Heat transfer in pulse-stabilized fluidization - Part 1: overall cylinder and average local analyses

The present study examines the effect of an opposing oscillatory flow on heat transfer from an immersed horizontal cylinder in a bubbling gas-fluidized bed. This opposing oscillatory flow creates a state of fluidization termed pulse-stabilized fluidization. Heat transfer rates were measured for a monodisperse distribution of particles for fluidization ratios ranging from 1.1 to 2.7. Overall heat transfer measurements from a submerged horizontal cylinder show that the heat transfer characteristics are significantly altered by an opposing oscillatory flow. A modified form of the Strouhal number effectively characterizes the particle Nusselt number. Time-averaged local heat flux measurements showed that the local heat transfer distribution was altered by the hydrodynamics induced by the opposing oscillatory flow.     

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.     

Effect of operating parameters on heat transfer in the lean phase region of a CFB riser column

The radial variation of the heat transfer coefficient across the bed width, including the effect of fins and fin shapes (surface roughness), on the heat transfer characteristics in the lean phase region of a circulating fluidized bed riser column are investigated. Three test sections (bare horizontal tube, V-fin tube and square fin tube) are employed for the investigation. The experimental unit consists of a riser column of 102×102 mm in bed cross-section, 5·25 m in height with a return leg of the same dimensions, and both made up of plexiglas columns. For the same operating conditions, the provision of fins results in a drop in heat transfer coefficient, but increases total heat transfer owing to increased surface area. The present experimental data are compared with published literature and good agreement has been observed. The experimental data also corresponds to the two-zone hydrodynamics (i.e. the core–annulus structure) of a circulating fluidized bed. © 1997 John Wiley & Sons, Ltd.     

Flow boiling heat transfer in circulating fluidized bed

In order to enhance heat transfer and mitigate contamination in the boiling processes, a new type of vapor-liquid-solid (3-phase) circulating fluidized bed boiling system has been designed, combining a circulating fluidized bed with boiling heat transfer. Experimental results show an enhancement of the boiling curve. Flow visualization studies concerning flow hydrodynamics within the riser column are also conducted whose results are presented and discussed.     

Flow boiling heat transfer in circulating fluidized bed

In order to enhance heat transfer and mitigate contamination in the boiling processes, a new type of vapor-liquid-solid (3-phase) circulating fluidized bed boiling system has been designed, combining a circulating fluidized bed with boiling heat transfer. Experimental results show an enhancement of the boiling curve. Flow visualization studies concerning flow hydrodynamics within the riser column are also conducted whose results are presented and discussed.     

循环流化床流体动力学研究进展

论述了国内外在循环流化床流体动力学研究领域的进展,对循环流态化颗粒流动问题、颗粒聚集及传热以及循环流态经数学模型等热点问题进行了重点回顾,并指出了当前循环流化床流体动力学研究的一些新动向。     

Thermo-Hydrodynamics of a Helical Coil Heat Exchanger Operated with a Phase-Change Ice Slurry as a Refrigerant

The thermal performance of helical-coil heat exchangers can be significantly enhanced when operated with ice slurry as a phase-change refrigerant. It is essential to also consider the hydrodynamics of ice slurry flow to determine the overall performance of the heat exchanger. This study presents a detailed numerical investigation of the thermo-hydrodynamic performance of a helical coil heat exchanger operated with a laminar and non-Newtonian flow of ethyl-alcohol ice slurry subject to phase change. The Bingham plastic model is used to reflect the non-Newtonian behavior of ice slurry. The phase change of ice slurry is modelled using the enthalpy-porosity method. The pressure drop and heat transfer of ice slurry in a double-turn helical coil are determined in terms of ice mass fraction and Dean number. The results show that an increase in the ice mass fraction and Dean number results in an increase of the heat transfer rate. This is, however, associated with an increase in pressure drop. The entropy generation analysis is introduced to evaluate the overall performance of the heat exchanger, taking into account the opposing effects of heat transfer and pressure drop. It is evident that, at certain ice mass fractions, there exists an optimal value of the Dean number that leads to minimum irreversibility and maximum overall performance.     

Mixed convection boundary layer flow over a thin vertical cylinder with localized injection/suction and cooling/heating

The effects of localized cooling/heating and injection/suction on the mixed convection flow on a thin vertical cylinder have been studied. The localized cooling/heating and (or) injection/suction introduce a finite discontinuity in the mathematical formulation of the problem which increases its complexity. In order to overcome this difficulty, a nonuniform distribution of wall temperature (heat flux) and surface mass transfer is considered at certain sections of the cylinder. The nonlinear coupled parabolic partial differential equations governing the mixed convection flow under boundary layer approximations have been solved numerically by using an implicit finite-difference scheme. The effects of the localized cooling/heating and (or) injection/suction on the heat transfer are found to be significant, but the effects of cooling/heating on the skin friction are comparatively small. The positive buoyancy force which assists the flow and the curvature parameter increase the skin friction and heat transfer.     

Heat transfer enhancement via liquid–liquid phase separation

The heat transfer and flow phenomena during phase separation of partially miscible liquid solvent system were investigated experimentally. The experiments were conducted with a three components system which has an upper critical solution temperature, using critical and off-critical compositions of the solvent mixtures. The convective heat transfer rates were studied for laminar flow in a small diameter horizontal tube and for free convection from its outer surface. It was found that with phase separation the forced convective heat transfer can be augmented by up to 130% compared to heat transfer rates obtained in single phase flow (without phase separation). However, for low quenching rate and depth associated mainly with experiments conducted with critical compositions, deterioration of the heat transfer rates was observed. The free convection heat transfer coefficients were found to be augmented up to 100%. Macro- and micro-flow visualization were also conducted to follow the flow phenomena during the phase separation, and the mechanisms responsible to the heat transfer enhancement are discussed.     

Study of conjugate natural convection between vertical coaxial rectangular cylinders

Laminar natural convection between two coaxial vertical rectangular cylinders is numerically studied in this work. The outer cylinder is connected with vertical rectangular inlet and outlet pipes. The inner cylinder dissipates volumetric heat. The fluid flow and heat transfer characteristics between the cylinders are analyzed in detail for various Grashof numbers. The heat transfer rates on the individual faces of the inner cylinder are reported. The bottom face of the inner cylinder is found to associate with much higher heat rates than those of the other faces. The average Nusselt number on bottom face is more than 2.5 times of the Nusselt number averaged on all the faces. At a given elevation, local Nusselt number on the inner cylinder faces increases towards cylinder edges. The effect of thermal condition of the walls of outer cylinder, inlet and outlet on the natural convection is analyzed. The thermal condition shows strong qualitative and quantitative impact on the fluid flow and heat transfer. The variation of induced flow rate, dimensionless maximum temperature and average Nusselt numbers with Grashof number is studied. Correlations for dimensionless buoyancy-induced mass flow rate and temperature maximum are presented.     

Periodic heat transfer in directly opposed free and forced convection flow

An experimental study of the heat transfer from small circular cylinders placed horizontal to a downward flowing air stream is reported. Based on heat-transfer measurements and flow visualization, a model for directly opposed free and forced convection was developed. Three modes of flow were observed. For very low velocities the free convection, buoyant plume dominates the heat transfer. At a “lower critical” Reynolds number, when the free and forced convections are of the same order of magnitude, a well defined periodic heat transfer was obtained. The periodic heat transfer was due to the build-up of the buoyant forces to a magnitude where they overcame the downward force of the air flow. At an “upper critical” Reynolds number the periodic heat transfer abruptly ceases. For velocities greater than the upper critical limit the forces due to the air flow dominate. A potential like, laminar sheet forms, as a shroud around the thermal layer of the hot cylinder. The average heat transfer from the cylinder decreases with increasing Reynolds number for both the case of dominant free convection and the periodic heat-transfer regime. The minimum value of the heat transfer occurred at the upper critical Reynolds number.     

Modeling gas oil spray coalescence and vaporization in gas solid riser reactor

The sprayed feed droplet behavior, including coalescence and vaporization into gas–solid flow, is complex especially near the atomizer region in fluid catalytic cracking (FCC) riser reactor. A three dimensional CFD model of the riser reactor has been developed, which takes into the account three phase hydrodynamics, heat transfer and evaporation of the liquid droplets into a gas–solid flow as well as phase interactions. A hybrid Eulerian–Lagrangian approach was applied to numerically simulate the collision and vaporization of gas oil droplets in the gas–solid fluidized bed. This numerical simulation accounts the possibility of coalescence of feed spray droplets in computing the trajectories and its impact on droplet penetration in the reactor. The modeling result shows that droplet coalescence mainly occurs at the initial part of the atomizing region and where three phase flow hits the reactor wall and bounces back. The model has the ability of inspecting the effects of feed injector geometry on the overall reactor hydrodynamic and heat transfer. The CFD simulation results showed that the evaporated droplet gas caused higher local velocities of the gas and solid particles and gas–solid flow temperature reduction.     

Analysis of local convective heat transfer in a spark ignition engine

A computational fluid dynamics (CFD) code is applied to simulate fluid flow, heat transfer and combustion in a four-stroke single cylinder engine with pent roof combustion chamber geometry, having two inlet valves and two exhaust valves. Heat flux and heat transfer coefficient on the cylinder head, cylinder wall, piston, intake and exhaust valves are determined with respect to crank angle position. Results for a certain condition are compared for total heat transfer coefficient of the cylinder engine with available correlation proposed by experimental measurement in the literature and close agreement are observed. It was found that the local value of heat transfer coefficient varies considerably in different parts of the cylinder, but they have equivalent trend with crank angle. Based on the results, new correlations are suggested to predict maximum and minimum convective heat transfer coefficient in the combustion chamber of a SI engine.     

Corona discharge augmentation of the catalytic combustion of hydrogen in the diffusion controlled regime

An experimental study of the catalytic combustion of hydrogen in air at 1 atm has been conducted for laminar and turbulent flows in an annular reactor. The reactor comprises a small-diameter inner cylinder and an outer cylinder of platinized alumina. Measurements of the overall rate of combustion have been made wherein the inner cylinder is maintained at a positive d.c. potential with respect to the outer cylinder such that a steady corona discharge takes place. For nearly fully developed flows for Reynolds numbers from 300 to 6500 and corona discharge currents up to 1.1 mA, the reaction rate increases with corona current. Augmented reaction rates are correlated in terms of the ratio of electrical body forces due to corona discharge and the momentum forces of the mean flow. The reaction order is found to be unity with respect to the concentration, both with and without corona discharge present. Overall first-order rate constants are much lower than true rate constants, indicating a dominance of mass transfer. Rate constants are found to increase markedly with corona current but cannot be correlated in the same way as heat transfer coefficients. The fractional increase in overall reaction rate at constant corona current, is found to increase rapidly for low Reynolds numbers, but after reaching a peak, decreases rapidly. This decrease occurs across the laminar-to-turbulent flow transition.     

Visualization on flow patterns during condensation of R410A in a vertical rectangular channel

The visualization experiments on HFC R410A condensation in a vertical rectangular channel （14.34mm hydraulic diameter, 160mm length） were investigated. The flow patterns and heat transfer coefficients of condensation in the inlet region were presented in this paper. Better heat transfer performance can be obtained in the inlet region, and flow regime transition in other regions of the channel was also observed. Condensation experiments were carried out at different mass fluxes （ from 1.6 kg/h to 5.2 kg/h） and at saturation temperature 28~ C. It was found that the flow patterns were mainly dominated by gravity at low mass fluxes. The effects of interfacial shear stress on condensate fluctuation are significant for the film condensation at higher mass flux in vertical flow, and con- sequently, the condensation heat transfer coefficient increases with the mass flux in the experimental conditions, The drop formation and growth process of condensation were also observed at considerably low refrigerant vapor flow rate.     

Constructal multi-scale cylinders in cross-flow

This paper reports a new concept for maximizing heat transfer density in assemblies of cylinders in cross-flow: the use of cylinders of several sizes, and the optimal placement of each cylinder in the assembly. The heat transfer is by laminar forced convection with specified overall pressure difference. The resulting flow structure has multiple scales that are distributed nonuniformly through the available volume. Smaller cylinders are placed closer to the entrance to the assembly, in the wedge-shaped flow regions occupied by fluid that has not yet been used for heat transfer. The paper reports the optimized flow architectures and performance for structures with 1, 2 and 3 cylinder sizes, which correspond to structures with 1, 2 and 4 degrees of freedom. The heat transfer rate density increases (with diminishing returns) as the optimized structure becomes more complex. The optimized cylinder diameters are relatively robust, i.e., insensitive to changes in complexity and flow regime (pressure difference). The optimized spacings decrease monotonically as the driving pressure difference increases. The multi-scale flow architectures optimized in this paper have features and qualities similar to tree-shaped (dendritic) designs, where the length scales are numerous, hierarchically organized, and nonuniformly distributed through the available space.     

Energetic performances of a refrigerating loop using ice slurry

The consideration of environmental constraints in production, transport and distribution of cold energy resulted in reconsidering the practices of installations dimensioning in particular. Their containment led to the development of secondary refrigerants such as ice slurries to store, transport and distribute the cold energy. These heat transfer fluids should have good thermophysical properties, giving high transport capability, high heat transfer ability as well as low pressure drops. The use of ice slurries can lead to lower flow rates and smaller pumping power compared to single phase fluid. The purpose of the presented work is to study the distribution network of indirect cold systems thanks to a model allowing the evaluation of the influence of various parameters on the operating behaviour of the installation. The available domain for the use of secondary heat transfer fluid (whether in their single phase or two phase form) is determined considering the best design from an energetic point of view. Because of the essential role of the fluid distribution between the production site and consumers, we focus our study on pressure drops and pumping power due to the fluid flow in cooling loops. For each investigated case, the minimum consumption power is obtained with the two phases (solid–liquid) heat transfer fluid (ice slurry).     

Effect of riser exit geometry on bed hydrodynamics and heat transfer in a circulating fluidized bed riser column

This paper reports the variation of suspension density along the riser column and the effect of riser exit geometry on bed hydrodynamics and heat transfer in the upper region of a circulating fluidized bed (CFB) riser column. The experiments are conducted in a CFB riser column which is 102 mm × 102 mm in bed cross‐section (square), 5.25 m height, with a return leg of the same dimension. The unit is made up of interchangeable plexiglass columns. The superficial primary air velocity is varied between 4.2 and 6.4 m/s. The suspension density profile along the riser height is influenced by the exit geometry. With a 90° riser exit geometry, the suspension density profile in the upper region of the CFB riser column increases towards the riser exit. This particular trend has been observed for about 2 m length in the top region of the riser. The change in suspension density profile in the top region influences the variation of heat transfer coefficient. With a 90° riser exit geometry, the suspension density increases towards the riser exit, which in turn increases the heat transfer coefficient. The effect of riser exit geometry on hydrodynamics and heat transfer is significant for about 2 m length in the upper region of the riser column. Copyright © 2001 John Wiley & Sons, Ltd.     

Endwall heat transfer and film cooling measurements in a turbine cascade with injection upstream of leading edge

Detailed heat transfer measurements were conducted on the endwall surface of a large‐scale low‐speed turbine cascade with single and double row injection on the endwall upstream of leading edge. Local film cooling effectiveness and the heat transfer coefficient with coolant injection were determined at blowing ratios 1.0, 2.0, and 3.0. In conjunction with the previously measured flow field data, the behaviors of endwall film cooling and heat transfer were studied. The results show that endwall film cooling is influenced to a great extent by the secondary flow and the coverage of coolant on the endwall is mainly determined by the blowing ratio. An uncovered triangle‐shaped area with low effectiveness close to pressure side could be observed at a low blowing ratio injection. The averaged effectiveness increases significantly when injecting at medium and high blowing ratios, and uniform coverage of coolant on the endwall could be achieved. The averaged effectiveness could be doubled in the case of double row injection. It was also observed that coolant injection made the overall averaged heat transfer coefficient increase remarkably with blowing ratio. It was proven that film cooling could reduce endwall heat flux markedly. The results illustrate the need to take such facts into account in the design process as the three‐dimensional flow patterns in the vicinity of the endwall, the interactions between the secondary flow and coolant, and the augmentation of heat transfer rate in the case of endwall injection. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(3): 141–152, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20007     

Bed-to-wall heat transfer modelling in the top region of a CFB riser column with abrupt riser exit geometries

The paper presents a mechanistic model to predict bed-to-wall heat transfer coefficient in the top region of a circulating fluidized bed (CFB) riser column by considering the riser exit geometry effects on bed hydrodynamics. With abrupt riser exit geometry, some solids will reflect back in to the riser column, thereby increasing the solids concentration in the top region of the riser column of a CFB. This in turn results in higher bed-to-wall heat transfer coefficients in the top region. At present, not much information exists in the literature to predict bed-to-wall heat transfer coefficient in the top region of a riser column with riser exit geometry effects. In the present work, a mechanistic model is proposed to estimate bed-to-wall heat transfer coefficient with riser exit geometry configurations. The length of influence of gas–solid flow structure from the riser exit due to various riser exit geometries is also presented. The solids reflux ratio is an important parameter, which influences the heat transfer rate in the top region. For the same operating conditions the bed-to-wall heat transfer coefficient increases with the abrupt riser exit geometry configuration compared to a smooth riser exit in the top region. The proposed model predictions are compared with the published experimental data for right angle exit configuration and a reasonable agreement is observed.