采用液相还原法,以抗坏血酸（C6H8O6）为还原剂,还原六氢氧化铂酸(H2Pt（OH）6)、醋酸银（CH3COOAg）制备了水基银铂合金纳米流体。研究流量、温度、表面活性剂(聚乙烯吡络烷酮（PVP）)物质的量比等对纳米流体黏度、稳定性、热导率的影响。实验结果表明,在PVP物质的量比为5、流量为0.5 mL/min、反应温度为50℃时,纳米流体的稳定性最好,热导率最高;在流体温度为60℃时,热导率相较于基液增加了16.92%。热导率随PVP物质的量比、反应温度、流量的增加均呈现先增大后减小的趋势。黏度随表面活性剂浓度的增加而增加,随反应温度、流量的增加呈现先减小后增加的趋势。反应温度为50℃时,纳米流体黏度最小,在流体温度为50℃时,相较于去离子水增加了36.51%。

Micro/mini-channel heat sinks are extensively utilized in heat dissipation systems that involve high heat flux components. These heat sinks are preferred due to their compact size and excellent heat transfer efficiency. The selection of a refrigerant condensation system for the heat dissipation system under vapor conditions is crucial. In order to investigate the heat transfer characteristics of flow condensation in a mini-channel, a visual experiment system was used with a hydraulic diameter of 2 mm and a volume flowrate of 30 mL/min, 60 mL/min, 90 mL/min, 120 mL/min respectively. The study focused on the effect of inlet vapor quality on the flow pattern and heat transfer characteristics of flow condensation in the mini-channel. The results indicate that the flow pattern in flow condensation process can be classified into annular flow, transition flow, slug flow and bubble flow in sequence along the flow direction. As the inlet vapor quality (xin) increases, the dominance of annular flow gradually increases, making it difficult to capture the bubble flow with high volume flowrate and high inlet vapor quality. This phenomenon is observed under experimental conditions when: 1) xin is equal to or greater than 0.75 at a volume flowrate of 60 mL/min, 2) xin is equal to or greater than 0.55 at a volume flowrate of 90 mL/min, and 3) xin is equal to or greater than 0.25 at a volume flowrate of 120 mL/min. The increase in inlet vapor quality is beneficial to the improvement of condensation heat transfer coefficient. In this experiment, it was observed that the local heat transfer coefficient was decreased by up to 83.7 % when comparing the lowest vapor quality to the highest at the same volume flowrate, emphasizing the significance of inlet vapor quality in the flow condensation during the heat transfer process. © 2023 Elsevier Ltd

To meet the increased heat dissipation demands of 5G base stations, a flat plate heat pipe array (FPHPA) of 500 × 200 × 3 mm3 comprising of 19 independent channels is developed. To investigate the thermal performance of the FPHPA under different filling ratios (FR) and heat powers (30–300 W), a multi-channel independent filling system is designed to conduct two experimental cases: uniform or asymmetric filling of all channels. Results indicate that 30 %–70 % FR exhibits better thermal performance and can replace the aluminum plate over a broader heat power range. Furthermore, separate tests are conducted to evaluate the heat transfer and start-up performance of the FPHPA. The optimal filling ratio interval of single channel at different heat fluxes is derived through asymmetric filling. Subsequently, a mixture model is proposed and compared with the uniform filling ratio case, resulting in optimized heat transfer performance. The optimized FPHPA provides better heat transfer performance, effectively reducing the area averaged wall temperature and ensuring the normal operation of the 5G base station. © 2023 Elsevier Ltd

The thermophoresis of nanoparticles suspended in gas is investigated in the transition regime by molecular dynamics simulations. It is found that there exists significant discrepancy between the simulation results and the theoretical predictions for the thermophoretic force, which is attributed to the adsorption of gas molecules on nanoparticles and the gas-particle non-rigid body collisions. By using the effective particle radius, the simulation results and Talbot et al.'s equation could agree with each other in the transition regime. In addition, the effect of the finite system size of the molecular dynamics simulations is non-negligible, and the simulation results modified by effective particle radius can coincide with Phillips' equation quite well. Therefore, for particles of a few nanometers, the non-rigid body collision effect and the adsorption of gas molecules and the effective radius of the nanoparticle under strong gas-particle coupling should be taken into account in the theoretical model. The investigation presented in this paper provides guidance for the application of nanoparticles in aerosol science.

Thermal rectification refers to the phenomenon by which the magnitude of the heat flux in one direction is much larger than that in the opposite direction. In this study, we propose to implement the thermal rectification phenomenon in an asymmetric solid-liquid-solid sandwiched system with a nano-structured interface. By using the non-equilibrium molecular dynamics simulations, the thermal transport through the solid-liquid-solid system is examined, and the thermal rectification phenomenon can be observed. It is revealed that the thermal rectification effect can be attributed to the significant difference in the interfacial thermal resistance between Cassie and Wenzel states when reversing the temperature bias. In addition, effects of the liquid density, solid-liquid bonding strength and nanostructure size on the thermal rectification are examined. The findings may provide a new way for designs of certain thermal devices.

Liquid evaporation from micro/nanoscale pores is widely encountered in cutting-edge technologies and applications. Due to its two- or three-dimensional features, the nano-porous evaporation is less understood compared to the one-dimensional evaporation of a planar liquid surface. This paper reported a novel study of the inter-pore interference effect in nano-porous evaporation, clarifying the variation in the net evaporation rate from individual nanopores when the inter-pore distance, neighboring nanopore diameter, or liquid temperature were changed. Molecular simulation results showed that the reduction in inter-pore distance could enhance the evaporation rate from nanopores by augmenting the vapor convection effect and suppressing the condensation flux. This interference effect was more pronounced at lower evaporation intensity with the evaporation flux being different by up to 25% from the one-dimensional case. The inter-pore interference was equally observed for Knudsen numbers of 0.1 and 10. Additionally, the non-uniformity in nanopore size distribution had no influence on the evaporative mass flux within the present parameter range. The non-uniformity in nanopore temperatures, however, could affect the net evaporation from individual nanopores, similarly by modulating the vapor convection magnitude in adjacent to the interface and the condensation flux. The effect of inter-pore interference is found to be essential at low evaporation intensity, which is highly relevant in industrial applications such as water evaporation under atmospheric pressure.

In this work, the negative differential thermal resistance effect has been proposed in a solid-liquid-solid sand-wiched system with a nanostructured cold surface. Non-equilibrium molecular dynamics simulations demon-strate that the heat flux in the present sandwiched system increases with the temperature bias for low temperature bias, while for high temperature bias, the heat flux decreases counter-intuitively with increasing temperature bias. Based on the analysis of the interfacial thermal resistance and the density depletion length at the solid-liquid interface, the negative differential thermal resistance effect at high temperature bias is attributed to the suppressed solid-liquid interfacial thermal conductance with decreasing temperature. In addition, it is found that the negative differential thermal resistance effect can be tuned by the size of the nanostructure.

Manifold microchannel heat sinks (MMCHSs) are essential in thermal management. Conventional Z-type MMCHS encounter flow maldistribution and large temperature gradients on the heating surface. This work proposes an innovative MMCHS with restrictors to reduce flow maldistribution and compared with other optimization schemes. Through the numerical simulation, we optimize three dimensionless parameters (the width ratio α and length ratio β of the restrictor and inlet-to-outlet width ratio of manifold γ). Introducing the restrictor into the MMCHS can reduce flow maldistribution. In the research scope, the maximum mass flux discrepancy of MMCHS with α = 0.3 decreased from 327.17% to 64.51% compared to MMCHS with α = 1 when the volumetric flow rate is 8.51 mL/min. The reason is that restrictors increase the ratio between pressure drop in microchannels and total pressure drop(Δpc/Δp). Additionally, the coupling effect of the restrictor and γ on thermal-hydraulic in MMCHS are complicated. Therefore, the thermal resistance ratio (Rt/Rt,0), the pumping power ratio (Pp/Pp,0) and performance evaluation criteria (PEC) are taken as references to consider the thermal-hydraulic performance of MMCHS comprehensively. The results display that the MMCHS with α = 0.5– 0.6, β = 0.4 and γ = 0.8– 1 can further control the synchronization of Rt/Rt,0 and Pp/Pp,0 at about 0.75. The PEC values are all greater than 1.59. © 2023

本文通过液相还原法制备了水基银铜合金纳米流体,并对其吸光度、热导率和黏度进行了实验测试。结果表明,在前驱体注射流量为0.5 mL/min、反应温度为70℃和表面活性剂聚乙烯吡络烷酮(PVP)摩尔比为7的制备条件下,纳米流体的吸光度峰值呈

采用数值计算方法研究了两种结构微通道内由于入口速度的正弦变化而发展的非稳态层流流动与换热特性.分别研究了脉动频率、振幅以及雷诺数对流体在微通道内流动与传热的影响.研究结果表明，在雷诺数为100～400时，脉动流动对矩形通道底面温度与换热性能影响较小，但对三角凹穴结构通道有着显著影响.随着脉动频率的增加，通道底面温度先增加后减小.证明存在一极限频率使得小于该频率时通道底面温度升高，大于该频率时则降低.随着振幅的增加，通道底面温度在减小，换热不断增强.但是，随着雷诺数的增加，脉动流动的作用逐渐减小.脉动频率与振幅的增加都会使得通道的压降增加.