Pseudophase change effects in turbulent channel flow under transcritical temperature conditions

Abstract

We have performed direct numerical simulations (DNS) of compressible turbulent channel flow at supercritical pressure with top and bottom isothermal walls kept respectively at a supercritical (T top > T pb ) and subcritical temperature (T bot < T pb ), where T pb is the pseudoboiling temperature. The DNS are conducted using a high-order discretization of the fully compressible Navier-Stokes equations in conservative form closed with the Peng-Robinsion (PR) state equation. Bulk density is adjusted to obtain a bulk pressure of approximately p b = 1.1p cr where p cr is the critical pressure of the working fluid. Top-to-bottom temperature differences investigated are ∆T = 5 K, 10 K, and 20 K, where T top/bot = T pb ± ∆T /2; buoyancy effects are neglected. Varying ∆T modifies the average location of pseudophase change from y pb /h = −0.23 (∆T = 5 K) to 0.89 (∆T = 20 K), where h is the channel half-height and y = 0 the centerline position. Real-fluid effects cause visible deviations from classical scaling laws in the mean velocity profile. Enstrophy generation due stretching and tilting decreases with ∆T . The proximity to the pseudotransitioning layer inhibits the intensity of the velocity fluctuations, while enhancing the density and temperature fluctuations. Conditional probability analysis reveals that the sheet of fluid undergoing pseudophase change is characterized by a dramatic reduction in the kurtosis of density fluctuations and becomes thinner as ∆T is increased. Instantaneous visualizations show dense fluid ejections from the pseudoliquid viscous sublayer, some reaching the channel core, causing positive values of density skewness in the respective buffer-layer region (vice versa for the top wall).

Investigation of species-mass diffusion in binary-species boundary layers at high pressure using direct numerical simulations

“…Toki, Teramoto & Okamoto (2020) also simulated a planar turbulent channel flow of supercritical nitrogen and introduced a new transformation equation for the temperature profile in the logarithmic region. Transcritical channel flow of R-134a was simulated by Kim, Hickey & Scalo (2019) and it was reported that even the recent improvements to the scaling laws cannot accurately capture the velocity distribution in that transcritical flow.…”

Targeted turbulent structure control in wall-bounded flows via localized heating

“…A fourthorder Runge-Kutta scheme is used for the time integration. The solver has been rigorously validated in the case of shock-turbulence interactions 27 and wall-bounded flows, 29 and the accuracy of the numerical schemes is well established. 30…”

Structure of the thermal boundary layer in turbulent channel flows at transcritical conditions

“…At turbulent conditions in the transcritical regime, the presence of both liquid-like and gas-like thermodynamic property dependencies yields two spatial regions in the fluid domain, each with distinctive behaviour as influenced by the thermodynamics. Compared with classical constant-property trends, observations of turbulence in the gas-like region have shown -among other features -increased overall streamwise and spanwise anisotropy, enlarged spanwise spacing of large-scale near-wall structures (Patel et al 2015), and intensified relative turbulent fluctuation levels of thermodynamic transport properties (Kim, Hickey & Scalo 2019). Reverse trends have been observed in the liquid-like regime.…”

“…Additionally, in realistic transcritical flows, these coupled effects of modulated turbulence and property variations can be combined with a number of other physical phenomena, including thermoacoustic oscillations and buoyancy, all of which contribute towards further modification of the resultant turbulence dynamics and heat transfer, as discussed in Kim et al (2019) and Nemati et al (2015). The presence of buoyancy effects can cause transcritical fluids to undergo significant reduction in the magnitude of heat transfer.…”