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.

利用流动沸腾换热的微通道热沉具有所需工质流量小、换热系数高、温度分布均匀等优势,是适用于高热流密度微电子器件的一种极具应用前景的散热技术。但随着热流密度不断攀升,流动沸腾的不稳定性逐渐成为制约微通道散热技术发展的重要因素,其引发的流量、压降振荡和壁面热点会导致一系列控制问题和安全问题。因此,迫切需要深入地研究微通道热沉中流动沸腾不稳定性的产生机理,并设计和开发出换热系数高和稳定性好的新型微通道热沉。本文设计和加工了不同基材和结构的微通道热沉,搭建了微尺度流动沸腾可视化实验台,采用实验与理论建模相结合的方法,对微通道热沉内连续两相流型不稳定沸腾和压降振荡型不稳定沸腾进行研究,分析其成因,并提出提高流动沸腾稳定性的方法。首先,设计和加工了硅基并联矩形截面直微通道和折线型微通道,并搭建了一个同步多传感器流动沸腾实验台,该实验台采用高压氮气推进,因此微通道上游具有极低的可压缩性。在对两种微通道热沉进行流动沸腾换热实验发现:在相近的工况条件下,折线型微通道热沉的流动沸腾换热系数最高可达直微通道的1.25倍,其沸腾起始点的壁面温度也较低,证明设置弯折结构可以有效地提高微通道中的流动沸腾换热系数,但折线型微通道的平均压降比直微通道要高。实验结果还表明,直微通道中的单相/两相界面清晰且整齐,而折线型微通道热沉中个别通道内的返流现象比较严重,这容易引起流量分配不均;与直微通道相比,折线型微通道入口附近存在大量额外的核化穴,这削弱了流动沸腾的稳定性,特别是高质量通量下,其沸腾稳定性更差。其次,利用高速显微摄影系统和高速红外热像仪,研究了微通道在不同沸腾状态下的流型转变和壁面温度分布变化,根据单相强制对流换热系数预测关联式和两相摩擦压降预测关联式建立了微通道中流动沸腾稳定性边界的预测模型,计算结果表明:降低入口温度、提高微通道上游换热系数以及在入口加装节流结构都可以提高流动沸腾的稳定性,但由于节流件会显著提高入口压力,因此节流件在设计之初应平衡流动阻力和稳定性的关系。利用热网络法,结合实际流型转变过程,推导出了适用于硅基并联矩形截面直微通道的连续两相流型不稳定沸腾的动态仿真模型。通过分析模型及其计算结果可知,通过强化单相强制对流换热系数和流动沸腾换热系数可以有效地消除固/液温差-热流密度关系曲线上负斜率区,从而避免工质与壁面发生自持地能量交换,最终消除了连续两相流型不稳定沸腾。再次,设计和加工了6063铝合金基矩形截面直微通道热沉实验件,并搭建了一个可调节系统上游阻力配置和压缩性的流动沸腾实验台,该实验台采用精密注射泵推进,以提高实验结果的准确性。在6063铝合金基微通道热沉中进行的流动沸腾换热实验表明,若系统上游可压缩性较大,微通道会发生压降振荡型不稳定沸腾,流量和压降会发生大振幅、长周期振荡,并且微通道内主要是单相液/汽液两相转换流;增加系统上游的沿程阻力损失可以降低压降振荡的振幅和周期,并且提高稳定流动沸腾边界的干度,尤其对低质量通量条件下稳定边界的干度的提升效果相对较好;增加系统上游的局部阻力损失对压降振荡的振幅和周期影响较小,但流动沸腾的稳定性边界仍会得到显著提高。最后,设计和加工了三个具有不同深度和入口结构的6063铝合金基矩形截面直微通道热沉,搭建了一个循环供液式流动沸腾实验台,研究了微通道结构对压降振荡型不稳定沸腾的影响。研究结果表明,工质在微通道中的压降-质量通量关系曲线中存在一个负斜率区,该区域的平均斜率随着通道深度的增加而增加;负斜率区的平均斜率越高,越不利于微通道中的工质与可压缩容积发生动量交换,提高负斜率可增强流动沸腾的稳定性;入口引导结构会削弱微通道热沉的并联稳定性,使微通道间更容易发生动态的流量分配不均,也导致工质在低干度条件下的负斜率升高。通过比较不同结构微通道的实验数据后发现,入口引导结构可以有效提高流动沸腾稳定性边界的干度,提高微通道在低干度工况下的流动沸腾稳定性。

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