Effect of heat transfer conditions on the critical pressure of metal ignition in oxygen

Experimental data on the critical pressure of ignition of titanium alloy fragments in gaseous oxygen are analyzed. The fragments are obtained after fracture of alloy samples in the dynamic mode $(p_2^*)$ and under natural convection conditions $(p_1^*)$. The results are analyzed with allowance for the heat transfer coefficients from material ignition initiators under similar conditions. Based on the shape of the experimental thermograms of plate cooling, the coefficient of heat transfer from microcraters with a juvenile surface formed due to knockout of metal particles from the plate by the high-velocity flow is found: $\alpha_2\approx11$ W(m$^2$ $\cdot$ K). The value of $\alpha_2$ is close to the value of this coefficient calculated with the use of the coefficient $\alpha_1\approx5$ kW/(m$^2$ $\cdot$ K) of heat transfer from titanium rod microfragments (with the size of the order of metal grains) formed during titanium rod fracture in oxygen under conditions of natural convection with allowance for the ratio $(p_2^*)/(p_1^*)$.