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

Temperature fluctuations caused by the mixing of hot and cold streams at tee junctions may lead to high cycle thermal fatigue (HCTF) failure. It is necessary to evaluate the integrity of structures where the HCTF may occur. Therefore, the Japan Society of Mechanical Engineers (JSME) published “Guideline for Evaluation of High Cycle Thermal Fatigue of a Pipe (JSME S017),” in 2003, which provides the procedures and methods for evaluating the integrity of structures with the potential for HCTF. In JSME S017, one of the important procedures of thermal fatigue evaluation is to classify the flow patterns at tee junctions, because the degree of thermal fatigue damage is closely related to the flow pattern downstream of the mixing junction. The conventional characteristic equations for classifying flow patterns are only applicable to 90-deg tee junctions (T-junctions). However, angled tee junctions other than 90 deg (Y-junctions) are also used in chemical plants and refineries for reducing the pressure drop in the mixing zone and for weakening the force of the impingement of the branch pipe stream against the main pipe. The aim of this paper is to develop general characteristic equations applicable to both T- and Y-junctions. In this paper, general characteristic equations have been proposed based on the momentum ratio for all angles of tee junctions. Further, the validity of the proposed characteristic equations and their applicability to all angles of tee junctions have been confirmed using computational fluid dynamics (CFD) simulations. The results have also highlighted that the angle of the branch pipe has a significant effect on increasing the velocity ratio range for 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 HCTF may be a concern. In addition, categorization 3 is recommended as a more proper method for classifying flow patterns at tee junctions when evaluating the potential for thermal fatigue.

Thermal Stress Response to Boundary Oscillation Between Hot and Cold Fluid Temperature

In nuclear power plants, high cycle thermal fatigue induced by temperature fluctuation of the coolant is one of frequent failure modes. To ensure the safety of nuclear power plant systems, it is important to prevent thermal fatigue failure. Typical causes of high cycle thermal fatigue are thermal striping at Tee-junction and thermal stratification oscillation. In order to evaluate thermal stress caused by thermal striping, a frequency response function has been developed. This function was derived from a heat transfer and thermal elastic theories, and can adequately evaluate thermal stress induced by temperature gradient into wall-thickness direction. However, this theoretical method cannot adequately evaluate thermal stress by thermal stratification oscillation, because this phenomenon has the fluid temperature distribution gradient along axial direction. To investigate the mechanism of thermal stress generated by oscillation of thermal stratification, two types of models were studied. In the first type, fluid temperature oscillates with sinusoidal history at the same location, and in the second one, the boundary layer of hot and cold fluid temperature moves with sinusoidal velocity. Through clarification of the stress generation mechanism, the frequency response function was improved to evaluate stress by the thermal stratification oscillation. Applicability of this function was verified through agreement with finite element simulations.