In addition to their low cost and weight, polymer heat exchangers offer good anticorrosion and antifouling properties. In this work, a cost effective air-water polymer heat exchanger made of thin polymer sheets using layer-by-layer line welding with a laser through an additive manufacturing process was fabricated and experimentally tested. The flow channels were made of 150 μm-thick high density polyethylene sheets, which were 15.5 cm wide and 29 cm long. The experimental results show that the overall heat transfer coefficient of 35-120 W/m 2 K is achievable for an air-water fluid combination for air-side flow rate of 3-24 L/s and water-side flow rate of 12.5 mL/s. In addition, by fabricating a very thin wall heat exchanger (150 μm), the wall thermal resistance, which usually becomes the limiting factor on polymer heat exchangers, was calculated to account for only 3% of the total thermal resistance. A comparison of the air-side heat transfer coefficient of the present polymer heat exchanger with some of the commercially available plain plate fin heat exchanger surfaces suggests that its performance in general is superior to that of common plain plate fin surfaces.
This paper focuses on the study of an innovative manifold microchannel design for air-side heat transfer enhancement that uses additive manufacturing (AM) technology. A numerical-based multi-objective optimization was performed to maximize the coefficient of performance and gravimetric heat transfer density (Q/MΔT) of air–water heat exchanger designs that incorporate either manifold-microchannel or conventional surfaces for air-side heat transfer enhancement. Performance comparisons between the manifold-microchannel and conventional heat exchangers studied under the current work show that the design based on the manifold-microchannel in conjunction with additive manufacturing promises to push the performance substantially beyond that of conventional technologies. Different scenarios based on manufacturing constraints were considered to study the effect of such constraints on the heat exchanger performance. The results clearly demonstrate that the AM-enabled complex design of the fins and manifolds can significantly improve the overall performance, based on the criteria described in this paper. Based on the current manufacturing limit, up to nearly 60% increase in gravimetric heat transfer density is possible for the manifold-microchannel heat exchanger compared to a wavy-fin heat exchanger. If the manufacturing limit (fin thickness and manifold width) can be reduced even further, an even larger improvement is possible.
Over the last decade, rapid development of additive manufacturing techniques has allowed the fabrication of innovative designs which could not have been manufactured using conventional fabrication technologies. One field that can benefit from such technology is heat exchanger fabrication, as heat exchanger design has become more and more complex due to the demand for higher performance systems. One specific heat exchanger design that has shown significant performance enhancement potential over conventional designs and can greatly benefit from additive manufacturing technology is a manifold-microchannel heat exchanger. It is a design that combines careful fluid distribution through appropriate manifolds with an enhanced heat transfer surface design to achieve specific thermohydraulics performance expectations. Additive manufacturing allows fins as thin as 150 μm to be fabricated, which is an important enabler feature for the heat exchanger thermal performance. In addition, additive manufacturing allows the manifold and the microchannel sections to be fabricated as a single piece, which eliminates the need to fuse those sections together through a subsequent bonding process. As part of this work, we fabricated and experimentally tested a high-performance titanium alloy (Ti64) air-water heat exchanger that utilizes manifold-microchannel design. The heat exchanger was fabricated using direct metal laser sintering (DMLS) fabrication technique. The air-side implemented a manifold-microchannel design, while the water side used multiple rectangular channels in parallel. This was because the major thermal resistance occurs on the air side. The pressure drop and heat transfer performance of this heat exchanger were evaluated. The experimental results showed a noticeable performance reduction compared to the ones projected by numerical simulation due to an inaccuracy and low fidelity in printing of thin fin profile. However, despite this manufacturing inaccuracy, compared to a conventional wavy-fin surface, 15%–50% increase in heat transfer coefficient was possible for the same pressure drop value. Compared to a plain plate-fin surface, 95%–110% increase in heat transfer coefficient was possible for the same pressure drop value. The air-side heat transfer coefficient in the range of 100–450 W/m2K was achievable using manifold-microchannel technology for air-side pressure drop of 90–1800Pa. Since metal based additive manufacturing is still in the developmental stage, it is anticipated that with further refinement of the manufacturing process in future designs, the fabrication accuracy can be improved.
Compared to state-of-the-art heat exchangers, manifold-microchannel heat exchangers (MMHX) have shown superior heat removal density at moderate pressure drops. However, MMHX made of Ni-based superalloys or other tough-to-machine materials can be a challenge to fabricate using conventional fabrication methods, because of the inherently complex manifold microchannel geometry, as well as the required small feature sizes. In this study, a direct metal laser sintering (DMLS) additive manufacturing (AM) technique was used to fabricate the compact high-temperature MMHX reported here. The AM MMHX was fabricated as a single object, which significantly simplifies the fabrication process. In this work, three different AM machines were used to study the effect of laser power, powder size, and layer thickness on the fin and channel sizes of the fabricated MMHX. To evaluate the minimum wall thickness able to hold the design pressures, pressure containment tests were performed. As a result, a wall thickness of 0.3 mm was shown to withstand 340 kPa and be leakage-free. A detailed analysis of different printing orientations and their effect on the MMHX's design was also performed. Lastly, a 76×76×76 mm3 MMHX was successfully fabricated with fin thickness of 0.13 mm out of maraging steel. A second unit with dimensions of 94×87.6×94.4 mm3 was successfully fabricated with fin thickness of 0.22 mm out of Inconel 718. Details of the fabrication process and key takeaway results are discussed.
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