Flow Characteristics Of Falling Film Research Articles

Numerical and experimental study on the falling film flow characteristics with the effect of co-current gas flow in hydrogen liquefaction process

Liquid hydrogen storage and transportation is an effective method for large-scale transportation and utilization of hydrogen energy. Revealing the flow mechanism of cryogenic working fluid is the key to optimize heat exchanger structure and hydrogen liquefaction process (LH2). The methods of cryogenic visualization experiment, theoretical analysis and numerical simulation are conducted to study the falling film flow characteristics with the effect of co-current gas flow in LH2 spiral wound heat exchanger. The results show that the flow rate of mixed refrigerant has a great influence on liquid film spreading process, falling film flow pattern and heat transfer performance. The liquid film of LH2 mixed refrigerant with column flow pattern can not uniformly and completely cover the tube wall surface. As liquid flow rate increases, the falling film flow pattern evolves into sheet-column flow and sheet flow, and liquid film completely covers the surface of tube wall. With the increase of shear effect of gas-phase mixed refrigerant in the same direction, the liquid film gradually becomes unstable, and the flow pattern eventually evolves into a mist flow.

Effect of rhombus mesh on 3D falling film flow characteristics over microchannel flat tube for LiBr (Lithium bromide) absorber

Effect of rhombus mesh on 3D falling film flow characteristics over microchannel flat tube for LiBr (Lithium bromide) absorber

Numerical study on the effect of co-current gas flow on the falling film flow characteristics outside the horizontal tube

Gas-liquid interactions are a ubiquitous feature of falling film exchangers in the nuclear industry. In the current research, a 2D model is established to explore the impact of co-current gas flow on the hydrodynamic characteristics of the falling film outside the horizontal pipes. This study examines different factors, including film flow rate, tube diameter, co-current gas flow velocity, transverse pitch ratio, distributor height, and inlet liquid temperature. The results revealed that the co-current gas flow significantly reduces the film thickness in the strong gas-liquid interaction zone and intensifies the fluctuating motion in the weak portion. Lead to the location of the minimum film thickness shifting upwards to around 90°, which highlights the need to monitor this region to prevent the occurrence of film rupture or dry spots that negatively impact heat transfer performance. When the gas flow velocity is constant, the lower liquid film flow rate, the larger tube diameter, and the distributor height enhance the impact of gas flow velocity on the falling film, resulting in reduced liquid film thickness. A film thickness correlation applicable to both with and without co-current gas flow is established, and 91% of 900 prediction data fell within ±15% of the numerical results.

Numerical study on the effect of surface wettability on the falling film flow characteristics of the spiral wound tube heat exchanger

Falling film flow is critical in the shell-side heat transfer performance of spiral-wound heat exchangers, and the wettability of the tube surface has a significant effect on the falling film flow characteristics. Therefore, a three-dimensional numerical model with two rows of tubes was established to study the influence of surface wettability on the liquid film distribution outside the tubes of a spiral wound heat exchanger. The transient spreading characteristics of the liquid film under different wettabilities were studied, the thickness of the liquid film along the axial and circumferential directions of the tube surface was quantitatively measured, and the effects of the tube spacing and Reynolds number on the flow characteristics of the falling film were analyzed. The results indicate that for the upper half-tube, increasing the contact angle to a certain extent is beneficial for improving the uniform distribution of the liquid film, whereas for the lower half-tube, decreasing the contact angle can suppress the fluctuation of the liquid film. The influence of the contact angle on the axial spreading of the liquid film was greater than that in the circumferential direction. When the contact angle does not exceed 30°, increasing Re can improve the wetting ratio, otherwise, increasing Re number has a limited effect on improving the wetting ratio. A smaller tube spacing can increase the thickness of the liquid film outside the tube. When the tube spacing changed from 4 mm to 8 mm, the average liquid film thicknesses of the circular, elliptical and egg-shaped tube decreased by 4%, 30.7% and 59.6% respectively.

Effect of Structural Optimization of Scrubbing Cooling Rings on Vertical Falling Film Flow Characteristics.

In order to study the influence of the structural optimization of the scrubbing cooling ring in the scrubbing cooling chamber on the flow characteristics of the vertical falling film, the flow characteristics of the turbulent falling film in the rising section of the development region at different internal platform heights of the scrubbing cooling ring and a high Reynolds number were studied by FLUENT software. First, the correctness of the model was verified by the maximum error of simulation and experimental results of no more than 9.836%. Then, the distribution of liquid film thickness (δ), velocity (V), and turbulence intensity (Iz) at 0° of the tube in the axial direction x = 0–500 mm were calculated and obtained when the platform height (H) was 0–30 mm and the liquid film Reynolds number (Rel) = 1.1541 × 104–3.4623 × 104. The results showed that δ in the entrance region increased sharply due to the “jet” effect with solid wall constraints formed by the structure of the water inlet pipe and the scrubbing cooling ring. On the contrary, the liquid film in the fully developed region showed a stable fluctuation trend due to the weakening of the “jet” effect. When H = 30 mm, the change of δ was relatively stable and the change of Iz was small, indicating that this platform height is conducive to the stable and uniform distribution of the liquid film. In addition, when Rel < 1.1541 × 104, the liquid film was unstable due to the low flow rate and insufficient cohesion of the liquid film, but V increased slightly. In addition, with the increase of Rel, δ did not change significantly along the axial direction, that is, the Plateau–Rayleigh hindered the growth of δ. Finally, the empirical formula for δ applicable to Rel = 1.1541 × 104–3.4623 × 104 at the axial fixed position was fitted for the first time.

Optimization of Corrugated Sheet Packing Structure Based on Analysis of Falling Film Flow Characteristics

The falling film flow characteristics of a liquid on the surface of corrugated sheet packing are crucial for its mass transfer performance in various industrial applications. In this study, a falling film flow experiment with laser-induced fluorescence technology was conducted to validate the flow characteristics of a falling film simulated using computational fluid dynamics (CFD). The influences of Reynolds number (Re) and the packing structure on flow characteristics were analyzed with quantitative film thickness and wetted area obtained through three-dimensional simulation. The results show that the CFD model can accurately predict the liquid falling–film flow behavior and calculate the characteristic parameters. For sinusoidal corrugated sheets, when Re reaches 500, the groove flow changes into a rivulet flow along the adjacent ripples and the wetted area is at its largest, about 0.022 m2. However, relative to the geometric area of the corrugated sheet, the wetted area can only reach 20% of the surface area, and the overall wetting performance is still poor. Triangular and trapezoidal corrugated sheets were further proposed and proved to improve the wetting area compared with the sinusoidal sheet, with maximum increases of 23% and 9%, respectively. On this basis, extensive research was carried out on the corrugation angle. The results show that a triangular corrugated sheet with a 75° corrugated angle was more conducive to the flow of the liquid film, and the wetted area was 38.8% of the surface area.

Experimental and numerical study on the falling film flow characteristics outside circular tube applied in floating liquefied natural gas (FLNG) under offshore conditions

Spiral wound heat exchanger (SWHE) relying on falling film evaporation and boiling is often used for FLNG. The performance of SWHE can be impacted strongly by the motion of the FLNG caused by the wave and typhoon. The falling film characteristics of SWHE outside circular tube are studied experimentally and numerically by a visualization experimental device based on the high-speed camera and a numerical model based on the dynamic grid. The results show that the wave crest of the liquid film moves to the titled side under offshore conditions. The evolution process of falling film flow pattern outside circular tube with the tilt angle of 9° can be divided into four stages: droplet formation and migration, liquid column formation and migration, liquid column coalescence, liquid sheet formation. A correlation permitting the prediction of the falling film flow pattern outside circular tube and the other one permitting the prediction of the average film thickness of circular tube are developed respectively based on the experimental and numerical data.

Falling film flow characteristics on horizontal tubes and their effects on scale formation in seawater evaporators

Falling film flow characteristics on horizontal tubes and their effects on scale formation in seawater evaporators

Three-dimensional numerical simulation of gas-liquid falling film flow characteristics on the airside of finned-tube heat exchanger with a typical large fin pitch

The closed-type heat-source tower (CTHST) heat pump system is a more promising alternative to the conventional air-source heat pump (ASHP) unit compared with the open-type heat-source tower (OTHST) heat pump system. However, the heat transfer efficiency of the CTHST is significantly lower than that of the OTHST, resulting from its indirect heat transfer way, liquid film break-up and high gas phase pressure drop. To provide a theoretical basis for the heat-mass transfer enhancement and structural optimization design of the high-performance CTHST, a three-dimensional (3D) model taking gas-liquid interfacial friction force into consideration was established in the present paper to study counter-current and co-current gas-liquid falling film flow characteristics on the airside of the finned-tube heat exchanger (FTHE) with a typical large fin pitch used in the CTHST. The predicting results of liquid film thickness based on the established model show good agreement with those of experiments in the reference, with a maximum deviation less than 5%, meaning that this model is reliable. Then the effects of contact angle, air flow direction, air Reg and liquid film Rel on liquid film characteristics were studied. The prediction correlation of average liquid film thickness was presented. The pressure drop characteristics of gas phase were analyzed in the co-current gas-liquid flow under three different liquid film Rel. Meanwhile, the change law of interfacial friction force with air Reg was revealed. The numerical results show that the critical mass flow rate of complete film flow on the finned-tube surface with contact angle of 30° is 0.06 kg/(m⋅s). The average liquid film thickness on the surface of the FTHE is obviously higher than that on the vertical plate. There is a close and positive quadratic curve relation between average interfacial friction force and air Reg. To obtain a complete film flow and small air pressure drop, the air Reg should be less than 2190.7 and 3286.0 in the counter-current and co-current gas-liquid falling film flow, respectively.

Experimental and theory investigations on falling film flow characteristics and heat extraction performance of spray heat exchanger for sewage heat pump system

The flow characteristics of sewage directly affect the technical and economic performance of sewage-source heat pump systems in heat extraction from sewage. To prove there is significant improvement in heat extraction from sewage with the spray heat exchanger for sewage heat pump system (SHPS): (1) The flow states of the falling film on and between the multiple horizontal tubes were investigated experimentally. The correlations of Reynolds number (range from 100 to 600) and the tube distances to the flow were derived; (2) A model was developed for the falling film flow state analysis of the horizontal elliptical tube of the spray heat exchanger using the ellipse plotting method, which was constructed using approximately three circular arcs. The laser-induced fluorescence technology was used to measure the horizontal tube falling film thickness of water. This experiment focused on the film thickness distribution characteristics of Al-brass tubes with outside diameters of 19 mm and 25.4 mm in column flow with the tube spacing ranging from 20 mm to 40 mm and the Reynolds number (Re) ranging from 184 to 368. The images of the gas-liquid interface were captured with a digital camera. The image processing technology was used to quantitatively describe the liquid film thickness around the horizontal tube. The comparisons between the falling film thickness calculation results and experimental data were in good agreement for the variation of the angle of the elliptical surface and the specific liquid rate. Furthermore, a novel correlation was developed to describe the Nusselt number which was subjected to the examined tube shapes; (3) The heat extraction performance of SHPS with spray heat exchanger was experimentally investigated. The temperature of the sewage ranged from 10 to 30℃. The heat supply, input power, COP, and output water temperature of the condenser of the SHPS were discussed. The results of the proposed calculation method were in conformity with the experimental data and gave good predictions with maximum deviation being lower than 8.6%. Thus, the proposed method is helpful in improving the design and the performance of the SHPS.

Numerical study on falling film flowing characteristics of R113 inside vertical tube under different structural conditions

Abstract To develop an appropriate falling film evaporation device for organic fluid cogeneration, a numerical study on the gas–liquid two-phase counter-current flow characteristics of R113 inside a vertical tube under different structural conditions was conducted using the Fluent software. The effects of the tube length, tube diameter, and annular gap on the falling film flow characteristics were determined, respectively. The results indicated that under a certain spray density, the falling film thickness in the region of the steady section was almost constant with different structural parameters for the tube diameter, tube length, and annular gap. In addition, a smaller tube diameter resulted in a steadier film flow. When the tube diameter decreased to a specific value, the film thickness showed a uniform distribution along the wall surface. This indicated that a best falling tube diameter exists. Meanwhile, the film fluctuation was enhanced with an increase in the tube length. When the tube length was greater than 1.2 m, the falling film splashed and could not completely wet the wall surface. The film fluctuation was enhanced by decreasing the annular gap, and the film could not be formed when the annular gap was smaller than 1.2 mm.

Experimental study of falling film flow characteristics outside shaped tubes related to a spiral‐wound heat exchanger

AbstractIn this paper, the falling film flow and the effects of the marine inclination on the shell‐side flow pattern outside the sinusoidal corrugated tube and the square‐corrugated tube related to a spiral‐wound heat exchanger are experimentally studied using n‐pentane as the testing fluid. The flow field phenomenon and the film thickness are investigated with the utilization of high‐speed camera technology. The results show that the critical Re of the flow regime transition of the falling film outside the square‐corrugated and sinusoidal corrugated tubes is quite different from the critical Re of the flow regime transition of the falling film outside the horizontal circular tube. The tube shape has a certain impact on the falling film flow characteristics. Moreover, under the inclination conditions, the liquid column and liquid film will move toward the inclined side. The resistance ability of the inclination of the square‐corrugated tube is better than the resistance ability of the sinusoidal corrugated tube due to the dragging effect.

Numerical Simulation for Falling Film Flow Characteristics of Refrigerant on the Smooth and Structured Surfaces

Flow characteristics of a liquid film flowing over a smooth surface and structured surface with the Reynolds number range from 10 to 1121 are studied. The mixture of R21 and R114 refrigerants is used as the test liquid. The 3D transient simulations are taken to capture the liquid film’s dynamic characteristics and spatial distribution. Effects of the inlet dimension, inlet flow rates, surface tension, and surface structuring on the wettability, average velocity, and film thickness are studied systematically. The obtained results show that surface tension is essential for an accurate simulation, while inlet width has no effect on the liquid film parameters in the steady-state flow regime. For low flow rates, wetting area and film thickness both are small, and a suggested range of Reynolds number is chosen to simulate further heat transfer in order to balance the film thickness and dry spots generation. It is shown that a ripple surface structure hinders the liquid film movement, reflected in a lower velocity and a larger film thickness compared to the smooth surface. Lateral movement of a liquid film can also be observed at the structured surface.

Influence of oval-shaped tube on falling film flow characteristics on horizontal tube bundle

Falling liquid film flow on a horizontal tube is an important process which plays an important role in heat transfer in heat exchangers. The aim of the present study is to investigate experimentally the effect of the oval-shaped tube on flow characteristics of liquid falling film. The effects of the tube type, Reynolds number, and intertube spacing on the falling film flow modes, dimensionless wavelength, and film thickness for a horizontal tube bundle were studied. Compared with the circular tube, the transitional Reynolds number of the shaped tube is smaller for the oval-shaped tube. The falling wavelength increases with the decrease of Reynolds number and firstly increases then tends towards stability with increasing of intertube spacing. The falling wavelength of the oval-shaped tube is smaller than that of the circular tube. The minimum value of the film thickness appears approximately at the angular position of 135° for the oval-shaped tube, while the angular position of the circular is 105°. Finally, these conclusions obtained in this article provide a valuable reference to the application of the shaped tube in the falling film evaporator for the seawater desalination system.