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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. [ABSTRACT FROM AUTHOR]

Falling film technology has been widely applied in various industrial processes, but vapor shearing effects cause significant problems. The mechanism by which vapor affects the hydrodynamics of a falling film has not yet been fully understood, particularly for cross vapor streams. In this paper, a three-dimensional model is developed to investigate the hydrodynamics of a falling film on a horizontal tube with a cross vapor stream. The results indicate that (1) there are valley and peak zones during the spreading of the liquid film; (2) the effect of cross vapor flow on the hydrodynamics of the falling film depends on the film flow rate, the cross vapor velocity, and the position on the tube; (3) with increasing vapor velocity, the peripheral film thickness increases on both the upwind and downwind sides for values of z* (the dimensionless axial coordinate) from 0 to 0.1, while decreasing on the upwind side but increasing on the downwind side for values of z* from 0.2 to 0.5; and (4) under the action of the cross vapor stream, liquid film offset occurs, characterized by the formation of new stagnation and detachment zones on the downwind side of the tube. [ABSTRACT FROM AUTHOR]

The peripheral heat transfer correlation of the subcooled falling liquid film outside the horizontal tube was established using the present numerical data under different liquid flow rates, heat fluxes, tube sizes, distributor heights, and liquid temperatures. The results indicated that the present correlation can predicate 93% of 2654 data within ±25% in θ = 2°–15°, 92% of 2254 data within ±15% in θ = 15°–165°, and 80% of 1442 data within ±30% in θ = 165°–178°. The peripheral heat transfer correlation can predict 74% of 903 data and 80% of 700 data, respectively, within 0°–180° and 15°–165° with the deviations of ±30%. The sensitivity analysis indicated that the parameter angle and the distributor height have the most and least significant impact on the peripheral heat transfer correlation, respectively. [ABSTRACT FROM AUTHOR]

Nitrogen oxides conversion rate, ammonia slip, and urea crystallization are major problems in the application of the selective catalytic reduction systems of diesel engine. Based on the bench test, the selective catalytic reduction system model was proposed with the code AVL Fire to investigate these problems. The research results indicate that the increasing ammonia/nitrogen oxides feed ratio contributes to reduce the emission of nitrogen oxides. However, the excessive supply of urea-water solution causes the increase of ammonia slip and liquid film. Nitrogen oxides conversion rate reaches a higher level when the exhaust temperature is 450°C. The performance of the system goes worse as the exhaust flow rate increases. The location of the mixer has a huge impact on the flow streamline. The formation of the liquid film and the interaction between the pipe and the droplets depend on the flow streamline. The mixer location is optimized to reduce the quantity of ammonia slip and liquid film based on the numerical study. [ABSTRACT FROM AUTHOR]

• Thoroughly studied the variables affecting falling film dehumidification on corrugated plates. • Provided the optimum shape of the corrugated plate for falling film dehumidification performance. • Proposed a mass transfer correlation based on the current numerical data. • Clarified the enhancement mechanism of the corrugated surface for falling film dehumidification. Due to a number of benefits, liquid desiccant dehumidification has received a lot of attention lately. In comparison to previous improvement strategies, corrugated plates have demonstrated higher efficacy in liquid desiccant dehumidification. This study employs computational fluid dynamics to examine the dehumidification performance of falling film liquid desiccant dehumidification on over sinusoidally shaped corrugated plates. The effects of plate geometries, solution parameters, and moist air (concentration of humidity, temperature, and flow rate) were discussed. Additionally, a mass transfer correlation was proposed based on the current numerical data. It is indicated that the corrugated plate significantly enhances dehumidification performance due to the liquid film waves intensification, mass transfer area expansion, and induction of vortexes close to the gas-liquid interface. The most effective falling film dehumidification is achieved using a corrugated plate with a wavelength of 5.0 mm and an amplitude of 0.5 mm. By raising the solution or air inlet temperature, the dehumidification rate is reduced while being strengthened by raising the solution or air inlet concentration, velocity, or air humidity. While decreasing when solution intake concentration or air velocity rises, the efficacy of dehumidification rises as solution velocity does. The proposed mass transfer correlation accurately predicts 93.3 % of the 45 data, with an error of 11 % or less. In the current parameter scopes, the magnitude has a more significant effect on Sh air than the wavelength. Adjustments to air velocity and solution concentration can cause significant fluctuations in Sh air , while air and solution temperature have little effect on Sh air. [ABSTRACT FROM AUTHOR]

• Tested cross vapor effect on the falling film evaporation of low pressure refrigerant. • Conducted larger vapor velocity effect on falling film evaporation in tube bundle. • Compared the cross vapor effect on falling film evaporation of R123 and R134a. • Visualized the cross vapor effect on falling liquid film flow and heat transfer. The falling film evaporation and the effect of cross vapor stream in falling film evaporator with triangular tube bundle using R123 was experimentally investigated. The variations of local and bundle average heat transfer coefficients were described. The tube bundle consisted of 4 × 3 (column × row) of triangular horizontal copper tubes. The tests without vapor effect were conducted with nominal heat fluxes of 20 to 60 kWm–2, saturation temperatures of 6 to 16 °C and film flow rates of 0.016 to 0.18 kgm–1s–1. Cross vapor stream effect experiments were operated at three heat fluxes of 20, 30 and 40 kWm–2 and two film flow rates of 0.035 and 0.07 kgm−1s−1, and the vapor velocity at the narrowest interstice in the tube bundle varies from 0 to 5.0 ms−1. It is indicated that the heat transfer is seriously influenced by the bundle effect while less affected by changing of saturation temperature. With the increase in vapor velocity, the heat transfer performance is generally weakened; the cross vapor stream has a strong influence on falling film evaporation of R123. [ABSTRACT FROM AUTHOR]

• Examined surface tension model effect on falling film flow on a horizontal tube. • Surface tension is indispensable in calculation of gravity-driven liquid film flow. • Given microscopic insights of falling film flow for different surface tension models. • Both CSF and CSS models can reliably reproduce the falling liquid film hydrodynamics. Computational fluid dynamics (CFD) modelling is extensively and successfully used for the reproduction of liquid film hydrodynamics. Surface tension significantly influences the numerical reproduction of falling liquid film flow. In this study, the hydrodynamics of the liquid film flow over a horizontal tube and a vertical plate is explored with different surface tension models. The results reveal that the neglect of surface tension leads to quantitative and qualitative distortions in the numerical simulation of the thin liquid film. Both continuum surface force (CSF) and continuum surface stress (CSS) models can produce the hydrodynamic behaviors of the falling film on the horizontal tube and the vertical plate, including pendant droplets, column necking or necking break, thickness of the film, vortex, liquid film detachment, as well as the values and trends of the velocity components. However, the CSF and CSS models produce different transient phase contours and streamlines, as well as different profiles of the time-averaged velocity components of the falling films on the horizontal tube and the vertical plate. Besides, there is no obvious difference between CSF and CSS in the perspective of time-averaged film thickness on a horizontal tube. [ABSTRACT FROM AUTHOR]

The effect of vapor flow on the falling film evaporation of refrigerant R134a outside a horizontal tube bundle is investigated with an experimental approach. The test space is a cube with a rectangular cross section of 0.575 m (length) × 38.8 mm (width). The tube bundle was 3 × 6 (columns × rows) of staggered horizontal finned tubes made of copper. The longitudinal tube pitch is 22.5 mm and the transverse is 19.9 mm. The external fin density of test tube is 45 fpi (fins per inch), and outside diameter is 19.05 mm. The vapor flow velocity can be adjusted in the range of 0–3.1 m/s. Liquid falling film flow rate ranges from 0.07 to 0.2 kg/m·s. Experiment is firstly conducted at saturation temperature of 6 °C without the effect of additional vapor flow at the heat flux of 20, 60, 100 and 180 kW/m 2 (for the first tube row). Vapor flow effect experiment was carried out at three heat fluxes 20, 40 and 60 kW/m 2 . It is found that falling film flow rate is an important factor to influence the evaporating heat transfer coefficient. With the effect of vapor flow, both positive and negative effects are observed as the increment of vapor velocity. Positive effects are predominant for the two tubes in the top positions and higher vapor velocity. [ABSTRACT FROM AUTHOR]

Falling film heat and mass transfer, representatives of advanced and low-carbon heat and mass transfer technologies, has been applied in a variety of thermal processes due to its great prospect for heat and mass transfer enhancement. However, the hydrodynamics, heat and mass transfer are significantly influenced by the gas (vapor or air) streams during the operation of falling film heat exchangers. To acquire the detailed profiles of flow, temperature and concentration and to explore the influence mechanism of gas stream shearing on the falling film hydrodynamics and heat and mass transfer, researchers have adopted experimental, analytical and numerical methods for different attempts. This paper presents a comprehensive summarization of the relevant work concerning the hydrodynamic, heat and mass transfer characteristics of falling film components, including the evaporator, absorber, evaporative cooler and dehumidifier. Considering the specific applications, the cocurrent, countercurrent, and cross gas streams shearing on the liquid film on a horizontal tube and bundle, a vertical tube and a vertical plate are reviewed. The objective of this review is to cover sufficient knowledge of the results, findings and insights of gas stream impacts on the hydrodynamics, heat and mass transfer in building utilized heat exchangers to provide guidance on how to maximize the positive effect or at least minimize the negative impact of gas shearing on the falling film heat and mass transfer. Schematics of gas stream models encountered in various applications. [Display omitted] • Provide the first review of the gas stream effect on falling liquid film flow. • Clarified mechanisms of gas effect on liquid film flow, heat and mass transfer. • Summarized studies of gas stream effect on falling film flow, heat and mass transfer. • Concerned film thickness, heat and mass transfer prediction under gas shearing. • Proposed some recommendations and improvements for future research. [ABSTRACT FROM AUTHOR]

In this study, thin liquid film boiling heat transfer characteristics of R134a on a plain and two finned tubular surfaces were experimentally investigated. The dependence of heat transfer coefficient (HTC) on the composite effects of microstructure and hydrophobic coating was characterized. It is found that HTC increases monotonously with increasing heat flux until reaching a threshold heat flux (THF), beyond which the HTC starts to descend. The THF is highest for the plain surface and lowest for the boiling-enhanced surface due to the effect of reentrant cavities. The boiling-enhanced surface shows larger HTC under low heat fluxes (less than 60 kW m−2) compared with the condensation-enhanced counterpart, whereas the latter one is superior when the heat flux exceeds 60 kW m−2. Hydrophobic coating can substantially intensify the liquid film boiling heat transfer on boiling-enhanced finned surface with reentrant cavities. The intensification of heat transfer is more prominent in cases of high heat flux. [ABSTRACT FROM AUTHOR]

• Detailed characterization of ionic liquid drops impacting on heated porous surfaces. • Increased temperature and velocity accelerate the droplet penetration. • Ionic liquid drop exhibits smaller diameter, height, and more stability than water. • Evaporation alters the relationship between residual volume and contact time. The impingement dynamics of a promising ionic liquid absorbent, namely, 1-ethyl-3-methylimidazolium acetate ([EMIm]Ac) in droplet falling film flow modes on heated porous surfaces in absorption refrigeration were investigated experimentally. The influences of impact velocity and surface temperature on the ionic liquid droplet spreading diameter, dynamic contact angle, spreading height, and residual volume against the heated porous surfaces were explored. The results indicated that the residual volume generally decreased in linear with increasing contact time at 323.15 K, while the dependence exhibited a polynomial behavior at higher heating temperatures. Compared to the impingement behaviors of deionized water droplets, such as oscillations and capillary waves were not observed in [EMIm]Ac droplets. The residual volume of the deionized water droplets varied exponentially with the contact time owing to the combined effects of the evaporation, penetration, and variation of the thermophysical properties of the droplet. Moreover, increases in the impact velocity and surface temperature enlarge the spreading diameter and accelerate the droplet penetration for the ionic liquid. [ABSTRACT FROM AUTHOR]