Falling Film Heat Exchangers Research Articles

Falling film hydrodynamics and heat transfer under vapor shearing from various orientations

Vapor shearing is a common issue encountered in the operations of falling film heat exchangers. The vapor stream effect depends on its orientation. This study investigates liquid film hydrodynamics and heat transfer performance under the influence of vapor streams from different orientations. The results indicate that both orientation and velocity of vapor determine the encountering time and position of the films on the tube's two sides. The liquid film thickness uniformity and the liquid column deflection vary significantly depending on the orientation and velocity of the vapor. Zones of accelerated liquid film, climbing liquid film, liquid stagnation, and transition of liquid film flow pattern are observed. The gradient of film thickness along the tube axis and the deflection in time-averaged peripheral film thickness increase as the vapor orientation varies from 0° to 90° and subsequently decrease as the vapor orientation varies from 90° to 180°. Vapor streams have more pronounced effects on time-averaged peripheral film thickness in regions close to the liquid inlet and outlet. Vapor streams result in changes in peripheral heat transfer coefficients toward the downstream side depending on the orientation and velocity of the vapor. The impact of vapor streams on the overall heat transfer coefficient does not directly correlate with the velocity of the vapor when maintaining the same orientation.

Heat transfer and irrigating liquid hydrodynamics on one-row vertical bundle of inclined tubes in falling film heat exchanger

Spiral-wound heat exchangers widely used currently in various industries provide efficient heat transfer between media through spiral tube winds at evaporation and boiling of falling liquid film. The investigated bundle in the form of one-row vertical bundle of inclined tubes (5 degrees to the horizon) simulates a part of a spiral-wound heat exchanger. Heat transfer and hydrodynamics processes are studied at falling film irrigation by liquid R21 refrigerant at 3 bar of absolute pressure with water as heat carrier. The bundle is placed inside of rectangular heat exchanger with optical windows. Heat transfer measurements are performed for tubes at different positions on the bundle height. Irrigation densities corresponded to falling film Reynolds numbers 300 – 1100, heat flux values within the range of 5000 – 10000 W/m2.

A numerical study on fluid flow and heat transfer characteristics of oily wastewater falling film outside an elliptical tube

Falling film heat exchangers have recently been widespread as a heat transfer technique for oily wastewater. Researching the characteristics of fluid mechanism for heat transmission is crucial. In this research, numerical models are applied to simulate the heat transmission and flow of falling film outside ellipse tube. In order to emulate fluid flow and heat transmission, the oily wastewater falling film is analyzed utilizing the VOF approach under circumstances of constant thermal flow. On the flow properties and heat transfer coefficient distribution of liquid film, the impacts of the input height, contact angle, liquid film flow rate, and oil content are examined. These computational outcomes are attained: (1) More so than film thickness, according to the simulation results, increasing liquid film flow significantly affects velocity;(2) One of the main causes of how oil concentration affects heat transmission is the complex disturbance process of liquid film and the force change around tube structure;(3) Around 40°, inlet height has the greatest impact on local heat transfer coefficient;(4) The overall heat transmission will deteriorate due to the hydrophobic impact brought on by increased contact angle.

An improved dynamic model of MED-TVC system by considering changes in ejector conditions

Most MED transient behavior studies have failed to cover the actual ejector conditions and falling film heat exchanger capacity over a wide range of ejector changes. An unsteady and steady models of an industrial-scale MED with an ejector have been investigated in this study by considering these two issues. In order to solve differential equations, the variable-step, variable-order technique (VSVO) in MATLAB-Simulink is utilized. After validation of the proposed model, the impact of the falling film heat exchanger heat transfer capacity on the MED effects during overload to the system, and the effect of ejector malfunction at pressures lower than the ejector design motive pressure have been investigated. Also, the start-up from ambient conditions to steady-state conditions has been modeled. Results show that by the lowest reducing the motive pressure from the ejector design pressure, the thermo-compressor performance is disrupted and the GOR is reduced by 33 %. Also, a 10 % reduction in motive flow rate causes a 5-fold increase in the saltwater height level of primary effect, while a 10 % reduction in pressure of the motive vapor results in a 10-fold increase; which causes flooding and submerges the heat exchanger tubes, which can alter the performance and might cause failure.

Thermodynamic performance and heat and mass transfer analysis of air source absorption heat pump for heating

To lower CO2 emission and obtain better heating efficiency compared with conventional gas boilers, this paper proposed a solar air source absorption heat pump (SAAHP) with falling film heat exchanger for district heating. The thermodynamic model is established to analyze the performance of the proposed system. The falling film generator as the key heat exchange component has been designed carefully, and then the numerical models of the falling film generation process which include one and two-dimensional numerical models are developed. According to the thermodynamic analysis, the primary energy efficiency (PEE) and heating capacity of the system are 1.65 and 45.02 kW at the generation temperature of 190 °C and ambient temperature of −7 °C, respectively, which can save 41.69 t of CO2 emissions from heating. The two-dimensional numerical model indicates the main heat and mass transfer occur between the falling film height x = 0 mm and x = 200 mm, and the interface heat transfer decreases from 74.6 kJ/(m2·s) to 40.5 kJ/(m2·s) in a typical vertical falling film generation process. In addition, the energy transfer as the inter-diffusion is corrected in the energy equation which improves the prediction accuracy by about 1.5% of the two-dimensional model. The research provides a reference for the optimization design of the generator, where one-dimensional models can quickly guide the design and selection of equipment, modified two-dimensional numerical models and Computational fluid dynamics (CFD) models can provide references for the refined optimization design, which attributes to the improvement of thermodynamic performance of the solar absorption heat pump system.

Experimental and numerical investigations of heat transfer characteristics of vertical falling film heat exchanger in data center cabinets

To reduce the energy consumption in data centers, this paper proposes a novel structure that integrates a cabinet with a vertical falling film heat exchanger in data center to perform natural evaporative cooling. First, an experimental setup is built to verify the heat and mass transfer characteristics of a falling film in a vertical tube and to obtain the mass transfer coefficient 0.05. Then, the characteristics of tube length direction and heat and mass transfer are analyzed using compiled user-defined functions for different parameters of moist air and water in the tube. The maximum growth rates of heat transfer coefficient and mass transfer rate are 67.65% and 55.23% at increasing inlet moist air Rema,i, respectively. Finally, the heat transfer characteristics of the falling film heat exchanger with staggered and in-line are compared for inlet air velocities in the range of 1–3.5 m·s−1. The corresponding results demonstrate that the heat flow rate, heat transfer coefficient, and overall heat transfer performance PEC of the staggered are better than those of the in-line as the inlet air velocity of the heat exchanger continues to increase. The maximum PEC of the staggered heat exchangers is 40.07% higher than that of the in-line heat exchangers.

Dropwise and liquid-jet laminar flow of subcooled water falling over horizontal tube banks

Falling-film heat exchangers are widely used to promote heat and mass transport within industrial environments. In contemporary condensers, for example, liquid films continuously form over a tube row and impact multiple tubes beneath, resulting in a rich variety of two-phase flow phenomena, including the classical Rayleigh-Plateau instability. It is widely known that altering the contact angle of the tubes influences the heat exchangers’ heat and mass transfer characteristics by changing the falling films’ flow behaviour (i.e. dropwise or jet). Therefore, resolving interfacial flow characteristics, such as the Rayleigh-Plateau instability, is crucial for falling film studies as this instability may change the state of the falling film from being jet flow to dropwise/jet flow or even just dropwise. To explore this aspect, three-dimensional direct numerical simulations are used in the present work to replicate the jet instability behaviour. By employing certain mesh refinement strategies demonstrated from various case studies, the interfacial flow behaviour and jet instability have been captured, showing good agreement with experimental and theoretical data previously published. This is determined from the increased jet bifurcation and a maximum error of 2.2% of the film thickness in comparison to Nusselt's result within the bulk region of the tube. In addition, the average and local heat transfer coefficients across the first tube were compared with numerical solutions to give an average error of 3.2 and 14 %. Jet instability is found to be directly related to the amplitude of the axial pressure variations which is augmented by the growth rate of the capillary wave that travels upstream. Specifically, for contact angles of 30 and 60°, the flow is in a dropwise and dropwise/jet state due to the emergence of the Rayleigh-Plateau instability. Notably, the additional satellite droplets that materialize from the unstable jet lead to an increase in the local heat transfer coefficient of the film compared to without the Rayleigh instability improvement.

Experimental Research on Heat Transfer Performance of CO2 Tube Bundle Falling Film Evaporator

Abstract CO2 is a promising refrigerant substitute, and falling film evaporator is widely used in high-capacity heat pump systems. In this article, a CO2 heat pump experimental device is built. According to experimental parameters, a CO2 falling film heat exchanger is fabricated. The falling film heat exchanger is used in heat pump evaporator, and its heat transfer characteristics are analyzed and tested. The outer diameter of the heat exchanger is 219 mm, outer diameter of the inner tube is 15.88 mm, and total outer surface area of the inner tube is 0.58 m2. Number of internal tubes was 12, each with a length of 0.97 m. During the test, change of heat transfer coefficient of each tube was analyzed when evaporation temperature, CO2 mass flowrate, and heat flow density changed. The influence of the position of heat exchange tube inside falling film evaporator on heat transfer coefficient is analyzed. The order of heat transfer coefficient of internal heat exchange tube is obtained. Finally, database of CO2 falling film evaporator is established. Through data sorting, heat exchange correlation formula is obtained. The deviation of 90% data in database is less than 5%. Heat exchange correlation formula has good accuracy.

Film breakdown onset Reynolds number of an absorption heat transformer with falling film heat exchangers: Modeling and estimation

This work presents the development of a physical model for an absorption heat transformer with falling film heat exchangers. The absorption heat transformers are devices for increasing moderate temperatures of heat sources to more useful levels to be used in some processes as water purification. The process units are modeled in lumped form to reduce the model complexity. The phenomena that appear in the falling film heat transfer are analyzed. The laminar flow with partial wetting describes the behavior of the units. The falling film reaches full wetting efficiency at the film breakdown onset Reynolds number, which is considered a parameter. Sensitivity analysis is applied to the AHT model to identify the sensible variables that are affected by the film breakdown onset Reynolds number of each unit, then these parameters are estimated. The film breakdown onset Reynolds number estimated for the units are from 8.90 to 141.28, which depend on the unit geometries, the fluid properties, and the physical phenomenon. The evaporator lumped model of the process of this work is compared with a detailed evaporator model specific regarding each round of coil presented in previous research. From this comparison, it is recommended to model the first tube of the evaporator individually while the other tubes can be modeled in lumped form. The model results are compared with experimental results to validate the model. The comparison shows low errors from 0.22 to 6.13 %, except for the pressure of the evaporator–absorber chamber with an error of 13.83 %. Finally, some tests at different generator temperatures varying the cooling water temperature from 33.8 °C to 31.8 °C decrease the pressure by approximately 0.53 kPa and increase the flow of refrigerant by 0.00011 kg/s. The establishment of the model and the estimation of the unknown parameters would contribute to the development of accurate dynamic models for absorption heat transformers to implement strategies to improve control process.

Time-evolving dry patches in a flowing film around a horizontal tube

Dry patches could occur on the heat exchanger wall even if the initial liquid film covers the tube surface entirely. Here, we propose a simple three-dimensional model to predict the formation and evolution of dry patches in a water film coating on a horizontal tube, considering thermocapillary and evaporation of the liquid film. Results indicate that the film thickness distribution along the axis is not uniform, convex at ends and concaves at the center. Under the action of axial thermocapillary force, the growth rate of the dry patch in the circumferential direction is more significant than that in the axial direction and finally forms an elliptical shape. Comparative analysis reveals that the appearance of dry patches is delayed as the spray temperature decreases or Re increases. Tube radius has little influence on the drying of liquid film, which is not recommended to control dry patch formation by reducing the tube radius. This work could be beneficial to assist the feasibility design of falling film heat exchangers.

Design and experimental demonstration of enabling components for simultaneous water purification and space conditioning using thermally driven absorption heat pumps

• Absorber/desorber for water-purifying absorption heat pump designed and tested. • 90% removal of organic contaminants from graywater achieved. • Up to 99% removal of dissolved solids from NaCl solution demonstrated. • Good agreement (AAD = 8.6%) between desorber model and experiments. A horizontal-tube, falling-film heat exchanger capable of acting as an absorber or desorber in a small-scale (300-W cooling, 10.5 kg day −1 water purification) water-purifying absorption heat pump cycle was designed with LiBr as the absorbent. A design model for the heat exchanger that accounted for both heat transfer and mass transfer effects in the falling liquid film during desorption and absorption was formulated. Parametric studies were conducted for tube lengths ranging from 0.12 to 0.30 m and for absorber equilibrium solution outlet temperatures ranging from 35 to 45 °C. Based on the results of these simulations, a tube length of 0.24 m and an absorber equilibrium solution outlet temperature of 37.5 °C were selected. Batchwise experiments were conducted to validate the component model and determine the water purification potential of the cycle. Three feed solutions were tested: DI water, synthetic graywater, and 3.5 wt% NaCl. Measurements showed 90% removal of contaminants from synthetic graywater and 99% removal of NaCl from the 3.5 wt% NaCl solution. The measured heat transfer, after correcting for the presence of air in the chamber, demonstrated an average absolute deviation (AAD) of 18.6% for operation as an absorber and an AAD of 8.6% for the desorber.

Numerical simulation of falling film flow hydrodynamics over round horizontal tubes

The performance of a falling film heat exchanger depends significantly on the flow characteristics and the associated heat transfer in the liquid film flowing over its horizontal tube surfaces. In this study, three-dimensional numerical simulations based on the Volume of Fluid method are conducted to investigate the hydrodynamic behavior of a falling film over horizontal round tubes with diameter and tube-to-tube spacings each of 16 mm. Using a mixture of water and ethylene glycol, the simulations comprise a range of flow rates (Re = 15 to 210) that covers all basic intertube flow modes i.e., droplet, jet (in-line and staggered), and sheet mode. InterFoam, a two-phase incompressible flow solver in OpenFOAM, has been used to simulate the flow field. The computational model agrees well with the available experimental data. The numerical results present the variation of the liquid film thickness and the film interface velocity over the tube surface for all the flow modes. In the droplet mode, the impact of the droplet on the tube produces liquid waves that interact with the neighboring waves and are seen to travel on the tube surface downstream of the impingement region and the region between the impingement sites. The movement of these waves causes a significant change in the film thickness over the tube surface, quantitatively, by more than 350% during a short interval of the droplet mode cycle. After the droplet impact, the liquid bridge formed in between the tubes undergoes necking and finally bifurcates under the Raleigh-Plateau instability mechanism at a location of spacing-to-tube-diameter ratio of 0.8 from the top surface of the impinging tube and produces satellite droplets in between the tubes. In the steady in-line jet mode, the liquid from the neighboring jets do not interact with each other, and the bulk liquid flows downstream of the impingement region. A dimple around the jet impingement region is formed where the film thickness changes by around 40%. The base of the impinging jets was seen to possess ripples due to the Raleigh-Plateau instability with wavelengths ranging from 0.3 - 1.0 times the capillary length scale. For the steady staggered jet mode, the liquid jet impinges on the tube surface, which then spreads rapidly and interacts with the adjacent jet liquid to develop crest and stable segments along the tube length direction. The ratio of the film thickness between the crest and the stable segments was found to be 1.7. Finally, for the sheet mode, due to the formation of surface waves in the liquid sheet, interfacial waves are seen to travel along the tube periphery with amplitudes of about ±20% of the nominal film thickness. These flow mode characteristics alter the film thickness and flow velocity distribution over the tube surface and are expected to play an essential role in the surface heat transfer behavior over round horizontal tubes. A set of correlations have been presented to predict the film thickness and interfacial velocity over the tube surface for each of the studied modes of flow with reasonable accuracy.

Experimental and numerical study of a falling film heat exchanger used in a vapour absorption refrigeration system

Falling film evaporators have found applications in a wide range of industries, varying from chemical industries, petroleum refinery, desalination and food process industries. These heat exchangers are preferred mainly because of high heat exchange rates with low temperature difference between the heat exchange fluids at low film fluid flow rates with low evaporation temperature. This study is focused on numerical and experimental investigation of vapour generator (falling film heat exchanger) of a vapour absorption refrigeration (VAR) system. The influence of key operating parameters like mass flow rate of fluid, inlet temperature and the concentration of the thermic fluid as well as the coolant, are experimentally studied and are compared with the numerical results. The deviation between the experimental and numerical results was found to be 8.72%, with a lesser deviation at lower temperatures. The minimum percentage error is 2.15%, for a thermic fluid temperature at 50 °C. There is a stronger agreement in the results for lower values of the glycol water mixture inlet temperature, than at high temperatures. A maximum of 1000 W/m2 heat is transferred to the aqua-ammonia with ammonia concentration of 0.6 by volume is observed for thermic fluid inlet temperatures of 80 °C. The effect of various operating parameters were numerically studied and interpreted. The results obtained were in correlation with the concepts of heat transfer.

Wettability Effects on Falling Film Flow and Heat Transfer Over Horizontal Tubes in Jet Flow Mode

Abstract The performance of a falling-film heat exchanger is strongly linked to the surface characteristics and the heat transfer processes that take place over the tubes. The primary aim of this numerical study is to characterize the influence of surface wettability on the film flow behavior and its associated surface heat transfer in the jet-flow mode. Volume of fluid (VOF) based simulations are carried out for horizontal tubes with different surface wettabilities. The wettability of the tube surfaces is represented using the Kistler's dynamic contact angle model. Surface wettability effects ranging from superhydrophilic to superhydrophobic are studied by varying the equilibrium contact angle from 2 deg to 175 deg. Two different liquid mass flow rates of 0.06 and 0.18 kg/m-s corresponding to the inline and staggered jet flow modes are studied. Results are presented in terms of the liquid film thickness, the contact areas between the different phases (solid–liquid and liquid–air), and the heat transfer coefficient or Nusselt number. The resistance imposed by the increasing contact angles inhibits the extent of the liquid spreading over the tube surface, and this, in turn, influences the liquid film thickness, and the wetted area of the tube surface. A significant decrement in the heat transfer rate from the tube surfaces was observed as the equilibrium contact angle increased from 2 deg to 175 deg. The local distributions of the Nusselt number over the tube surface are strongly influenced by the flow recirculation in the liquid bulk.

都市域における下水管路網を活用した下水熱利用・熱融通システムの研究 (第10報) 下水環境下における流下液膜式交換器試験結果

都市域における下水管路網を活用した下水熱利用・熱融通システムの研究 (第10報) 下水環境下における流下液膜式交換器試験結果

Falling Film Transitions on Horizontal Enhanced Tubes: Effect of Tube Spacing

Horizontal falling film heat exchanger is widely applied to industrial area. In this work, falling film flow mode of tube bundles test bed was built for solving the basic problem of falling film heat transfer. Three tube spacing(s=6mm, 10mm, 15mm,) and three types of tube(smooth, two(2D) and three (3D) dimensional finned enhanced)were tested using water. Changing flow rate to obtain the effect of tube spacing on falling film Reynolds number(Re) at the same flow mode, the effect of tube spacing on flow mode transitions were investigated for three types tube. The result showed: (1)Increasing of tube spacing(s=6mm, 10mm, 15mm) has small effect for the Re of smooth, and lacking of enough surface tension appeared instable flow modes(s=20mm, 30mm). (2)For the Re of 2D enhanced tube, it decreased with tube spacing increased occurred in droplet and droplet-column modes, but for the column and column-sheet modes, Re was increasing (3)The tube spacing has great effect for the Re of 3D enhanced tube, and Re increased with tube spacing. The research gives guidelines to develop new efficient heat transfer equipment.

Numerical Research on Flow Characteristics in a Plate-type Falling Film Heat Exchanger for a LiBr/H2O Absorption Heat Pump

An optimum designed plate-type falling film heat exchanger is the key technique to scale down the size of a LiBr/H2O absorption heat pump. Flow characteristics have the largest impact on the performance of a plate-type falling film heat exchanger. However, there is no research on falling-film flow characteristics of LiBr solution on the corrugated plate in vacuum atmosphere. Numerical flow models based on a plate-type falling film heat exchanger were constructed with 2-D and full 3-D spatial and temporal resolution using the Volume of Fluid technique. Flow characteristics of horizontal columns with different degrees of arc were successfully modelled with 2-D spatial model and the optimum arc degree is found. The falling-film flow patterns under different Reynolds number are thoroughly analyzed with full 3-D spatial model.

Experimental studies on a direct-steam-generation absorption heat transformer built with vertical falling-film heat exchangers

This study presents an experimental rig of direct-steam-generation single-stage heat transformer (SSHT), which was built with four main vertical heat exchangers and a plate heat exchanger. This rig uses a LiBr-water binary working fluid and is driven by low-grade hot water. Data from six test runs were analysed. The results show that bilateral falling film made it easier for the absorber to generate steam directly with pressure changing from 190kPa to 260kPa, which corresponded to an absorber heat load from 15.5kW to 7.2kW, coefficient of performance (COP) from 0.38 to 0.20, and exergy coefficient of performance (ECOP) from 0.48 to 0.27, respectively. The absorber temperature can be as high as 124°C, with calculated internal gross temperature lift (GTL) of 34.8K and external GTL of 29.6K. The economizer was a conventional plate heat exchanger with effectiveness around 0.73 in six test runs.

Experimental assessment of an absorption cooling system utilizing a falling film absorber and generator

This study presents the results of the evaluation of an ammonia/lithium nitrate absorption cooling system. The generator and the absorber are shell and tubes falling film heat exchangers while the rest of the components are compact plate heat exchangers. A parametric study was carried out in order to determine the coefficients of performance and cooling capacities at different operating conditions. Also, an analysis was carried out to determine the influence of the absorber solution recirculation on the system performance. The generator temperatures varied between 80°C and 100°C, while the cooling water temperatures varied from 20°C to 34°C. Cooling capacities up to 4.5kW and evaporator temperatures as low as 4°C were achieved with the system. The internal coefficients of performance varied between 0.3 and 0.62 depending on the system operating temperatures. The system also showed good stability and repeatability.

Investigation and testing of new polymeric-based working pairs for absorption chillers

Absorption chillers are a potential technology for using solar energy or waste heat for cooling and air-conditioning purposes. State-of-the-art working pairs require elaborate system designs and an appropriate material selection to deal with such problems as formation of non-condensable gases, corrosion, and crystallization. The investigation of alternative working pairs is one important possibility to reduce or avoid such problems. A new approach is to use linear- or hyper-branched polymers in combination with water as a working pair. A selected polymer used in this work has the capability to absorb water at suitable vapor pressure levels. Furthermore, the new working pair is expected to potentially diminish the problems of corrosion, formation of non-condensable gases, and damage of the machine due to crystallization and to relax at least the pricing situation. On the other hand, the high viscosity and the low heat conductivity compared to conventional working pairs are crucial for its application. To prove the feasibility of the polymeric-based working pair, experiments have been carried out in a lab-scale single-stage absorption chiller. The lab-scale chiller used falling-film heat exchangers and had a cooling capacity between 60 and 100 W. It is shown that under equal operating conditions, the new working pair is able to reach about one-third of the cooling power compared to lithium bromide/water at vaporizer temperatures suitable for cooling ceilings applications. If used as a drop-in replacement, the cooling power of the new working pair does not reach the performance of lithium bromide/water. Nevertheless, the usage in adapted machines for specific applications should be discussed in the future.