Spiral Heat Exchanger Utilizing Dimpled Primary Surface

Paper presents a small demonstrator of the heat exchanger technology based on spiral counter-flow arrangement of dimpled primary surfaces. The heat exchanger has been developed as result of collaborative effort between Ukrainian and US scientists, authors of the paper. The heat transfer surfaces were fabricated out of stainless steel foil with arrays of spherical dimples augmenting heat transfer on the hot (gas) sides and corresponding spherical protrusions enhancing heat transfer on the cold (air) sides of the heat exchanger passages. About thirty primary surface geometries have been studied prior to selection of the presented configuration. Protruding diagonal ribs were introduced between lines of dimples on each side of the low-pressure passages, which were sandwiched between high-pressure passages, to prevent collapsing of the heat transfer surfaces under the pressure difference of 3.5 Bar. The counter-flow heat exchanger was assembled out of 24 (12 pairs) spiral rectangular cross-section channels, which were formed between dimpled surfaces. Development work was focused on optimizing thermal effectiveness and pressure losses within the heat exchanger core with little attention devoted to the entry and exit sections of the heat exchanger. The paper provides details of the experimental rig that was build for testing of the heat exchanger simulating operational parameters, which were representative for microturbine application. The overall recuperator effectiveness of 80–82% at total pressure loss of 10% was measured, including entry and exit losses. Excessive pressure losses in the entry-exit sections and headers were found to be main contributors to these losses. It was revealed that losses in the headers were related to inadequate structural support of the foil surfaces in the lower pressure passages, causing these passages to be partially closed. Design improvement measures addressing this issue are being evaluated with a goal of achieving 90% effectiveness with total pressure losses of less than 5% of the air inlet pressure.