Thermoacoustic waves and Piston Effect inside a microchannel with Carbon Dioxide near critical conditions

Abstract

• Adiabatic thermalization inside microchannels. • Thermoacoustic waves inside microchannels. • Computational Fluid Dynamics study of Piston Effect ( PE ) under terrestrial and Zero-gravity conditions. • Existence and Quantification of PE in forced convection. • PE in near-critical carbon dioxide. • Comparison of acoustic time scales and diffusion time scales. Heat transfer near the critical condition of Carbon Dioxide due to thermo-acoustic waves in a 100-µm high microchannel was numerically studied. The fluid temperature outside the thermal boundary layer was compared between computational fluid dynamics (CFD) models and a pure conduction model. The comparison revealed that the CFD model predicted a temperature increase in the bulk fluid much faster than the time constant required for such increase purely by conduction. It is believed that another heat transfer process, termed the piston effect ( PE ), which is associated with pressure waves in the fluid, was responsible for this increase. Explicit unsteady fluid model indicated that propagation of pressure waves due to the rapid expansion of the boundary layer and the associate change in the fluid density distribution resulted in this temperature rise. It was confirmed that natural convection wasn’t responsible for the temperature increase under quiescent conditions and it was revealed that the PE time scales are 5 to 6400 times faster than diffusion time scales. In addition, it was discovered that the PE is significant for certain forced convection conditions that transfer 5-7% of the total power input to the bulk fluid. A new correlation was proposed to correlate the temperature rise in the bulk fluid in terms of Fourier and Reynolds numbers.