Porous burners have gained increasing attention as efficient alternatives to conventional free-flame burners for domestic cooking applications owing to their enhanced thermal efficiency, improved temperature uniformity, and reduced pollutant emissions. By stabilizing combustion within or near a porous ceramic matrix, these burners promote internal heat recirculation and intensify heat transfer to the cookware. Despite these advantages, their performance is highly sensitive to operating parameters, among which the equivalence ratio plays a pivotal role. Variations in equivalence ratio can significantly alter flame stability, combustion regime, temperature distribution, heat-transfer mechanisms, and pollutant formation. Although previous studies have investigated the influence of equivalence ratio on selected performance indicators, a comprehensive experimental assessment encompassing all major thermal, combustion, and emission characteristics under identical operating conditions remains limited. The present study addresses this gap by experimentally examining the effect of equivalence ratio on the overall performance of a porous ceramic burner developed for domestic cooking applications.
Experiments were conducted at a constant thermal input of 1.83 kW, corresponding to the minimum rated power for household cooking burners, over an equivalence-ratio range of 0.5 to 1.2, covering lean, near-stoichiometric, and rich combustion regimes. All tests were performed in accordance with relevant Iranian national standards to ensure reproducibility and practical relevance. The investigated parameters include flame temperature, porous-ceramic surface temperature, preheating-zone temperature, thermal efficiency, partitioning of convective and radiative heat transfer to the cooking vessel, flame stability, combustion regime, and CO emissions.
The results indicate that the equivalence ratio exerts a dominant influence on flame location and combustion regime, which subsequently governs the thermal and environmental performance of the burner. As the equivalence ratio increases from lean conditions toward stoichiometry, flame temperature rises substantially. Specifically, the flame temperature increases from approximately 1016 °C at an equivalence ratio of φ = 0.7 to about 1080 °C near stoichiometric conditions, reaching a maximum of around 1100 °C under rich combustion. A similar increasing trend is observed for the surface temperature of the porous ceramic. This behavior is attributed to the progressive submergence of the flame within the porous structure, which enhances conductive heat transfer between the flame and the ceramic matrix and increases radiative heat emission from the heated surface.
Beyond near-stoichiometric conditions, however, the rate of increase in flame temperature diminishes despite further enrichment of the mixture. This trend reflects the onset of oxygen-deficient combustion and the increasing influence of incomplete oxidation, which limits further temperature rise. Under rich conditions, although the ceramic surface temperature remains elevated due to enhanced heat recirculation within the porous medium, the effective combustion intensity no longer increases proportionally.
The preheating-zone temperature exhibits a similar dependence on the equivalence ratio. Under lean combustion conditions, the high velocity of the fuel–air mixture leads to short residence times within the porous structure, restricting effective upstream heat transfer and preheating. As the equivalence ratio increases, the mixture velocity decreases, allowing more intense thermal interaction between the combustion zone and the preheating region. Consequently, the preheating-zone temperature increases noticeably, which promotes improved flame stabilization and facilitates flame anchoring within the porous matrix. This enhanced preheating effect plays a critical role in shaping the observed combustion regimes.
Thermal efficiency, a key performance indicator for cooking applications, shows a non-monotonic variation with equivalence ratio. The maximum thermal efficiency of 63.2% is achieved at an equivalence ratio of φ = 0.7. At equivalence ratios lower than this value, thermal efficiency decreases due to excessive mixture velocity, which induces flame instability, partial lift-off, and reduced heat-transfer effectiveness. Under these conditions, a considerable fraction of the released thermal energy is lost with the exhaust gases rather than being transferred to the cooking vessel.
As the equivalence ratio increases beyond φ = 0.7 thermal efficiency gradually declines, reaching approximately 58% at φ = 1.2. This reduction occurs despite the higher ceramic surface temperatures measured under richer conditions. These findings demonstrate that thermal efficiency is not governed solely by ceramic temperature or radiative intensity. Instead, flame position relative to the cookware and the velocity of combustion products play a decisive role in determining the net heat-transfer rate. When the flame becomes fully submerged within the porous medium, convective heat transfer to the cooking vessel is significantly weakened, resulting in lower overall efficiency.
Analysis of the heat-transfer mechanisms further elucidates this behavior. Increasing the equivalence ratio leads to an increase in radiative heat transfer due to elevated ceramic temperatures and higher surface emissive power. Conversely, convective heat transfer decreases as a result of reduced exhaust-gas velocities and weaker forced convection around the cooking vessel. At an equivalence ratio of φ = 0.7, an optimal balance between convective and radiative heat transfer is established. In this condition, convective heat transfer remains substantial owing to relatively high gas velocities, while radiative heat transfer contributes effectively without becoming dominant. This balanced heat-transfer regime explains the observed maximum in thermal efficiency.
CO emissions are found to be strongly dependent on the equivalence ratio. Under lean combustion conditions, CO concentrations remain extremely low, typically in the range of 1–3 ppm, indicating near-complete combustion facilitated by excess oxygen availability. As the equivalence ratio increases toward rich conditions, CO emissions rise due to oxygen deficiency and incomplete oxidation of carbon-containing species, reaching a maximum of approximately 25 ppm. Importantly, across the entire equivalence-ratio range investigated, CO emissions remain below the permissible limit specified by the BS EN 30-1-1 standard for domestic gas burners, confirming acceptable safety and environmental performance.
Flame stability observations are consistent with the thermal and emission trends. At very lean equivalence ratios (φ = 0.5–0.6), the flame becomes unstable and prone to blow-off as a result of excessive mixture velocity. In the equivalence-ratio range of φ = 0.7–0.8, a stable blue flame attached to the ceramic surface is consistently observed, corresponding to optimal operating conditions. At higher equivalence ratios, the flame progressively becomes submerged within the porous structure and exhibits yellowish streaks characteristic of rich combustion. No flashback phenomena are observed throughout the tested operating range, indicating a high level of operational safety.
Overall, the results identify an equivalence ratio of approximately φ = 0.7 as the optimal operating condition for the investigated porous burner, at which maximum thermal efficiency, stable flame behavior, and minimal CO emissions are simultaneously achieved. The findings highlight the critical importance of precise equivalence-ratio control in the design and operation of porous burners for domestic cooking applications and provide valuable guidance for the development of high-efficiency, low-emission cooking technologies.