Accurate in-furnace temperature measurements are essential for temperature-based CFD validation and NOx risk assessment, yet intrusive probes can under-read in regions with strong gradients and evidence for DME-based blends across flame and MILD operation remains limited. This study investigates the sensitivity and accuracy of intrusive (suction pyrometer probe) versus non-intrusive (planar laser Rayleigh scattering, LRS) thermometry in measuring temperature, using CFD simulations as a complementary benchmark for interpretation and model assessment, in a semi-industrial scale furnace. By comparing temperature data across three techniques, it is shown how fuel composition and burning regime (conventional flame vs. MILD combustion) affect measurement agreement and accuracy. The analysis covers pure DME, DME/CH4, and DME/H2 mixtures under both flame and MILD conditions, extending earlier work in this furnace to a broader fuel-regime matrix. LRS successfully captured the thermal field for all fuel compositions in both flame and MILD modes, generally aligning well with CFD-predicted temperature distributions. Intrusive probe measurement yielded similar overall trends but showed significant deviations in regions of steep temperature gradients, particularly near burner jets and in highly reactive, hydrogen rich flames. In these zones, the probe-measured temperatures were up to 200 K lower than LRS values, a discrepancy far exceeding experimental uncertainty and attributed to the probe's volumetric averaging and flow disturbance effects. The effect was most pronounced for the DME/H2 blend under flame-like conditions, reflecting the increased measurement sensitivity to fuel reactivity. The numerical simulations capture the overall combustion behaviour across fuel mixtures, though modelling of the mixtures with high H2 required adjusting turbulence-chemistry parameters to capture its mixing-controlled flame characteristics and NOx emissions. Overall, the results provide a semi-industrial benchmark comparison of LRS and suction-probe thermometry for DME-based blends across flame and MILD regimes, clarifying the fuel- and regime-dependence of diagnostic bias and its implications for temperature-based CFD validation and diagnostic selection in practical combustion systems.
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