The unstrained planar diffusion flame in a chamber, which has served as an ideal theoretical construct for decades, has recentlybeen realized in a new burner- a Porous Plug Counterdiffusion (PPCD) burner. In this burner, one reactant is supplied through aporous plate (76 × 20 mm) at the bottom of the chamber and is transported to the flame with the uniform stream flowing upwards,and the other reactant diffuses from the top of the chamber to the flame against the upward flow of products. The supply of bothreactants is essentially uniform over the burner cross section such that the velocity, which is directed everywhere upwards, andall other physical quantities depend only on the vertical coordinate normal to the planar flame. Since the uniform supply of thereactant diffusing against the bulk flow is a major experimental challenge, laboratory investigations of planar diffusion flameshave been until now limited to counterflow burners where the flow is non-uniform and the flame strained. For our design, the topreactant is uniformly supplied over the chamber cross section (77 × 21 mm) through an equally spaced cartesian array of 41 by11 stainless steel tubes (1.0 mm O.D., 0.8 mm I.D.), allowing for the product gases to uniformly escape upwards in the spacesbetween the tubes. Although the flow field and reactant concentration at the needle exits is three-dimensional on the scale ofthe tube spacing, the spatial nonuniformities are limited to a thin layer near the needle exits which is comparable to the needles’diameter.Past theoretical studies have shown that the reactant convected to the flame zone has a significant influence on the conditionsfor thermo-diffusive instabilities as well as the extinction limits of a diffusion flame. The new burner, which has been shownto essentially produce a planar diffusion with practically no strain, appears to be well suited to test these theories. Since hydrodynamiceffects are trivial in this one-dimensional configuration, the roles of the transport and chemical descriptions can bedirectly assessed. In this work, we examine some characteristics of diffusion flames and compare the experimental data of theunstrained diffusion flame that we obtained with theoretical predictions of the one-dimensional model. In particular, we discussthe dependence of the extinction limits on the fuel and oxidizer properties, the degree of dilution, and the supply conditions atthe two ends of the chamber. An additional objective is to further explore the effects of detailed chemistry and transport on thestructure of an unstrained diffusion flame.
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