Heat Exchanger Design
Aavid designed the heat exchanger based on stringent performance, dimensional, and weight specifications required for a typical bleed air engine application.
The plate fin design consists of folded fin layered in alternate directions, with side rails to complete the loop (Figure 1). Wavy fin was selected as it has good thermal transfer properties and is practical to manufacture from titanium. Alternatives such as lanced offset fin are difficult to make due to the material properties of titanium. Geometric dimensions met envelope specifications and the number of passes was designed for optimal heat transfer.
Aavid investigated a variety of titanium alloys. C.P. Ti Grade 4 was selected for the fin stock and tube plate. This is a commercially pure titanium alloy with high tensile and yield strengths. Wavy fin was manufactured at Aavid using a Robinson fin machine with a custom fin-folding die (Fig. 2).
Braze fillers of various material compositions and forms (pastes, foils, and powders) were evaluated. A pre-alloyed titanium powder developed by MRi provided the highest quality braze joints. Several methods of application were tested and a suspension spray method yielded the most even coverage.
Heat exchanger cores produced with spray-applied titanium alloy braze material showed excellent joining at all observable edges and internal fin-plate joints, and there was no plugging of passages (Fig. 3.) Tensile pull, lap shear, and burst tests demonstrated good consistency in braze strengths. Metallographic cross sections of the various fin-plate joints showed that the structure of the braze joint was sound with no voiding (Fig. 4). As with all plate-fin heat exchangers, well designed braze fixtures are critical to ensuring high quality brazed joints and good yields from the vacuum brazing process.
Aavid experimented with various header designs and methods of attachment. The best results were achieved with a header that was directly welded in a glove box.