Abstract
It is well established that when a turbulent flow is subjected to a non-uniform body force, the turbulence may be significantly suppressed in comparison with that of the flow of the same flow rate and hence the flow is said to be laminarised. This is the situation in buoyancy-aided mixed convection when severe heat transfer deterioration may occur. Here we report results of direct numerical simulations of flow with a linear or a step-change profile of body force. In contrast to the conventional view, we show that applying a body force to a turbulent flow while keeping the pressure force unchanged causes little changes to the key characteristics of the turbulence. In particular, the mixing characteristics of the turbulence represented by the turbulent viscosity remain largely unaffected. The so-called flow laminarisation due to a body force is in effect a reduction in the apparent Reynolds number of the flow, based on an apparent friction velocity associated with only the pressure force of the flow (i.e. excluding the contribution of the body force). The new understanding allows the level of the flow ‘laminarisation’ and when the full laminarisation occurs to be readily predicted. In terms of the near-wall turbulence structure, the numbers of ejections and sweeps are little influenced by the imposition of the body force, whereas the strength of each event may be enhanced if the coverage of the body force extends significantly away from the wall. The streamwise turbulent stress is usually increased in accordance with the observation of more and stronger elongated streaks, but the wall-normal and the circumferential turbulent stresses are largely unchanged.
Highlights
Convective heat transfer, or convection, is commonplace in thermal energy systems
In a partially laminarised flow, turbulent kinetic energy and shear stress are both reduced in magnitude, and the reduction becomes stronger with the increase of the body force
We show that the body-force-influenced flow behaves as a ‘standard’ undisturbed turbulent flow but at a lower apparent Reynolds number; and this apparent Reynolds number is directly associated with the pressure gradient of the body-force-influenced flow
Summary
Convective heat transfer, or convection, is commonplace in thermal energy systems. He demonstrated that the velocity profiles are flattened in flows with influences of weak and medium buoyancy forces when the flow is partially laminarised, and become M-shape when the buoyancy is very strong and turbulence and heat transfer effectiveness regain their strength These were further confirmed by Carr, Connor & Buhr (1973) and Polyakov & Shindin (1988), both using air, and Kurganov & Kaptilny (1992) using CO2 at a supercritical pressure. The scope of the paper It is clear from the brief review provided above that buoyancy may significantly suppress turbulence resulting in a severe heat transfer deterioration This is attributed to the modification of the near-wall shear stress balance, resulting in a modified velocity profile. Results for cases shown in table 2 are based on 100 flow fields
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