Mean Mass Flux Research Articles

Characterizing the long-term occurrence and anthropogenic drivers of per- and polyfluoroalkyl substances in surface water of the Rhine River

Per- and polyfluoroalkyl substances (PFAS) raise significant concerns due to their persistence, bioaccumulation potential, and toxicity to both ecosystems and human health. However, the long-term trends of PFAS in aquatic environments remain inadequately explored. In this study, we systematically assessed the spatiotemporal distribution, periodic fluctuations, source apportionment, and risk evaluation of 12 PFAS in the Rhine River based on the long-term measuring data collected from 2007 to 2019. The study revealed that the mean concentration and mass flux of total PFAS during this period were 32.83 ng L−1 and 6.36 × 104 μg s−1, declining at an annual rate of 3.70% and 3.82%, respectively. Wavelet analysis demonstrated that the most prominent periodic oscillation of PFAS was 40–60 months. Regarding the sources of PFAS, we employed the self-organizing map (SOM) and the positive matrix factorization (PMF) model for source apportionment. The results indicated that the primary sources of PFAS were agrochemical, pharmaceutical and textile industries, accounting for 38.1% of the total concentration. The contribution from household contamination, tannery industry, and coating materials has increased annually. In contrast, the share of electrochemical fluorination and chemical recycling has shown a continuous decline. The risk quotient (RQ) and hazard quotient (HQ) calculations for three age groups indicated that PFAS exposure did not pose a significant risk to ecological or human health. Implementing source-oriented mitigation strategies is crucial to effectively reduce the ecological and human health risks of PFAS in receiving waters.

Condensation of R290 + R13I1 zeotropic mixture in a plate heat exchanger with offset-strip fin geometry

Energy-efficient refrigerants with a low global-warming potential (GWP) is becoming increasingly popular, particularly in the automobile sector. In this sense, despite its higher flammability, R290 can meet environmental regulations with outstanding thermal properties. Studies have been conducted to reduce the flammability of R290 by combining it with flame retarding agents, and it has been discovered that the use of R13I1 has good control over R290's flammability and offering good thermal performance. However, research into the two-phase behavior of R290 + R13I1 combinations is extremely important for building compact heat-pumps. Therefore, this study experimentally investigated the condensation behavior of a new low-GWP R290 + R13I1 refrigerant mixture in a plate heat exchanger with an offset-strip fin flow passage. The effects of the heat flux (HF), mean vapor quality (xm), saturation temperature (Ts), and mass flux (MF) on the condensation heat transfer coefficient (HTC) and two-phase frictional pressure drop were explored. The results show that the peak condensation HTC increased by 66.2% and 19.7% with an increase in HF and MF, respectively, although it deteriorated by 26.7% in the case of Ts. Moreover, the dominance of the mass transfer resistance (MTR) and surface tension was observed at lower xm values, whereas the shear force dominated at higher xm values. The MTR was estimated with Silver–Bell–Ghaly (SBG) approximation, and the values were in the range of 0.0000342 K.m−2.W−1 and 0.000067 K.m−2.W−1. Moreover, the Nusselt number (Nu) predicted using the SBG approximation correlations was lower than the experimental value. Hence, two types of new Nu correlation were made with and without SBG approximation, and they showed reasonably good Nu prediction with 9.6% and 10.8% mean absolute deviation.

Experimental Evaluation of Two-Phase Heat Transfer Coefficient Under Oscillatory Flow Conditions and Correlation Development

Abstract Natural circulation boiling water reactors (NCBWRs) are prone to flow instabilities. Due to instabilities, heat transfer deteriorates. However, the extent to which the heat transfer coefficient (HTC) deteriorates is not well established. In this work, experiments are conducted under controlled flow oscillations to evaluate the impact of the time period, mean mass flux, and amplitude ratio on HTC. Existing correlations for HTC are compared with experimental results and are found to be not satisfactory. To improve the predictions of HTC, correction factors (CFs) are proposed for existing correlations. Further, an improved HTC correlation has been developed, incorporating new nondimensional numbers.

Characteristics of Chemical Solutes and Mineral Dust in Ice of the Ablation Area of a Glacier in Tien Shan Mountains, Central Asia

Diverse microbial communities live on glacial surfaces, with abundances and diversities dictated by the chemical and physical conditions of the supraglacial environment. Chemical solutes and mineral dust on glacial surfaces are generally derived from the atmosphere as aerosols, but they can also be supplied from glacial ice. In this study, a 56-m ice core from the ablation area of a mountain glacier in the Tien Shan Mountains of Central Asia was analyzed to characterize the chemical solutes and mineral dust in glacial ice. Soluble chemical ion and mineral dust analysis in the ice core showed that their concentrations varied with depth. The most dominant ion was Ca2+ (mean: 79.8 μEq L−1), followed by Cl−, SO42-, NH4+, and NO3−. The mean dust concentration in the glacial ice was 2.5 ×105 number ml−1. Chemical solute and mineral dust concentrations in the samples indicate that melting glacial ice could potentially affect supraglacial conditions. The annual fluxes of the major ions outcropping from glacial ice were estimated based on their concentrations and the observed melt rate of the ice surface. Consequently, they were comparable to or higher than those from the atmosphere. The mean mass flux of mineral dust from glacial ice was greater than that from the atmosphere. Our results showed that glacial ice supplies chemical solutes and mineral dust to the supraglacial environment and that changing melting rates of glacial ice would affect the chemical conditions on the glacier surface and the growth of photoautotrophs on the ablating ice surface.

Evaporative heat transfer of R410A flow in an annular duct under oscillatory wall heat flux

In this investigation, the evaporative heat transfer characteristics of R410A flow in a slender annular duct under an oscillatory heat flux are studied experimentally with emphasis on the influences of mean mass flux, heat flux, and oscillating amplitude and period of heat flux. The applied heat flux is varied in ranges of q = 5,000 to 15,000 W m−2 and the mean mass fluxes of 300 and 400 kg.(m−2s−1) are considered, respectively. The results reveal that increasing either oscillating amplitude or period of heat flux enhances the oscillation of surface temperature and evaporation heat transfer coefficient (HTC). The prevailing influence of heat flux oscillations on the evaporation heat transfer causes notable variations in the vapour quality and liquid film thickness. At a high level of vapour quality, the thickness of refrigerant liquid film on the hot surface is decreased, leading to the increase of the evaporation HTC. Furthermore, the oscillation in the wall temperature is slightly stronger as the period of heat flux oscillation boosts in the range of 120 s to 600 s. Based on the present measured data, some correlations are provided for the evaporation HTC of R410A as the function of the heat flux and vapour quality.

Bubble characteristics during alternating subcooled flow boiling with oscillated refrigerant flow rates – An experimental investigation

In this paper, a comprehensive experimental investigation is performed to study the bubble characteristics during the alternating subcooled flow boiling with the oscillated refrigerant flow rates. The R-407C refrigerant is selected as the refrigerant. Different parameters related to the bubble characteristics are investigated during the temporal alternating R-407C flow boiling. In addition, a number of relationships are proposed for these parameters as the function of R-470C mass flux. The flow regime map for this type of boiling is also investigated. The results indicated that the intermittent boiling can be observed for the wider ranges of the Boiling number. For the long oscillation period of the mass flux, the oscillation vibration of the subcooled flow boiling is quite strong. The bubble departure frequency boosts with increasing the mean mass flux. Eventually, the active nucleation site density is enhanced with increasing the mean saturation temperature of refrigerant.

Measuring Shallow Convective Mass Flux Profiles in the Trade Wind Region

AbstractMass flux is a key quantity in parameterizations of shallow convection. To estimate the shallow convective mass flux as accurately as possible, and to test these parameterizations, observations of this parameter are necessary. In this study, we show how much the mass flux varies and how this can be used to test factors that may be responsible for its variation. Therefore, we analyze long-term Doppler radar and Doppler lidar measurements at the Barbados Cloud Observatory over a time period of 30 months, which results in a mean mass flux profile with a peak value of 0.03 kg m−2 s−1 at an altitude of ~730 m, similar to observations from Ghate et al. at the Azores Islands. By combining Doppler radar and Doppler lidar measurements, we find that the cloud-base mass flux depends mainly on the cloud fraction and refutes an idea based on large-eddy simulations that the velocity scale is in major control of the shallow cumulus mass flux. This indicates that the large-scale conditions might play a more important role than what one would deduce from simulations using prescribed large-scale forcings.

Unsteady phenomena in saturated flow boiling heat transfer – An experimental study on oscillating refrigerant mass flux

The present work pertains to experimental studies on the unsteady phenomena in the saturated flow boiling of R407C stemming from the oscillatory refrigerant flow rate with the triangle shape in the horizontal slender annular channel. Attention is mainly paid to examine the effects of different parameters, including mean mass flux, amplitude and period of the oscillating mass flux, saturation temperature of R407C, input thermal flux, and gap size on the boiling diagrams and heat transfer coefficients. Firstly, the steady-state saturated flow boiling is studied as baseline cases. It was concluded that beyond the onset of nucleate boiling region, the mass flux of refrigerant has little influences on the boiling diagrams, which indicates that the fully developed nucleate boiling is the major mechanism. The saturated flow boiling heat transfer coefficient is enhanced as the gap size is reduced. Then, the experiment focuses on the temporal changes in the wall temperature Tw and the heat transfer coefficient hr stemming from oscillatory flow conditions. When the mass flux is oscillated periodically with the triangle shape, the Tw and hr are also oscillated temporally and enhanced with increasing in either the amplitude or frequency of oscillating mass flux. At higher input thermal flux, Tw decreases with the decreasing mass flux, which has opposite tendency compared to the single-phase convection with lower thermal flux. For intermediate exerted thermal flux, there is a negligible oscillation in the Tw and hr for most cases; the amplitudes of oscillations are lower than 0.2 °C. Besides, the significant oscillations in the wall temperature and heat transfer coefficient are noted the case with large period of the oscillating mass flux.

Condensation heat transfer performance and integrated correlations of low GWP refrigerants in plate heat exchangers

In this study, the effects of mass flux, heat flux, and condensation pressure according to mean vapor quality on the condensation heat transfer coefficient and frictional pressure drop of R124 were analyzed for plate heat exchangers with different chevron angles. The heat transfer coefficient increased with an increase in the mean vapor quality, mass flux, and heat flux. However, it decreased with an increase in the condensation pressure. The frictional pressure drop exhibited a similar trend to that of the heat transfer coefficient, but it was not significantly affected by a change in the heat flux. Furthermore, Nusselt number and friction factor correlations were developed based on the experimental data of R124. Two types of integrated correlations, namely those for the heat transfer coefficient and friction factor, were developed considering R1233zd(E) and R1234ze(E), based on previous studies. Subsequently, integrated correlations were developed for R124, R1233zd(E) and R1234ze(E). For the integrated correlations, the correction factors C1 and C2 are introduced and compared with those proposed in previous studies. For the validation of the correction factors, the final integrated correlations including R134a and R410A were developed, and the correlations predicted 88.1% and 73.2%, respectively within the error range of 25%.

An empirical model of snowdrift based on field measurements: Profiles of the snow particle size and mass flux

Snowdrift often occurs in cold regions due to the combination of wind and snowfall. The drift formation and movement processes are complex due to the mixture of air, snow and sometimes water as well as the interaction of the temperature, humidity, pressure and flow. To better understand the relevant motion of blowing snow and the vertical snow distribution at an open terrain, the present study creates an empirical model of blowing snow, with the snow particle size and snow flux profile obtained by field measurements in Xinjiang autonomous region in northwest China. Based on the current data, we summarize and analyze the snow particle size probability distribution, the mean equivalent diameter profile and the snow mass flux profile in drifting snow. The results show that the probability distribution of the equivalent diameter of different snow particles (e.g., new snow, deposited snow, and blowing snow) follows the gamma distribution, and these parameters significantly vary for different types of snow particles. The mean equivalent diameter and mass flux of blowing snow considerably decrease with height. The friction velocity is an important factor that affects the profiles of the snow particle size and snow mass flux. The empirical model of the distribution of the particle diameter and mass flux proposed in the present study considering the friction velocity agrees well with field measurements.

Transition induced by streamwise arrays of roughness elements on a flat plate in Mach 3.5 flow.

The flow behind streamwise arrays of roughness elements was examined with a hot-wire probe. The roughness elements had heights of approximately 20% and 40% of the boundary layer thickness and different spacings and orientations of these roughness elements were tested. The circular roughness elements were spaced two diameters apart or four diameters apart from center to center. Transition moved upstream only when the roughness elements were spaced four diameters apart. The rectangular roughness elements were oriented so that they were at a 45-degree angle relative to the leading edge of the plate. Tandem rectangular elements either had the same orientation or opposing orientation. Mean mass-flux and total-temperature profiles of the flow field downstream of the roughness elements were examined for mean-flow distortion. Mass-flux fluctuation profiles showed that a 45-kHz odd-mode disturbance was present downstream of the shorter circular roughness elements. The dominant instability downstream of the taller circular roughness elements was a 65-85 kHz even-mode disturbance. Mass-flux fluctuation profiles showed that the dominant mode downstream of the tandem rectangular roughness elements with the same orientation was similar to that of a single roughness element and centered at a frequency of approximately 55 kHz. The 55-kHz instability appeared to correspond to increased spanwise shear, and thus was determined to be an odd-like mode. The dominant instability downstream of the tandem roughness elements with opposing orientation was centered at a frequency of 65 kHz and did not transition in the measurement region.

Aerosol vertical mass flux measurements during heavy aerosol pollution episodes at a rural site and an urban site in the Beijing area of the North China Plain

Abstract. Due to excessive anthropogenic emissions, heavy aerosol pollution episodes (HPEs) often occur during winter in the Beijing–Tianjin–Hebei (BTH) area of the North China Plain. Extensive observational studies have been carried out to understand the causes of HPEs; however, few measurements of vertical aerosol fluxes exist, despite them being the key to understanding vertical aerosol mixing, specifically during weak turbulence stages in HPEs. In the winter of 2016 and the spring of 2017 aerosol vertical mass fluxes were measured by combining large aperture scintillometer (LAS) observations, surface PM2.5 and PM10 mass concentrations, and meteorological observations, including temperature, relative humidity (RH), and visibility, at a rural site in Gucheng (GC), Hebei Province, and an urban site at the Chinese Academy of Meteorological Sciences (CAMS) in Beijing located 100 km to the northeast. These are based on the light propagation theory and surface-layer similarity theory. The near-ground aerosol mass flux was generally lower in winter than in spring and weaker in rural GC than in urban Beijing. This finding provides direct observational evidence for a weakened turbulence intensity and low vertical aerosol fluxes in winter and polluted areas such as GC. The HPEs included a transport stage (TS), an accumulative stage (AS), and a removal stage (RS). During the HPEs from 25 to 31 January 2017, in Beijing, the mean mass flux decreased by 51 % from 0.0049 mg m−2 s−1 in RSs to 0.0024 mg m−2 s−1 in the TSs. During the ASs, the mean mass flux decreased further to 0.00087 mg m−2 s−1, accounting for approximately one-third of the flux in the TSs. A similar reduction from the TSs to ASs was observed in the HPE from 16 to 22 December 2016 in GC. It can be seen that from the TS to the AS, the aerosol vertical turbulent flux decreased, but the aerosol particle concentration within the surface layer increased, and it is inferred that in addition to the contribution of regional transport from upwind areas during the TS, suppression of vertical turbulence mixing confining aerosols to a shallow boundary layer increased accumulation.

Heat transfer, void fraction and pressure drop during condensation inside inclined smooth and microfin tubes

This paper presents experimental heat transfer and pressure drop measurements during the condensation of R134a at a mean saturation temperature of 40 °C inside a microfinned copper tube with an inner diameter of 8.92 mm and a helix angle of 14°. Experiments were conducted for mean qualities from 0.1 to 0.9 at different inclination angles ranging from −90° (vertically downwards) to +90° (vertically upwards), at mass fluxes of 200–600 kg/m2s. Heat transfer coefficients were calculated directly from measured data, while the frictional pressure drops were obtained from the experimental data using the Bhagwat and Ghajar void fraction correlation developed in 2014. Results were compared with those obtained from a similarly sized smooth tube having an inner diameter of 8.38 mm to obtain the relative microfin heat transfer enhancement and pressure penalty factors. For both tubes, it was found that the heat transfer coefficient increased significantly with the mean vapour quality and mass flux. For the microfin tube, the highest heat transfer coefficients were obtained at tube inclinations of between −15° and −5° (downward flows), while for the smooth tube, the highest heat transfer coefficients occurred between inclinations of between −30° and −15° (downward flows). The heat transfer enhancement factor for the microfin tube was between 0.98 and 2.38 depending on the inclination angle. For both tubes, it was found that higher frictional pressure drops occurred at higher mass fluxes. In most cases, higher vapour qualities produced higher frictional pressure drops depending on the flow pattern. The lowest frictional pressure drop occurred at either horizontal tube positions or vertical downward flow inclinations. Microfin pressure drop penalty factors ranged between approximately 0.8 and 4 depending on the mass flux, inclination and vapour quality.

Long‐Term Mean Mass, Heat and Nutrient Flux Through the Indonesian Seas, Based on the Tritium Inventory in the Pacific and Indian Oceans

AbstractThe Indonesian Throughflow (ITF), the only oceanic tropical pathway linking the Pacific and Indian Oceans, plays a critical role in the redistribution of heat and mass, affecting both the regional and global climate systems. Based on the distributions and changes in the tritium inventories, and tritium budgets in and between the South Pacific and South Indian Oceans from the Geochemical Ocean Sections Study and World Ocean Circulation Experiment programs, the long‐term mean water mass transport of the ITF is estimated. The total throughflow of the ITF is estimated as 16.2 ± 3.5 Sv. The North Pacific contributes 14.1 ± 2.7 Sv to the total throughflow, of which the layers between 0–250, 250–650, and 650–1,000 m carry 7.3 ± 0.6, 5.4 ± 1.3, and 1.4 ± 3.0 Sv, respectively. Along with the water transport, the North Pacific component of the ITF transfers 0.84 ± 0.14 PW of heat into the Indian Ocean, of which almost 90% exits from the Indian Ocean at 30°S. The net nitrate flux into the photic zone associated with the ITF is 2.71 ± 2.60 mmol·m−2·d−1 in the Indonesian Seas, ~90% of which is induced by vertical diffusion.

Physical flow properties for Pollard-like internal water waves

We present a study of the physical flow properties for a recently derived three-dimensional nonlinear geophysical internal wave solution. The Pollard-like internal wave solution is explicit in terms of Lagrangian labelling variables, enabling us to examine the mean flow velocities and mass flux in the three-dimensional setting. We show that the Pollard-like internal water wave does not have a net wave transport.

A Numerical Study of Convective‐Scale Structures in the Outer Cores of Sheared Tropical Cyclones: 1. Updraft Traits in Different Vertical Wind Shear Magnitudes

AbstractThis study investigates the outer‐core convective‐scale updraft features of numerically simulated tropical cyclones under vertical wind shear of different magnitudes. Updrafts in the outer core were weighted in favor of downshear formation, and the increase in the shear value led to more short‐lived updrafts. The tops of detected updrafts tended to cluster at either 4‐ to 6‐km or 12‐ to 14‐km height. Downshear‐left updrafts mostly moved radially inward, and updrafts on the left side of the shear vector moved more rapidly azimuthally. Longer‐lived updrafts gained greater vertical mass fluxes during their life cycles, with those in strongly sheared tropical cyclones having larger vertical mass fluxes for the same updraft life span. The height of stronger vertical velocity in convective bursts decreased as shear increased, whereas larger vertical velocity within nonconvective‐burst cells held between 3 and 4 km. The values of several environmental parameters (such as the supercell composite parameter and the energy‐helicity index) associated with updrafts in the downshear‐left quadrant were larger than those in the downshear‐right quadrant for the weakly sheared tropical cyclone, whereas they became much larger in the downshear‐right quadrant for highly sheared vortices. There were no relationships between the parameters and updraft mean mass fluxes, so the use of these parameters for predicting the likelihood of updraft strength in tropical cyclones requires caution.

Condensation heat transfer characteristics of R245fa in a shell and plate heat exchanger for high-temperature heat pumps

In this study, the condensation heat transfer characteristics of R245fa in a shell and plate heat exchanger (SPHE) are measured and analyzed by varying the mean vapor quality from 0.16 to 0.86, mass flux from 16.0 to 45.0 kg m−2 s−1, heat flux from 1.3 to 9.0 kW m−2, and saturation temperature from 90 to 120 °C for high-temperature heat pumps (HTHPs). The condensation heat transfer coefficient and frictional pressure drop are shown to increase with increasing mean vapor quality and mass flux and with decreasing saturation temperature. The condensation heat transfer coefficient of R245fa in SPHE is on average 5.9% lower than that in the brazed plate heat exchanger (BPHE) with a similar effective heat transfer area. However, the average two-phase frictional pressure drop of R245fa in SPHE is 16.7% lower than that in BPHE. Empirical correlations for the condensation heat transfer coefficient and two-phase frictional pressure drop in SPHE are developed with mean absolute errors of 8.2% and 9.4%, respectively.

Scaling properties of food flow networks.

Food flows underpin the complex food supply chains that are prevalent in our increasingly globalized world. Recently, much effort has been devoted to evaluating the resources (e.g. water, carbon, nutrients) embodied in food trade. Now, research is needed to understand the scientific principles of the food commodity flows that underpin these virtual resource transfers. How do food flows vary with spatial scale? To address this question, we present an empirical analysis of food commodity flow networks across the full spectrum of spatial scales: global, national, and village. We discover properties of both scale invariance and scale dependence in food flow networks. The statistical distribution of node connectivity and mass flux are consistent across scales. Node connectivity follows a generalized exponential distribution, while node mass flux follows a Gamma distribution across scales. Similarly, the relationship between node connectivity and mass flux follows a power law across scales. However, the parameters of the distributions change with spatial scale. Mean node connectivity and mass flux increase with increasing scale. A core group of nodes exists at all scales, but node centrality increases as the spatial scale decreases, indicating that some households are more critical to village food exchanges than countries are to global trade. Remarkably, the structural network properties of food flows are consistent across spatial scales, indicating that a universal mechanism may underpin food exchange systems. In future research, this understanding can be used to develop theoretical models of food flow networks and to model food flows at resolutions for which empirical information is not available.

Mean and turbulent mass flux measurements in an idealised street network.

Pollutant mass fluxes are rarely measured in the laboratory, especially their turbulent component. They play a major role in the dispersion of gases in urban areas and modern mathematical models often attempt some sort of parametrisation. An experimental technique to measure mean and turbulent fluxes in an idealised urban array was developed and applied to improve our understanding of how the fluxes are distributed in a dense street canyon network. As expected, horizontal advective scalar fluxes were found to be dominant compared with the turbulent components. This is an important result because it reduces the complexity in developing parametrisations for street network models. On the other hand, vertical mean and turbulent fluxes appear to be approximately of the same order of magnitude. Building height variability does not appear to affect the exchange process significantly, while the presence of isolated taller buildings upwind of the area of interest does. One of the most interesting results, again, is the fact that even very simple and regular geometries lead to complex advective patterns at intersections: parametrisations derived from measurements in simpler geometries are unlikely to capture the full complexity of a real urban area.

Wind Tunnel Test of Snow Redistribution on Roofs

A series of wind tunnel tests were carried out to predict snow distribution on roofs. A typical stepped roof and two gable roofs were investigated. Model snow depth distributions on the roofs were measured at various time intervals. The effects of velocity and wind direction were studied, and mass transport rates on the roofs were examined as well. The results show that, the saltation of particles decreases the turbulence intensity nearby the surface, and the effective aerodynamic roughness-lengths of the field are proportional to the 2nd power of the friction velocities. At the wind direction of 0°, the snow depth distribution on the stepped roof is well consistent with the field observation. Snow depth distributions on the stepped roof are quite unbalanced under oblique winds, especially at wind direction of 135°, the snow depth coefficients range from 0 to 1.7. The leeward side of the gable roofs may accumulate more snow at wind direction of 0°. The maximum coefficients are 1.7 and 1.2 for slope 10° and 20° respectively. The mean mass flux rates on the roofs are proportional to velocities, implying that the mass transport rate increases with the 3rd power of the friction velocity.