The objective of this research is to analyze the performance of a passive hybrid powerplant control system to be implemented in a lightweight unmanned aerial vehicle capable to ascend up to the high troposphere (10,000 m). The powerplant is based on a high-temperature PEM fuel cell connected in parallel to a set of lithium-polymer batteries and regulated by two power diodes. Test performed in steady state demonstrates that the use of the hybrid system increases the efficiency of the stack by more than 7% because the voltage at the main DC bus is limited by the batteries. The robustness of the passive control system is proved in a longterm test in which random perturbations of ±15% are applied to the average power that would be demanded during the ascent flight. The hybridization of the stack with the batteries eliminates sudden peaks in the current generated by the stack, which are responsible for prompt degradation phenomena that drastically reduce its useful lifetime. The study demonstrates that with the passive hybrid powerplant it is possible to reach the target height with the gas storage system considered in the application, contrary to what happens with the simple power plant.
Top coating are usually moulded, painted or sprayed onto the wind blade Leading-Edge surface to prevent rain erosion due to transverse repeated droplet impacts. Wear fatigue failure analysis based on Springer model has been widely referenced and validated to quantitatively predict damage initiation. The model requires liquid, coating and substrate speed of sound measurements as constant input parameters to define analytically the shockwave progression due to their relative vibro-acoustic properties. The modelling assumes a pure elastic material behavior during the impact event. Recent coating technologies applied to prevent erosion are based on viscoelastic materials and develop high-rate transient pressure build-up and a subsequent relaxation in a range of strain rates. In order to analyze the erosion performance by using Springer model, appropriate impedance characterization for such viscoelastic materials is then required and represents the main objective of this work to avoid lack of accuracy. In the first part of this research, it is proposed a modelling methodology that allows one to evaluate the frequency dependent strain-stress behavior of the multilayer coating system under single droplet impingement. The computational tool ponders the operational conditions (impact velocity, droplet size, layer thickness, etc) with the appropriate variable working frequency range for the speed of sound measurements. The second part of this research defines in a complementary paper, the ultrasonic testing characterization of different viscoelastic coatings and the methodology validation. The modelling framework is then used to identify suitable coating and substrate combinations due to their acoustic matching optimization and to analyze the anti-erosion performance of the coating protection system.
h i g h l i g h t sHeat transfer coefficients to refrigerate a HT-PEMFC stack are calculated. Experiments are performed in 2 wind tunnels, for 3 form factors and real conditions. The calculated heat transfer coefficient varies from 8 to 44 W m À2 K À1 . Results at sea level are suitably extrapolated for a target altitude of 10 km. Flow area is optimized as a function of the power required to cool the stack down.
a b s t r a c tThe objective of this research is to calculate the heat transfer coefficients needed for the further design of the optimal cooling system of a high-temperature polymer electrolyte membrane fuel cell (HT-PEMFC) stack that will be incorporated to the powerplant of a light unmanned aerial vehicle (UAV) capable of reaching an altitude of 10,000 m. Experiments are performed in two rectangular tunnels, for three different form factors, in experimental conditions as close as possible to the actual ones in the HT-PEMFC stack. For the calculations, all the relevant thermal processes are considered (i.e., convection and radiation). Different parameters are measured, such as air mass flow rate, inlet and outlet air temperatures, and wall temperatures for bipolar plates and endplates. Different numerical models are fitted revealing the influence of the diverse relevant non-dimensional groups on the Nusselt number. Heat transfer coefficients calculated for the air cooling flow vary from 8 to 44 W m À2 K À1 . Results obtained at sea level are extrapolated for a flight ceiling of 10 km. The flow section is optimized as a function of the power required to cool the stack down to the temperature recommended by the membrane-electrode assembly (MEA) manufacturer using a numerical code specifically developed for this purpose.
In the present study, the optimal design of a high-temperature proton exchange membrane fuel cell (HT-PEMFC) that will be used to power an unmanned aerial vehicle (UAV) in a high altitude mission is performed. The use of PEMFCs for service ceiling above 10 km implies overcoming a number of problems caused by the harsh environmental conditions. Among them, new strategies to manage the heat generated by electrochemical reactions are needed. The maximum power required by the UAV was determined solving the aerodynamic problem, and the design of the lightweight HT-PEMFC, including its cooling system, was optimized. To perform the numerical solution of the heat transfer problem, a computational code was implemented using the EES software. The decisions adopted resulted in a 40-cells stack with an electric power above 1 kW and a weight around 3.65 kg. Besides, it is demonstrated that, for the configuration considered in the study, a passive cooling system without any additional fan system can be used to maintain the stack temperature in 160°C.
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