Mass Flux Research Articles

Performance optimization of non-isothermal endoreversible chemical pump via Lewis analogy

By considering heat and mass transfer (HMT) coupling, an endoreversible non-isothermal chemical pump cycle model is established and the optimal performance is studied herein. Assuming that the HMT follows Lewis analogy (heat transfer (HT) process obeys Newton's HT law and mass transfer (MT) process obeys diffusion MT law), rate of energy-pumping (Σ) and its corresponding vector coefficient of performance (χT and χμ, which are defined herein) are deduced analytically. Based on the general optimization results, the special examples of endoreversible Carnot heat pump with Newton's HT law and the endoreversible isothermal chemical pump with diffusion MT law are further deduced. The effects of heat flux rate (q1) and mass flux rate (g1) on the optimization results are analyzed. The Σ increases with the increases of q1 and g1. The surface of Σ to [χT,χμ] is a monotonically decreasing plane, and the Σ decreases with the increases of χT and χμ. The special cases of endoreversible Carnot heat pump with Newton's HT law and endoreversible isothermal chemical pump with diffusion MT law are further obtained. The Σ and χμ obtained by endoreversible non-isothermal chemical pump are larger than those obtained by endoreversible isothermal chemical pump. Coupling effect of HMT increases Σ and χμ, but correspondingly decreases χT.

Video-based measurements of the entrainment, speed and mass flux in a wind-blown eruption column

On May 4 2010 a wind-blown ash plume issued from Eyjafjallajökull volcano in Iceland. Analysis of a 17-minute-long video recording of the eruption suggests that, within 2–2.5 km of the vent, the flow was moving with the wind and rising under buoyancy, following a trajectory directly analogous with laboratory experiments of turbulent buoyant plumes in a cross-flow. The radius of the time-averaged ash cloud grew with height z at a rate r = 0.48 z, corresponding to an entrainment coefficient 0.4, again consistent with laboratory experiments. By analysing the frames in the video and comparing the shape of the plume to that predicted by the model, we estimate that during the 17 minutes recorded, the eruption rate gradually decreased by about 43% from an initial rate of 1.11 × 104 kg s−1 to 0.63 × 104 kg s−1. The analysis reported herein opens the way to assess eruption rates and eruption column processes from video recordings during explosive volcanic eruptions.

Sand Supply Affects Wind Erosion Efficiency and Sand Transport on Sand-Cemented Body Mulch Bed

Sand-cemented bodies (SCBs) are naturally distributed in some interdune corridors in the central Taklimakan Desert, northwest China. In this study, field-collected SCB particles were used as the experimental material, and wind tunnel experiments were conducted with different sand supplies, wind velocities, and SCB coverages to evaluate SCB wind erosion efficiency and vertical mass flux. The results showed that wind erosion efficiency decreased as SCB coverage increased. When the SCB coverage was above 40%, sand deposition processes occurred only under saturated sand flow, while sand transport remained unaffected by increases in SCB coverage under unsaturated sand flow. Under saturated flow, the highest concentrations of transported sand were found at 0–6 cm above the surface, and the main sand bed process was deposition. The sand bed process changed from aeolian erosion to deposition with increasing SCB coverage and tended to remain stable until the SCB coverage exceeded 40%. By contrast, under unsaturated sand flow, the sand bed process was primarily aeolian erosion, and the highest concentrations of transported sand were found at 0–4 cm above the surface. At high SCB coverage levels (more than 40%), a general balance between aeolian erosion and deposition processes was reached. In summary, increasing SCB coverage had a significant impact on surface wind erosion processes. Thus, SCBs can be used as a novel sand retention material.

Lagrangian Measurements Of Surface Water Waves: Relation Between Drift Velocities And Set-Down

Since the work of Stokes on steady progressive surface waves (Stokes, 1847), there has been interest in fluid particle trajectories and associated mass flux. The original result obtained by Stokes was based on linear theory, and implied that there is a net forward drift in the fluid beneath a propagating surface wave. In the non-dimensional case, the drift velocity for a sinusoidal wave on a fluid of depth h is given by Recent works suggest that in many cases, particularly in waves propagating over a shear flow, a net Eulerian flow may develop, which is opposed to the Stokes drift. Monismith et al. (2007) suggested that in a deep-water setting, the net Eulerian backflow may cancel the Lagrangian Stokes drift on a pointwise basis. Further, Grue & Kolaas (2017) found good agreement with the theoretical findings of Longuet-Higgins (1953), except near the bottom and near the free surface, where boundary layers have a discernable impact on the induced flow. In recent field measurements, it was observed that the drift velocity correlates positively with the local average fluid depth (Bjørnestad et al., 2021). In other words, a wave with a set-up features a large forward drift, while a wave with a set-down features a negative net drift. The present work aims at investigating what observed in the field by means of laboratory experiments carried out in a wave flume, where monochromatic and bichromatic waves of different characteristics have been run.

A numerical study on multi-nozzle spray cooling of downward-facing heater surface

An experimental study on multi-nozzle spray cooling of a downward-facing heater surface has been carried out in the SPAYCOR facility at KTH, to provide data assessing the feasibility of spray cooling for in-vessel melt retention (IVR) in light water reactors. To help understand the characteristics and influential factors of the liquid film formed on the heater surface in spray, a numerical study on the dynamics of an isothermal liquid film on the heater surface has also been performed by adopting the OpenFOAM platform, and Eulerian and Lagrangian methods for liquid film and droplets, respectively. The present study is an extension of the previous modeling from hydrodynamics to thermal-hydraulics of the spray cooling problem, via adding heat flux of the heater and two convective heat transfer models between the heater wall and the liquid film. Moreover, droplets-film interaction model is modified. The SPAYCOR experiment is simulated by the numerical models, and the simulation results show a good agreement between the numerical and experimental data, in particular when the modified droplets-film interaction model is applied. After the validation of the numerical models against the SPAYCOR experiment, the numerical models are employed to investigate influential factors on heat transfer, such as mass flux, nozzle-to-surface distance, and nozzle matrix layout. The results indicate that heat transfer is enhanced by increasing mass flux and decreasing nozzle-to-surface distance, and the change of nozzle matrix from inline to staggered layout has little impact on heat removal capacity or temperature distribution of the multi-nozzle spray cooling.

Analysis of subcooled flows boiling in horizontal rectangular mini-channels under microgravity conditions

In the aerospace field, microgravity significantly impacts flow boiling heat transfer performance and the bubble behavior. This study uses water as the working fluid and employs numerical methods to investigate flow boiling heat transfer in rectangular mini-channels under microgravity conditions. A coupled Volume-of-Fluid and Level-Set method (VOSET) is utilized for precise tracking of vapor–liquid interfaces. The study reproduces flow patterns and heat transfer characteristics under various heat fluxes and flow velocities, revealing the effects of microgravity on flow boiling heat transfer. At low mass flux density of 100 kg/(m2·s) and heat fluxes of 300 kW/m2, the absence of buoyancy in microgravity causes bubbles to adhere to the heating wall, forming dry patches. Conversely, under normal gravity, buoyancy promotes bubble detachment from the wall, leading to lower pressure drop and better heat transfer. As the heat flux increases to 400 kW/m2, bubbles rapidly grow into large bubbles, eventually reaching a size comparable to the mini-channel dimensions. Under both gravitational conditions, bubbles become confined by the mini-channel walls, resulting in similar dry patches areas, thus diminishing the impact of microgravity on heat transfer performance and pressure drop. Subsequently, the study explored the impact of microgravity on heat transfer performance, with mass flux densities varying from 100 kg/(m2·s) to 1000 kg/(m2·s) and heat fluxes ranging from 300 kW/m2 to 500 kW/m2. As mass flux density increases, inertial forces become more dominant. Notably, when mass flux density exceeding 500 kg/(m2·s), the heat transfer performance and pressure drop under different gravitational conditions becomes consistent, diminishing the influence of microgravity on heat transfer.

Formation of low-mass protostars and their circumstellar disks

Understanding circumstellar disks is of prime importance in astrophysics; however, their birth process remains poorly constrained due to observational and numerical challenges. Recent numerical works have shown that the small-scale physics, often wrapped into a sub-grid model, play a crucial role in disk formation and evolution. This calls for a combined approach in which both the protostar and circumstellar disk are studied in concert. We aim to elucidate the small-scale physics and constrain sub-grid parameters commonly chosen in the literature by resolving the star-disk interaction. We carried out a set of very high resolution 3D radiative-hydrodynamics simulations that self-consistently describe the collapse of a turbulent, dense molecular cloud core to stellar densities. We studied the birth of the protostar, the circumstellar disk, and its early evolution ($<6\ yr $ after protostellar formation). Following the second gravitational collapse, the nascent protostar quickly reaches breakup velocity and sheds its surface material, thus forming a hot ($ K $), dense, and highly flared circumstellar disk. The protostar is embedded within the disk such that material can flow without crossing any shock fronts. The circumstellar disk mass quickly exceeds that of the protostar, and its kinematics are dominated by self-gravity. Accretion onto the disk is highly anisotropic, and accretion onto the protostar mainly occurs through material that slides on the disk surface. The polar mass flux is negligible in comparison. The radiative behavior also displays a strong anisotropy, as the polar accretion shock was shown to be supercritical, whereas its equatorial counterpart is subcritical. We also find a remarkable convergence of our results with respect to initial conditions. These results reveal the structure and kinematics in the smallest spatial scales relevant to protostellar and circumstellar disk evolution. They can be used to describe accretion onto regions commonly described by sub-grid models in simulations studying larger-scale physics.

Hydrology and oxygen addition drive nutrients, metals, and methylmercury cycling in a hypereutrophic water supply reservoir

Impaired water quality in Mediterranean climate reservoirs is mainly associated with eutrophication and internal nutrient loading. To improve water quality in hypereutrophic Hodges Reservoir, California, United States, a hypolimnetic oxygenation system (HOS), using pure oxygen gas, was implemented in 2020. This study encompasses 3 years of pre-oxygenation data (2017–2019) and 2 years of post-oxygenation data (2020–2021) to understand the cycling of nutrients, metals, and mercury in the reservoir. During the wet year of 2017, mildly reduced conditions lasted until mid-summer in the enlarged reservoir. Nutrients and metals were seen in the hypolimnion including ammonia (~2 mg-N/L), manganese (~0.5 mg/L), phosphate (~0.5 mg-P/L), and sulfide (~10 mg/L). Production of methylmercury (MeHg), an important bioaccumulative toxin, was favored from April to June with a hypolimnetic accumulation rate of around 200 ng/m2·d. In contrast, the dry year of 2018 exhibited higher hypolimnetic concentrations of ammonia (~4 mg-N/L), manganese (~1 mg/L), phosphate (>0.5 mg-P/L), and sulfide (>15 mg/L). The rapid onset of highly reduced conditions in 2018 corresponded with low MeHg hypolimnetic accumulation (~50 ng/m2·d). It seems that mildly reduced conditions were associated with higher MeHg accumulation, while sulfidic, reduced conditions impaired inorganic mercury bioavailability for MeHg production and/or promoted microbial demethylation. Sulfide also appeared to act as a sink for iron via FeS precipitation, and potentially for manganese via MnS precipitation or manganese coprecipitation with FeS. Mass flux estimates for 2017–2019 indicate that much of the nutrients that accumulated in the hypolimnion moved via turbulent diffusion into the epilimnion at loading rates far exceeding thresholds predicting eutrophic conditions. After oxygenation in 2020–2021, the reservoir water column was highly oxidized but showed a lack of thermal stratification, suggesting reservoir operations in combination with HOS implementation inadvertently mixed the water column in this relatively shallow reservoir. Post-oxygenation, concentrations of ammonia, phosphate, manganese, and mercury in bottom waters all decreased, likely in response to oxidized conditions. Oxygenated bottom waters exhibited elevated nitrate, a byproduct of ammonia nitrification, and iron, a byproduct of FeS oxidation, indicating a lake-wide response to oxygenation.

Impacts of spatial heterogeneity of anthropogenic aerosol emissions in a regionally refined global aerosol–climate model

Abstract. Emissions of anthropogenic aerosol and their precursors are often prescribed in global aerosol models. Most of these emissions are spatially heterogeneous at model grid scales. When remapped from low-resolution data, the spatial heterogeneity in emissions can be lost, leading to large errors in the simulation. It can also cause the conservation problem if non-conservative remapping is used. The default anthropogenic emission treatment in the Energy Exascale Earth System Model (E3SM) is subject to both problems. In this study, we introduce a revised emission treatment for the E3SM Atmosphere Model (EAM) that ensures conservation of mass fluxes and preserves the original emission heterogeneity at the model-resolved grid scale. We assess the error estimates associated with the default emission treatment and the impact of improved heterogeneity and mass conservation in both globally uniform standard-resolution (∼ 165 km) and regionally refined high-resolution (∼ 42 km) simulations. The default treatment incurs significant errors near the surface, particularly over sharp emission gradient zones. Much larger errors are observed in high-resolution simulations. It substantially underestimates the aerosol burden, surface concentration, and aerosol sources over highly polluted regions, while it overestimates these quantities over less-polluted adjacent areas. Large errors can persist at higher elevation for daily mean estimates, which can affect aerosol extinction profiles and aerosol optical depth (AOD). We find that the revised treatment significantly improves the accuracy of the aerosol emissions from surface and elevated sources near sharp spatial gradient regions, with significant improvement in the spatial heterogeneity and variability of simulated surface concentration in high-resolution simulations. In the next-generation E3SM running at convection-permitting scales where the resolved spatial heterogeneity is significantly increased, the revised emission treatment is expected to better represent the aerosol emissions as well as their lifecycle and impacts on climate.

An Experimental Investigation of R600a Condensation in a Multiport Microchannel

This study aims to provide condensation heat transfer coefficients of R600a (isobutane) refrigerant under mass fluxes between 50 and 98 kg/m2·s at saturation temperatures of 35 °C, 40 °C and 45 °C. Additionally, experiments are conducted with varying inlet vapour quality to understand its effect on the condensation heat transfer measurement. An aluminium multiport microchannel with a hydraulic diameter (Dh) of 0.399 mm is used, where a plexiglass cover is mounted on the top of the microchannels to observe the flow conditions. A 1D heat transfer through the aluminium block is assumed, and heat flux through the refrigerant to the coolant is measured to obtain condensation heat transfer coefficients of R600a. The results showed that decreasing saturation temperature and increasing vapour quality increase the condensation heat transfer coefficient. Increasing refrigerant mass flux increases the heat transfer coefficient up to a specific mass flux. It is observed that the effect of inlet vapour quality becomes significant as introduced quality decreases due to increasing fluctuation.

Engagement of modified heat and mass fluxes on thermally radiated boundary layer flow past over a stretched sheet via OHAM analysis

This work investigates advanced heat transfer mechanisms in boundary-layer flow of nanofluids over a stretchable sheet. The comprehensive model integrates radiation effects, Brownian motion, thermophoresis, Cattaneo-Christov heat flux and Magnetohydrodynamics (MHD). Nanofluids, composed of a base fluid with suspended nanoparticles, exhibit distinct thermophysical properties crucial for influencing heat and mass transfer dynamics. Through rigorous numerical analysis, key parameters such as nanoparticle volume fraction, stretching sheet velocity, Cattaneo-Christov parameter, radiation parameter, and magnetic field strength are examined. The findings reveal intricate interactions among these parameters, providing a thorough understanding of their combined impact on temperature and velocity characteristics inside the boundary layer. This research contributes significantly to fundamental knowledge in nanofluid dynamics and provides practical insights for optimizing heat transfer processes and fluid behavior over-stretching surfaces, especially in the context of advanced heat transfer mechanisms. The implications extend across diverse engineering disciplines and materials science, offering valuable guidance for designing and improving processes involving nanofluid flows over-stretching sheets. With respect to the magnetic field parameter, the fluid velocity exhibits a decreasing attitude.The temperature function enhances with higher R\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathcal{R}$$\\end{document} values but diminishes with increasing Pr\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathcal{P}\\mathcalligra{r}$$\\end{document} values.

Condensation Heat Transfer of Zeotropic Refrigerant Mixtures R407C and R448A in a Horizontal Smooth Tube

Zeotropic refrigerant blends have attracted wide interest due to their promises to reduce global warming potential as well as their potential ability to improve the performance of heat pumps and air conditioners. The complex phenomenon of non-isothermal phase change complicates condensation of zeotropic mixture refrigerants and merits extensive investigation to better understand their performance profiles. The present work experimentally studied condensation heat transfer of R407C and R448A – two R134a-based zeotropic refrigerant mixtures – in a horizontal smooth tube, condensed by counter-current water cooling. Measurements for mass flux were in the range of 120 to 320 kg m−2 s−1 and saturation temperature in the range of 42 to 64 °C. Variation in refrigerant temperature and composition shift during condensation were quantitatively presented. The experimental data showed the effects of mass flux and saturation temperature on the heat transfer coefficients of R407C and R448A in different vapor quality regions. The analysis of heat and mass transfer resistances related to the flow regimes and refrigerant thermophysical properties provides a deep understanding of the condensation heat transfer of zeotropic refrigerant mixtures. The experimental data presented in this work are of value for the design of condensers.

Using Shelf‐Edge Transport Composition and Sensitivity Experiments to Understand Processes Driving Sea Level on the Northwest European Shelf

AbstractVariability in ocean currents, temperature and salinity drive dynamic sea level (DSL) variability on the Northwest European Shelf (NWES). It is dominated by mass variations, with steric signals relatively small. A mechanistic explanation of how ocean dynamics relates to the mass component of NWES sea level variability is required. We use regional ocean model experiments to isolate sources of variability and then investigate the effect on monthly to‐interannual DSL variability together with the simulated momentum budgets along the shelfbreak. Regional (local) wind and non‐regional (remote) forcing are important on the NWES. For the local wind forcing, the net mass flux onto the shelf, which drives a shelf‐mean mode of DSL variability, results from a combination of surface Ekman, bottom Ekman and geostrophic flows and explains 73% of the variance in transport across the shelf‐edge. The geostrophic flow is closely related to wind stress with a flow about half that of surface Ekman transport but in the opposite direction. For the remotely forced mass‐flux across the shelf‐edge, the geostrophic component explains 62% of the variance and bottom friction plays an important indirect role. The remotely forced variability, while relatively spatially uniform, is more important for explaining DSL variance over the western NWES. This mode of variability is sensitive to signals propagating northward via a thin strip of the southern boundary near the Portuguese coast, consistent with coastal trapped wave signals. It also appears to drive steric height in the Bay of Biscay, which is related to DSL on the shelf.

Influence of the thermophysical properties of working liquids on heat transfer performance during flow boiling in microchannels

A thorough understanding of the governing interfacial interactions during flow boiling in a microchannel would provide important insights into the underlying physics of fluid dynamics and heat transfer mechanisms. In this study, flow-boiling experiments were performed in a single rectangular microchannel with hydraulic diameter of 0.18 mm. Three working fluids (water, ethanol, and HFE7100) with significantly different thermo-physical properties were tested under an inlet subcooling of 25°C, mass flux of 50–330 kg·m−2·s−1, and effective heat fluxes up to 400 kW·m−2. Non-dimensional groups were used to quantify the relative effects of the forces governing the liquid–vapor interface. The bubble dynamics, flow pattern transition, heat transfer, and pressure drop were compared for the three different working fluids, and a mechanistic explanation of the flow boiling performance was proposed based on a force scaling analysis. Meanwhile, through investigating the influence of dimensionless parameters on heat transfer coefficient, a new basic heat transfer correlation that applies to a wide range of working fluids was developed, with 92.5% of data points falling within ±30% error bands and with a mean absolute error of 14.7%. HFE7100 exhibited the minimum increase in pressure drop with heat flux owing to its smallest liquid-to-vapor density ratio, low viscosity, and lowest interfacial tension effect. This study lays the foundation for the development of more reliable theoretical correlations and provides important insights into optimal design principles.

Intelligence modeling of the flow boiling heat transfer of nanorefrigerant for integrated energy system

To promote the application of nanorefrigerant in Organic Rankine Cycle and Integrated Energy System a reliable model with simple structure and favorable accuracy for predicting the flow boiling heat transfer coefficient (HTC) of nanorefrigerant is essential. In this work, four intelligence models—the radial basis function (RBF), multilayer perceptron (MLP), least squares support vector machine (LSSVM), and adaptive neuro fuzzy inference system (ANFIS)—were developed to predict the flow boiling heat transfer coefficient using nanorefrigerants, based on 765 experimental samples. The performances of these artificial intelligence models were comprehensively evaluated through accuracy analysis, variation trend analysis, and sensitivity analysis. Results indicated that the comprehensive performance of the RBF model was superior than those of other intelligence models and the existing empirical models. The RBF model accurately captured the variation trend of the output as the input variables were varied. Meanwhile, the impact degrees of all input variables in decreasing order were nanoparticle concentration (φ), mass flux (G), thermal conductivity of nanoparticle (kp), and vapor quality (x).

Time-dependent AGN disc winds – I. X-ray irradiation

ABSTRACT We study active galactic nucleus (AGN) line-driven disc winds using time-dependent radiation hydrodynamics. The key criterion for determining wind launching is the coupling strength of the ultraviolet radiation field via the spectral lines of the gas. The strength of these lines in turn relies crucially on the gas ionization state, determined by the local X-ray intensity. We consider a suite of models where the central ionizing radiation is affected by scattering, absorption, and re-emission by the intervening gas. In a pure attenuation model, the disc launches an episodic wind, as previous studies have shown. Including scattering or re-emission tends to weaken the wind, lowering the mass flux and outflow velocity and, if sufficiently dominant, suppressing the outflow entirely. However, the exponential nature of radiative attenuation means that only a modest, factor of a few, increase in the absorption cross-section can overcome the wind suppression due to scattering and re-emission. We find mass outflow rates of ∼20 per cent or more of the assumed inflow rate through the disc, indicating that radiation-driven winds may significantly alter the structure of the accretion flow. The winds also supply a large, time-varying column of material above the nominal constant disc scale height, which will determine the geometry of reprocessed emission from the central source. Our results suggest the need for accurate photoionization modelling, radiation transport, and accretion disc physics, to study their effects on the AGN disc winds.

Numerical investigation of bubble dynamics and flow boiling heat transfer in cylindrical micro-pin-fin heat exchangers

Micro-pin-fin evaporators are a promising alternative to multi-microchannel heat sinks for two-phase cooling of high power-density devices. Within pin-fin evaporators, the refrigerant flows through arrays of obstacles in cross-flow and is not restricted by the walls of a channel. The dynamics of bubbles generated upon flow boiling and the associated heat transfer mechanisms are expected to be substantially different from those pertinent to microchannels; however, the fundamental aspects of two-phase flows evolving through micro-pin-fin arrays are still little understood. This article presents a systematic analysis of flow boiling within a micro-pin-fin evaporator, encompassing bubble, thin-film dynamics and heat transfer. The flow is studied by means of numerical simulations, performed using a customised boiling solver in OpenFOAM v2106, which adopts the built-in geometric Volume of Fluid method to capture the liquid–vapour interface dynamics. The numerical model of the evaporator includes in-line arrays of pin-fins of diameter of 50μm and height of 100μm, streamwise pitch of 91.7μm and cross-stream pitch of 150μm. The fluid utilised is refrigerant R236fa at a saturation temperature of 30 °C. The range of operating conditions simulated includes values of mass flux G=500–2000kg/(m2s), heat flux q=200kW/m2, and inlet subcooling ΔTsub=0–5K. This study shows that bubbles nucleated in a pin-fin evaporator tend to travel along the channels formed in between the pin-fin lines. Bubbles grow due to liquid evaporation and elongate in the direction of the flow, leaving thin liquid films that partially cover the pin-fins surface. The main contributions to heat transfer arise from the evaporation of this thin liquid film and from a cross-stream convective motion induced by the bubbles in the gap between the cylinders, which displace the hot fluid otherwise stagnant in the cylinders wakes. When the mass flow rate is increased, bubbles depart earlier from the nucleation sites and grow more slowly, which results in a reduction of the two-phase heat transfer. Higher inlet subcooling yields lower two-phase heat transfer coefficients because condensation becomes important when bubbles depart from the hot pin-fin surfaces and reach highly subcooled regions, thus reducing the two-phase heat transfer.

Multi-year mesozooplankton flux trends in Kongsfjorden, Svalbard

AbstractWe conducted this study to investigate the relationship between environmental stressors and mesozooplankton fluxes in inner Kongsfjorden, Svalbard. The ongoing Arctic amplification, characterized by phenomena such as increased temperatures, glacial and watershed runoff, and diminishing ice cover, poses significant challenges to marine ecosystems. Our multi-year time-series analysis (2010–2018) of mesozooplankton, collected from a moored automatic sediment trap at approximately 87 m depth, aims to elucidate seasonal and interannual variations in fluxes within this Arctic fjord. We integrate meteorological, hydrological, and chemical datasets to assess their influence on zooplankton populations. Principal component analysis reveals the impact of seawater characteristics on mesozooplankton fluxes and composition, while two-way ANOVA highlights the role of seasonality in driving variations in our dataset. We observe a decrease in swimmer fluxes following the maxima mass flux event (from 2013 onwards), coupled with an increase in community diversity, possibly attributed to copepod decline and functional diversity. Notably, sub-Arctic boreal species such as Limacina retroversa have been detected in the sediment trap since 2016. Our continuous multi-year dataset captures the physical, chemical, and biological dynamics in this extreme environment. With Arctic amplification in Kongsfjorden and increasing submarine and watershed runoff, we anticipate significant shifts in mesozooplankton communities in the medium to long-term. This underscores the urgency for further research on their adaptation to changing environmental conditions and the potential introduction of alien species.

Time in DNB experimental study on Cr coated zircaloy cladding

Since the Fukushima-Daiichi accident effort has been put in by the nuclear community to develop Accident Tolerant Fuel (ATF) cladding materials to reduce the risk of clad oxidation and, consequently, hydrogen generation and potential major accidents. Among designed materials, the use of a chromium coating on top of zirconium-alloy cladding showed reasonable results in preliminary tests, being a favorable cladding solution for Pressurized Water Reactors. This paper provides highly detailed temperature data obtained during time in departure from nucleate boiling (DNB) experiments under prototypical reactor conditions of temperature, pressure, mass flux, and heat flux profile on a Cr-coated zirconium-alloy cladded heater rod simulating the fuel pin. Data were obtained at mass fluxes from 1300 to 1700 kg/m2s, pressure of 16 MPa, and inlet temperature of 333 °C (subcooling degree of 14.4 °C). The change in dry time was obtained by increasing the spike in wall temperature necessary to shut down the power. These tests showed that increasing the spike in temperature to characterize the critical heat flux from 50 °C to 400 °C resulted in variations of the departure from nucleate boiling measured value within the uncertainty range of their measurements, hence, validating the use of a spike of 50 °C, a commonly used value in this kind of study, as an indicator of the critical heat flux. Within the dry region, the temperature was found to increase along the direction of the flow for a very short length, reaching a peak, from which it reduces linearly until the surface reaches a temperature that is no longer able to sustain a dry patch. It was noticed that the dry region can grow against the flow direction due to the cosine profile of the heat flux, and that small dryout events can occur without leading to departure from nucleate boiling, i.e., being fully suppressed. However, two consecutive events may sum up and cause the critical heat flux. Rewet time was found to increase with peak cladding temperature and decrease with mass flux. A maximum peak cladding temperature of 844 °C was found, with the surface remaining dry for approximately 14.1 s. The minimum film boiling temperature was measured during the rewet process, showing absolute values of around 580 °C, and difference between wall and inlet temperatures of about 240 °C. A post-inspection using microscope and X-ray images showed no damage to the cladding and heater structure, differently than what was noticed for the bare zircaloy in a previous study. This indicated that the presence of chromium coated cladding can improve the oxidation resistance of the cladding not only in short critical heat flux transients but also in longer events associated with major accidents, like a Loss of Coolant Accident.

Numerical investigation of flow boiling characteristics of R134A in a smooth horizontal tube

Three-dimensional transient simulations using ANSYS Fluentʼs mixture multiphase model was conducted to investigate the impact of heat flux, mass flux, and saturation temperature variations on pressure drop behaviour and heat transfer characteristics during flow boiling of R134a refrigerant in smooth horizontal tubes of different diameters. The simulations employ an implicit numerical method to solve the governing equations, with relevant source terms for mass and energy transfer, incorporating the continuum surface force (CSF) and shear stress transport (SST) κ-ω models to account for surface tension and flow turbulence respectively. The results are presented in both dimensional (heat transfer coefficient and pressure drop) and non-dimensional (Nusselt number and two-phase frictional factor) forms. The Nusselt number shows a direct relationship with the Reynolds number, while the two-phase friction factor exhibits different behaviour. At higher vapor quality, mass flux significantly affects heat transfer performance and pressure drop. Heat flux plays a crucial role in heat transfer, but its impact on pressure drop is negligible. Higher saturation temperatures enhance heat transfer while reducing pressure gradients. The obtained results demonstrate good agreement with existing correlations, with mean absolute deviations (MAD) as low as 1.2 % for the heat transfer coefficient and 23.7 % for the pressure drop behaviour. This highlights the model's accuracy in predicting flow boiling behaviour of R134a in smooth horizontal channels, providing valuable insights for thermal analysis. The findings contribute to the understanding of flow boiling processes and aid in designing and optimizing efficient heat exchangers for various engineering applications.