Heat Transfer to Kraft Black Liquor in a Liquid?Solid Fluidized Bed

Wall-to-bed heat transfer coefficients to pulp mill Kraft black liquor in a nominal 25-mm tubular, liquid?solid fluidized bed have been determined. The fluidizing particles were stainless steel balls of 4.0, 4.76, and 6.35 mm diameter. The heat transfer coefficients were determined for conditions where porosity, bulk temperature, and liquor concentration were varied. The best correlation was obtained when the data were split into two groups of low and high liquor concentrations, i.e., 30?40% and 50?65% total dissolved solids. The data were also compared with the predictions obtained from 26 published correlations, of which the best 10 are presented and discussed in this article. The results of this investigation are relevant to other fluidized bed systems with high liquid viscosity, such as sugar concentrations, hydrocarbons, and polymers.     

Heat transfer from a horizontal tube in a magnetofluidized bed

Heat transfer coefficient values are reported for a horizontal Nylon 24.1 mm diameter heated probe immersed in an iron shot bed of 1511 μm average diameter and exposed to an external uniform magnetic field collinear with the fluidizing air velocity. The electromagnet produces a maximum constant magnetic-field intensity of 27,850 A/m in a cylindrical central region, 152.4 mm in diameter and 152.4 mm in height, within a constancy of 1%. Total and local heat transfer coefficients are measured at different axial and angular positions over a range of air velocity (0.28–2.50 m/s) and magnetic-field intensity (0.0–19,974 properties, viz., minimum fluidization and bubbling velocities and mean bed voidage.     

Heat Transfer from Spiral Tubing in an Air-Fluidized Bed

Coefficients of heat transfer from a coiled Turbotec spiral tube to an air-fluidized bed were measured. Experimental parameters included particle diameter, number of flutes, flute pitch, groove depth, and air fluidizing velocity. Results indicated that the heat transfer coefficient generally increased with increasing fluidizing velocity. A maximum coefficient was observed in some cases. The heat transfer coefficient increased with decreasing particle size. The spiral tube with three flutes had the highest heat transfer coefficients. The spiral tube with four flutes and a pitch of 5.66 cm had gains in heat transfer duty as large as 40% compared to plain tubes. A correlation was formulated which fit the data within the range of experimental error.     

Parametric studies on a metal hydride based single stage hydrogen compressor

Influence of operating parameters such as heat source and sink temperatures, operating pressures, pressure ratios, cycle time, and bed parameters such as overall heat transfer coefficient, bed thickness and thermal conductivity, on the performance of a single stage hydrogen compressor is presented. Coupled heat and mass transfer and reaction kinetics are considered for the analysis. An AB₂-type alloy, Ti_0.98Zr_0.02V_0.43Fe_0.09Cr_0.05Mn_1.5 is chosen as an example. At a given pressure ratio, the hydrogen throughput increases with hot fluid temperature and decreases with increase in cold fluid temperature. Optimum values of thermal conductivity and overall heat transfer coefficient exist for any given hot and cold fluid temperatures. Among the variables studied, heat transfer fluid temperature, bed thickness and supply pressure are found to exert significant influence on the compressor performance.     

Heat transfer from immersed vertical tube in a fluidized bed of group A particles near the transition to the turbulent fluidization flow regime

Experiments were performed in a 0.29 m ID fluidization column to investigate heat transfer from a vertical tube immersed in a bed of 70 μm FCC particles in the range of superficial velocities close to the transition to the turbulent fluidization regime. The results show that the transition is a gradual process and that the changing hydrodynamics affect the heat transfer. The highest heat transfer coefficients were found in the range of superficial gas velocities where the transition to turbulent regime occurred. Radial profiles of heat transfer coefficient were almost flat in the turbulent fluidization regime and changed very little with increasing superficial gas velocity.     

脉动流化床传热特性

在讨论脉动燃烧对传热特性影响的基础上，基于脉动流化燃烧概念，试验研究了脉动流化床中的传热特性，得到不同空截面风速、静止床层高度和声波扰动下的传热系数，并与同一运行参数条件下非脉动流化床及纯气流脉动燃烧器中的传热特性进行对比．总结影响脉动流化床传热特性的各种因素，分析不同运行参数对传热的影响作用．     

A design methodology for small-scale bubbling fluidized-bed furnaces

A detailed methodology is presented for the design of small-scale fluidized-bed furnaces (SSFBF) in the 15–250 kW capacity range for domestic space and hot water heating. These bubbling fluidized-bed furnaces burn a beneficiated coal-water slurry mixture, and do not have heat exchange tubes in the bed or freeboard regions. Algorithms are developed, based on this methodology, to calculate furnace design and performance parameters. The heat transfer coefficients are calculated for the dense bed and freeboard of the SSFBF. These are then compared with theoretical and experimental heat transfer coefficients in the published literature. Excellent agreement is found between those in the SSFBF and those in the literature. The finding of this work is that the methodology and the heat transfer coefficients obtained provide excellent design tools for the scale-up of bubbling fluidized-bed furnaces in the 15–250 kW range.     

Numerical study of different gas–solid flow regimes effects on hydrodynamics and heat transfer performance of a fluidized bed reactor

In this study, a two‐?uid Eulerian–Eulerian model has been carried out applying the kinetic theory of granular flow (KTGF) to study the hydrodynamics and heat transfer behavior of a fluidized bed reactor simultaneously. The effects of different gas–solid flow regimes on the operating conditions and heat transfer rate between the hot air and two types of low and high‐density inert particles are investigated in a fluidized bed dryer. Different gas–solid flow regimes for wood and glass particles of groups A, B, and D of Geldart's classification are simulated to introduce the most optimal flow regime in terms of heat transfer rate and operating costs. The compromise between the heating rate, the height required for the reactor, and the ratio of the final mass to the initial mass of solid particles, which specifies the need for a cyclone separator showed that the bubbling regime of Geldart B powder for low‐density particles and the turbulent regime of Geldart D powder or bubbling regime of Geldart B powder for high‐density particles are the optimal operating conditions and flow regimes. Furthermore, it was concluded that the convective heat transfer is the dominant mechanism, which increases with increasing the air velocity and decreasing the particle diameter in each group.     

用于太阳能超临界CO2布雷顿循环的流态化颗粒换热试验与模拟

搭建30 kW浅层多级流态化颗粒换热试验台,在约1.5倍临界流化速度、换热器采用直管管束逆流形式布置时颗粒侧换热系数可达590～860 W/(m2·K).采用双欧拉流体模型对流化床内水平埋管管束换热进行数值模拟,模拟结果与试验结果偏差在10％以内.利用析因设计与线性回归模型研究颗粒粒径、颗粒导热系数和流化气体速度对流态...     

Bed-to-wall heat transfer behavior in a pressurized circulating fluidized bed

An experimental investigation has been made to study the effect of pressure and other relevant operating parameters on bed hydrodynamics and bed-to-wall heat transfer in a pressurized circulating fluidized bed (PCFB) riser column of 37.5 mm internal diameter and 1940 mm height. The experiments have been conducted with and without bed material for the consideration of frictional pressure drop due to gas density at elevated pressures. The pressure drop measured without sand particles is assumed as the pressure drop due to gas density for the calculation of bed voidage and suspension density profiles. The specially designed heat transfer probe is used to measure the bed-to-wall heat transfer coefficient. The experimental results have been compared with the published literature and good agreement has been observed. The axial bed voidage is less in the bottom zone of the riser column and is increasing along the height of the bed. With the increase in system pressure, the bed voidage is found to be increasing in the bottom zone and decreasing in the top zone. The heat transfer coefficient increases with the increase in system pressure as well as with the gas superficial velocity. The heat transfer coefficient is also observed to be increasing with the increase in average suspension density.     

Heat transfer to a horizontal tube bundle located in the freeboard of a bubbling fluidized bed combustor

The characteristics of heat transfer from bubbling gas-fired fluidized bed to a horizontal staggered water-tube bundle located in the freeboard region is experimentally investigated. The purpose is to demonstrate the effect of bed temperature on the coefficients of heat transfer by the different modes to each of the four rows of the bundle, which experiences heat transfer by convection from flue gases, luminous radiation from bed material and non-luminous radiation from gases. The bed temperature itself is varied and controlled through the fuel–air mass ratio. Sixteen runs have been conducted with bed temperature ranging from 1114 to 1429 K, resulting in an overall heat transfer coefficient in the range 74·0–105·0 W m⁻² K⁻¹ for the first row and 58·0–65·0 W m⁻² K⁻¹ for the last. An overall convective heat transfer coefficient from gases, and possible carried over sand particles, to the bundle is formulated. © 1997 by John Wiley & Sons, Ltd.     

Effect of pressure on thermal aspects in the riser column of a pressurized circulating fluidized bed

In the present paper the effect of pressure on bed‐to‐wall heat transfer in the riser column of a pressurized circulating fluidized bed (PCFB) unit is estimated through a modified mechanistic model. Gas–solid flow structure and average cross‐sectional solids concentration play a dominant role in better understanding of bed‐to‐wall heat transfer mechanism in the riser column of a PCFB. The effect of pressure on average solids concentration fraction ‘c’ in the riser column is analysed from the experimental investigations. The basic cluster renewal model of an atmospheric circulating fluidized bed has been modified to consider the effect of pressure on different model parameters such as cluster properties, gas layer thickness, cluster, particle, gas phase, radiation and bed‐to‐wall heat transfer coefficients, respectively. The cluster thermal conductivity increases with system pressure as well as with bed temperature due to higher cluster thermal properties. The increased operating pressure enhances the particle and dispersed phase heat transfer components. The bed‐to‐wall heat transfer coefficient increases with operating pressure, because of increased particle concentration. The predicted results from the model are compared with the experimentally measured values as well as with the published literature, and a good agreement has been observed. The bed‐to‐wall heat transfer coefficient variation along the riser height is also reported for different operating pressures. Copyright © 2005 John Wiley & Sons, Ltd.     

Study of wall-to-bed heat transfer in a bubbling fluidised bed using the kinetic theory of granular flow

Research into heat transfer modelling in fluidised beds is very limited due to its complexity. The kinetic theory of granular flow (KTGF) has been applied successfully to hydrodynamic modelling in the past but its application in heat transfer modelling has not been tested extensively. A two-fluid Eulerian–Eulerian model has been carried out applying the KTGF to a wall-to-bed reactor. The local heat transfer coefficients are compared against experimental data for two drag models, namely the Gidaspow and the Syamlal–O’Brien drag models. Furthermore, a parametric study is carried out for a variety of coefficients of restitution, particle diameter sizes and inlet velocities. Near wall analysis is carried out in both dense and dilute regions. Both drag models detect the passage of the bubble reasonably well but they predict the complete transition of the bubble past the sensors occurs at slightly different times. The heat transfer coefficients obtained with the Syamlal–O’Brien model showed more local fluctuations than the Gidaspow model because the Syamlal–O’Brien models was developed based on the particle terminal velocities which would indicate a slight sensitivity to a microscopic scale. Extension of the simulation for a longer period makes it possible to reveal that a periodic distribution occurred after 1.5 s and the local heat transfer coefficients gradually reduced to agree better with the experimental results which were previously over estimated. The study shows that a regular dynamic pattern is established in the bubbling fluidised bed only after 1.5–2 s.     

CFD modeling of heat transfer of CO2 at supercritical pressures flowing vertically in porous tubes

Computational fluid dynamics (CFD) tool has been used for investigation of convective heat transfer of CO₂ in two porous tubes. Effects of some important parameters such as pressure, inlet temperature, mass flow rate, wall heat flux and porosity on temperature distribution and local heat transfer coefficients have been studied numerically. Near the supercritical conditions, these parameters are very effective on temperature gradient and local heat transfer coefficients. For example at p = 9.5 MPa, under the same conditions, the heat transfer coefficient in a tube with particle diameters of 0.1–0.12 mm is about 20–30% higher than when the particle diameter of 0.2–0.28 mm were used. The heat transfer coefficient increases with decreasing pressure and increasing mass flow rate. Also the porosity of the bed has the important role on the heat transfer. The CFD predictions have been compared to the experimental data and showed pretty good agreement.     

An investigation of direct condensation of steam jet in subcooled water

The direct contact condensation phenomenon, which occurs when steam is injected into the subcooled water, has been experimentally investigated. Two plume shapes in the stable condensation regime are found to be conical and ellipsoidal shapes depending on the steam mass flux and the liquid subcooling. Divergent plumes, however, are found when the subcooling is relatively small. The measured expansion ratio of the maximum plume diameter to the injector inner diameter ranges from 1.0 to 2.3. By means of fitting a large amount of measured data, an empirical correlation is obtained to predict the steam plume length as a function of a dimensionless steam mass flux and a driving potential for the condensation process. The average heat transfer coefficient of direct contact condensation has been found to be in the range 1.0∼3.5 MW/m²−°C. Present results show that the magnitude of the average condensation heat transfer coefficient depends mainly on the steam mass flux. By using dynamic pressure measurements and visual observations, six regimes of direct contact condensation have been identified on a condensation regime map, which are chugging, transition region from chugging to condensation oscillation, condensation oscillation, bubbling condensation oscillation, stable condensation, and interfacial oscillation. The regime boundaries stable condensation, and interfacial oscillation condensation. The regime boundaries are quite clearly distinguishable except the boundaries of bubbling condensation oscillation and interfacial oscillation condensation.     

A study on heat transfer in a conical fluidized-bed reactor with an immersed cylindrical heater

In this paper, numerical and experimental analyses of the heat transfer between an immersed heater and a cone bed of sand particles were carried out. A three-dimensional (3D) model using the Eulerian–Eulerian model coupled with the kinetic theory for granular flow was used to simulate heat transfer and the related bed flow characteristics. The effects of different inlet gas velocities, represented by the fluidizing number (the ratio between inlet gas velocity to minimum fluidizing velocity), and different particle-wall boundary conditions on heat transfer and hydrodynamics were investigated. Both the experiments and numerical simulation results showed that the heat transfer coefficient and the bed expansion ratio increased with increasing the inlet gas velocity. For the particle-wall boundary condition, applying the no-slip condition showed the best agreement in the heat transfer coefficient and the bed expansion ratio to the experimental results.     

A study on heat transfer for immersed tube in internally circulating fluidized bed

INTRODUCTIONThemethodhowtodealwiththedomesticandindustrialwasteswithoutfurthercontaminationisoneofthemostimportantenvironmentalissues.Fluidizedbedcombustor(FBC)hasadvantagesofhighcombustionefficiency)lowpollution,convenienceinpreprocessingbeforefedin...     

Heat transfer investigations for in-bed and freeboard vertical U-tubes in a fluidized bed at moderate temperatures

The heat transfer coefficients for a single U-tube immersed vertically in fluidized beds of silica sands ($\text{d}\text{?}{}_{\mathrm{p}}\text{= 222, 488, and 778 μ}\text{m}$) and for a bundle of six hair-pin shaped nested vertical tubes mounted in the freeboard have been measured as a function of fluidizing velocity at bed temperatures ranging from 365 to 535 K. Simultaneous measurements of bed voidage were also made in each case. The effects of bed temperature, fluidizing velocity and particle size on in-bed and freeboard heat transfer coefficients and voidage are examined and explained.     

基于小波变换的鼓泡流化床压力波动信号的分析

运用小波变换对鼓泡流化床内压力信号进行多尺度分解 ,并计算各尺度高频信号的功率谱积分 ,由此来对鼓泡床压力波动信号所含的丰富信息进行分析 ,这种分析方法充分体现了小波分析的优越性。通过对GeldartD类床料的实验、研究得出分解时选用的最佳小波基随流化速度变化的规律 ;尺度 6及尺度 5对应的高频信号分别可以表征气泡在床表面的破裂及在布风板附近的形成 ,尺度 4的高频信号是由气泡沿床层上升而引起的 ,这在以往使用傅立叶变换或只用功率谱分析是无法得出的。结果表明 ,小波变化结合功率谱分析是一种分析鼓泡流化床内压力脉动信号的有效方法     

Enhancement of pool boiling heat transfer in water and ethanol/water mixtures with surface-active agent

Surface tension of ethanol/water mixtures is measured over the whole ethanol fraction range and the effect of the surface-active agent on surface tension is also measured in the mixtures, in order to gain basic data related to enhancement of the heat transfer coefficient in water and the mixtures. The boiling heat transfer coefficient, the onset of boiling and the critical heat flux in water and ethanol/water mixtures, with and without the surface-active agent, have been measured on a horizontal fine heated wire at a pressure of 0.1 MPa. The experiment was carried out in the whole range of the ethanol fraction and in a surfactant concentration of 0-5000 ppm. The experiment shows that the coefficients were enhanced in a lower ethanol fraction (C ? 0.5) and in low heat flux which is slightly higher than heat flux at the onset of boiling. It is also found that the enhancement due to the surfactant disappears over 1000 ppm. Finally, we clarify that depression of the surface tension by the surfactant remarkably enhances the heat transfer coefficients in the nucleate pool boiling.