Partially Premixed Flames

Partially premixed flames are created by adding air in less than stoichiometric proportions to the fuel streams prior to the reaction zone. They occur either due to deliberate addition to the of air to fuel tube for enhancing stability of flame or due to flame liftoff from the fuel injector. In turbulent regimes they often occur because of local extinction and reignition events. Many researchers have reported their studies (1-8) to analyze the behavior of partially premixed flames. Libby and Economos (1) predicted a finitely thick reaction zone for partially premixed flames in contrast to a thin reaction sheet. Tsuji and co-workers (2-4) performed experimental investigations into the structure of partially premixed flames revealing the presence

of two separate reaction zones depending on the stoichiometry of the premixed stream. Several experimental and analytical investigations (6-8) have been reported to understand the dynamics and extinction limits of the partially premixed flames.

Based on the fuel types, NOx emissions have been reported to increase with partial premixing caused by reduced radiative heat loss (9) or due to availability of oxygen and radicals responsible for prompt NOx production over a larger region (10). With increasing partial premixing NOx emission can be decreased by reducing the residence time of the fuel at the burner exit (9, 11). Gore (12) reported minimum NOx emissions for laminar flames of methane-air at an optimal level of partial premixing. Gore and Zhan (13) found minimum NOx emission index for equivalence ratio of 2.0. Kim et al. (14) reported minimum NOx emission index at equivalence ratio of 2.2 for ethane-air flames. Some of the recent developments in the field of partial premixed flames reported the effects of partial premixing on NOx emissions from laminar and turbulent flames (9, 11). Driscoll et

al. (11) found that NOx emission properties of turbulent partially premixed methane-air flames are very complicated and do not follow a definite pattern. Turns et al.

(9) studied turbulent partially premixed flames of methane, propane and ethylene with air. They found that NOx emission indices for methane flames increased first and then decreased with increased level of partial premixing. Owing to multiple complexities, experimental efforts to gain insight into the NOx generation behavior for partially premixed flames pose a great challenge. This provides a need to study, numerically the behavior of NOx emission. Williams and co-workers (15, 16) reported a non-monotonic behavior for NOx emission index as a function of fuel stream equivalence ratio, qualitatively in agreement with earlier experimental observations. However, quantitatively the two chemical mechanisms used in (15) and (16) produced different values of emission indices. Thus, with considerable amount of investigations to analyze NOx generation, there is a need to study methods to reduce it. Articles reporting NOx emission for burning rich hydrocarbon mixtures in premixed flames, using porous medium has already been known. Premixed flames in porous medium is known to produce low NOx emissions. Thus, a combination of premixed and partially premixed flames can be a useful way of checking NOx

generations.
To serve this aspect filtration combustion of premixed gases in inert porous media combustors has been extensively studied some time ago (17-20). With many other technological advancements achieved till date, combustion in porous media seems to be one of the most promising findings mainly because of its low pollutant emission and low temperature burning of fuels. Filtration combustion of gaseous mixtures in porous media differs from the homogenous oxidization mainly due to the ability of the porous medium to recuperate heat internally. This ability is attributed to the two reasons namely: highly developed inner surface of the porous media resulting in efficient heat transfer between the hot gases and the solid particles and the intense mixing of gas flow through the porous media which in turn increases the effective diffusion and heat transfer in the gas phase (21). The insertion of a solid medium into the combustion regime of a premixed gaseous mixture comes with the concept of radiation-controlled flame (22). The theory of filtration combustion presents a new genre of flames with exothermic chemical transformation coupled with fluid motion in porous medium. The properties of the flame front are defined by the strong interdependence of the two phases present. Kennedy et al. (23) studied NOx generation for lean and rich premixed methane-air flames in laminar flow regime. They used a micro combustor filled with alumina porous medium and found non-monotonic behavior of NOx emission index. Experimental observations reported shows, NOx emission index increased initially to a maximum value before decreasing again as the equivalence ratio is varied from lean to rich regime. The maximum is obtained close to stoichiometric conditions. These results help in generating a technology for cleaner technology for burning rich hydrocarbons.
There are two variants of flame waves that develop in filtration combustion namely: stationary and transient combustion zone systems. Some of the work with stationary waves in filtration combustion are reported in (24-27). The second approach of travelling wave can be found in Refs. (21, 23, 28). The travelling waves are observed in unsteady combustion zones. Strong interfacial heat transfers and low degree of thermal non-equilibrium between the solid and gas phase and the relative displacement of the combustion zones generates positive and negative enthalpy fluxes between the reacting gas and solid phase resulting in combustion temperatures that are significantly lowered from adiabatic conditions. Other factors that play a significant role in determining the combustion temperatures are the heat transfer mechanisms and the reaction chemistry model. These conditions are common to the low velocity regime of filtration gas combustion (17). The upstream propagation of the combustion wave results in underadiabatic combustion temperatures (29) while the downstream movement of the wave generates superadiabatic combustion regimes with temperatures higher than the adiabatic flame temperature (30).
With a substantial amount of work devoted to predict the NOx generation for rich premixed mixture of hydrocarbons, there lies a great opportunity to focus in the laminar regime of ultrarich premixed methane-air combustion. The present study would help researchers to gain insight into the strategies to limit NOx generation for ultrarich blends of methane-air. Furthermore, generalized computational models stands validated with supporting experimental results.