Enhancing thermal mixing of supercritical water through a confined co-flowing planar jet

Previous studies have reported that the thermal mixing of supercritical water (SCW) would be inhibited by the density gradient in jet flow. The confined co-flowing planar jet which has one central inlet and two outer inlets is expected to enhance thermal mixing through stronger turbulent entrainment induced by double mixing layers. Direct numerical simulations (DNS) of planar jet of supercritical water (653-843 K, 25 MPa) are performed. The effects of the density ratio ρ_r (1.1, 3, 6) between jet and ambient fluids, the Reynolds number based on the density, velocity, diameter, and viscosity of central inlet R e in = ρ in U in D in / μ in (1000-4000), and the buoyancy on thermal mixing properties are investigated. We find that increasing ρ_r results in the decay of turbulence near the double mixing layers and the attenuation of thermal mixing. The self-similar behavior for co-flowing planar jet of supercritical water can be more likely to achieve for the mean field than for the turbulence field. While increasing Re_in results in the amplification of turbulence production in the far-field region due to the vortex stretching mechanism induced by larger velocity gradient, the enhancement of thermal mixing is insignificant. The gravity wave along the normal direction leads to density stratification and inhibition of turbulent mixing near the mixing layers when Re_in less than 2000. The gravity effect can be neglected when Re_in greater than 2000 due to the increasing turbulence production. Finally, we find that the enhancement of thermal mixing can be achieved by increasing the turbulent intensity of outer inlets.