LAUSR

Abstract This experimental study was aimed at identifying the near-flammable limit and the unstretched flame speed of outwardly propagating spherical flames according to changes in the mole fraction of oxygen in the oxidizer ( R O ) and mole fraction of R245fa in the fuel ( R R ) for R245fa/CH 4 /O 2 /N 2 mixtures in a constant volume combustion chamber. The flames observed according to changes in R O and R R of the R245fa mixtures could be divided into three regimes, with typical, transition, and buoyancy-induced behaviors. These representative flame behaviors were similar to those observed in previously reported experiments on R134a mixtures. The R245fa mixtures particularly showed a more expanded near-flammable limit than R134a. The rising velocity U r and the rising length L r in the buoyancy-induced regime with the near-flammable limit were used to identify the buoyancy effect on the flame that rose and propagated outward under normal gravity conditions. In addition, as the slope of the linear relationship between the stretch rates ( k ) and stretched flame speeds ( S d ), the Markstein length ( L b ) of the R245fa mixture tended to be positive in the typical regime and negative in the transition and buoyancy-induced regimes. Such characteristics are similar to those of R134a. However, in the typical regime, the results from a regression analysis on the unstretched flame speeds ( S u ) derived from the same linear relationship showed that the flame inhibition effect from mixing R245fa was smaller than that of R134a.