Zhan-Chao Hu

A supercritical CO2 thermoacoustic pump is a novel device driven by periodic pressure oscillations from a thermoacoustic engine, which employs check valves to rectify the oscillatory flow for directed transport of the working fluid. Given the inherent advantages of thermoacoustic engines, including no moving parts and structural simplicity, it is essential to develop check valves of simplicity and high reliability to preserve these system-level benefits. This work proposes and evaluates two check valve concepts through numerical and experimental studies. The first is a no-moving-part check valve based on topology optimization. It generates optimized solid flow-disturbing structures within a design domain to achieve rectification via asymmetric flow resistance. Numerical results indicate that orifice-like structures within the flow path are a key contributor to enhanced diodicity. However, experiments reveal that under the high-amplitude pressure oscillations typical of thermoacoustic pumps, its performance remains limited. The second concept incorporates a simple spherical moving element (ball-type check valve). Its performance was significantly improved by optimizing the valve seat profile, adding a low-stiffness spring, and controlling the ball stroke. Consequently, this design delivers rectification performance comparable to the reference precision check valve. In summary, this study delineates two design pathways for check valves in supercritical CO2 thermoacoustic pumps: a fully no-moving-part approach and a simple moving-part approach. The former suits applications prioritizing ultimate reliability and maintenance-free operation, while the latter offers improved rectification with only a modest complexity increase. The conclusions provide clear guidance for optimizing check valves in thermoacoustic pumps and related devices.

Cross-diffusion effects, including Soret and Dufour effects, are enhanced around the pseudo-critical temperature (Tpc) of a binary mixture. Their influences on heat transfer at supercritical pressure have been scarcely studied. To bridge this gap, large-eddy simulations (LES) are conducted to investigate forced convective heat transfer of a CO2–ethane mixture at supercritical pressures in a circular pipe subject to a uniform heat flux. Both heating and cooling conditions, along with varying initial concentrations and thermodynamic pressures, are included in the simulations. The LES results reveal that the Soret effect causes concentration separation, resulting in a concentration boundary layer. The magnitudes of the thermodiffusion factor (kT) and the radial temperature gradient control the intensity of separation, which is more pronounced at near-critical pressure and high heat flux. Since kT is significant only around Tpc, downstream decay of the concentration separation is observed as the loci of T=Tpc migrate away from the wall so that the local radial temperature gradient diminishes. The primary factors affecting heat transfer are the variations in thermal conductivity and isobaric specific heat resulting from concentration separation. In contrast, the Dufour effect and the accompanying inter-diffusion play negligible roles. In deterioration scenarios, the bulk Nusselt number (Nub) shows a maximum relative drop of 8%, whereas in enhancement scenarios, Nub shows a maximum relative increase in 10%, with both deterioration and enhancement decaying downstream. Cross-diffusion effects have negligible impacts on density and streamwise velocity, but noticeably alter streamwise velocity fluctuation and turbulent kinetic energy.