ECN: Thermoacoustics
What is thermoacoustics?
A sound wave is associated with changes in pressure, temperature en density of the medium through which the sound wave propagates. In addition, the medium itself is moved around an equilibrium position. These fluctuations are that small in the sounds we hear day-to-day that they are unnoticed. The sound waves that are common in thermoacoustics however are extreme in magnitude, resulting in noticeable fluctuations. The temperature fluctuations for example can amount to tens of degrees. However, this is still too little to be of any use. That is why an acoustic wave is brought into interaction with a porous structure with a much higher heat capacity compared to the medium through which the sound wave propagates. This porous structure acts as a kind of heat storage (regenerator).
Within thermoacoustics a distinction is made between a thermoacoustic engine or prime mover (TA-engine) and a thermoacoustic heat pump (TA-heat pump). The first relates to a device creating an acoustic wave by a temperature difference while in the second an acoustic wave is used to create a temperature difference.
TA-engine
When a temperature gradient is imposed across a regenerator by for example a cold and a hot heat exchanger, the following happens with a parcel of gas when an acoustic wave passes by from the cold side.
The gas is being compressed by the passing pressure wave (compression). Successively the gas parcel is moved to a hotter part of the regenerator. Since the temperature over there is higher than the gas parcel, the gas is heated (heating). Then the pressure wave that first compressed the gas parcel is now expanding it (expansion). Finally, the gas parcel is moved back to its original position. The parcel of gas is still hotter than the structure (regenerator) resulting in heat transfer from the gas to the structure (Cooling).
During this cycle the gas is being compressed at low temperature, while expansion takes place at high temperature. This means that work is performed on the gas. The effect of this work is that the pressure amplitude of the sound wave is increased. The thermodynamic cycle just described resembles the well-known Stirling cycle. The acoustic wave has the function of both pistons normally present in a Stirling engine. In this way it is possible to create and amplify a sound wave by a temperature difference. The thermal energy is converted into acoustic energy that can be regarded as a kind of mechanical energy.
TA-heat pump
The reverse process of what has been describe above happens in a TA-heat pump. The thermodynamic cycle is run in the reverse direction, meaning that acoustic energy is converted into thermal energy. In this case heat is pump from a low temperature to a high temperature. Since the medium that is used is a gas (air, noble gas) this kind a of heat pump has virtual no limitations in its applications.
