Evaporation Surface Temperature Research Articles

Experimental study on the thermal performance of aluminum three-dimensional vapor chamber heat sink with a louvered-fin stacked evaporator wick for data center servers

Under increasing heat loads and stricter energy consumption limits for data center servers, traditional heat sinks are becoming inadequate. In this paper, a novel aluminum three-dimensional vapor chamber heat sink of air cooling is developed, in which the evaporator and condenser are connected internally to form a three-dimensional flow channel for working fluid so that two heat transfer methods, phase change and single-phase, are combined in one device. Furthermore, a novel louvered-fin stacked evaporator wick structure and its fabrication process are proposed, which greatly improve thermal performance and temperature uniformity. Thermal characteristics under different heat loads, fan input power, and tilt angles are studied experimentally. Results show that the heat sink has a maximum heat dissipation capability of more than 450W, at which the maximum evaporator surface temperature is within 75 °C. The fan input power can distinctly improve thermal performance, heat load impacts overall thermal resistance slightly and the minimum is 0.083 °C/W. The tilt angle impacts thermal resistance slightly but significantly affects both the evaporator surface and condenser fin temperature uniformity. Under a tilt angle of 90°, regardless of heat loads, the condenser fin temperature is much lower than other tilt angles.

Experimental analysis of a solar interfacial evaporation under high power concentrator

Solar-driven interfacial evaporation in solar distillers is recently intensively investigated. This work presents a new distiller system based on bottom-heating/interfacial evaporation process driven by high power concentrated solar energy. This system applies Annual Fresnel solar concentration and a circular Fresnel lens, increasing input solar energy. The 1 D water supply wick is elected as the interfacial evaporator to satisfy water evaporation demand by capillary suction. Solar distillers with high-power concentrator are experimentally tested and the results are discussed in this work. This work aims at analyzing the bottom-heating/interfacial evaporation performance under concentrated sunlight. The concentration ratio of an Annual Fresnel solar concentration and a circular Fresnel lens is 62.4. Further optimization of the distiller is carried out according to the concentrators and low ambient temperature requirements of interfacial evaporation. Performances of different distillers are compared. The results show that, the cumulative freshwater productivity of the interfacial evaporation and bottom-heating distiller are 21.25 kg/(m2·day) and 15.98 kg/(m2·day), respectively. The maximum gain output ratio of the interfacial evaporation is 72.1%, higher than the bottom-heating distiller. The maximum temperature of interfacial evaporation surface is 81.53 C∘.

Contribution of anthropogenic aerosols to persistent La Niña-like conditions in the early 21st century

The discrepancy between the observed lack of surface warming in the eastern equatorial Pacific and climate model projections of an El Niño-like warming pattern confronts the climate research community. While anthropogenic aerosols have been suggested as a cause, the prolonged cooling trend over the equatorial Pacific appears in conflict with Northern Hemisphere aerosol emission reduction since the 1980s. Here, using CESM, we show that the superposition of fast and slow responses to aerosol emission change-an increase followed by a decrease-can sustain the La Niña-like condition for a longer time than expected. The rapid adjustment of Hadley Cell to aerosol reduction triggers joint feedback between low clouds, wind, evaporation, and sea surface temperature in the Southeast Pacific, leading to a wedge-shaped cooling that extends to the central equatorial Pacific. Meanwhile, the northern subtropical cell gradually intensifies, resulting in equatorial subsurface cooling that lasts for decades.

Role of Interfacial Evaporation Process in Thin Vapor Chambers With Variable Pillar Spacing and Nonuniform Heat Flux

This paper reports the results of our numerical studies on the performance of thin film evaporation, mostly seen in the evaporator of a thin vapour chamber, using micropillar arrays of varying pitch and under both uniform and non-uniform heat flux conditions. The dry-out heat flux and evaporator surface temperature have been used as the performance metrics. A numerical model has been formulated to solve the fluid flow and phase change heat transfer considering the capillary pumping action (wicking action). The model is next exercised to study the performance of the evaporator with variable pitch of the micropillar array and with both uniform and non-uniform heat flux at the base. The pitch variations are done either by maintaining the mean pitch to be constant (same number of pillars) or by keeping the limiting pitch (maximum) equal to the uniform pitch scenario. The results show that a linearly decreasing pillar spacing arrangement along the downstream direction, i.e., along the flow direction (+ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> direction), results in significant enhancement of the cooling limit up to 1.5 times along with the reduction in overall evaporator surface temperature when compared to uniform pillar spacing where increase in cooling limit is accompanied with increase in overall evaporator surface temperature. In addition, the interplay of variable spacing and non-uniform heat flux results in design guidelines for pillar arrangements for given power maps while throwing new insights into the complex fluid flow and heat transfer phenomena encountered in this process. A higher concentration of pillars is found to result in lower evaporator surface temperatures, thereby recommending more number of pillars in high heat flux zones.

Critical role of climate factors for groundwater potential mapping in arid regions: Insights from random forest, XGBoost, and LightGBM algorithms

Groundwater potential mapping (GPM) provides the valuable information on groundwater volume that can be withdrawn from the aquifer without affecting the environmental conditions. In this study, GPM was innovatively explored by including three typical climate factors - precipitation (PRE), evaporation (EVA), and ground surface temperature (GST) - and by taking into consideration a total of 23 conditional groundwater factors during the model construction. Three ensemble learning models are used for the modeling process: random forest (RF), XGBoost, and LightGBM. The study was conducted in the southern regions of Yinchuan Plain, which is known for its temperate continental climate with low precipitation and high evaporation. It was found that all six adopted models (i.e., RF-C, XGBoost-C, LightGBM-C, RF, XGBoost, LightGBM) made reasonable predictions of groundwater potential, with areas with high and very high potential concentrated in the southwest region, where the Yellow River enters the study area. The LightGBM-C model performs the best (OA: 0.769, F1 score: 0.667, AUC: 0.921), while the RF model performs the worst (OA: 0.654, F1 score: 0.4, AUC: 0.757). According to the LightGBM-C model, there are more productive wells in areas with high groundwater potential (22 wells, 0.0152 per km2), while fewer non-productive wells are found (3 wells, 0.0021 per km2). The performance of LightGBM-C and RF-C models has been notably enhanced by climate factors (AUC + 0.073, OA + 0.025, F1 score + 0.052 for LightGBM, and AUC + 0.073, OA + 0.051, F1 score + 0.149 for RF). The further cumulative importance results indicate that the three ensemble models which considered climate factors demonstrated a substantial sensitivity to PRE (27.78%), EVA (28.14%), and GST (23.34%). In arid and semi-arid regions, PRE and EVA are highly recommended factors for GPM, while DD (35.17%), EV (35.22%), NDVI (28.06%) and GWD (25.77%) are also confirmed to be important when climate factors are not taken into account.

Climate change effect on water temperature and evaporation in Lake Qarun

ABSTRACT Climate change can have significant effects on the water and heat budgets of lakes and reservoirs. This study examined the effect of climate change on Lake Qarun through analyzing the temporal variation in lake surface temperature and evaporation from the lake. The analysis was based on the remote-sensing dataset MOD11A1 and the ERA5 climate reanalysis dataset. Daily surface temperatures for 1-km MOD11A1 pixels were filtered to remove outliers, averaged to obtain mean lake temperature, and then aggregated to compute the monthly average lake temperature. Lake evaporation was estimated by applying an un-mixing algorithm to ERA5 evaporation fluxes for mixed land-lake pixels and land-only pixels. The resulting monthly evaporation from the lake was corrected using a regression model based on evaporation estimates available in the literature for Lake Qarun. The regression model performance was confirmed through the Nash–Sutcliffe efficiency coefficient, correlation coefficient, and root mean square error. Over the period 1980 to 2019, findings show that annual evaporation ranged between 140 and 190 cm/year. Maximum average monthly evaporation for the study period is 21 cm in July while the minimum is 6.7 cm in February from 1980 to 2019. For the average lake surface temperature, it varied between ~31.9 and 33.9°C over the period 2000 to 2019. For the same time period, average maximum and minimum monthly temperatures were 44°C in August and 20.1°C in January. It is concluded that the average increase in evaporation and lake surface temperature is 37 mm/decade and 0.4°C/decade, respectively. The increase in both lake surface temperature and evaporation is likely due to climate change effects. However, using datasets with a larger temporal record for lake surface temperature, further studies can be applied for confirmation. Also, further confirmation can be obtained by applying the same analysis to other lakes in Egypt.

THE EFFECT OF LIQUID PROPERTIES ON HEAT TRANSFER AND EVAPORATION CHARACTERISTICS OF THE LIQUID FILMS FORMED BY UNSTEADY SPRAY-WALL IMPACTS

For intermittent spray-cooling purpose, it is essential to study the unsteady aspects of film evaporation and heat-transfer characteristics. In the present study, total evaporation time and surface temperature variations are investigated for four different liquid films (water, ethanol, n-octane, and n-hexane). The evaporation process is analyzed using a three-dimensional spray-wall impact with Lagrangian wall-film model. The evaporation process occurs in three stages; at the initial moments, most of the heat is used to raise the film temperature, and slight evaporation also exists. The film temperature rises until it reaches the liquid saturation point to evaporate at a constant rate. In the last stage, the evaporation rate decreases with time due to the accumulation of vapor in the bulk flow. The effect of heat flux and initial film thickness on the total evaporation time and the slope of its changes are investigated. The results show that the total evaporation time increases linearly with the initial thickness. Also, the molecular weight and saturation point of liquids are influential parameters after the enthalpy of evaporation. The surface temperature rises to a maximum value before reducing by the film evaporation. The maximum amount of the wall temperature depends on the liquid thermal conductivity and the evaporation rate. Finally, the effect of the initial value of the film temperature is investigated, and a correlation for estimating the total evaporation time is extracted.

A highly efficient bio-inspired 3D solar-driven evaporator with advanced heat management and salt fouling resistance design

Solar-driven evaporators offer a promising approach to obtaining fresh water due to their environmental protection and pollution-free operation, and because they require no other energy input. In this work, we prepared a low-cost fabric roll-based bionic evaporator (FRBE) with advanced heat management and salt pollution resistance based on a three-dimensional (3D) spiral groove structure design for the solar evaporation of seawater. The spiral groove structure, inspired by the vertical water supply of natural trees, had a super-strong regional water circulation ability between the yarns and fibers. The heat energy of the photothermal surface was quickly transmitted to the cold evaporation surface to realize rapid water evaporation, and the low evaporation surface temperature effectively reversed the usual heat loss phenomenon. Under 1.0 sun irradiation, the evaporation rate and evaporation efficiency of the prepared FRBE-4–2 reached 2.89 kg·m−2·h−1 and 130.04 %, respectively. In the evaporation tests with 10 wt% and 20 wt% NaCl solutions, the regional water circulation effectively promoted the dissolution and diffusion of salt on the evaporation surface, reflecting excellent salt pollution resistance performance. Therefore, the 3D spiral evaporation surface design developed in this study provided a new idea for efficient and sustainable steam generation.

Condensation on a Vertical Plate With Sinusoidal Microfins

Abstract A theoretically based, approximate solution for condensation on horizontal, microfinned tubes has been used as a basis to obtain a relation between heat flux and vapor-surface temperature difference for condensation on a vertical plate with sinusoidal microfins. The earlier result was in excellent agreement with experimental data with regard to magnitudes of the heat-transfer coefficient, dependence on fin and tube dimensions, and for a range of fluids with widely different properties. The new equation for condensation on a vertical plate with sinusoidal microfins takes account of the vertical gravity force and the lateral surface tension-induced pressure gradient over the curved fin surface. The equation reduces to the Nusselt result when fin height is zero and to simple area enhancement of the Nusselt result when surface tension is set to zero. Comparisons are made with experimental data for condensation of nitrogen on a vertical surface with sinusoidal microfins.

Experimental investigation of aluminum fins on relative thermal performance of sintered copper wicked and grooved heat pipes

Heat pipes are one of the most modern thermal devices used for high heat flux applications like fuel engines and thermal boilers to hydrogen storage tanks etc. Various kinds of thermal performance enhancement techniques have been employed to increase heat transfer through these pipes. For this purpose, manufacturing of a finned heat pipe is one of the famous techniques which maximizes heat transfer and increases operating range of heat pipes too. This study compiles the experimental results; 1) of installing Aluminum fins on the condenser section of heat pipes, 2) and a comparison between finned and finless distilled water based sintered wicked as well as grooved heat pipes, by keeping all other parameters same. The distribution of surface temperatures, thermal conductivity and thermal resistance is considered for comparison purposes. It is clearly observed that power surface temperature has an inverse relationship with the evaporator thermal resistance that is; if the power surface temperatures increases then the evaporator thermal resistance decreases. Also, installation of fins is more significant in sintered heat pipes as compared to grooved heat pipes. Due to installation of fins, there is an increase in the operating range which was observed 47% for sintered heat pipes and 30.1% for grooved heat pipes, when compared at 20 W. It has also been noted that the thermal conductivity of sintered heat pipes increased up to 73.2% due to the installation of fins and unexpectedly decreased up to 58.8% in case of grooved heat pipes. Thermal resistance of finned sintered heat pipes was 42% lower than finless type, and surprisingly it has increased up to 1.43 times in grooved heat pipes. Finally, a through comparison of finless sintered and grooved heat pipes results that finless grooved heat pipe shows the best performance and up to a 4.7% lower evaporator surface temperature was observed.

Satellite-Based Monitoring of the Algal Communities of Aras Dam Reservoir: Meteorological Dependence Analysis and the Footprint of COVID-19 Pandemic Lockdown on the Eutrophication Status.

Aras Dam Lake is a strategic aquatic ecosystem in Iran and there are reports of toxic phytoplankton blooms in this reservoir. This study was performed to determine the effect of meteorological variables on the formation and expansion of toxic phytoplankton communities in Aras dam reservoir. The data of this project have been obtained using field studies and satellite data (MODIS and Sentinel-2). Sampling to determine the composition of phytoplankton communities in the area was carried out seasonally in two time periods from 2003 to 2014, and environmental assessments were also performed based on meteorological and satellite data over an 18-year period (2003–2020). The Chlorophyll-a content was obtained from MODIS and correlated with meteorological data. The statistical analysis showed that the highest coefficient of determination is related to the correlation of chlorophyll-a and Evaporation (R2 = 0.86). Also, the relative root mean square error is equal to 18%, 18.1% and 21.2% for the chlorophyll-a -SST, chlorophyll-a -wind and chlorophyll-a -Evaporation relations, respectively. Moreover, in a supplementary study, correlation between the chlorophyll-a content with selected meteorological variables including evaporation, wind speed and water surface temperature were investigated seasonally. The results showed that the trend of changes in chlorophyll-a content with three considered variables are parabolic functions and chlorophyll-a -Evp (R2 = 0.86, MAPE = 15.2%) model indicates better performance. The results also showed that the eutrophication rate of the reservoir during lockdown period increased in comparison with the same time at pre-pandemic period, which can be related to increase of incoming waste loads in this reservoir.Graphical

Assessment of a novel defrost method for PV/T system assisted sustainable refrigeration system

Energy consumption has continuously increased depending on the rapidly growing human population, enlarging economies, advancing technologies, and improving living standards. A noteworthy share of the energy consumption has been arising from the buildings all across the world. Refrigeration, heating, and air conditioning systems have accounted for a significant portion of the energy consumption in the buildings. Therefore, it is possible to both reduce energy consumption, and mitigate the carbon footprints by efficiently designing, constructing, and operating these systems. In this framework, the present research has centered on the refrigeration systems, and aimed to develop a novel defrost method for photovoltaic thermal (PV/T) assisted sustainable refrigeration systems. In the conventional refrigeration systems, the frost process occurs when air condenses on the evaporator surface as a result of the evaporator surface temperature being below the freezing point of water or the dew point temperature of the air in the conditioned space. Differently in the present work, PV/T system is used to prevent the frost process in the refrigeration system, unlike the conventional systems. Accordingly, the efficiency loss caused by the temperature increment will be prevented by cooling the PV module, and it is aimed to be more efficient by reducing the daily power consumption as an alternative solution method to the frost that occurred on the evaporator in refrigeration systems. On this purpose, a novel evaporator design is developed, and used for defrosting in this study. Accordingly, this novel design includes a refrigerant line inside the evaporator and a hot water line from the PV/T in this design. In the results, it is noticed that the system designed for winter conditions could be used for defrosting. While an average of 605 W for heat energy was used for each defrost process, the average defrost duration was recorded to be approximately 4 min. While the average electrical efficiency of the PV module was found to be 13.6%, the average total efficiency was found to be 38%. Besides, Average PV module surface temperature was determined as 36.4 °C, average water storage tank temperature was determined as 26.4 °C. In addition, the coefficient of performance (COP) of the refrigeration system is calculated to be 4.18. COP increased by an average of 9% during defrosting. Furthermore, the environmental economic cost was calculated to be 14.6 $/h. In the conclusion, it is proven that the novel defrost method proposed in the present work can be used for refrigeration systems, and contribute to both the reduction of energy consumption and mitigation of carbon emissions arising from the buildings.

Electro-osmosis Aided Thin-Film Evaporation from a Micropillar Wick Structure.

The heat-dissipating capacity of a surface having micropillar wick structures, which resembles the evaporator section of a vapor chamber, is mainly limited by the liquid flow rate through the porous structure (permeability) and the capillary pressure gradient. The efficacy of a regular vapor chamber is determined from two parameters, namely, the dry-out heat flux and temperature of the evaporator surface. These two parameters possess a counter relation to each other. The work described herein introduces and evaluates the performance of a novel idea of electro-osmosis-aided thin-film evaporation from a micropillar array structure. This study is conducted using a discretized approach that is validated against the thin-film evaporation model and additionally the electro-osmotic flow model with pre-existing pressure gradient conditions. The unique feature of this approach is that it results in an increment in the magnitude of dry-out heat flux without significantly changing the surface temperature, wherein the increase in permeability is due to the addition of electro-osmotic flow. This comprehensive model considers various geometries, zeta potentials, and extremal electric fields and establishes the beneficial effects of the application of an external electric field. The results are used to predict the sensitivity and the dependence of the dry-out heat flux and the evaporator surface temperature on these parameters. For a host of electro-osmotic parameters considered herein, a maximum increment of up to 320% in the dry-out heat flux is observed for an external electric field of 105 V/m. The study, therefore, conclusively demonstrates the beneficial impact of electro-osmosis in enhancing the dry-out heat flux without any significant Joule heating.

Experimental and numerical investigation of thermal performances of R290 and R1234yf refrigerants in a cold room

In today’s world, most of the currently used refrigerants in refrigeration systems have an important influence on global warming. This study presents the experimental and numerical consumption of thermal and energy performances comparison of new generation low GWP R290 and R1234yf refrigerants for a cold room. Evaporator temperatures of −9 °C, −6 °C, −3 °C, and condenser temperatures of 25 °C, 30 °C, 35 °C were utilized as the operational parameters. Results revealed that the highest COP values in the system for R290 and R1234yf were 3.99 and 3.41, respectively. An average 11.1% increase in the cooling capacity was determined with R290 compared to the R1234yf refrigerant. In the second part of the study, evaporator surface temperature and air velocity distributions in the cold room for R290 and R1234yf refrigerants occurring until the cold room temperature reached from the ambient temperature to 0 °C were numerically shown using Ansys Fluent 18.1. Numerical analysis using experimental data indicated that the evaporator surface had a lower temperature with R290 refrigerant than R1234yf. Results of the study highlighted the usability of R290 refrigerant instead of R1234yf in refrigeration systems in terms of thermodynamic performance.

Over 11 kg m-2 h-1 Evaporation Rate Achieved by Cooling Metal-Organic Framework Foam with Pine Needle-Like Hierarchical Structures to Subambient Temperature.

Solar steam generation has become a hot research topic because of its great potential to alleviate the drinking water crisis without extra energy input. Although some efforts focusing on designing spatial geometry have been made to multiply the evaporation performances of up-to-date three-dimensional evaporators, they still have some shortcomings, such as low material and space utilization efficiencies, complex spatial geometry, energy loss due to the hot solar absorption surface, and salt crystallization due to inefficient water supply. Herein, a biomimetic copper-based metal-organic framework (Cu-Cu(OH)2-MOF) foam sheet with interconnected pores and pine needle-like hierarchical structures consisting of Cu(OH)2 nanowires and MOF nanowhiskers is fabricated. The pine needle-like hierarchical structures of Cu-Cu(OH)2-MOF foam contribute to absorbing solar energy and supplying sufficient water by trapping incident light and enhancing the capillary force, respectively. Inspired by drying clothes outside under solar irradiation, through exposing one end of the Cu-Cu(OH)2-MOF foam to air, the biface evaporator achieves a subambient evaporation surface temperature and an evaporation rate of up to 3.27 kg m-2 h-1 under only one sun illumination. Furthermore, when coupled with an air flow, the biface evaporator realizes an excellent evaporation rate of 11.58 kg m-2 h-1 with an energy efficiency of 160.07% even in seawater, ensuring its great application prospect to be used in drinking water production and seawater desalination.

Experimental and numerical comparison of thermodynamic performances of new and old generation refrigerants in the same cooling system

The use of new generation low GWP refrigerants in cooling systems is becoming increasingly common due to thermal performance and less environmental impact. In this study, the thermal performance of the new generation R290 refrigerant and the old generation R404A refrigerant in the same cooling system were investigated experimentally and numerically. The thermal performance values of both gases were compared, provided that the temperature of the cold room cooled by the vapor compression cooling system drops to 0?C. ?5?C, 0?C, 5?C evaporator temperatures and 25?C, 30 ?C, 35 ?C condenser temperatures were used as operating parameters. The experimental results revealed that the highest COP values in the system for R290 and R404A were 4.68 and 3.94, respectively. An average of 9.38% increase in cooling capacity was detected with R290 compared to R404A refrigerant. In the numerical analysis part of the study, the evaporator surface temperature and air velocity distributions of both refrigerants in the cold room were shown numerically by using ANSYS FLUENT 19.1. As a result of the study, it was stated that R290 had higher thermal performance than R404A, according to both experimental and numerical analysis results.

Experimental Study of Evaporation Flux, Salt Precipitation, and Surface Temperature on Homogeneous and Heterogeneous Porous Media

Salt precipitation in porous media is widespread, which has garnered great research attention. However, the mechanisms governing the salt precipitation, water flux, and surface temperature changes in homogeneous and heterogeneous porous media remain unclear. This study investigated the dynamics of salt precipitation, evaporative loss, and surface temperature in homogeneous fine sand (0.1–0.25 mm), coarse sand (1‐2 mm), and a heterogeneous column with fine and coarse sand. All sand columns were initially saturated with NaCl solution. The experimental results showed that the salt was precipitated as efflorescence above the surface of the fine sand, whereas it was mainly precipitated as subflorescence below the surface of the coarse sand, causing the unconsolidated sand to form a strong stone‐like mass. The evaporated loss was significantly higher in heterogeneous than in homogeneous sand, but this difference in evaporation was insignificant in the stage where vapor diffuses through the precipitated salt to the external air. The salt crust formed not only decreased the surface temperature due to increased albedo by salt precipitation, but also resulted in a more discrete temperature distribution in the porous media. Our research results can promote further understanding of salt precipitation and evaporation in porous media.

Performance evaluation of evaporation from micropillar arrays with different pillar topologies

This paper reports the results of our studies on the performance of thin film evaporation, mostly used in the evaporator of an ultra-thin vapor chamber using micropillar arrays. The dry-out heat flux and evaporator surface temperature have been used as the performance metrics. A numerical model has been formulated to solve for the fluid flow and phase change heat transfer considering the capillary pumping action (wicking action) using a cell-by-cell forward approach and validated with results reported in the literature. The solutions of the model equation are then utilized to study the effects of geometry and arrangement of the micropillars within the vapor chamber and to gain further insights into the interplay of the various forces. It is seen that these variations strongly influence the pumping action as well as the shape of the liquid vapor interface thereby impacting the pressure field, flow rate and the dry-out heat flux. A conical shape of the micropillar (having the same volume as that of other shapes) has a significant impact on the dry-out heat flux e.g., reducing the top diameter from 10 to 8 μm results in an 18% reduction. For a rectangular arrangement of the micropillars, the performance is quite sensitive to the transverse and axial spacing. In the range of 30–40 μm of axial and transverse spacing, the dry out heat flux can be altered by as much as 20%. Higher transverse pitch with lower axial pitch is found to be a better combination for a given overall geometry and number of pillars.

Investigation of the evaporation in the boundary layer

THE PURPOSE. Consider a stationary diffusion problem when a pure liquid evaporates from a flat surface of evaporation into a laminar boundary layer of a forced gas flow (in the absence of deepening of the evaporation surface and wave formation on it) at the number. In the classical model of the diffusion problem of the flow of mass from a flat surface into the laminar boundary layer, only the additional slowing down effect arising in this case is taken into account. However, the resulting solution does not correspond to the general case of evaporation, since in this case the mass transfer can significantly depend on the thermal conditions of the problem, conjugate in phases; in criterion form, this circumstance is expressed by the appearance of an additional parameter [1-3]. Note that this parameter is related to the value of the derivative of the relative concentration along the transverse coordinate on the evaporation surface. In the course of the proposed solution, the temperature of the evaporation surface and, accordingly, the value of this parameter were taken constant. METHODS. When solving the problem, we used approximate numerical methods for integrating the diffusion equation (Euler's method, integro-differential equation method, and also the method of successive approximations). In this case, the retarding effect of the vapor flow from the surface of the phase transition was assumed to be relatively insignificant in our case (which corresponded to the experimental data used in [1-3]. RESULTS. The article analyzes the well-known classical solution of the diffusion equation according to the Hartnett - Eckert model and notes that the result obtained in this case does not correspond to the general case of evaporation, when the mass transfer in the gas phase also depends on the complex. Based on the solution obtained in our work, we come to the conclusion that the effect of this parameter manifests itself in an increase in the thickness of the diffusion boundary layer. In addition, this effect is also associated with the value of the longitudinal coordinate, being more noticeable at its small values. CONCLUSION. The indicated evaporation pattern can be physically explained by a relatively larger amount of evaporated substance than in the “standard” case (since values, in turn, are associated with higher values of the evaporation surface temperature). It can also be assumed that in the region of the gas flow immediately adjacent to the evaporation surface, these factors manifest themselves in a similar way in the case of turbulent flows.

Effect of temporal evolution of the evaporation surface temperature on the plume expansion under pulsed laser ablation

The effect of the temporal evolution of the evaporation surface temperature on the neutral plume expansion under pulsed evaporation into vacuum has been studied. Two-dimensional calculations have been performed based on the direct simulation Monte Carlo (DSMC) method. The Gaussian distribution of the temporal evolution of the surface temperature is assumed. The regimes with the constant and varying temperature of the evaporation surface have been compared. It is shown that for evaporation of more than ten monolayers the varying temperature leads to a considerable change in the plume dynamics with up to 9% decrease in the average energy of particles passing through a time-of-flight detector on the normal to the surface.