Review on liquid film flow and heat transfer characteristics outside horizontal tube falling film evaporator: Cfd numerical simulation

doi:10.1016/j.ijheatmasstransfer.2020.120440

International Journal of Heat and Mass Transfer

Volume 163, December 2020, 120440

https://doi.org/10.1016/j.ijheatmasstransfer.2020.120440 Get rights and content

Highlights

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CFD numerical calculation method of horizontal-tube falling film evaporation is summarized.
•
Microscopic mechanisms of thermodynamic, flow and structural parameters on liquid film flow characteristics and heat transfer are analyzed by summarized CFD simulation results.
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Improvement and research directions of CFD simulation are suggested.

Abstract

The horizontal-tube falling film evaporation is an efficient heat transfer technology for energy-saving, with good economic and environmental benefits. In the falling film evaporation technology, the thickness and uniformity of liquid film are considered as important hydrodynamic characteristics that influence the heat transfer of fluid greatly. However, through experimental methods, it is difficult to obtain the microscopic characteristics and subtle changes of the velocity and temperature distribution inside the liquid film, and hard to describe the gas-liquid interface during the evaporation process of the liquid on the tube surface. Therefore, CFD simulation is used in the researches on the falling film flow and heat transfer characteristics outside the horizontal tube. This paper mainly reviewed the CFD simulation work of falling film evaporation outside the horizontal tube in the existing literatures. The models used by researchers were made a summary. The effects and microscopic mechanisms of flow parameters such as spray density, thermodynamic parameters such as the surface tension of liquid, structural parameters such as the liquid distributor and evaporation characteristics of heat transfer tubes and tube bundle were summarized and analyzed. Based on the researches’ CFD simulation results of hydrodynamics and heat and mass transfer of falling film evaporation, it is found that these parameters have their own reasonable ranges to ensure the efficient operation of falling film evaporators under specific operation conditions. Beyond the reasonable range, the formation of liquid film on the tube surface is to be destroyed, it will result in the decrease of heat transfer performance and the dryout. In addition, by adjusting the magnitude of the surface tension, wall adhesion, inertia force and other forces, the change of these parameters leads to the generation of vortex and recirculation phenomenon, the decrease of liquid film thickness and the enhancement of liquid film fluctuation, so as to reduce the thermal boundary layer thickness and enhance the heat and mass transfer performance of the film evaporation process. It is considered that under the comprehensive action of multi-parameters, the three-dimensional distribution of flow characteristics and heat transfer coefficient of multicomponent working fluid, as well as the change of flow pattern between tube bundles, are the improvement directions of current CFD simulation. And the research directions of future CFD simulation are the microscopic hydrodynamic and heat transfer characteristics caused by wall structure reconstruction and the influence of auxiliary components on the falling film evaporation process.

Section snippets

Abbreviations

two-dimensional

three-dimensional

CFD

computational fluid dynamics

CSF

continuum surface force

FVM

finite volume method

HTC

heat transfer coefficient, W/(m²•K)

PRESTO

pressure staggering option

PISO

pressure-Implicit with splitting of operators

UDF

user-defined function

VOF

volume of fluid

SIMPLE

semi-implicit method for pressure linked equations

Assumptions

Because the flow behavior and the heat and mass transfer of falling film evaporation process are very complex, it is difficult to simulate realistic flow conditions according to the actual state. Therefore, it is necessary to simplify the physical model by including various assumptions in the model. In the followings, the assumptions adopted by different researchers in the falling film evaporation simulations are classified.

(1)
The fluid is continuous and incompressible Newtonian fluid [18,34].
(2)
The

Influencing factors of liquid film flow and heat transfer characteristics

The falling film flow and heat transfer characteristics outside the horizontal tube are affected by the flow parameters such as spray density, spray temperature and heat flux, thermodynamic parameters such as surface tension and viscosity, as well as the structural parameters such as liquid distributor, tube bundle, heat transfer tube. These parameters produce effects on flow pattern between tubes and liquid film thickness distribution on the tube by adjusting the magnitude of surface tension,

Concluding remarks

Scholars used CFD technology to capture the details of flow field, and analyzed the mechanism of falling film flow and heat transfer from the micro-level. However, through the review of the existing literature, it was found that there are some improvement directions of current CFD simulation. Based on the comprehensive discussion of hydrodynamics and heat and mass transfer of falling film evaporation, the research directions of future CFD simulation were put forward, as well as the issues worth

Author statement

All authors certify that they have participated sufficiently in the work to take public responsibility for the content, including participation in the concept, design, analysis, writing, or revision of the manuscript. Furthermore, each author certifies that this material or similar material has not been and will not be submitted to or published in any other publication before its appearance in the International Journal of Heat and Mass Transfer.

Declaration of Competing Interest

The authors declared that there is no conflict of interest.

Acknowledgments

This research was supported by the National Natural Science Foundation of China (No. 51876135).

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Horizontal-tube falling-film evaporator is regarded as an effective alternative to full-liquid evaporator because of its higher heat transfer coefficient and lower refrigerant charge. In the process of falling-film evaporation, the thin liquid film thickness (δ) is an important parameter for characterizing the hydrodynamics, and accurate prediction of δ can effectively prevent the reduction in heat transfer efficiency caused by local wall dryout. In this study, non-azeotropic refrigerant R32/R134a was taken as the research object. By incorporating the multi-component phase change model and contact angle model into the governing equations, a two-dimensional numerical model of falling-film evaporation outside a horizontal circular tube was established. The effects of the spray height (H), tube diameter (d), Reynolds number (Re), inlet temperature (T_inlet), and mass fraction of R32 in the liquid-phase mixture (M_{R32_liquid}) on δ were analyzed under sheet flow. The results show that an increase in T_inlet, H, and d is favorable for increasing the average δ (δ_ave), whereas an increase in M_{R32_liquid} and Re leads to a reduction in δ_ave. Besides, the decreasing rate of the δ_ave decreases gradually as H and d increase. When H is 4 mm, the local δ (δ_local) first decreases and then increases as the circumferential angle (Φ) increases. When H ranges from 6 mm to 13 mm, as Φ increases, δ_local first increases slightly, then decreases, and finally increases significantly near the wake region. The δ_local of a larger tube diameter is thicker than that of a smaller tube diameter near the impact region, whereas in the subsequent regions, the δ_local of a larger tube diameter is thinner than that of a smaller tube diameter. The minimum δ_local occurs at Φ between 130° and 140°, and with an increase in H and d, Φ corresponding to the minimum δ_local decreases.
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ReviewReview on liquid film flow and heat transfer characteristics outside horizontal tube falling film evaporator: Cfd numerical simulation

Highlights

Abstract

Section snippets

Abbreviations

Assumptions

Influencing factors of liquid film flow and heat transfer characteristics

Concluding remarks

Author statement

Declaration of Competing Interest

Acknowledgments

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Review
Review on liquid film flow and heat transfer characteristics outside horizontal tube falling film evaporator: Cfd numerical simulation