Bulk Temperature Research Articles

On the Numerical Investigation of Two-Phase Evaporative Spray Cooling Technology for Data Centre Applications

Two-phase evaporative spray cooling technology can significantly reduce power consumption in data centre cooling applications. However, the literature lacks an established methodology for assessing the overall performance of such evaporation systems in terms of the water-energy nexus. The current study develops a Lagrangian–Eulerian computational fluid dynamics (CFD) modelling approach to examine the functionality of these two-phase evaporative spray cooling systems. To replicate a modular system, a hollow spray cone nozzle with Rosin–Rammler droplet size distribution is simulated in a turbulent convective natural-air environment. The model was validated against the available experimental data from the literature. Parametric studies on geometric, flow, and climatic conditions, namely, domain length, droplet size, water mass flow rate, temperature, and humidity, were performed. The findings indicate that at elevated temperatures and low humidity, evaporation results in a bulk temperature reduction of up to 12 °C. A specific focus on the climatic conditions of Dublin, Ireland, was used as an example to optimize the evaporative system. A new formulation for the coefficient of performance (COP) is established to assess the performance of the system. Results showed that doubling the injector water mass flow rate improved the evaporated mass flow rate by 188% but reduced the evaporation percentage by 28%, thus reducing the COP. Doubling the domain length improved the temperature drop by 175% and increased the relative humidity by 160%, thus improving the COP. The COP of the evaporation system showed a systematic improvement with a reduction in the droplet size and the mass flow rate for a fixed domain length. The evaporated system COP improves by two orders of magnitude (~90 to 9500) with the reduction in spray Sauter mean diameter (SMD) from 292 μm to 8–15 μm. Under this reduction, close to 100% evaporation rate was achieved in comparison to only a 1% evaporation rate for the largest SMD. It was concluded that the utilization of a fine droplet spray nozzle provides an effective solution for the reduction in water consumption (97% in our case) for data centres, whilst concomitantly augmenting the proportion of evaporation.

Kinetic study of microwave heating-assisted chemical looping dry reforming of methane over magnetite

Dry reforming of methane carried out via the chemical looping concept and employing microwave heating is a sustainable syngas (a mixture comprising hydrogen and carbon monoxide) production technology. Understanding the intrinsic reaction kinetics of reduction and oxidation is essential for successful scale-up of this technology. By employing magnetite as a microwave absorber oxygen carrier, we investigated the reaction kinetics at bulk temperatures in the range of 650–800 °C for reduction and 500–650 °C for oxidation. Results indicated both reactions followed a phase-boundary controlled (contracting sphere) reaction mechanism. Upon developing the reaction kinetics based on solid temperature relevant to microwave-heated particles, we estimated the activation energy to be 85 kJ/mol for the reduction reaction and 22 kJ/mol for the oxidation reaction. By developing the reaction kinetic of the reduction under microwave heating and based on bulk temperature, we estimated 68 kJ/mol as the activation energy of the reduction reaction. Comparing these values with the activation energy of magnetite reduction by methane under conventional heating (around 90 kJ/mol) indicated that microwave irradiation apparently decreased the activation energy. Consequently, by developing the reaction kinetics based on an appropriate temperature, i.e., solid temperature, we demonstrated that microwave primarily had a thermal effect in our study, increasing the reaction rate constant, rather than a non-thermal effect, like altering the activation energy.

Catalyst pellets with Gaussian activity distribution under forced periodic operation for reactions with Langmuir-Hinshelwood kinetics

This research investigates how combining forced periodic operation with spatially distributed catalyst activity can enhance heterogeneous catalytic processes. It focuses on analyzing reaction-diffusion phenomena within porous catalyst pellets. These pellets exhibit a Gaussian distribution of active sites, and the study investigates how externally forced periodic variations in bulk reactant concentration and temperature affect the reaction process. The paper establishes a mathematical model for a non-isothermal reaction based on Langmuir-Hinshelwood kinetics. This model is then transformed into its dimensionless form for numerical analysis. Numerical simulations are employed to investigate the impact of various parameters on the concentration and temperature profiles within the pellet, as well as on the pellet productivity. These parameters include the position and width of the Gaussian distribution of active sites, the Thiele modulus, the mass and heat Biot numbers, the Arrhenius number for reaction, the energy generation function, the ratio of characteristic times for diffusion and heat conductivity, and frequencies and amplitudes of periodic variations. The simulations reveal complex relationships between the spatial and temporal profiles of concentration and temperature within the pellets. Using porous granules with a non-uniform catalyst activity profile alongside forced periodic operations for reaction-diffusion processes enables higher productivity compared to granules with a uniform activity profile and subject to the steady-state operation. This study demonstrates the potential for optimizing catalytic processes in porous pellets with non-uniform activity profiles under forced periodic operation, offering valuable insights into enhancing process efficiency.

Magnetic biochar generated from oil-mill wastewater by microwave-assisted hydrothermal treatment for sonocatalytic antibiotic degradation

The pollution risks of biomass-containing and antibiotics-containing wastewater mean that it must be treated before discharge into the environment. In a process following circular economy principles, oil-mill wastewater (OMWW) was treated in a microwave-assisted hydrothermal process and with simultaneously fabricated magnetic biochar (MBC), which was used as a catalyst for the sonocatalytic degradation of antibiotics metronidazole (MET) and ciprofloxacin (CIP) in aqueous solutions. FeCl3·6H2O, FeCl2·4H2O, and KOH were added to the OMWW, which was then treated in a SynthWave microwave reactor under either a nitrogen or air atmosphere. The UV254 and COD removal efficiencies of the OMWW reached 42.3% and 60.6%, respectively, after treatment at 250 ℃ for 3h. The MBC obtained from the OMWW treatment with air exhibited the highest adsorption and catalytic activity for antibiotics degradation. The synergistic effects of combining adsorption and sonolytic removal were in the ranges of 0.76-3.07 (MET) and 0.75-3.17 (CIP), resulting in significant degradation rate constants of pseudo-first-order kinetics (k1) that ranged from 0.010 to 0.086min-1 (MET) and from 0.018 to 0.050min-1 (CIP). Hydrophobic MET underwent faster degradation than hydrophilic CIP. The •OH is crucial for removing the model antibiotics. Initial antibiotics concentration, reaction volume, ultrasonic frequency, and power had clearer impacts on antibiotics degradation than bulk temperature. Furthermore, the possible sonocatalytic degradation pathways have also been proposed herein. The absorbents and catalysts were derived during the purification of OMWW and subsequently used for antibiotics removal from water, becoming an innovative case of circular economy and sustainable waste treatment.

Pulsed microwave induced super-heating in graphitic carbon domains drives high-efficiency 5-hydroxymethylfurfural synthesis

Precise heat delivery to localized active sites for energy-efficient chemical reactions remains a challenge so far since existing strategies for heat transfer rely on the temperature gradient. The coupling of microwave irradiation and microwave-responsive catalysts (MRCs) offers a great opportunity to spatially align heat generation and utilization at targeted positions. In this study, a series of MRCs were fabricated with graphitic carbon in bulk-phase and sulfonation structures at the surface. Raman deconvolution analysis (RDA) and HRTEM revealed that Fe-catalyzed MRC formed loose and curly graphite nanosheets and sulfonic groups grafted onto the graphitized edges, while Co-catalyzed and Ni-catalyzed MRC formed closely packed nanosheets and disordered carbon nanostructures at edges. The structural evolution of carbon altered the dielectric properties of the material and further influenced the microwave response and interfacial reaction. Results indicated that Fe-catalyzed MRCs reached 80 mol% HMF yield in 4 pulsed cycles, reflecting a high energy efficiency (18.5 mmol/KJ·L·g). This outstanding performance stems from the super-heating behavior of MRCs under microwave irradiation, causing the interface reaction temperature to be higher than the bulk temperature. This study clarifies the relationship between carbon nanostructure and microwave response at interfaces and offers practical strategies for enhancing the efficiency of microwave energy utilization in heterogeneous catalysis.

HALOBATES: An autonomous surface vehicle for high-resolution mapping of the sea-surface microlayer and near-surface layer on essential climate variables

Abstract Essential climate variables (ECVs) are physical, chemical, or biological parameters supporting the characterization of the climate system. The oceanic ECVs temperature and salinity have been defined for the sea surface (upper 1 mm) and bulk water (upper 2 meters). Direct observation and sampling of the sea surface from ships and buoys is limited due to the potential loss of integrity of the sea surface and contamination by the research platform. The design, development, and operation of a research vehicle with autonomous capabilities based on commercially available components are described in this study. The autonomous operation and fully automated sampling enable high-resolution mapping of the sea surface microlayer (SML), i.e., the upper < 1mm layer, and near-surface layer (< 1.2 m). Sampling the SML is based on a rotating glass disk sampler. The autonomous surface vehicle HALOBATES has been equipped with multiple conductivity, temperature, and depth sensors (CTDs) in a flow-through system to measure both sea surface and bulk temperature and salinity. The performance of the autopilot under various scenarios is demonstrated. Typical anomalies of ECVs within the SML and near-surface layer are evident and forced by both atmospheric and oceanic processes. Data analyses reveal HALOBATES' ability to resolve thermohaline structure on different spatial and temporal scales, for example, small-scale freshwater lenses and thermohaline changes across oceanic fronts. The integration of an Acoustic Doppler Current Profiler supports the interpretation of thermohaline structures. HALOBATES is a platform with cutting-edge technology for understanding climate-related processes between the ocean and atmosphere and providing high-resolution sea surface data for validating satellite products of ECVs.

Heat Transfer Analysis and Friction Factor of Ternary Nanofluids with Twisted Tape Inserts

This study investigates the heat transfer efficiency and friction factor of ternary nanofluids (Al₂O₃-TiO₂-SiO₂) in a simple tube equipped with a twisted tape. Ternary nanofluids, prepared using a volume-based composition ratio of 20:16:64, were tested at various volume concentrations ranging from 0.5% to 3.0%. Forced convection heat transfer experiments were conducted under varying Reynolds numbers (2,300 to 12,000) and a bulk temperature of 70 °C. The results indicate that the maximum viscosity occurs at a volume concentration of 3.0%. The highest increase in heat transfer for ternary nanofluids in a simple tube with twisted tape (H/D = 2.0) was achieved at a volume concentration of 3.0%, reaching 225.35%. Compared to a plain tube, the average thermal performance factor (TPF) of the twisted tape was significantly improved, with a further increase observed when the volume concentration rose from 2.73% to 3.22%.

Homotopic dynamic solution of hydrodynamic nonlinear natural convection containing superhydrophobicity and isothermally heated parallel plate with hybrid nanoparticles

Abstract The purpose of this work is to investigate the effect of thermal radiation on convective heat transfer for an electrically conducting hybrid nanofluid moving perpendicularly through a microchannel with parallel plates heated under isothermal conditions, while subjected to a transverse magnetic field. In this case, one surface exhibited a superhydrophobic slip and temperature jump, whereas the others did not. The objective of the study was to determine the impacts of thermal radiation, heat generation, and magnetism on the volume flow rate, velocity, and bulk temperature when either surface is heated by a constant wall temperature. Our findings reveal that the radiation parameter significantly influences both the Nusselt number and skin friction differently depending on the surface conditions, while also reducing flow rate and bulk temperature. The heat generation parameter similarly affects these variables but varies with the type of surface being heated. Additionally, both the velocity and temperature profiles increase with the porosity parameter and heat generation coefficient, regardless of the heated surface. Statistical analysis confirms the significance of magnetic and heat generation parameters in determining skin friction and the Nusselt number, and streamlines and isotherms illustrating the effects of thermal radiation and magnetism on two different hybrid nanofluids when heated on nonslip and superhydrophobic surfaces were provided. These insights provide valuable information for the design and maintenance of mini- and microdevices in the fields of nanoscience and nanotechnology.

Moisture dependence of responses in physical–chemical property and CH4 ad-/desorption capability of coals to microwave radiation

For purpose of understanding feasibility of coalbed methane (CBM) production in practical water (H2O)-containing reservoirs by using microwave radiation, the dielectric property highly associated with microwave-absorbing and resultant temperature-rising capabilities of coals were addressed primarily; afterward, the temperature-rising rule of coals and its dependence on moisture were examined; finally, the impacts of moisture on changes about main physical–chemical property and resultant CH4 ad-/desorption performance of coals were investigated under microwave field. Results indicated that the coals display microwave-absorbing potential to raise temperature. The moisture initially increases the temperature-rising rate owing to its strong microwave-absorbing capability while decreases the final bulk temperature because of its endothermal evaporation. The resultant rising temperature decomposes and transforms minerals. Moreover, the temperature rise upgrades crystallite structure of the dried coals. As for the wetted coals, the rapid temperature rise disorders crystallite structure of the Sihe and Yima coals due to potential steam explosion. Furthermore, the radiation decreases the aromaticity and the total surface oxygenic groups of the dried coals; the moisture whittles down these decreasing trends. In general, the micro- and mesopore surface area and volume of the coals drops after radiation. The microwave radiation smooths pore surface of the wetted coals while roughens that of the dried coals. The complexity of the coal pore structure almost reduces after radiation. Besides, the microwave radiation significantly reduces the CH4 adsorption capability of the dried coals by 6.74–36.69 %; the moisture further reduces the CH4 adsorption capability by 20.48–43.03 %. Ultimately, the microwave radiation almost enhances the ad-/desorption hysteresis of CH4 on the dried coals while weakens that of the wetted coals. Such alterations depend on the responses of pore abundance and pore surface roughness to microwave. These findings confirm that microwave radiation is a promising option to produce CBM, particularly for H2O-bearing CBM reservoirs.

Numerical and experimental investigation of calcium sulfate crystallization fouling under upward subcooled flow boiling

In this paper, a quasi-unsteady mathematical model was presented to examine the crystallization fouling of the calcium sulfate aqueous solutions under upward subcooled flow boiling (SFB). An experimental set-up is also fabricated to collect experimental data for validating the model results. The developed model can axially and radially determine the temperature variation. An asymptotic predictive model was also used to calculate the heat transfer coefficients. Theoretical results of the bulk and surface temperatures at any desired location of the test section were coupled with the fouling formation equations to provide an integrated model for prediction of variable fouling resistance and deposit thickness along the heat transfer surface. Besides, variation of local heated surface and deposit surface temperatures as well as the pressure drop were theoretically evaluated over time while the deposit grows on the heated surface. The simulation results showed that reaction is the controlling mechanism when SFB is predominant.

Parametric study of free turbulent slurry jet: influence of particle size and particle concentration on the flow dynamics and thermal behavior of high-speed slurry jet

A detailed computational investigation was conducted to explore the dynamics of high-speed free slurry jets, with a focus on how variations in abrasive size and concentration affect their behavior. The study yielded several significant observations: Firstly, it was observed that slurry jets containing larger particles exhibited notably higher average velocities, attributed to their inherent self-similar characteristics. Specifically, at the far field of the jet, slurry jets with larger particle sizes demonstrated a 15% increase in velocity compared to those with smaller particles. Additionally, an analysis of turbulent intensity revealed that beyond a certain axial distance (x/D = 10), turbulence levels progressively increased. However, intriguingly, for slurry jets with a high concentration (Co = 15%), there was a notable decrease (28%) in turbulent intensity compared to water jets, indicating a complex interplay between particle size and concentration. Secondly, the study found that as the concentration of particles in the slurry jet increased, there was a corresponding rise in the bulk temperature of the jet. This phenomenon was primarily attributed to the heightened thermal conductivity resulting from the increased density of particles in the water. Furthermore, an examination of the Nusselt number revealed interesting trends. While the Nusselt number exhibited a peak near the nozzle exit, indicative of enhanced heat transfer in this region, it showed a decremental trend along the axial direction of the jet, attributable to jet divergence. In summary, the computational analysis provided valuable insights into the behavior of high-speed slurry jets, highlighting the intricate relationships between abrasive size, concentration, velocity distribution, and thermal characteristics. These findings contribute to a deeper understanding of slurry jet dynamics and have significant implications for various industrial applications.

Dielectric relaxation and electrical conductivity in lead-free organic ferroelectric diisopropylammonium bromide (dipaBr)

In this communication, we explore the dielectric relaxation behavior of the novel organic salt Diisopropylammonium Bromide (dipaBr), focusing on the universal relaxation law. We conduct dielectric relaxation and conductivity analyses across a broad spectrum of temperatures (325K–443K) and frequencies (20 Hz–20 MHz). The irregularity in peak broadening observed in complex modulus spectroscopy indicates a distribution of relaxation periods with varying time constants, confirming that the relaxation is of the non-Debye type. To interpret the experimental findings, we utilize the Havriliak-Negami (HN) formula in our investigation of dielectric relaxation. The parameters derived from the model are discussed. The relaxation process in the material appears to depend on temperature. The ac conductivity profile follows Jonscher's universal power law, and the relationship between bulk DC conductivity and temperature follows an Arrhenius-like pattern.

Thermal and hydrodynamic analysis inside a channel partially filled with porous material

AbstractIn this work, the transfer of heat and changes in fluid pressure within a rectangular channel fairly packed with porous material have been studied numerically for various Darcy numbers and dimensionless porous layers. The model was run with the following assumptions: laminar flow, forced convection, isotropic porous material, local thermodynamic equilibrium, constant wall temperature boundary condition, and no thermal dissipation. The study covers a broad range for the dimensionless porous layer, 0 ≤ hr < 1, and the Darcy number, 10−4 < Da < 10−2. The fully developed and developing flow over the channel is investigated in numerical study. It was observed that the inertia effect may be disregarded when Da < 10−4. The local dimensionless bulk temperature distribution, pressure drop, and velocity profiles were all shown to be impacted by the Darcy number and dimensionless porous layers, according to the numerical analysis results. The maximum heat transfer rate was attained when the ratio of the porous layer inside the channel was 0.8, and the pressure gradient was the highest. Partial packing of the channel with a porous material has two advantages: it increases the rate heat transmission rate and results in a much smaller pressure drop than a filled porous medium.

(Invited) Electrochemical Processes Drive Membrane-Based Recovery of N and P

We expend a large amount of energy and money on the treatment of municipal and industrial wastewater, relying on redox reactions to transform contaminants (e.g, ammonium, reduced carbon) into other less/non-harmful species (N2/NO3 -, CO2), or on biological accumulation approaches that sequester contaminants into bacteria (e.g., biomass, PO4 -3). In the worst case, these contaminants are simply released into the environment where they cause harm and are lost. Yet, many of these contaminants are actually valuable resources (e.g., NH3, PO4 -3). In an age where environmental sustainability and climate change mitigation are of utmost importance, it makes little sense to invest large amounts of energy into the conversion and elimination of these valuable resources, only to turn around and use fossil fuels to produce more. In addition, the value of these resources is often higher than the value of the treated water, making their recovery an economically-attractive prospect.Here, we discuss our recent work on membrane-based separations of ammonia, phosphate, and volatile fatty acids (VFAs) from different waste streams. Critically, in all cases we achieve separation by leveraging electrochemical reactions and processes. For ammonia recovery, we utilize an electroactive membrane contactor that is used as a cathode – the application of cathodic potentials to the membrane surface increases the local pH at the membrane/water interface, which transforms ammonium to ammonia and enables its effective volatilization and recovery across the membrane. For VFA recovery, we use a composite electroactive membrane that combines electrothermal heating and an electrochemical pH swing to protonate and extract these volatile species from water. Finally, for phosphate separation, we engineer a phosphate-specific pathway through a cation exchange membrane, and use electrodialysis to pull phosphate ions from a mixed anion solution. Together, these approaches can be used to recover valuable products from complex waste streams, such as municipal and industrial wastewater, without the need to rely on bulk pH and temperature modification.

Modification of the turbulence properties at the bow shock: statistical results

Turbulent solar wind is known to be a main driver of the processes inside the magnetosphere, including geomagnetic storms and substorms. Experimental studies of the last decade demonstrate additional ways of interplanetary plasma transport to the magnetosphere, including small-scale processes in the magnetosphere boundary layers. This fact implies that properties of the solar wind turbulence can affect the geomagnetic activity. However, in front of the magnetosphere are a bow shock and a magnetosheath region which contribute to the changes in the properties of the solar wind turbulence and may result in destructions of the association between solar wind turbulence and the magnetosphere. The present study provides the statistics of two-point simultaneous measurements of the turbulence properties in the solar wind and the magnetosheath based on Wind and THEMIS spacecraft data. Changes in the turbulence properties are analyzed for different background conditions. Solar wind bulk speed and temperature are shown to be the main factors that influence the modification of turbulence at the quasi-perpendicular bow shock at frequencies higher than the break frequency (ion transition range). Inside the magnetosheath, significant steepening of spectra occurs with an increase in temperature anisotropy without a connection to the upstream spectrum scaling that underlines the crucial role of the instabilities in turbulence properties behind the bow shock.

Effect of Chevron Angles on the Thermal Performance of Corrugated Plate Heat Exchanger (CPHEs)

Corrugated plate heat exchangers (CPHEs) possess a greater surface area for heat transfer and a heightened level of turbulence as a result of the corrugations. This study presents an experimental analysis comparing the thermal performance of a flat plate heat exchanger and a corrugated plate heat exchanger (CPHE) with varied corrugation angles. The analysis was conducted using water as the test fluid. The comprehensive testing has been carried out on CPHEs with different chevron angles (β) of 30°/30°, 45°/45°, and 60°/60°. Experimental data is collected under steady- state conditions, with a single phase (water-water) system, covering Reynolds numbers (Re) ranging from 900 to 1250. The test fluid's mass flow rate, ranging from 0.05 to 0.16 kilograms per second, is measured together with the accompanying steady state temperatures. Each plate was equipped with four thermocouple sensors to detect the bulk temperature of the cold and hot fluids at the intake and output. The performance was evaluated using both parallel and counterflow configurations. Experimental observations are utilized to estimate the temperature difference (∆T) between the inlet and output streams, the logarithmic mean temperature difference (LMTD), and the exergy (E). The measured ∆T and E values for corrugation angles (30°, 45°, and 60°) of CPHE were compared to those of flat plate heat exchangers. For corrugation angles of 30°, 45°, and 60°, the values of ∆T and E (efficiency) increase as the mass flow rate of the fluid increases. An increase in the corrugation angle leads to an increase in turbulence in the flow, which in turn enhances heat transmission. In addition, the thermal effectiveness (ε) was calculated using the NTU method and compared for all of the plates. Key Words: Corrugated plate heat exchanger, Thermal performance, Chevron angle, heat transfer coefficient.

Grain boundary segregation for the Fe-P system: Insights from atomistic modeling and Bayesian inference

In this work we re-assess experimental data for grain boundary (GB) segregation of P in bcc Fe with thermodynamic and statistical methods. The data are based on Auger-Electron-Spectroscopy (AES) measurements which have provided P GB concentrations for various bulk contents and temperatures for ferrite. While in the previous investigations of this system a single-site McLean equation was used to extract segregation enthalpy and entropy, we employ multi-site segregation models as suggested by atomistic simulations. We use a recently introduced methodology based on inter-atomic potentials to calculate the segregation energy spectrum, as well as vibrational entropy and solute-solute interactions of P in polycrystalline Fe, thereby obtaining a three-dimensional distribution of energy, entropy and interactions. Using this trivariate distribution we predict P GB concentrations and compare them to the experimental measurements. Furthermore, we calibrate the parameters of physical models of increasing complexity to the experimental data with an inverse modeling approach based on Bayesian inference. While it is not possible to seamlessly link the results from AES to atomistic modeling, our investigation provides significant new insights for thermodynamic modeling of GB segregation. The vibrational entropy deduced from experiment differs from physics based atomistic simulations even when taking the spectral nature of energy, entropy and interactions into account. However, the correlations of the quantities are in good agreement when considering the trivariate model. Our investigation highlights the need for new and more accurate experimental datasets of GB segregation.

Fully developed flow through shrouded-fin arrays: exact and asymptotic solutions

The flow resistance, i.e. friction factor times Reynolds number ( $\,f\,{Re}$ ), of longitudinal-fin heat sinks with or without clearance between a shroud and the tips of the fins is an important parameter in thermal design. This is because it dictates the caloric resistance of the heat sink, i.e. change in bulk temperature of the fluid flowing through it. When there is no clearance and the common and oft-valid assumption of negligible fin thickness is invoked, $f\,{Re}$ corresponds to simply that of a rectangular duct. However, with clearance, only numerical results are available as per the well-known study by Sparrow, Baliga and Patankar (ASME J. Heat Transfer, vol. 100, 1978). We develop analytical formulae for $f\,{Re}$ for fully developed flow with clearance. The exact solution is provided by an integral formula derived via conformal mappings. Additionally, simple formulae are derived via asymptotic expansions in three cases: (1) the fin spacing is small compared to the fin height and clearance; (2) the clearance is small compared to the fin spacing, which is small compared to the fin height; (3) the same as case (2) but valid for larger clearances. The different asymptotic formulae are compared to the exact formula, and together cover almost the entire relevant parameter range (for fin spacing and clearance) with errors of less than 15 %. A formula for the limiting case of no clearance is shown to be more accurate, for any fin spacing, than a widely used correlation from the literature.

New insights into kinetics and mechanisms of acid sulphite delignification of Eucalyptus globulus wood

The kinetics of Eucalyptus globulus wood delignification in Mg-base acid sulphite pulping of dissolving pulp was studied using a pilot-scale batch digester simulating the industrial process. The behavior of lignin during impregnation, heating-up and bulk delignification at the final temperature was evaluated by the analysis of acid-insoluble lignin in wood and wood residue after pulping. The dynamics of lignin dissolution was followed by its structural analysis in solid residues and in solution by wet chemistry and 1D and 2D nuclear magnetic resonance (NMR) spectroscopy. About 30 % and 20 % of the lignin were removed in the impregnation and heating-up pulping steps, respectively. About 45 % of the lignin was removed at the final pulping temperature in bulk and 3 % in the residual delignification stages. The kinetics of bulk delignification followed a pseudo-first-order kinetics with an effective activation energy of 138.4 kJ/mol in the temperature range of 140–148 °C. Bulk delignification follows an undefined overall order in the hydrogen and bisulfite ions and depended on the partial pressure of SO2 at ca. 0.60 order within the range of combined SO2 in pulping acid of 1.14–1.66 % and total SO2 loads of 6.2–7.0 %. Minimum degree of lignin sulphonation to be dissolved in the pulping liquor was suggested to be about 0.35 sulphonic groups per one phenyl propane unit. Lignin removal during pulping showed dependence on its location in different morphological parts of the fibers and related structural characteristics, which were discussed accordingly. A role of the polysaccharide complex in the accessibility of lignin was highlighted. A noticeable cleavage of β-O-4 linkages occurred in lignin already dissolved in pulping liquor at the end of cooking.

Numerical Simulation and Experimental Comparison of System Analysis Module 1D Mixing Model for Cold Shock Transients in the Gallium Thermal-Hydraulic Mixing Facility

Liquid metals are being investigated as coolants in many advanced reactor designs because of their high thermal conductivity and effectiveness at high temperatures. However, they often pose challenges to reactor operation and safety because of the complex thermal mixing and stratification in the plenum of pool-type reactor designs. The advanced system analysis code System Analysis Module (SAM) currently under development at Argonne National Laboratory aims to develop and implement thermal mixing models to accurately capture these complex thermal fluid behaviors. In this study, the SAM thermal mixing model was compared against experimental data from the Gallium Thermal-Hydraulic Experiment facility, a scaled liquid metal test facility that uses gallium as a surrogate fluid to investigate the stratification and thermal mixing of low-Prandtl-number fluids in the upper plenum of a liquid metal–cooled reactor. Two cold shock transient cases were used: one with stable stratified flow (Ri = 32) and one with stronger thermal mixing (Ri = 0.5). The resultant temperatures were then compared with the experimental temperatures over the entire plenum to assess the ability of the mixing models to capture the thermal behavior and to better correspond mixing parameters to various flow scenarios. Generally, the zero-dimensional mixing model was more capable of capturing the bulk temperature of the component modeled assuming that an accurate mass flow rate was provided, but it was inherently unable to capture thermal gradients in space. The one-dimensional mixing model was capable of capturing that the thermal gradients provided accurate selection of the mixing coefficients. The temperature at the outlet junction was compared over time for each of the mixing models with the recorded experimental temperature. The implemented mixing models demonstrated the ability to effectively capture the overall thermal behavior for stronger mixing scenarios but struggled with more stably stratified flows. It was found that a system analysis code’s covering of the entire range of different operating conditions still remains a challenging task, and it is suggested that further model and closure improvements are necessary to accurately capture complex thermal mixing and stratification phenomena.