Experimental study of the effects of air flow through Helmholzt cavity on heat transfer within a smooth round tube

When the air flows through the Helmholzt oscillation cavity, it will be pulsing. The inlet and outlet diameters of Helmholzt cavity remain const, but the length of cavity is changed so that, the Reynolds number at the inlet changes from 22000 to 82000. The experimental investigation was conducted under equiflux conditions. Comparison was made between the heat transfer coefficient of steady air flow and unsteady air flow. The results show that: The pulse air flow can increase the heat transfer, but the enhancement of heat transfer takes place mainly at the distance of 10 times of the diameter along the axial direction of the round tube. The ratio of enhancement lies between 1.1 to 1.9. This ratio decreaser significantly as the distance becomes longer. Beyond the distance of 10 times of the diameter, the ratio of enhancement fluctuates slightly as the Reynolds number increases. The experimental investigation also shows that the heat conduction coefficient is influenced by the length of the Helmholzt cavity.