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Characteristics of interfacial oscillation caused by condensation of steam jet in subcooled water flow

EXPERIMENTAL THERMAL AND FLUID SCIENCE 2019年 109卷 109864-000页

作者： Xu, Qiang Chu, Xiaona Yao, Tian Liu, Weizhi Guo, Liejin Xi An Jiao Tong Univ State Key Lab Multiphase Flow Power Engn Xian 710049 Shaanxi Peoples R China

An experimental study is conducted to investigate interfacial characteristics of direct contact condensation of steam jet in counter-current subcooled water flow in confined conditions. A customized visualization window and a high-speed camera are combined together to capture and record the dynamical interface behavior of the condensing jet. The transient interface behavior of the jet is analyzed and discussed quantitatively by utilizing digital image processing technology. With increase of steam mass flux, the bubbling and jetting regime appear successively at subsonic conditions and sonic or supersonic conditions, respectively. The transient radial interface position along all axial locations downstream of the jet is obtained quantitatively for these two typical condensation regimes. The probability distribution function and power spectrum density of the radial interface position along all axial locations of a certain jet follow almost the same principles in the time-frequency domain. Significant variations of the probability distribution function and power spectrum density between the bubbling and jetting regimes happen at transonic condition. With increase of axial distance away from the nozzle exit, the probability of the pinch-off events first rise and then drops in subsonic and sonic conditions. While the jet interface is very stable and no pinch-off events occur in supersonic condition.

关键词： Steam condensation Interface position Jet stability Condensation regime multiphase flow

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Role of solid-liquid interaction energy on anomalous thermal conductivity enhancement in well-dispersed dilute nanofluids studied by equilibrium molecular dynamics

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CHEMICAL PHYSICS 2020年 539卷 110943-110943页

作者： Wang, Xin Jing, Dengwei Xi An Jiao Tong Univ State Key Lab Multiphase Flow Power Engn Xian 710049 Peoples R China Xi An Jiao Tong Univ Int Res Ctr Renewable Energy Xian 710049 Peoples R China

In this study, Green-Kubo formulism was employed to quantitatively assess thermal conductivity of nanofluids with different solid-liquid interaction energy. Anomalous thermal conductivity enhancement was observed in well-dispersed dilute nanofluids with strong solid-liquid interaction energy. The simulation results indicated that solid-liquid interaction is the key factor for the understanding of the heat conduction mechanism in nanofluids. Green-Kubo theory and Brownian motion were studied in detail to derive the physical origin of thermal conductivity enhancement. The results suggested that the phonons across the solid-liquid interface or micro-convection induced by Brownian motion of nanoparticles is responsible for thermal conductivity enhancement. In addition, a wide range of thermal conductivity enhancement and nonlinear increasing with volume fraction was observed, which is consistent with experiment investigations. The effect of solid-liquid interaction energy on the viscosity of nanofluids was also studied and was found to be an insignificant factor. Our results indicated that the nanofluids with strong solid-liquid interaction energy should have more significant application potential.

关键词： Molecular dynamics Nanofluids Thermal conductivity Viscosity Micro-convection Brownian motion

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Central-pointy to central-concave icing transition of an impact droplet by increasing surface subcooling

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INTERNATIONAL COMMUNICATIONS IN HEAT AND MASS TRANSFER 2019年 108卷 104326-000页

作者： Shang, Yuheng Liu, Xiufang Bai, Bofeng Zhong, Xin Xi An Jiao Tong Univ State Key Lab Multiphase Flow Power Engn Xian 710049 Shaanxi Peoples R China

The effect of substrate temperature is investigated experimentally on the ice profile of an impact water droplet. A hydrophilic substrate, a relatively high impact velocity which can lead to a larger contact area, and a low surface temperature are employed aiming at enhancing the heat transfer rate through the droplet. The frozen droplet is uniquely central-concave at low surface temperatures, in contrast to the widely observed central-pointy ice formed at cold surfaces. The formation of the central-concave icing is attributed to the additional interior circular-like freezing front advancing both upwards and outwards during the solidification. Upon decreasing the surface temperature, the nucleation is triggered earlier, and the freezing rate is higher than that of a quasi-steady droplet freezing on a similar subcooled surface due to the impact-induced flattening of the droplet. A freezing regime map described by the dimensionless number representing the heat conduction rate through the substrate and Weber number consists of two regimes for the formation of the central-pointy and central-concave ice profile respectively.

关键词： Impact droplet Freezing Ice profile Spread Heat conduction rate

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Hybrid Photoelectrochemical Water Splitting Systems: From Interface Design to System Assembly

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ADVANCED ENERGY MATERIALS 2020年第11期10卷

作者： Niu, Fujun Wang, Degao Li, Fei Liu, Yanming Shen, Shaohua Meyer, Thomas J. Xi An Jiao Tong Univ Int Res Ctr Renewable Energy State Key Lab Multiphase Flow Power Engn Xian 710049 Shaanxi Peoples R China Univ N Carolina Dept Chem CB 3290 Chapel Hill NC 27599 USA Dalian Univ Technol State Key Lab Fine Chem Dalian 116024 Peoples R China

Photoelectrochemical (PEC) water splitting has attracted increasing attention due to its potential to mitigate energy and environmental issues. Hybrid PEC systems containing semiconductor photosensitizers and molecular catalysts are reported to be highly active and stable for water splitting with great potential for facilitating clean fuels production. In this review, following a showcasing of the fundamental details of hybrid PEC systems for water splitting, semiconductor/molecular catalyst interface designs are highlighted, with a focus on interfacial physicochemical interactions and binding, and interfacial energetics and dynamics for efficient charge transfer. Recent advances in hybrid system assemblies for PEC water splitting are also briefly introduced. Finally, future challenges and directions in the field of hybrid PEC water splitting for solar energy conversion are reviewed. The current review provides state-of-the-art strategies for optimized interface design for creating highly active and stable PEC water splitting assemblies.

关键词： artificial photosynthesis interface design photoelectrode assembly water splitting

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Characteristics of frictional pressure drop of two-phase nitrogen flow in horizontal smooth mini channels in diabatic/adiabatic conditions

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APPLIED THERMAL engineering 2019年 162卷 114312-114312页

作者： Chen, Xingya Hou, Yu Chen, Shuangtao Liu, Xiufang Zhong, Xin Xi An Jiao Tong Univ State Key Lab Multiphase Flow Power Engn Xian 710049 Shaanxi Peoples R China

The two-phase frictional pressure drop of nitrogen under diabatic and adiabatic conditions is investigated experimentally in two horizontal mini channels. The effects of factors in a variety, including the channel diameter, mass flux, heat flux, inlet pressure and vapor quality are examined on the frictional pressure drop and liquid-phase multiplier of the two-phase flow. The two-phase frictional pressure drop predicted by previous correlations from other studies are compared with the experimental results of this study, and the lowest mean absolute deviation is 18.3% for adiabatic conditions and 10.9% for adiabatic conditions. Based on the experimental results of nitrogen flow, two new correlations in the form of the Chisholm-C correlation are developed to predict the two-phase frictional pressure dropby involving the effect of surface tension, and the mean absolute deviation of which is 12.3% and 10.4% for the adiabatic and diabatic conditions respectively. The new correlations are also shown able to well predict the experimental frictional pressure drop of other studies under different experimental conditions.

关键词： Frictional pressure drop Liquid nitrogen Mini channel Two-phase flow Correlation

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Effect of alkali and alkaline metals on gas formation behavior and kinetics during pyrolysis of pine wood

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FUEL 2021年 290卷 120081-120081页

作者： Li, Jinhu Burra, Kiran G. Wang, Zhiwei Liu, Xuan Gupta, Ashwani K. Univ Maryland Dept Mech Engn Combust Lab College Pk MD 20742 USA China Univ Geosci Fac Engn Wuhan 430074 Hubei Peoples R China Henan Univ Technol Coll Environm Engn Zhengzhou 450001 Peoples R China Xi An Jiao Tong Univ Sch Energy & Power Engn State Key Lab Multiphase Flow Power Engn Xian 710049 Peoples R China

This study examines the acid and alkaline treatment of pine wood to help understand the effect of alkali and alkaline earth metals (AAEMs) content on the pyrolysis behavior at different temperatures. Acid and alkali pretreatment of pine wood were conducted to modify AAEMs content by ion-exchanging method. Thermal kinetic behavior of the pretreated samples was first conducted by a thermogravimetric analyzer (TGA) at different heating rates that provided activation energies of pyrolysis. Gas formation behavior of the samples for different extents of conversion was carried out in a fixed-bed reactor at two different temperatures of 823 K and 1073 K. Evolutionary behavior of the gas components during pyrolysis, including flow rate, and yield and their energy content were measured and compared. Results showed that the values of activation energies increased with the extent of conversion for all the pretreated samples examined. The effect of AAEMs on pyrolysis behavior of biomass varied with the extent of conversion. The presence of AAEMs in biomass decreased the decomposition energy at low conversion while it greatly improved under high conversion. Besides, alkali treated sample with higher AAEM content enhanced the gas and char yield while it reduced the bio-oil production at low temperature with low conversion. However, at high temperature the opposite trend was observed. Presence of AAEMs was favorable for the generation of H-2, CO, CO2 and CnHm at low temperature, while it showed an inhibition effect on CO and CnHm yield and syngas energy at high temperature. The catalytic mechanism of AAEMs on the pyrolysis behavior at different temperatures was discussed based on activation energy and gaseous formation. Results revealed decomposition of carboxylate at low temperature and formation of stable biomass-Na (BM-Na) structure at high temperature that led to the variation of activation energy and changed gaseous products yield and syngas energy. Acid washing pretreatment w

关键词： Kinetic behavior Mechanism Ion-exchanging Gas components Activation energy Syngas yield

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Experimental study on evolutions of temperature and height of waterfilm during static flash

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INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER 2018年 121卷 223-232页

作者： Zhang, Dan Yang, Qingzhong Liang, Ting Yan, Junjie Xi An Jiao Tong Univ Sch Energy & Power Engn State Key Lab Multiphase Flow Power Engn Xian 710049 Shaanxi Peoples R China

The temperature and height of waterfilm were a pair of coupled characteristic parameters indicating the instant heat/mass transfer during static film flash of pure water. Their evolutions and interactions were studied through experiments with initial height of waterfilm ranging between 0.15 and 0.3 m, initial temperature between 80.2 and 93.0 degrees C, initial pressure of vacuum chamber between 0.013 and 0.061 MPa. Results suggested both height and decrease rate of temperature of waterfilm experienced a rapid increase to peak value and a gradual decline after that. Considering most of heat transfer happened within waterfilm, volumetric evaporation coefficient was introduced as the total heat flux transferred through all bubble interfaces within unit volume of waterfilm under unit superheat. This coefficient increased significantly with rising volumetric void fraction of waterfilm. Except vapor generation, mass transfer during static flash also included the overflowing of vapor. Its overflowing speed increased with initial height of waterfilm. According to these results, a model was set up to calculate the evolutions of temperature and height of waterfilm from given evolution of saturation temperature corresponding to the pressure of flash chamber. (C) 2018 Elsevier Ltd. All rights reserved.

关键词： Static film flash Waterfilm temperature Waterfilm height Evaporation Vapor overflow

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Non-monotonic behavior of flame instability of 1,3-butadiene/O2/He mixture up to 1.5 MPa

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FUEL 2019年 255卷 115749-000页

作者： Lu, Xin Hu, Erjiang Xu, Zhaohua Ge, Ruihan Huang, Zuohua Zeng, Ke Xi An Jiao Tong Univ State Key Lab Multiphase Flow Power Engn Xian 710049 Shaanxi Peoples R China

Non-monotonic behavior of Markstein lengths of 1,3-butadiene/O-2/He mixtures was studied in a constant pressure combustion chamber at initial temperature of 298 K, initial pressure of 0.5-1.5 MPa and equivalence ratios of 0.8-1.5. Markstein lengths of 1,3-butadiene/air mixtures were also measured at initial temperature of 298 K, initial pressure of 0.1-0.5 MPa for comparison. Non-monotonic behavior was also observed in both theoretical Markstein lengths and Markstein numbers of 1,3-butadiene/O-2/He mixtures, but was not observed in those of 1,3-butadiene/air mixtures. The effects of factors on non-monotonic behavior were studied. It is concluded that the rapid increase of Zel'dovich number in rich condition leads to the non-monotonic behavior of Markstein numbers of 1,3-butadiene/O-2/He mixtures, and further leads to the rapid increase of Markstein length. Both experimental and theoretical critical flame radius and Peclet number were obtained and showed similar non-monotonic phenomenon to Markstein lengths and Markstein numbers, respectively. It is indicated that the cellular structure will first appear near stoichiometric ratio, thus the laminar burning velocities of helium diluted mixtures under elevated pressure are easier to measure under both rich and lean conditions.

关键词： Markstein length 1,3-Butadiene Non-monotonic behavior Elevated pressure Critical flame radius

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Heat transfer performance and optimization of a close-loop R410A flash evaporation spray cooling

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APPLIED THERMAL engineering 2019年 159卷 113966-000页

作者： Lin, Yan-Ke Zhou, Zhi-Fu Fang, Yu Tang, Hong-Lin Chen, Bin Xi An Jiao Tong Univ State Key Lab Multiphase Flow Power Engn Xian 710049 Shaanxi Peoples R China

Flash spray cooling has been subject to increased attention because of its high heat dissipation capacity at low surface temperature in the application of high power technologies. In this study, experiment was conducted to study the effects of spray distance and nozzle diameter on heat transfer performance in a closed-loop R410A flash spray cooling system for the first time. Five spray distance from 10 mm to 30 mm and three nozzles with same internal structure but different diameters of 0.51, 0.56 and 0.69 mm were employed. The experiment results indicated the critical heat flux (CHF) value firstly increased and then deceased with the increase of spray distance, which is consistent with previous research of spray cooling with FC-72 and FC-87. The highest CHF value reached 264 W/cm(2) while maintaining surface temperature below 30 degrees C and heat transfer coefficient (HTC) was about 210 kW/(m(2).K) at 25 mm, which were 60% higher than those at 10 mm spray distance. The nozzle with medium orifice diameter of 0.56 mm showed a superior cooling performance, instead of larger nozzle with higher refrigerant flow rate. Therefore, there existed a counterbalance between the mass flow and outlet velocity in determining the optimum nozzle orifice diameter.

关键词： Flash spray cooling Spray distance Nozzle diameter R410A Electronics thermal management

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Gas flow rate measurement in low-quality multiphase flows using Venturi and gamma ray

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EXPERIMENTAL THERMAL AND FLUID SCIENCE 2019年 100卷 319-327页

作者： Pan, Yanzhi Li, Chao Ma, Yugao Huang, Shanfang Wang, Dong Xi An Jiao Tong Univ State Key Lab Multiphase Flow Power Engn Xian 710049 Shaanxi Peoples R China Haimo Technol Grp Corp Lanzhou 730050 Gansu Peoples R China Tsinghua Univ Dept Engn Phys Beijing 100084 Peoples R China

Online flow rate measurements are encountered in many areas with a great need for high accuracy, especially in low-quality (0-0.1) flows. This paper concerns gas flow rate measurements using a Venturi meter and gamma-ray attenuation technique. A linear correlation is developed to predict the gas flow rates with wide ranges of void fractions (0-95%). The correlation predictions were compared to experimental data in which the root mean square errors of the oil-air-water multiphase flow prediction results are 7.72%, 8.93% and 9.11% for the gas flow rate, the quality and the gas-liquid slip ratio, respectively. The effect of the Venturi size was tested, and the metering accuracy is found to increase with the inlet diameter. The prediction accuracy is improved for three-phase flows than for the corresponding two-phase flows due to the oil-water stirring in the three-phase flows. The analyses of the results indicate that the measurement accuracy is higher with stable flow regimes than intermittent flow regimes. The present method can be widely applied to both two-phase and three-phase flows in the oil industry even for different Venturis and fluid media.

关键词： multiphase flow Venturi meter Gamma ray Gas flow rate flow regime

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