Classification of Flow Patterns in Angled T-Junctions for the Evaluation of High Cycle Thermal Fatigue

Abstract

Temperature fluctuations generated by the mixing of hot and cold streams at T-junctions can cause high cycle thermal fatigue failure. One of the important parameters for determining the degree of thermal fatigue damage is related to the flow patterns downstream of the mixing junction. Many papers have been published identifying the characteristic equations for classifying the flow patterns at the T-junctions, which have been shown to be related to the momentum ratios between the main pipe flow and the branch pipe flow. The flow patterns and governing equations studied in the past are only applicable to 90-degree tee junctions (T-junctions). The intention of this paper is to extend the work of others to include angled tee junctions other than 90 degrees (Y-junctions). Y-junctions especially with 45-degree angle are also used in the chemical and refinery plants of our concern for lowering the pressure drop in the mixing zone and weakening the impingement of the branch pipe stream against the main pipe. This paper presents the proposal for a set of extended characteristic equations for classifying the flow patterns in mixing tees which can be applied to all angles of tee junctions. The modified characteristic equations have been verified using computational fluid dynamics (CFD) simulations. Models with branch angles other than 90 degrees have been used to confirm the robustness and validity of the modified equations. The effect of the pressure drop caused by the varying opening sizes has been shown to play only a minor role in determining the overall flow patterns, with the key variables being the velocity ratio and the interaction area between the flow in the main pipe and the jet exiting from the branch pipe. The results have also highlighted that the angle of the branch pipe has significant impact on increasing the velocity ratio range for the less damaging deflecting jet flow pattern, which is an important finding that could be used to extend the current design options for piping systems where high cycle thermal fatigue may be a concern.