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Experimental investigation into the effects of different pilot amounts of dimethyl ether (DME) on the performance and emission of a single-cylinder directinjection DME engine is conducted. The results show that a DME engine can operate at a wider range of speeds and loads at quasi-homogenous charge compression ignition (QHCCI) mode. The brake thermal efficiency increases while the exhaust temperature decreases. NOx emission decreases by about 30%–50% although there is a slight increase in HC and CO emissions. NOx, HC and CO emissions increase with an increase in the amount of DME pilot. QHCCI is a good way to increase thermal efficiency and decrease NOx emission.

AbstractAu@Pd core–shell nanobricks (CNBs) with concave surfaces and Pd shells with a thickness of approximately 5 nm were synthesized by co‐reduction of HAuCl4 and H2PdCl4 in the presence of Au seeds and Ag ions. These as‐synthesized concave CNBs exhibit significantly enhanced catalytic activity for the electrooxidation of ethanol in alkaline media compared to the commercially‐used Pd black. The improved performance of the Au@Pd CNBs can be attributed to the exposed stepped surfaces, high‐index facets, and the synergistic effects of the core and shell metals.

Audible noise of can-type filter capacitors in high- voltage direct current converter stations originates from the vibration of capacitor cases, so it is meaningful to investigate the vibration characteristics. However, no experiment under practical excitations or thorough theoretical analysis was presented for the vibration characteristics. This study theoretically and experimentally investigated the vibration characteristics of can-type filter capacitors with stacked elements parallel to bottom. The vibration amplitude was found to be proportional to voltage squared. If the frequency spectrum of excitation voltage included several harmonics (more than three), vibration amplitude on some specific frequencies was related with the phases of these voltage components. Each capacitor surface had a concavo-convex vibration shape and the vibration of the bottom surface and the side surfaces were in opposite phases. The vibration amplitude of the bottom surface was the largest. Moreover, frequency characteristics of capacitor vibration were studied. The frequency response of the bottom surface was bigger than that of narrow and broad side surfaces for lower frequencies, while was smaller for higher frequencies. It was inferred that bottom surface vibration was mainly caused by elastic forces from direct contact and side surfaces vibration mainly came from damping forces from dielectric liquids.

Abstract To effectively reduce heat leakage, foam insulation is widely used in refrigeration, energy, chemical, cryogenic system and pipe engineering. The present study is aimed to investigate the thermal performance of foam on cryogenic storage system under variable environmental parameters. The heat transfer model through the foam is introduced in detail. The minimum thicknesses of foam for different cryogens are calculated under the influence of the external free convection during the terrestrial condition. Meanwhile, with carbon dioxide diffusion out of and air invasion in the foam cells, the related unsteady process is studied with coupling the mixture gas heat conduction and the external free convection. While during the liftoff of rocket, foam insulation is subjected to the forced aerodynamic heating. The transient thermal performance of foam is particularly investigated under the influence of aerodynamic heating, with the variable physical properties, flight velocity and acceleration, and environmental parameters considered. Finally, the performance comparison between foam and foam/Multilayer insulation is made. Some valuable conclusions are obtained. The present study is of significance to the insulation design for cryogenic fuel storage.

Abstract To analyze the behavior of PWRs fuel rods under steady operation conditions, some models such as modified Forsberg-Massih model and TUBRNP model were applied to FROBA-ROD code, which was developed by Xi'an Jiaotong University in 2012. Then more significant fuel rod behaviors could be simulated by FROBA-ROD code, which facilitated the safety analysis of fuel elements especially under accident conditions. To conduct transient thermal calculations and to improve the simulation accuracy, existing thermal models in FROBA were modified with the new fuel rod thermal conductivity model and burnup distribution model. The new models were validated by comparing it with the results of authorized commercial codes FRAPCON-4.0 and ABQUS and by comparing it with IFA-432 Rod 1 experimental benchmark data conducted by Oak Ridge National Laboratory. The relative calculation errors of centerline temperature calculated by modified FROBA-ROD, original FROBA-ROD and FRAPCON were 5.44%, 11.68% and 9.47% separately, which indicated that the modification and update of FROBA-ROD were valid. Modified FROBA-ROD code was applied to perform the behavior analysis of the AP1000 fuel rod. The simulation results indicated that there were enough safety margins for fuel rod behavior of AP1000 at rated operation conditions. The maximum oxidation thickness of cladding appeared at the upper part of the cladding tube in AP1000, which indicated that this area suffered a higher risk of cladding failure. Modified FROBA-ROD code is expected to deal with fuel rod scenarios under accident conditions after some additions of transient models.

Abstract The source term calculation capability is implemented in the high-fidelity neutronics code. With this high-fidelity tool, the accuracy of the two-step source term calculation method is analyzed in this work, including decay heat, the activation photon source term, and the neutron source term. The accuracy of NECP-X for the source term calculation is demonstrated through the comparison with SCALE6.1 and STREAM for a set of cases, and the results show good consistency between different codes. The analysis of the two-step source term calculation method is conducted by the control-variable method, and the approximations of generating cross-sections with the 2D lattice calculations and the homogenization process are analyzed for a full-core case. The numerical results show that approximations may result in some difference for the decay heat and the photon source results for the small core, and the difference is investigated with the high-fidelity source term calculation capability.

The first stage of GE-E3 turbine is employed to investigate effect of casing purge flow upstream rotor blade tip. Three-dimensional Reynolds-averaged Navier-Stokes (RANS) equations and standard k-ω model are solved to obtain tip heat transfer simulations. The results reveal that: heat transfer coefficient of blade tip surface can be significantly reduced when casing purge flow is set. Tip averaged heat transfer coefficient of cases with and without swirly velocity casing purge flow decrease 3.5% and 3.4% compared with the case without casing purge flow. Compared with case which blowing ratio equals to 0.5, it can be found that averaged tip heat transfer coefficient of cases which blowing ratio equals to 1.0 and 1.5 decrease 2.3% and 1.8%, respectively. Setting blowing ratio as 1.0 can best cool tip surface without wasting cold air resources. Increasing rotating speed can induce cold air entering tip trailing region and improve local cooling effect. Flow structure inside the tip clearance are also revealed and discussed.