Food smoke generation by frictional heating

Smoke can be generated by frictional heating where a plank of wood is pressed against a spinning wheel. Here, a plank of kānuka (12 × 12 × 150 mm), a common New Zealand food smoking wood, is pressed against a wheel (diameter 123 mm) with a knurled surface (asperities, 200 μm). Wheel speed (2500–3500 rpm), pressing force (9.8–68.7 N) and chamber temperature (ambient − 200 °C) were varied. Measurements were performed of the plank wear rate, power draw and, in the ambient trials, the frictional interface temperature. Three distinctive wear behaviours appear: no smouldering with no wear, steady smouldering at an intermediate rate (ca. 0.1 mm/s), and steady smouldering at a high rate (ca. 0.4 mm/s). The transition between these two steady wear behaviours occurs sooner when pressing force and chamber temperature are increased and are characterized by marked reductions in the coefficient of friction, from ca. 0.6–0.9 to < 0.1. The transition is also associated with an order of magnitude change from ca. 1 to 10 mm of wear per kilojoule of expended frictional energy. While the precise frictional interface temperature and the heat sinks are difficult to quantify, the transition between these steady wear behaviours signals an exothermic shift in the heat of reaction, indicating the increased contribution of secondary reactions to the overall energetics. These results underlie the difficulty in achieving steady and consistent smoke production in frictional food smoke generators.