Heat losses in a smouldering system: The key role of non-uniform air flux

Abstract

Smouldering combustion is emerging as a valuable tool for energy conversion purposes. However, the effects of radial/lateral heat losses, while critical to its viability, are not well understood. It is known that heat losses weaken the smouldering reaction near the walls. It is less known that these losses generate non-uniform air flux across the system cross-section, potentially changing conversion rates and quenching limits. This study integrated: (i) highly instrumented smouldering experiments across numerous scales, (ii) a novel method of estimating non-uniform air flux in the experiments, (iii) analytical modelling to predict non-uniform cooling, and (iv) energy balance calculations to quantify the non-uniform heat of smouldering. Altogether, this work demonstrates that heat loss-induced non-uniform air flux is significant, affecting key smouldering propagation and cooling characteristics. The uniform air flux injected at the base became redistributed with a ~50% decrease at the centreline and a ~50% increase at the wall. This was shown to cause a concave (in the direction of air flow) smouldering front and a concave cooling front. The former was shown to cause radial heat transfer inwards, leading to super-adiabatic heating towards the centre of the reactor. The latter was shown to inhibit cooling along the centreline, which progressed ~40% slower than expected during propagation. Altogether, the multiple and integrated analyses used reveal the magnitude and significance of heat losses in smouldering systems. This insight is valuable to better harness smouldering for engineering applications.