Experimental Study of Heat Transfer from the Heated Rib-Roughed Wall to a Steady or Pulsating Flow

The heat exchanger performance is mostly limited by the poor gas side heat transfer. This motivates attempts to increase the effectiveness of gas side heat transfer processes in numerous types of heat exchangers in automotive industry mostly used for cooling. The efficiency of compact heat exchangers can be improved by means of boundary layer modification. The objective of this reported work is to study the mechanism of convective heat transfer from the heated wall of a duct to a flow of air. The surface of the heated wall is flat or rib-roughened. Rib-roughened surfaces are used to induce turbulence and to enhance the heat transfer. Interferometer, schlieren and PIV methods assisted by high-speed video camera are used to study the mechanism of heat transfer from the heated wall to the flow. These methods are accompanied by measurements of the flow velocity, instant temperature, turbulence and frequency analysis of turbulence parameters. The range of Reynolds numbers studied was from 1000 to 12000. Finally the influence of a steady, oscillatory and pulsating flow on the heat transfer is investigated and compared. The low frequency oscillations from 0.083 to 0.25 Hz are applied in experiments to be comparable with frequencies used in literature. Frequencies of the pulsating flow are used in the range from 0.2 to 5 Hz. The results are presented in a form of a time-averaged Nusselt number as a function of a Reynolds number calculated on the basis of the oscillatory and pulsating flow parameters. It is found that turbulence promoters on the heated wall enhance heat transfer, while low frequency pulsation of the mean flow deteriorates it. Both effects are explained by the processes in the boundary layer sticking to the heated wall of the investigated channel.