW-OFF (a) (b) Total Volumetric Flow Rate, Nml/min Fig. 4: Stability limits of the meso scale combustor when burning different fuels. (a) hydrogen, (b) Methane, Propane and Butane. Equivalence Ratio,! Butane Propane Methane BLOW-OFF STABLE Thermal Loss Fraction, Pl/Pin Total Volumetric Flow Rate, Nml/min Fig. 5: Thermal loss fraction (P l /P in ) as a function of the total flow rate (fuels: C 3 H 8 ). Values computed through Eq. (3) at the reach blow-out limit (for φ>1 P = m& LHV! ); Values computed through Eq. (3) at the lean blow-out limit. in f 3.2. Chemical and Thermal Efficiencies Chemical efficiency, η c, is estimated according to the two methods explained in section 2, i.e. Eq.(1) and Eq.(2). Results evidenced that both methods gave quite similar results, see Fig. 6(a)-(b). When the meso-scale combustor is fed with methane the chemical efficiency is about, while for propane or butane η c is higher than. Moreover when burning methane the combustion efficiency decreased by increasing the total flow rate, while when burning butane and propane η c stayed approximately constant. Values of thermal efficiency, η th, estimated according to Eq. (4) were lying in the range -, see Fig. 7. Thermal efficiencies increased by increasing the total volumetric flow rate. At fixed φ an increase in the total 4

5 Italian Section of the Combustion Institute volumetric flow rate corresponds to an increase of the input thermal power; thus relevance of thermal losses is decreasing as the input thermal power increased Chemical efficiency, hc Methane ER= Propane ER= Butane ER= (a) (b) Fig. 6: Chemical Efficiency of the meso scale combustor when burning hydrocarbon fuels. (a) η c from Eq.(1); (b) η c from Eq.(2). ER = equivalence ratio. Thermal efficiency, hth 0.9 Methane ER= Propane ER= Butane ER= Chemical efficiency, hc Methane ER= Propane ER= Butane ER= Fig. 7: Thermal Efficiency of the meso scale combustor when burning hydrocarbon fuels. ER = equivalence ratio Pollutant emissions Pollutant emissions (NO x ) at the exhaust of the combustor are measured for butane at different total volumetric flow rate and at constant equivalence ratio, φ=. The experimental results shown in Fig. 8 evidenced an increase of NO x concentration as the total (air+butane) volumetric flow rate increases BUTANE! = NOx, 3% O Fig. 8: NO x emissions when burning butane, equivalent ratio is kept constant and equal to. 4. Conclusions Stability range, thermal and chemical efficiencies of a non premixed meso scale swirl combustor burning different hydrocarbon fuels are experimentally investigated. The mesoscale combustor has a cylindrical combustion chamber with a volume of 254 mm 3. Methane, propane and butane are used as gaseous fuels. Combustion efficiency is estimated through the measured values of CO, CO 2 and O 2 molar fractions in the exhaust gas, while thermal 5

6 30th Meeting on Combustion efficiency is computed from the estimated convective and radiative heat losses from the combustor chamber. Stable gas-phase combustion is achieved in a wide range of operating conditions for all the tested fuels. Wider stability limits are evidenced by the combustor when burning propane or butane as compared to burn methane. Likely due to thermal losses, all the hydrocarbon fuels has shown a minimum mass flow rates below which combustion can not be sustained. Chemical conversion efficiency was generally between and 0.9, and the highest values were observed when burning propane and butane. Thermal losses from the combustion zone were fairly high and their value were about 50%-30% of the thermal power released by combustion. 5. References 1 Fernandez-Pello, A. C.: Proceedings of the Combustion Institute, 29:883 (2002). 2 Waitz I.A., Gautam G., Tzeng Y-S.: J Fluids Eng, 120:109 (1998). 3 Epstein A.H., Senturia S.D., Al-Midani O., Anathasuresh G., Ayon A., Breuer K., Chen K.-S., Ehrich F.F., Esteve E., Frechette L.: 28th AIAA Fluid Dynamics Conference, (1997). 4 Dumand C., Guidez J., Orain M., Sabel nikov V. A.: Europeean Conference for Aerospace Science EUCASS, Moscow, July (2005). 5 Bruno C.: 37th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Salt Lake City, Utah, USA, July (2001). 6 Chigier N., Gemci T.: 41st AIAA Aerospace Sciences Meeting and Exhibit, Reno, Nevada, USA, January (2003). 7 Dunn-Rankin D., Martins Leal E., Walther D. C.: Prog Energy and Combustion Science, 31:422 (2005). 8 Yetter R.A., Yang V., Wang Z., Wang Y., Milius D., Peluse M., Aksay I.A., Angioletti M., and Dryer, F.L: 41st AIAA Aerospace Sciences Meeting and Exhibit, Reno, Nevada, USA, January, (2003). 9 Wu Ming-hsun, Yetter R.A., Yang V.: 42nd AIAA Aerospace Sciences Meeting and Exhibit, Reno, Nevada, USA, January, (2004). 10 Cozzi F., Coghe A., Olivani A.: Third International Conference on Green Propellants for Space Propulsion, Poitiers, France, September (2006). 11 Cozzi F., Olivani A., Coghe A., Lucchetti A., Tonazzo F.: The 29th Meeting of The Italian Section of The Combustion Institute, Pisa, Italy, June (2006). 12 Tanda G., Misale M.: J Heat Transfer 128: 302 (2006). 13 Glassman I.: Combustion, Third Edition, Academic Press, San Diego, California, USA (1996). Acknowledge This work is supported by the Italian Ministry for University and Research (MIUR) under contract n _002. The authors are grateful for the assistance provided by Mr. Paolo Sesana and Mr. David Capelli during the conduction of the experimental activity. 6