Effect of self-entrainment and porous geometry on stability of premixed LPG porous burner

Abstract

This paper presents findings on combustion stability of a self-entrained, rich-premixed LPG porous burner. Alumina (Al2O3) porous foams, with 15ppi carved into bulked and hollowed cylindrical shapes were placed at a convertible burner tip, while the conventional porous-free burner was used as a base case for comparison. Visual flame propagation, temperature and the percentage of oxygen in a premixed zone were monitored at different firing rates. Initially, the tests were carried out based on basic geometry of bulky porous cylindrical foam under a free flame mode. At this part, the limitation of burner operation was realized in term of premixed equivalence ratio which was a result from self-entrainment. Later, an improvement was made on porous geometry and its effect on burner performance was considered with the previous version. It was found that propagation of flame within a porous matrix had significant contribution to temperature of the premixed zone as a result of radiation. Discussion was made relating to two competing factors controlling flame propagation within the porous burner i.e.; local flame speed and local convective effect. Combustion stability was found with domination of flame speed over convection when Φ<4.1 in the premixed zone corresponding to illumination of the porous medium with the temperature of premixed zone being greater than 200°C. Beyond 980kW/m2 (equivalent to 4L/min of LPG) where convection effect dominated, heat recirculation within porous domain was broken down resulting in formation of soot burning outside porous medium corresponding to considerably drop of temperature. At this range, such short characteristic resident time, lower than 2s, was observed with Φ>4.1 in premixed zone. The hollowed cylinder, however, yielded stable flame throughout the operation range at the central port while propagation was maintained in a porous matrix which acted as a flame holder even at high firing rate. The temperature of premixed zone was also observed higher compared with bulked porous burner while the central port could eliminate flow blockage which otherwise gave significant adverse effect on primary air entrainment.