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 simulation of a closed wet cooling tower with novel design

A closed wet cooling tower with novel design was proposed and numerically investigated. The studied cooling tower consists of two main parts: one heat and mass transfer unit (HMTU) and one heat transfer unit (HTU). In the HMTU, copper tubes are arranged as heat transfer tubes while plastic tubes are collocated to enlarge the mass transfer area between the spray water and the airflow. In the HTU, only copper tubes are adopted as heat transfer tubes. Heat and mass transfer process takes place among the process water, airflow and spray water in the HMTU, while in the HTU only heat transfer between the process water and the spray water is observed. A transient one dimensional distributed-parameter model was adopted to evaluate the cooling tower performance under different operating conditions. Determination of heat and mass transfer coefficients, as well as the influence of Lewis number on the cooling tower performance, was presented.     

Shallow solar pond: State-of-the-art

This review article deals with the various aspects of shallow solar ponds (SSP) suitable for domestic purposes and for supplying industrial process heat. The introduction gives a general idea of the status of the SSP technology among various solar energy applications. This part also surveys designs and performance of both water bag type and large scale SSP, already experimentally studied by other workers. The analysis of thermal process of SSP takes the input energy, the mechanism of thermal absorption and various losses in the system, into consideration. The methods for the improvement of the system performance suggests the materials that can be used and designs that can be incorporated in the various components of SSP. Here, a detailed study of the module, the glazing, the glazing supports and the insulation is done. The auxiliary systems viz, the mode of water transfer and water storage, play an important part in the optimum performance of the SSP systems. The “modes of operation” gives an account of the three different ways of circulating the liquid through the system-batch heating, closed cycle continuous flow heating, and open cycle continuous cycle flow heating. The theoretical analysis of the thermal performance of the SSP is done by making use of the Hottel-Willier-Bliss model for the flat plate collectors. A computation based on this model is employed in evaluating the values of monthly average daily collections efficiency with respect to the ambient temperatures for various initial water temperatures. The results obtained from the experiments conducted by Lawrence Livermore Laboratory, gives an idea of the system performance. The review also looks into the effects of various parameters, such as, the mass flow rate of the liquid, the total mass of water required per day, water depth, radiation intensity, average day-time ambient temperature, number of glazings, total heat loss coefficient, etc. The different modes of flow of the liquid are compared. An incorporation of the reflector in the SSP by various workers proved to provide a marked improvement in the system performance. The aspects such as, the cost effectiveness, maintenance and reliability of the SSP are briefly dealt with. It was felt that the review would be incomplete without a mention of the limitations and potential applications of the SSP. Based on these studies on SSP, done by various workers, the conclusion was that the performance of the system can be improved by the proper choice of the material, and by optimizing the design and the modes of operation.     

Effectiveness of heat and mass transfer in packed beds of liquid desiccant system

A liquid desiccant system (using CaCl₂) is presented for air dehumidification using solar energy or any other low grade energy to power the system. The system utilizes two packed beds of counterflow between an air stream and a solution of liquid desiccant for the processes of air dehumidification and solution regeneration. To simplify the prediction of the performance of the system an effectiveness of heat transfer and an effectiveness of mass transfer in the packed beds are defined. A finite difference model is developed to model the heat and mass transfer in packed beds during the air dehumidification mode and the solution regeneration mode. This finite difference model is used to calculate the effectiveness of heat and mass transfer in the packed beds at various bed heights, various air and solution flow rates, various inlet temperatures of air and solution to the bed, and various concentrations of CaCl₂ solution at the bed entrance. Charts of the effectiveness of heat and mass transfer are presented in a convenient form. A designer of a liquid desiccant system may use the charts in predicting the performance of these systems without having to use the finite difference model for this purpose.     

Heat and mass transfer in a non-isothermal fixed bed solid adsorbent reactor: a uniform pressure-non-uniform temperature case

A uniform pressure model is presented to describe the heat and mass transfer in a fixed bed of solid adsorbent in a finned reactor. This model neglects the resistance to mass diffusion but takes into account the resistances to heat diffusion through two coefficients: the heat conductivity of the adsorbent bed and the heat transfer coefficient between the adsorbent bed and the fins. An experiment has been conducted to validate this model and the two heat transfer coefficients are obtained by an identification technique. When the temperature of the closed reactor is modified on one side of the reactor, large temperature inhomogeneities inside the reactor are observed and mass transfer occurs through a heat pipe effect: the model explains that effect which is observed experimentally. That uniform pressure model is more adapted to describe the history of solid adsorbent reactors used in thermal processes than uniform temperature models proposed by other authors.     

Granular phase change materials for thermal energy storage: Experiments and numerical simulations

The present paper reports on the utilization of granular phase change composites (GPCC) of small particle diameter (1–3 mm) in latent heat thermal energy storage (LHTES) systems. The phase changing parameters (phase change temperature, latent heat, and energy storage capacity) of GPCC have been determined using differential scanning calorimeter (DSC) and temperature-history methods. Further analysis of measurement results has been conducted to describe the evolution of latent heat with temperature during phase change in terms of liquid fraction–temperature relationships. Charging and discharging packed bed column experiments have been also carried out for different operating conditions to analyze the potential of GPCC for packed bed thermal energy storage. The present column results clearly demonstrate the dependence of temperature variation along the packed bed and the overall performance of the storage unit on the phase change characteristics of GPCC. Small and non-uniform particles diameters of GPCC and heterogeneity of the bed material complicate the phenomena of heat transfer and evolution of latent heat in the packed bed. Mathematical modeling of the packed bed that considers the GPCC and air as two separate phases with inter-phase heat transfer is presented. Comparisons between experimental and numerical results are used to evaluate the sensitivity of numerical simulations to different model parameters.     

Thermal modeling of solar stills: an experimental validation

Expressions for water and glass temperatures, yield and efficiency of both single and double slope multiwick solar distillation systems in quasi-steady state conditions have been derived. The analysis is based on the basic energy balance for both the systems. A computer model has been developed to predict the performance of the solar stills. Experimental validation of the thermal model has been carried out by using modified heat transfer coefficients. Internal heat transfer coefficients have been evaluated based on both inner and outer glass cover temperatures for typical days namely January 22, and June 19, 2001 in Delhi. A fair agreement has been observed between theoretical and experimental results by using the modified internal heat transfer coefficients based on inner glass cover temperature.     

Validation of the use of heat transfer models in liquid/solid fluidized beds for ice slurry generation

Heat transfer coefficients in a liquid/solid fluidized bed heat exchanger are investigated for application in ice slurry generators. A range of temperature driving forces is determined in which ice slurry generation is stable. In this range ice crystal formation or growth does not affect heat transfer coefficients. A model is proposed that accurately predicts heat transfer coefficients in the fluidized bed ice slurry generator. Due to lower temperatures and higher viscosity in ice slurry generation, heat transfer coefficients measured are lower than predicted with heat transfer correlations specific for liquid/solid fluidized bed heat exchangers. Heat transfer coefficients measured are however significantly higher than for single phase fluid flow.     

A non-uniform temperature non-uniform pressure dynamic model of heat and mass transfer in compact adsorbent beds

This paper discusses a new dynamic two-dimensional model for the simulation of innovative consolidated-type adsorbent beds to use in adsorption energy systems. It consists of a cylindrical pipe, conveying the thermal vector fluid, coated with a layer of consolidated zeolite.The governing equations take into account with detail the transport phenomena and are solved according to advanced numerical methods in the time and space domain.A parametric analysis is carried out for the evaluation of the overall system performance sensitivity to the most meaningful parameters, such as adsorbent bed thickness, water vapour permeability and heat transfer coefficients. A critical discussion is also made about the most credited adsorbent bed arrangements, i.e., pure powder, consolidated powder and metal bound consolidated powder. It was possible to demonstrate that the adsorbent bed, of consolidated powder type, proposed by the CNR-ITAE Lab, performs better than other bed arrangements available in the literature.     

Pool boiling heat transfer to electrolyte solutions

Pool boiling heat transfer coefficients were measured for solutions of salts with positive solubility in water. The effect of the dissolved salts on nucleation site density, bubble departure diameter and bubble frequency was also investigated. The results show that at low heat fluxes heat transfer coefficients can be considerably lower than corresponding values for distilled water. At high heat fluxes the negative effect of the dissolved electrolyte gradually decreased and finally some improvement in heat transfer coefficient was observed. A correlation was developed for nucleate boiling of aqueous solutions from salts with positive solubility. Assuming that the mass transfer resistance is limited to the liquid side, the proposed model allows the prediction of heat transfer coefficients from boiling point data of the respective solutions. Comparison with a significant number of experimental data for different systems indicates that the model should be sufficiently accurate for most practical applications.     

Heat and mass transfer during adsorption of ammonia in a cylindrical adsorbent bed: thermal performance study of a combined parabolic solar collector, water heat pipe and adsorber generator assembly

In this paper we present the study of adsorption refrigerator which use an activated carbon-pair ammonia. The ability of activated carbons to adsorb large mass of ammonia makes them ideal for use in adsorption refrigeration and pump systems. These systems have not reasonable efficiency. In order to make these systems economically viable, their size must be reduced. This implies a need for a rapid heating and cooling the adsorbent/refrigerant pair. However, the main problems to be overcome is related to the poor heat transfer in the adsorbent bed. So, it is necessary to study and understand the heat and mass transfer within the bed and to improve it. A detailed model of heat and mass transfer into the generator has been developed. For a given heat flux, temperature and adsorbed mass have been computed in every point at each step time along the adsorbed bed (generator). Experimental installation simulating an adsorption machine working within a temperature ranging from 20 to 250 °C and pressure ranging from 0 to 2.5 × 10⁶ Pa, allows for identification of the generator's equivalent thermal conductivity and internal heat transfer coefficient. These two parameters are then used to simulate thermal performance of a design whose features include the insertion of stainless steel water heat pipe (HP's) condensers into the generator. The HP's evaporator heat input is of solar origin using a compound parabolic collector (CPC). Nominal Solar coefficient of performance, COPs =14.37% obtained through both Adimensional Exergy Loss (AEL), and COP study, shows the competitiveness of the proposed design.     

Heat Transfer and hydrodynamics in Annular Chromatography：CFD—Simulation and Experiments

IntrotctiouAImular chrDmatography Provides the POssibility ofseparating multicomponent fixtures continuously inone single unit. The rotating annulus of thechromatotw can be filled with arbitw stationalsPhases depending on the existing separation Problem.The feed is introduced at a fixed and stationary sechonat the top of the unit, while the eluent is distributedeverywhere else around the upper circumference. At thebottom the separated Products can be collected atcendn stationals exit angle…     

Investigation of radiative contribution in a high temperature fluidized-bed using the alternate-slab model

The modified alternate-slab model of Gabor is examined for the prediction of radiative contribution to the total heat transfer from a high temperature fluidized-bed system of air-sand to an immersed surface. The results are compared with the predictions of other models and experimental data on average heat transfer coefficient, and percentage radiative contribution as a function of various influencing parameters. The heat transfer coefficients are overestimated by the model within reasonable limits and approach the experimental data for high values of heat transfer surface temperature. The percentage radiative contribution is substantial for large values of particle diameter, surface and bed temperatures. The model is found reliable and simple to handle over a wide temperature range. Results are also presented for air-ash and air-dolomite systems in view of their practical significance.     

Heat and mass transfer in a flat plate solar solid adsorption refrigeration ice maker

A uniform pressure model is presented to describe the heat and mass transfer in an adsorbent bed for a flat plate solar ice maker. This model accounts for heat and mass transfer in a porous bed in a two-dimensional transient process. An experiment has been conducted to validate this model and the calculated results are in good agreement with experiments. With the help of this model, the transient analysis and performance prediction of an intermittent solar powered solid refrigerator can be presented.     

Modified Wilson Plots for Enhanced Heat Transfer Experiments: Current Status and Future Perspectives

Heat transfer coefficients for enhanced tubes are typically measured indirectly using the “Wilson plot” method to first characterize the thermal performance of the one side (heating or cooling supply) and then to obtain the heat transfer data for the enhanced side based on the Wilson plot results. A brief history of the Wilson plot evolution and alternative methods to the Wilson plot, including the advantages and disadvantages, are discussed as applied for enhanced heat transfer. A slight modification to the Briggs and Young (1969) method is proposed so that the experimental errors can be propagated through the method, allowing us to estimate the error in the generated correlations. Furthermore, a new method based on unconstrained minimization is proposed as an alternative to the least-squares regression. As an example, both methods have been applied to two enhanced boiling tubes (the most recent generation) and heat transfer coefficients were compared against direct wall temperature based heat transfer coefficient measurements made on the same tubes for water flow with high-performance internal helical ribs. Both the unconstrained minimization method and the modified Briggs and Young (1969) method performed well and predicted the same heat transfer performance within experimental uncertainty for two databases taken on two different experimental facilities. Furthermore, if the presently modified Wilson plot method is utilized, and the form of the correlating equation is chosen judiciously and is only applied within the range of experimental conditions tested, the results garnered from the analysis can very adequately predict the local heat transfer performance.     

微纳结构铜表面低液位池沸腾实验研究

采用两步电镀法,在改变电流密度的情况下制备出具有不同微纳结构和润湿特性的A、B两个表面,并应用于低液位饱和池沸腾的实验研究中.通过与铜表面对比,发现两个表面在低热流密度情况下,传热系数要高于铜表面,但液位降低时传热系数提升幅度较小,原因在于铜表面沸腾气泡较大,液位降低气泡脱离有很大影响,而表面A、B沸腾气泡较小,液位降...     

New design equations for liquid/solid fluidized bed heat exchangers

Liquid/solid fluidized bed heat exchangers have originally been developed for desalination plants. However, due to their substantial benefits with respect to significantly improved heat transfer and fouling reduction, successful applications also exist in areas such as petrochemical, minerals and food processing as well as in the paper and power industries. The excellent performance of fluidized bed heat exchangers is related to the interaction between particles and heat transfer surface and to mixing effects in the viscous sublayer. In this paper, the results of experimental investigations on heat transfer for a wide range of Newtonian and non-Newtonian fluids are presented. New design equations have been developed for the prediction of bed voidage and heat transfer coefficients. The predictions of these correlations and of numerous correlations recommended by other authors are compared with a large database compiled from the literature.     

Numerical heat transfer analysis of the packed bed latent heat storage system based on an effective packed bed model

Due to the complexity of the fluid flow and heat transfer in packed bed latent thermal energy storage (LTES) systems, many hypotheses were introduced into the previous packed bed models, which consequently influenced the accuracy and authenticity of the numerical calculation. An effective packed bed model was therefore developed, which could investigate the flow field as the fluid flows through the voids of the phase change material (PCM), and at the same time could account for the thermal gradients inside the PCM spheres. The proposed packed bed model was validated experimentally and found to accurately describe the thermo-fluidic phenomena during heat storage and retrieval. The proposed model was then used to do a parametric study on the influence of the arrangement of the PCM spheres and encapsulation of PCM on the heat transfer performance of LTES bed, which was difficult to perform with the previous packed bed models. The results indicated that random packing is more favorable for heat storage and retrieval as compared to special packing; both the material and the thickness of the encapsulation have the apparent effects on the heat transfer performance of the LTES bed.     

The Numerical Comparison of Heat Transfer in a Coated and Fixed Bed of an Adsorption Chiller

Ecological adsorption technology is becoming a focus of attention by industry due to the utilization of low grade thermal energy sources for cooling production. It can be a promising part of sustainable development concept of the global economy. Therefore, research aiming at improving their performance i.e. Coefficient of Performance (COP) by optimizing the construction of sorption beds with a built in heat exchanger system is crucial. The heat transfer characteristics between the bed of porous media (sorbent) and surface of the heat exchanger system determine the heating power of an adsorption chiller. The HP increase can be obtained by heat transfer intensification due to the increase in the thermal conductivity of the sorbent layer in the vicinity of the heat exchanger’s surface. The novel modification of the sorbent layer structure is proposed in the paper in order to improve the heat transfer processes in the heat exchanger boundary layer. The analysis of desorption process conditions in the parametric model of a coated and fixed adsorption bed design is presented in the paper. The computational fluid dynamics (CFD) with conjugate heat transfer analysis is used to determine the crucial input parameters (temperature distribution in the sorbent bed) for further analytical calculations. The commercial code Ansys Fluent was used to perform numerical simulations. The developed computational model consisted of three subdomains representing heating water, heat exchanger material (copper) and sorbent (silica gel). The comparison of a novel coated design and a conventional fixed bed is discussed in the paper. The numerical analysis is based on experimental thermal conductivity measurements of the sorbent layer in different configurations, which were performed using Laser Flash Method.     

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.