Influence of topological characteristics of a brazed joint formation on joint thermal integrity

Adequate thermal integrity of brazed joints is a critical feature of a state-of-the-art compact heat exchanger design. New generations of aluminum compact heat exchangers with highly augmented heat transfer surfaces (such as micro-channeled extruded tubes in conjunction with composite AA3003/AA4343 multi-louvered fin foils) can provide significantly enhanced thermal performance - if the related manufacturing process, e.g., controlled atmosphere brazing (CAB) is performed flawlessly. However, empirical evidence indicates that often, even under a very tight brazing process control, design requirements imposed by heat transfer considerations lead to a more or less poor joint formation. This paper deals with an analysis of thermal features of a fin-tube joint as a function of topological alterations of the AA4343 joint filet size formed by materials processing during brazing. These alterations are caused by poor wetting and/or pronounced Si diffusion and/or increased liquid clad penetration. These phenomena lead to a less than optimal joint formation and will be illustrated by both empirical evidence and theoretical predictions. Based on numerical predictions of a joint topology formed by the surface tension driven reactive flow of molten metal, and subsequently verified by empirical evidence gathered through both laboratory and industrial testing, several topology alterations were devised as being representative for thermal integrity studies, and thermal characteristics of corresponding fin-tube joints were determined in terms of the spreading thermal contact resistance and corresponding temperature fields. Numerical predictions of joint topology were devised with an in-house developed finite element code and verified by Surface Evolver code. Temperature distributions established within selected fin-tube joint configurations were simulated using CFD software (FLUENT). Experimental data were obtained using a computer controlled transparent hot zone with an ultra high purity nitrogen background atmosphere.