Pressure Wave Generators

Our TwinSTARTM pressure wave generators (PWG) are valveless electroacoustic devices - like extremely powerful audio speakers - that produce high-intensity acoustic power. They are designed to alternately compress and expand inert gases to drive acoustic loads such as thermoacoustic Stirling (‘pulse tube’) and Stirling cryocoolers. PWG’s are sometimes used to drive other loads that require oscillating pressure and flow, such as thermoacoustic refrigerators, or experiments to characterize conceptual Stirling cryocoolers or engine components. Pressure amplitudes up to 25% of mean pressure to 30 bar and volume amplitudes of 300 litre/second are possible with matched, pressurized loads.

TwinSTARs are balanced, high-efficiency resonant drivers with non-wearing, oil-free, low-noise and low-vibration operation at frequencies from 30-120Hz. For product specifications and more detailed information, click on the Pressure Wave Generator of interest at the bottom of this page.

How It Works

The below illustration shows schematically what’s inside a PWG. There are two patented STAR™ linear reciprocating motors with pistons of area AP, both addressing a common space (the front or compression volume, VC). Each motor is enclosed by a back volume VB. The motors are wired in parallel and run by a common voltage, which causes them to alternately compress and expand the gas in VC and deliver acoustic power to the load ZAC. All of the pressure force on the back sides of the pistons, and part of the force on the front side (that part that is in phase with displacement) act as gas-spring forces on the motors, raising their natural frequencies. It is important for the user to know how much gas spring is provided by the load, to determine the natural or ‘resonance’ frequency of the system and ensure that the PWG is run at or near this frequency. “Off-resonance” operation is only recommended when the power rating of the PWG far exceeds the power requirements of an application, and even then, caution is advised.

Pressure Wave Generator Motor

The linear motors are moving magnet transducers, characterized by

The PWG as a whole is characterized by the aforementioned piston area AP and volumes VC and VB, plus the seal length (piston length) L, the front side and back side surface areas SC and SB, and the tiny annular gap between the pistons and the cylinders, which we call the clearance gap δ. Together with the ratio of specific heats γ = cP/cV of the working gas, these parameters define the dynamics and the efficiency of the PWG.

  • Transduction coefficient Bl in Newtons/Amp
  • Electrical resistance of motor coil Re in Ohm
  • “Mechanical resistance” Rm in N•s/m
  • Coil inductance Le in Henrys
  • Moving mass m in kg
  • “Mechanical” (mostly magnetic) stiffness Km in N/m

 

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