The present invention relates to apparatus and methods in the field of heat exchangers and, more particularly, to apparatus and methods for controlling the flow through a heat exchanger to minimize incidence of low cycle fatigue metal damage.
The usable life of a typical high temperature cross- or counter-flow, plate-fin heat exchanger, e.g., an aircraft precooler, is often limited by low cycle fatigue (LCF) induced by large air temperature or air mass flow excursions which occur in the flow through the heat exchanger during repeated vehicle maneuvers, repeated engine start/stop cycles, or repeated ground maintenance cycles.
Analysis, testing and field operations have identified that the vulnerable areas for LCF are in the extreme corner areas of one of the inlet faces, typically the corners of the hot inlet face. Thermal/stress analysis has indicated that stack-wise temperature gradients generated at the corner of the alternating stacked passages are correlated with high strain ranges which accelerate the initiation and propagation of cracks in the metal of the separator plates (tube-sheets) which segregate the alternating hot/cold flows. Leakage between passages initiates and increases until some specified loss of performance ends the useful life of the heat exchanger.
Typical strategies for extending the useful LCF life of a heat exchanger involve increasing the gauge (thickness) of various metal components used to construct the entire heat exchanger. These strategies usually add significant weight to the heat exchanger and often degrade overall performance since metal is added to areas that are not associated with a described localized problem. Because these are “whole” heat exchanger approaches to a localized problem, they quickly become limited in the extent to which they can mitigate the scale of the localized problem. They are also limited in applicability since design changes become more difficult to implement at the point in the timeline when LCF issues become illuminated analytically.
As can be seen, there is a need for a targeted, localized approach to attenuating a localized stack-wise temperature gradient which has greater leverage to abate a large range of gradients while having minimal impact on heat exchanger design, weight and performance.