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 been recently 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, analyses supercritical microconfined turbulence by computing DNS of high-pressure ($P/P_c = 2$) N$_2$ at transcritical conditions imposed by a temperature difference between the bottom (${T/T_c}=0.75$) and top (${T/T_c}=1.5$) walls for a friction Reynolds number of $Re_\tau=100$ (bottom wall).The results obtained indicate that microconfined turbulence can be achieved under such conditions, leading to mixing and heat transfer increments up to $100\times$ and $20\times$, respectively, with respect to equivalent low-pressure systems.In addition, it is found that the near-wall flow physics deviates from single-phase boundary layer theory due to the presence of a baroclinic instability in the vicinity of the hot/top wall.This instability strongly modifies the flow behaviour in the vicinity of the wall and renders present 'law of the wall' transformation models not accurate.

The technological opportunities enabled by understanding and controlling microscale systems have not yet been capitalized to disruptively improve energy processes. The main limitation corresponds to the laminar flows typically encountered in microdevices, which result in small mixing and transfer rates. This is a central unsolved problem in the thermal-fluid sciences, in what some researchers refer to as “quot;ab-on-a-chip and energy - microfluidic frontier”. Therefore, this work focuses on analyzing the potential of supercritical fluids to achieve turbulence in microconfined systems by studying their thermophysical properties. In particular, a real-gas thermodynamic model, combined with high-pressure transport coefficients, is utilized to characterize the Reynolds number achieved as a function of supercritical pressures and temperatures. The results indicate that fully-turbulent flows can be attained for a wide range of working fluids related to heat transfer applications, power cycles and energy conversion systems, and presenting increment ratios of O(100) with respect to atmospheric (subcritical) thermodynamic conditions. The underlying physical mechanism to achieve relatively high Reynolds numbers is based on operating within supercritical thermodynamic states (close to the critical point and pseudo-boiling region) in which density is relatively large while dynamic viscosity is similar to that of a gas. In addition, based on the Reynolds numbers achieved and the thermophysical properties of the fluids studied, an assessment of heat transfer at turbulent microfluidic conditions is presented to demonstrate the potential of supercritical fluids to enhance the performances of standard microfluidic systems by factors up to approximately 50x.

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