No abstract

No abstract

In the present paper, the influence of vorticity layer on the turbulent separated flow and heat transfer in a cross-flow cavity was experimentally examined. The vorticity layer was generated by a miniturbulator installed in the upstream region of the flow separation point. As the miniturbulator, a small cross-flow rib was used whose height was one order of magnitude smaller than the cavity depth. The variable parameters were the angle of wall inclination in the cavity, the rib height, and the rib-to-cavity separation. The additional vortical disturbances introduced into the recirculation zone were found to exert an appreciable influence on the vortex formation pattern and on the distribution of pressure and heat-transfer coefficients. The experimental data were compared to computation data obtained with the Fluent 6 software. Numerical data on the dynamic and thermal characteristics of flows past a system comprising a sudden pipe expansion and a low-height diaphragm installed in the upstream region of the flow separation point are also presented. It is found that such a diaphragm, used to modify the characteristics of the separated flow, results in a change of the length and intensity of the eddying flow in the separation zone. The vortex sheet produced by the diaphragm interacts with the primary eddy, makes the separation zone more extended, and shifts, even to a greater extent, the point at which the heat-transfer coefficient attains its maximum in the downstream direction. The maximum heat-transfer coefficient turns out to be increased in comparison with undisturbed flow. Both the location of the diaphragm and the diaphragm height strongly affect the heat-transfer characteristics.

Results of an experimental and numerical study of the flow structure and turbulent heat transfer in a rectangular channel with longitudinal pressure gradient behind the backward-facing step. With the channel expansion the angle of rotation of the upper wall is 1.43; 2.86 and 4 ○ , and with the channel convergence the rotation angle is 3; 5.7 and 7.6 ○ . Experiments are carried out at Reynolds numbers, calculated on the step height and velocity in front of the backward-facing step, Re H = 4 000; 8 000; 12 000. It was determined, with the increase of pressure gradient the maximum value of Nusselt number increases for the converging channel and decreases for the expanding channel.

The process of interaction of two separated flows of various scales in a tube with a sudden expansion is investigated. It has been determined that an additional turbulizing element in the form of a small diaphragm leads to dramatic changes in the structure of the recirculation zone in the channel behind a step, shift in the flow attachment point, and, accordingly, redistribution of heat and mass transfer coefficients. The approach of a mini-turbulizer to the separation point increases the size of recirculation zone and heat transfer intensity. An increase in the height of a mini-turbulizer affects similarly the characteristics of a separated flow.