Heat flux measurements in a scramjet combustor using embedded direct-write sensors

Embedded heat flux sensors fabricated using direct-write technology have been developed and tested within a direct-connect gaseous hydrocarbon-fueled scramjet combustor under Mach 5 flight conditions. The sensors employed a multilayer thermopile architecture fabricated via the additive direct-write thermal spray process, which enables their integration directly onto the components to be tested. The sensors were embedded within a 0.50-mm-thick (0.020-in.-thick) thermal barrier coating on test plugs that were inserted into the tunnel for testing. Fuel equivalence ratios and dynamic pressures were varied from φ = 0.6 to 1.0 and Q = 24 and 48 kPa (500 and 1000 psf), respectively, with increases in the global heat flux found to depend on the tunnel location. Heat flux values ranging from 30 to 75 W/cm² (26 to 66 Btu/ft² · s) were measured downstream of the cavity flame holder, 20 to 150 W/cm² (18 to 132 Btu/ft² · s) in the cavity flame holder, and 20 to 40 W/cm² (18 to 35 Btu/ft² · s) upstream of the flame holder. Direct-write sensors reported heat flux trends that were consistent with separate area-averaged calorimetric measurements, and the local nature of the direct-write sensors enabled the detection of subtle transient phenomena not captured by calorimetry.