Dimensional Effect on Self-Sustaining Catalytic Combustion of n-Heptane in Micro/Meso Tubes

Abstract

As one of the potential candidates for fuel surrogates, n-heptane has been the focus of numerous oxidation and combustion studies. In this work, n-heptane combustion in cylinder tubes with diameters of 2, 3, and 4 mm was investigated experimentally with the use of packed catalyst beds of Pt/Ce 0.8 Zr 0.2 O 2 to study the mechanism of self-sustaining combustion and the dimensional effects of the inner diameter of a tube combustor. The region of self-sustaining stable combustion, the tube wall temperature, and the combustion efficiency were examined. n-Heptane can maintain self-sustaining stable combustion in the three tubes even if the wall temperature was below 430 K. As the equivalence ratio (Φ) increased, the stability region of the Reynolds number (Re) was expanded at the expense of fuel wastage. The lower stability limit of Re decreased as the tube diameter increased, and the value could reach as low as 12 in the tube with a diameter of 4 mm. The equivalence ratio was the dominant factor of the wall tube temperature at the lower boundary, whereas Re was significant at the upper boundary. An optimal Re allowed for the achievement of the highest wall temperature, and the optimal Re was high with a small diameter. The tube with a large diameter presented both a high combustion efficiency and high heat release rate. Combustion efficiency was improved by 40% when the diameter of the tube increased from 2 to 4 mm, and over 80% combustion efficiency was achieved in the d = 4 mm tube at Φ = 2. Under a fuel-rich condition, the tube combustor outputted heat energy at a low rate and emitted heat to the atmosphere at a high rate. This resulted in the wastage of a massive ratio of the input fuel.