495 research outputs found
Combustion in microspaces and its applications
PhD research can be divided in three main parts: part one and two related to the development of some of the most important aspects of the catalytic combustion in microspaces, part three related to a possible application of the catalytic combustion in microspaces. Part 1: The combustion of gaseous HC fuels in a small confined space could represent an alternative way to produce thermal and electrical energy. The combustion of CH4 and its lean mixtures with H2 on catalytic monoliths was studied and optimized. 2% Pd/(5% NiCrO4), 2% Pd/(5% CeO2ZrO2), 2% Pd/(5% LaMnO3ZrO2) and 2% Pt/(5% Al2O3) catalysts, suitably developed, were deposited on SiC monoliths via in situ SCS and tested in a lab-scale microreactor by feeding only CH4, only H2, and three lean CH4/H2 mixtures with increased content of H2 and constant thermal power density of 7.6 MWth m-3. Monolith with 2% Pt/(5% Al2O3) was very appropriate for the combustion of only CH4 or H2, but its performance worsen when H2 was added to the reactive mixture. On the contrary, the Pd-based catalysts were most suitable for the combustion of the CH4/H2 lean mixtures, with the best behavior shown by 2% Pd/(5% NiCrO4) followed by 2% Pd/(5% CeO2ZrO2). Monolith coated with 2% Pd/(5% LaMnO3ZrO2), instead, showed the worse performance, both in terms of CH4 combustion only and of the various mixtures; moreover, it displayed quite high CO emissions, not compatible with the environmental issues. In particular, the catalytic reactivity towards CH4 combustion of the Pd- based raised by increasing the H2 content in the reactive mixture. The observed enhancement in reactivity of the mixture when the CH4 fuel was enriched with H2 could be explained by an increase of the OH• radicals in the gas mixture. Part 2: The present work deals with the investigation on the performance of catalyst 2% Pd/ 5% LaMnO3•ZrO2 (PLZ), lined on silicon carbide (SC, with thermal conductivity of 250 W m-1 K-1) or cordierite (CD, with thermal conductivity of 3 W m-1 K-1) monoliths, for the CH4/H2/air lean mixtures oxidation. The bare and coated monoliths were tested into a lab- microreactor designed to provide a favorable environment for microscale combustion of CH4/H2/air lean mixtures to reach high power density (7.6 MWth m-3; GHSV 16,000 h-1). Various CH4/H2 mixtures were tested in heating and cooling phases on the various monoliths, by studying both the homogenous and heterogeneous reactions. The relative percentages of methane and hydrogen were mutually varied (maintaining the sum of the two fuels equal to 100%), in order to always assure a constant power density. The air was always fed with equal to 2. The main aim of the catalytic combustion tests was to select the best settings to achieve at the minimum temperature full CH4 conversion with the minimum H2 concentration in the reactive mixture, accompanied by the lowest possible CO concentration. Depending on the thermal conductivity of the tested monoliths, the existence of a steady-state multiplicity was verified, mainly when the hydrogen concentration was quite low. Basically, microburners with low wall thermal conductivity (CD monoliths) exhibited shorter ignition times compared to the higher thermal conductivity ones (SC monoliths) due to the formation of spatially localized hot spots that promoted catalytic ignition. At the same time, the CD material required shorter times to reach steady-state. But SC materials assured longer time on stream operations. The presence of the catalyst lined on both monoliths allowed reaching lower CO emissions. The best results belonged to the catalytic SiC monolith, with a low hydrogen concentration in the fed mixtures. Part 3: The idea was to realize an autothermal steam reforming reaction. This was made by coupling a combustion reaction (exothermic), which provided the heat necessary, with a steam reforming reaction (endothermic) in a same specific built micro reactor. The total reagents chosen for the two reactions were methane (used both as fuel and as a reactant for the steam reforming), air and steam (produced by heating water). The main advantage of this system: producing enough energy, for example, to power auxiliary transportation of vehicles, reducing consumption and pollutant emissions; at the same time, because of the overall limited dimensions, reducing the risk of explosion if compared to the hydrogen "on board " storage. The development was a stainless steel reactor consisting of two plates with microchannels, containing the catalyst (Pt/AlO3), in which the reactions took place. These plates were placed in indirect contact, separated by a middle plate made of stainless steel, so to conduct the heat from the combustion side to the steam reforming, and also to avoid the mixing of the fluids. The sealing of both sides were ensured by two ceramic gaskets, suitable to withstand high temperatures. The sizing was performed first theoretically assuming a S / C = 4 (Steam to Carbon), and taking into account the maximum flow rates that could be set to the mass flow controllers. It was then calculated the theoretical thermal power necessary to sustain the steam reforming process, and then calculated the flow of methane and air to be sent to the combustor, to obtain an autothermal reforming. The catalyst used was chosen because of its catalytic activity for both types of reaction. Once it was determined the best side for the steam reforming, it was decided to experiment the coupled reactions. After having reached 900 °C in oven, with complete methane combustion, oven heat was no more provided: combustion was able to be sustained because of a mixture of 7% CH4 in air (inside the flammability limit) and reagents for the steam reforming were sent in a steam/carbon 4:1 replacing nitrogen flow. Results show how the performance of the reactor was affected by thermal dissipation; hence the material used as insulating, in order to wrap up the reactor, plays a key role for performing tests. Tests were carried out increasing thermal power from combustion side to balance the heat dissipations, so to obtain a balance between heat generated and used by the reaction of steam reforming and the heat lost in the environment. It has been showed the way for producing good quality data on coupling combustion and steam reforming reactions in this reactor. In a future, it could be possible using a GC instead of the ABB analyzer in case of new tests with high CH4 not reacted, or of course improving methane conversion choosing a better catalyst for steam reforming, composing a reactor with multiple plates for optimizing the process as shown in Vlachos' simulations, and trying to run flows in either concurrent or countercurren
Optimal efficiency of the Q-cycle mechanism around physiological temperatures from an open quantum systems approach
The Q-cycle mechanism entering the electron and proton transport chain in
oxygenic photosynthesis is an example of how biological processes can be
efficiently investigated with elementary microscopic models. Here we address
the problem of energy transport across the cellular membrane from an open
quantum system theoretical perspective. We model the cytochrome protein
complex under cyclic electron flow conditions starting from a simplified
kinetic model, which is hereby revisited in terms of a quantum master equation
formulation and spin-boson Hamiltonian treatment. We apply this model to
theoretically demonstrate an optimal thermodynamic efficiency of the Q-cycle
around ambient and physiologically relevant temperature conditions.
Furthermore, we determine the quantum yield of this complex biochemical process
after setting the electrochemical potentials to values well established in the
literature. The present work suggests that the theory of quantum open systems
can successfully push forward our theoretical understanding of complex
biological systems working close to the quantum/classical boundary.Comment: 13 pages, 6 figures. Pre-submission manuscript, see Journal Reference
for the final versio
Electromechanical Quantum Simulators
Digital quantum simulators are among the most appealing applications of a
quantum computer. Here we propose a universal, scalable, and integrated quantum
computing platform based on tunable nonlinear electromechanical
nano-oscillators. It is shown that very high operational fidelities for single
and two qubits gates can be achieved in a minimal architecture, where qubits
are encoded in the anharmonic vibrational modes of mechanical nanoresonators,
whose effective coupling is mediated by virtual fluctuations of an intermediate
superconducting artificial atom. An effective scheme to induce large
single-phonon nonlinearities in nano-electromechanical devices is explicitly
discussed, thus opening the route to experimental investigation in this
direction. Finally, we explicitly show the very high fidelities that can be
reached for the digital quantum simulation of model Hamiltonians, by using
realistic experimental parameters in state-of-the art devices, and considering
the transverse field Ising model as a paradigmatic example.Comment: 14 pages, 8 figure
Pulse-efficient quantum machine learning
Quantum machine learning algorithms based on parameterized quantum circuits
are promising candidates for near-term quantum advantage. Although these
algorithms are compatible with the current generation of quantum processors,
device noise limits their performance, for example by inducing an exponential
flattening of loss landscapes. Error suppression schemes such as dynamical
decoupling and Pauli twirling alleviate this issue by reducing noise at the
hardware level. A recent addition to this toolbox of techniques is
pulse-efficient transpilation, which reduces circuit schedule duration by
exploiting hardware-native cross-resonance interaction. In this work, we
investigate the impact of pulse-efficient circuits on near-term algorithms for
quantum machine learning. We report results for two standard experiments:
binary classification on a synthetic dataset with quantum neural networks and
handwritten digit recognition with quantum kernel estimation. In both cases, we
find that pulse-efficient transpilation vastly reduces average circuit
durations and, as a result, significantly improves classification accuracy. We
conclude by applying pulse-efficient transpilation to the Hamiltonian
Variational Ansatz and show that it delays the onset of noise-induced barren
plateaus.Comment: 8 pages, 6 figure
Guy de Maupassant, Le Colporteur et autres nouvelles
In questo articolo, Henri Mitterand ripercorre lo stretto legame che univa i due grandi artisti Émile Zola e Paul Cézanne. Egli intende dimostrare come molti testi dedicati alla loro vita ed amicizia siano privi di attendibilità: ciò accade a causa della superficialità delle analisi svolte, per l’incompletezza degli elementi biografici, e per l’effettiva manipolazione dei medesimi scritti. L’esempio più evidente concerne il pensiero di Claude Perruchot: infatti, quest’ultimo sostiene che Paul..
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