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70 and mechanical aspects of design. Ansys was Guimbard’s team’s primary CFD simulator of choice, while Catia was used more heavily for designing the engine’s mechanical parts to exact geometries and tolerances. Perhaps the biggest factor in the development of the heat exchanger design has been Turbotech’s ongoing (and exclusive) partnership with Le Guellec, whose production centre makes microtubes for aerospace applications. While it had supplied mainly tubes and hoses for climate control systems (such as in Airbus passenger aircraft), it is also capable of manufacturing metallic microtubes, which are the key enabling technology in the heat exchanger. “When we made our technological requirement clear to them – particularly our heat exchanger’s need for at least 60-70% thermal efficiency, low cost and low weight – they decided to become not just our supplier but a partner and investor as well,” Guimbard explains.  “Microtubes in general are a much lighter thermal-exchange medium than plate-type heat exchangers. That, and Le Guellec’s design approach, keep our microturbine systems competitive with piston engines in terms of price, weight and fuel efficiency.”  Fauvet adds, “They also manufacture their microtubes very cost-effectively. Their in-house production lines are arranged to lower the price by a factor of about four or five compared with the typical market price of such microtubes.” CFD studies and analyses of their testbench results have enabled Turbotech to determine the ideal shaping and pathways for the microtubes through their heat exchangers relative to the flow of hot exhaust gases. This was necessary to optimise the recycling of heat and minimise the loss of intake pressure as well as the stresses from thermal expansion and contraction differentials – the tubes are hotter the closer they run to the outlet of the turbine chamber.  “Many of the failed attempts at aircraft turbine heat exchangers from the 1960s and ’70s used relatively wide pipes for connecting the heat exchanger to the turbomachinery, which caused them to lose a lot of air pressure as the air travelled from their compressors to their combustors,” Guimbard says. “That meant lower engine power output and efficiency. “And the tubes are made from Inconel, for thermal efficiency as well as structural integrity against thermal expansion and contraction. We’ll also be testing stainless steel microtubes soon to see how they perform over time.” The exact length, diameter and number of heat exchanger microtubes can also be customised to fit the engine efficiency and volume requirements of end-users. Overall, 60% of the engine (including rotating parts and all outer enclosures) is manufactured by subcontractor GCA (based outside Paris), before being assembled in-house by Turbotech. Combustion and exhaust The engine is spark-ignited. At the moment, a high-tension spark plug is used, but the company plans to switch to April/May 2020 | Unmanned Systems Technology The annular air intake duct on the TP-R90 sits between the gearbox (left) and the turbine housing, on the right (Author’s image) Many of the failed attempts at aircraft turbine heat exchangers used relatively wide pipes, which caused them to lose a lot of air pressure