**2.3. Reciprocating flow**

530 Numerical Simulation – From Theory to Industry

combustion efficiency, was proven.

**2.1. Stability of flame** 

number:

PM with lean and rich mixtures, significant reduction in pollutants and increasing

In recent years many researchers investigated the PM combustion technology both experimentally and theoretically. The most of researches are in field of steady combustion in PM and few of them are about transient flame propagation, both approaches are employed in PM combustion. Steady combustion is widely used in radiant burners and surface combustor-heaters due to its high radiant emissivity of the solid. The combustion zone is stabilized by its solid. The other, transient leads to an unsteady reaction zone freely propagate as a filtration combustion wave in the downstream direction. Combustion in PM differs considerably from the homogeneous flames flame front. Considerable features of PM for application of combustion technology are: large specific surface area, excellent heat transfer properties, heat capacity, transparency for fluid flow, thermal resistance,

The flame stabilization and propagation in a PM are governed by the modified Peclet

ܲ݁ ൌ ሺௌಽௗఘሻ

where *SL* is the laminar flame speed, *dm* is the equivalent diameter of the average hollow

gas mixture and *k* is the thermal conductivity of the gas mixture. For flame propagation through a porous material, the critical Peclet number of 65 has been found. Thus, *Pe < 65* for

PM may work with a premixed flow or with a non-premixed fuel flow. Premixed porous burners consist of two zones: the premixed fuel–air mixture first enters a hot solid matrix, where it is heated until it enters to the second hot solid matrix. Depending on its application, a third section, a compact heat exchanger may be added to the burner. The schematic of a

A premixed mixture of methane-air and hydrogen-air were used in different burners with two section PM. Measurements show considerable reduction in the concentrations of NOx, CO in the fluid gases. Also the effects of hydrogen addition to methane, were investigated. For the porous burners hydrogen was found to lower the NOx emissions slightly, while for the other burners an increase, or no obvious effect, was found. The enhancement of the radiation flux from PM burners operating with non-premixed flames by using a vane-rotary burner, in which the swirling fuel flow was confined by an air duct, is necessary. They also studied

emission characteristics of ceramic foam burners operating with non-premixed flame.

, (1)

ߩ

is the density of the

mechanical resistance, recuperation of energy and electrical properties.

space of the porous material, *Cp* is the specific heat of the gas mixture,

**2. Background of combustion in porous media** 

quenching, and *Pe > 65* for flame propagation.

**2.2. Premixed and non-premixed mixture** 

two-layer premixed combustor is shown in Fig. 1.

For the excess enthalpy combustion, a combustion system using reciprocating flow in PM was introduced. By the reciprocating flow, the combustion gas enthalpy is regenerated into increase in enthalpy of the combustible gas through the PM, which store heat. For this technique a new arrangement of the PM that stabilized flame for a wide operating condition, was used. The mixture first flow in, and the gas and solid temperatures reaches to a maximum at the exit side. Then the flow direction is reversed by means of valves. On the reverse flow half-cycle, the fresh mixture encounters much higher solid temperatures at the entering side. Therefore, the amount of heat recycled becomes larger than that with the single flow direction. Hence in the reciprocating flow system, the heat transfer from the combustion gases raises the solid temperatures from both directions.
