Explanation of temperature gradients induced by a helium jet flushing into ambient air. Contribution of the Dufour Effect – a simulation and experimental approach

Abstract When a helium jet gas stream expands into the air at a flow rate of several Standard Liters per Minute (SLM), experiments have shown that the temperature in the core of the gas jet rises of several degrees Kelvin. This unexpected heating has so far remained unexplained. The Dufour effect is a non-equilibrium thermodynamics process resulting in heat transfer and occurring when a concentration gradient exists in a mixture of gases. In the case of helium jets admixing with air, the Dufour effect would cause heat transfer towards the helium-rich region.
Order-of-magnitude calculations were performed to assess whether the Dufour and Joule-Thomson effects might explain the temperature increase. 2D-axisymmetrical Computational Fluid Dynamics (CFD) simulations were carried out for various gas flows and outlet diameters, using helium or argon as an alternative inlet gas. Experimental results were obtained and compared with the simulation for a 2 mm diameter jet with 2 and 4 SLM. The experiments provide temperature maps on planes perpendicular to the direction of the jet for various distances from the outlet. The observed temperature patterns are similar to those predicted by the Dufour effect. For argon jets, the temperature gradients are reversed and have a smaller amplitude. Overall, these findings demonstrate that the Dufour effect is the main phenomenon driving the temperature gradient of helium and argon gas jet in the air.