Analytical Modelling of a Spray Column Three-Phase Direct Contact Heat Exchanger (original) (raw)
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A numerical study on heat transfer characteristics in a spray column direct contact heat exchanger
Journal of Mechanical Science and Technology, 2002
A reliable computational heat transfer model has been investigated to define the heat transfer characteristics of a spray column direct contact heat exchanger, which is often utilized in the process involving counterflows for heat and mass transfer operations. Most of the previous studies investigated are one-dimensional unsteady solutions based on rather fragmentary experimental data. Development of a multidimensional numerical model and a computational algorithm are essential to analyze the inherent multidimensional characteristics of a direct contact heat exchanger. The present study has been carried out numerically and establishes a solid simulation algorithm for the operation of a direct contact heat exchanger. Operational and system parameters such as the speed and direction of working fluid droplets at the injection point, and the effects of aspect ratio and void fraction of continuous fluid are examined thoroughly as well to assess their influence on the performance of a spray column.
Computational Analysis of Single Drops and Sprays for Spray Cooling Applications
Spray cooling is a key technology in the thermal management of the next generation electronic, aircraft and spacecraft systems. There have been relatively fewer computational studies of spray cooling because simulating all the detailed physics and dynamics of a spray consisting of millions of drops per second is computationally very expensive. In this study, computational approaches have been used to analyze single drops and sprays for spray cooling applications. The commercially available Computational Fluid Dynamics (CFD) code ANSYS Fluent (versions 14, 14.5, 15) has been used to perform single drop and spray simulations on two desktop workstations and the High Performance Computing (HPC) cluster at West Virginia University (WVU). Single drop impingement on wet surfaces has been studied using the 2D axisymmetric Volume of Fluid (VOF) model in ANSYS Fluent 14 and 14.5. The free surface shape and hydrodynamics of single drops after they impact on wet surfaces have been validated with the experiments performed by members of the WVU Spray Cooling team in the Mechanical and Aerospace Engineering Department (MAE) at WVU. Initial film thickness, initial drop diameter, initial drop shape and gravity effects have been investigated for water at room temperature. It has been concluded that gravity has significant effects on the drop and film dynamics while drop shape does not have any significant effects. The 2D axisymmetric Discrete Phase Model (DPM) with the wall film submodel in ANSYS Fluent 14 has been used to perform simulations of spray impact on flat surfaces. The effects of the nozzle-to-surface distance, spray half angle, spray coolant, spray mass flow rate and gravity on spray variables (e.g. average drop diameters, drop velocities, etc.) have been analyzed for a full cone spray based on the Spraying System 1/8 G nozzle operating at 40 psi which has been used in the spray experiments performed by members of the WVU Spray Cooling team. Full cone 40 psi water spray cooling simulations with phase change have been performed in 3D coordinates using the DPM, Eulerian Wall Film (EWF) and the Species Transport Model (STM) in ANSYS Fluent 15. The free surface shape and hydrodynamics of the film have been analyzed. The film thickness results have been compared with experiments. The effects of the surface temperature, spray temperature and air temperature on the film characteristics (e.g. film thickness, film velocity magnitude) and heat transfer (e.g. surface heat flux) have been studied. It has been concluded that air temperature does not have a significant effect on the film characteristics and heat transfer whereas spray temperature has significant effects. Increasing the spray temperature 50 K (from 300 K to 350 K) causes a 62% decrease in the surface heat flux. Full cone 40 psi water spray cooling simulations have been also performed in 2D axisymmetric coordinates using the Eulerian Multiphase (EM) model in ANSYS Fluent. The computed average surface heat flux value was 8% different compared to the 3D DPM-EWF-STM model. However, there has been a large discrepancy in the film characteristics between these two models and also between the EM model and experiments. In conclusion, the 3D DPM-EWF-STM model is the preferred method in order to analyze spray cooling at the present time. i TABLE OF CONTENTS
Experimental investigation of droplet dynamics and heat transfer in spray cooling
Experimental Thermal and Fluid Science, 2003
Experimental investigation on droplet dynamics and heat transfer in spray cooling was conducted. Water and water solutions with different surfactant additions (sodium dodecyl sulfate) were used to cool a 10 mm in diameter, horizontal copper surface. A multi-nozzle spray system was constructed for studying the effects of mass flux on spray cooling. This multi-nozzle system provides a variable mass flux spray from 0.156 to 1.20 kg/m 2 s with a diameter and velocity variation less than 20%. The incoming and outgoing droplets were characterized in situ with a newly developed laser phase-Doppler anemometry (PDA). It was found that the heat transfer process in spray cooling can be divided into four regimes using the expulsion rate defined as the ratio of local outgoing to incoming mass fluxes. The advantage of the surfactant addition in spray cooling has also been investigated. Besides adding surfactant results in relative small diameters for both impinging and expulsing droplets, the slope of the droplet expulsion rate at the transition to the critical heat flux (CHF) regime also changes sharply with surfactant addition which results in a almost constant heat removal rate near the CHF regime. This character may provide an additional safety mechanism for a heat transfer device to avoid burnout.
AIChE Journal, 1966
Transfer characteristics are presented for a perforated plate-spray column in which a volatile dispersed phase evaporates while rising in the continuous, counterflowing, immiscible phase. Optimal column heights, volumetric transfer coefficients, holdup, and foam heights are reported as functions of flow rate and temperature approach for a pentane-water system, A comparison with related studies is presented. Recently multiphase exchangers where latent heat rather than sensible heat is transferred between immiscible fluids have been effectively used in water desalination by direct contact freezing. Some of the technical and economical aspects of utilizing these multiphase exchangers were reported by Umano (1), Wiegandt (2, 3) , and others (4). Experimental direct freezing pilot plants (5) are presently in the process of accumulating valuable technical know-how. Unlike the multiphase exchangers, where both the dispersed and the continuous fluids undergo change of phase, three-phase exchangers may be used for heat transfer at various temperature levels. The latter obviously depends on the choice of the transfer fluid. Stagewise operation of three-phase exchangers in a closed evaporation and condensation cycle was recently proposed by Harriott and Wiegandt (6) for simultaneous cooling and heating of sea water and desalined water streams in countercurrent flow. Some experimental data of condensation of methylene chloride in water flowing concurrently in a two-stage packed bed and a single-stage sieve-plate column were reported (6). Limited data were also reported on cocurrent flow evaporation of methylene chloride or pentane in water in a 2-in. diameter column. Wilke et al. (7) studied evaporation of water from sea water flowing concurrently in direct contact with hot Aroclor in an horizontal 3-in. pipe and steam condensation in direct contact with Aroclor in packed columns. Studies of condensation of steam in Aroclor in a simulated cocurrent spray cohmn were also reported (8). Despite increasing interest in technical information on direct contact heat transfer with change of phase, very little is known regarding the basic mechanism and heat transfer characteristics associated with evaporation of dispersions in immiscible liquids. Studies of single-drop evaporation in immiscible liquid media were recently reported by Sideman et al. (9, 10). These, however, may not be directly extended to population of drops, where the onset of nucleation is not simultaneous (and should be dependent on the vapor holdup and degree of turbu
A simplified model for bi-component droplet heating and evaporation
International Journal of Heat and Mass Transfer, 2010
A simplified model for bi-component droplet heating and evaporation is developed and applied for the analysis of the observed average droplet temperatures in a monodisperse spray. The model takes into account all key processes, which take place during this heating and evaporation, including the distribution of temperature and diffusion of liquid species inside the droplet and the effects of the non-unity activity coefficient (ideal and non-ideal models). The effects of recirculation in the moving droplets on heat and mass diffusion within them are taken into account using the effective thermal conductivity and the effective diffusivity models. The previously obtained analytical solution of the transient heat conduction equation inside droplets is incorporated in the numerical code alongside the original analytical solution of the species diffusion equation inside droplets. The predicted time evolution of the average temperatures is shown to be reasonably close to the measured one, especially in the case of pure acetone and acetone-rich mixture droplets. It is shown that the temperatures predicted by the simplified model and the earlier reported vortex model are reasonably close. Also, the temperatures predicted by the ideal and non-ideal models differ by not more than several degrees. This can justify the application of the simplified model with the activity coefficient equal to 1 for the interpretation of the time evolution of temperatures measured with errors more than several degrees. Crown
Numerical investigation of droplet evaporation phenomena in the spray methods
IOP Conference Series: Materials Science and Engineering
Physical phenomena of the droplet evaporation occurred in the spray pyrolysis reactor tube has been studied. This model simulation was expected to predict the process occurred in the reactor tube and predicted the size and morphology of particle. The dynamical model of particles influenced by macroscopic parameters such as reactor wall temperature, flow rate carrier gas and concentration of precursor have been successfully simulated. The size of the particle, the number of aqueous vapors around droplet, air temperature around the droplet, temperature of droplet and solution concentration profile of particle in the reactor tube were simulated. The desired final particle can be achieved by controlling these factors.
2010
This paper presents a numerical investigation of heat and mass transfer inside a wet cooling tower with forced air draft, which find application in energy process industries and oil refineries. The mathematical model consists of mass, momentum and energy conservation equations, water droplet trajectories and their interaction with the gas phase, the computational domain and boundary conditions. Numerical distributions of air velocity, air temperatures, water vapor fractions and evaporation rates are shown and discussed. The wet cooling tower achieves an efficiency of around 80%, which can be improved by optimizing the value of the water droplet size, nozzle spray angle and water-to-air flow rate ratio. The water droplet size has a dominant effect on the cooling tower efficiency, whereas small droplets improve the efficiency up to 10%. On the other hand, the spray angle and the water-to-air ratio lead to slight improvements, about 2-3% in the best case.
A Note on Heat and Mass Transfer to a Spray Droplet
Nuclear Technology, 1988
Large scale simulation models can aid in improving the design of spray dryers. In this work an Eulerian-Lagrangian model with coupled gas phase and droplet heat and mass transfer balances is used to study airflow dynamics, temperature and humidity profiles at different positions in the spray. The turbulent gas flow is solved using large eddy simulation (LES). A turbulent dispersion model accounts for the stochastic subgrid fluid velocity fluctuations along the droplet trajectory. The dispersed phase is treated with Lagrangian transport of droplets, and collisions between droplets which are detected with a stochastic Direct Simulation Monte Carlo (DSMC) method. The outcome of a binary collision is described by collision boundary models for water and milk concentrates. The drying of droplets is modeled by the reaction engineering approach (REA). The effect of the inlet air conditions and of droplet viscosity on the temperature and humidity distributions are analyzed. Most of the heat and mass transfer occurs in the first 10-20 cm from the nozzle where the slip velocities and temperature and humidity driving forces are higher. The droplets size increases, both in the axial and radial direction, because of the dominance of coalescence over separation in the droplet spray studied here. Because the spray domain considered in this work is relatively small, the droplet residence time is small, and consequently the amount of evaporation is still low. The droplet size distributions of milk concentrates are affected by the predominance of coalescence over separation events. The coalescence dominated regime increases when the droplet viscosity is higher.
Numerical modelling of thermal spray systems
The purpose of this paper is to present a brief overview of the MD* thermal spray process, review the current existing models of heat transfer and fluid mechanics relevant to this process, outline a simplified thermal model implemented together with the results, and discuss the activities required for the implementation and verification of an accurate temperature model of the MD* thermal spray process.
Heat transfer in the film boiling regime: Single drop impact and spray cooling
International Journal of Heat and Mass Transfer, 2017
In this study a model for the heat transfer into a single drop impacting onto a hot solid substrate in the film boiling regime is developed. The model accounts for the expansion of the thermal boundary layers in the spreading drop and in the solid substrate, and for the evaporation of the liquid phase, leading to the creation of a thin vapor layer. An explicit expression for thickness of the vapor layer is obtained from the energy balance at the liquid-vapor and vapor-solid interfaces. The theory allows prediction of the heat transferred from the hot substrate to the single drop during impact. This quantity is then used for the development of a model for an average heat transfer coefficient for spray cooling in the film boiling regime. The model accounts for the probability of drop interactions on the wall, when the droplet number density in the spray is high. The theoretical predictions for the heat transfer coefficient agree well with existing experimental data.