Laminar Film Research Articles

Vitamin D3 formation in milk by UV treatment – Novel insights into a rediscovered process

Vitamin D3 is essential for several functions in the human body and the demand is usually covered by natural reactions in skin with UV radiation delivered by the sun. But living beyond a latitude of 35° can lead to a lack of sufficient exposition to the deciding wavelength. Here, many countries fortify their milk prophylactically with artificial vitamin D3. However, the precursor molecule of vitamin D3 (7-deydrocholesterol) is already naturally located in the milk fat globule membrane. Thus, this study deals with the transformation of the naturally occurring 7-dehydrocholesterol into vitamin D3 through UV treatment of the milk - a mechanism that was observed a century ago only indirectly. Different parameters such as temperature (10 - 50°C), fluid flow regimen (turbulent vs. laminar thin film, i.e., 0.6 mm) and wavelength (254, 280 and 313 nm) were investigated in this study for their efficiencies. The UV dose of each experiment was measured with chemical actinometry delivering the actually applied dose reaching the milk. Thus, the connection between applied UV dose and generated vitamin D3 content in the milk measured quantitively with LC-MS/MS was evaluated here that both were not possible a hundred years ago. The experimental results revealed that temperature generally promotes the vitamin D3 formation at 254 nm. Further, a turbulent flow is not as efficiently treated as a laminar thin film flow that is as narrow as 0.6 mm. As expected from absorbance spectra of the precursor molecule 7-dehydrocholesterol, 280 nm turned out to be the most efficient wavelength, followed by intermediate success through irradiation with 254 nm and almost no effect by 313 nm. Generally, it was shown that vitamin D3 concentration of milk was easily increased by UV treatment with today's technologies and that adjustment of certain physical parameters have a significant effect on the efficiency.

Development of innovative equipment for milk microflora inactivation

The article is dedicated to the development of innovative equipment for the inactivation of milk microflora. It provides an analysis of existing methods to suppress the growth and destruction of microorganisms in milk, aiming to enhance product quality indicators. The advantages and disadvantages of current pasteurization equipment are highlighted. An overview of alternative methods for milk inactivation is presented, focusing on the impact of ultraviolet radiation on bacteria and lower organisms. It is concluded that a major advantage of ultraviolet (UV) milk treatment, in addition to its bactericidal effect, is its minimal impact on proteins under efficient operating conditions. Considering the limited penetration of UV rays into milk, a device is proposed—a milk disinfection separator that separates microorganisms from the main product by placing them in the form of a thin laminar film accessible for effective irradiation. Special attention is given to justifying the parameters of the UV irradiation section. The methodology for hydraulic calculation of device movement is presented, along with a method for determining the bactericidal effectiveness of UV irradiation on microorganisms and the necessary lethal dose for this purpose. By specifying the system's productivity, knowing the bactericidal dose required to deactivate bacteria, and considering the irradiation intensity of the UV source, the dimensions of the irradiation section and the required electrical power of the equipment are determined. The article proposes modern LED arrays with the necessary UV wavelength range, along with diagrams for their connection and equipment control.

Study on temperature distribution of vertical liquid falling film heat transfer with Al2O3 nanofluid

This study presents a laminar smooth 2-D theoretical model of the falling liquid film using an in-house code and validated with experimental results. The numerical model considers thermophysical property enhancements with nanoparticles Al2O3 to different liquids such as water, ethylene glycol (EG), and LiBr-H2O. An experimental setup has been developed to validate the pure heat transfer process in the falling liquid film, using the measurement of the falling film interface temperature. The simulation results agreed well with the experimental results for various liquids at low flow rates. The discrepancy increases as the flow rate increases due to the limitations of the laminar smooth film model in the numerical simulation. With a volumetric concentration of ϕ=4% nanoparticle Al2O3, the heat transfer coefficient of nanofluid water, EG, LiBr-H2O increased around 15%–20% compared with the baseline fluid. A new correlation was obtained to predict the average heat transfer coefficient based on experimental data. The experimental results have an uncertainty at 9.3% for the surface temperature and 5.5% for the falling film bulk temperature. The maximum deviation between the calculated heat transfer coefficient by the model and experimental data is around 25%, at Re<1000. The average error has decreased by 23% when comparing the new correlation considering Ka and the conventional form.

Heat transfer correlation of smooth inverted annular film boiling in a circular tube

Experiments on steady film boiling heat transfer are conducted in a circular tube with test conditions covering the range of 10–50 K inlet subcooling, 0.2–2.5 MPa pressure, and 100–1000 kg/m2s mass flux. Through analysis of experimental data and comparison with corresponding predictions by the existing correlations, they are demonstrated to be insufficient for accurately capturing the varying trends of the experimental data within inverted annular flow boiling (IAFB) regime. It is discovered that IAFB is divided into three regions, namely, smooth IAFB, rippled IAFB and turbulent IAFB, by the distribution of wall temperature and its rate of change along axial direction, which is consistent with relevant observations from previous study. Only the vapor film in the smooth IAFB can be attributed to laminar vapor film flow, which leads to the discrepancy in the overall trend of existing correlations and experimental data. As fluid flows downstream, interface interaction due to velocity difference between vapor film and liquid core accounts for the fluctuations. The assumption that transition between smooth and rippled IAFB is result from Kelvin-Helmholtz (K-H) instability accounts for the flow characteristics of vapor film in smooth IAFB. Heat transfer at interface is obtained based on vapor–liquid velocity. Additionally, a new heat transfer correlation for smooth IAFB is proposed. The MAPE of the new correlation is 16.4%, and the RMSPE is 18.5%. The new correlation could more accurately describe the heat transfer characteristic in smooth IAFB.

Rapid and reversible humidity-dependent colour change by water film formation in a scaled springtail.

Colour is often not a static trait but can change over time either through biotic or abiotic factors. Humidity-dependent colour change can occur through either morphological change (e.g. to feather barbules in birds) or by the replacement of air by water causing a shift in refractive index, as seen in arthropod multi-layer cuticles or scales. The scaled springtail Lepidocyrtus cyaneus has scales that produce colour largely via thin film interference from their lamina. We observed a marked colour change from golden to violet/purple coloration in humid conditions. Light microscopy, micro-spectrophotometry, contact angle goniometry and optical modelling indicate that the formation of a thin film of water on top of the hydrophilic scales increases their laminar thin film thickness, causing a shift towards violet/purple colour. Evaporation of the water film causes the metallic golden colour to return. This constitutes a remarkably rapid colour change (in the order of seconds), only limited by the speed of water film condensation and evaporation, that may serve as inspiration for new dynamically coloured materials and sensors.

Model for core catcher ablation and cavity shape in the film regime

During core meltdown in a nuclear reactor the corium may propagates toward the lower head and hit the vessel or structural elements as a coherent liquid jet. This could occur for instance in sodium fast reactors (SFR) if discharge tubes and an in-vessel core-catcher are used as mitigation devices. This can result in the ablation of the core-catcher and potential loss of its integrity. During ablation, a cavity is formed within the solid. Therefore the ablation of a solid structure by a liquid jet is studied here, with a focus on the cavity shape to improve future core-catcher designs. During the cavity formation, liquid flows over the cavity being formed as a liquid film in the film ablation regime, and ultimately results in the formation of a liquid pool. In the film ablation regime, two types of cavity shapes have been identified previously. To describe the process of the cavity formation, a mathematical model valid in this film ablation regime is here proposed which links the local melting rate to the local curvature of the cavity. It is applied with two sets of assumptions applicable to the two types of cavity previously identified. For the first assumption set, the heat transfer coefficient varies along the liquid/solid interface while the liquid temperature is held constant, it is representative of laminar boundary layer growth in laminar film. For the second assumption set, the heat transfer coefficient is held constant while the temperature varies along the liquid/solid interface, this is consistent with a turbulent liquid film. The two modeling approaches are compared with experimental data acquired from the HAnSoLO experimental setup. Numerical predictions show promising agreements with experimental observations, particularly for the laminar boundary layer growth zone of the cavity.

Hydrodynamic and thermal model for gravity-driven smooth laminar film flow undergoing flash evaporation cooling: Case study and correlation development

Hydrodynamic and thermal analyses have been carried out for gravity-driven smooth laminar film flow, undergoing flash evaporation at the free surface. A classical one-dimensional semi-analytical approach has been adopted to address a unique problem where hydrodynamic and thermal boundary layers (TBLs) approach from opposite directions and eventually intersect each other. This occurs due to the rapid evaporation cooling at the film-free surface exposed to the low-pressure ambiance, leading to the growth of a TBL from the free surface. In contrast, the hydrodynamic boundary layer (HBL) grows from the solid wall over which the film flow occurs. The intersections between the TBL and HBL edges, HBL edge and the free surface, and TBL edge and the wall, in conjunction with the attainment of a fully developed hydrodynamic condition, result in the division of the overall film domain into three distinct hydrodynamic and five distinct thermal sub-zones requiring zone-specific formulations. The model is successfully validated for hydrodynamic formulations with the existing experimental data. However, the lack of available experimental studies limits the validation of the proposed thermal model. Correlations for relevant thermal and hydrodynamic parameters, such as local Nusselt number, local free surface temperature, local bulk mean temperature, and local film thickness, are developed based on the model predictions. The proposed model and the correlations derived from its predictions are anticipated to serve as crucial benchmarks for optimizing the design of thermal management and desalination systems that are fundamentally driven by the film evaporation process.

ANALYTICAL INVESTIGATION ON THE CONDENSATION PERFORMANCE OF VERTICAL, TILTED AND HORIZONTAL SURFACES FOR PASSIVE WATER HARVESTING

Condensate harvesting is the phenomenon of obtaining water from water vapor in the humid air condensing on a surface. The idea is a passive technique with no additional energy consumption, yet condensation performance of a surface varies with many parameters. This study analytically investigates the condensation performance of the vertical, tilted, and horizontal water harvesting systems. First, viscous effects, inertia, and gravitational forces are examined in detail to express the evolution of boundary layer in condensation film. Then the condensation performance of each surface orientation and tilt angles are documented and compared for all flow conditions. Although vertical surfaces have higher condensate harvesting performance compared to the tilted and horizontal systems, the condensation rate is only about 2% lower up to 15⁰ tilt angle. When the harvesting surface is tilted at 30⁰, the condensation rate of the laminar film decreases by 3.5%, while the reduction is 4.7% in wavy-turbulent film condensation. The results indicate that the change in condensation rate is more evident just after 45⁰ tilt. Furthermore, 89⁰ tilted surfaces experience 63.7% and 74.1% lower condensate harvesting in laminar and wavy-turbulent regimes, respectively. In addition, identical horizontal surfaces produce only one fifth condensation rate of a vertical system.

The characteristics of laminar film condensation of pure vapor flow in a vertical tube

In this study, we investigate the characteristics of laminar film condensation of pure vapor flow in a vertical tube, taking into account (i) the vapor flow rate reduction owing to condensation, (ii) the finiteness of the vapor boundary-layer thickness, (iii) the curvature of the vapor layer, and (iv) the pressure variation. An approximate integral two-phase boundary-layer equation model is derived, and a condition that pressure recovery occurs is determined from the asymptotic analysis around the tube inlet. Furthermore, the present model is further simplified to obtain an analytical solution if the degree of subcooling is sufficiently large. The asymptotic solution for sufficiently large degrees of subcooling gives a good approximation to the numerical solution. The tube length of complete condensation is estimated. Comparison with vertical plane condensation demonstrates that the film thickness and the Nusselt number increased and decreased, respectively, by the vapor flow reduction for large degrees of subcooling.

Numerical simulation of condensation from isobutane vapour – air mixture

Hydrocarbon condensation has been a topic of interest in recent years due to its applications in refrigeration, chemical processing and air conditioning. The condensation of friendly fluids reduces the Global Warming Potential and Ozone Depletion Potential, which can be used as an alternative to many hydrofluorocarbon (HFC) refrigerants. The main objective of this study is to investigate the combined heat and mass transfer of laminar film condensation from isobutane vapour in the presence of a small percentage of non-condensable gas (air) inside a vertical tube. The external tube wall is subjected to a constant temperature. To achieve our goal, a mathematical model based on mass conservation, momentum, and energy in both gas and liquid phases is established. The discretization of equations in each region with the boundary conditions is realised using an implicit finite difference scheme. The validity of the numerical model developed is verified in comparison with the results of the literature. The obtained results show that the condensation process is enhanced by increasing the inlet velocity and the inlet-to-wall temperature difference. Moreover, the results indicate that the presence of non-condensable gas reduces the efficiency of heat and mass exchanges.

Thermal distribution analysis of rectangular fin considering multiboiling heat transfer modes

AbstractIn this investigation, a numerical method is used to compute the thermal distribution analysis of a rectangular fin with surface emissivity and internal heat generation. Here, the thermal conductivity, heat generation, emissivity at the surface, and coefficient of heat transfer depend on temperature linearly. The role of four distinct multiboiling heat transfer modes such as laminar film boiling (condensation), laminar convection, turbulent convection, and nucleate boiling are discussed in detail and the corresponding outcomes are displayed graphically. Isolated (insulated) and convective tip boundary conditions for the fin tip are employed in this study. The solution is obtained using shooting technique involving Runge Kutta Fehlberg method. It is emphasized that the thermal distribution shows a diminishing trend for the convective tip condition compared to the insulated tip. In addition to this, it is illustrated that laminar film boiling and laminar convection are two effective modes of heat transfer in comparison with turbulent convection and nucleate boiling for a finned surface in boiling liquids. The study on fin efficiency shows that fin efficiency increases with the increase in internal heat generation number.

Comparison and prediction of UV-C inactivation kinetics of S. cerevisiae in model wine systems dependent on flow type and absorbance

UV-C (254 nm) treatment can be an alternative or supplement method to adding sulfite to wine for stabilization. There are various types of UV-C reactors using different flow types. However, literature data often refer to specific microorganisms in specific food using specific UV-C reactors. Therefore, a statement about the influence of the flow conditions on the inactivation of microorganisms is difficult. This study examined the inactivation of Saccharomyces cerevisiae dependent on the flow conditions. Different flow conditions were realized in three UV-C reactor types. In order to minimize the influence of the natural fluctuations of the ingredients in wine, a model medium was introduced and validated to mimic the inactivation of S. cerevisiae in wine. The absorbance of the model solution was adjusted to resemble wine available on the market by applying variable concentration of food coloring (sunset yellow). These model solutions with inoculated microorganisms were UV-C treated using different flow conditions (Dean vortices, turbulent and laminar tube flows, and laminar annular thin film flows with a 3 mm and a 0.6 mm gap). Mathematical models were established to predict the inactivation of S. cerevisiae dependent on the flow type, the absorbance of the (model) wine and the UV-C dose.

Theoretical study on forced convection filmwise condensation inside converging channels in the presence of a non-condensable gas

In this paper, a theoretical study is performed on forced convection filmwise condensation inside a vertical converging channel in the presence of a non-condensable gas. Two solution schemes are developed with consideration of the turbulence effect in gas mixture and a laminar condensate film flow. Various heat transfer mechanisms are revealed by examining condensation heat transfer inside converging channels. In the present study, steam and air are taken as the vapor and non-condensable gas, respectively. It is found that the film velocity transits from a linear profile to a parabolic profile for a straight channel along the channel axial direction under the combined influence of gravitational force and interface shear stress. However, for a converging channel, the transition process from linear to parabolic velocity profiles is substantially delayed along the axial direction due to the enhanced interfacial shear stress in the latter condensing section. The film thickness decreases along the channel axial direction at the latter condensing section for both natural and forced convection condensation due to the substantially decreased local condensation rate and/or the enhanced interfacial shear stress in a converging channel. The combined influence of the curvature effect and enhanced interfacial shear stress leads the local heat transfer enhancement to increase substantially at the latter condensing region for forced convection condensation inside a converging channel. A uniform enhancement on the normalized local heat transfer coefficient in the inlet condensing section was found for both natural and forced convection condensation. However, the reduced condensation area has led the normalized local heat transfer coefficient to decrease substantially along the axial direction for converging channel as compared to straight channel in the case of natural convection condensation. Comparatively, the increased interfacial shear stress results in the normalized heat transfer coefficient along the entire channel to be enhanced. The average heat transfer coefficient increases as the converging angle or the Reynolds number increase. The turbulence effect can enhance the condensation heat transfer. However, the presence of air can cause a significant decrease in the condensation heat transfer. For the case of pure vapor condensation, the enhancement ratio increases as the Reynolds number increases. When the air is present in the system, an insufficient vapor supply is observed when the Reynolds number is low. Therefore, the enhancement ratio increases substantially as the converging angle increases. When the air mass fraction is high (W=0.4), the enhancement ratio appears to be insensitive to the Reynolds number in the laminar and turbulent regime, respectively. In addition, the enhancement ratio generally increases as the air mass fraction increases.

Jet impingement on the underside of a superhydrophobic surface

Water patch topology and momentum loss resulting from a jet impacting on the underside of a flat plate with varied hydrophobicity were studied. The jet's Reynolds and Froude numbers ranged from 3700 to 31 000, and from 1 to 23, respectively. Hence effects of gravity were expected to be non-negligible, and data suggest that this is the case for hydrophobic surfaces. The interplay of gravity, surface tension, inertia and viscosity resulted in two distinct behaviours. On hydrophilic surfaces, water spread uniformly. Friction reduced momentum, and led to accumulation at the edges of the patch until gravity overcame surface tension and produced droplets. On hydrophobic surfaces, two rims formed, enclosing a thin laminar film. The patch shape was mostly determined by the balance of kinetic and surface energy. Dewetting occurred in most cases when the two rims merged, but for a narrow parameter, range water detached soon after impact and formed a type of skewed water bell. The transition in detachment topology was predicted reasonably by a simple model considering whether an attached or detached rim minimizes energy. Due to promotion of dewetting by gravity, the water patch area decreased compared to that reported in previous studies, which considered jet impingement on vertical surfaces and tops of horizontal surfaces. Owing to the application that motivated this study, the streamwise force on the plate was also measured. On hydrophobic surfaces the reduction in force correlated with the reduction in water patch area. Water patch area and momentum loss were both found to scale best with the contact-angle-modified Weber number.

Magnetohydrodynamic Thin Film Flow through a Porous Stretching Sheet with the Impact of Thermal Radiation and Viscous Dissipation

In this paper, the Darcy–Forchheimer laminar thin film flow with MHD and heat transfer on an unsteady horizontal stretched surface is investigated. The impact of thermal radiation and viscous dissipation is also considered for thin film flow. The analysis of heat source with thermal radiation in a boundary layer flow can play a great role in manufacturing engineering procedures such as the production of electric power, solar energy modernization, and astrophysical flow. By using similarity transformation, the system of PDEs is converted to ODEs. Then HAM is applied for the solution of the problem. Moreover, the velocity and temperature profile for various embedded variables are discussed through graphs while the numerical solution of concerned physical quantities such as skin friction and Nusselt number are discussed through tables. The analysis shows that velocity profile is reducing function of M , K 1 , S , and F 1 . The temperature profile is an increasing function S and E c while a reducing function of Pr and R . The overall behavior of the proposed system is highlighted analytically.

Measurements of the liquid film thickness for annular flow during flow condensation in a circular tube

Liquid film thicknesses of R245fa and zeotropic mixtures of R134a/R245fa were measured during flow condensation in circular micro-channel using a CCD camera and a Laser Focus Displacement Meter (LFDM). The fluctuation characteristics were studied along with the average liquid film thickness and the thermal resistance. The average liquid film thickness had no obvious association with the temperature glide. The liquid film fluctuation frequency was most strongly affected by the vapor-liquid interface shear force with the liquid film thickness determining the fluctuation range. Larger mass fluxes weakened the thinning effect of the mass flux. With increasing R134a mass fraction, the impact of mass flux was much obvious. With increasing R134a mass fraction, the liquid film thermal resistance increased. The laminar liquid film momentum equation was used to develop a simple method to predict the mean liquid film thickness.

MHD dissipative Casson nanofluid liquid film flow due to an unsteady stretching sheet with radiation influence and slip velocity phenomenon

Abstract The problem of non-Newtonian Casson thin film flow of an electrically conducting fluid on a horizontal elastic sheet was studied using suitable dimensionless transformations on equations representing the problem. The thin film flow and heat mechanism coupled with mass transfer characteristics are basically governed by the slip velocity, magnetic field, and the dissipation phenomenon. The present numerical analysis by the shooting method was carried out to study the detailed, fully developed heat and mass transfer techniques in the laminar thin film layer by solving the competent controlling equations with eight dominant parameters for the thin liquid film. Additionally, the predicted drag force via skin-friction coefficient and Nusselt and Sherwood numbers were correlated. In view of the present study, a smaller magnetic parameter or a smaller slip velocity parameter exerts very good influence on the development of the liquid film thickness for the non-Newtonian Casson model. Furthermore, a boost in the parameter of unsteadiness causes an increase in both velocity distribution and concentration distribution in thin film layer while an increase in the same parameter causes a reduction in the film thickness. Likewise, the present results are observed to be in an excellent agreement with those offered previously by other authors. Finally, some of the physical parameters in this study, which can serve as improvement factors for heat mass transfer and thermophysical characteristics, make nanofluids premium candidates for important future engineering applications.

Detailed modeling of laminar film condensation from zeotropic binary mixtures in a vertical tube

Zeotropic binary mixtures have the potential to be suitable replacements for phased-out refrigerants. In this work, a detailed numerical model of film condensation from a binary mixture is developed as a potential tool to assist evaluating the performance of such replacements. Downward co-current laminar film condensation of a zeotropic binary mixture inside a vertical tube is modelled numerically using the full two-dimensional parabolic governing equations in both phases. The model predicts the condensation behavior of a binary mixture at any composition with properties as functions of the local pressure, temperature, and composition. Detailed results of the heat and mass transfer and the local velocity, temperature, and mass fraction in the entire domain are produced as well as the axial variation of the pressure and the film thickness. Detailed results for zeotropic mixtures of R290/R600A and mixtures of R23/R116 with an azeotrope are presented. A non-monotonic variation of the vapor mixture axial velocity is observed during the axial development of the film and the depletion of the vapor mass flow. It was found that the local heat transfer coefficient depended mainly on the liquid thermal conductivity and the film thickness. For the two refrigerant pairs studied in this work, the local heat transfer coefficient increased with the mass fraction of the less condensable component. The difference in the condensation rates of the constituting components are quantified with and without an azeotrope.

Effect of Porosity on the Laminar Condensation Type Thin Film on a Vertical Wall in Forced Convection

Effect of Porosity on the Laminar Condensation Type Thin Film on a Vertical Wall in Forced Convection

Effects of Navier slip on film condensation heat transfer over upward facing horizontal flat surfaces with free edges

Transport phenomena involving condensate liquids generated from the phase-change heat transfer in microchannels and in engineered superhydrophobic surfaces require consideration of slip effects. Herein, the laminar film condensation over upward facing flat slabs and circular disks of finite sizes with free edges in the presence of wall slip effects is investigated. By considering the Navier slip model and extending the classical Nusselt analysis, the mass, momentum, and energy of the liquid film in two-dimensional/axisymmetric coordinates are solved for the film thickness and the heat transfer rate in non-dimensional form. Numerical solution yields the local structure of the condensate film profile and the Nusselt number for different values of slip coefficient. The results reveals that the condensate film on horizontal surfaces becomes thinner and the overall heat transfer rate is enhanced with an increase in the slip coefficient. In particular, an analysis of the results indicates a power law dependence of the Nusselt number on the non-dimensional slip coefficient with an exponent close to 0.5. Significant enhancement in phase change heat transfer follow from the modification of the local velocity profiles within the condensate film, resulting from the additional momentum gain near the wall surfaces due to increases in slip effects.