Abstract: Multi-effect distillation is an important principle in desalination technology, in which the thin film evaporation of sea water takes place at the outer surface of bundle of tubes, normally arranged in horizontal. Heat of condensation inside the tube is transferred to the falling water film by conduction, thereafter convective evaporation of film takes place. Heat transfer on the outside is unpredictable due to the uncertainty in nature of film, film thickness, film dryout, etc. and film coefficient is considerably less compared to inside. Thermal spray coating on metallic surface with molten metal particles create porous surface which enhances heat transfer due to higher turbulence and nucleation sites, if sufficient temperature difference available. This paper discusses the experimental studies carried out on a 3 × 5 bundle of 25.4 mm ∅ Al horizontal tubes with thermal spray coating of aluminum. Influences on temperature profile, heat transfer coefficient, mass evaporation, etc. are studied. A comparison of sea water and fresh water is carried out for the performance of the system. The experimental setup is established at IIT Madras, Chennai, India. © 2014 Balaban Desalination Publications. All rights reserved.

A Mani

Department of Mechanical Engineering

Desalination

Evaporation

experimental study

FRESHWATER

Heat transfer

SEAWATER

TEMPERATURE PROFILE

CHENNAI

India

Tamil Nadu

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Experimental studies on thermal spray-coated horizontal tubes for falling film evaporation in multi-effect desalination system

Desalination and Water Treatment

In falling film evaporation process, the heat is transferred from the condensing fluid side to the liquid flowing on evaporating side. Tube geometry and tube size have an important role on the performance of the falling film evaporators. The current article presents a two-dimensional computational fluid dynamics study of water falling film evaporation on a thermal spray metal coated vertical corrugated conduits. Two-phase flow simulation is carried out by using a finite volume method-based commercial software, using k-ω equations with shear stress transport. Sinusoidal corrugations with different porosity have been selected for this study. Heat and mass transfer during falling film evaporation are included through user defined functions. Effect of Reynolds number, wall superheat, and surface roughness on heat transfer coefficient is presented. Numerical results are compared with the results of horizontal circular tube, as well as corrugated plate conduits falling film evaporation from literature. An enhancement of film heat transfer coefficient of about 15% is observed for the vertical corrugated plate conduits. Copyright © 2017 ASHRAE.

Fulltext

Science and Technology for the Built Environment

Falling film evaporation on a thermal spray metal coated vertical corrugated plate conduits

Horizontal tube falling film evaporation finds wider applications in multi-effect distillation system in recent years. The latent heat released inside the tube due to condensation of vapor is transferred to the falling film on the outer surface of the tube resulting in convective evaporation of water film. The evaporator consists of multiple rows and columns of horizontal tubes. Heat transfer from condensing film inside the tube bundle is more or less uniform while there is a large variation in heat transfer outside the tube bundle. This paper focuses on Computational Fluid Dynamics (CFD) analysis of falling film evaporation of seawater on a single tube and bundle of tubes using ANSYS Fluent 13.0®. CFD results are validated with published data available in the literature and also with experimental studies carried out. The effect of feed rate, tube diameter, wall temperature, etc. on the heat transfer is studied. Local film coefficient around the tubes of 19.05, 25.4 and 50.8 mm θ for different film Reynolds numbers is discussed. It is observed that convective evaporation heat transfer performance increases with feed rate, but decreases with tube diameter. In-line tube configuration is found to be better compared to staggered tube configuration. © 2015 Elsevier B.V.

Heat transfer characteristics in horizontal tube bundles for falling film evaporation in multi-effect desalination system

Horizontal tubefalling film evaporators find various applications like multi effect distillation for sea water desalination, power and process applications, refrigeration applications, etc. In this system, latent heat released inside the tube due to condensation is transferred to the falling film on the tube surface resulting in convective evaporation. Among many heat transfer enhancement techniques, thermal spray coatings enjoy diverse applications with economic advantage for commercial applications. This paper focuses on Computational Fluid Dynamic (CFD) analysis of falling film evaporation of water on horizontal tubes under the influence of gravity at vacuum. ANSYS Fluent with VOF two-phase model is used for the falling film studies on the horizontal tube. Half section of the tube is modeled with gravitational flow of sea water from the top. This model is used to predict the enhancement in convective evaporation on thermal spray coated tube surface with varying roughness. These CFD results are validated with published experimental data available in the literature. From this study, it is observed that heat transfer coefficient is increased by 10-15% due to increased roughness for laminar convective film boiling under vacuum. The film coefficient enhancement is higher for the larger diameter tubes. The tubes in the tube bank located beneath the top tube exhibit reduction in performance, which may be due to partial drying out of film on tube surface or due to evaporation and maldistribution of flow. At high heat flux and low feed rate, the decrease in film coefficient is more predominant. The effect of feed rate, tube diameter, wall temperature, heat flux, tube arrays, etc. on the heat transfer characteristics is studied and presented in this paper. © 2013 Desalination Publications.

Effect of flame spray coating on falling film evaporation for multi effect distillation system

International Journal of Heat and Mass Transfer

Influences of solution subcooling, wall superheat and porous-layer coating on heat transfer in a horizontal-tube, falling-film heat exchanger

Experimental study of falling film evaporation heat transfer coefficient on horizontal tube

Distribution characteristics of falling film thickness around a horizontal tube

Falling water film evaporation on newly-designed enhanced tube bundles

Journal	Data powered by TypesetDesalination and Water Treatment
Publisher	Data powered by TypesetTaylor and Francis Inc.
ISSN	19443994
Open Access	No