Self-tuning Mechanism in a Looped Tube Thermoacoustic Engine

We constructed a looped tube thermoacoustic engine equipped with a stack made of ceramics having many square pores and two heat exchangers. We employed three stacks with different pore sizes and studied the effect of changing the pore size on the acoustic field excited in the present engine through the pressure and velocity measurements. By analysing the experimental data, we are led to conclude that the present thermoacoustic engine automatically tunes the acoustic field in such way that the energy conversion is executed in the most efficient way under the given condition. Introduction When a steep temperature gradient set up along the stack exceeds some critical value, a gas column in the tube begins to oscillate. This phenomenon is caused by the thermoacoustic energy conversion from heat flow Q into work flow I in the stack.[1] The thermoacoustic energy conversion between Q and I can be controlled by two parameters. One is ωτ [1], where ω is an angular frequency of an acoustic wave and τ is the thermal relaxation time required for thermal equilibrium to develop in the cross-sectional area of the flow channel and given as τ = r/2α by the radius r of the flow channel and the thermal diffusivity[2] α. The parameter ωτ characterizes the degree of the thermal interaction between an oscillating gas and a solid wall in the stack. If ωτ << π, the gas parcel in the channel moves reversibly, equilibrating at the local wall temperature, whereas if ωτ >> π, the gas motion becomes isentropic. In the flow channel having ωτ ~ π, the gas parcel has irreversible thermal contacts with wall due to incomplete heat transfer. As another parameter, we refer to the phase lead Φ of cross-sectional mean velocity ( ) i t U ue ω +Φ = relative to pressure