Polymer conformation is the molecular basis underlying essentially all physical properties of polymers, and chain conformation and conformational energy play central roles in crystalline structure formations and structure transitions of polymers.

Structures of polymers depend on the degree of polymerization and chain conformation, and the chain conformation plays a key role on the structure formation and structure transition of an existing polymer, especially for the crystalline polymers. DSC and WAXD measurements were performed to investigate crystallization behavior and structure of polylactide (PLA) with different thermal histories. A similar Mpemba effect was found during the crystallization process, which showed that the PLA sample quenched from “hot” crystallizes faster than that quenched from the “cold”. The obtained crystallization half-times of isothermal crystallization, and the crystallization temperatures, glass transition temperatures, and melting points during the cold nonisothermal crystallization process with a high heating rate, decrease with holding temperature. The Mpempa effect was discussed and attributed to the temperature dependent chain conformation or local structure of the amorphous and confined effect on the conformation adjustment before PLA crystallization. The unusual crystallization phenomenon of PLA would be helpful to understand the polymer crystallization and control the crystalline structure of PLA.

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Summary Implemented optimization and full life cycle economic analysis on the inter‐stage pressure ratio were rarely paid attention to in supercritical carbon dioxide multi‐stage recompression. In this article, a thermodynamic model of supercritical carbon dioxide recompression Brayton cycle was established based on gas‐cooled fast reactor and the sensitivity analysis was performed. With the aim of seeking the optimal pressure ratio for two‐stage, three‐stage, and four‐stage recompression configuration, the full life cycle economic indicators were predicted, respectively. Parameter sensitivity analysis indicated that the thermodynamically optimized result could not coincide with that of the economically optimized. The optimal PR distribution scheme may result in the improvement of the initial cycle system efficiency from 46.89% to 48.07%. Meanwhile the levelized cost of electricity decreased from 57.40$/MWh to 55.87$/MWh. The approaching equilibrium trend of thermoeconomic parameters confirmed the existence of the optimal number of recompression stages, which could also be derived from the prediction tendency based on full life cycle economic indicators. Furthermore, it was discovered that reducing the year of repayment and the loan proportion enhanced the net present value of nuclear power plant, but the former method gave rise to longer payback period and lower internal rate of return. The present work preserves the guiding significance for nuclear power plant investors and the utilization of the supercritical carbon dioxide Brayton system.

The nonaxisymmetric endwall profiling has been proven to be an effective tool to reduce the secondary flow loss in turbomachinery. In the present work, first, without considering the endwall midgap in the real machine, an endwall optimization design procedure for reducing secondary flow losses has been developed, which allowed complete threedimensional parameterization turbine endwall design. The profile of the endwall has been designed using automatic numerical optimization by means of an improved efficient global optimization algorithm based on kriging surrogate model. Next, a large-scale linear cascade with a low-speed wind tunnel has been chosen for the experimental validation of the optimization results. The experimental measurements and numerical simulations both demonstrated that the total pressure loss and secondary flow intensity were reduced with the nonaxisymmetric endwall used in the cascade passage. Then, in order to evaluate the ability of the optimized nonaxisymmetric endwall with the midgap, the midgap was added in for both the baseline flat endwall and the optimized nonaxisymmetric endwall in the numerical simulations analysis. The entropy generation rates analysis were used for the investigation of loss distribution in the passage. For the cascade in the present work, with the midgap added in, the optimized nonaxisymmetric endwall did not perform as well as the situation without the midgap in the loss reduction. In addition, comparing to the baseline flat endwall, the optimized nonaxisymmetric endwall needed more net leakage flow to avoid the ingress of passage flow into the midgap.