Advances in Electronics Cooling

This article highlights the advantages of on-chip microchannel cooling technology, based on first- and second-law analysis and experimental tests on two types of cooling cycles, the first driven by an oil-free liquid pump and the second by an oil-free vapor compressor. The analysis showed that the drivers of the fluid were the main culprits for major losses. It was further found that when energy recovery is of importance, making use of a vapor compression cycle increases the quality of the recovered energy, hence increasing its value. This was demonstrated by analyzing the synergy that can exist between the waste heat of a data center and heat reuse by a coal-fired power plant. It was found that power-plant efficiencies can be increased by up to 6.5% by making use of a vapor compression cycle, which results not only in significant monetary savings, but also in the reduced overall carbon footprints of both the data center and the power plant.     

Effects of Thermodiffusion and Nanoparticles on Convective Instabilities in Binary Nanofluids

In this article, the effects of thermodiffusion of nanoparticles and solute in binary nanofluids and nanoparticles on the convective instabilities of a binary nanofluid is theoretically investigated. Thermodiffusion implies that mass diffusion is induced by thermal gradient, which is the so-called Soret effect. In order to analyze the convective instabilities of a binary nanofluid, a new stability criterion is obtained based on the linear stability theory and new factors g and f are proposed. The results show that the Soret effect of solute makes the binary nanofluids unstable significantly and the convective motion in a binary nanofluid sets in easily as the ratio of Soret coefficient of nanofluid to that of binary basefluid δ₄ increases for δ₄ > ?1. It is also found that with an increase of the volume fraction of nanoparticles, the nanofluid becomes stable, but at or near ψ _bf = ? 0.3 the state of nanofluid changes from stable to unstable. The results from the addition factor analysis show that an asymptotic point of ψ _bf where the maximum value of g diverges infinitely exists in the range of ? 1.2 < ψ _bf < ? 1.1 with given conditions. The binary addition factor g is always higher than the normal addition factor f, which means that the heat transfer enhancement by the Soret effect in binary nanofluids is more significant than that in normal nanofluids.     

Convective Heat Transfer and Resistance Characteristics of Nanofluids with Cylindrical Particles

Numerical research on convective heat transfer and resistance characteristics of TiO₂/water nanofluids with cylindrical particles in laminar channel flow are performed by solving the governing equations of fluid flow with the additional term of cylindrical nanoparticles, the equation of probability density functions for cylindrical nanoparticle orientation, and general dynamics equation for nanoparticle volume concentration. The nonuniformity of nanoparticle distribution is considered and the effects of both particle volume concentration and Reynolds number on friction factor and local Nusselt number are mainly analyzed. The results show that the friction factor of nanofluid flow increases with an increase in particle volume concentration. And the friction factor decreases with increasing Reynolds number and is not dependent on the volume concentration at high Reynolds numbers. The Nusselt number declines when the Reynolds number decreases, and finally approaches an asymptotic value after the Reynolds number falls to a certain value. The Nusselt number is higher in the entrance region than at the downstream locations, and will become steady at somewhere downstream when the flow is thermally and hydraulically developed.     

CuO纳米流体作为柴油机冷却介质的数值计算分析

用CuO纳米流体作为柴油机的冷却介质,运用计算流体力学(CFD)方法对CuO粒子质量分数为1%,3%和5%的CuO纳米流体在柴油机冷却水套内的流动和换热过程进行三维数值模拟,并采用湍流随机跟踪方法,对固液两相流离散项纳米粒子的运动进行轨迹追踪,得到了不同CuO质量分数的纳米流体在柴油机水套内的CuO粒子分布,速度场分布,换热总量以及水套进出、口之间的压降变化。计算结果表明,CuO纳米流体作为介质可以显著提高柴油机的散热性能,随着纳米粒子的增加,柴油机散热能力增强,水泵功率损失小范围增加。     

Performance of Power Plant Spray Cooling Modules and Systems

The detailed three-dimensional local interaction model developed previously for spray units is used here to predict the cooling performance of both a pass of sprays coaxially aligned with the wind, and a multipass spray system. Since the model predicts the local changes in temperature and humidity of the air-vapor phase as well as droplet trajectories and temperatures, it is capable of determining the degree of interference between adjacent spray units as a function of upwind conditions and spray spacing without recourse to empirical interference factors. Model predictions are compared with droplet cooling and wet-bulb temperature data for spray passes consisting of units of the Power Spray Module. The thermal efficiency of a pass of sprays is examined by use of a theoretically generated dimensionless performance chart.. Such charts are utilized in a computer model of a multipass spray canal system. The model shows excellent agreement with system data. System design and performance charts are presented for industrial use.     

Heatline Analysis on Heat Transfer and Convective Flow of Nanofluids in an Inclined Enclosure

This article presents a heatline method to analyse the transport mechanism of heat transfer and convective flow of nanofluids in an inclined square enclosure, where a heated thin plate located in the middle of the enclosure. The fluid flow, heat transfer, and heat transport characteristics are illustrated using streamlines, isotherms, Nusselt number and heatlines. Results show that fluid flow and temperature fields strongly depend on Rayleigh number, inclination angle, solid volume fraction, types of nanoparticles and the plate length, and the maximum strength of heatfunction increases as the inclination angle and Rayleigh number increase.     

A Novel Approach for Energy and Water Conservation by Using Silver-Carbon Quantum Dots Hybrid Nanofluids in Wet Cooling Towers Systems

This study presents the use of Silver-Carbon Quantum Dots (Ag-CQD) hybrid nanofluids,prepared by a facile wet chemical method,for heat transfer enhancement of w...     

燃气轮机入口空气冷却系统的技术经济性能

符号说明P—发电出力Q—燃料输入热量T—大气环境温度W—燃气轮机年净增发电量price—能源价格TR—冷吨Cost—运行能耗成本下标amb—大气环境peak—峰电期间aver—平电期间fuel—燃料saving—年所节省的1前言燃气轮机作为一种启动迅速、易于实现能源梯级利用及污染排放相对较少     

纳米流体强化内燃机冷却系统流动的基础研究

利用高导热率、传热性能好的传热工质(纳米流体)替代传统冷却介质应用于内燃机冷却系统中,通过纳米流体流动特性的基础研究,为其在内燃机冷却系统中的应用提供理论基础支持.因此,利用试验方法对纳米流体在波壁管内的流动进行可视化研究,以期对纳米流体的流动机理进行详细的探讨,从而推动纳米流体在内燃机冷却系统中的应用.研究发现:纳米流体的黏度增加值不大,且随着温度的升高,增加值降低;而相同入口速度状态下,纳米流体在波壁管内的流动比纯水更为活跃,漩涡数量增多,质量传递特性增强,且随纳米颗粒浓度的增加,流动湍流效应增大.通过分子动力学方法发现纳米颗粒在纳米流体流动过程中存在强烈的旋转作用,从而出现微湍流流动效应,进一步强化了纳米流体的湍流流动效果.     

纳米流体对内燃机冷却系统强化传热的数值模拟研究

将Cu-水纳米流体应用到内燃机冷却系统中,并利用大型通用CFD软件STAR-CD针对不同浓度纳米流体的内燃机冷却系统进行三维数值模拟计算.通过计算可以得到传热工质的流场、压力场及壁面换热系数的空间分布.结果表明,以Cu-水纳米流体作为传热工质可以显著提高内燃机的散热性能,且散热量随着纳米粒子浓度的增大而增大.同时,纳米粒子的加入也引起了内燃机冷却系统泵功的增加,但是与散热量的显著提高相比是可以接受的.     

Energy Performance Analysis of Ice Thermal Storage for Commercial HVAC Systems

Ice thermal storage is a promising technology to reduce energy costs by shifting the cooling cost from on-peak to off-peak periods. The paper investigates the application of ice thermal storage and its impact on energy consumption, demand and total energy cost. Energy simulation software along with a chiller model is used to simulate the energy consumption and demand for the existing office building located in central Florida. Furthermore, the study presents a case study to demonstrate the cost saving achieved by the ice storage applications. The results show that although the energy consumption may increase by using ice thermal storage, the energy cost drops significantly, mainly depending on the local utility rate structure. It found that for the investigated system the annual energy consumption increases by about 12% but the annual energy cost drops by about 3 6%.     

焦化装置冷切焦水系统运行分析及优化改造

九江石化1.0Mt/a延迟焦化装置冷、切焦水系统采用密闭处理技术,进行除油、除焦粉,并经冷却后循环使用。分析认为,该系统存在安全、环保运行隐患,水中含油、含焦多,油品损失大,新鲜水耗量大。为此,在原有工艺流程优化操作基础上,通过在冷焦水储罐增设氮气气封,放水线增设排放线和过滤器,沉淀池增设隔油设施,循环水场外排污水回用,富余的切焦水补充冷焦水系统,以及放空冷却塔含油污水回收方式改进等工艺流程技术优化改造,解决了冷切焦水系统长周期安全稳定运行的隐患,实现了该系统新鲜水消耗量为零的目标,每年可节约新鲜水8×104t,缓解了外排污水的环保治理压力,促进了装置节水减排工作。优化后的污油回收方式可实现年回收污油1200t,既改善了冷焦水的水质,又减少了油品损失。针对该系统存在的其他问题,提出进一步完善改进的技术建议。     

Optimal Ground Tube Length for Cooling of Electronics Shelters

This paper presents a theoretical, numerical, and experimental study to investigate the possibility of optimizing the configuration (geometry) of underground heat exchangers for maximum heat transfer. The first part of the study identifies a novel fundamental optimization principle for maximizing heat transfer between a tube and its surroundings, which is expected to be present in any buried tube heat exchanger design. The second part presents a practical application of the fundamental principle: a simplified physical model to determine the temperature field inside an electronics shelter that uses an earth-air heat exchanger and the soil as a heat sink. A volume elements methodology is employed to obtain a system of ordinary differential equations with time as the independent variable that combines principles of classical thermodynamics and heat transfer. This allows the computation of the temperature and relative humidity fields at every instant inside the shelter. The numerical results obtained with the proposed model are validated by means of direct comparison with experimental temperature and relative humidity measurements. It is shown that the tube length can be optimized such that the maximum temperature reached inside the shelter is minimal. The results also demonstrate the potential of the utilization of buried tubes for cooling electronic packages. Since accuracy and low computational time are combined, the model is shown to be efficient and could be used as a tool for simulation, design, and optimization of electronic packages cooled by underground heat exchangers.     

Thermomagnetic Convection in Square and Shallow Enclosures for Electronics Cooling

Combined temperature and magnetic field gradients established in ferrofluids induce thermomagnetic convection. We report a comparative numerical analysis of steady-state convection carried out for a square and a shallow enclosure under zero-gravity conditions. Two symmetrically placed, discrete, constant-flux flush-mounted heaters on the bottom wall of the cavities represent power-dissipating devices in electronics/MEMS applications. The sidewalls serve as heat sinks. A line dipole placed below the bottom wall is the field-source. The role of the magnetic field, the thermal boundary conditions, and the enclosure dimensions in influencing heat transfer are revealed through a comparative thermal-fluidic analysis in different convection-dominated operational regimes.     

冷却塔冷却性能的评价模型

冷却塔是汽轮发电机组重要的冷端设备之一，其运行性能对电站的经济和安全运行有重要的影响。通过建立自然通风冷却塔出水温度和冷却性能的计算模型，为冷却塔的经济运行提供依据，并且还可以为冷却塔的检修和改造提供指导。     

An Investigation of Convective Cooling of an Array of Channel-Mounted Obstacles

A numerical investigation of forced convective cooling of an array of obstacles was performed to synthesize the effects of various pertinent parameters on the cooling performance. Reynolds number, channel clearance height-to-element length ratio, spacing-to-channel height ratio, geometric ratio of the blocks, and total number of obstacles were varied to estimate their influence on the cooling process. Two generalized sets of Nusselt number correlations were developed for the obstacles in the channel based on a very large number of computational simulations. The numerical data match the correlation equations quite well.     

Experimental Study on Thermosyphon for Shipboard High-Power Electronics Cooling System

The closed-loop thermosyphon (CLT) has advantages of simple structure and reliability for transporting heat in long distances with small decrease in temperature. It is considered a promising cooling device for power electronics onboard ships. In this research, CLT for cooling of power electronics onboard ship was developed, and the performance was experimentally examined using a CLT apparatus. The performance was investigated for steady-state heat transfer under a wide range of pressures and heat loads from 18.3 kPa to 35.3 kPa and from 88.9 W to 616.2 W, respectively. The fill charge rates were 27% and 45%. The circulation coolant temperature at the condenser was set to 15°C. The measured data for each rated heat input were registered by a data logger in every 5-s increment of sampling data for a 30-min period. During the steady-state operation, CLT could maintain the system pressure and produced the vapor bulk temperature at around saturation boiling regime. The temperature distributions of the system were measured from each probed thermocouple along the loop. It is understood that higher heat inputs around above 349 W could keep the bulk vapor in an almost constant temperature from evaporation process up to the inlet position of the condenser. The condenser of the direct hull cooling method could also maintain the condensation process with a temperature decrease of around 30°C from the inlet vapor temperature of the condenser. It was clarified that the CLT has good thermal performance in the higher heat loads with low thermal resistance and provides a steady circulation loop from each two-phase process of heating in the evaporator and cooling during condensation.     

Boiling Heat Transfer Enhancement Using Micro-Machined Porous Channels for Electronics Cooling

Boiling heat transfer enhancement for a passive electronics cooling design is presented in this paper. A novel pool boiling enhancement technique is developed and characterized. A combination of surface modification by metallic coating and micro-machined porous channels attached to the modified surface is tested and reported. An experimental rig is set up using a standard BGA package with 12 mm × 12 mm thermal die as a test surface. The limiting heat flux for a horizontally oriented silicon chip with fluorocarbon liquid FC-72 is typically around 15 W/cm². Boiling heat transfer with the designed enhancement techniques is investigated, and the factors influencing the enhancement are analyzed. The metallic coated surface at 10°C wall superheat has a heat flux six times larger than an untreated chip surface. Micro-machined porous channels with different pore sizes and pitches are tested in combination with the metallic coated surface. The boiling heat flux is seven times larger at lower wall superheat compared to the plain chip surface. Maximum critical heat flux (CHF) of 38 W/cm² is obtained with 0.3 mm pore diameter and 1 mm pore pitch. A ratio of pore diameter and pore pitch is found to correlate well with the heat transfer enhancement obtained by experiments. Structures with smaller pore diameter to pitch ratio and larger pore opening are found to have higher heat transfer enhancement in the tested combination.     

Stability,Thermal Conductivity and Photothermal Conversion Performance of Water-Based ZnO Nanofluids

Water-based Zn O nanofluids were prepared via the two-step method, and the influences of dispersant type and mass fraction on the stability and particle size distribution of the nanofluids were investigated. The spectral transmittances of the nanofluids were obtained by a UV-Vis-NIR spectrophotometer. Thermal conductivities of the nanofluids with various mass fractions were measured at different temperatures and a fitted nonlinear correlated equation was established for low-concentration application and compared with existing models. The photothermal conversion performance of the nanofluids was evaluated by theoretical and experimental methods at three optical depths. The results showed that, CTAB as the dispersant provides better physical stability of water-based Zn O nanofluids than SDBS or GA does. Effect of the temperature on the nanofluid thermal conductivity is remarkable with the increase of nanoparticle mass fraction, especially in the range 55°C to 75°C. The maximum thermal conductivity of the studied nanofluids is 0.9488 W/(m·°C) at 75°C, 43.61% higher than that of water. The minimum thermal conductivity of the studied nanofluids is 0.6376 W/(m·°C) at 25°C, 5.16% higher than that of water. The photothermal conversion performance of the nanofluids is quite good with a maximum average absorption efficiency of 0.47, 135% higher than that of water(η=0.2), and the maximum SAR is 527.5 W/g.