Investigation of the thermal structure of the burning zone in condensed mixtures by fine thermocouples

Abstract

Temperature profiles through combustion waves in mixtures of ammonium perchlorate (AP) with polymethylmethacrylate (PMM), paraform (PF), and sulphur were obtained by fine W5%Re–W20%Re thermocouples embedded in the strands. The surface temperature, T s , increased with pressure, p (for AP + PF, α = 1 mixture-from 460 °C at p = 0.3 atm to 580 °C at p = 1 atm) and there was a corresponding rise in the burning rate. In contrast, the addition of a catalyst (Cu 2O) had no effect on T s despite a considerable (1.7–1.95 times) increase in the burning rate. The slopes of ln( T − T 0) = f( x) curves reveal a marked increase at T = T ∗ (where T ∗ ranges from 200 °C to 310 °C), probably due to the phase change AP (orthorhombic) → AP (cubic). The departure of T ∗ from the well-known value 240 °C may be ascribed to errors in the thermocouple measurements and to the gasification of PF at T < T ∗ at low pressures. The profiles T( x) and ϕ( x) (where ϕ = dT dx ) were obtained theoretically from a solution of the heat flow equation for a medium with a phase change. An estimate was obtained for the value of ( ϕ 240°+ dT − ϕ 240°− dT ) equal to 1.69 × 10 5 ṁ deg C/cm for the preheat zone in pure AP (where m is mass burning rate). Two maxima of ϕ were observed in the gas phase. The first appears when the thermocouple traverses the NH 3 + HClO 4 flame. The value ( ϕ max) 1 increased with pressure at p ⩽ 1 atm (for AP + PMM, α = 0.75 mixture—from 2.1 × 10 5 deg C/cm at p = 0.34 atm to 4.9 × 10 5 deg C/cm at p = 1 atm). At p = 1 atm ( ϕ max) 1 decreased with increasing p, probably due to thermal inertia of the thermocouple. A simple relationship was obtained for the maximum thermal gradient recorded by a thermocouple. The heat balance at the burning surface is considered. For the mixture under investigation the heat production in the condensed phase at p≤1 atm was insignificant. The addition of Cu 2O increased the thermal gradient in the gas phase near the surface and the burning velocity by about the same extent.