Deposition of magnetite particles from flowing suspensions under flow‐boiling and single‐phase forced‐convective heat transfer

The deposition rate of colloidal magnetite particles was measured under both single‐phase forced‐convective and flow‐boiling conditions. All measurements were made at alkaline pH where both the heat transfer surface and the surface of the magnetite particles appear to be negatively charged. For single‐phase forced convection, the deposition rate constant is lower than the mass transfer coefficient for colloidal particles, and the difference is attributed to the force of repulsion between the negatively charged surfaces of the particle and substrate. The deposition rate measured under flow‐boiling conditions is lower than that reported for the deposition of colloidal particles at neutral pH. The difference is, again, attributed to the force of repulsion between the particle and substrate. Particle removal rates were significantly lower than deposition rates; analysis using the theory of turbulent bursts suggests a removal efficiency of only 10^?9% for each turbulent burst. The low removal efficiency is consistent with the particle diameter being significantly smaller than the thickness of the laminar sublayer in these tests.     

Forced convection subcooled boiling of binary mixtures

A mode based on an additive mechanism of heat transfer is proposed for forced convection subcooled boiling of binary mixtures. The contributing modes of heat transfer are: (i) the heat transferred as latent heat by the rising bubbles, (ii) the heat transferred as the heat contained in the superheated thermal layer that is removed from the surface in the wake of the rising bubbles and (iii) the single phase forced convection heat transfer from the heating surface not influenced by the bubbles. Experimental data from the literature on binary systems show good agreement with the model, validating the postulated mechanism.     

单相流体通过多孔金属换热器换热性能的理论分析

泡沫金属具有非常大的比表面积和良好的导热性能,在强迫对流情况下,具有很强的换热能力.通过建立泡沫金属在强迫对流的情况下的换热模型,得到了泡沫金属高度、孔密度、孔隙率和空气流速的变化对其换热性能的影响.分析结果显示:增大泡沫金属换热器的高度、孔密度、空气流速和减小孔隙率,都能提高换热器的换热性能;当这4个参数各自变化到一...     

空气横掠沉没于多孔泡沫金属中错列管束的对流传热

对空气横掠沉没于多孔泡沫金属中正三角错列传热管束的对流传热进行了试验研究。结果显示,多孔泡沫金属使得流体的流动阻力明显增加,但阻力增加的倍数随Reynolds数的增大而减小;多孔泡沫金属对对流传热具有明显的强化作用,强化效果随Re的增大而减小。与横掠错列光管束相比,本试验范围内,流动阻力增加的最大倍数为12.727,最小倍数为10.109;对流传热的最大强化倍数为2.78,最小强化倍数为2.08。总结出了适合于试验范围的对流传热的气流量纲1阻力系数的表达式和传热计算关联式。综合强化对流传热及增加流动压降两方面可知,多孔泡沫金属较适合于流体较小流速的场合。     

汽液固三相流动沸腾传热计算与实验研究

对汽液固三相循环流化床中流动沸腾传热进行了理论分析和实验研究 ,在此基础上结合渐进模型及表面更新机理建立了汽液固三相流动沸腾传热模型 ,模型计算值和实验数据吻合较好 ,最大偏差在 18%以内     

Roughness and constriction effects on heat transfer in crystallization fouling

This contribution addresses apparent negative fouling resistances for crystallization fouling. Two effects contribute to this phenomenon; surface roughness enhances heat transfer in the roughness controlled phase. In the crystal growth phase, surface roughness as well as the constriction of flow cross section due to the fouling layer build-up is taken into consideration. Fouling experiments were carried out in a double pipe heat exchanger with a supersaturated aqueous CaSO₄ solution at a Reynolds number of 17,500 corresponding to a flow velocity of 0.65 m s⁻¹. The measured pressure drop between inlet and outlet allowed the calculation of the integral friction factor for the current surface roughness. With the given friction factor it was possible to estimate the actual heat transfer coefficient of the inner tube. Accounting for the increase in heat transfer caused by surface roughness in the roughness controlled phase, the fouling resistance was recalculated. In the subsequent growth phase flow acceleration due to constriction effects is considered in addition to the roughness effect. Overall the integral fouling resistance and consequently the deposit thickness are underestimated by a factor of up to 2.5 when simply using heat balance. With the proposed approach apparent negative fouling resistances can be eliminated quantitatively.     

垂直多孔表面管内高沸点工质强化流动沸腾换热的数值分析

在对流动特性和换热机理进行分析的基础上,建立了垂直多孔表面管内高沸点工质强化流动沸腾换热的数学模型和数值计算方法.该模型认为环状流区域同时存在强制对流与核态沸腾两种换热方式.液膜厚度、速度与温度等参数通过求解液膜的质量守恒、动量守恒、能量守恒方程获得.核态沸腾换热则包括气泡脱离带走的热量及用于加热气化核心影响区流入液体的热量两部分.提出了流动沸腾情况下的多孔层气泡脱离直径及气化核心密度计算方法.对异丙苯在火焰喷涂型垂直多孔表面管内向上流动时的沸腾换热进行了数值预测,大部分工况的计算值与实验结果符合良好.在低干度区(x=0.1),核态沸腾在总换热中的比例约为50%.随着干度的增加,核态沸腾受到越来越大的抑制,当x为0.5时,这一比例降低到约15％.     

肋化结构对两相受限式阵列射流冷却的影响

为了利用两相沸腾换热提高阵列射流冷却热沉性能,使其可以用于更高热流密度散热场景,基于肋化结构对相变换热的促进作用,本文提出了两种复合不同肋化表面的受限式阵列射流冷却热沉结构,并对优化后的热沉的传热、流动特性展开了相关实验研究。两种肋化表面结构分别为：光滑切割针肋（SL1）、外覆烧结多孔层的粗糙针肋（SL2）,针肋尺寸均为0.6mm×0.6mm×1mm（长×宽×高）,SL2中多孔层颗粒粒径为120~150μm,厚度约为2倍颗粒直径。分布式阵列射流孔板构成5×5的射流单元,每个单元对应4×4针肋阵列,热源总面积30mm×30mm,针肋共计400个。实验使用无水乙醇为工质,得到了热沉在工质流量2.6~12.7mL/s、入口温度283~313K范围时的沸腾曲线和传热曲线。结果表明,两种针肋结构均可以有效实现单相强迫对流换热向两相沸腾换热的转变;增加入口温度或降低工质流量均可以有效地促进相变的发生。在相同工质流量、温度时,SL1较SL2具有更优的单相换热性能,随着加热热流密度的增加,SL2可以在更低壁面过热度下达到沸腾起始点实现过冷沸腾,表现出更佳的传热特性,但SL1可以达到更高的临界热流密度。     

Effect of Surface Treatment on Flow Boiling Heat Transfer Coefficient in CaSO4 Containing Water

This paper reports the influence of heat transfer surface treatment on the formation of calcium sulphate deposit during flow boiling heat transfer. The surface of several test heaters was treated by surface modification techniques, such as dynamic mixing magnetron sputtering [DLC （diamond-like carbon）, DLC-F （diamond-like carbon-fluorine） and AC （amorphous carbon）] and polishing to reduce surface energy. The results showed that heat transfer surface with low surface energy experienced significant reduction of formation of CaSO4 deposit. （1） Magnetron sputtering stainless steel heat transfer surface with DLC, DLC-F and plasma arc sputtering with AC did not change the surface roughness, but they reduced surface energy and improved heat transfer coefficient, so hindered CaSO4 deposit formation significantly. The DLC-F surface performed better than the DLC surface. （2） Surface energy played an important pole in improving heat transfer coefficient. The less the surface energy the more significant the heat transfer coefficient improved with other experimental conditions identical. （3） The polished surface improved the roughness of the heater, but owing to the high surface energy it was not better than the DLC-F surface for a long-term consideration on improving the heat transfer coefficient.     

MOMENTUM INTEGRAL METHOD FOR FORCED CONVECTION IN THERMAL NONEQUILIBRIUM POWER-LAW FLUID-SATURATED POROUS MEDIA

Forced convection heat transfer for power-law fluid flow in porous media was studied analytically. The analytical solutions were obtained based on the Brinkman-extended Darcy model for fluid flow and the two-equation model for forced convection heat transfer. As a closed-form exact velocity profile is unobtainable for the general power-law index, an approximate velocity profile based on the parabolic model is proposed by subscribing to the momentum boundary layer integral method. Heat transfer analysis is based on the two-equation model by considering local thermal nonequilibrium between fluid and solid phases and constant heat flux boundary conditions. The velocity and temperature distributions obtained based on the parabolic model were verified to be reasonably accurate and improvement is justified compared to the linear model. The expression for the overall Nusselt number was derived based on the proposed parabolic model. The effects of the governing parameters of engineering importance such as Darcy number, power-law index, nondimensional interfacial heat transfer coefficient, and effective thermal conductivity ratio on the convective heat transfer characteristics of non-Newtonian fluids in porous media are analyzed and discussed.     

Investigation of wax deposit ‘sloughing’ from paraffinic mixtures in pipe flow

Deposit ‘sloughing’ from ‘waxy’ crude oils has been described in the literature as a possible mechanism, leading to partial or complete dislodging of the deposit from the pipe wall due to changes in flow parameters. A bench‐scale flow loop apparatus was used to investigate ‘sloughing’ with prepared single‐phase ‘waxy’ mixtures of a multicomponent paraffinic wax dissolved in a multicomponent solvent. Experiments were performed to study the changes in the deposit‐layer thickness due to step increments in the ‘waxy’ mixture flow rate, the mixture temperature, and the coolant temperature. It was observed that the deposit‐layer thickness decreased with an increase in each of the three parameters; however, a complete or sudden dislodging of the deposit‐layer did not occur in any of the experiments. A steady‐state heat‐transfer model was used to predict the variation in the deposit mass or thickness due to changes in the selected parameters. In each case, the step‐wise decrease in the deposit thickness, as observed experimentally, was predicted to be caused by changes in the thermal resistance and/or thermal driving force.     

羟基化多壁碳纳米管/R141b纳米流体核沸腾

向多壁碳纳米管引入羟基基团,改善了其在制冷剂R141b中的分散性和稳定性。同时研究了不同质量分数纳米流体热导率、表面颗粒沉积、接触角变化对核沸腾传热性能的影响。结果表明:羟基化碳纳米流体强化沸腾传热,强化率随质量分数的增加而增加,沸腾后期有所下降。在测试浓度范围内,质量分数为0.05%,热通量为87.4 kW·m^-2时,强化率达到最大168%。流体的热导率随着质量分数的增加而增大,质量分数为0.10%时其热导率是纯R141b的1.18倍。分析认为:纳米流体热导率的增加、表面沉积颗粒及纳米颗粒扰动是强化传热的主要影响因素,接触角变化的影响可忽略不计。结论由质量分数为0.03%纳米流体沸腾过程高速成像得到验证。     

AN ANALYSIS OF FLOW FILM BOILING OVER AN INCLINED SURFACE EMBEDDED IN POROUS MEDIA

The rate of heat dissipation during flow film boiling from inclined surfaces embedded in porous media is investigated both theoretically and experimentally. The theoretical model relies on the well established Darcy model for the flow both inside the vapor layer and inside the forced convection layer surrounding the vapor film. The main focus is to study the effects of wall temperature, liquid subcooling, wall suction, and orientation of the heated surface on the heat transfer phenomenon. The resulting similarity equations are integrated numerically by use of the fourth-order Runge Kutta method. Systematic “shooting” is required to satisfy the boundary conditions at the liquid-vapor interface and at infinity. Results are reported for the behavior of the vapor film thickness and both the local and average heat transfer coefficients as a function of vapor superheat and liquid subcooling. Experiments were conducted in Freon-113 and a porous medium consisting of 2.8 mm glass beads to verify the theoretical findings, and excellent agreement was found between theory and experiments.     

磁场调控磁性纳米流体流动和传热研究进展

磁性纳米流体在实现能量高效和可控传递领域极具发展潜力。本文综述了磁场作用下磁性纳米流体对流换热及沸腾换热的最新进展,主要包括强制对流换热、混合对流换热、自然对流换热、池沸腾换热及管内沸腾换热等方面的实验研究,分析了磁场类型、强度、梯度、频率、方向及磁铁位置等对磁性纳米流体流动和热传输特性的影响,指出可通过改变外加磁场来实现对磁性纳米流体流动和传热的控制,并探讨了磁性纳米流体流-磁耦合作用下的传热机理以及目前所面临的挑战。在此基础上,提出了未来磁场调控磁性纳米流体对流换热和沸腾换热的主要发展方向：制备稳定的磁性纳米流体,建立系统有效的流动和传热理论模型,并从微介观尺度诠释热-流-磁耦合协同换热机理。     

Boiling Heat Transfer Enhancement of Water/Salt Mixtures on Roll‐Worked Enhanced Tubes in Compact Staggered Tube Bundles

An experimental study was performed for the boiling heat transfer enhancement of water/salt mixtures on both plane and roll‐worked enhanced tubes in compact staggered tube‐bundle evaporators under atmospheric and increased pressure conditions. The effects of tube spacing, position of tubes, test pressure and salt‐water concentration on the boiling heat transfer characteristics in restricted spaces of compact tube bundles consisting of plane and roll‐worked tubes were investigated. The experimental results indicate that the single roll‐worked tubes in a bulk liquid have a greater boiling heat transfer promotion than the single plane tubes. For the plane tubes in compact tube bundles, the effect of tube spacing on the boiling heat transfer is very significant. The boiling heat transfer has a maximum enhancement when the optimum tube spacing is selected. For the roll‐worked tubes in compact bundles, the effect of tube spacing on the boiling heat transfer is also significant as the tube spacing is small. The boiling heat transfer still has a maximum value and a compound enhancement effect of the boiling heat transfer from both the optimum tube spacing and the surface treatment is observed for the enhanced tube bundles.     

Boundary-layer flow and heat transfer in porous beds

This paper describes the transport phenomenon for boundary-layer flow and heat transfer in packed beds. It focuses on both natural and forced convection studies, and also emphasizes boundary-layer applications for chemical reactors. Transport in porous beds often involves non-Darcian flow effects such as the solid-boundary resistance, high flow rate inertial losses, near-wall porosity variation, and thermal anomalies such as dispersion or inter-pore mixing. These effects significantly alter velocity and temperature profiles in boundary-layer and confined flows. This work discusses these effects and demonstrates the variation in heat transfer predictions found when using theoretical models which include these non-Darcian effects.     

Thermal Diffusion and MHD Effects on Combined Free‐Forced Convection and Mass Transfer of a Viscous Fluid Flow Through a Porous Medium with Heat Generation

The problem of thermal diffusion and magnetic field effects on combined free‐forced convection and mass transfer flow past a vertical porous flat plate, in the presence of heat generation is studied numerically. The governing momentum, energy and concentration equations are converted into a system of nonlinear ordinary differential equations by means of similarity transformations. The resulting system of coupled nonlinear ordinary differential equations is solved numerically by using the Shooting method. Numerical results are presented for velocity, temperature and concentration profiles within the boundary layer for different parameters of the problem including suction parameter, heat generation parameter, Soret number, Dufour number, magnetic parameter, etc. In addition, the effects of the pertinent parameters on the skin friction and the rates of heat and mass transfer are discussed numerically and illustrated graphically.     

复合粉末多孔表面管的沸腾传热

采用管内去离子水加热管外丙酮沸腾的方法对复合粉末多孔表面管的沸腾传热特性进行了实验研究,获得了沸腾传热系数和K值随温度和几何参数的变化情况,并与国内几种多孔表面管的实验结果进行了比较,同时对多孔管的抗垢性能进行了分析.实验结果表明,多孔层孔隙率对沸腾传热系数的影响最大,多孔管的沸腾传热系数是同类型的光滑管的14倍,多孔管具有一定的抗垢性能.所得结果为该种类型多孔管的工业化应用提供了科学依据.     

水平细圆管内非共沸混合工质的流动沸腾

以非共沸混合工质R32/R134a为实验工质,进行了水平细圆管内流动沸腾换热实验.在获取大量实验数据的基础上,分析了质量干度、热通量和质量通量密度对沸腾换热的影响,讨论了各种工况下的换热机理,比较分析了细圆管和常规管道内流动沸腾换热性能.实验结果表明：在本实验范围内,水平细圆管内流动沸腾换热主要受热通量的影响,绝大部分实验工况下核态沸腾占主导地位.尺度效应是引起微细通道内流动沸腾换热特性不同于常规管道的主要原因.     

水泥窑窑体表面换热系数的计算方法

提出了一种计算水泥窑窑体面换热系数的新方法。这种方法充分考虑了强制对流，自然对流，热辐射和窑体大小对换热系数的影响。推导了窑体表面强制对流与自然对流的换热关系式，通过互相比较得到了可忽略强制对流域自然对流的条件。结果表明，大多数情况下窑体表面的强制流换热数量极相等。因此利用向量总和法建立了混合对流换热关系式。进一步的研究发现，辐射散热所占的比例很大，采用黑度小的材料可显著降低熟料热耗。该方法...