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68 chest to the combustion chamber. These tubes run considerably hotter than the first set, as they are the first to see the turbine exhaust gases. To drive the turbine at 80,000 rpm, the gas exiting the combustion chamber must be heated to 900 C. In ordinary turbine engine configurations, the air coming from compressor is at about 200 C before combustion, meaning a 700 C increase in temperature is needed from combustion. In Turbotech’s design though, air leaves the heat exchanger and enters the combustor at roughly 530 C. That greatly reduces the temperature increase required, and by extension the amount of fuel needed to heat and pressurise the charge air for optimal power output.  “The recuperated turbine design approach we’re using has been applied in industry for a long time,” Fauvet says. “Large marine vessels, stationary electric power plants and even American battle tanks have integrated heat exchangers into their turbine engines.” However, a common theme between these three applications is that none of them are particularly constrained by weight. The problem facing aircraft turbines is that heat exchangers could add considerable mass, so the challenge is to maximise the efficiency-to-weight ratio. Turbotech’s solution is the matrices of microtubes connected directly to the turbo machinery. “Many aerospace companies, including Rolls-Royce, Allison and Pratt & Whitney, have tried since the 1950s to introduce heat exchangers on turbines, but they were never deployed in practice because, as well as being unable to get the weight down, they were far too complex and expensive to produce,” Guimbard says. “The fuel they saved simply couldn’t compensate for the increased engine weight and cost of production at the time.” Many other attempts at recuperated microturbine designs have also failed, often because the mechanical and thermal stresses inflicted on their heat exchangers prevented them from meeting customer requirements for service life or TBOs. “The key to a successful aircraft microturbine therefore is to build the heat exchanger’s channels using careful design and the right kind of microtubes,” Guimbard says. “They need to be as light as possible, and have a long life cycle. “The latter requires that they’re designed and manufactured to resist vibration, as well as the mechanical stresses that come from variations in thermal expansion along their lengths, because the exhaust gases flow in unequal quantities and at different temperatures from one end to the other.” Naturally, it was critical that Turbotech could accurately model the airflow at each stage of the engine, and design the heat exchanger, the compressor, the turbine wheels, chambers and exhaust section accordingly.  In-depth studies and various iterations were carried out using Ansys and Catia (a software suite developed by Dassault Systems for CFD, CAD and CAE), with Guimbard leading the aerodynamic April/May 2020 | Unmanned Systems Technology Dossier | Turbotech TP-R90 and TG-R55 The compressor wheel used for drawing in air on the TP-R90 and TG-R55 (Author’s image) The key to a successful microturbine is to build the heat exchanger’s channels using the right microtubes – they need to be as light as possible