Horizontal Falling Film Evaporators Research Articles

The simulation of droplet impact on liquid film evaporation in horizontal falling film evaporator based on smoothed particle hydrodynamics

PurposeIn this paper, the complex flow evaporation process of droplet impact on the liquid film in a horizontal falling film evaporator is numerically studied based on smoothed particle hydrodynamics (SPH) method. The purpose of this paper is to present the mechanism of the water treatment problem of the falling film evaporation for the high salinity mine water in Xinjiang region of China.Design/methodology/approachTo effectively characterize the phase transition problem, the particle splitting and merging techniques are introduced. And the particle absorbing layer is proposed to improve the nonphysical aggregation phenomenon caused by the continuous splitting of gas phase particles. The multiresolution model and the artificial viscosity are adopted.FindingsThe SPH model is validated qualitatively with experiment results and then applied to the evaporation of the droplet impact on the liquid film. It is shown that the larger single droplet initial velocity and the smaller single droplet initial temperature difference between the droplet and liquid film improve the liquid film evaporation. The heat transfer effect of a single droplet is preferable to that of multiple droplets.Originality/valueA multiphase SPH model for evaporation after the droplet impact on the liquid film is developed and validated. The effects of different factors on liquid film evaporation, including single droplet initial velocity, single droplet initial temperature and multiple droplets are investigated.

Convective heat transfer of falling film around the horizontal half-oval tube with reverse airflow in an evaporative condenser

Horizontal falling-film evaporation is frequently used in industrial air conditioning system to improve its thermal performance. Transient heat transfer characteristics on the surface of a circular tube have been investigated, while the non-circular tube such as half-oval tube has been rarely reported. This study aims to investigate the thermal performance of liquid film around a half-oval tube by adopting transient simulations with laminar and SST k-ω models, respectively. Simulations consider varying wind velocity, liquid Reynolds number, heat flux and length-width ratio of the tube. Simulated results are quantitively validated by comparing with reported experimental results, and the average differences are below 9.5 %. Results indicate that the local convective heat transfer coefficient (CHTC) increases with wind and the average CHTC is at least 5.98 % higher than the case without wind. Although the influences of liquid Reynolds number and heat flux show slight differences, they still can improve the convective heat transfer. The local CHTC exhibits an escalating trend with length-width ratio of 1–5. Meanwhile, the average CHTC for the four tested half-oval tubes is at least 6.8 % higher than the values of the circular tube. An empirical correlation for predicating the average CHTC is established by regression fitting with a mean deviation of 4.6 %. These findings help to design the falling-film evaporation system by adopting the half-oval tube in industrial air conditioning system.

Modeling and simulation of a helical falling film evaporator to improve its performance and design

This work presents a computational model of a helical falling film evaporator of 13 tubes based on physical principles. The momentum and energy transfer of the system are modeled. The work aims to understand the behavior of a horizontal falling film evaporator and to improve the evaporator design by analyzing the heat transfer. In the evaporator, the wetting efficiency varies in each tube because the evaporation of the falling film is performed gradually. Therefore, the behavior of the evaporator variables, such as the wetting efficiency in each tube, is studied. Furthermore, the model granularity based on the analysis of the subcooling and saturation zones is investigated. The effect of the film breakdown onset Reynolds number and the mass flow rate on the evaporator performance is studied. This analysis shows that the maximum heat transfer rate of the simulation is reached when the falling film Reynolds numbers of the tubes are as close as possible and pass by the film breakdown onset Reynolds number value. Additionally, the results indicate that at a low mass flow rate, the heat transfer is carried out almost completely in the first five tubes, and at a higher mass flow rate, the heat transfer is more significant in the last tubes. Finally, a comparison between the evaporator of 13 tubes and an arrangement of two evaporators of five tubes each one is performed. From this analysis, the results show that the heat transfer rate of the arrangement of two evaporators is 21.16 % and 18.69 % higher than the evaporator of 13 tubes at 0.00529 and 0.00793 kg/s⋅m, respectively.

Numerical Study of Liquid Film Formation around Tubes of Horizontal Falling Film Evaporator

Falling film evaporative heat exchangers are extensively used in processing industries; broad areas of application being refrigeration, desalination and food processing industries. The fundamental aspect of this type of heat transfer process is to extract the process heat in the form of latent heat by liquid which is sprayed over the surface of the process tubes. Formation of liquid film over a fully wetted horizontal round tube of falling film evaporator has been numerically simulated here. Two numerical approaches, Volume of Fluid (VOF) technique and the Eulerian multiphase model are applied to compare their results. The effect of varying flow and geometrical parameters on the film thickness is investigated. Two horizontal tubes of diameter 19.05mm and 25.04mm with three different uniform spacing have been selected for simulation. Film Reynolds numbers 650, 950 and 1250 are considered for the above set of parameters. Ii is observed that the geometrical and flow parameters considerably influence the film thickness. Transient analysis of the film formation has been carried out and parameters like pathline of liquid film and the velocity profile have been obtained for understanding the flow behavior in a better manner. All the simulated results agree well with the published data.

Review on CFD analysis of horizontal falling film evaporators in multi-effect desalination plants

Review on CFD analysis of horizontal falling film evaporators in multi-effect desalination plants

Seawater Desalination Pilot Plant: Optimal Design and Sizing of Solar Driven-Four Effect Evaporators Combined with Heat Integration Analysis

This paper puts forward the results and methodology used for thermal analysis, sizing and design of four effects pilot scale desalination unit combined with solar thermal energy and using falling film evaporators for a production capacity of 7m3/day. The modelling equations are given at steady-state conditions and are based on mass and heat balances equations, heat transfer equations, and thermodynamic and physical properties of each stream. MATLAB programming is used for the resolution of the developed algorithm. The impact of the important parameters, such as evaporator type, tube dimensions, heating medium and top brine temperatures on the performance ratio and heat transfer rate, has been studied. Besides, this work is completed by Pinch analysis to provide the optimal Heat Exchanger Network (HEN) taking into account three different configurations and the impact of the minimum temperature difference ∆Tmin. Results show that it is beneficial to use the horizontal falling film evaporators allowing high heat transfer coefficient values ranging between 3.18 and 3.32kW/m2/K. In addition, the optimal HEN enables a 62% reduction of thermal power needed to feed seawater heating. Also, in this study, several suggestions and criteria for evaporators and HEN design and optimization have also been dealt with.

Numerical simulation of heat transfer in a horizontal falling film evaporator of multiple-effect distillation

The presented work describes a mathematical model for the simulation of the heat transfer in a horizontal falling film evaporator for sea water desalination in the context of a multi-effect distillation process. The simultaneous evaporation and condensation of falling films on the inside and outside of a horizontal tube is mathematically described and solved numerically. For the description of radial heat transfer, the pipe circumference is divided into an impingement zone and a zone of laminar flow. Different model equations are implemented for both zones. Moreover, the accumulation of condensate at the bottom of the pipe, as well as liquid motion in axial direction is described. The inside pressure drop along the tube, as well as the temperature changes were calculated.The model of outer heat transfer is validated for a limited range of Reynolds numbers. It is compared to experimental data and numerical results from literature and it is shown how the application of the model may be extended to a larger range of outside-Reynolds numbers. Regarding the phenomena inside the tube, the calculated results indicate a significant relevance of the pressure drop on the heat transfer for increased pipe lengths. The reducing influence of accumulating condensate on the overall heat transfer is identified as comparably low. Finally, various combinations of pipe diameters and lengths are analyzed and the associated transmitted heat flows are graphically summarized.

MED 담수기내 수평관 강하막식 증발기의 3D 수치해석적 연구

In the present work, a numerical study of a horizontal falling film evaporator in a multi-effect distillation (MED) plant is performed. Tube bundles in the evaporator are described as porous media, and a volume-averaged method is applied. To calculate the fluid flow and phase change in the evaporator due to heat transfer in the system, FLUENT and user-defined functions (UDF) are used. To observe the performance of the evaporator under different operational conditions, tests are conducted for a steam mass flux ranging from 0.5 to 2.5 in the horizontal tube, for mass fraction of the noncondensable gas in the tube inlet ranging from 0% to 1%, and for film Reynolds numbers ranging from 100 to 1,000 for the falling film. The evaporation rate increases with the steam mass flux and Reynolds number. In contrast, the evaporation rate decreases by 0.87% with a 1% increase in the mass fraction of the noncondensable gas in the tube.

Experimental Study on the Liquid Distributor of Horizontal Tube Falling Film Evaporator

A calandria liquid distributor device was designed for horizontal falling film evaporation. The guide plate was introduced in this, which is easy to manufacture and can effectively reduce splash phenomenon. This paper studies the liquid distribution from the relation between distributive unwell-distributed coefficient and liquid flux of distributor tube and distributor hole. The result showed that the distributive unwell-distributed coefficient of in liquid distributor was decreased along with accretion of liquid flux, as apposed to homogeneous perforated. The unwell-distributed hole coefficient of distributor tube No.0 was increased along with accretion of flux, as apposed to distributor tube No.1 and distributor tube No.2.

Falling film evaporation on enhanced tubes, part 1: Experimental results for pool boiling, onset-of-dryout and falling film evaporation

Horizontal falling film evaporators have the potential of displacing flooded evaporators as the standard in the refrigeration industry due to compelling advantages, such as smaller refrigerant charge and lower pressure drop. However, there is a need to improve the understanding of falling film evaporation mechanisms to provide accurate thermal design methods. In this work, falling film evaporation measurements on a single tube, a vertical row of ten horizontal tubes and a small tube bundle with three rows of 10 tubes each are presented. Two enhanced boiling tubes, the Wolverine Turbo-B5 and the Wieland Gewa-B5, were tested using R-134a and R-236fa. The tests were carried out at a saturation temperature of Tsat = 5 °C, film Reynolds numbers ranging from 0 to 3000, and heat fluxes between 15 and 90 kW m−2 in pool boiling and falling film configurations. A visualization study was performed under diabatic conditions to study the flow.

Case study on the CO 2 release in horizontal falling film evaporator

The release of non-condensable (NC) gases, especially CO 2 in multiple-effect distiller (MED) for seawater desalination has been barely investigated and has very less publications available in it. NC gases which are carbon dioxide (CO 2), oxygen (O 2) and nitrogen (N 2) notably affect the heat transfer for condensation zone, energy consumption, the performance and material life span of the distillers. The understanding of CO 2 release in MED is very important knowledge for design and better operation of desalination distillers. The present paper describes a model of CO 2 release and the carbonate system in falling film horizontal tube-evaporators, physical properties of seawater and following correlations. The model was applied 5-effects MED, operating at top brine temperature (TBT) of 60–75 °C. The CO 2 release rates as well as depend on the salinity and top brine temperature of the inlet seawater and the specific CO 2 release decreased first to last effect.

Comparison of heat transfer coefficient correlations for thermal desalination units

This paper presents several simulator models allowing studying the behavior of different types of thermal desalination plants [1]. Three types of evaporators have been distinguished in thermal desalination plants: horizontal falling film evaporators (HFF), vertical falling film evaporators (VFF) and vertical rising film evaporators (VRF). An in depth study of the correlations adopted to calculate the heat transfer coefficients (HTC) of the selected evaporator/condenser has been made, in order to predict the plant performance of dual-purpose plants. Once the sensitivity analysis of the possible HTC has been done, the selected HTC has been implemented in a complex simulation model of MSF and MED/VC/TVC desalination units. Validation (if possible) of the calculated HTC was also done with the real data encountered in the literature.