Microconfined high-pressure transcritical fluid turbulence

Abstract

Microfluidics technology has grown rapidly over the past decades due to its high surface-to-volume ratios, flow controllability, and length scales efficiently suited for interacting with microscopic elements. However, as a consequence of the small rates of mixing and transfer they achieve due to operating under laminar flow regimes, the utilization of microfluidics for energy applications has long been a key challenge. In this regard, as a result of the hydrodynamic and thermophysical properties they exhibit in the vicinity of the pseudo-boiling region, it has recently been proposed that microconfined turbulence could be achieved by operating at high-pressure transcritical fluid conditions. Nonetheless, the underlying flow mechanisms of such systems are still not well characterized, and, thus, need to be carefully investigated. This work, consequently, analyzes supercritical microconfined turbulence by computing direct numerical simulations of high-pressure (P/Pc = 2) N2 at transcritical conditions imposed by a temperature difference between the bottom (T/Tc = 0.75) and top (T/Tc = 1.5) walls for a friction Reynolds number of R e τ = 100 (bottom wall). The results obtained indicate that microconfined turbulence can be achieved under such conditions, leading to mixing and heat transfer increments up to 100 × and 20 ×, respectively, with respect to equivalent low-pressure systems. In addition, it is found that the near-wall flow physics deviates from a single-phase boundary layer theory due to the presence of a baroclinic instability in the vicinity of the hot/top wall. This instability is generated by the combination of the external force driving the flow and the large variation of density across the pseudo-boiling region, which strongly modifies the flow behavior in the vicinity of the wall and renders present "law of the wall"transformation models inaccurate.