Film Flow Characteristics Research Articles

Refrigerant film flow and heat transfer characteristics on the elliptical tube under constant wall temperature

• CFD models of falling film on horizontal tubes are verified by experimental data. • Effects of wall heating conditions on heat transfer of falling film are explored. • 11 refrigerants film flow and heat transfer under different temperature is studied. • Correlations of Nusselt number and film thickness for elliptical tubes are fitted. In this paper, the 2D model of the elliptical tube is established to simulate the film hydrodynamics and heat transfer performance. The volume of fluid (VOF) is used and the model is verified by the experimental data in the previous literature. The effects of the heating condition, fluid medium, inlet temperature ( T in ) on the film thickness and heat transfer coefficients are explored. The studied 11 refrigerants include ethane, propane, R123, R1234yf, R1234ze(E), R125, R134a, R143a, R152a, R227ea, R245fa at T in = –30, –10 and 10 °C. The results show at Re = 2000, the local heat transfer coefficients of the propane at T in = 10 °C under fixed heat flux are 19% greater than those under fixed wall temperature. Both the local film thickness and local heat transfer coefficients increase with Reynolds number ( Re ). The overall external heat transfer coefficients increase with Re , but the growth rate slows down at Re ≥ 1500. Fluids have different results of film thickness and heat transfer coefficients even they have a similar Kapitza number ( Ka ). Different from the water and seawater, the heat transfer coefficients reduce as the inlet temperature increases for the refrigerants. A correlation is proposed to predict local film thickness on both circular and elliptical tubes. A correlation is acquired by the fitting method to predict the overall external heat transfer coefficients under the heating condition of fixed wall temperature, which can capture 89% of 132 data within the deviations of ±10%.

Experimental research on falling film flow and heat transfer characteristics outside the vertical tube

• Flow and heat transfer characteristics of falling film are studied experimentally. • The experimental data show large differences with values of existing correlations. • Correlations of heat transfer coefficient and dry burning condition are developed. • The study provides a reference for the research on evaporator of MVR. The falling film flow has attracted much attention recently due to its advantages of low temperature difference, low flow rate, high heat transfer coefficient and high heat flux density. In this paper, a series of experiments were carried out to investigate the falling film flow and heat transfer characteristics outside the vertical tube based on the established experimental system. Flow patterns, the trend of heat transfer coefficients along the tube as well as the falling film breakdown phenomenon were described. Two new correlations were obtained to predict the heat transfer coefficient and falling film dry burning based on experimental data, and the maximum deviation between the experimental data and predicted values were −13.36% and 16.81%, respectively. The new correlations can provide a reference for the design of the heater in the Mechanical vapor recompression systems.

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.

Effects of Film Flow and a Surfactant on Scale Formation in Falling Film Evaporators for Seawater Desalination

Horizontal tube falling film evaporators are commonly used in multiple-effect distillation (MED) plants for seawater desalination. Scale formation on the evaporator tubes strongly depends on film flow characteristics. The thickness of the liquid film on a horizontal tube was measured in a test rig with a high-resolution optical micrometer. Furthermore, scale formation was studied with artificial seawater in a horizontal tube falling film evaporator at pilot plant scale. The effects of a polyoxyalkylene triblock copolymer surfactant on falling film flow and scale formation were investigated at different wetting rates. Film thicknesses on the bottom of the tube exceed film thicknesses on the top by an order of magnitude. The thinnest film is formed on the tube sides. The mean film thickness increases with increasing wetting rate. The scale formation on the top of the tube is stronger than that at the bottom. The highest scale thickness was measured at the tube sides. Scale mass and scale layer thickness increase with decreasing wetting rate. As the minimum and the mean film thickness increase and surface waves are mostly damped in presence of the surfactant, the mass transfer resistance is higher and, thus, scale formation is reduced.

Experimental Study of Flow Boiling and Condensation in Tubes with Pin-Fin and Metallic Foam Structures

Appropriate arrangement of the fin structures in the evaporator and condenser not only enhances the heat transfer coefficient but also suppresses the increase of pressure drop. Due to this reason, a new method of using large-scale pin fins is explored to increase the heat transfer area and to modify the liquid film flow characteristics in the enhanced tubes. In the present study, four enhanced tubes and one plain tube were manufactured by selective laser melting. The working fluid is R407C and the enhanced tubes are eight short pin fins tube, five long pin fins tube, twisted pin fins tube and a tube filled with metallic foam. The results show that the shorter fin tube has the largest heat transfer coefficient for condensation at 3967 W/m2⋅K and the metallic foam tube demonstrates the largest heat transfer coefficient for boiling at 10606 W/m2⋅K. However, the shorter fins have the highest efficiency indices based on the heat transfer coefficient-pressure drop evaluation criterion and the heat transfer coefficient-area evaluation criterion. For condensation, the indices are 1.28 and 1.35, respectively. For boiling, the indices are 2.55 and 1.62, respectively.

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

A critical review of filmwise natural and forced convection condensation on enhanced surfaces

The use of enhanced surfaces is an efficient method to improve filmwise condensation. Due to their immense potential, many enhanced structures were developed and investigated with the aim of improving natural and forced convection condensation heat transfer coefficients. This has resulted in large collections of predictive models and experimental data being reported. In this review, the developments in this field of research in the past few decades and the recent advances are collated and examined. This paper focuses on the review of natural convection condensation on the external surfaces of enhanced flat plates and tubes and forced convection condensation in enhanced tubes. The various models predicting the heat transfer coefficients on plain and enhanced surfaces are evaluated. For natural convection condensation, the liquid film-based models, semi-empirical models and numerical modes are reviewed whereas, for forced convection condensation, the gravity-dominated and vapor shear-dominated models are discussed. The effects of these enhanced structures on the liquid film and two-phase flow characteristics are analyzed and the various types of enhanced tubes and flat plates investigated are categorized. The manufacturing techniques employed to fabricate these surfaces are identified. A detailed evaluation of the heat transfer and pressure drop performances of the various enhanced surfaces is performed. In addition, their thermal performances are summarized and compared, and their associated heat transfer mechanisms are elucidated. For external condensation on a single tube row, three-dimensional fin structures were found to provide better thermal performance than two-dimensional structures, with some three-dimensional fin structures exhibiting more than 6 times the heat transfer coefficients of a plain surface. However, in a tube bundle, the heat transfer coefficient of three-dimensional fin tubes decreases more significantly with increasing tube row as compared to two-dimensional fin tubes. For convective condensation in circular tubes, the herringbone and pin fin tubes demonstrated better thermal and pressure drop performances than other internally enhanced tubes. Their efficiency indices were between 1.25 and 1.28. Based on the literature surveyed, the various experimental results are compared, existing research gaps are identified and frameworks for future research work are provided.

Influence of a soluble surfactant on the wave characteristics of a liquid film flow

In this paper we present results of experimental investigation of the influence of soluble surfactant (Triton X-100) on wave characteristics of vertically falling liquid film in the range of film flow Reynolds number 25 < Re < 95. Shadowgraph technique is used to analyze wave patterns on the whole length of the test section (140 cm). Film thickness fields obtained with the help of high-speed Laser Induced Fluorescence (LIF) technique in two areas in the bottom part of test section allow analyzing statistical characteristics of the film flow. Expectedly, adding surfactant suppresses wave motion. For some surfactant concentrations the absence of waves is observed in the whole test section. For other concentrations the wave repetition rate decreases with distance from the liquid inlet. Analysis of spectral characteristics reveals that for water with some concentrations of surfactant, as for pure liquids, the steady-state three-dimensional wave regimes are observed. The characteristics in such flow regimes differ from the characteristics of pure water flow.

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.

Annulus Cold Film Flow Characteristics of Can Combustor with Liner Temperature Rising

Annulus Cold Film Flow Characteristics of Can Combustor with Liner Temperature Rising

Three‐dimensional large eddy simulation of wave characteristics of liquid film flow in a spinning disk reactor

AbstractThe waves of liquid film flow play an important role on the process intensification in spinning disk reactors (SDRs). However, the mechanism of wave formation was still unclear. In this work, a three‐dimensional large eddy simulation was developed to investigate the mechanism, as well as the characteristics of waves in the SDR. Agreed with the imaging results, different wave patterns were identified as: smooth film, concentric, and spiral waves in spreading direction; sine‐like and pulse‐like waves in fluctuating direction. The radial and tangential relative movements among the layers of liquid film were found to dominate the formation of different wave patterns. Local average film thickness (havg) and local wave amplitude (Δh) ranged from 0 to 500 μm and 0 to 200 μm, respectively. The waves can improve the turbulent intensity and enlarge the specific surface area, resulting in the intensification of transfer processes.

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 and experimental investigation on the condensing heat transfer of R134a outside plain and integral-fin tubes

Condensing heat transfer of R134a on horizontal single plain and integral-fin tubes was investigated by both computational and experimental methods. The VOF model and Lee condensation model were utilized in the simulation. Validation of the model was performed with two different refrigerants. Condensing heat transfer coefficient for plain and integral-fin tubes were calculated in comparison with the experimental data and Nusselt analytical solution. Instantaneous film flow characteristics of condensation on horizontal single plain and integral-fin tubes were discussed respectively. Contours of liquid volume fraction for the plain tube and condensing heat transfer coefficient for the plain tube at different time with Tw = 303 K are presented. For integral-fin tubes, contours show that the thickness of liquid film at lateral fin surface is much smaller than that at root fin surface. Comparison between R134a and R11 were made and it was found that different surface tension could lead to different liquid film distribution which could result in different heat transfer performance. Experiments involving the same fin structure were also conducted to verify the theoretical model and a satisfactory agreement between simulation and experimental results was found.

Experimental investigation of liquid film cooling in hypersonic flow

Film cooling is generally considered as a promising active cooling technology for developing thermal protection systems of hypersonic vehicles; however, most of experimental and numerical studies of film cooling mainly concentrated on gaseous film cooling. Since the phase change of liquid coolants can absorb a large amount of latent heat, liquid film cooling should have more potential advantages, especially for severe environments accompanied by hypersonic flight. To address this issue, the film cooling using water as a coolant was experimentally investigated in hypersonic flow. Experiments were carried out in a detonation tunnel, at a hypersonic Mach number of 6 using a 25° apex-angle wedge. Characteristic physical quantities, such as surface temperature rise, shock wave structure, film thickness, and cover area, are measured by thermocouples, schlieren, and a specially devised liquid film measurement system. The experimental results verify that the liquid film cooling is feasible in hypersonic flow and also indicate that it is featured with maintaining aerodynamic performances due to the weak effect on the main flow caused by coolant injection. Inspired by these results, liquid film flow characteristics and its influencing factors including mass flow rate, dynamic pressure, coolant injection direction, and surface tension are investigated to guide the design of a thermal protection system.

Energy saving from furnace slag: An analysis of free-surface film flow characteristics of liquid slag on the rotary cup

Blast furnace (BF) slag is regarded as a high-grade waste heat resource. Dry centrifugal granulation technology became an important method for its low-energy consumption, recovering waste heat, etc. In this paper, the analytic formulas of liquid film thickness and the average radial velocity were obtained by micro-element force analysis. The influence of various factors on liquid film was analyzed by computational fluid dynamics (CFD)simulation. It was revealed that liquid film produced a slip when the cup diameter was 50 mm and the rotary speed was 1500 rpm. The critical rotary speed of the slip increased with an increase in the cup diameter. The liquid film thickness changed almost linearly with the inlet mass flow rate. With the amount of slag constant, the influence of the slag inlet conditions on the liquid film at the edge of the cup was negligible. The surface tension mainly affected the initial spreading process of the liquid film. With the increase of the cup edge inclination angle, liquid film thickness increased.

Study on liquid film thickness and flow characteristics of falling film outside an elliptical tube

Study on liquid film thickness and flow characteristics of falling film outside an elliptical tube

Investigation on flow and breakdown characteristics of water film on vertical corrugated plate wall

Steam and water separation effect in the corrugated plate dryer plays a decisive role in the economic indicators of nuclear power plants. Investigation on flow characteristics of water film on vertical corrugated plate wall is beneficial to the research of steam-water separation mechanism and also plays an important role in the analysis of the water film flow characteristics and instability. In this paper, the water film thickness is measured by Planar Laser Induced Fluorescence (PLIF) technique. The relationship between the free falling water film thickness and time and the relationship between the average water film thickness and the water film Reynolds number are obtained, respectively. With probability density function (PDF), power spectral density (PSD) and wavelet analysis methods, the flow fluctuation instability of water film on the corrugated plate wall is analyzed. Characteristic frequency and vibration amplitude of water film flow fluctuation are obtained and the relationship between the water film thickness and the Reynolds number is fitted. Results reveal that the water film thickness has a certain regularity with the change of the Reynolds number and the two have a positive correlation. As the Reynolds number increases, the generation of solitary waves increases the water film volatility. The water film thickness is multi-frequency and has no obvious periodicity. When the water film Reynolds number in the experiment is less than 4055 and the flow is in a certain range of flow resistance, the water film breakdown will not happen owing to its own gravity. It indicates that the gravity have little influence on phenomenon of breakdown of a water film with a stable flow in a vertical corrugated plate. Therefore, the force which the water film is subjected to at the corrugated plate corner is analyzed. The model is established and the theoretical equation of the critical airflow velocity of water film breakdown and the water film thickness is derived as well.

Hydrodynamics of New Structured Packings: an Experimental and Micro-scale CFD Study

Corrugations on the packing surface affect the efficiency of mass transfer in packing column. The influences of corrugations on film flow characteristics were investigated by numerical and experimental methods in this paper aiming to investigate the influence of geometric configuration on the mass transfer efficiency. In the simulations, a multiphase solver in the open-source software OpenFOAM was used to track the free surface position with VOF method. To validate the simulations quantitatively, the velocity profiles in the liquid film fields in 2-D simulations are compared with the experimental results. The flow fields of three different types of packing structures, namely sinusoidal, triangular and rectangular corrugated plates, in the experiments correspond well with the simulation results. For the shape optimization design, the new structures of plates were proposed and the hydrodynamics of the new corrugations plates were analysed. The order of mass transfer results of different types of corrugated plates is arranged.