Yilmaz, Harun; Karyeyen, Serhat; Tepe, Ahmet Ümit; Brüggemann, Dieter

Colorless distributed combustion technique enables less pollutants emissions for a fuel/oxidizer mixture to be combusted. In this study, colorless distributed combustion characteristics (CDC) of hydrogen/air mixtures were numerically investigated in a micro combustor to overcome the difficulties associated with micro combustion and to obtain a more uniform temperature profile on the outer wall. To that end, hydrogen/air combustion was simulated by using ANSYS/Fluent CFD code at a constant equivalence ratio (1.0) and thermal input (100 W) and different mixture inlet temperatures of 300 K, 600 K and 1000 K to seek colorless distributed combustion. CDC conditions were tried to be achieved by decreasing the oxygen concentration from 21 % to 9 % at an interval of 3 %. In conclusion, colorless distributed combustion conditions could be achieved at different O2 concentrations, irrespective of the mixture inlet temperature. At 300 and 600 K, such conditions did not make any positive contribution to the power output of the micro combustor. Nevertheless, at 1000 K and 9 % O2 concentration, all combustion and emission performance metrics were improved in a manner that could increase the power output of the micro power generator.

Yilmaz, Harun; Cam, Omer; Yilmaz, Ilker

In this study; effects of fuel composition, equivalence ratio and swirl number on flame behavior (dynamic and static) of premixed 100% CNG, CNG-H2, CNG-H2-CO and CNG-H2-CO2-CO mixtures under externally altered acoustic conditions were experimentally investigated in a laboratory scale swirl stabilized combustor. During experiments, the amount of CNG in all gas mixtures tested was set as 20% and 40% by volume except for the CNG-H2 mixture. Moreover, H2/CO ratios of CNG-H2-CO mixtures were arranged to be able to provide low, medium or high heating value. To better represent synthetic gas, CNG-H2-CO mixture was then diluted with the same amount of CO2, and mixtures of CNG-H2-CO2-CO with varying H2/CO ratios were achieved to evaluate effects of CO2 dilution (in addition to flame behavior) on flame characteristics (i.e. performance metrics such as flame temperature and emissions). Acoustic field of the combustor was altered via side mounted loudspeakers to trigger combustion instabilities, and under these circumstances, flame behavior was evaluated by examining instantaneous flame images, pressure and luminous intensity profiles. Data obtained from pressure transducers and photodiodes was also utilized to detect whether dynamic instabilities, i.e. thermoacoustic instabilities, excite static instabilities such as blowout, flashback, and flame liftoff etc. or not. Results of this study showed that fuel composition, equivalence ratio and swirl intensity are determinant parameters on flame behavior. Two main inferences of this study are: swirl number has a monotonic impact on flame stability characteristics (1); under externally excited acoustic conditions, CO2 presence in gas mixtures makes flames more resistant to dynamic instabilities (less axial and radial oscillations) and blowout but it increases flashback tendency (2).

Yilmaz, Harun; Cam, Omer; Yilmaz, Ilker

In this study, combustion and emission characteristics of output synthetic gas mixtures of two different power stations, namely Schwarze Pumpe and Fife, were investigated in a swirl stabilized combustor. For this purpose, an experimental test rig which is equipped with a premixed burner that can combust synthetic gases in a wide gas composition range was built and utilized. Tested synthetic gas mixtures are 61.9%H2-26.2%CO-6.9%CH4-5.0%CO2 (Schwarze Pumpe - H2/CO: 2.36) and 34.4%H2-55.4%CO-5.1%CH4-5.1%CO2 (Fife - H2/CO: 0.62). Actually, these gas mixtures contain slight amount of N2 and Ar. Therefore, right amount of CO2 was added to each mixture to represent these inert constituents. The comparison of combustion behavior of respective synthetic gas mixtures was conducted via examining axial and radial temperature values throughout the combustor. Besides, emission behavior was interpreted by analyzing measured CO, CO2 and NO values at combustor chimney. Results of this study showed that 61.9%H2-26.2%CO-6.9%CH4-5.0%CO2 mixture performs better in respective combustor with regard to temperature distribution, and emitted CO and NO levels.

Yilmaz, Ilker; Yilmaz, Harun; Cam, Omer; Ilbas, Mustafa

Main challenges for a fuel efficient micro scale combustion are rooted from size restriction of micro combustors which results with inappropriate residence time of fuel/air mixture and intensified heat losses due to relatively high surface to volume ratio of such devices. One way of increasing energy output of micro combustors is to optimize its geometry by considering simplicity and easy manufacturability. In this study, effect of combustor geometric properties on combustion behavior of premixed hydrogen/air mixtures was numerically investigated. For this purpose, an experimentally tested micro combustor's geometric properties were modified by establishing a backward facing step which is varying distance from combustor inlet and has varying step height, and adding opposing cavities which are varying distance from combustor inlet and have constant length to depth ratio into the flow area. Modeling and simulation studies were performed using ANSYS Design Modeler and Fluent programs, respectively. Combustion behavior was analyzed by means of centerline and outer wall temperature distributions, amount of heat transferred through combustor wall, conversion ratio of input chemical energy to utilizable heat, and species distributions. Turbulence model used in this study is Renormalization Group (RNG) k-ε. Multistep combustion reaction scheme with 9 species and 19 steps was simulated using Eddy Dissipation Concept model (EDC). Results showed that backward facing step in the flame region alters reaction zone distribution, flame length and shape, and consequently temperature value and distribution throughout the combustor. Lastly cavity was found to slightly increase peak temperature value.