Experimental Investigation of Heated-Tube Wall Ignition in a HAN-Based Green Monopropellant

This paper details the design and development of a liquid propellant test stand intended for thermal ignition studies of aqueous Hydroxylammonium Nitrate (HAN) solutions. The stand uses uniformly heated stainless-steel tubing with an outer diameter of 0.25 inches, wall thickness of 0.0245 inches and a length of 3 feet. This experimental section was instrumented with pressure transducers and thermocouples to detect and characterize the thermal ignition conditions of the HAN propellant. Utilizing the same geometry as the experimental tube, target conditions were defined based on the results of parametric transient thermal computational fluid dynamics (CFD) analyses evaluating estimated ignition conditions based on mass flow rates ranging from 0.25 g/s to 1 g/s and external wall temperatures from 200℃ to 500℃. Flow rates less than 1 g/s were shown to have no effect on the bulk outlet temperature of the propellant, with the external wall temperature having a more significant effect. Additionally, centerline and inner wall axial temperature profiles of the propellant were developed to predict the location of ignition along the heated tube. Both the centerline and inner wall temperature profiles indicated ignition for the 300℃ to 500℃ conditions. These simulations were evaluated under the boundary conditions of ambient propellant temperature at the inlet and atmospheric outlet pressure at the tube exit. The stand features a variety of active and passive safety systems, including manual and automatic flow control valves and over-pressure relief hardware. A custom NI LabView program was developed to remotely manage the necessary hardware actuation for flow, purging, and venting sequences.