Validated Numerical Analysis of CaSO4 Fouling

In scaling experiments, the formation of fouling layers on heat transfer surfaces usually proceeds in a non-uniform manner. The result is a non-uniform layer, and hence a varying thermal resistance over the area covered with scale. Consequently, a non-uniform heat flux distribution results over the heat transfer surface. To evaluate the changes in the heat flow distribution resulting from a non-uniform scale layer, numerical calculations have been performed using a case where CaSO₄ scales form on a heated copper plate subjected to a shear flow. The calculated heat flux is used to calculate fouling resistances from measured temperatures. The results of the numerical calculations confirm that a non-uniform heat flux distribution occurs over the surface when the plate is partially covered with scale. Further, it is seen that the heat flux, the surface temperature, and the driving force all decrease with increase in scale accumulation.     

两种柱形涡流发生器CaSO_4析晶污垢特性实验研究

为得到涡流发生器污垢规律,采用质量浓度为2 100mg/L的硫酸钙过饱和溶液进行了传热过程中的污垢生成实验,并通过离线称重法得到了装有涡流发生器试片表面单位面积污垢沉积量生长曲线.结果表明：涡流发生器直径和排列间距对壁面污垢沉积量有显著影响.当涡流发生器直径（4mm）一定时,试片表面单位面积污垢沉积量均随着涡流发生器排列间距的增大而增加;在排列间距（10mm）一定时,污垢沉积量随着涡流发生器尺寸的增大而减少.     

Characterization of Pool Boiling of Seawater and Regulation of Crystallization Fouling by Physical Aberration

ABSTRACT

As fresh water becomes increasingly scarce desalination has become an important technique to meet the portable water requirements around the world. Thermal distillation continues to be one of the most important and widely used methods of desalination currently used. Scale formation, corrosion of the heater surface, and the subsequent degradation of the heat exchanger is one of the biggest challenges in thermal desalination. In this paper, pool boiling of seawater is characterized using standard artificial sea water. Various boiling characteristics such as critical heat flux and heat transfer coefficient are analyzed. The nature of the scales formed on the heater surface and their effect on the heat transfer efficiency are studied. A passive method to reduce the thermal resistance due to scale buildup from crystallization fouling using stainless steel beads is examined.     

Investigating the Impact of Boiling Conditions on the Fouling of a Crude Oil

ABSTRACT

The impacts of nucleate boiling and pressure on crude oil fouling are factors that have not been heavily investigated in previous research. Variables such as wall temperature and fluid velocity/shear are often a main focus, as they are key variables for predictive fouling models, which provide insight to fouling thresholds. Research detailed in this report shows that nucleate boiling and pressure greatly impact the measured fouling rate of a crude oil tested using the Heat Transfer Research, Inc., rotating fouling unit. When nucleate boiling is occurring, the use of fouling resistance plots to measure fouling rates is not a reliable method due to the impact boiling has on the heat transfer coefficient. Visual inspection of fouling deposits to validate fouling resistance data has also been found to be critical. Images of fouling deposits are included. Fouling under nonboiling conditions was shown to increase with increasing pressure.     

Bubble Dynamics and Heat Transfer During Pool Boiling on Wettability Patterned Surfaces

Surfaces with spatial wettability patterns have been proven to enhance heat transfer coefficient and critical heat flux in pool boiling. To understand the physical mechanism behind this phenomenon and obtain the correlation among some critical parameters (bubble departure frequency, bubble size, nucleation site density, surface tension), pool boiling experiments were conducted. A Pyrex glass with a layer of indium-tin-oxide was used as the substrate. Hydrophobic patterns will serve as nucleation sites. Experiments were conducted in deionized water under atmospheric pressure at a relatively low heat flux. The processes of nucleation, growth, and departure of individual bubbles were visualized by using a high speed camera through the bottom of the heater surface. It has been found that the patterned surface performed the best in heat transfer for subcooled pool boiling when compared with hydrophilic and hydrophobic surfaces. The nucleation site density of the biphilic surface was much higher, when compared with that of the homogeneous surface. The individual bubbles always nucleate on the edge of the hydrophobic and hydrophilic area, and then move onto the hydrophobic pattern. Most of the individual bubbles detach from the wettability patterned surface in the diameter range from 300 µm to 450 µm (around 77.3%). The bubble departure periods scatter in the range from 80 ms to 1500 ms.     

Thermal Fouling of Heat Exchanger Tubes due to Heavy Hydrocarbon Droplets Impingement

ABSTRACT

This work discusses fouling in the vapor–steam mixture overheater in the convection section of an industrial steam cracker due to the thermal degradation of heavy hydrocarbon droplets deposited on the tube wall. A spray of heavy hydrocarbon multicomponent droplets is injected in a tube of the vapor–steam mixture overheater and the path of the droplets through the tube is followed by an Eulerian–Lagrangian computational fluid dynamics simulation. To study tube fouling, the droplet impingement behavior on the wall, the evaporation of the deposited liquid, and a coking model describing thermal coke formation due to degradation of heavy hydrocarbons are required. To describe the droplet impingement behavior, a regime map for single component millimeter-sized droplets is taken from the literature. Two simulations are performed to study fouling problems in a vapor-mixture overheater tube. Simulation results are found to be grid sensitive. By analyzing and comparing simulation results it is concluded that reliable fouling data require a regime map for the impingement of multicomponent heavy hydrocarbon micron-sized droplets.     

Nucleate Pool Boiling of Pure Liquids and Binary Mixtures：PartI－Analytical Model for Boiling Heat Transfer of Pure Liquids on Smooth Tubes

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Influence of Thermophysical Wall Properties During Pool Boiling on Large Diamond and SiC Heaters

The results of pool boiling experiments with synthetic diamond and silicon carbide (SiC) heaters are presented for water as the boiling liquid. The diamond and SiC heaters varied considerably in thermal conductivity, but they had smooth, nearly identical surfaces, which was also the case in regard to their contact angles for water. Temperature sensors and electric heating wires were directly vapor-deposited underneath the surfaces. The experiments were carried out with comparable large heaters (15 mm × 15 mm) for pure water under atmospheric pressure (1 bar) in a pool boiling cell. The heat transfer characteristics including the corresponding boiling curves were obtained. In prior work, it was found that the influence of the thermophysical wall properties might be substantial in the case of special heater geometries, leading to trapped bubbles, but no significant differences between both materials were observed in the case of conventional heater configurations.     

Investigation on Active Thermal Control Method with Pool Boiling Heat Transfer at Low Pressure

In order to maintain a desirable temperature level of electronic equipment at low pressure, the thermal control performance with pool boiling heat transfer of water was examined based on experimental measurement. The total setup was designed and performed to accomplish the experiment with the pressure range from 4.5 kPa to 20 kPa and the heat flux between 6 kW/m~2 and 20 kW/m~2. The chosen material of the heat surface was aluminium alloy and the test cavity had the capability of varying the direction for the heat surface from vertical to horizontal directions. Through this study, the steady and transient temperature of the heat surface at different pressures and directions were obtained. Although the temperature non-uniformity of the heat surface from the centre to the edge could reach 10℃ for the aluminium alloy due to the varying pressures, the whole temperature results successfully satisfied with the thermal control requirements for electronic equipment, and the temperature control effect of the vertically oriented direction was better than that of the horizontally oriented direction. Moreover, the behaviour of bubbles generating and detaching from the heat surface was recorded by a high-resolution camera, so as to understand the pool boiling heat transfer mechanism at low-load heat flux. These pictures showed that the bubbles departure diameter becomes larger, and departure frequency was slower at low pressure, in contrast to 1.0 atm.     

Two-Phase Flow Characteristics During Convective Boiling of Halocarbon Refrigerants Inside Horizontal Small-Diameter Tubes

Two-phase flow observations were performed for R134a and R245fa in 1.1- and 2.32-mm ID horizontal tubes. The tests were run for mass velocities ranging from 100 to 600 kg/m²-s and saturation temperatures of 22, 31, and 41°C. Additionally, an objective method to characterize two-phase flow patterns was developed. This method is based on simultaneous processing of signals from the following devices: a pair of diodes laser-sensors with a transparent tube between them within which the two-phase flow occurs, a micro-piezoelectric pressure transducer to determine the variation in the local pressure, and a microthermocouple within the fluid. The method was developed based on the k-means clustering algorithm, which consists of the gradual agglomeration of data of similar average characteristics. Simultaneous images of two-phase flow were obtained through a high-speed camera (10,000 frames/s) and used to identify the following flow patterns: bubbly, elongated bubbles, churn, and annular flows. The maps obtained by the objective method were compared against flow pattern results segregated based on flow visualization and a reasonable agreement was obtained between them. The vapor quality for the transition between churn and annular flow pattern decreases with decreasing the tube diameter, whereas the vapor quality for the transition between elongated bubbles and churn flow decreases with increasing tube diameter. Effects of saturation temperature and mass velocity were also verified. Additionally, elongated bubble velocities, frequencies, and lengths were determined based on the analysis of high-speed videos and the processing of signals of the diode/laser-sensor. The elongated bubble velocity was correlated as a linear function of the two-phase superficial velocity. A new image treatment method was developed to automatically identify the entrainment frequencies, which were segregated according to two groups: low and high frequency. The former group was characterized by frequencies lower than 20 Hz and the later by 50–500 Hz frequency ranges.     

Fundamental Issues of Critical Heat Flux Phenomena During Flow Boiling in Microscale-Channels and Nucleate Pool Boiling in Confined Spaces

This article presents a comprehensive literature review on the fundamental issues of critical heat flux (CHF) during flow boiling and nucleate pool boiling in microscale channels and confined spaces. First, distinction between macro- and micro-scale channels is discussed. Then the CHF mechanisms are discussed. Next, experimental and theoretical studies of subcooled flow boiling CHF in microscale channels together with the prediction methods are reviewed and analyzed. Following this, experimental and theoretical studies on saturated flow boiling CHF together with the prediction methods are summarized and discussed. Furthermore, experimental and theoretical studies on nucleate pool boiling CHF in confined spaces together with the relevant prediction methods are reviewed as well. So far, limited studies on CHF microscale channels and confined spaces are available in the literature. There are numerous discrepancies in the existing studies on CHF results, mechanisms, and prediction methods. Furthermore, there are no generalized prediction methods for CHF in microscale channels and confined spaces. According to this review, future research needs for the experiments, mechanisms, and prediction methods of CHF phenomena in microscale channels and confined spaces have been addressed.     

Initiation of CASO4 Scale Formation on Heat Transfer Surfaces under Pool Boiling Conditions

The success of innovative fouling mitigation techniques such as ion implantation depends upon the early stage of scale formation on the heat transfer surface. This is because the first crystalline nuclei that appear on the surface during the initial period dictate how fouling would develop in latter stages. In this study, the initial period of deposition of calcium sulfate on heat transfer surfaces has been investigated under pool boiling conditions. The independent variables were heat flux and calcium sulfate concentration. The experimental results show that the time until the heat transfer coefficient reaches its intermediate maximum decreases with an increasing concentration and heat flux, and is also significantly affected by the surface finish. Neural network architectures were utilized to correlate the experimental results during the initial deposition period. A satisfactory agreement between predicted and measured heat transfer coefficients has been achieved with an average error of 8.7%.     

Effect of Nanostructured Microporous Surfaces on Pool Boiling Augmentation

ABSTRACT

Nanostructured microporous surfaces were electrodeposited at various electrolyte temperatures on copper substrate to investigate the saturated pool boiling enhancement of distilled water at atmospheric pressure. Surface structure topography and wickability were analyzed to investigate their relation to critical heat flux. Scanning electron microscope showed that the micro-clusters have nanostructures from cubic at 5°C to dendritic at 60°C electrolyte temperature. Rate-of-rise experiments demonstrated that dendritic copper structure has the best capillary performance. The experimental results of pool boiling heat transfer indicate that the critical heat flux increased with surface wickability. Electrodeposited porous surface in hot electrolyte showed the highest critical heat flux and heat transfer coefficient of the 124 W/cm² and 17 W/cm²K, respectively, which is 50% and 270% higher than that of plain surface. However, the two-step electrodeposition and annealing were used in fabrication of surfaces, but the mechanical strength of layer needs more improvement by changing the electrochemical process parameters.     

Pool Boiling of Distilled Water on PTFE-Coated Heating Tube

The article describes results of investigations of heat transfer and pressure drop during saturated bubbly boiling of new environment-friendly refrigerating media. Taking advantage of the experimental results, an analytical model of heat transfer and pressure drop is worked out. An essential feature of the model is that it accounts for components of the heat flux density in the boundary sublayer due to convection, interaction of heat sinks, and turbulence effects. The comparison of computational results and experimental data, from the present study and from other investigations, shows that the proposed method of determination of heat transfer coefficients and pressure drop can be applied to a number of refrigerating media with a wide range of heat and flow parameters, assuring accuracy to - 20%. Therefore, the method can be used in design of heat exchangers for refrigeration units as well as complete refrigeration units.     

沸腾表面凹坑的尺度分布特征

在沸腾表面上储气(或汽)凹坑是形成活化核心的关键因素。文中从类似Sierpinski地毯的经典分形表面出发,成功构造了凹坑以不同尺度分布的表面。研究结果表明凹坑的尺度分布雄数与表面形貌的分形维数是两个含义不同的概念,机械表面凹坑(或凸点)的尺度分布特征符合分形理论。分析活化核心的测量结果,表明活化核心的尺度分布同表面上凹坑的尺度分布一样具有分形特征。     

The Effect of Novel Plasma-Coated Compact Tube Bundles on Pool Boiling

Boiling heat transfer of R-134a on a porous, plasma-coated tube bundle was investigated experimentally to determine the effects of the number of tube rows and the total tube number. The bundle consists of up to four tubes with a pitch-to-diameter-ratio of 1.33. Heat transfer coefficients for a single tube with a porous copper coating were up to four times higher than for a smooth tube. Observations showed that the plasma coating enhanced the heat transfer coefficient in tube bundles as well. The bundle factor of the coated tube bundle showed a similar, slightly increased trend as the smooth tube bundles. The enhancement effect of the coated tubes decreases to a certain extent with an increasing heat flux and decreasing saturation temperature. However, it is significantly less pronounced than trends that have been reported from other investigations. The aim of a stable enhanced coating was confirmed by long-term experiments with steady results.     

不凝结气体对多壁碳纳米管换热表面的池沸腾性能的影响

为揭示不凝结气体对多壁碳纳米管（Multi-walled Carbon Nanotube, MWCNT）纳米结构表面核态池沸腾过程的影响,使用气体沉积法（Chemical Vapor Deposition, CVD）在硅表面制作MWCNT纳米结构表面,并使用光滑硅表面进行对比实验研究。实验操作中,将驱气前后的工作液体应用于两种表面的池沸腾实验,换热表面过热度控制在0℃-35℃,工作液体过冷度分为40℃和50℃。实验结果表明,液体中含气量的变化对MWCNT纳米结构表面影响较小,而对光滑硅表面的影响较大;对比硅表面,MWCNT纳米结构表面能够有效提升沸腾传热效果,对于驱气后的工作液体提升效果更为明显。     

石墨烯纳米制冷剂核态池沸腾换热的实验研究

为分析单层石墨烯纳米片对核态池沸腾换热的影响机理,对基液为R141b、分散相为单层石墨烯纳米片的纳米制冷剂的核态池沸腾换热特征进行了测定,采用Hot Disk热物性分析仪和铂金板法分别测定了石墨烯纳米制冷剂的热导率和表面张力,采用接触角测量仪和扫描电子显微镜（SEM）观测了沸腾后加热表面的润湿性和形貌特征。实验中,单层石墨烯纳米片的质量百分含量（ω）为0.02%~0.50%,实验压力为一个标准大气压,热流密度为20~200 kW/m2。实验结果表明：单层石墨烯纳米片的加入,使制冷剂R141b的核态池沸腾换热得到强化;当ω=0.2%时,换热系数提高比例出现峰值,为57.7%。伴随ω的增加,石墨烯纳米制冷剂的热导率增大、表面张力减小,沸腾表面润湿性增强且微腔数先增后减,综合作用的结果导致存在一个最佳的单层石墨烯纳米片浓度（即ω=0.2%）使换热系数最高。     

Effects of Acidity and Method of Preparation on Nucleate Pool Boiling of Nanofluids

Past research has shown contradicting trends in the rate of heat transfer during pool boiling of nanofluids, which could be attributed either to their stability or to their method of preparation or to both. An experimental study has been conducted to investigate the effects of electrostatic stabilization and preparation method of nanofluids on their pool boiling rate of heat transfer. Nanofluids made from water and alumina nanoparticles at 0.1 vol% concentration were used. The effect of electrostatic stabilization was investigated by changing the pH value from 6.5, neutral, to 5, acidic. The effect of preparation method has been investigated by using nanofluids prepared from dry particles and from ready-made suspensions. Compared with water, all nanofluids investigated resulted in deterioration in the rate of heat transfer during pool boiling. Neutral nanofluids made from ready-made suspensions and from dry particles resulted into almost the same deterioration in the rate of heat transfer of 49% and 45%, respectively, with respect to that of pure water. The most significant effect of electrostatic stabilization was found in the case of acidic nanofluids made from dry particles, which resulted in deterioration in the rate of heat transfer of 31%. However, acidic nanofluids made from ready-made suspensions resulted in a deterioration of 46%, which is almost the same as that of suspension-made and dry particles-made nanofluids. These results indicate that electrostatic stabilization using acid addition is most effective with nanofluids made from dry particles.     

Experimental Study on Pool Boiling Heat Transfer for R22, R407c,and R410a on a Horizontal Tube Bundle With Enhanced Tubes

An experimental test rig for study of the pooling-boiling heat transfer performance of pure and mixed refrigerants was designed and established. The test section is a horizontal tube bundle evaporator with nine mechanically fabricated porous surface tubes in a triangular layout. With this test system, the heat transfer coefficients of the nucleate boiling in the evaporator were measured for R22, R407c, and R410a. Extensive experimental measures were made for those pure and mixed refrigerants at different heat fluxes from 10 kW m^?2 to 43 kW m^?2 at saturation temperature of 9°C. Comprehensive measured data are presented in this paper. From experimental results, it is found that the pool boiling heat transfer coefficient increases with increasing the heat flux. It is also found that boiling heat transfer coefficients for R410a are 1.25–1.81 times and 6.33–7.02 times higher than that for R22 and R407c, respectively. The experimental correlations for the pool boiling heat transfer coefficients of R22, R407c, and R410a on the present enhanced tubes bundle are developed. The thermal resistance analysis reveals that the thermal resistance of the water side is a controlling factor for the evaporator for R22 and R410a. However, for R407c, the thermal resistance of the refrigerant side is slightly higher than that of the water side. To further improve the overall heat transfer coefficient in the evaporator of R22 and R410a, the enhancement for both the inside and outside is equally important, and the effectively enhanced boiling surface must be developed for the evaporator of R407c.