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 22 000 to 82 000.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 experimeotal investigation also shows that the heat conduction coefficient is influenced by the length of the Helmholzt cavity.