HEAT TRANSFER ENHANCEMENT IN TURBULENT RIBBED-PIPE FLOW

초록

Understanding heat transfer mechanisms in turbulent pipe flow is essential for solving many engineering problems. From the view point of controlling heat transfer, installing surface roughness is a popular way of enhancing heat transfer in turbulent pipe flow, relevant to heat exchangers and nuclear reactors to name a few. Such a surface roughness is often modeled with a rib. In the current investigation, we perform Large Eddy Simulation (LES) of turbulent heat transfer in circular ribbed-pipe flow in order to study the effects of periodically-mounted square ribs on heat transfer enhancement. The ribs are implemented within a cylindrical coordinate system by using an immersed boundary method, and dynamic subgrid-scale models are used to model Reynolds stresses and turbulent heat flux terms. A constant heat flux is imposed on the pipe wall. The Reynolds number (Re) based on the bulk velocity and pipe diameter is 24,000, and Prandtl number (Pr) is 0.71. The blockage ratio is fixed with 0.125, while the pitch ratio based on the rib interval and rib height is varied with 2, 4, 6, 8, 10, and 18. Since the pitch ratio is the key parameter that can change flow topology and turbulence characteristics, we focus on its effect on the flow and temperature fields. Profiles of mean velocity components, mean temperature, root-mean-squares (rms) of velocity and temperature fluctuations are presented at the selected streamwise locations downstream of a typical rib. An Octant analysis is performed to elucidate structures of turbulent heat transfer. We also present the budget of temperature variance in the form of contours. In comparison with the straight-pipe case at the same Re and Pr, the effects of the ribs can be clearly identified, leading to overall enhancement of turbulent heat transfer. Our LES results shed light on a complete understanding of the heat-transfer mechanisms in turbulent ribbed-pipe flow which has numerous applications in engineering.