Structure Of Turbulent Premixed Flames Research Articles

Characteristics of Mixture Turbulence and Structure of Opposed Jet Burner Flames.

In the present study, a flow field developed in an opposed jet burner and its turbulence characteristics were measured in detail using a hot-wire anemometer. The flame structure was clarified by schlieren photographs and instantaneous temperature measurements with a thermocouple. Extremely strong turbulence of small scale is generated by impingement of two mixture flows. For the case of a low mixture supply velocity, a flame within the transition regime from a wrinkled laminar flame to a distributed reaction zone is produced in the opposed jet burner. With increasing the mixture supply velocity, the structure of the flame zone changes to the distributed reaction zone. These two types of structure correspond to those of region 2 and region 3 of the 3-region model of the turbulent premixed flame structure.

Effects Of Equivalence Ratio On the Extinction Stretch Rate Of Cylindrical Premixed Flames

Abstract The actual flame stretch rates of the stretched cylindrical premixed flame and counterflow twin flames were measured with LDV for the methane-air and propane-air mixtures over a wide range of equivalence ratios. The stretch rate at the flame center at the point of flame extinction was considered to be the maximum stretch rate for the existence of cylindrical and plane ftamelets. It was shown that the actual flame stretch rate at the point of flame extinction for the stretched cylindrical flame was smaller than that for the counterflow twin flames except for the case of very rich propane-air mixtures. The conditions for the existence of the cylindrical flamelet in turbulent premixed flames, which has a similarity to the fine structure of turbulent premixed flames proposed by Chomiak (1970, 1972, 1977), were discussed using the stretch rate data at the extinction point and the turbulence parameters. It was found that the cylindrical flame let could not exist in highly turbulent flames where the dis...

Elementary processes governing behavior of turbulent premixed flames

Abstract

Characteristics of Mixture Turbulence and Structure of Opposed Jet Burner Flames.

In the present study, a flow field developed in an opposed jet burner and its turbulence characteristics were measured in detail by a hot wire anemometer. The flame structure was elucidated by schlieren photographs and instantaneous temperature measurements with a thermocouple. Extremely strong turbulence of small Scale is generated by impingement of two mixture flows. For the case of low mixture supply velocity, a flame within the transition regime from wrinkled laminar flame to distributed reaction zone is produced in the opposed jet burner. With increasing mixture supply velocity, the structure of the flame zone changes to the distributed reaction zone. These two types of structure correspond to region 2 and region 3 of the 3-region model of turbulent premixed flame structure.

Recent Advances in Experimental Studies of Turbulent Premixed Flames

Many physical and chemical processes are included in turbulent premixed combustion and interact with each other in a complex way. These are characterized by the turbulence Reynolds number and the Damkohler number. Various regimes of turbulent premixed combustion are identified depending on these dimensionless parameters. In general, two important regimes are identified, namely the wrinkled laminar flame and the distributed reaction zone. Using newly developed experimental techniques, the characteristic features of the wrinkled laminar flame are being revealed and studies on the distributed reaction zone are under way. In the present review, essential problems relating to the modeling of physical processes in the wrinkled laminar flame are first described; secondly, new experimental findings relating to the distributed reaction zone are presented, and finally the transition mechanism of the turbulent premixed flame structure is discussed.

Structure of opposed jet premixed flame and transition of turbulent premixed flame structure

A distributed reaction zone was achieved successfully in the laboratory by an opposed jet burner. This burner consists of two opposed mixture supply tubes with water-cooled disks. An annular turbulent premixed flame is produced around the impinging mixture jets. In the present paper, the structure of the opposed jet flame was characterized and, by comparing with the tube burner flame, the transition mechanism from wrinkled laminar flame to distributed reaction zone was discussed. On the unburned gas side of the opposed jet burner flame, the essential feature of the temperature pdf is bi-modal with relatively high probability of intermediate temperature. In this region, a thick wrinkled (not laminar) flame structure is found in which small dissipative eddies are embedded. In the middle and the burned gas regions, the probability of unburned mixture disappears and that of intermediate temperature increases, which suggests that the flame zone consists of the small eddies of completely burned and partially reacted gases. By velocity measurements with an LDV and a hot wire anemometer, it was found that, for low turbulence Reynolds numbers, a fully developed wrinkled laminar flame is produced by increasing the Damkohler number. Also it was found that, for nearly constant Damkohler numbers, the fully developed wrinkled laminar flame structure changes to the distributed reaction zone with increasing the turbulence Reynolds number.

An evaluation of the Klimov-Williams criterion

Different expressions of the Klimov-Williams criterion are explored and the relations between them are established. Five different calculation methods of the K-W criterion are proposed. The sensitivity of the criterion to its quantitative evaluation based on cold flow or flame turbulence parameters and to the use of various definitions of the laminar flame front thickness is assessed by using experimental data coming from V-shaped turbulent premixed flames. It is shown that when the Zeldovich characteristic length is adopted, the values of the K-W criterion are scattered around 0.2 either when the cold flow or the flame turbulence parameters are used. Its values range around unity when the total visible flame front thickness is used. Other implications for the interpretation of the structure of turbulent premixed flames of these various calculation methods are discussed.

Comment on the paper “Flame quenching by turbulence,” J. Chomiak and J. Jarosiński, Combust. Flame, 48:241–249 (1982)

Different expressions of the Klimov-Williams criterion are explored and the relations between them are established. Five different calculation methods of the K-W criterion are proposed. The sensitivity of the criterion to its quantitative evaluation based on cold flow or flame turbulence parameters and to the use of various definitions of the laminar flame front thickness is assessed by using experimental data coming from V-shaped turbulent premixed flames. It is shown that when the Zeldovich characteristic length is adopted, the values of the K-W criterion are scattered around 0.2 either when the cold flow or the flame turbulence parameters are used. Its values range around unity when the total visible flame front thickness is used. Other implications for the interpretation of the structure of turbulent premixed flames of these various calculation methods are discussed.

Packaging trade-offs for an LSI-oriented very high-speed computer, the HITAC M-200H: Tsuneyo Chiba, Akira Masaki, Kennichi Furumaya and Satoshi Hososaka. IEEE Trans. Components, Hybrids, Mfg Technol.Chmt-4, (2) 166 (June 1981)

Methane/air turbulent premixed flames diluted with superheated water vapor at high-pressure and high-temperature were experimentally investigated to explore the effects of recycled water vapor on turbulent flame characteristics from the viewpoint of applying high-temperature air combustion (HiTAC) to high-load combustors as well as elucidating those effects of exhaust gas recirculation (EGR) in IC engine. A newly devised water evaporator was installed in a high-pressure chamber and superheated water vapor was successfully supplied to air up to 1.0 MPa and 573 K. The maximum dilution ratio defined as the ratio of the molar fraction of H2O to those of air and H2O was 0.1. Turbulent burning velocity, mean volume and the structure of the turbulent flame region were compared with those of flames diluted with CO2, reported previously, which is another major species in recycled burnt gas. Results showed that the effects of superheated water vapor dilution on turbulent burning velocity, ST, normalized by laminar burning velocity, SL, was much weaker than that of CO2 dilution. The mean volume of the turbulent flame region defined as the region between 〈c〉 = 0.1 and 〈c〉 = 0.9, was scarcely changed either. This means that the effects of recycled burnt gas on the structure of turbulent premixed flames at high pressure and high-temperature is predominated by CO2 but not by superheated water vapor, indicating that suppression of combustion oscillation in premixed-type gas-turbine combustors by extension of the volume of heat release region is due to recycled CO2. The emission indices of CO and NOx were also measured at high pressure, and it was proved that water vapor dilution is effective to restrain CO emission, which is a possible defect of HiTAC when it is applied to gas-turbine combustors.

Computer-aided design of VLSI circuits: Arthur Richard Newton. Proc. IEEE69 (10), 1189 (October 1981)

Methane/air turbulent premixed flames diluted with superheated water vapor at high-pressure and high-temperature were experimentally investigated to explore the effects of recycled water vapor on turbulent flame characteristics from the viewpoint of applying high-temperature air combustion (HiTAC) to high-load combustors as well as elucidating those effects of exhaust gas recirculation (EGR) in IC engine. A newly devised water evaporator was installed in a high-pressure chamber and superheated water vapor was successfully supplied to air up to 1.0 MPa and 573 K. The maximum dilution ratio defined as the ratio of the molar fraction of H2O to those of air and H2O was 0.1. Turbulent burning velocity, mean volume and the structure of the turbulent flame region were compared with those of flames diluted with CO2, reported previously, which is another major species in recycled burnt gas. Results showed that the effects of superheated water vapor dilution on turbulent burning velocity, ST, normalized by laminar burning velocity, SL, was much weaker than that of CO2 dilution. The mean volume of the turbulent flame region defined as the region between 〈c〉 = 0.1 and 〈c〉 = 0.9, was scarcely changed either. This means that the effects of recycled burnt gas on the structure of turbulent premixed flames at high pressure and high-temperature is predominated by CO2 but not by superheated water vapor, indicating that suppression of combustion oscillation in premixed-type gas-turbine combustors by extension of the volume of heat release region is due to recycled CO2. The emission indices of CO and NOx were also measured at high pressure, and it was proved that water vapor dilution is effective to restrain CO emission, which is a possible defect of HiTAC when it is applied to gas-turbine combustors.