Study on gas holdup in a fluidized flotation column from bed pressure drop

This paper investigated the overall gas holdup characteristics in a cocurrent three-phase fluidized flotation column with liquid as the continuous phase. The air, water, and glass beads with a diameter of 3 mm were, respectively, used as the gas, liquid, and solid phases in the flotation column. The gas holdup studies were carried out in a plexiglass column with 0.05 m in internal diameter and 2.2 m in height. Bed pressure drop measurements were used to calculate the fractional gas holdup. During the measurements, the superficial gas and liquid velocities, respectively, varied from 0.42 to 2.55 cm/s and from 6.47 to 10.82 cm/s. Detailed experimental investigations were carried out to study the effects of static liquid height, initial static bed height, gas velocity, liquid velocity, and frother concentration on gas holdup in a cocurrent three-phase fluidized flotation column. It was found that the gas holdup increased with the flow rate of air and decreased with an increase in the water flow rate. Certain effect of the static bed height on gas holdup was observed when the gas velocity varied. But the increase in the static liquid height resulted in the decrease in gas holdup when the gas velocity varied.     

Heat extraction from salinity-gradient solar ponds using heat pipe heat exchangers

This paper presents the results of experimental and theoretical analysis on the heat extraction process from solar pond by using the heat pipe heat exchanger. In order to conduct research work, a small scale experimental solar pond with an area of 7.0 m² and a depth of 1.5 m was built at Khon Kaen in North-Eastern Thailand (16°27′N102°E). Heat was successfully extracted from the lower convective zone (LCZ) of the solar pond by using a heat pipe heat exchanger made from 60 copper tubes with 21 mm inside diameter and 22 mm outside diameter. The length of the evaporator and condenser section was 800 mm and 200 mm respectively. R134a was used as the heat transfer fluid in the experiment. The theoretical model was formulated for the solar pond heat extraction on the basis of the energy conservation equations and by using the solar radiation data for the above location. Numerical methods were used to solve the modeling equations. In the analysis, the performance of heat exchanger is investigated by varying the velocity of inlet air used to extract heat from the condenser end of the heat pipe heat exchanger (HPHE). Air velocity was found to have a significant influence on the effectiveness of heat pipe heat exchanger. In the present investigation, there was an increase in effectiveness by 43% as the air velocity was decreased from 5 m/s to 1 m/s. The results obtained from the theoretical model showed good agreement with the experimental data.     

Flow structure of subcooled boiling flow in an internally heated annulus

Local measurements of flow parameters were performed for vertical upward subcooled boiling flows in an internally heated annulus. The annulus channel consisted of an inner heater rod with a diameter of 19.1 mm and an outer round pipe with an inner diameter of 38.1 mm, and the hydraulic equivalent diameter was 19.1 mm. The double-sensor conductivity probe method was used for measuring local void fraction, interfacial area concentration, and interfacial velocity. A total of 11 data were acquired consisting of four inlet liquid velocities, 0.500, 0.664, 0.987 and 1.22 m/s and two inlet liquid temperatures, 95.0 and 98.0 °C. The constitutive equations for distribution parameter and drift velocity in the drift-flux model, and the semi-theoretical correlation for Sauter mean diameter, namely, interfacial area concentration, which were proposed previously, were validated by local flow parameters obtained in the experiment.     

Pressure resistance to air flow during ventilation of different types of wood fuel chip

Of two types of willow chip (28 and 50 mm cutting length), two types of willow chunk (100 and 200 mm cutting length) and one type of forest chip (28 mm cutting length), the pressure resistance to air flow during ventilation was measured. The influence of air velocity and particle size has been determined and the results are described by a mathematical equation.The measured pressure resistance correlated very well with the size distribution of the different chip and chunk types. The lowest pressure resistance was found in 200 mm long willow chunk at only 2 Pa/m at an air flow rate of 0.1 m/s, while the highest was found in the 28 mm chip of willow and forest material at 39 Pa/m.The pressure resistance was shown to be a factor 10 to 100 lower for these wood fuels than for agricultural crops like rape seed and grain.     

喷管内传递特性对微尺度扩散火焰的影响

对均匀空气流中微尺度甲烷扩散燃烧进行了数值模拟,重点考察微喷管内的流动和传热传质对微尺度燃烧特性的影响.结果表明,在低流速下,内径为0.3 mm的微喷管内进气速度为1.0 m/s时燃料与空气的混合已经发生,混合气被管外的热量预热,同时火焰的热损失增加.在喷管直径一定时,减小燃料喷出速度,传热传质现象对微尺度甲烷扩散火焰特性的影响增强;当进气速度为0.5 m/s时,甲烷在微喷管内开始燃烧,放出热量.在进行微尺度解析计算时,必须包含一定的喷管区域.     

Mechanistic Modeling of the Convective Heat Transfer Coefficient in Gas-Liquid Intermittent Flows

The development of a mechanistic procedure to estimate the convection heat transfer in horizontal gas-liquid intermittent—or slug—flow is presented. In broad terms, the mean convective heat transfer coefficient is calculated following an averaging procedure based on the unit cell model of the slug flow pattern. The flow parameters (i.e., unit cell frequency, liquid slug and elongated bubble length and velocity, and liquid hold-up) were obtained from empirical data for air/water flows in a 15 m-long, 25.4 mm ID copper pipe and for natural gas (mostly methane and ethane) and oil or water flows in an actual size, 200 m-long, 150 mm ID steel pipe. A time-averaging procedure based on the unit cell parameters was then used to calculate the mean convective heat transfer coefficient. The slug flow parameters taken on the small scale air/water loop and the actual size pipeline were used for comparisons. Heat transfer data from the small scale air/water loop were used to validate the results calculated using the averaging procedure. Finally, the approach herein proposed also showed good agreement with previously published data and well-known correlations.     

闭式深层海水供冷系统换热特性及优化设计研究

针对低纬度岛屿地区全年温度高、湿度大,空调全年运行时间长而常规能源运输成本高,传统高能耗空调系统难以适应的问题,提出一种利用深层海水供冷的闭式空调系统。根据换热特性的差异,将换热管道分为沿程垂直换热管道和海底换热盘管两部分,通过数值模拟研究管径、流速等参数对垂直管段和海底换热盘管段传热性能的影响。研究结果表明：垂直管段管径在0.6 m以内,流速在1~2 m/s范围可保证较高的换热性能;对于海底换热盘管段,最佳管径为0.025~0.050 m,最佳流速为0.4~0.8 m/s。在此基础上建立适用于垂直管段优化设计的费用年值数学模型,计算其比摩阻、流速,形成设计用水力计算表,并给出适用于海底换热盘管段工程设计的设计线算图及其修正公式。     

管流中沉降微粒团再悬浮控制因素分析

沉降微粒再悬浮受微粒团直径、气流速度、微粒团与壁面间的黏附状况以及管道情况等多种因素的作用,是柴油机排气微粒净化捕集及微粒采样过程中的一个重要现象.以管流中的柴油机排气沉降微粒为研究对象,利用建立的沉降微粒团再悬浮静态受力模型,对管流中沉降微粒团再悬浮的控制因素进行了分析.研究结果表明,微粒团黏附角、平均流速以及管径等因素对管流中沉降微粒团的再悬浮具有重要的影响.通过研究,得到了微粒团黏附角、平均流速以及管径等控制因素对管流中沉降微粒团再悬浮的影响规律.     

Local flow measurements of vertical upward bubbly flow in an annulus

Local measurements of flow parameters were performed for vertical upward bubbly flows in an annulus. The annulus channel consisted of an inner rod with a diameter of 19.1 mm and an outer round tube with an inner diameter of 38.1 mm, and the hydraulic equivalent diameter was 19.1 mm. Double-sensor conductivity probe was used for measuring void fraction, interfacial area concentration, and interfacial velocity, and laser Doppler anemometer was utilized for measuring liquid velocity and turbulence intensity. A total of 20 data sets for void fraction, interfacial area concentration, and interfacial velocity were acquired consisting of five void fractions, about 0.050, 0.10, 0.15, 0.20, and 0.25, and four superficial liquid velocities, 0.272, 0.516, 1.03, and 2.08 m/s. A total of eight data sets for liquid velocity and turbulence intensity were acquired consisting of two void fractions, about 0.050, and 0.10, and four superficial liquid velocities, 0.272, 0.516, 1.03, and 2.08 m/s. The constitutive equations for distribution parameter and drift velocity in the drift-flux model, and the semi-theoretical correlation for Sauter mean diameter namely interfacial area concentration, which were proposed previously, were validated by local flow parameters obtained in the experiment using the annulus.     

Experimental and numerical study of downward bubbly flow in a pipe

An experimental and numerical study of the local structure of downward gas–liquid flow in a vertical pipe with 20-mm inner diameter is reported. In the experiment, the electrodiffusion technique was used in combination with electrical conductivity measurements. To examine the effect of gas-phase dispersion on flow characteristics, two different gas–liquid mixers were used capable of producing large-diameter (>1 mm) and small-diameter (<1 mm) gas bubbles at identical rate characteristics of the flow. The unified heterogeneous-medium mechanics approach was used to develop, in the Eulerian two-velocity approximation, a calculation model for downward turbulent liquid/air bubble flows. It is shown that, as the volumetric gas flow rate of the mixture at the inlet to the pipe increases, local maxima of continuous phase velocity and bubble concentration emerge in the near-wall zone of the flow, with liquid turbulence suppressed in the wall zone and enhanced in the core of the flow.     

A new database on the evolution of air–water flows along a large vertical pipe

A comprehensive database was obtained for stationary upward air–water flows in a vertical pipe with an inner diameter of 195.3 mm using the wire-mesh sensor technology. During the experiments the sensor was always mounted on the top of the test section while the distance between gas injection and measuring plane was varied to up to 18 different L/D by using gas injection chambers at different vertical positions. The gas was injected via holes in the pipe wall. The pressure was kept at 0.25 MPa (absolute) at the location of the active gas injection while the temperature was constant at 30 °C ± 1 K. This procedure exactly represents the evolution of the flow along the pipe, as it would be observed for an injection at a constant height position and a shifting of the measurement plane. The experiments were done for 48 combinations of air and water superficial velocities varying from 0.04 m/s to 1.6 m/s for water and 0.0025 m/s to 3.2 m/s for air. From the raw data time-averaged data as: radial gas volume fraction profiles, bubble size distributions, radial volume fraction profiles decomposed according to the bubble size and the radial profiles of the gas velocity were calculated. Due to the combination of the new experimental procedure with the high spatial and temporal resolution of the wire-mesh sensor technology the data have new quality especially regarding their consistency in the evolution with increasing L/D. This closes a gap for data suitable for CFD code development and validation for two-phase flows, especially for models on bubble coalescence and break-up.     

Analysis of cold flow fluidization test results for various biomass fuels

A systematic theoretical and experimental study was conducted to obtain hydrodynamic properties such as particle size diameter, bulk density, fluidizing velocity, etc. for locally available biomass residue fuels in Malaysia like rice husk, sawdust, peanut shell, coconut shell, palm fiber as well as coal and bottom ash. The tests were carried out in a cold flow fluidization bed chamber of internal diameter 60 mm with air as fluidizing medium. The height of the chamber could be raised up to 630 mm by five separate cylindrical rings. Bed-pressure drop was measured as a function of superficial air velocity over a range of bed heights for each individual type of particle. The data were used to determine minimum fluidization velocity, which could be used to compare with theoretical values. The particle size of biomass residue fuel was classified according to Gildart's distribution diagram. The results show that Gildart's particle size (B) for sawdust, coal bottom ash, coconut shell have good fluidizing properties compared to rice husk, type (D) or palm fiber, type (A). The bulk density and voidage are found to be main factors contributing to fluidizing quality of the bed.     

高浓度水煤浆直管内流动的数值模拟

通过管流法试验得出质量浓度为65.3%的兖州煤水煤浆为剪切增稠的幂流体。将试验得出的流变模型和参数作为计算的依据,运用FLUENT软件提供的非牛顿流体模块,对水煤浆在直管中的流动进行了数值模拟。计算得出水煤浆在管道中流动产生滑移的临界速度,并得到临界速度与管径的变化关系。提出滑移速度新的定义方法,计算得出三个管径中不同平均流速下的滑移速度均为0.02 m/s,表明水煤浆的滑移速度与平均流速和管径呈弱相关。通过对滑移速度进行修正,得出滑移修正后的单位长度压差与实测值相吻合,表明计算模型是正确的。计算得到管道截面水煤浆的表观粘度变化曲线,从管壁到管道中心先缓慢减少再急剧降低。     

椭圆形换热单管积灰特性的数值模拟研究

椭圆形换热管作为一种强化换热元件,在抗积灰性能方面具有一定优势.本文基于ANSYS FLUENT软件平台建立了一套模拟程序,针对椭圆形换热单管的积灰特性展开了数值模拟研究.重点研究了换热管的椭圆度、烟气流速以及飞灰颗粒粒径对飞灰沉积特性的影响.研究表明,当换热管的椭圆度在1～2之间变化时,5～100μm粒径的颗粒沉积率...     

Effect of Channel Length on the Gas–Liquid Two-Phase Flow Phenomena in a Microchannel

An optical measurement system was used to investigate the effect of microchannel length and inlet geometery on adiabatic gas–liquid two-phase flow. Experiments were conducted with 146-mm- and 1571-mm-long, circular microchannels of 100 μm diameter. Void fraction and gas and liquid plug/slug lengths and their velocities were measured for two inlet configurations for gas–liquid mixing: (a) reducer and (b) T-junction. The superficial gas velocity was varied from 0.03 to 14 m/s, and superficial liquid velocity from 0.04 to 0.7 m/s. The test section length was found to have a significant effect on the two-phase flow characteristics measured at the same axial location (37 mm from the inlet) in both microchannels. The mean void fraction data for the short (146 mm) microchannel with the reducer inlet agreed well with the equation previously proposed by Kawahara et al. (2002). On the other hand, the mean void fraction data for the long (1571 mm) microchannel obeyed the homogeneous flow model and Armand's equation for both the reducer and T-junction inlet configurations. Many long and rapidly moving gas plugs/slugs and long, slowly moving liquid plugs/slugs were observed in the short microchannel compared to the long microchannel, leading to the differences in the time-averaged void fraction data. The mean velocity of liquid plugs/slugs generally agreed well with Hughmark's equation and the homogeneous flow model predictions, regardless of the inlet configurations and microchannel lengths. Thus, both the microchannel length and inlet geometry were found to significantly affect the two-phase flow characteristics in a microchannel.     

Two-phase flow patterns of nitrogen and nanofluids in a vertically capillary tube

Two-phase flow patterns of nitrogen gas and aqueous CuO nanofluids in a vertically capillary tube were investigated experimentally. The capillary tube had an inner diameter of 1.6 mm and a length of 500 mm. Water-based CuO nanofluid was a suspension consisted of water, CuO nanoparticles and sodium dodecyl benzol sulphate solution (SDBS). The mass concentration of CuO nanoparticles varied from 0.2 wt% to 2 wt%, while the volume concentration of SDBS varied from 0.5% to 2%. The gas superficial velocity varied from 0.1 m/s to 40 m/s, while the liquid superficial velocity varied from 0.04 m/s to 4 m/s. Experiments were carried out under atmospheric pressure and at a set temperature of 30 °C. Compared with conventional tubes, flow pattern transition lines occur at relatively lower water and gas flow velocities for gas–water flow in the capillary tube. While, flow pattern transition lines for gas–nanofluid flow occur at lower liquid and gas flow velocities than those for gas–water in the capillary tube. The effect of nanofluids on the two-phase flow patterns results mainly from the change of the gas–liquid surface tension. Concentrations of nanoparticles and SDBS have no effects on the flow patterns in the present concentration ranges.     

Flame stability and emission characteristics of propane-fueled flat-flame miniature combustor for ultra-micro gas turbines

An engineering model of a propane-fueled miniature combustor was developed for ultra-micro gas turbines. The combustion chamber had a diameter of 20 mm, height of 4 mm, and volume of 1.26 cm³. The flat-flame burning method was applied for lean-premixed propane–air combustion. To create the stagnation flow field for a specific flat-flame formation, a flat plate was set over the porous plate in the combustion chamber. A burning experiment was performed to evaluate the combustion characteristics. The flame stability limit was sufficiently wide to include the design operation conditions of an equivalence ratio of 0.55 and air mass flow rate of 0.15 g/s, and the dominant factors affecting the limit were clarified as the heat loss and velocity balance between the burning velocity and the premixture flow velocity at the porous plate. CO, total hydrocarbons (THC), and NO_x emission characteristics were established based on the burned gas temperatures in the combustion chamber and the temperature distribution in the combustor. At an air mass flow rate of less than 0.10 g/s, CO and THC emissions were more than 1000 ppm due to large heat loss. As the air mass flow rate increased, the heat loss decreased, but CO emissions remained large due to the short residence time in the combustion chamber. NO_x emission depended mainly on the burned gas temperature in the combustion chamber as well as on the residence time. To reduce emissions despite the short residence time, a platinum mesh was placed after the combustion chamber, which drastically decreased the CO emissions. The combustor performance was compared with that of other miniature combustors, and the results verified that the present combustor has suitable combustion characteristics for a UMGT, although the overall combustor size and heat loss need to be reduced.     

旋流格栅式新型烟气脱硫装置研究

基于喷淋、冲击、自激、旋流的原理,综合了旋流加格栅的特点,以强化气液间的传质。针对影响脱硫效率的主要因素,如液封高度、喷淋液pH值、烟气含湿量及内塔穿孔气速,进行了一系列试验,试验表明,当喷淋量为110.68 m3/h,液封高度调节到1230 mm,烟气的含湿量控制在5.56左右,pH值维持在8～9之间,内塔穿孔气速为14.5 m/s左右时,脱硫效率可达98.5%,是一个较好的工况点。工业应用表明该装置脱硫效率大于97%,设备阻力小于1200 Pa。     

An experimental study of air-solid two-phase flow in a 90° bend using LDV system

The measurements of the mean streamwise and radial velocities, the associated turbulence and the relative particle densities were made in an air-solid two-phase flow in a square sectioned (30mm×30mm) 90° vertical to horizontal bend using laser Doppler velocimetry. The radius ratio of the bend was 2.0. Glass beads of 100μm in diameter were employed to form the solid phase. The measurements of air and solid phases were performed separately at the same bulk velocity 19.34m/s, corresponding to a Reynolds number of 3.87×10⁴. The mass ratio of solid to air was 1.6%. The results indicate that the particle trajectories are very close to straight lines. The streamwise velocity profiles for the gas and the solids cross over near the outer wall with the solids having the higher speed. At θ=30° and 45°, particle-wall collisions happen mostly in the region from θ=30° to θ=75°, and cause a sudden change in solid velocity. The particles tend to move towards the outer wall in 90° bend. The particle concentration near the outer wall is much higher than that near the inner wall in the bend, and there are few particles in the inside of the bend. The bend leads to apparent phase separation: at θ=45°, the solids concentrate in the half of the duct near the outer wall. After θ=60° the second peak concentration appears, and goes gradually towards the inner wall.     

微尺度甲烷扩散火焰及其熄灭特性

用不同直径微/小尺度圆管对甲烷在空气中的扩散燃烧进行实验研究,分析了微/小尺度火焰的结构,考察了影响火焰高度的相关因素,详细探讨了火焰的熄灭极限特点,并拟合了淬熄速度与喷管出口直径d之间的经验关系式.结果表明,火焰的高度与喷管出口速度呈线性关系,随d减小而减小;H/d(火焰高度/喷口直径)与出口处Re值成正比,与d无关;随尺度d的减少,下限(淬熄速度)增大,火焰的稳燃区间变小,稳定燃烧条件苛刻.