Thermal Transport and Entropy Production Mechanisms in a Turbulent Round Jet at Supercritical Thermodynamic Conditions

Abstract

Abstract:In the present paper, thermal transport and entropy production mechanisms in a turbulent round jet of compressed nitrogen at supercritical thermodynamic conditions are investigated using a direct numerical simulation. First, thermal transport and its contribution to the mixture formation along with the anisotropy of heat fluxes and temperature scales are examined. Secondly, the entropy production rates during thermofluid processes evolving in the supercritical flow are investigated in order to identify the causes of irreversibilities and to display advantageous locations of handling along with the process regimes favorable to mixing. Thereby, it turned out that (1) the jet disintegration process consists of four main stages under supercritical conditions (potential core, separation, pseudo-boiling, turbulent mixing), (2) causes of irreversibilities are primarily due to heat transport and thermodynamic effects rather than turbulence dynamics and (3) heat fluxes and temperature scales appear anisotropic even at the smallest scales, which implies that anisotropic thermal diffusivity models might be appropriate in the context of both Reynolds-averaged Navier-Stokes (RANS) and large eddy simulation (LES) approaches while numerically modeling supercritical fluid flows.

Entropy Generation Analysis and Thermodynamic Optimization of Jet Impingement Cooling Using Large Eddy Simulation

“…This allows quantification of the overall entropy generation of a system and to examine how irreversibilities are distributed locally throughout the system [24]. Based on the concept of minimal entropy generation and CFD, causes of irreversibilities have been analyzed for a wide range of thermo-fluid processes including laminar and turbulent heat transfer in wall-bounded flows [25][26][27][28][29][30][31][32][33], flows under supercritical thermodynamic conditions [34][35][36][37], reacting flows [38][39][40][41] and also in heat transfer in impinging flows [9,42,43]. Contributions of the theory and application of entropy generation analysis using CFD for different types of engineering systems are reviewed in [24,44,45].…”

“…Focusing on entropy generation analysis of turbulent heat and fluid flows using CFD, only a few direct numerical simulations (DNS) are reported in the literature (e.g., [9,34,35,41,46,47]). All these DNS studies are restricted to heat and fluid flow problems at low-to-moderate Reynolds numbers and simple geometries due to the high computational cost of DNS.…”

Turbulence Modeling Insights into Supercritical Nitrogen Mixing Layers

“…However, when entering the transition region, instabilities start to appear, and turbulent dissipation begins to increase. [10] reports a maximum of density fluctuations in this region as pockets of dense fluid start to smear the potential core. The same authors also discuss how the heat absorbed to overcome the intermolecular attraction leads to an increase in the heat entropy production with a maximum already closer to the self-similar region.…”

“…In between these two regions lies the transition zone, where the turbulent and diffuse mixing is most relevant. As instabilities begin to appear, dense pockets of liquid-like nitrogen are separated from the jet core, causing an increase in density fluctuations [10,11]. As a result, the density sharply decreases, and the energy dissipation is significant.…”

“…In terms of Direct Numerical Simulation (DNS), the work of [10] stands out, in which a lower inlet velocity is used, reducing the Reynolds number. A merged PISO/ SIMPLE algorithm allows for the study of entropy production rates, dominated by heat transfer [21].…”