PREDICTION OF HEAT TRANSFER IN A RIBBED CHANNEL: EVALUATION OF UNSTEADY RANS METHODOLOGY

摘要

Heat transfer in a straight channel with rib turbulators on one wall is predicted numerically with an unsteady Reynolds-averaged Navier-Stokes (URANS) methodology and compared to code-validation quality experimental data from the literature. Additionally, for comparison, steady simulations of the problem are conducted using two popular turbulence closure models, a Realizable k-ε model and a differential Reynolds-stress model. Closure in the URANS simulation is provided by a new eddy-viscosity-based model that was developed in the Advanced Computational Research Laboratory at Clemson University. This new model consists of three transport equations, and it is designed specifically to promote natural unsteadiness in the flow without the need for artificial forcing. In all cases, the Reynolds number, based on hydraulic diameter, is equal to 24,000. Eight square ribs, orthogonal to the flow direction, are equally spaced on the bottom wall of the channel. For the URANS simulation, after the flow becomes fully-developed in the streamwise direction, the predicted Nusselt number on the ribbed wall follows the trend of measured data from the modeled experimental study. However, the unsteady simulation slightly overpredicts the distance to the peak heat transfer aft of each rib. Also, the heat transfer prediction is very dependent on the grid resolution aft of the ribs. Therefore, efficient refinement of the unstructured mesh and grid-independence issues are discussed. Results of both steady simulations show a significant underprediction of Nusselt number over the entire ribbed wall, with the Reynolds-stress model giving the better result of the two steady closure models. The results of this study clearly show that unsteady vortex shedding off of the ribs is important in the physics of this problem, and a systematic, unsteady methodology is necessary to accurately predict ribbed-channel heat transfer.