Experimental and numerical study of near-wall temperature fields in a pellet boiler: Impact of excess air ratio and heat-transfer optimization with a flow deflector

CFD is widely used to analyze pellet-boiler combustion, yet its predictive value for chamber design depends on validation against spatially resolved thermal data, not only global flue-gas and heat-output metrics. This study combines near-wall temperature mapping with CFD to quantify the coupled effects of excess-air ratio (λ) and a compact flow deflector. Spatially resolved near-wall temperature maps are used to evaluate CFD beyond bulk metrics and quantify the impact of passive flow shaping. Experiments used 30 near-wall thermocouples, flue-gas analysis, and heat output measurement while varying λ. Increasing λ from 2.26 to 3.93 cooled the chamber, diluted CO 2 , and reduced heat output from 10.89 to 8.97 kW. An operational optimum occurred near λ ≈ 2.54, where CO reached 510 mg·m −3 at 10% O 2 and heat output was 10.47 kW. Near-wall mapping revealed pronounced wall-heating maldistribution, with the wall-heating maldistribution index ∣D∣ reaching 33.3% (50% fan setting, λ ≈ 3.57). The CFD model captured the cooling trend and peak wall-temperature levels. The deflector was evaluated at an operating point (40% fan setting, λ ≈ 3.40), where it homogenized the thermal field and improved oxidation: ∣D∣ decreased from 27.8% to 3.2%, maximum near-wall temperature dropped from 584 to 474 °C, CO decreased from 1255 to 512 mg·m −3 , and heat output increased from 9.45 to 9.70 kW, while NO x increased from 144 to 202 mg·m −3 . Overall, operating near λ ≈ 2.5 with passive flow control lowers CO and peak near-wall temperatures and slightly increases heat output, with a trade-off in NO x . • 30 near-wall thermocouples revealed uneven wall heating around the chamber. • Wall asymmetry reached D = 39.6%; a deflector cut it to D = 3.3%. • CO minimum 510 mg/m 3 (10% O 2 ) at λ ≈ 2.54 with 10.47 kW output. • Deflector reduced CO 1255 → 512 mg/m 3 and T max 584 → 474 °C. • CFD predicted wall-temperature drop with increasing airflow, consistent with tests.