Fluid Residence Time Research Articles

Investigation of the thermal behavior of water flow within parallel tubes in a solar collector

This paper discusses the numerical simulation of the thermal behavior of steady-state water flows within parallel tubes in a plane solar collector. The CFD modeling technique, which is based on the finite volume method, has been implemented with the SIMPLE algorithm. The governing equations of the flow are those of Navier-Stokes, coupled with the energy conservation equation, and solved by using the computer software FLUENT. The profiles of water temperature are analyzed for different mass flow rates. A considerable increase in the heat transfer between the absorber wall and the tubes was observed at the low velocity of water flows. The lower mass flow rate allowed a longer residence time of fluid, and the slower mass flow yielded an increased outlet temperature.

Hydrodynamic behavior of a novel 3D-printed nature-inspired microreactor with a high length-to-surface ratio

Microreaction technology has experienced almost exponential growth in the last decades due to significant practical benefits when compared with conventional reactors. However, the predominantly laminar flow impedes the required mixing in some processes. A novel design inspired by differential growth organic shapes in nature for improving mass transfer in microreactors is proposed. Firstly, this geometry allowed an area utilization of 16,1% and 27,2% higher than conventional serpentine and spiral channels, with the same hydraulic diameter, which can be useful for surface contact processes as in electrochemical reactions. Secondly, the performance of the microreactor was analyzed in terms of fluid flow and residence time distribution (RTD) for Reynolds in a range from 1-100. The continuous bending in the differential growth shape geometry leads to the formation of secondary vortices while approaching to the plug flow regime for the studied flow rates. These results were obtained by numerical analysis executed with the software COMSOL Multiphysics by solving three-dimensional Navier-Stokes equations with a convection-diffusion model for the species concentration. The validation of the RTD numerical model was conducted in a one-body 3D-printed microreactor by injecting a pulse of rhodamine B in water and measuring the response with colorimetric techniques. These organic shapes open a window to further research in microfluidics, as its parametric design can be adapted to a wide variety of reaction conditions.

Numerical simulation and modeling of viscous fluid residence time distribution in the melt spinning pack

AbstractMelt spinning is an important method in fiber production. The hydrodynamic performance of fluid in the melt spinning process's core device, spinning pack, can directly affect the quality of fiber products. To guarantee the uniform distribution of flow in the spinning pack, the residence time distribution (RTD) characteristics of high viscosity fluid in the spinning pack were investigated experimentally and numerically in this work. Peak and trailing of RTD curves indicate the existence of short circuit and dead zone in the spinning pack. Results show that the porous media and gland can reduce the dead zone, and baffles could reduce the short circuit. Based on the experiment and numerical simulation results, a one‐dimensional axial diffusion model was established to describe the RTD characteristics of viscous fluid in the spinning pack. This work can be valuable references for the understanding and optimization of the spinning pack in melt spinning process.

Experimental investigation of low-temperature autoignition in turbulent premixed swirling flames

The role of low-temperature chemistry/ignition (LTC/LTI) in aero engine-like combustors that feature confined bluff-body and swirling flows inside has been studied in the present work. A series of CH4/DME/air mixtures were experimentally investigated at ϕ=0.65 with the volume fraction of DME in the fuel blend (αDME) ranging from 0% to 100% to take LTI into account. To this end, OH-PLIF and CH2O-PLIF measurements combined with PIV and thermocouple methods are used to capture high-temperature and low-temperature flames (HTFs and LTFs) and the flow field and temperature in the outer recirculation zone (ORZ). It is found that adding DME into the lean CH4/air mixture has the potential to introduce LTF in the ORZ under certain conditions. There are three flame regimes in the bluff-body swirl burner in terms of DME enrichment (αDME): (1) When αDME<50%, neither of LTF and HTF exist in the ORZ, and only a V-shape HTF is observed between the inner shear layer (ISL) and inner recirculation zone (IRZ), denoted as regime I. (2) When 50%≤αDME≤70%, a stable LTF in the ORZ can co-exist with the above V-shape HTF, denoted as regime II. (3) When αDME>70%, besides the V-shape HTF, the LTF in the ORZ and a new HTF front between the ORZ and outer shear layer (OSL) can exist intermittently, i.e., the LTF first occurs in the ORZ and then transitions to the new HTF between the ORZ and OSL, denoted as regime III. Furthermore, using an ignition Damköhler number (Daig), defined as the ratio of maximum fluid residence time and evaluated shortest first-stage or second-stage ignition delay time in the ORZ, is able to reasonably classify the flame regimes. These results open up the possibility of employing LTI to extend the stability margin of lean swirling flames.

Fluid flowrates and compositions and water–rock interaction in the Hikurangi margin forearc, New Zealand

The decrease in interseismic coupling and slip rate deficit from south to north, from locked to slipping, of the Hikurangi margin is attributed from interpretation of geophysical data to more fluid-saturated sediments in the north. To assess this interpretation, fluid flowrates were measured in springs of the subaerial forearc, structures correlated with flowrates and the fluids geochemically and isotopically evaluated to understand their origins and source depths. Extrapolations from on-ground measurements show that ∼95% of aqueous fluids and ∼98% of gas are expelled to the surface in the slipping zone, within the northern subaerial forearc. The relative fluid residence time, from the equilibration of CH4 and CO2 and measured ascent rates, is 3x shorter in the slipping zone suggesting less constricted migration pathways than in the locked zone. Fluids are expelled not only from the sediments that produce Na–Cl fluids and thermogenic gas, but also from the oceanic igneous mafic slab that releases high-Cl fluids and abiotic CH4 from serpentinization, giving rise to Na–Ca–Cl fluids. Chloride-rich surface emissions consist of ∼70% by volume Na–Cl and ∼30% Na–Ca–Cl fluids. Higher rates of fluid expulsion in the north coincide with higher permeability and greater production of aqueous fluids from clay dehydration, serpentinization and a higher degree of diagenesis in sedimentary rocks. Mantle gas, subduction interface fluids and porewaters have the most direct access to the surface in the slipping Center, where the degree of water–rock interaction and homogeneity of water compositions suggest more recent fluid generation than other sectors in the subaerial forearc. Porewaters and thermogenic gas equilibrate at 110° + 20 °C with a slight increase in temperature from south to north. Overall, ∼120 Tg/a water is subducted in the Hikurangi margin and <0.5 Tg/a CH4 is being emitted to the atmosphere. This study shows the added impact of waters of serpentinization on the overall fluid compositions, aqueous fluid subduction budget, extent of fluid distribution in the oceanic slab and, indirectly, the temperature estimate of 185° + 25 °C for fluids ascending from the subduction interface in the Hikurangi margin.

The Whole Process CFD Numerical Simulation of Flow Field and Suspended Solids Distribution in a Full-Scale High-Rate Clarifier

To further reduce the contents of phosphorus and suspended solids (SS) in the effluent, the high-rate clarifier is adopted by some wastewater treatment plants (WWTPs) to upgrade their processes. However, due to the complex phosphorus species and unclear hydraulic characteristics after the addition of the high-rate clarifier, the removal efficiencies of total phosphorus and the optimization strategies of SS removal remain unclear. The high-rate clarifiers are mainly composed of the mixing zone and the settling zone. Only the hydraulic characteristics of the mixing zone have been studied. It is necessary to focus on the hydrodynamics of the settling zone to understand the sedimentation process and pollutant removal mechanism in depth. Therefore, this study focused on the whole process of high-rate clarifier of the Wuhan Nantaizi Lake WWTP in China. The distributions of flow velocity and SS in the full-scale high-rate clarifier were simultaneously simulated by the solid-liquid two-phase whole process computational fluid dynamics (CFD) model which was established in this study. The simulated results of the developed CFD model were in good agreement with the experimental data obtained in the high-rate clarifier. The normalized standard error was less than 7.66%. The overall flow velocities in the settling tank were much smaller than that in the reaction tank. After the fluid passed through the diversion effect of the overflow wall and the under-through channel, the overall kinetic energy loss was relatively large. Meanwhile, the residence time of fluid in the settling tank was longer than that in the reaction tank. The sludge sedimentation happened at the edge of the settling tank due to the vertical angle between the plug-flow fluid and the outlet. The successful construction of the CFD model could lay the foundation for the next step of the high-rate clarifier optimization operation and research on pollutant removal.

The Use of Noble Gases to Constrain Subsurface Fluid Dynamics in the Hydrocarbon Systems

Geochemical techniques have been widely applied to study the generation and migration of hydrocarbons in sedimentary basins over the last several decades. Diagnostic biomarkers and stable carbon and hydrogen isotopes (such as δ13C and D) are generally used to identify the sources and thermal maturity of hydrocarbons and to reveal the accumulation process and mechanism of oil and gas reservoirs. However, some questions, such as secondary migration processes and pathways of oil and gas, and the relationship between groundwater flow and hydrocarbon transport, remain unclear and challenging. The low abundance and chemical inertness properties allow noble gases to serve as robust tools for tracing subsurface fluid flow. Additionally, they can be used for identifying and quantifying the role of the concomitant groundwater related to the generation and migration of hydrocarbons. This paper reviews the previous modeling work on using noble gases to study the fluid flow, flow paths, and gas/oil-water interactions in hydrocarbon systems. Noble gases from various sources can be readily identified due to their distinct isotopic and elemental signatures. Atmosphere-derived noble gases can be used to evaluate the amount of involved aquifer water associated with the hydrocarbon system and determine the groundwater migration paths and flow rates. Radiogenic noble gases accumulate over time, providing information about the subsurface fluid residence time. Questions concerning the specific trapping sites and mechanisms that affect heavy noble gas adsorption into organic sediments are still unresolved. Investigating the hydrocarbon generation, migration, and subsurface crustal fluid interactions in the hydrocarbon reservoirs can improve our understanding of noble gases as useful tracers in the subsurface environment and provide valuable geological evidence for the exploration and production of petroleum sources.

Online measurement of mean residence time of supercritical-pressure fluid with/without chemical reaction in pipe flow: A particle image statistics method considering optical distortion and radial uneven distribution of tracer particles

This paper proposed a nonintrusive, in-situ experimental method for visualizing and measuring the mean residence time (MRT) of supercritical-pressure pipe flow with/without chemical reactions based on the high-frequency sampling of laser-induced particle images. Normal and oblique optical arrangements (NOA & OOA) for the incident laser were applied to induce particle scattering in the JP-10 pipe flow, using SiO2 microsphere as tracers. Optical distortion caused by drastic change in fluid properties was analyzed. The relative deviation of the proposed particle image statistics (PIS) method was within ±4% with NOA after considering optical distortion and uneven radial distribution of particles. Although the same level of accuracy was achieved with OOA without the need for correction, the noise in images became significant when the fuel approached pseudo-critical temperature. When there was no influence from pyrolysis, the MRT simulated using 1D subroutine agreed well with measured data. However, the relative deviation increased after pyrolysis.

Upscaling calcite dissolution rates in a tight reservoir sandstone

Calcite is a highly abundant mineral in the Earth’s crust and occurs as a cement phase in numerous siliciclastic sediments, where it often represents the most reactive component when a fluid percolates through the rock. Hence, the objective of this study is to derive calcite dissolution rates on different scales in a reservoir sandstone using mineral surface experiments combined with vertical scanning interferometry (VSI) and two types of core plug experiments. The 3D geometry of the calcite cement phase inside the rock cores was characterized by X-ray micro-computed tomography (µXCT) and was used to attempt dissolution rate upscaling from the mineral surface to the core scale. Initially (without upscaling), our comparison of the far-from-equilibrium dissolution rates at the mineral surface (µm-mm-scale, low fluid residence time) and the surface normalized dissolution rates obtained from the core experiments (cm-scale, high fluid residence time) revealed differences of 0.5–2 orders of magnitude. The µXCT geometric surface area connected to the open pore space left( {GSA_{{Cc,{text{open}}}} } right) considers the fluid accessibility of the heterogeneously distributed calcite cement that can largely vary between individual samples, but greatly affects the effective dissolution rates. Using this parameter to upscale the rates from the µm- to the cm-scale, the deviation of the upscaled total dissolution rates from the measured total dissolution rates was less than one order of magnitude for all investigated rock cores. Thus, GSA_{{Cc,{text{open}}}} showed to be reasonably suitable for upscaling the mineral surface rates to the core scale.

A Comprehensive Compendium on Passive Augmentation Techniques for Enhancement of Single-Phase Heat Transfer Coefficients in Heat Exchanger Tubes under Laminar and Turbulent Flow Conditions

Several passive techniques can substantially improve the heat transfer performance of conventional heat exchangers. Passive methods persistently use heat transfer augmentation inside tubular sections, which tackle dominant thermal resistance. The thermal resistance in turbulent flow is primarily due to a thin viscous sublayer near the tube wall. This proliferates into the cross-section of the conduit in the case of laminar flows due to the existence of a relatively thicker boundary layer. This urges disturbance in the entire fluid throughout the cross-section in laminar flows, whereas augmentation devices are usually located close to the wall for turbulent flows. Most of these methods yield increased fluid residence time within the system by inducing swirling motion. The form of disturbance in the flow field is the characteristic of the passive technique used to prolong the fluid residence time and is exhibited differently in both flow regimes. The present article showcases a comprehensive review of heat transfer enhancement through thermo hydraulic performance assessment of these methods reported in the literature. The comparison is based on ratios of Nusselt numbers, first at the same Reynolds numbers, and then, at equal pumping power with constant heat transfer area.

Timing Earth's Abiotic Kitchen: Short Hydrothermal Fluid Residence Times in Serpentinizing Oceanic Crust

AbstractHydrothermal alteration of ultramafic rock (serpentinization) creates extremely reducing (H2‐rich) fluids in the oceanic crust, resulting in strong thermodynamic drives to reduce CO2 to organic molecules in the absence of life. Timescales on which such hydrothermal fluids circulate (thus produce or destroy such organic molecules) have remained enigmatic. In their new publication, Moore et al. (2021, https://doi.org/10.1029/2021JC017886) present compelling radioisotope‐based estimates of fluid residence times in a widely known site of purported abiotic synthesis—The Lost City Hydrothermal Field. Using a model that accounts for the sorptive behavior of Ra radionuclides during circulation, they find that fluids at Lost City must have remarkably short residence times, averaging 0.5–2 years or less. The study represents a critical step forward in our understanding of Earth's abiotic organic “kitchen”, as it now places a constraint on the timeframe in which organic molecule creation should occur in such fluids (if at all) prior to venting at the seafloor.

Downscaling reverse osmosis for single-household wastewater reuse: towards low-cost decentralised sanitation through a batch open-loop configuration

Abstract There is a significant demand for water recycling in low-income countries. However, wastewater infrastructure is primarily decentralised, necessitating the development of affordable household-scale reclamation technology. In this study, a batch open-loop reverse osmosis (RO) system is therefore investigated as a low-cost clean water reclamation route from highly saline concentrated blackwater. In a single-stage configuration, increasing feed pressure from 10 to 30 bars improved selective separation at water recovery exceeding 85%, whereas lower cross-flow velocity improved product recovery, reducing specific permeate energy demand from 21 to 4.8 kWh m−3. Rejection achieved for total phosphorous (99%), chemical oxygen demand (COD, 96%), and final pH (8.7) of the RO permeate was compliant with the ISO30500 reuse standard for discharge. However, the rejection of total nitrogen in the RO permeate was non-compliant with the reuse standard due to the transmission of low-molecular weight (MW) uncharged organic compounds. It is suggested that rejection may be improved by increasing feed pressure to rebalance selectivity but may also be controlled by reducing fluid residence time (storage) to constrain the hydrolysis of urea. The economic analysis identified that a high-pressure 1812 element cost of ∼US$30 meets the sanitation affordability index of US$0.05 capita−1 day−1. However, the unit cost of a high-pressure feed pump must be reduced to ∼US$500 to obtain an affordable system cost. These unit costs can be achieved by manufacturing 1812 elements at economies of scale, and by adopting pumping solutions that have been developed for other applications requiring high pressures and low flows. Overall, our findings suggest that RO in the batch open-loop configuration has the potential to deliver affordable and safe water production from blackwater in a decentralised (single-household) context.

Experimental Investigation of Polymer-Induced Fouling of Heater Tubes in the First-Ever Polymer Flood on Alaska North Slope Part II

Summary A polymer flood pilot has been ongoing in the Schrader Bluff viscous oil reservoir at Milne Point on the Alaska North Slope. The results from the pilot are encouraging. However, a major concern of the operator is the influence of polymer on the production system after breakthrough, especially the fouling in heat exchangers. This work applies a multiexperimental approach to study the severity of polymer-induced fouling in both dynamic and static states of produced fluids to determine safe operating conditions. Dynamic scale loop (DSL) tests were conducted to study fouling due to polymer at different skin temperatures (165–350°F) in a dynamic state of fluid flow where the fluids’ flow mimics the residence time of fluids in the heat exchanger of the field pilot. Static deposit tests were also conducted at similar skin temperatures of 165–250°F using a novel experimental apparatus designed and built in-house. It was found that at higher skin temperatures of 250–350°F, tube blocking was observed in the DSL tests, whereas the tests at 165–200°F did not show any tube blocking, even in a more extended test period. The deposit test showed that the deposit rate generally increases with skin temperature, and the presence of polymer aggravates the fouling. The copper tube performs best when the skin temperatures are 165–200°F, while the stainless steel tube performs best at a skin temperature of 250°F. These experiments also manifested the influence of the cloudpoint of the solution as the deposit rate increased significantly when the skin temperature was higher than the solution cloudpoint. The study provides a source of practical guidance to the field operations.

Equilibrium and kinetic controls on molecular hydrogen abundance and hydrogen isotope fractionation in hydrothermal fluids

Molecular hydrogen (H2) is among the main components in hydrothermal fluids and exerts a major role in geochemical and biological processes. Nevertheless, the understanding and quantification of factors controlling H2 abundance and isotope composition remain uncertain. Sub-aerial hydrothermal systems provide a unique opportunity to study sources and reactions of H2. Here, we present hydrogen fugacity (fH2) and δD values of H2 and H2O in hydrothermal fluids from such systems worldwide (Greece, Iceland, Kenya and New Zealand), representative of mid-ocean ridge and arc systems and sourced by seawater and meteoric water. The hydrothermal fluid temperatures are 226-359°C, and the fH2, δD-H2 and δD-H2O values are 0.002-3.3 bar, -646 to -391‰ and -94.1 to +11.3‰, respectively. Comparison of measured data with geochemical modeling reveals that H2 is predominantly sourced from the reduction of H2O, enabled through the oxidation of aqueous Fe+II to Fe+III and precipitation of Fe+III-bearing hydrothermal minerals (e.g., epidote). We argue that fH2 in hydrothermal fluids is controlled by metastable mineral-fluid and fluid-fluid equilibria upon progressive rock alteration and depend on temperature, rock-to-water ratio (r/w), source water composition and volcanic gas input. Consequently, fH2 in these systems is not fixed by a sole redox buffer. Our data demonstrate that δD-H2 values of reservoir fluids at depth are controlled by the δD value of the source water and the equilibrium isotope fractionation characteristic of the hydrothermal reservoir temperatures. Attainment of isotopic equilibrium at reservoir conditions is supported by fast isotopic equilibration times at T > 200°C, on the order of minutes to few hours, relative to hydrothermal fluid residence times within the reservoirs of years to decades or even longer. During its ascent to the surface, the isotopic composition of H2 can be modified due to isotopic re-equilibration with H2O. The extent of re-equilibration depends on the interplay between fluid flow velocity, cooling rate and kinetics of D-H exchange between H2 and H2O. For fast flowing well discharges (∼0.1 to >10 m/s), negligible changes occur, whereas for slowly flowing (<0.06 m/s) fumaroles, complete re-equilibration at surface temperatures (∼100°C) is sometimes observed. Based on the extent of hydrogen isotope disequilibrium relative to reservoir conditions, fluid reservoir-to-surface travel times are estimated to range from few minutes to <3 hrs for well fluids and few hours to days for fumarolic vapor.

Activities of 223Ra and 226Ra in Fluids From the Lost City Hydrothermal Field Require Short Fluid Residence Times

AbstractThe residence time of fluids circulating through deep‐sea hydrothermal systems influences the extent of water‐rock reactions and the flux of major and minor elements to the ocean. While the fluid residence times in numerous basaltic and gabbroic systems have been determined, those of the less studied ultramafic systems are currently unknown. Fluids that interact with mantle rocks have fundamentally different chemistries and therefore have unique influences on seawater chemistry. In this first investigation of radium isotopes in a serpentinite‐hosted system, vent fluids were discovered to contain 10–100 times greater activities of 223Ra (half‐life = 11.4 days) than observed in high‐temperature basalt‐hosted systems. The 223Ra activities of 10–109 dpm L−1 produce 223Ra/226Ra activity ratios ranging from 9 to 109. These extremely high 223Ra activities, which are accompanied by low activities of 226Ra, place significant constraints on fluid residence times and the adsorption coefficient of radium between fluid and rock. Our data constrain the nondimensional retardation factor (R) to very low values between 1 and 4, reflecting the extent to which Ra is transported more slowly than fluids due to adsorption and other processes. These results suggest that the residence time of fluids in contact with serpentinite is less than 2 y and perhaps as low as 0.5 y. They are surprisingly similar to those of basalt‐hosted systems. Thus, fluids in hydrothermal systems share similar hydrogeological characteristics despite differences in rock types, underlying porosity, and heat sources, enabling larger‐scale models of hydrothermal biogeochemistry to be developed across systems.

Local and Global Sensitivity Analysis of a Reactive Transport Model Simulating Floodplain Redox Cycling

AbstractReactive transport models (RTMs) are essential tools that simulate the coupling of advective, diffusive, and reactive processes in the subsurface, but their complexity makes them difficult to understand, develop and improve without accompanying statistical analyses. Although global sensitivity analysis (SA) can address these issues, the computational cost associated with most global SA techniques limits their use with RTMs. In this study, we apply distance‐based generalized sensitivity analysis (DGSA), a novel and computationally efficient method of global SA, to a floodplain‐scale RTM and compare DGSA results to those from local SA. Our test case focuses on the impact of 17 uncertain environmental parameters on spatially and temporally variable redox conditions within a floodplain aquifer. The input parameters considered include flow and diffusion rates, geochemical reaction rates, and the spatial distribution of sediment facies. Sensitivity was evaluated for three distinct components of the model response, encompassing both multidimensional and categorical output. Parameter rankings differ between local SA and DGSA, due to nonlinear effects of individual parameters and interaction effects between parameters. DGSA results show that fluid residence time, which is controlled by aquifer permeability, generally exerts a stronger control on redox conditions than do geochemical reaction rates. Sensitivity indices also demonstrate that sulfate reduction is key for establishing and maintaining reducing conditions throughout the aquifer. These results provide insights into the key drivers of heterogeneous redox processes within floodplain aquifers, as well as the main sources of uncertainty when modeling complex subsurface systems.

Effects of organic matter complexation on partitioning of transition metals into calcite: Cave-analogue crystal growth experiments

We highlight the potential for first row transition metals to carry paleohydrological signals in speleothems, and argue that these metals constitute valuable proxies for climate reconstructions. Metal availability to speleothems is hypothesised to be restricted by organic complexation, which strongly limits the free ion activity of transition metals (Co2+, Ni2+ and Cu2+) in dripwater, thereby creating a kinetic overprint on metal concentrations and isotope ratios in speleothem calcite.This study presents the results of the first cave-analogue experiments of transition metal partitioning into calcite in the absence and presence of organic ligands. The Geological Microclimate (GeoMic) system establishes homeostatic control of air temperature and atmospheric pCO2 (set to 20 ± 0.1 °C and 1000 ± 10 ppm, respectively), and regulates relative humidity to cave-analogue levels (>90%). Calcite was precipitated from flowing aqueous films on inclined glass plates to assess the effect of prior calcite precipitation (PCP) on metal partitioning. We report speleothem-specific inorganic Kd values for Co, Ni and Cu of ∼4, 1, and 44, respectively. Pronounced PCP effects were observed in Co and Cu in the inorganic experiments, leading to strong reduction in Cu/Ca and Co/Ca, whereas Ni/Ca molar ratios remained constant.Introduction of the organic chelating ligand nitrilotriacetic acid (NTA) overturned the behaviour of all three metals, leading to stable solution Cu/Ca ratios and progressive enrichment of Co/Ca and Ni/Ca ratios as PCP progressed and apparent Kd values <1 for all three metals. This result is confirmed in a Cu-fulvic acid experiment. We show that in the presence of organic ligands with stability constants close to those observed in natural systems, the partitioning of transition metals into calcite is dependent on the fluid residence time and the stability of the metal–ligand complexes. Therefore, drip rate should be the first-order control on transition metal concentrations in stalagmites: opening the way to a novel and potentially quantitative paleohydrological proxy.

Experimental thermal performance and enviroeconomic analysis of serpentine flow channeled flat plate solar water collector.

The thermal performance of a flat plate solar water collector (FPSWC) depends on the amount of solar energy absorbed by the absorber, the quantity of heat transferred to the heat transfer fluid (HTF), and the fluid residence time in the collector. In this real-time experimental study, the thermal efficiency of the serpentine flow channeled FPSWC is compared with that of a conventional collector. The heat transfer coefficients (HTC) and heat loss coefficients of both configurations are evaluated at three different water mass flow rates (0.0083kg/s, 0.0167kg/s, and 0.025kg/s). The results show that the serpentine flow channeled collector offers higher energy and exergy efficiencies of 78.9% and 6.47%, respectively, at a mass flow rate of 0.025kg/s due to the continuous surface contact of the HTF with the absorber plate. The conventional collector yields energy and exergy efficiencies of 66.28% and 4.58%, respectively, at similar operating conditions. The peak HTC of the serpentine flow collector is 210W/m2K, which is 27.3% higher than that of the conventional collector at a maximum flow rate. The maximum HTC is observed at a higher mass flow rate and lower absorber temperature. The heat loss increases when solar radiation intensity increases; the HTC reaches its peak value at the maximum solar radiation intensity. The proposed collector shows a cleaner production of hot water with a lower payback period when compared to a conventional collector, as evident from the enviroeconomic analysis. The findings can contribute to more successful deployments of solar thermal systems.

Chemical and isotopic constraints on hydrological processes in Unzen volcanic geothermal system

Geothermal systems developed in volcanic regions release magmatic volatiles that may forecast volcanic eruptions. These volatiles can be emitted directly in gaseous form to the atmosphere or can be absorbed by condensed geothermal fluid and groundwater that discharges at surface thermal features. The subsurface flow dynamics of these fluids are therefore crucial for the interpretation of their chemical and isotopic compositions. Furthermore, since geothermal fluids are globally used as energy and cultural resources, such information is also important for their sustainable management. In this context, we investigated the subsurface residence times of fluids from three geothermal areas in Shimabara peninsula (Unzen, Shimabara and Obama) by measuring their tritium and 36Cl activities, along with their chemical and stable isotopic compositions. At Shimabara, the trace tritium activities of the geothermal fluids indicate that magmatic volatiles were transported by pre-nuclear (residence time >60 years) groundwaters. Tritium and δD-δ18O data indicate that the steam feeding the Unzen geothermal field is also derived from pre-nuclear meteoric water, and contributing about a quarter of the water budget. The 36Cl/Cl ratio of the geothermal fluids in Obama exceeds that of seawater, indicating subsurface addition of nucleogenic chloride during prolonged water-rock interaction.

Seasonal variability in silicate weathering signatures recorded by Li isotopes in cave drip-waters

Silicate weathering is a critical process in Earth’s carbon cycle, but the fundamental controls on weathering are poorly understood and its response to future climate change is uncertain. In particular, the potential for changes in seasonality or extreme weather events to control silicate weathering rates or mechanisms has been little studied. Here, we use lithium (Li) isotope measurements in bimonthly sampled drip-waters from two caves in the Yorkshire Dales (U.K.) to assess the response of silicate weathering processes to changes in temperature and hydrology over seasonal timescales. While the caves are contained in limestone bedrock, the drip-water Li isotope signal predominantly reflects silicate weathering of the overlying soils that are dominated by glacial till.Drip-water Li isotope compositions record spatial and temporal variability ranging from δ7Li values of +1 to +17‰, with a mean of +11‰. These values are significantly higher than local lithogenic inputs (δ7Li = −1 ± 1‰), consistent with isotope fractionation during secondary mineral formation. Comparison to temperature, precipitation, drip rates, and drip-water chemistry enables the controls on the Li isotope weathering signatures to be explored, revealing possible roles for both temperature and fluid residence time in setting the balance between primary rock dissolution and secondary mineral formation. Specifically, our Li isotope data are consistent with a scenario in which cooler temperatures and/or longer fluid residence times lead to enhanced secondary mineral formation relative to silicate dissolution.Overall, our results indicate the potential for Li isotope variability over short temporal and spatial scales, which is important to consider when interpreting past changes in weathering processes or fluxes from paleo-records. In addition, the seasonal changes in Li isotopes suggest that weathering processes may be sensitive to seasonality or to extreme weather events, rather than responding only to the mean climate state. With warmer temperatures and more intense rainfall events expected in future, both increased primary rock dissolution and enhanced weathering efficiency (due to reduced secondary mineral formation) could potentially contribute to increased carbon dioxide drawdown by silicate weathering.