Experimental and Numerical Study of Single-Phase Heat Transfer Enhancement in a Tube

Abstract

Fuel-element appendages affect the thermalhydraulic performance of a CANDU fuel channel by increasing its pressure drop and enhancing fuel-to-coolant heat transfer. The high cost of characterization of these effects through experiments provides incentive to develop reliable numerical methodologies by which to quantify relevant parametric trends. Recent studies at Chalk River Laboratories indicate that the current capabilities of Computational Fluid Dynamics (CFD) are adequate to assess the fuel-appendage effects under normal operating (single-phase) conditions from simulations alone. This paper describes the experimental work undertaken to provide data for validation of CFD models used in the numerical assessment of appendage effects, and presents preliminary results of a validation study completed with the TASCflow software. Heat transfer and pressure drop in single-phase flow through a vertical pipe obstructed by a cylindrical ring were investigated experimentally in the MR-7A loop at Chalk River Laboratories using Freon-134a as a coolant. The test section was a vertical 8mm ID directly heated tube, made of Inconel 600. Two ring-shaped flow obstructions were tested; they reduced the flow area by 17.8% and 30%. The presence of the flow obstructions enhanced heat transfer up to 38% and 61%, respectively. These significant changes were observed in the downstream region, over a very short distance behind the obstruction. The tests were carried out within the mass flux range between 1 and 6 Mg/m2s, and the highest relative heat-transfer enhancement occurred at 1 Mg/m2s. The pressure drop increase due to the flow obstructions was characterized by the form loss factor, K, which was found to be 0.18 and 0.45, for the 17.8% and 30% rings, respectively. The heat-transfer enhancement and pressure-drop experiments were subsequently simulated numerically using TASCflow software. All simulations were performed assuming axisymmetric flow conditions, and employing the "standard" k-E model for turbulent flows. Forced convection in Freon and conjugated heat conduction in the ring and the tube walls were modelled. The predictions generated by the code agreed well with the experimental results for both heat transfer and pressure drop. This demonstrates the merit of using computational, CFD-based methodology to assess appendage effects in single-phase forced convection flows.