Shear rate dependence of thermal conductivity and its effect on heat transfer in a Non-Newtonian flow system

The purpose of this research was to investigate the extent to which the thermal conductivity of non-Newtonian fluids is affected by fluid motion, and then the effect of this shear-rate-dependent thermal conductivity, measured in Lee [1995], on the heat transfer for a typical convective system. Such information would have important implications in the design and analysis of non-Newtonian thermal systems such as are found in food processing operations, polymer processing, paint manufacturing, biological systems and many others. A simple parallel plate flow model with temperature-independent properties gave increases in heat transfer on the order of 30–80 % compared to the heat transfer with shear-rate-independent thermal conductivity in Newtonian fluid flow over the entire temperature range (20–50 °C) of CMC solutions depending on the inlet average velocity due to the effect of the shear-rate-dependent thermal conductivity.     

Effects of colloidal properties on sensible heat transfer in water-based titania nanofluids

Nanofluids have attracted considerable attention in recent years as effective working fluids in heat transfer applications. Nanofluids are essentially suspensions of nanoparticles in a base fluid and exhibit higher thermal conductivity than conventional heat transfer fluids.     

Heat transfer in lid driven channels with power law fluids in a hydrodynamic fully developed flow field

We present a finite element numerical study of heat transfer in lid driven channels with fully developed axial flow for non-Newtonian power law fluids. The effect of channel aspect ratio and material properties on temperature distribution and wall heat transfer are studied. The results show that in comparison with Newtonian fluids the shear thinning property of the fluids acts to reduce the local viscous dissipative heating and as a result the axial local fluid temperature is reduced. Applications of the results to scraped-surface heat exchanger design and operation are recommended.     

Al2O3-based nanofluids: a review

Ultrahigh performance cooling is one of the important needs of many industries. However, low thermal conductivity is a primary limitation in developing energy-efficient heat transfer fluids that are required for cooling purposes. Nanofluids are engineered by suspending nanoparticles with average sizes below 100 nm in heat transfer fluids such as water, oil, diesel, ethylene glycol, etc. Innovative heat transfer fluids are produced by suspending metallic or nonmetallic nanometer-sized solid particles. Experiments have shown that nanofluids have substantial higher thermal conductivities compared to the base fluids. These suspended nanoparticles can change the transport and thermal properties of the base fluid. As can be seen from the literature, extensive research has been carried out in alumina-water and CuO-water systems besides few reports in Cu-water-, TiO₂-, zirconia-, diamond-, SiC-, Fe₃O₄-, Ag-, Au-, and CNT-based systems. The aim of this review is to summarize recent developments in research on the stability of nanofluids, enhancement of thermal conductivities, viscosity, and heat transfer characteristics of alumina (Al₂O₃)-based nanofluids. The Al₂O₃ nanoparticles varied in the range of 13 to 302 nm to prepare nanofluids, and the observed enhancement in the thermal conductivity is 2% to 36%.     

相变微粒乳液自然对流强化传热与稳定性研究进展

人工合成的潜热型功能性热流体同时具有流动性与高的蓄热或蓄冷能力,在自然对流的流动与传热中表现有复杂的非线性传热和流动现象。本文总结了该类流体在自然对流传热方面的最新研究进展,包括：自然对流的强化传热及其稳定性,乳液中微粒的粒径及其乳液热物性的影响,对该方面将来的发展趋势进行了展望。     

板翅式换热器翅片通道中流体流动与传热的计算流体力学模拟

针对平直形和锯齿形两种不同翅片类型,利用计算流体动力学(CFD)软件FLUENT对板翅式换热器的微小通道进行了模拟,得出了通道中流体的流动与传热特性.计算结果表明,在相同情况下,平直形和锯齿形翅片中冷热流体的局部传热系数的最大值都出现在入口处;在锯齿形翅片的相邻2个锯齿的交错面上,流体的局部传热系数和压力存在突变,流体的边界层厚度要薄于在平直形翅片中的厚度,流体的局部换热系数要和压损大于其在平直形翅片中的值.     

水/FC-72纳米乳液振荡热管传热特性研究

通过超声方法将水和FC-72乳化制备得到了不同体积浓度的水/FC-72纳米乳液,对以水/FC-72纳米乳液为工质的振荡热管开展了传热特性的实验研究。结果表明,使用纳米乳液后振荡热管冷热段温差明显下降,传热性能得到增强,且存在与充液率有关的最佳水相体积浓度,30%、50%和70%充液率下的浓度最佳值分别为11%、9%和9%。与使用纯FC-72时相比,充注最佳水相体积浓度纳米乳液的振荡热管在充液率为30%、50%和70%时冷热段温差最大可分别下降约3.3℃(或18.7%)、3.1℃(或13.7%)和4.3℃(或23.5%)。分析发现,纳米乳液的有效热导率和比热容均随着纳米水滴体积浓度的增加而明显增大,这可能是引起振荡热管传热能力增强的主要原因。     

High temperature heat transfer equipment

Various alternative methods of high temperature heat transfer in the range of 300 to 750 F, such as direct firing, forced circulation of heat transfer fluids such as water, oil, Therminol, Dowtherm and other fluids are outlined. Advantages and disadvantages of each are discussed. Important considerations include design of heaters, temperature uniformity, heat transfer rates, safety precautions, hardware required, control sequences, fluid degradation and velocities. Presented at the AOCS Meeting, New Orleans, April 1970.     

基于理论的有机工质管壳式冷凝器构形优化

建立考虑传热耗散率和总泵功率的线性加权复合函数,在总冷凝率和总传热面积一定的条件下以复合函数最小为目标,对有机工质的管壳式冷凝器进行构形优化,得到最小复合函数和冷凝管最佳外径。结果表明：冷凝器最优构形与初始设计结构相比,总耗散率提高了10.70%,而总泵功率和复合函数分别降低了54.94%和6.46%。这说明复合函数牺牲了一定的传热性能,使得冷凝器流动性能显著提高,最终使得其综合性能得到提高。选择合适的冷凝管数目可实现复合函数二次最小化。将理论应用到有机工质管壳式冷凝器的构形优化中,为其结构优化设计提供了新的指导,这一方法可进一步推广到有机工质循环系统的优化设计中。     

Research progress of low GWP working fluid HFO-1234ze(Z) for high temperature heat pumps

High temperature heat pump can effectively recover industrial waste heat to achieve the purpose of energy saving,emission reduction and environmental protection. At present, novel working fluids for the high temperature heat pumps are studied to replace CFC-114 and HFC-245fa. The alternative working fluid should have low global warming potential (GWP) and good working performance. The eco-friendly working fluid HFO-1234ze(Z), whose GWP < 1 and critical temperature above 423 K, is considered as a potential heat pump replacement working fluid. This paper summarizes the research of HFO-1234ze(Z) including the synthesis technology, thermophysical properties, transport properties, and heat transfer performance. The feasibility of application of HFO-1234ze(Z) in high temperature heat pump system is analyzed. It is considered that HFO-1234ze(Z) has good performance and development prospect in high temperature heat pump.     

新型低GWP高温热泵工质HFO-1234ze(Z)的研究进展

高温热泵可以有效回收工业余热,达到节能减排和保护环境的目的。目前,有关高温热泵技术的研究热点在于寻找一种全球变暖潜能值(GWP)低、使用性能良好的工质,以替代现有CFC-114、HFC-245fa工质。对新型环境友好型工质HFO-1234ze(Z)进行了综述,其GWP<1,临界温度高于423 K,是一种潜在的高温热泵替代工质。总结了近年来国内外学者对HFO-1234ze(Z)的合成技术、热力学性质、输运性质、传热性能等方面的研究,并分析了HFO-1234ze(Z)在高温热泵系统中应用的可行性,认为HFO-1234ze(Z)在高温热泵中具有较好的工作性能和发展前景。     

Natural convection on a vertical plate with variable heat flux in supercritical fluids

The present article investigates laminar free convection with uniform or non-uniform prescribed surface heat flux over a vertical flat plate in supercritical fluid, numerically. A new equation for thermal expansion coefficient in a supercritical fluid is derived based on Redlich–Kwong equation of state as a function of pressure, temperature and the compressibility factor. Calculated values of thermal expansion coefficient have been compared with the experimental results which show better accuracy in comparison with van der Waals ones. The governing systems of partial differential equations were solved numerically using the finite difference method. The local Nusselt number was calculated and plotted as a function of the local Rayleigh number. Using supercritical fluids in constant heat flux free convection, decrease wall temperature in comparison with under critical fluids. It was observed that positive and negative slopes of surface heat flux distribution increases and decreases the heat transfer coefficient, respectively.     

Constructal optimization for an organic fluid shell-and-tube condenser based on entransy theory

A complex function composed of linear weighted sum of the entransy dissipation rate (EDR) caused by heat transfer and total pumping power is established in this paper. Under the conditions of fixed total condensation rate and heat transfer area, constructal optimization of a shell-and-tube condenser (STC) with organic fluid is carried out by taking minimum complex function as optimization objective. The minimum complex function and the optimal external diameter of the condensation tube are obtained. The results show that compared with the initial design construct, the optimal construct of the STC increases the total EDR caused by heat transfer by 10.70%, and reduces the total pumping power and complex function by 54.94% and 6.46%, respectively. This illustrates that the complex function sacrifices a certain heat transfer performance, and improves the fluid flow performance evidently, which leads to the overall performance improvement of the STC. The twice minimization of the complex function can be realized by choosing an appropriate number of the condensation tubes. New guidelines for the optimal structure designs of the STCs are provided by introducing entransy theory into constructal optimizations of the organic fluid STCs, and this method can be further extended to the optimal designs of the cycle systems with organic fluids.     

Pseudocritical heat transfer inside vertical tubes

Heat transfer to fluids with pressures slightly above the critical is strongly influenced by the variation of the fluid properties occurring in the so-called pseudocritical region. The heat transfer can either be improved or reduced, depending on the flow conditions. The improvement is due to the enhanced heat transport caused by the increased heat capacity of the fluid. In the case of reduced heat transfer the turbulence is weakened by gravitational effects. In the present paper the heat transfer due to the complex interaction between gravity and turbulence in the so-called pseudocritical region is predicted by a numerical solution of the conservation equations for a turbulent pipe flow. The numerical results are compared with published experimental data for an electrically heated vertical pipe with the fluid R12.     

非共沸工质换热匹配特性影响热泵性能的高级分析

针对非共沸工质非线性复杂相变传热过程,基于高级分析方法,推导出表征热泵系统性能的评估指标模型——温度匹配度（TMD）,探讨了非共沸工质与换热流体之间的换热匹配特性,实验验证了模型的准确性与适用性。选用了三组不同温度滑移程度的非共沸工质（M1、M2、M3）为对象,探究了TMD与换热夹点、系统COP、效率η以及换热器实际损占比ε之间的关系。结果表明：TMD越小,换热流体间的温度匹配越好,系统COP、效率越大,换热器实际损占比越小,反之亦然;且当TMD最小时换热器内夹点总是出现在饱和气态点处。     

Detailed Thermal Evaluation of Brazed Plate Heat Exchanger Using Infrared Thermography

Infrared thermography of brazed plate heat exchangers for evaporators and condensers recently was used to quantify maldistribution. With the knowledge of the secondary fluids heat transfer coefficient and a given secondary fluids distribution, the local heat transfer coefficient of the primary fluid (refrigerant) and its distribution to the channels can be calculated. A sensitivity analysis shows ± 10 % difference of the surface temperature to the wall center temperature at the expected ratios of heat transfer coefficients. The method is presented in this paper using an exemplary infrared picture.     

纳米技术在强化传热中应用的研究进展

采用纳米技术,通过改变传热流体的物性和流动特征以及传热表面的特性,能有效地强化传热。本文综述了纳米流体、纳米胶囊粒子潜热型热流体以及纳米涂层表面在强化传热中应用的研究进展,分析了各种纳米技术的强化传热机理,指出了现有研究工作存在的问题和今后研究的方向。     

过冷中间流体气化器换热过程实验

中间流体气化器(IFV)是液化天然气浮式储存与再气化装置中关键设备之一。IFV较多采用丙烷作为中间流体,利用其蒸发、冷凝相变过程将LNG气化,而过冷中间流体换热流程可以减小气化器体积。通过对过冷丙烷换热流程进行实验,充分结合实际FSRU海平面运行背景,验证中间流体丙烷在水平面和倾斜角度下的换热特性及对整个换热过程的影响因素。实验发现海平面的略微晃动对其换热有一定的增强;当海水温度随四季发生改变升高时,中间流体提供给LNG的气化换热量也会增大,海水进出口温度差也会增大;当海水流量变大时,整个过程的换热强度增大。     

非牛顿流体中的气泡行为

非牛顿流体广泛存在于各学科领域（如化工、食品和生物医学等）,也与许多工业过程及人类的生命过程密切相关。非牛顿流体中的气泡行为直接影响流体传质、传热及化学过程的快慢。因此,了解和研究非牛顿流体中的气泡行为具有重要意义。本文分别从气泡生成、聚并和破裂3个方面对非牛顿流体中气泡行为的研究进展进行评述。     

板式换热器在高粘度流体强化传热的工程研究

通过对高粘度流体强化传热的工程研究,确定传热系数与流量、流体阻力与流速的关联式,实验证明采用板式换热器代替反应釜夹套撤热是行之有效的,高效换热器在高粘度流体的工程应用上可达到强化换热的效果。