Droplet Distribution Research Articles

The implication of PMMA molecular weight on compatibility of SAN/PMMA blends containing GO-g-PMMA hybrid compatibilizers

The molecular weight of a polymer is one of the most important factors defining material properties and performance. Within the current study, poly(styrene-co-acrylonitrile)/poly(methyl methacrylate) (SAN/PMMA) blends containing hybrid particles of graphene oxide-g-poly(methyl methacrylate) (GO-g-PMMA) were designed so that the molecular weight of the PMMA chains tethered on the GO particles was correlated with the molecular weight of the PMMA matrix. Hybrid particles were prepared using surface-initiated atom transfer radical polymerization (ATRP). The number average molecular weight of the matrix PMMA was Mn = 20,100 or 130,900 g/mol, while in the case of hybrids it was Mn = 5600, 27,300 or 191,600 g/mol. Two blend compositions were investigated: SAN/PMMA= 70/30 and 90/10 wt/wt, and the content of GO-g-PMMA was 1 wt%. Based on rheological studies it was demonstrated that higher molecular weights of the matrix PMMA promoted the compatibility of SAN/PMMA due to the similar viscosity of both blend components. Furthermore, the length of the PMMA chains grafted to the GO influenced the viscoelastic response of the material, resulting in different relaxation behavior as confirmed by mean relaxation times calculated from Cole-Cole plots. In addition, the effect of neat GO and GO-g-PMMA particles on the blend morphology and size distribution of PMMA droplets was discussed based on rheology and TEM analyses.

Chia Oil Microencapsulation Using Tannic Acid and Soy Protein Isolate as Wall Materials.

The use of proteins to produce oil-containing microcapsules has been previously analyzed; however, their chemical modification, in order to improve their performance as wall materials, is a strategy that has not been widely developed yet. This study aimed to analyze the chemical modification of the proteins through cross-linking reactions with tannic acid and to evaluate their performance as wall materials to the microencapsulation of oils rich in polyunsaturated fatty acids. The cross-linking reaction of isolated soy protein and tannic acid was carried out at pH 10-11 and 60 °C. Subsequently, emulsions were made with a high-speed homogenizer and microcapsules were obtained by spray drying. Microcapsules were characterized by particle size, morphology (SEM), total pore area and % porosity (mercury intrusion methodology), superficial properties (contact angle), and size distribution of oil droplets (by laser diffraction). Additionally, encapsulation efficiency was determined as a function of total and surface oil. Oil chemical stability and quality were studied by Rancimat, hydroperoxide values, and fatty acid profiles. In addition, a storage test was performed for 180 days, and released oil and polyphenols were determined by in vitro gastric digestion. Moreover, the fatty acid composition of the oil and the total polyphenol content and antioxidant capacity of polyphenols were analyzed. The results showed that spray-dried microcapsules had an encapsulation efficiency between 54 and 78%. The oxidative stability exhibited a positive correlation between the amount of polyphenols used and the induction time, with a maximum of 27 h. The storage assay showed that the peroxide value was lower for those cross-linked microcapsules concerning control after 180 days. After the storage time, the omega-3 content was reduced by 49% for soy protein samples, while cross-linked microcapsules maintained the initial concentration. The in-vitro digestion assay showed a decrease in the amount of oil released from the cross-linked microcapsules and an increase in the amount of polyphenols and a higher antioxidant capacity for all samples (for example, 238.10 mgGAE/g and 554.22 mg TE/g for undigested microcapsules with TA 40% versus 322.09 mgGAE/g and 663.61 mg TE/g for digested samples). The microcapsules showed a high degree of protection of the encapsulated oil, providing a high content of polyunsaturated fatty acids (PUFAS) and polyphenols even in prolonged storage times.

Research progress on the calculation model of critical liquid carrying flow of gas well

AbstractThe accumulation of liquid in gas wells has become a prevalent problem in the middle and later stages of gas well development with different wellbore structures, which seriously affects the production efficiency of gas wells. To precisely forecast the generation time of liquid accumulation in gas wells and timely adopt drainage gas production technology, the literatures on gas critical liquid‐carrying flow calculation models for vertical wells, inclined wells and horizontal wells were analyzed and summarized. The results show that the research on the calculation models for gas critical liquid‐carrying flow rate in vertical and inclined wells is relatively mature. However, because of the unique wellbore structure of horizontal wells, the gas‐liquid two‐phase flow pattern and distribution of droplets and liquid‐film are complex, leading to a lack of understanding of gas liquid carrying mechanism throughout the whole wellbore of existing horizontal wells, and we established a calculation model of gas critical liquid‐carrying flow rate in horizontal wells considers single factors. Based on the above summary and analysis, the research direction of the calculation model for gas critical liquid‐carrying flow rate in the entire wellbore of horizontal wells is pointed out. Combining the gas liquid‐carrying test of entire wellbore of the horizontal well, considering the distribution form of droplets and liquid‐film, the aggregation and fission between droplets, and the energy exchange between gas and liquid, the gas liquid‐carrying mechanism of the entire wellbore of horizontal well is revealed, clarifying the most easy to liquid‐accumulating well sections in the whole horizontal well, and a calculation model of gas critical liquid‐carrying flow rate of horizontal well considering multiple factors is established, providing crucial theoretical foundation and technical support for the drainage and gas recovery of a horizontal well.

Effect of low fuel temperature on combustion deterioration of kerosene swirling spray flames using OH-PLIF

The low fuel inlet temperature poses challenges to the flame stability of aircraft engine combustors. In order to investigate the combustion deterioration of swirl-stabilized kerosene spray flames under conditions of low fuel inlet temperature (T < −16 °C), a comprehensive system was established, comprising a burner commonly used in aircraft engines and a cooling setup. Upon analyzing OH-PLIF images, it becomes evident that sub-zero Celsius temperatures significantly lead to a decrease in OH radical concentration, resulting in a diminished heat release rate. The strongest signals decrease by a minimum of 5 times. Additionally, through the analysis of LIF spectra and detuned images, the differences between OH-LIF and kerosene-LIF are beneficial for recognizing the kerosene particles from the OH-LIF signals at the same time. For atomization assessment, employing the LOG operator blob detection method, it is found that at sub-zero Celsius temperatures, the overall count of fuel LIF particles experiences a slight reduction, while the count of larger particles increases across various air flow rates. This observation highlights an inferior droplet distribution at lower fuel temperatures. The image processing of kerosene LIF particles opens up the possibility for a rapid evaluation of atomization, especially when assessing combustion deterioration in kerosene spray flames through the use of a simple OH-PLIF technique.

Atomization Characteristics of a Hollow Cone Nozzle for Air-Assisted Variable-Rate Spraying

During variable-rate spraying in orchards, the atomization characteristics and distribution of droplets in and out of the target area can be affected by the sprayer pressure. In this study, a variable-rate spraying control system test bench was designed, and a hollow cone nozzle QY82.317.22 was selected. The droplet atomization characteristics, including volume median diameter (Dv0.5), the relative span of the droplet spectrum, and droplet velocity at different spray pressures, were studied at distances ranging from 0.4 to 2.4 m from the nozzle orifice with an air velocity of 10 m/s at the nozzle orifice position. The effects of longitudinal distance, transverse distance, and spray pressure on Dv0.5, relative span, and droplet velocity were analysed by multiple linear regression analysis, and the regression model was established. The experimental results show that at a longitudinal distance of 1.8 m, Dv0.5 ranges from 120 to 150 μm, meeting the requirements for optimal droplet size for controlling crawling pests and plant diseases on crop leaves; and the relative span is 1.2, indicating a wide droplet spectrum. At different pressure conditions, Dv0.5 decreases as pressure increases. Through multiple linear regression analysis, the longitudinal distance, the transverse distance, and the spray pressure have high significance for Dv0.5 and the droplet velocity. The longitudinal distance and the transverse distance have a highly significant effect on the relative span. In this study, the mathematical relational model of droplet characteristics at different spatial positions and different pressures was established, providing an agricultural reference for predicting the droplet characteristics at different spatial positions to achieve the best application effect. This model is conducive to the effective use of pesticides and reduces environmental pollution.

Self-Lubricating and Shape-Stable Phase-Change Materials Based on Epoxy Resin and Vegetable Oils.

Palm or coconut oil is capable of dissolving in a mixture of bisphenol A-based epoxy resin and a high-temperature hardener (4,4'-diaminodiphenyl sulfone) when heated and then forms a dispersed phase as a result of cross-linking and molecular weight growth of the epoxy medium. Achieving the temporary miscibility between the curing epoxy matrix and the vegetable oil allows a uniform distribution of vegetable oil droplets in the epoxy medium. This novel approach to creating a dispersed phase-change material made a cured epoxy polymer containing up to 20% oil. The miscibility of epoxy resin and oil was studied by laser interferometry, and phase state diagrams of binary mixtures were calculated according to theory and experiments. A weak effect of oil on the viscosity and kinetics of the epoxy resin curing was demonstrated by rotational rheometry. According to differential scanning calorimetry and dynamic mechanical analysis, the oil plasticizes the epoxy matrix slightly, expanding its glass transition region towards low temperatures and reducing its elastic modulus. In the cured epoxy matrix, oil droplets have a diameter of 3-14 µm and are incapable of complete crystallization due to their multi-component chemical composition and non-disappeared limited miscibility. The obtained phase-change materials have relatively low specific energy capacity but can be used alternatively as self-lubricating low-noise materials due to dispersed oil, high stiffness, and reduced friction coefficient. Palm oil crystallizes more readily, better matching the creation of phase-change materials, whereas coconut oil crystallization is more suppressed, making it better for reducing the friction coefficient of the oil-containing material.

In-situ observation of phase separation dynamics for immiscible aqueous solution within ultrasonic field

A high-speed imaging technique was used to observe the phase separation process of water (H2O)-20 %succinonitrile (SCN) immiscible solution within ultrasound field. Combining with numerical simulation, the effects of ultrasonic cavitation and acoustic streaming on the fragmentation and migration of secondary droplets were revealed. It was found that the previously spherical or near-spherical secondary H2O-rich droplets formed under static condition were dynamically transformed into several novel forms, such as tadpole-like, string-beads, gourd-like, and threadlike patterns. The calculated results showed that the cavitation could fragment micron-scale H2O-rich droplets because of the produced higher shock wave pressure than the droplets’ Laplace pressure, and the subsequent droplet morphology evolution mainly depended on the liquid ejection volume determined by the distance between the droplets and the collapsing bubbles. Meanwhile, acoustic streaming, which generated shear force exceeding the surface tension of H2O-rich phase, stretched, split and finally fractured millimeter-sized or even larger secondary droplets into several smaller spherical sub-droplets. In comparison, the observed secondary droplet distribution characteristics in H2O-20 %SCN solution were similar to the Bi-rich particles in the ultrasonic solidification microstructures of Al-30 %Bi immiscible alloy, confirming that this work provided a deep understanding of the liquid phase separation mechanism within ultrasonic field.

Spark discharges at the interface of water and heptane: emulsification and effect on discharge probability

Spark discharges in liquid have shown great potential for use in numerous applications, such as pollutant degradation, precision micromachining, and nanomaterials production. Herein, spark discharges are initiated at the interface of two immiscible liquids, heptane and water. This leads to the formation of an emulsion via mechanisms akin to bubble dynamics and instabilities at the gas–liquid. At high discharge number, an additional mechanism contributes to emulsion formation, resulting in an increase in the number of smaller heptane droplets in water. Analyses of the current–voltage characteristics show that high probability of discharge occurrence is obtained when the electrodes are aligned with the interface. This result is correlated with the low erosion rate of the electrodes. In the case of discharges at the interface, we observed that beyond a certain number of discharges, the breakdown voltage drops; far from the interface, it increases with the discharge number. Based on 2D simulation with a Monte Carlo approach to consider various droplet distribution in water, the electric field distribution is determined. The results support the fact that the decrease in breakdown voltage may be attributed to the intensification of the E-field in water close the heptane droplet. Therefore, spark discharges generated at the interface of a heptane/water system produce an emulsion of heptane in water, which facilitates the occurrence of subsequent discharges by intensifying the electric field and reducing the breakdown voltage.

Analysis of high-speed drop impact onto deep liquid pool

The present work is devoted to the analysis of drop impact on a deep liquid pool, focusing on the high-energy splashing regimes caused by large raindrops at high velocities. Such cases are characterized by short time scales and complex mechanisms, thus they have received very little attention until now. The BASILISK open-source solver is used to perform three-dimensional direct numerical simulations. The capabilities of octree adaptive mesh refinement techniques enable capturing of the small-scale features of the flow, while the volume of fluid approach combined with a balanced-force surface-tension calculation is applied to advect the volume fraction of the liquids and reconstruct the interfaces. The numerical results compare well with experimental visualizations: both the evolution of crown and cavity, the emanation of ligaments, the formation of bubble canopy and the growth of a downward-moving spiral jet that pierces through the cavity bottom, are correctly reproduced. Reliable quantitative agreements are also obtained regarding the time evolution of rim positions, cavity dimensions and droplet distributions through an observation window. Furthermore, simulation gives access to various aspects of the internal flows, which allows us to better explain the observed physical phenomena. Details of the early-time dynamics of bubble ring entrapment and splashing performance, the formation/collapse of bubble canopy and the spreading of drop liquid are discussed. The statistics of droplet size show the bimodal distribution in time, corroborating distinct primary mechanisms of droplet production at different stages.

Numerical investigation on spray and flow field characteristics of elliptical nozzle in supersonic crossflow

ABSTRACT It has been suggested that using an elliptical nozzle could enhance the atomization and mixture quality. In order to explore the elliptical liquid jet on spray and flow field characteristics in supersonic airflow, the VOF-Spray one-way coupling model was adopted to study the nozzle exit patterns, size, and velocity distribution of droplets. The research also compared the flow field characteristics in Mach 2.85 air crossflow under the application of different orifices. The numerical results showed that elliptical 4 nozzle (Aspect ratio of 4) had better flow performance than circular nozzle. The cavitation area and turbulent disturbance of elliptical 4 nozzle were mainly concentrated in the major axis direction. The elliptical 4 jet always had a lower penetration depth than circular jet. At the flow range of 0 ~ 50 mm, the penetration depth of elliptical 4 jet is roughly 12% less than that of circular jet. Another finding was that the spanwise extension range of elliptical 4 spray was larger than the circular spray. The spanwise extension range of elliptical 4 spray increased by about 19.4% at most in the flow range of 0 ~ 50 mm. Additionally, the elliptical 4 spray had the smallest Santer Mean Diameter (SMD). The elliptical 4 jet had a 2.9% smaller Santer Mean Diameter than the circular jet. Due to the greater windward surface area of the elliptical 4 orifice, the gas streamwise velocity near the bottom wall was reduced. A reverse vortex pair with distinct characteristics could be observed under the application of elliptical 4 orifice. Furthermore, compared with the circular orifice, the shock wave angle generated by elliptical 4 orifice reduced by 9.7%, and the total pressure recovery coefficient rose by 2.3%.

Bovine bone protein-quercetin conjugates for improved physical and oxidative stability of oil-in-water emulsions

To enhance the functional properties of bovine bone proteins (BBP) for food-related applications, their quercetin conjugates (BBP-QC), with antioxidant and emulsifying properties, were prepared by alkaline induction. The structural changes of BBP-QC conjugates were investigated, and their different concentrations were examined for the physical and oxidative stability of oil-in-water (O/W) emulsions. Structural characterization using Fourier transform infrared spectroscopy (FT-IR) and fluorescence spectroscopy confirmed the covalent binding of QC with BBP resulting in a conformational change in BBP. Particle size and ζ-potential measurements showed that 0.6% BBP-QC effectively improved the physical stability, increased the net charge of the O/W emulsion. Fluorescence microscopy observation of emulsion microstructure showed that the distribution of oil droplets in the BBP-QC stabilized emulsion was smaller and more uniform than that without the conjugates and effectively alleviated the phase separation in the emulsion. The 0.6% BBP-QC emulsion had a peroxide value (PV) and thiobarbituric acid reactive substances (TBARS) value of only 1.45 mmol/kg and 3.32 μmol/kg after 12 days. Additionally, BBP-QC stabilized emulsions exhibited excellent antioxidant capacity in vitro antioxidant experiments. This study highlights the excellent properties of BBP-QC conjugates as novel emulsifiers with high antioxidant activity and potential applications in the food industry.

Statistical Characteristics of Droplets Formed due to the “Bag Breakup” Fragmentation Event at the Interface between Water and High-Speed Air Flow

Abstract Recent studies indicate that the dominant mechanism for generating sprays in hurricane winds is a “bag breakup” fragmentation. This fragmentation process is typically characterized by inflation and consequent bursting of short-lived objects, referred to as “bags” (sail-like pieces of water film surrounded by a rim). Both the number of spray droplets and their size distribution substantially affect the air–sea heat and momentum exchange. Due to a lack of experimental data, the early spray generation function (SGF) for the bag breakup mechanism was based on the assumed similarity with resembling processes. Here we present experimental results for the case with a single isolated bag breakup fragmentation event. These experiments revealed several differences from similar fragmentation events that control the droplet sizes, such as secondary disintegration of droplets in gaseous flows and bursting of bubbles. In contrast to the bubble bursting, the film thickness of the bag canopy is not constant but is random with lognormal distribution. Additionally, its average value does not depend on the canopy radius but is determined by the wind speed. The lognormal size distribution of the canopy droplets is observed in conjunction with the established mechanism of liquid film fragmentation. The rim fragmentation results in two types of droplets, and their size distribution has been found to be lognormal distribution. The constructed SGF is verified by comparing it with experimental data from the literature. The perspectives of transferring the results from laboratory to field environment have also been discussed. Significance Statement The “bag breakup” fragmentation is the dominant mechanism for generating spray in hurricane winds. The number and the sizes of the spray droplets substantially affect the heat transport from the ocean to the atmosphere and, thereby, the development of hurricanes. This paper presents experimental data and analysis that demonstrate how droplet formation occurs during bag breakup fragmentation. It also shows analysis of the quantity and size of droplets formed during a single fragmentation event. This work demonstrates how obtained experimental results can be applied to real field conditions in the context of hurricane prediction models.

Modeling Multivariate Spray Characteristics with Gaussian Mixture Models

With the increasing demand for efficient and accurate numerical simulations of spray combustion in jet engines, the necessity for robust models to enhance the capabilities of spray models has become imperative. Existing approaches often rely on ad hoc determinations or simplifications, resulting in information loss and potentially inaccurate predictions for critical spray characteristics, such as droplet diameters, velocities, and positions, especially under extreme operating conditions or temporal fluctuations. In this study, we introduce a novel approach to modeling multivariate spray characteristics using Gaussian mixture models (GMM). By applying this approach to spray data obtained from numerical simulations of the primary atomization in air-blast atomizers, we demonstrate that GMMs effectively capture the spray characteristics across a wide range of operating conditions. Importantly, our investigation reveals that GMMs can handle complex non-linear dependencies by increasing the number of components, thereby enabling the modeling of more complex spray statistics. This adaptability makes GMMs a versatile tool for accurately representing spray characteristics even under extreme operating conditions. The presented approach holds promise for enhancing the accuracy of spray combustion modeling, offering an improved injection model that accurately captures the underlying droplet distribution. Additionally, GMMs can serve as a foundation for constructing meta models, striking a balance between the efficiency of low-order approaches and the accuracy of high-fidelity simulations.

Numerical studies of gasoline direct-injection sprays (ECN Spray G) under early- and late-injection conditions using Large Eddy Simulation and droplets-statistics-based Eulerian–Lagrangian framework

This paper presents an approach that uses Large Eddy Simulation under the Eulerian–Lagrangian framework to model the gasoline direct-injection (GDI) spray from Engine Combustion Network (ECN) multi-hole and counter-bore injectors. The approach considers the significant role that cavitation plays in primary atomization due to the counter-bore configuration. The local Weber number (We) and Reynolds number (Re) for GDI sprays are much smaller than those for a diesel spray, which renders conventional diesel spray approaches unsuitable. The proposed one uses a statistics-based droplet distribution function to address this and incorporates necessary information from a DNS inner-nozzle flow simulation. The corresponding models for drag, heat transfer, evaporation, and breakup consider the effects of viscous and distorted spray droplets. The approach has been applied to two different operating conditions, with simulation results that agree well with experimental data. Compared to the blob method assuming the ejected droplets are at the size of the injector diameter, the proposed CFD framework can reproduce a more accurate plume shape in the Spray G3 case. Applying the late-injection condition (Spray G1), the proposed approach improves the projected liquid volume (PLV) extinction compared to the blob method by providing a more accurate droplet evaporation prediction. These findings demonstrate the significance of the proposed approach for modeling GDI sprays and highlight its potential for future research in this area.

Impact of respiratory aerosol size and number distribution on the relative importance of different routes in SARS-CoV-2 transmission.

Exploring the relative importance of different routes in SARS-CoV-2 transmission is crucial in infection prevention. However, even in the same environmental setting, the relative importance of different routes has varied in different studies. We hypothesize that respiratory aerosol size and number distribution might play a key role. In this study, size and number distribution of respiratory droplets emitted from breathing, talking, and coughing were identified from PubMed and Web of Science. The infection risk of SARS-CoV-2 via airborne, droplet, and fomite transmission routes was modeled in a household and a healthcare setting. The relative importance of three routes varied with different size distributions in both settings. Generally, the contribution of the airborne route increased with the volume percentage of respirable droplets emitted. And the increase of the total number of emitted droplets leads to an increase in the contribution of tdroplet route. In the healthcare setting, as the total number of emitted droplets increased from 110 to 4,973, the contribution of droplet route increased from 62.24% to 98.11%. Next, by considering the combination of breathing, coughing, and talking when the infected person was asymptomatic, the airborne route predominated over the droplet and contact routes. When the infected person had developed symptoms, that is, cough, the droplet route played a dominant role in SARS-CoV-2 transmission. In conclusion, risk analyses will be improved with improved sampling methods that enable characterization of viruses within respiratory droplets of different sizes.

INVESTIGATION OF THE INFLUENCE OF LIQUID FUEL INJECTION RATE ON THE COMBUSTION PROCESS USING KIVA–II SOFTWARE

In this work, the ignition of liquid fuel was studied at various rates of fuel injection into the combustion chamber. The studied processes are described by the following equations: continuity, momentum, energy, k-ɛ turbulence model and others. The computational experiment was carried out using the KIVA-II software. The numerical method is efficient and takes into account various factors such as multistage chemical chain reactions, transfer of momentum, heat and mass by convection, radiation, turbulence, etc. In order to select the optimal parameters of the organization of these processes influence of the heptane injection rate into the combustion chamber on the process of combustion and self-ignition of heptane was studied. The injection rate of heptane varied from 160 m/s to 400 m/s an interval of 30 m/s. The size and temperature distributions of heptane droplets before evaporation, the concentration fields of carbon dioxide, and the temperature fields in the combustion chamber depending on the rate of fuel injection through the injector into the combustion chamber were obtained. The optimal range of heptane injection speed was determined, which is 250 m/s - 280 m/s, in order to increase the efficiency of the combustion chamber and reduce the negative impact on the environment.

EVOLUTION OF THE DISTRIBUTION FUNCTION OF DROPS IN AN OIL EMULSION

Oil emulsions are polydisperse media and have a droplet size in the macro and micro range. Macro and microemulsions can be found in both water-in-oil emulsion types and oil-in-water emulsion types. Taking into account the size distribution of water-oil emulsion droplets has a great influence on the accuracy of calculations when modeling the process of oil dehydration. Droplet size and droplet size distribution are important for the stability and viscosity properties of the emulsion. In the article, based on the rate of coalescence and fragmentation of particles, an expression is proposed for the rate of change in the number and size of particles per unit volume. On the basis of the Fokker-Planck equation, an expression for the distribution function of water droplets in an oil emulsion is found and, on its basis, an estimate of the distribution is investigated

Experimental study on heat transfer characteristics of the phase change chambers for heat flux enhancement

One promising way to increase the output power and the thermoelectric conversion efficiency of an automotive exhaust thermoelectric generator (TEG) is to add a phase change chamber between the exhaust channel and the thermoelectric (TE) modules with the chamber surface contacting with the exhaust channel where boiling occurs being larger than that with the module surfaces where the condensation occurs. The power is increased because a phase change chamber significantly increases the heat flux through the modules; thus, increasing the temperature difference between the module hot and cold sides. Although there is a significant body of literature on phase change chambers, most prior research has focused on chambers with larger condensation areas than boiling areas since they act as heat spreaders, while little is on chambers with condensation areas smaller than boiling areas for heat flux enhancement. These chambers have great potential to improve TEG power generation by enhancing the heat flux, therefore, more studies are needed to understand the chamber heat transfer characteristics. This research experimentally investigated the heat transfer characteristics of three chambers with condensation to boiling area ratios of 1/5, 2/5, and 3/5 for boiling heat fluxes from 4.50 to 22.70 kW/m2 and deionized water filling ratios from 33 to 50%. The results show that the boiling heat transfer coefficient increases with increasing heat flux or decreasing area ratio and is weakly influenced by the filling ratio. The condensation heat transfer characteristics depend on the distribution of droplets which is strongly affected by both the heat flux and the area ratio. The influence of the filling ratio on the condensation heat transfer coefficient increases with increasing area ratio. The thermal resistance of the chamber decreases with increasing heat flux and the lowest thermal resistance is obtained at 1/5 area ratio at lower heat fluxes and at 2/5 or 3/5 area ratio at higher heat fluxes.

An experimental study on edge-affected frosting characteristics on a vertical cold plate at different surface temperatures

Frosting has received significant attention in various fields due to its potential threat. To accurately predict and control the frosting process on vertical cold plate surface considering the edge effect, the frosting characteristics with surface temperature between −15.0 °C and −5.0 °C are experimentally studied under forced convection. The results show that as the cold plate temperature decreases, the durations of droplet solidification stage in the edge-affected region decreases slowly. Meanwhile, the area-average equivalent contact diameter and the coverage area ratio of edge-affected droplets both increases. The density difference of droplet distribution between the edge-affected and unaffected regions increases from 1.02 × 107 to 3.58 × 108 per m2. The average frost layer thickness reaches 7.41 × 10−4 m for −7.5 °C at 2,400 s, and it increases by 21.20%, 38.40%, and 82.08% when the temperature decreases to −10.0 °C, −12.5 °C, and −15.0 °C, respectively. Results of this study are expected to be meaningful for the optimization of frost detection and defrosting technologies.

Experimental study of relative humidity effect on the edge effect of frosting characteristics on a vertical cold plate surface

As a common phenomenon, frosting often brings negative impacts in various fields. To gain more insight into the frosting process on a vertical cold plate considering the edge effect, a specific experimental system is designed and built to perform frosting experiments under forced convection conditions with relative humidity varying from 40% to 80%. The results show that as the relative humidity increases, the droplet solidification stage period in the unaffected region decreases, but the equivalent width of edge-affected region, area-average equivalent contact diameter of droplets, and coverage area ratio of droplets increases and then decreases. The area-average equivalent diameter of droplets in edge-affected region increases by 41.25%, 214.63%, 137.98%, 159.03%, and 30.30%, when compared to the unaffected region from Case 1 to Case 5, respectively. Besides, the difference between the droplet distribution density in the edge-affected and unaffected regions gradually decreases, but the average frost layer thickness in the edge-affected region increases significantly as the relative humidity increases. The frost layer growth rate decreases first and then increases as relative humidity increases. From Case 1 to Case 5, the frost growth rates at 1,800 s decrease by 90.05%, 70.19%, 56.08%, 62.28%, and 82.39%, respectively, compared to the initial ones. Results of this study can provide a reference for the anti-frosting and defrosting control technologies in the field such as refrigeration and air source heat pump units.