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

doi:10.1016/j.desal.2015.06.018

Desalination

Volume 375, 2 November 2015, Pages 129-137

https://doi.org/10.1016/j.desal.2015.06.018 Get rights and content

Highlights

•
2 dimensional modelling of falling film evaporation
•
Prediction of film coefficient and evaporation for different tube sizes
•
Experimental studies on falling film evaporation
•
Comparison of in-line staggered tube layouts in tube bundle
•
Comparison of CFD, experimental data and published literature

Abstract

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 ∅ 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.

Introduction

Falling film evaporators find more applications in refrigeration and desalination industries due to less liquid inventory, high heat transfer coefficient, low temperature drop, low pressure drop, etc. Falling film evaporation takes place outside the tube geometry with two-phase heat transfer. Convective evaporation as well as nucleate boiling occurs in the film as it flows over the tube depending on the heat flux conditions. There are three modes of flowing film which are drop mode, column mode and sheet mode which is related to the fluid flow rate. Heat transfer coefficient around tube surface is influenced by the film Reynolds number and the mode of the film. Film mode in turn depends on film Reynolds number. Heat transfer in tube bundles compared to a single tube is significant as evaporators and condensers made of in-line or staggered tubes of considerable numbers. Most of the theoretical and experimental research on falling film evaporation is based on a single tube. In multi-effect desalination (MED) system, seawater is sprayed over the tube bundle and flows as a thin film over the tubes due to gravity under vacuum conditions. Liquid mal-distribution and local dry out are common problems in such systems. Normally upper tubes experience excess flow rate and film thickness while the lower strata in tube bundle have dry out conditions. Experimental investigations conducted in the past show different trends in heat and mass transfer due to the simultaneous occurrence of nucleate boiling and convective evaporation. At lower heat flux only convective evaporation takes place and normally film coefficient increases with film Reynolds number. At high heat flux conditions nucleate boiling takes place as observed in experimental studies and the film Reynolds number has no significant influence. Falling film evaporators normally work under low heat flux conditions with convective evaporation. This paper discusses the Computational Fluid Dynamics (CFD) studies as well as experimental investigation on horizontal tube bundle for heat and mass transfer under various operating conditions.

A review of investigations on tube bundle was published by Ribatski and Jacobi [1]. Working fluids used in most of the investigations are refrigerants such as R 134a and ammonia. Fujita and Tsutsui [2] carried out experimental studies on 5 vertical tubes with refrigerant R-112 and the highest film coefficient was observed at the topmost tube. Zeng et al. [3] carried out experimental studies on in-line and staggered tubes with ammonia. It was reported that rectangular pitch configuration gave better results. Experimental studies of Moeykens et al. [4] carried out with R-134a and R-123 reported that staggered pitch gave better results compared with in-line tubes at high heat flux conditions. Roques et al. [5] carried out experimental studies with 10 tubes in vertical array using R-134a and observed that film coefficient reaches a stabilized value at Re_f = 500. It is also reported that the lower tubes showed a higher film coefficient. Habert M. [6] carried out experimental with R134a and R236fa to study the effects different working fluids, type of tubes, feed rates and heat flux conditions. It was reported that tubes present in the 4^th to 6^th row gave better film coefficients.

Sharma R. and Mitra S.K. [7] conducted studies on tube bundles with 120 rows of 22 mm tube size adopting triangular tube configuration for brackish water at atmospheric conditions. Tube drying and reduction in heat transfer coefficient was observed below Re_f = 400. Lower tubes in the bundle experienced reduction in film coefficient and liquid evaporation. Theoretical and experimental investigations on tube or tube bundles were presented by Yang and Wang [8], We Li et al. [9], [10], Mu et al. [11], Jani et al. [12] and Shen et al. [13] in recent times. In certain cases the film coefficient increased with feed rate, stabilized or decreased beyond a limit. There observed a gradual reduction in heat transfer coefficient with bundle depth and film dry out occurred in low feed rates. Present study, using CFD and experimental methods, was due to the lack of information on tube bundles compared to single tube and to study the reduction in performance in tubes in the vertical direction in the bundle. A comparison of in-line and staggered tubes were carried out with sea water and fresh water which has applications in seawater desalination.

Section snippets

Factors influencing film heat transfer

In falling film evaporation, seawater is sprayed on the top of the tube and allowed to flow along the curved tube surface. According to Chyu and Bergles [14] flow region can be divided into four. Liquid jet is impinging on top of the tube where stagnation region is formed. At the impingement zone, the flow takes a sharp turn and because of this the film coefficient is high due to the high velocity gradient. This is followed by the developing region, where the heat transfer coefficient decreases

CFD modeling

CFD is a modern tool to study complex two-phase heat transfer phenomenon such as falling film evaporation of water under vacuum conditions. For the present studies, horizontal tube of 19.05, 25.4 and 50.8 mm OD are considered. Tube wall is maintained at constant temperature of 64.6 °C and thin film of sea water is made to flow over it by gravity. The saturation temperature of vapor at 62.6 °C and the feed water temperature are at 58.6 °C respectively. Convective evaporation takes place under vacuum

Experimental studies on tube bundles

Experimental studies were carried out on tube bundle with the same configuration as in CFD studies with aluminum tubes of 25.4 mm OD. The set up consisted of an evaporator with 0.5 m OD × 1.5 m long shell with 6 mm thickness. There were 15 numbers of aluminum 6061 tubes of mm OD × 1.4 m long. The tubes could be interchanged as it is fitted with special grommets. Tube configurations could be changed by replacing the tube sheets in the evaporator. Falling water film over the evaporator tubes was heated

Single tube

CFD studies were carried out on single tube as well as bundle of in-line and staggered configuration. Experimental study was carried out on tube bundle with in-line and staggered configuration. Both the studies used fresh water (ground water) and seawater at 35,000 ppm salinity as working fluid.

Conclusions

CFD study on single tube and a bundle of tubes was carried out and it shows that heat transfer coefficient is highest for the top tube in the bundle and decreases proportionately to the position of tube in the bundle. Bundle depth effect is observed within the first 10 rows of tubes. Heat transfer coefficient is stabilized within first ten rows. As the Reynolds number increases, the film heat transfer coefficient also increases and stabilizes due to the increase in film thickness. Film heat

Nomenclature

heat transfer area, m^− 2

Archimedes number, g d_o³ ν^− 3

specific heat, J kg^− 1 K^− 1

tube outside diameter, m

gravitational acceleration, m s^− 2

heat transfer coefficient, Wm^− 2 K^− 1

h_fg

latent heat of vaporization, kJ/kg

thermal conductivity, Wm^− 1 K^− 1

vertical position of tube

Nusselt number, hν^2/3 k^− 1 g^− 1/3

Prandtl number, μCpk^− 1

outer tube radius, m

Re_f

film Reynolds number, 4Γμ^− 1

tube pitch, m

temperature, K

velocity, m/s

jet width, m

Subscripts

bundle

developing

evaporation

liquid

fully developed

vapor

impingement

Acknowledgment

Authors acknowledge the financial support received from the National Institute of Ocean Technology, Chennai (NIOT/F&A/PROJ/EFW01/2010) for the studies.

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Performance model for a horizontal tube falling film evaporator

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View full text

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

Highlights

Abstract

Introduction

Section snippets

Factors influencing film heat transfer

CFD modeling

Experimental studies on tube bundles

Single tube

Conclusions

Nomenclature

Subscripts

Acknowledgment

Int. J. Refrig.

Int. J. Heat Mass Transf.

Int. J. Heat Mass Transf.

Int. J. Heat Mass Transf.

Int. Commun. Heat Mass Transfer

Desalination

Experimental investigation of falling film evaporation on horizontal tubes

J. Heat Transf.

Spray evaporation heat transfer performance of R-123 in tube bundles

ASHRAE Trans. Am. Soc. Heat. Refrig. Aircondition. Eng.

Falling film transitions on plain and enhanced tubes

J. Heat Transf.

Falling film evaporation on a tube bundle with plain and enhanced tubes

Laboratory of Heat and Mass Transfer (LTCM)

Performance model for a horizontal tube falling film evaporator

Int. J. Green Energy