Scalar transport in the near field between two coaxial square air jets

The scalar transport in the near field of two square coaxial free air jet flow has been experimentally investigated. In order to study the entrainment and mixing processes taking place between the two jets, heat was used as a scalar. A triple sensor probe was used consisting of an X-wire probe for the point measurement of two velocity components and a single cold wire probe, for the simultaneous recording of instantaneous temperature. From the velocity and temperature signals the momentum and heat flux components were calculated. Eddy diffusivities of heat and momentum were directly evaluated from the experimental data. These diffusivities for all streamwise positions reach a maximum in the interaction region between the two jets; the maximum values increase with the downstream distance in the near field. The quadrant splitting technique applied to turbulent fluxes of momentum and the scalar revealed that the positive contributions to these transport terms are much larger than the negative ones. The quadrant splitting data confirmed that the flow is dominated by large scale coherent structures, which are intermittent in character and permitted the association of the scalar transport with vortical structures occurring in the near field of the coaxial jet flow. A proposed simplified mechanistic flow picture, which shows the passage of a vortex series through the mixing layer in the near field of the jets, is in agreement with the dominant characteristics of the flow.