Supercritical water oxidation using hydrothermal flames at microscale as a potential solution for organic waste treatment in space applications – A practical demonstration and numerical study

•Demonstration of the Supercritical Water Oxidation process in microfluidic chip.•Experimental evidence of hydrothermal flames at microscale.•hydrothermal flames formation at low flow rate conditions.•Numerical modeling to understand the hydrothermal flame formation at microscale. Supercritical water oxidation (SCWO) with hydrothermal flames is well established for the treatment of aqueous organic waste as it not only overcomes the limitations of simple SCWO, such as precipitation of salts, but also exhibits many advantages over other waste treatment processes. Seeking these advantages, we propose to perform SCWO using hydrothermal flames in microfluidic reactors (μSCWO) for aerospace applications to be used in deep space/ISS missions. The novelty and highlight of this work are successful demonstration of realizing microreactors (channel width 200 μm), which can withstand pressure of 250 bar with temperature 400 °C, thereby presenting the feasibility to realize this technology. We present the first evidence of SCWO/hydrothermal in a flow microreactor of sapphire, which is captured through optical visualization. This is followed by a numerical investigation to understand the underlying physics leading to the formation of hydrothermal flame and thus differentiate it from a simple SCWO reaction. The simulations are performed in a 2D domain in a co-flow configuration with equal inlet velocity of fuel and oxidizer at two different inlet temperatures (350 °C and 365 °C), just below the critical temperature of water using ethanol and oxygen, the former acting not only as a model organic matter but also fuel for the formation of hydrothermal flames. It is observed that due to microscale size of the system, hydrothermal flames are formed at low inlet velocities (< 30 mm/s), while reaction at higher ones are characterized as simple SCWO reaction. This upper limit of inlet velocity was found to increase with inlet temperature. Finally, some key characteristics of hydrothermal flames - ignition delay time, flame structure, shape, and local propagation speed are analyzed.