Turbulence attenuation in simultaneously heated and cooled annular flows at supercritical pressure

Abstract

Heated or cooled fluids at supercritical pressure show large variations in thermophysical properties, such as the density, dynamic viscosity and molecular Prandtl number, which strongly influence turbulence characteristics. To investigate this, direct numerical simulations were performed of a turbulent flow at supercritical pressure (CO$_{2}$at 8 MPa) in an annulus with a hot inner wall and a cold outer wall. The pseudo-critical temperature lies close to the inner wall, which results in strong thermophysical property variations in that region. The turbulent shear stress and the turbulent intensities significantly decrease near the hot inner wall, but increase near the cold outer wall, which can be partially attributed to the mean dynamic viscosity and density stratification. This leads to decreased production of turbulent kinetic energy near the inner wall and vice versa near the outer wall. However, by analysing a transport equation for the coherent streak flank strength, it was found that thermophysical property fluctuations significantly affect streak evolution. Near the hot wall, thermal expansion and buoyancy tend to decrease streak coherence, while the viscosity gradient that exists across the streaks interacts with mean shear to act as either a source or a sink in the evolution equation for the coherent streak flank strength. The formation of streamwise vortices on the other hand is hindered by the torque that is the result of the kinetic energy and density gradients. Near the cold wall, the results are reversed, i.e. the coherent streak flank strength and the streamwise vortices are enhanced due to the variable density and dynamic viscosity. The results show that not only the mean stratification but also the large instantaneous thermophysical property variations that occur in heated or cooled fluids at supercritical pressure have a significant effect on turbulent structures that are responsible for the self-regeneration process in near-wall turbulence. Thus, instantaneous density and dynamic viscosity fluctuations are responsible for decreased (or increased) turbulent motions in heated (or cooled) fluids at supercritical pressure.

Characteristics of turbulent heat transfer in an annulus at supercritical pressure

“…The geometry and simulation methods were previously described in Peeters et al [6]. For the sake of completeness, we will reiterate the specifics of the geometry and briefly describe the numerical methods that were used.…”

“…Exact details of the grid size with respect to wall units and the Batchelor scale as well as spectra of the enthalpy fluctuations can be found in Ref. [6].…”

“…The direct effects of variations in thermal diffusivity, specific heat capacity, density, and molecular Prandtl number are less well studied. In an earlier study, Peeters et al [6], investigated how turbulence is attenuated by the variable thermophysical properties of a fluid at supercritical pressure in an annular geometry, using direct numerical simulations. In the present study, we wish to build upon that work by investigating how turbulent heat transfer is attenuated by variations of thermophysical properties at supercritical pressure conditions.…”

Scalar statistics in variable property turbulent channel flows

“…Some of the well-known examples which involve variable thermophysical properties include supersonic flows for aircraft and propulsion systems, strongly heated or cooled flows in heat exchangers, or chemically reacting flows in combustion chambers. Furthermore, recently, in order to increase the efficiency of power cycles, there is increased interest in studying turbulent heat transfer to fluids at supercritical pressure [6][7][8]. These fluids exhibit strong thermophysical property variations due to a strong dependence of properties on temperature.…”

Experimental and Numerical Investigation of Convective Heat Transfer of Supercritical Carbon Dioxide at Low Mass Fluxes

“…Bae et al [9,10] investigated the turbulent heat transfer of CO 2 at supercritical pressure flowing in heated vertical tubes using direct numerical simulation (DNS) at the inlet Reynolds numbers of 5400 and 8900. Nemati et al [11] and Peeters [12] studied the turbulent attenuation and the effect of thermal boundary conditions on the development of turbulent pipe flows with fluids at the inlet Reynolds numbers of 360 and 8000. Chen [13] numerically studied the heat transfer and various convection structures of CO 2 flow in microchannels.…”