Effect of Stretch on Local Burning Velocity of Premixed Turbulent Flames

Abstract

The effect of positive stretch on the local flame properties of turbulent propagating flames in the flamelet regime was investigated experimentally for methane-, hydrogen-and propane-air mixtures with lean and rich having nearly the same laminar burning velocity (S L0 =25cm/s). The ratio of the turbulence intensity u' to S L0 was varied as 1.4 and 2. A 2D laser tomography technique was used to obtain the temporal local flame configuration and movement in a constant-volume vessel. Some of the key parameters of turbulent combustion quantitatively measured are the local flame displacement velocity S F , curvature and stretch of turbulent flames. Additionally, the Markstein number Ma was obtained from outwardly propagating spherical laminar flames, in order to examine the effect of positive stretch on burning velocity. It was found that the obtained S F was distributed over a wide range with flame stretch as well as curvature, even for low turbulence (u'/S L0 =1.4). There also existed a good relationship between S F and the turbulent burning velocity. A quantitative relationship between Ma based on laminar flames and the S F for positive stretch and curvature of turbulent flames could be observed only for mixtures with Lewis number Le greater about one.

Experimental Study on Local Flame Properties of Hydrogen Added Hydrocarbon Premixed Turbulent Flames

“…4 that the values of η 0 /r, Ka T and S F are distributed over a wider range than those obtained in the laminar combustion (20), (26) , even under u'/S L0 =1.4. Additionally, the same interesting tendencies as those in previous studies (20), (26) can be observed from Figure 6 shows the variation of the mean value of S F on the whole flame front S F,m normalized by S L0 with δ H , Φ and fuel types. It is clear from Fig.…”

“…In our previous study (26) , it was obtained that there was a meaningful relationship between S F of turbulent flames and Markstein number Ma of laminar flames. So, this section examines the relation between S F and Ma.…”

“…Additionally, the obtained S F is not considered as the local gas velocity which is necessary for estimating the accurate local burning velocity such as the so-called fuel consumption velocity. In this study, the obtained S F is evaluated statistically as a scalar value (17), (20), (26) .…”

“…A means to remove its influence is adopted from an earlier study (20), (26) . That is, for the discussion of analyzed results, the flame images taken at the same condition as the progress rate (R A /R C ) 3 being about 0.018 are used, where R A and R C denote the equivalent radius based on the burned area of 2D flame image and that based on the chamber volume(R C =61 mm), respectively.…”

“…Experimental apparatus and procedure are the same as that used in our previous studies (20), (26) . The combustion chamber used as shown in Fig.…”