11 December 2021, Volume 61 Issue 12

    

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SPECIAL TOPIC:HEAT TRANSFER AND THERMAL SYSTEM FOR HEAT TRANSFER AND THERMAL SYSTEM FOR
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  Research progress in transpiration cooling with phase change

  XU Ruina, LI Xiaoyang, LIAO Zhiyuan, HU Haowei, ZHU Yinhai, JIANG Peixue

  Journal of Tsinghua University(Science and Technology). 2021, 61(12): 1341-1352. https://doi.org/10.16511/j.cnki.qhdxxb.2020.25.044

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  With the increasing speeds and flight times of aerospace vehicles, the high heat fluxes caused by the aerodynamics and combustion have led to aircraft component temperatures that far exceed the material limits. Efficient, reliable thermal protection methods are then crucial in aerospace components. Transpiration cooling is an efficient active thermal protection method first developed in the 1940s that is used for thermal protection of conventional materials on ultra-high temperature/heat flux surfaces of aerospace vehicles. This paper reviews international and domestic research including that of the authors' team on transpiration cooling with phase change in the last several years. The flow and heat transfer mechanisms of transpiration cooling with phase change for subsonic and supersonic mainstream flows are explained. The biomimetic self-pumping and self-adaptive transpiration cooling method and its optimal structures are also presented. This paper also describes the optimization of transpiration cooling in typical thermal structures of aerospace vehicles. Advanced materials will be combined with transpiration cooling with phase change with non-uniform heat fluxes and extremely high temperatures in future designs to provide reliable high speed aerospace vehicles.
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  Advances in condensation heat transfer enhancement

  WEN Rongfu, DU Bingang, YANG Siyan, LIU Yuanbo, LI Qixun, CHENG Yaqi, LAN Zhong, MA Xuehu

  Journal of Tsinghua University(Science and Technology). 2021, 61(12): 1353-1370. https://doi.org/10.16511/j.cnki.qhdxxb.2022.25.002

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  Efficient condensation heat transfer is critical in high-efficiency, highly integrated conventional and emerging energy systems for power generation, chemical processing, electronics cooling, building energy systems, food processing, and water treatment and purification. The formation, transport, and removal of the liquid condensate by interfacial structures significantly enhances the condensation heat transfer. Recent advances in micro/nano-fabrication techniques and advanced materials have led to the rapid development of many functional structured surfaces, exciting improvements in interfacial transport, and better understanding of the underlying mechanisms. This review presents an overview of the basic heat and mass transfer processes from the hot vapor to the cold substrate to illustrate the condensation heat transfer enhancement principles. Some new strategies and important studies for enhancing condensation are then reviewed to illustrate the power of surface modification and system optimization. Finally, this review analyzes the challenges and perspective for advanced heat transfer surfaces in future industrial applications.
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  Experimental study of the steady and dynamic efficiencies of a solar methanol steam reforming reactor filled with a phase change material for hydrogen production

  MA Zhao, CHENG Zedong, HE Yaling

  Journal of Tsinghua University(Science and Technology). 2021, 61(12): 1371-1378. https://doi.org/10.16511/j.cnki.qhdxxb.2021.25.002

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  Solar thermochemical reactors are a promising way to produce clean hydrogen energy. However, the operation of a reactor driven by solar irradiation will fluctuate as the solar irradiation changes. This study analyzed the thermal management of a solar methanol steam reforming reactor for hydrogen production with phase change material (PCM) in the reactor. The analyses considered two latent heat type thermochemical reactors. The thermal and hydrogen production characteristics of these thermochemical reactors were investigated experimentally to show the influence of the heat flux variations. Then, the models were used to study the effect of the phase change material position on the reactor hydrogen production. Finally, the dynamic efficiencies of the latent heat type reactors were compared with that of the original design to show that when the reactor surface heat flux reaches 7 kW/m², the H₂ proportion of the production rate is 0.713 and the methanol conversion efficiency is 0.956. Increasing the heat flux increases the methanol conversion efficiency. Adding the PCM in the shell reduces the required catalyst mass by 66.0% while adding the PCM in the tube side reduces the required catalyst mass by 13.5% for steady-state operation while the reactor continues to operate efficiently. For dynamic operation, the methanol conversion efficiency is reduced by 23.4% when the PCM is added to the shell side and by 13.7% when the PCM is added to the tube side while the reactor thermal inertia is improved to cope with sudden heat flux changes.
  
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  Development and application of dual-wavelength flash Raman measurement system

  FAN Aoran, MA Weigang, WANG Haidong, ZHANG Xing

  Journal of Tsinghua University(Science and Technology). 2021, 61(12): 1379-1388. https://doi.org/10.16511/j.cnki.qhdxxb.2020.25.034

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  Accurate, in situ measurements of nanomaterial thermophysical properties have become a key heat transfer research problem. Raman-based methods are nearly ideal non-contact methods for nanoscale measurements. However, the limited temporal and spatial resolutions of previous Raman-based methods significantly affected their measurement accuracy. This study developed a dual-wavelength flash Raman measurement system with 100 ps temporal resolution and 100 nm spatial resolution. A pulsed laser was used to heat the sample with another pulsed laser with a different wavelength and negligible heating effect used to excited the Raman scattering to simultaneously measure the sample and substrate temperatures. The time delay between the heating pulse and the probing pulse was varied to measure the transient temperature variations of the sample and substrate to determine the thermal diffusivity of the nanomaterial. The spot center distance between the probing and heating lasers was varied to obtain high spatial resolution temperature distributions to further improve the transient measurement sensitivity and to determine the thermal contact conductance between the substrate supported nanomaterials and the substrate. Measurements of the properties of supported bilayer graphene show the system advantages.
  
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  Effect of spectral-selectivity on the performance of thermochemical cycling driven by concentrated solar energy

  LIN Pengzhu, LOU Jiahui, LI Jianlan, HAO Yong

  Journal of Tsinghua University(Science and Technology). 2021, 61(12): 1389-1396. https://doi.org/10.16511/j.cnki.qhdxxb.2020.25.038

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  Splitting H₂O or CO₂ via solar-driven thermochemical redox cycles is important for solar fuel production. Since the reactor chamber temperature in thermochemical redox cycles is much lower than the surface temperature of the sun, secondary radiation from the reactor chamber to the ambient can be suppressed by spectral-selective transmissive coatings. These significantly reduce the irreversible losses, improve the solar thermal collection efficiency, and reduce the solar thermal collection cost. The cutoff wavelength is a key characteristic parameter of spectral-selective transmissive coatings which significantly affect the thermochemical performance of solar-driven thermochemical cycling. This study investigates the effect of the spectral-selective transmissive coatings on the solar-to-fuel efficiency based on experimental data for thermochemical splitting of CO₂ using reticulated porous ceria. This work also discusses the economic impact of the spectral-selective transmissive coatings on the solar thermal collector cost. The results show that for a solar-driven thermochemical redox cycle with a high temperature of 1 773 K, the optimal cutoff wavelength is 1 350 nm, which coincides with the steam and CO₂ absorption peaks in the solar spectrum (AM1.5). Spectral-selective transmissive coatings can increase the theoretical solar thermal collection efficiency of a blackbody cavity by 34.7%~85.2% and can significantly enhance the upper limit of the solar-to-fuel energy efficiency. The coatings can reduce the reduction half-reaction heating time by 13.7% and radiation losses by 36.7%. Finally, this work analyzes the economic impact of the spectral-selective transmissive coating on the solar thermal collector cost. The spectral-selective transmissive coatings can effectively reduce the solar thermal collector cost when the unit cost of the spectral-selective transmissive coatings is 330 times less than the cost of a dish concentrator.
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  Heat transfer correlation for subcooled flow boiling of saline solutions

  GU Junping, LIU Qi, WU Yuxin, WANG Qinggong, LYU Junfu

  Journal of Tsinghua University(Science and Technology). 2021, 61(12): 1397-1404. https://doi.org/10.16511/j.cnki.qhdxxb.2021.25.012

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  Accurate predictions of the heat transfer characteristics of saline solutions is of great importance for modeling the hydrodynamics and heat transfer in evaporative heat exchangers using saline solutions as the heat transfer medium. The heat transfer characteristics of NaCl solutions in a vertical heated tube were investigated experimentally to determine the heat transfer coefficients of NaCl solutions for single-phase forced convection and subcooled flow boiling conditions. The present experimental data was used to verify the prediction accuracies of existing heat transfer correlations for saline solutions. The experimental heat transfer data for saline solutions and pure water were compared to identify the main factors characterizing the heat transfer coefficient variations. A new heat transfer correlation is presented that accurately predicts the subcooled flow boiling heat transfer coefficient of saline solutions for the design and safe operation of industrial applications.
SPECIAL COLUMN:KEY CORE TECHNOLOGY
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  Surface modifications and performance enhancements of key friction pairs in aviation hydraulic piston pumps

  LI Yulong, HE Yongyong, LUO Jianbin

  Journal of Tsinghua University(Science and Technology). 2021, 61(12): 1405-1422. https://doi.org/10.16511/j.cnki.qhdxxb.2021.25.023

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  The hydraulic piston pump is the core component of hydraulic systems in aeronautical and astronautical systems that also presents severe technical challenges. Advanced spacecraft and aircraft need even higher performance hydraulic piston pumps with increased pressures, speeds, efficiencies, safety, and reliability. The lubricating and frictional characteristics of the three key friction pairs are the critical factors affecting the performance and service life of hydraulic piston pumps. This article reviews the development history and design methods of aviation hydraulic piston pumps, summarizes the lubrication theories and experiments of the key friction pairs, and analyzes traditional and advanced surface modification methods which can reduce the friction coefficient and increase the wear resistance. Future research directions are also discussed.
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  Advanced combustion technologies for future gas turbines

  LI Suhui, ZHANG Guihua, WU Yuxin

  Journal of Tsinghua University(Science and Technology). 2021, 61(12): 1423-1437. https://doi.org/10.16511/j.cnki.qhdxxb.2022.25.001

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  Gas turbine technology development trends have changed dramatically to meet increasingly stringent environmental regulations and reduce CO₂ emissions. However, current lean premixed combustion based on swirling flows cannot adapt to these changes. Therefore, advanced combustion technologies are reviewed here to identify new gas turbine designs by introducing their working principles, R&D results, and analyses of their readiness levels and key performance metrics such as NO_x emissions. A method is given to evaluate their overall performance and the ease-of-implementation to narrow the technology pathway choices and identify major research directions.
CORONA VIRUS
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  Air disinfection for SARS-CoV-2 and other pathogens: A review

  GUO Yuntao, ZHANG Dongheyu, ZHANG Liyang, PENG Siqi, LUO Haiyun, TIE Jinfeng, WANG Xinxin

  Journal of Tsinghua University(Science and Technology). 2021, 61(12): 1438-1451. https://doi.org/10.16511/j.cnki.qhdxxb.2021.25.004

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  SARS, MERS, SARS-CoV-2 and other pathogens have caused many pandemics in the world. These pathogens are often spread as aerosols in the air. Thus, fast, efficient air disinfection is essential for effectively limiting the spread of the pathogens. A low temperature plasma disinfection method that deactivates many kinds of bacteria, fungi, viruses, spores and other microorganisms has attracted much attention due to its efficiency and environmental friendliness. Disinfection methods can be divided into physical disinfection, chemical disinfection and comprehensive disinfection based on their key factors. This paper reviews the disinfection mechanisms, application scenarios, development progress and other characteristics of various disinfection methods. The review then focuses on the application of these technologies to the disinfection of pathogens such as SARS-CoV-2 with emphasis on plasma disinfection including the key methods and prospects of plasma disinfection in central air conditioning systems. Finally, the Gong Zi Ting performance center of Tsinghua University is used as an example to show the practicality of this surface discharge plasma disinfection method as an example for further applications. This method can significantly improve epidemic prevention and control, as well as the construction of national biosafety systems.
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  Communicable disease transmission model for the prevention and control of COVID-19 in Wuhan City, China

  DING Ying, ZHANG Jianqin, YANG Mu, GONG Peng, JIA Lipeng, DENG Shaocun

  Journal of Tsinghua University(Science and Technology). 2021, 61(12): 1452-1461. https://doi.org/10.16511/j.cnki.qhdxxb.2020.25.043

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  Epidemic prevention and control strongly affect people's lives in cities, but existing communicable disease models cannot accurately simulate the effects of prevention and control procedures. A city simulation model for the 2019 coronavirus epidemic was developed based on an Agent model for Wuhan, China to model the epidemic transmission process. The model includes the government control measures and the hospital diagnosis and treatment levels during the epidemic with analyses of the infection rates and spatial distributions for various epidemic control measures. The model was also used to model the active anti-epidemic impact of nucleic acid testing after people returned to work. The results show that this modeling method accurately reproduces the spatio-temporal transmission characteristics of the Wuhan epidemic. Thus, this method can be used to evaluate government control measures and to implement diagnosis and treatment plans for decision-making for infectious disease prevention and control.
ENVIRONMENTAL SCIENCE AND ENGINEERING
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  Effects of direct straw return and burning ash amendment on soil NO fluxes in a winter wheat cropland

  SU Yuanyuan, WANG Yanqiang, MEI Baoling, YAO Zhisheng

  Journal of Tsinghua University(Science and Technology). 2021, 61(12): 1462-1471. https://doi.org/10.16511/j.cnki.qhdxxb.2020.25.039

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  Returning straw to the soil rather than burning it can reduce negative environmental impacts such as air pollution. However, the incorporation of straw into the soil changes the soil physico-chemical properties, the biogeochemical C and N cycles and the associated environmental pollutant nitric oxide (NO) production and release. This study assessed how the soil NO fluxes respond to different methods of straw return in a winter wheat cropland. The assessment was based on measurements of the NO fluxes and auxiliary variables throughout the entire wheat-growing season (from October, 2016 to May, 2017) in a long-term purplish soil experimental platform with conventional fertilization (NPK), conventional fertilization + direct straw return (NPK+SR) and conventional fertilization + burning ash amendment (NPK+SB) as well as no nitrogen application as a control (CK) using a static opaque chamber and chemiluminescent analysis. The results showed pronounced NO peak fluxes with the fertilized treatments within the first 1-2 weeks after basal fertilization, which is comparable to the temporal trend of the soil mineral nitrogen. Thus, soil mineral nitrogen, specifically the soil ammonium (NH₄⁺) concentration, is the key factor controlling the NO flux variations with soil NO fluxes strongly positively correlated with soil NH₄⁺ concentrations. In comparison to NPK, NPK+SB did not significantly affect seasonal NO emissions, while NPK+SR greatly inhibited seasonal NO emissions by 49.0%. When the control emissions were deducted as background emissions, the direct NO emission factors were estimated to be 0.33%, 0.32% and 0.15% for NPK, NPK+SB and NPK+SR, respectively. Besides, both NPK+SB and NPK+SR treatment improve crop nitrogen use efficiency, and consequently enhancing wheat grain yields by 18.9% and 15.8%, respectively, comparing with NPK. The yield-scaled emissions (i.e., NO emission intensity) of NPK+SB were 19.7% less than those of NPK. Direct incorporation of the straw into the soil instead of burning further reduced the NO emission intensity from the winter wheat field by 45.6%. Therefore, for this winter wheat field, NPK+SR is recommended as the optimal fertilization management method which improves food security while reducing atmospheric pollutant NO emissions.