Measurement on falling film thickness distribution around horizontal tube with laser-induced fluorescence technology

Falling film on horizontal tube evaporators, of both Mechanical Vapor Compression (MVC) and the Multi-Effect Distillation (MED) desalination systems, plays an important role in the heat and mass transfer (evaporation) and accordingly the systems productivity. Falling film thickness is mainly influenced by the intertube space, circumferential angle and the film’s Reynolds number. This paper presents two-dimensional numerical study of falling film thickness around horizontal tube in MVC and MED evaporators. The study is based on computational fluid dynamics (CFD) using volume of fraction (VOF) as a multi-phase technique in ANSYS Fluent. The numerical model is developed in order to study the heat and mass transfer charactristics, the liquid falling film behaviour and thickness distribution around circular horizontal. Four CFD study cases are developed to simulate the falling film behaviour at circumferential angle range from 150 to 1650 with inter-tube spacing of 10 mm, 16 mm, 33 mm and 40 mm and for constant value of flow rate and at the same surrounding conditions. Simulations are conducted using a domain of only two tubes with 20 mm outer diameter.The results from the numerical models are compared with the published experimental correlations, showing a comparatively reasonable agreement. In addition, a parametric study is carried out to illustrate the effect of flow Reyonlds number (Re) and intertube space on the average circumferential film thickness and heat transfer rates.

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Three-dimensional film thickness distribution of horizontal tube falling film with droplet and sheet flow

International Journal of Multiphase Flow ◽

10.1016/j.ijmultiphaseflow.2021.103933 ◽

2021 ◽

pp. 103933

Author(s):

Xueshuo Chen ◽

Jie Wang ◽

Tao Lu ◽

Jiang Sheng ◽

Xue Chen

Keyword(s):

Film Thickness ◽

Thickness Distribution ◽

Three Dimensional ◽

Horizontal Tube ◽

Falling Film ◽

Sheet Flow

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NUMERICAL INVESTIGATION OF FILM DISTRIBUTION IN HORIZONTAL-TUBE FALLING FILM EVAPORATOR

International Journal of Air-Conditioning and Refrigeration ◽

10.1142/s2010132511000521 ◽

2011 ◽

Vol 19 (03) ◽

pp. 177-183 ◽

Cited By ~ 1

Author(s):

JIN-BO CHEN ◽

QING-GANG QIU

Keyword(s):

Numerical Simulation ◽

Fluid Flow ◽

Film Thickness ◽

Liquid Film ◽

Numerical Investigation ◽

Horizontal Tube ◽

Flow Density ◽

Falling Film ◽

Film Distribution ◽

Simulation Results

The technique of horizontal-tube falling film has been used in the cooling and heating industries such as refrigeration systems, heating systems and ocean thermal energy conversion systems. The comprehensive performance of evaporator is directly affected by the film distribution characteristics outside tubes. In this paper, numerical investigation was performed to predict the film characteristics outside the tubes in horizontal-tube falling film evaporator. The effects of liquid flow rate, tube diameter and the circular degree of tube on the film thickness were presented. The numerical simulation results were compared with that of the empirical equations for calculating the falling film thickness, and agreements between them were reasonable. Numerical simulation results show that, at the fixed fluid flow density, the liquid film is thicker on the upper and lower tube and the thinnest liquid film appears at angle of about 120°. The results also indicate that, when the fluid flow density decreases to a certain value, the local dryout spot on the surface of the tube would occur. In addition, the film thickness decreases with the increases of the tube diameter at the fixed fluid flow density.

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