Comprehensive numerical investigations on the thermal performance of heat recirculating micro combustor with pin fins for micro-thermal voltaic system applications

Aiming at improving the relatively low energy output and energy conversion efficiency of the micro-thermal voltaic (MTPV) system, an innovative heat recirculating micro combustor with pin fins is designed. The effects of pin fins arrangement, hydrogen/air equivalent ratio on the energy output and performance of CHMC, HMCP and HMCI are compared and investigated. The result shows that when the V_in is 6 m/s and Φ is 1.0, the emitter power of CHMC is 72.76W, and that of HCMP and HCMI micro combustor are 75.99W and 76.35W. and the emitter efficiency of CHMC, HCMP and HCMI is 41.93%, 43.26% and 44.01%. HMCI has better energy output capability compared with CHMC and HMCP. Even though, HMCI brings a higher pressure drop, it is within the acceptable range. When the V_in is 6 m/s, the pressure drop from the pin fins only accounts for 26.4% of the total pressure drop for HMCI. Through the study of equivalent ratio, it is found that HMCI has good adaptability in different equivalent ratio range. This work provides new ideas for the development of MTPV system in the future.     

Accelerated bioactive behavior of Nagelschmidtite bioceramics: Mimicking the nano and microstructural aspects of biological mineralization

The ability of the Nagelschmidtite (Nagel) phase to promote osteogenesis, cementogenesis, and angiogenesis increased the interest in using this calcium silicophosphate bioceramic for tissue regeneration and vascularization applications. Nagel phase is a solid solution with the general formula Ca_7-xNa_x(PO₄)_2+x(SiO₄)_2-x, which allows several substitutions being Ca₇(PO₄)₂(SiO₄)₂ the most reported stoichiometry. Inspired by the well-known 45S5 bioactive glass chemical composition, we developed a synthesis route to obtain a Na-rich Nagel single phase. The effect of this bioceramic chemical and structural properties on apatite formation and crystallization mechanism is reported. The structural aspects at the nano and microscale of the mechanism of apatite growth and crystallization from the Nagel phase were compared to the formation process of Extra-Cellular Matrix (ECM) deposits in biological systems, revealing a biomimetic behavior during the apatite biomineralization process from the bioceramic.     

Urban water capacity: Irrigation for heat mitigation

Our world has been continuously urbanized and is currently accommodating more than half of the human population. Despite that cities cover only <3% of the Earth's land surface area, they emerged as focal points of human activities, and confront numerous environmental challenges as a result of changes in landscapes, hydroclimate, ecosystems, and biodiversity. In particular, the built environment usually experiences exacerbated heat stress induced by global climate and landscape changes, commonly known as the urban heat island effect. Urban irrigation, as a climate adaptation and mitigation strategy, is effective in cooling the built environment, but exhibits large uncertainties in the trade-off between water use and heat mitigation capacity. Here we show the efficiency of cooling effect induced by irrigation of urban vegetation, represented by a novel metric, viz. urban water capacity, analogous to the heat capacity, across the contiguous United States (CONUS) during summertime via numerical simulations. The urban water capacity is calculated as the average irrigation depth per degree of urban temperature reduction; the values are 4.52 ± 0.77 mm day⁻¹ °C⁻¹ and 7.27 ± 1.27 mm day⁻¹ °C⁻¹ (mean ± standard deviation) for surface and near-surface air cooling, respectively, over the CONUS. The robustness of urban water capacity is further exemplified in an extreme heat wave event, during which the warming anomaly is partially offset by the additional cooling from urban irrigation. Estimates of water capacity provide a quantitative metric for evaluating the efficacy of irrigation in urban planning under current heat stress and future warming.     

Adaptive Synchronization of Time Delay Chaotic Systems with Uncertain and Unknown Parameters via Aperiodically Intermittent Control

International Journal of Control, Automation and Systems - Time delay and uncertainty are the two common phenomena in nonlinear systems, which seriously influence the control and synchronization of...     

水平裂缝对坚硬顶板失稳破断的影响规律研究

为了研究水力压裂形成水平裂缝后坚硬顶板的失稳破断规律,基于材料力学和关键层相关理论,结合数值模拟,对含水平裂缝坚硬顶板的变形破坏特征和失稳破断机制进行了研究。结果表明:当坚硬顶板存在水平贯穿裂缝时,无论上位与下位岩层是否发生同步运动,坚硬顶板的破断步距均小于其初始极限跨距。存在水平贯穿裂缝的坚硬顶板及其上覆岩层,会形成拉压复合破坏区,更易于发生失稳破坏。坚硬顶板块体咬合点区域的拉压复合破坏作用是造成块体局部失稳、回转加剧和整体破断的主要原因。     

焊后低温热处理对马氏体不锈钢组织和性能的影响

对ZG06Cr13Ni4Mo马氏体不锈钢进行了焊后低温热处理工艺试验(240、300 ℃),通过显微组织分析、拉伸及弯曲试验、硬度试验及残余应力测试对不同低温热处理下焊接接头的显微组织、力学性能、硬度和残余应力等进行了研究。结果表明,经低温热处理后,接头焊缝热影响区组织为回火马氏体及碳化物,接头焊缝区的组织为低碳马氏体+块状马氏体+碳化物,接头的抗拉强度变化不大,硬度略有下降,经240 ℃低温热处理后,焊接接头焊缝处的残余应力消除了69.1%。     

Numerical study on premixed hydrogen/air combustion characteristics and heat transfer enhancement of micro-combustor embedded with pin fins

Aimed at improving the energy output performance of the Microthermal Photovoltaic (MTPV) system, it is necessary to optimize the structure of the micro combustor. In this paper, micro combustor with in-line pin fins arrays (MCIPF) and micro combustor with both end-line pin fins arrays (MCEPF) were presented to realize the efficient combustion and heat transfer enhancement, and the influence of inlet velocity, equivalent ratio, and materials on thermal performance was investigated. The results showed that pin fins embedding is beneficial to improving combustion, and the combustion efficiency of MCIPF and MCEPF reaches 98.5% and 98.7%, which is significantly higher than that of the conventional cylindrical combustor (MCC). However, with the increase of inlet velocity from 8 m/s to 14 m/s, MCIPF exhibits the highest external wall temperature with a range of (1302–1386 K), while MCEPF maintains the best temperature uniformity. As the inlet velocity increases to 10 m/s, the external wall temperature and temperature uniformity reach the optimum. Besides, under the conditions of different equivalence ratios, both external wall temperature and heat flux increases first and then decreases, meanwhile the temperature uniformity of MCEPF is significantly improved compared with that of MCIPF, they all exhibit the highest external wall temperature with an equivalence ratio of 1.1, and the thermal performance is greatly enhanced. By comparing the heat transfer performance of combustors with different materials based on MCEPF, it is interesting to find that the application of high thermal conductivity materials can not only increase the external wall temperature, but also improve the temperature uniformity. Therefore, materials with high thermal conductivity such as Aluminum, Red Copper and Silicon Carbide should be selected for application in micro combustors and their components. The current work provides a new design method for the enhanced heat transfer of the micro combustor.     

Research on the solidified morphologies of successive pileup metal droplets

Journal of Mechanical Science and Technology - Metal micro-droplet 3D printing is an effective additive manufacturing technique to form micro pillar structures. However, the control mechanism of...