Heat transfer in a pulsed bubbling fluidized bed

Surface-to-bed heat transfer and pressure measurements were carried out in a 0.17 m ID pulsed bubbling fluidized bed with glass bead and silica sand particles having mean diameters ranging from 37 μm to 700 μm to investigate the effects of flow pulsation on heat transfer and bed hydrodynamics. A solenoid valve was used to supply pulsed air to the bed at 1 to 10 Hz. The bed surface was found to oscillate with the frequency of pulsation, the oscillation's amplitude decreasing with frequency. The standard deviation of the bed pressure drop in the pulsed bed was found to be larger than that in the conventional bed due to the acceleration force imposed by pulsation. For both Geldart B and A particles, high frequency pulsation (7, 10 Hz) enhances the heat transfer compared to continuous flow, the enhancement diminishing with superficial gas velocity and particle size. For Geldart B particles, the effect of pulsation on heat transfer ceases around U_o/U_mf = 3.5, whereas 24% improvement in heat transfer coefficient was obtained for 60 μm glass bead particles (Group A) at superficial gas velocities as high as U_o/U_mf = 27. Furthermore, in the fixed bed (U_o/U_mf < 1) for Geldart B particles, 1 Hz pulsation was found to be very effective resulting in two- to three-fold increase in heat transfer coefficient compared to continuous flow at the same superficial gas velocity. The flow pulsation loses its effect on heat transfer with increasing static bed height, i.e., when H_bed/D > 0.85.     

Heat transfer in a high temperature fluidized bed

The heat transfer characteristics between the bed and immersed tube in a high temperature fluidized bed (7.5 cm I.D.×70 cm H) were investigated with sand and iron ore particles. The heat transfer coefficients were measured at operating temperatures of 200–600°C and gas velocities of 1–10 U_mf. The bed emissivity measured by the radiation probe was found to be 0.8–0.9. The experimentally obtained radiative heat transfer coefficient was in the range of 30–80 W/m²K for the operating temperature of 400–800°C and the contribution of radiation to total heat transfer was about 13% and 18% for the operating temperatures of 400°C and 600°C, respectively.     

Heat and mass transfer characteristics in a gas-slurry-solid fluidized bed

The gas-slurry-solid fluidized bed is a unique operation where the upward flow of a liquid-solid suspension contacts with the concurrent up-flow of a gas, supporting a bed of coarser particles in a fluidized state. In the present study we measured the gas holdup, the coarse particle holdup, the cylinder-to-slurry heat transfer coefficient, and the cylinder-to-liquid mass transfer coefficient at controlled slurry concentrations. The slurry particles were sieved glass beads of 0.1 mm average diameter and their volumetric fraction was varied at 0, 0.01, 0.05 or 0.1. The slurry and the gas velocities were varied up to about 12 and 15 cm/s, respectively. The coarse particles fluidized were sieved glass beads of average diameters of 3.6 and 5.2 mm. The individual phase-holdup values were measured and served for use in correlating the heat and mass transfer coefficients. The heat and mass transfer coefficients in the slurry flow, gas-slurry transport bed, slurry-solid fluidized bed and gas-slurry-solid fluidized bed operations can be correlated well by dimensionless equations of a unified formula in terms of the Nusselt (Sherwood) number, the Prandtl (Schmidt) number and the specific power group including the energy dissipation rate per unit mass of slurry, with different numerical constants and exponent values, respectively, to the heat and mass transfer coefficients. The presence of an analogy between the heat and mass transfer from the vertically immersed cylinder in these slurry flow, gas-slurry transport bed and gas-slurry-solid fluidized bed systems is suggested.     

Modeling of grid region heat transfer in a shallow gas-solid fluidized bed

Heat transfer coefficients between the bed and an immersed horizontal tube in the grid-region of a shallow gas-solid fluidized bed were experimentally and theoretically studied. Experiments were carried out in two fluidized bed columns with inside diameters of 88 and 137 mm, respectively. The fluidized particles tested were sand, limestone and glass beads. Experimental parameters also included particle size, superficial gas velocity, tube diameter, tube location and distributor design. A mechanistic model considering the contributions of jet phase, emulsion phase and dead phase was derived for estimating the grid-region heat transfer coefficients. Most of the model predictions were found to be within 25% of the experimentally observed data.     

Heat transfer and bubble dynamics in a three-phase inverse fluidized bed

In this study, surface-to-bed heat transfer experiments were performed to gain insight on heat transfer and hydrodynamics in a three-phase inverse fluidized bed. Air, tap water or 0.5 wt.% aqueous ethanol, and polypropylene were, respectively, the gas, liquid and solid phases. The solid loading was varied from 0 to 30 vol.%, and the gas and liquid superficial velocities from 2 to 50 mm/s and 0 to 21 mm/s, respectively. Visual observations were associated with measured phase holdups and instantaneous heat transfer coefficients. Larger gas velocities lead to an increase in bubble size due to the transition to the coalesced bubble flow regime. The greater turbulence induced by the larger bubbles increases the average heat transfer coefficient. On the other hand, adding ethanol reduces the heat transfer coefficient. Solid concentrations up to ∼13 vol.% increase the average heat transfer coefficient whereas higher solid concentrations tend to lower it. The distribution of instantaneous heat transfer coefficient peak height is wider at higher gas and liquid velocities while the addition of a surfactant narrows it. Gas holdups and average heat transfer coefficients are both compared with existing correlations, which are then adjusted for a better fit.     

Heat transfer in a swirling fluidized bed with geldart type-D particles

A relatively new variant in fluidized bed technology, designated as the swirling fluidized bed (SFB), was investigated for its heat transfer characteristics when operating with Geldart type D particles. Unlike conventional fluidized beds, the SFB imparts secondary swirling motion to the bed to enhance lateral mixing. Despite its excellent hydrodynamics, its heat transfer characteristics have not been reported in the published literature. Hence, two different sizes of spherical PVC particles (2.61 mm and 3.65 mm) with the presence of a center body in the bed have been studied at different velocities of the fluidizing gas. The wall-to-bed heat transfer coefficients were measured by affixing a thin constantan foil heater on the bed wall. Thermocouples located at different heights on the foil show a decrease in the wall heat transfer coefficient with bed height. It was seen that only a discrete particle model which accounts for the conduction between the particle and the heat transfer surface and the gas-convective augmentation can adequately represent the mechanism of heat transfer in the swirling fluidized bed.     

Heat transfer from a horizontal tube to a fluidized bed in the presence of unheated tubes

An experimental study of heat transfer from a horizontal tube to a fluidized bed was carried out in the presence of nearby unheated tubes. The effect of the number of tubes, their size, configuration, and the type of fluidized materials was studied. Certain configurations caused as much as a 56% reduction in heat transfer over the most favorable case. The top of the tube was covered by stagnant solids, the bottom partially by a gas film, and the sides by a small region of vigorously agitated solids. Most of the heat transfer occurred in the side region. An overall heat transfer coefficient of 35 Btu/hr-ft²-°F. was found at the best conditions.     

Heat transfer between a gas-solid fluidized bed and a small immersed surface

In this paper are presented the results of an investigation of heat transfer from a small electrically-heated element to the bulk of a gas-fluidized bed. Mean and instantaneous values of the heat transfer coefficient were measured using various gases and solids at bed static pressures ranging from 0.03 to 1.48 MN/m². For all materials and conditions the heat transfer coefficient showed the same general dependence on gas flowrate. The heat transfer coefficient exhibited a maximum and the corresponding Nusselt number was related to the Galileo number and the Prandtl number of the gas by the equation Nu_max = 0.30 Ga^0.20 Pr^0.40 1<Ga<220From a trace of the element temperature against time, the corresponding instantaneous values of the heat transfer coefficient were calculated. At any gas flowrate the mean heat transfer coefficient could be expressed in terms of the coefficient at the incipient fluidization point and a term which was characteristic of the fluctuations.     

Heat and mass transfer in apple cubes in a microwave-assisted fluidized bed drier

In the present study, a pilot scale microwave assisted fluidized-bed dryer was designed and used to dry apple cubes. A model was developed to describe heat and mass transfer in apple cubes during drying in a combined microwave-assisted fluidized-bed dryer. A numerical solution based on the finite difference method was used to develop the model for moisture distribution and temperature variation of samples. The model was validated using experimental data, including average moisture content, center and surface temperature at various air temperatures and microwave power densities.     

Heat transfer in a gas fluidized bed assisted by an alternating electric field

The application of an alternating electric field to cause movement of particles charged naturally by contact electrification in a fluidized bed, has be     

Heat transfer mechanism due to particle convection in circulating fluidized bed

The nature of heat transfer in circulating fluidized beds (CFBs) by particle convection was revealed from the experimental results of the particle flux in the lateral direction. As particles contact and rebound from the heat transfer surface because of their lateral motion, they establish a large local temperature gradient. Heat from particles is then transferred primarily through the gas to the heat transfer surface. The data of the lateral particle flux provide a basis to establish a model of heat transfer in CFBs.     

Modeling of wet jet in fluidized bed

A wet jet zone is established in many applications wherever feeding and dispersing a liquid, solution or slurry into fluidized bed by gases is needed. In the present study, a simple mathematical model has been developed to simulate the wet jet in fluidized bed. The different stages involved inside the jet zone have been estimated and analyzed.The evaporation stage of traveling droplets through the jet flare has been treated. The rates of evaporation of each size at all positions along the jet flare have been estimated according to the velocities and surrounding conditions. The final droplet sizes have been determined. Moreover, the total evaporation rate from traveling droplets, before collision either with entrained sand particles or flare boundaries, has been estimated. The traveling droplets, partially evaporated, may collide and settle on entrained sand particles. The model predicts the settlement rates of liquid droplets on entrained sand particles. The total part evaporated from settled liquid has been estimated as well.The study has been applied to the pneumatic feeding of liquid fuel into fluidized bed combustors operating at . The model has been utilized to predict the ratio of fuel vapor that releases inside the jet flare. The remaining part is assumed to evaporate inside the emulsion phase. Three different liquid fuels have been considered: a heavy oil, diesel fuel and gasoline. The main independent variables are those related to the injection conditions including the initial velocity of dispersing air, u₀, and air-to-liquid mass ratio, ALR.The model results demonstrate that only very small droplets completely evaporate inside the flare. The liquid settling over the entrained sand particles plays an essential role in the fuel evaporation inside the flare. The phenomenon is dominant at conditions that result in generation of droplets of larger sizes, i.e., heavier fuel, lower u₀, and greater ALR. The ratio of vapor fuel released in jet flare increases with lighter fuel, higher u₀ and lower ALR. At and ALR=1.0 nearly all-liquid fuel evaporates inside the flare.     

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.     

Heat transfer to the ceiling of the riser of a circulating fluidized bed

Very little information on the heat transfer to the ceiling of a circulating fluidized bed (CFB) boiler is available in the published literature though it constitutes a significant part of the furnace heat absorption. So, to explore this less-known heat transfer process a series of experiments were conducted at four different superficial gas velocities and three external solids circulation rates in a CFB pilot plant with a riser having a height of 5 m and a cross section of . The experimental results suggest that both solids circulation rates and superficial gas velocities had a significant influence on the local heat transfer to the ceiling close to the riser exit to the gas solids separator. However, on the ceiling, opposite of the exit, solids circulation rates and superficial gas velocities had only a minor influence on the local heat transfer coefficients.     

Heat transfer between an isolated bubble and the dense phase in a fluidized bed

Heat transfer between the bubble and dense phases of a bubbling fluidized bed plays a very important role in the system performance, especially for applications involving solids drying and gas‐phase combustion. However, very few experimental data are available on this subject in the literature. An experimental and modelling investigation on the heat transfer behaviour of isolated bubbles injected into an incipiently fluidized bed is reported in this paper. A new single‐thermocouple technique was developed to measure the heat transfer coefficient. The effects of bed particle type and size, and bubble size on the heat transfer coefficient were examined. The heat transfer coefficient was found to exhibit a maximum as the bubble size increased in the bubble size range investigated. The bed particle size had a comparatively small effect on the heat transfer coefficient. A simple mathematical model was developed which provides good agreement with experimental data.     

Solids entrainment in a large gas fluidized bed

Entrainment from a gas fluidized bed of cracking catalyst in a 2-ft diameter column equipped with continuous solids recycle has been studied. Radial profiles of entrainment rate and mean particle size were relatively flat. The entrainment rate decreased rapidly with increasing height above the bed and with decreasing fluidizing velocity. The existence of a transport disengaging height above which the entrainment rate and mean particle size become essentially constant was confirmed experimentally. This may be taken as the height above the bed surface at which the inverse Froude number becomes equal to 1000.     

射流喷动流化床实验研究

射流喷动流化床 ,是在喷动床的基础上 ,通入流化气 ,降低喷动床柱体部分的高度 ,提高喷动速度的方法得到 ;作为一种新型流态化装置 ,可用于颗粒物料的干燥和包衣 ,粉状物料的造粒 ;同时可用于悬浮液的干燥。通过射流喷动流化床用于颗粒物料涂敷造粒的试验 ,对射流喷动流化床流型、压降及其影响因素进行了研究     

Wear on tubes by jet impingement in a fluidized bed

This study investigated wear on heat-transfer tubes resulting from impingement of particles entrained by a jet in a fluidized bed. The particular application is the fluidized-bed boiler for the Liquid Metal Fast Breeder Reactor in which a steam-tube failure might cause a high-velocity jet, accelerate particles, and erode adjacent tubes.Croloy and 304 SS tubes immersed in a fluidized bed were subjected to impingement wear from a sonic-jet issuing through a 1.17 mm diam. hole. Wear on target tubes was severe (when spaced near the leak source (1.6 cm)). With spacings of 20 cm, wear was scarcely measurable, being less than 0.002 cm/min.Impingement wear was measured in beds of rounded nickel, angular magnesia and angular iron. Results clearly show that increasing the hardness or angularity of particles increases impingement wear rate.An analysis of impingement wear was performed to project results of room-temperature tests to actual conditions. At 800 K and 1400 kPa, wear is projected to occur at 3 – 5 times the rate measured in tests.     

Study of gas-liquid jet boundaries in a gas-solid fluidized bed

The boundaries of gas-liquid jets formed during the air-assisted injection of liquids into gas-solid fluidized beds have been investigated using a small and a commercial scale fluidized bed and several injection nozzles. Two independent techniques (a triboelectric probe system and a thermal tracer method) have successfully allowed the characterization of the jet angle, of the jet penetration, and of the overall mapping of the jet cavity.The study demonstrated the effects of the nozzle geometrical configuration, of the air-to-liquid mass ratio fed through the nozzles, and of the scale of the fluidized bed on the jet boundaries.Finally, the experimental results obtained in this work confirmed the accuracy of an empirical correlation for jet penetration available in the literature.