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In this work, thermal ignition of laminar external natural convection flows by vertical cylinders is investigated. Effects of cylinder size are studied, with cylinder height ranging from 12.7 to 25.4 cm, and cylinder surface areas varying from 25 to 200 cm2. The minimum ignition threshold for a stoichiometric hexane-air mixture is 1019 K for a cylinder 25.4 cm long and 200 cm2 in surface area. The ignition threshold is found to have a weak dependence on surface area, in contrast to historical data. The ignition results are consistent with ignition temperature decreasing inversely with the logarithm of surface height. Experiments are performed with multi-component, heavy-hydrocarbon fuels including POSF-4658 Jet A and two surrogate fuels, Aachen and JI, as well as hexane. The setup is heated for multi-component fuel testing. POSF-4658 Jet A has an ignition temperature of 971 K at an ambient temperature of 333 K. All fuels investigated were found to have ignition thresholds within 38 K of the POSF-4658 Jet A ignition threshold. JI is found to be the most appropriate surrogate for POSF-4658 Jet A. [ABSTRACT FROM AUTHOR]