Hydrodynamics and heat transfer characteristics of passive decay heat removal systems: CFD simulations and experimental measurements

In nuclear safety operations, condensing heat transfer area is placed in a large pool of liquid to accommodate passive thermal decay. Such systems are subject to pool boiling and thermal stratification. Velocity and temperature measurements were carried out in a 300 mm i.d. vessel with a central tube as the heat transfer area. For this purpose, particle image velocimetry (PIV) and hot film anemometry (HFA) were employed. Further, CFD simulations of this system were performed. An excellent agreement was found between the experimental measurements and the CFD simulations. For modeling, the boiling was an extension of the model of Krepper et al. (2007). The lift force was described according to the recommendations of Zeng et al. (1993). The stratification occurring inside the pool has been quantified in terms of a dimensionless number (stratification number). It has been observed that, for higher heat input rates stratification occurs in a shorter time period. The effect of submergence of the condenser tube in the large pool has been studied and it has been found that, for any height of submergence, vapors form at the top of the pool but placing the condenser near the bottom may reduce stratification to a certain extent. The model was extended to the real size (50 000 mm I.D.) passive decay heat removal system.     

The condensation/polymerisation of dimethyl siloxane fluids in a three-phase trickle flow monolith reactor

For the production of siloxane fluids, the viability of using a multi-channel monolith as a catalyst support system in a three-phase reactor has been studied. The catalyst was tripotassium phosphate (K₃PO₄). Experiments were performed in a single-channel flow reactor (15 mm i.d. and 500 mm catalyst coated length). The rate of reaction was followed by monitoring the disappearance of the hydroxyl group (–OH). Reaction experiments were performed at a hydroxyl group concentration range from 150 to 170 mol m⁻³, T=373–413 K and P=7.9 kPa with a nitrogen purge. The maximum temperature of operation was restricted to 413 K to avoid the formation of undesirable by-products. In the regime controlled by chemical kinetics, reaction was of an apparent first order with respect to –OH concentration, and in the apparent rate constant, the pre-exponential factor was 4.19×10⁻⁴ ms⁻¹, and the apparent activation energy was 16.1 kJ mol⁻¹. These are only valid for the operating pressure and purge gas flowrate used, as both of these are shown to affect water removal from the liquid phase and, hence, reaction rates. Mass transfer coefficients from the liquid to the catalyst surface were estimated and these increased rapidly with flowrate and were higher than expected for a falling liquid film.     

The efficiency of short airlift pumps operating at low submergence ratios

An airlift pump operating at submergence ratios between 0.2 and 0.6 and using water as the pumping liquid, has been investigated. Circular pipes of constant short length of 1 m and inner diameters of 3 and 5 cm were used as riser tubes. The water flow rate measurements for various air flow rates allowed the study of the pumping efficiency as a function of the air flow rate, submergence ratio and certain geometrical parameters (riser tube diameter and injector design). The efficiency curves start from a zero efficiency at a minimum air flow rate, then increase rapidly with the air flow rate up to an efficiency maximum. For higher air flow rates, the efficiency decreases exponentially. Diagrams showing the influence of the riser tube diameter and injector design, are presented. Additionally, based on the experimental data, two equations are presented, which allow the prediction of the maximum efficiency and the corresponding water and air flow rates inside the experimental range.     

环隙内流动沸腾传热特性的研究

本文以水和粘性液体为工质,对环隙宽度为2mm的环隙内流动沸腾传热特性进行了研究.实验结果表明,在一定的热通量范围,环隙内流动沸腾传热系数较空管的平均提高80%,总传热系数平均提高60%.当以水为工质时,总传热系数的实验值与计算值吻合较好.     

A new gas to liquids (GTL) or gas to ethylene (GTE) technology

Natural gas is a clean-burning and abundant energy resource, but much of it resides in locations remote from an economic means of transporting it to market. A logical solution for the problem would be to liquefy the natural gas, but this option requires very low temperatures and involves considerable costs. Another solution is to convert the natural gas into hydrocarbon liquids using chemical processing. Fischer-Tropsch technology converts the natural gas into “syngas” (a mixture of carbon monoxide and hydrogen) followed by reaction to liquid fuels. Unfortunately, Fischer-Tropsch technology is expensive.

At Texas A&M University, a research team has conceived a radically new process for converting natural gas into hydrocarbon liquids. It is a “direct” conversion method that does not require producing syngas. The process is essentially three reaction steps and two separation steps to produce hydrocarbon liquids. The process consists of two reaction steps and one separation step to produce ethylene. The process can operate economically with natural gas flows of as low as 300 kSCMD up to any desired capacity.

It is possible to use the GTL technology essentially anywhere natural gas exists from offshore platforms to relatively uninhabited onshore sites. This technology offers an alternative to flaring natural gas when pipelines do not exist. The liquids can be transported in liquid pipelines or in trucks or in tankers. Thus, it offers the opportunity to monetize a resource as well as to reduce undesirable emissions into the atmosphere. The GTE technology is more nearly suited to a location near an existing chemical industry that requires ethylene and/or hydrogen.

SynFuels International Inc. has licensed the technology to commercialize it, and the company has constructed a pilot plant capable of processing 3 kMCMD. The cost of a commercial 300 kSCMD plant should be in the US$ 50–75 million range. The cost of the liquids should be about US$ 25–28 per barrel. Of course, larger capacity plants would require a larger investment but produce a less expensive product.     

Heat transfer to walls in a high temperature fluidized bed of group II particles

High temperature heat transfer data in the particle diameter range 0.73 – 1.41 mm and temperature range 490 – 918 °C are presented. The original measurement method is based on an overall thermal balance over a 0.40 m² heat exchange surface. Experimental data are analyzed using results from the literature and a ‘particle-based’ heat transfer model. It is found that gas convective heat transfer is significant for the largest particles (1.41 mm) but it may be neglected for 1.18 mm particles at temperatures above 700 °C. An analysis of this result on the basis of Saxena and Ganzha's powder classification scheme is proposed. A general correlation based on Archimedes and Planck numbers is presented. It takes into account the interaction between radiation and conduction at high temperatures, utilizing the experimental results for beds of corundum particles (0.28, 0.425, 0.6, 0.73, 1, and 1.18 mm) in the temperature range 500 – 900 °C.     

EXPERIMENTAL STUDY ON SMALL CYCLONES OPERATING AT HIGH FLOWRATES

A new set of experimental data on the particle collection efficiency of small cyclones operating at high flowrates is reported. Seven different cyclones were studied systematically covering a flowrate range 60–110 l min^-1 and a particle size range 0.026–3.6 μm. Special emphasis was given to the effects of the cylinder height and of the exit tube length on the particle collection efficiency. The length ratios of the cylinder height and exit tube length to the cyclone body diameter were varied from 0.75 to 4.5 and from 0.5 to 1.5, respectively. The experimental results suggest that the flowrate plays a significant role in cyclone collection efficiency. High flowrate can help to collect fine particles. As the cylinder height is increased, the collection efficiency increases. However, when the cylinder height is increased excessively, the collection efficiency appears to decrease to certain extent. An optimized exit tube length was also observed. Pressure drop decreased substantially either as the cylinder height became longer or as the exit tube length became shorter. It was also found that the difference between the cyclone cylinder height and the exit tube length affects the particle collection characteristics significantly.     

Experimental study of falling film evaporation heat transfer outside horizontal tubes

The heat transfer process of falling film horizontal evaporation includes evaporation outside tubes and condensation inside tubes, the heat transfer coefficient of the former is about 50% of that of the latter. So the overall heat transfer coefficient is influenced mainly by the falling film evaporation outside tubes. An experimental study of falling film heat transfer outside horizontal tubes was carried out in order to show how the heat transfer coefficient is affected by different parameters such as flow density evaporation temperatures, temperature difference between wall and saturation water, and mass concentration of the seawater. Experiments were conducted using 14 mm outer diameter Al-brass tubes heated by internal electric heaters so that a uniform heat flux was generated on the outside surface of tubes. The results show that when flow density Γ varies between 0.013 kg/ms < Γ< 0.062 kg/ms, the heat stransfer coefficient of falling film evaporation outside horizontal tubes h increases with the increase in liquid feeding, evaporation boiling temperature and heat flux. h also increases with an increase in distributor height, however there is a maximum height in which any height above this. Besides, the amount of non-condensing gas has significant effect on h. The difference of heat transfer coefficient between freshwater and seawater is small. These results contribute to further improving the performance of heat transfer process and developing new evaporator.     

带有中间分液结构的管壳式冷凝器实验研究

对于管内凝结而言,为保持蒸汽在换热管进口段的高效换热状态而进行中间分液,改变气液两相流的流型,以保持相对较高的换热系数是"短管效应"理论的技术举措。结合传统冷凝器的结构,本文设计了一种用于实验研究的气液分离卧式管壳式水冷冷凝器。通过布置在冷凝器两端分程隔板处的不同直径和数量的分液管来观察其换热效果,并与传统冷凝器进行整体换热系数、出口冷凝液温度及压力损失三方面的实验对比。结果表明:具有不同直径和数量的分液管的冷凝器具有相似的热力性能;具有不同直径和数量的分液管的冷凝器整体换热系数比传统冷凝器要高,出口冷凝液温度比传统冷凝器要低,且具有较小的压力损失;在测试工况下,右侧开启1个0.5mm、1个1mm,左侧开启2个1mm、1个0.5mm分液管的冷凝器表现出较好的综合换热性能。     

Study of Single Phase Flow Heat Transfer and Friction Pressure Drop in a Spiral Internally Ribbed Tube

The experimental results of single‐phase flow heat transfer and pressure drop experiments in the turbulent flow regime in a spirally ribbed tube and a smooth tube are presented in this paper. The ribbed tube has an outside diameter of 22 mm and an inside diameter of 11 mm (an equivalent inside diameter of 11.6 mm) and the smooth tube has an outside diameter of 19 mm and an inside diameter of 15 mm. Both tubes were uniformly heated by passing an electrical current along the tubes with a heated length of 2500 mm. The working fluids are water and kerosene, respectively. The experimental Reynolds number is in the range of 10⁴–5 · 10⁴ for water and is in the range of 10⁴–2.2 · 10⁴ for kerosene. The experimental results of the ribbed tube are compared with those of the smooth tube. The heat transfer coefficients of the ribbed tube are 1.2–1.6 fold greater than those in the smooth tube and the pressure drop in the ribbed tube is also increased by a factor of 1.4–1.7 as compared with those in the smooth tube for water. The corresponding values for kerosene are 2–2.7 and 1.5–2, respectively. The heat‐transfer enhancement characteristics of the ribbed tube are assessed. This tube is especially suitable for augmenting single‐phase flow heat transfer of kerosene. Correlations for the heat transfer and the pressure drop for the spirally ribbed tube are proposed, according to the experimental data.     

An experimental study of non-newtonian fluid flow in fluidized beds: minimum fluidization velocity and bed expansion characteristics

In this work, new experimental measurements of the minimum fluidization velocity and velocity-voidage characteristics are reported for a variety of liquid-particle systems in glass columns of two different diameters. Three types of liquids, namely, Newtonian, visco-inelastic, and visco-elastic fluids, were used to fluidize the beds of glass particles of four different sizes (1.27–15.8 mm). The results obtained with Newtonian liquids conform to the expected behaviour. The applicability of a variety of equations has been examined with a view to predicting the values of the minimum fluidization velocity and fluidization index for non-Newtonian systems. The experimental results reported herein embrace the following ranges of conditions: 1.27 < D_p < 15.8 mm; D_T = 50.8 and 101.6 mm, and 0.382 n 1.00.     

Studies on the catalytic dechlorination and abatement of chlorided VOC: the cases of 2-chloropropane, 1,2-dichloropropane and trichloroethylene

The conversion of monochloropropanes and dichloropropanes over acid catalysts has been investigated in the presence of oxygen. In the temperature range of 450–550 K, dehydrochlorination of monochloropropanes to propene and HCl occurs selectively over silica–alumina, while significant formation of chlorinated by-products is observed over ZSM5 zeolite catalyst even at higher temperatures. Dichloropropanes conversion over silica–alumina catalyst gives rise mainly to chloropropenes in the temperature range 500–700 K. CO_x are predominant products only at the highest reaction temperatures (just above 700 K). Water vapor in the feed only slightly affects conversions and selectivities. Deactivation processes occur upon dichloropropane conversion, mainly due to coke deposition.

The conversion of highly chlorinated compounds, such as trichloroethylene (TCE) has been tested over silica–alumina and over HY zeolite in the presence of water vapor in the so-called “steam reforming” conditions (HVOC:water=1:2). With diluted feed (1200 ppm) on HY, reaction occurs above 800 K and formation of chlorinated by-products is minimized, CO_x being the main reaction products. At higher HVOC concentrations conversion is obtained at even lower temperature (600 K), but no more negligible by-products formation has been detected. In our conditions zeolite catalyst is more effective in TCE total conversion than silica–alumina.     

Drop properties and pressure fluctuations in liquid–liquid–solid fluidized-bed reactors

Characteristics of size, rising velocity and distribution of liquid drops were investigated in an immiscible liquid–liquid–solid fluidized-bed reactor whose diameter was 0.102 and 2.5 m in height. In addition, pressure fluctuations were measured and analyzed by adopting the theory of chaos, to discuss the relation between the properties of liquid drops and the resultant flow behavior of three (liquid–liquid–solid) phase in the reactor. Effects of velocities of dispersed (0–0.04 m s⁻¹) and continuous (0.02–0.14 m s⁻¹) liquid phases and fluidized particle size (1, 2.1, 3 or 6 mm) on the liquid drop properties and pressure fluctuations in the reactor were determined. The resultant flow behavior of liquid drops became more irregular and complicated with increasing the velocity of dispersed or continuous liquid phase, but less complicated with increasing fluidized particle size, in the beds of 1.0 or 2.1 mm glass beads. In the beds of 3.0 or 6.0 mm glass beads, the effects of continuous phase velocity was marginal. The resultant flow behavior of liquid drops was dependent strongly upon the drop size and its distribution. The drop size increased with increasing dispersed phase velocity, but decreased with increasing particle size. The drop size tended to increase with approaching to the center or increasing the height from the distributor. The size and rising velocity of liquid drops and correlation dimension of pressure fluctuations have been well correlated in terms of operating variables.     

R410A-油混合物在7 mm强化管内流动沸腾的换热特性

实验研究了环保替代制冷工质R410A-润滑油混合物在强化管内的流动沸腾换热特性,探索了质流密度、干度和平均油浓度对换热特性的影响。实验测试管为内螺纹强化管,长度为2000 mm、外径为7.0 mm。实验结果表明,纯制冷剂R410A的传热系数随干度的增大先增大后减小,峰值出现在干度为0.7～0.8左右;对于R410A-油混合物,在干度小于0.5的工况下,油的存在增强换热,在干度大于0.6的高干度情况下,传热系数随平均油浓度和干度的增大迅速降低。基于混合物性开发了R410A-油混合物在7 mm强化管内流动沸腾的换热关联式,新的关联式预测值与89%的实验数据的误差在±30%以内,平均误差为17.3%。     

Dynamic methods and new reactors for liquid phase molecular catalysis

Kinetic data acquisition and screening of transition metal complexes for homogeneous liquid phase catalysis calls for numerous testing in multiphase G/L, L/L and G/L/L systems. It is shown first, with an example in asymmetric hydrogenation, why detailed kinetics must be performed. Then, new reactors leading to fast experimental techniques are proposed. A liquid–liquid centrifugal partition chromatography is evaluated for determining rate constants and partition isotherms by combining frontal analysis and elution chromatography, the catalyst being maintained in a stationary aqueous phase. Two microreactors offering short residence time have also been tested and compared with a fast test reaction (t_R ca. 5–20 s). The combination of reacting pulses, carrier liquids and micromixer is proposed as a general high throughput tool for the investigation of G/L, L/L and G/L/L catalytic systems in a fast sequential way.     

Enhanced Heat Transfer Characteristics of Upward Flow Boiling of Kerosene in a Vertical Spirally Internally Ribbed Tube

Experiments of upward flow boiling of kerosene in a vertical spirally internally ribbed tube and a vertical smooth tube were conducted, respectively, in the present study. The spirally internally ribbed tube has an inner diameter of 11 mm (an equivalent inner diameter of 11.6 mm) and an outer diameter of 22 mm. The smooth tube has an inner diameter of 15 mm and an outer diameter of 19 mm. The test tubes were uniformly heated by passing an electrical current along the tubes with an available heated length of 2500 mm. At the outlet of the test section, the experimental pressure was 3 bars. The experimental heat flux ranged from 28.5 to 93.75 kW/m². The experimental mass flux was 410, 610, and 810 kg/m²s, respectively. Both local and average flow boiling heat transfer coefficients were measured in the test tubes. The enhanced heat transfer characteristics of the flow boiling of kerosene in the spirally internally ribbed tube are presented by comparing the experimental heat transfer coefficients with those obtained in the smooth tube. It shows that the flow boiling heat transfer coefficients in the spirally internally ribbed tube are 1.6 to 2 times greater than those in the smooth tube. The physical mechanisms of the enhanced heat transfer characteristics of flow boiling in the spirally internally ribbed tube are analyzed. According to the experimental data, an expression for the flow boiling heat transfer coefficient of kerosene was found in terms of the Martinelli number for the spirally internally ribbed tube. The correlation is applicable to the design of heat exchange equipment, using the spirally internally ribbed tube as a heat transfer element under these test conditions.     

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.     

Thermophoretic particle deposition efficiency in turbulent tube flow

This study investigated the thermophoretic particle deposition efficiency numerically. The critical trajectory was used to calculate thermophoretic particle deposition in turbulent tube flow. The numerical results obtained in turbulent flow regime in this study were validated by particle deposition efficiency measurements with monodisperse particles (particle diameter ranges from 0.038 to 0.498 μm) in a tube (1.18 m long, 0.43 cm i.d., stainless-steel tube). The theoretical predictions are found to fit the experimental data of Tsai et al. [Tsai, C. J., J. S. Lin, S. G. Aggarwal, and D. R. Chen, “Thermophoretic Deposition of Particles in Laminar and Turbulent Tube Flows,” Aerosol Sci. Technol., 38, 131 (2004)] very well in turbulent flows. In addition, an empirical expression has been developed to predict the thermophoretic deposition efficiency in turbulent tube flow.     

Microstructure development and properties of novel Ba-doped phase Sialon ceramics

Herein, we report the microstructure and properties of the newly developed near monophasic S-Sialon ceramic, based on the composition of Ba₂Si_12−xAl_xO_2+xN_16−x (x = 20.2). Appropriate amount of the precursor powders (BaCO₃, -Si₃N₄, AlN, Al₂O₃) with a targeted composition of BaAlSi₅O₂N₇ was ball milled and hot pressed to full density in the temperature range of 1600–1750 °C for 2 h in nitrogen atmosphere. Extensive transmission electron microscopy (TEM) study has been conducted to understand the microstructure development and characterise the various morphological features in hot pressed S-Sialon. The sintering mechanism is based on the liquid phase sintering route, which involves the formation of a Ba–Al silicate liquid (<5%) with dissolved nitrogen at intergranular pockets. The experimental observation suggests that the S-phase crystallises in elongated platelet morphology with preferred growth parallel to the orthorhombic ‘c’ axis and primary facet planes parallel to (1 0 0) and (0 1 0). The Ba-S-phase ceramic has an acoustically measured Young modulus of 210–230 GPa, a hardness of 13 GPa and a fracture toughness of 4 MPa m^1/2, little lower than typical of a ceramic with morphologically anisotropic grains contributing to bridging and pullout mechanisms.     

Onset of Nucleate Boiling of Binaries in a Vertical Thermosiphon Reboiler

In the present study a semi empirical equation has been developed as a modification and extension of the previous analyses of pure fluids for the prediction of wall superheat for six binary liquid mixtures including the effect of liquid submergence. The wall superheat required for the onset of nucleate boiling of binary mixtures was related to thermo physical properties, heat flux and liquid submergence. The results predicted from the theoretical analysis are consistent with the experimental data available in the literature. The results were compared with the available correlations, indicating that the correlations based on the submergence are in better qualitative agreement. A unified correlation for the binary mixtures covering wide ranges of heat flux, submergence, inlet liquid subcooling and surface material was also obtained with enhanced deviation of 20.37%.