I. Borde

Theoretical Drying Model of Single Droplets Containing Insoluble or Dissolved Solids

A literature survey of published single-droplet drying models has exposed their various shortcomings. An advanced theoretical model of drying of single droplets, containing either insoluble or dissolved solids, has been developed and numerically solved. The validation of the developed model has been accomplished by comparison of the predicted temperature and mass histories with published experimental data for slurries of silica and skim milk solution under different conditions. The results of simulations showed a good agreement between the calculated curves and experimental points in all case studies.

Theoretical Models of Single Droplet Drying Kinetics: A Review

During the last decades, growing attention has been given to theoretical and experimental studies of drying behavior of single droplet containing solids. This research interest is motivated by the need for fundamental understanding of the drying phenomena in extensively used technological processes like spray drying, fluidized bed drying, pneumatic drying, etc., at drop-wise and particulate levels. The present literature review summarizes the developed theoretical models of single droplet drying kinetics, discovers their benefits and deficiencies, and identifies prospects for future research.

An experimental investigation of bubble pump performance for diffusion absorption refrigeration system with organic working fluids

An experimental investigation was undertaken to study the performance of the bubble pump for diffusion absorption refrigeration units. The bubble pump is the motive force of the diffusion absorption cycle and is a critical component of the absorption diffusion refrigeration unit. The purpose of the bubble pump (besides the circulation of the working fluid) is to desorb the solute refrigerant from the solution. Therefore the efficiency of the bubble pump will be set by the amount of the refrigerant desorbed from the solution. The performance of the diffusion absorption cycle depends primarily on the efficiency of the bubble pump. A continuous experimental system was designed, built and successfully operated. The experiments were performed in which some of the parameters affecting the bubble pump performance were changed. During the experimental investigation, photographs were taken showing that the bubble pump operates at slug flow regime with a churn flow regime at the entrance of the bubble pump tube. It was obtained that the performance of the bubble pump depends mainly on the motive head and on the heat input to the bubble pump.

Absorption system based on the refrigerant R134a

The possibility of using 1,1,1,2-tetrafluoroethane (R134a) as a refrigerant in combination with different organic absorbents, such as dimethylether tetraethyleneglycol (DMETEG), N-methyl ϵ-caprolactam (MCL) or dimethyl-ethyleneurea (DMEU), in absorption units was investigated. The procedures in this paper are presented for the model working pair R134a-DMETEG. Temperature-pressure-concentration curves were constructed on the basis of vapour-liquid equilibrium measurements. Excess thermodynamic properties of the mixture were calculated, and enthalpy-concentration diagrams were constructed. The density and viscosity of the working fluids were measured. Thermostability testing of R134a-DMETEG showed that no changes were found in either of the phases over the course of the test. The performances of the investigated working fluids were compared in terms of the coefficient of performance (COP) and circulation ratio (f), which were evaluated by a computerized simulation program based on a single-stage absorption cycle containing a jet ejector as a mixer and preabsorber. The values of the COP for the three combinations were similar (R134a-DMEU reached 0.49, R134a-MCL 0.47 and R134a-DMETEG 0.46), but the value of f for R134a-DMTEG was lower than for the other two pairs. R134a-DMETEG was therefore the best of the three candidate working pairs.

Working fluids for an absorption system based on R124 (2-chloro-1,1,1,2,-tetrafluoroethane) and organic absorbents

Abstract The possibility of using R124 (2-chloro-1,1,1,2,-tetrafluoroethane, CHClFCF3) and organic absorbents as working fluids in absorption heat pumps was investigated. Various classes of organic compounds, all commercially available, were tested as absorbents for possible combination with R124; the absorbents included DMAC (N′, N′-dimethylacetamide, C4H9NO), NMP (N-methyl-2-pyrrolidone, C5H9NO), MCL (N-methyl ϵ-caprolactam, C7H13NO), DMEU (dimethylethylene urea, C5H10N2O), and DMETEG (dimethylether tetraethyleneglycol, C10H22O5). To evaluate the performance of a candidate refrigerant-absorbent pair in a refrigeration or heat pump cycle, the thermophysical properties of the pure components and the mixture and the equilibrium and transport properties have to be determined, either from experimental data or by prediction methods. The thermal stability of the refrigerant-absorbent must also be tested. A method for the calculation of the concentration in the liquid and gas phases and the excess thermodynamic properties of the mixture as a function of the system temperature and pressure based on our experimental setup is described. On the basis of vapor-liquid equilibrium measurements, density and viscosity measurements and thermostability testing, enthalpy-concentration diagrams were constructed. The performance characteristics of the investigated working fluids in terms of the coefficient of performance (COP) and the circulation ratio (f) were calculated for a single-stage absorption cycle. In terms of overall performance (COP, f and stability) R124-DMAC was found to be the superior combination, followed by R124-NMP, R124-DMEU and R124-MCL (the three pairs for which stability problems were found at high temperatures), and finally by R124-DMETEG.

Mathematical Modeling of Drying of Liquid/Solid Slurries in Steady State One-Dimensional Flow

ABSTRACT A mathematical model of simultaneous mass, heat and momentum transfer for two-phase flow of a gas and a solid/liquid slurry was developed. The model was applied to calculation of the drying process of coal-water slurry droplets in a gas medium in a steady one-dimensional flow. The model was based on the well-known two-stage drying process for slurry droplets. After the first period of drying, in which the evaporation rate is controlled by the gas phase resistance, the evaporating liquid diffuses through the porous shell (crust) and then, by convection, into the gas medium. Inside the dry external crust of the drop, a wet central core forms, which shrinks as evaporation proceeds. The temperature of the slurry droplet rises. The process ends when the temperature of the dry outer crust reaches the coal ignition temperature in the case of combustion or when the moisture of the particle reaches the final required moisture. The developed model was based on one-dimensional balance equations of mass, ene...

Numerical study on the design parameters of a jet ejector for absorption systems

The purpose of incorporating a jet ejector into an absorption system is to improve the preabsorption of the refrigerant coming from the evaporator by the weak solution, i.e., to improve the overall absorption process. The mixing process in the jet ejector is very intensive as a result of spray generation of the liquid phase and of extensive subcooling of the weak solution in the solution heat exchanger. To facilitate the design of a jet ejector for absorption machines, a numerical model of simultaneous mass and heat transfers between the liquid and gas phases in the jet ejector was developed. The steady-state model was based on unidimensional balance equations of mass, energy and momentum for the liquid and gas phases. Polynomial correlations were employed to calculate the thermodynamic properties. On the basis of the developed model, we studied the following design parameters of the jet ejector as functions of the length and the cone angle of the diffuser: pressure recovery, temperature and concentration of the refrigerant in the solution, and velocities of the gas and liquid drops. The parametric study also involved examination and ways of augmentation of the mass transfer process in the diffuser with the ultimate aim of designing a compact and efficient unit. The calculations were performed for a mixture of the refrigerant R125 (pentafluoroethane) with organic absorbents.

Modeling of Droplet Drying in Spray Chambers Using 2D and 3D Computational Fluid Dynamics

Steady-state and unsteady-state 2D axisymmetric and 3D calculations of heat and mass transfer processes in cylinder-on-cone spray chamber have been performed. The theoretical model of the process is based on two-phase Eulerian-Lagrangian approach for gas-droplet flow. In steady-state, two cases are considered: flow of hot air only (no liquid spray) and heat and mass transfer between water spray and hot air. For unsteady-state, droplet-droplet interactions in the water spray are included. It has been found that 2D axisymmetric model is suitable for fast and low-resource consumption numerical calculations and it predicts values of velocity, temperature, and vapor mass fraction in the spray chamber with reasonable accuracy. However, due to its restrictions, 2D axisymmetric model fails to predict asymmetry of flow patterns and presence of the transversal air flow and it cannot provide an actual three-dimensional picture of particle trajectories inside the spray chamber. In the case when the above characteristics are important, the utilization of 3D model is essential.

Steady state one dimensional flow model for a pneumatic dryer

Abstract A one-dimensional steady-state mathematical model for dilute phase flow in a pneumatic dryer is presented. The model takes into account mass, momentum and heat transfer between the gas and the particle phases. The model was applied to the drying process of wet PVC particles in a large-scale pneumatic dryer. The rate of drying was controlled by a two-stage drying process. In the first drying stage, heat transfer controls evaporation from the saturated outer surface of the particle to the surrounding gas. At the second stage, the particles were assumed to have a wet core and a dry outer crust; the evaporation process of the liquid from a particle is assumed to be governed by diffusion through the particle crust and by convection into the gas medium. As evaporation proceeds, the wet core shrinks while the particle dries. The drying process is assumed to stop when: the moisture content of a particle falls to a predefined value; or when the particle riches the exit of the pneumatic dryer. The one-dimensional pneumatic drying model was solved numerically and two operating conditions, adiabatic and given pneumatic dryer wall temperature, were simulated. The prediction of the numerical simulation was compared successfully with experimental results of drying of wet PVC particles conveyed with air in a large-scale pneumatic dryer.

Droplet–Droplet Interactions in Spray Drying by Using 2D Computational Fluid Dynamics

A 2D axisymmetric model of the spray drying process is presented. The two-phase flow theoretical model is based on a combined Eulerian-Lagrangian approach and takes binary interactions (coalescence or bouncing) between spray droplets into consideration. Validation of the model (incorporated in FLUENT 6.3.26) demonstrated good agreement and consistency with the literature data. The results of transient simulations showed that droplet–droplet interactions displace the region of heat and mass transfer from the central core toward the periphery of the drying chamber. It was also found that insulation of the spray dryer can substantially affect temperature and humidity patterns, whereas its influence on the velocity flow field is less marked.