Study of a Thermal Barrier Coating in a Pulsed Detonation Engine

A pulse detonation engine (PDE) works by establishing regular purge-detonation cycles. During a cycle, a point inside the detonation tube downstream of the ignition source is exposed to extreme high temperature briefly as the detonation wave passes, followed by near room temperature purge gas. As is the case for many combustion systems, minimizing unwanted heat loss in a PDE is important to maximizing efficiency. For this reason, the effect of coating the inner wall of a detonation tube with a thin, low thermal conductivity coating was studied experimentally and computationally. A three inch diameter 304 stainless steel detonation tube with 1.65 mm wall thickness was coated on the inside with a 0.635 mm thick layer of 93% Zirconium Oxide and 7% Yttrium Oxide. Experimental results were obtained operating the PDE at a frequency of 10 Hz. Temperature was measured at four points on the exterior of the detonation tube between 33 and 45 inches downstream of the ignition source. At all locations measured, the exterior tube temperature would rise from an initial ambient temperature faster with the uncoated tube until a crossover point approximately 1000–1500 cycles after the start of the experiment. After this cross over point, outer wall temperature was higher for the coated tubes, which at this point was increasing faster for the tube with the thermal barrier. To better understand the physical reason why the insulated tube always reached a higher temperature after roughly 1000 cycles, a simplified computational model was created using a time-dependent, one-dimensional, implicit finite difference method to calculate the temperature at 20 evenly spaced nodes. The temperature profile of the tube was assumed to vary only radially, so the interior points were solved using the transient one-dimensional heat conduction equation. One dimensional convection boundaries were applied at the inner and outer walls of the tube. Boundary conditions of increasing complexity were applied, beginning with a constant temperature, natural convection condition at the outer wall and a forced convection boundary condition at the inner wall with a step change in temperature to simulate the detonation wave. Experimentally measured wall temperatures are presented and compared to computationally obtained temperature profiles. The focus is on understanding the physical reason for the behavior of the insulated tubes.