Unlike flaming combustion, smoldering involves oxidation directly at the surface of solid fuel particles.  Materials prone to smoldering (like grain, animal feed, coal, and insulation materials) are typically porous or in granular form.  The very incomplete combustion during smoldering leads to large emissions of toxic gasses and aerosols and huge damage to wildlands, industry, climate and human health.

In this contribution, we will demonstrate that smoldering fires allow a number of concepts from statistical physics to be explored experimentally in a new context.  A sample that undergo smoldering consists of a number of interacting parts at different combustion stages; with regards to oxygen concentration, fuel combustion state, and temperature.

The governing heat balance contains a non-linear term (heat generation from combustion) and linear terms (heat losses to surroundings, heat storage in sample) in temperature.  Smoldering fires survive under a surprising range of conditions, as in an almost completely oxygen-depleted atmosphere.

We report two striking phenomena that emerge from the noisy, long-lived, adaptive smoldering processes:  Spontaneous synchronization of the entire sample – and intermittent growth in combustion intensity under periodic quenching.

Synchronization

Synchronized, pulsating temperatures are observed experimentally in smoldering fires. The entire sample volume (1.8 l) participates in the pulsations between intense combustion at high temperatures and apparent extinguishment at low temperatures (pulse period 2-4 h).  The synchrony lasts up to 25 h and is followed by a spontaneous transition to either disordered combustion or self-extinguishment.

Pulsation frequency and amplitude increase towards synchronization breakdown and transition to intense, disordered combustion.  The synchronization is obtained when the fuel bed is cooled to the brink of extinguishment (Mikalsen et al., EPL 121, 50002 (2018)).

Intermittent growth

Once ignited, a smoldering fire often grows slowly in intensity (temperature), sometimes with a spontaneous transition to flaming fire.  We puncture this highly non-linear fire growth by periodically adding fresh, room-temperature fuel.  This leads to abrupt drops in combustion intensity (mass-loss rate), followed by recurrent growth.  We give a statistical description of the response to quenching, both at micro-level (single drops) and macro-level (evolution over many drops) (Meyer et al., in preparation).