Bulk Temperature Research Articles

Integrating the complex rheological Carreau model with the Graetz-Brinkman problem in a horizontal passage

ABSTRACT The current investigation emphasizes the influence of viscous dissipation in laminar forced convection internal flow for horizontal conduit using the non-Newtonian fluid. A Carreau fluid model is taken into account. The hydrodynamically developed velocity profile is obtained by employing a perturbation approach. A classical Graetz approach is utilized to achieve the solution of the developed energy equation while the MATLAB built-in command bvp4c is taken into account for the solution of the Eigenvalue problem. The negligible axial conduction and uniform physical characteristics are presumed. The influence of the viscous dissipation and model parameters on non-dimensional bulk temperature and Nusselt number is deliberated for both cooling and heating situations. The current analysis is valid for small values of Weissenberg number We ≤ 1 . The findings reveal that viscous dissipation has a significant impact on temperature distribution in both developing and fully established regions.

Simulations on Levitation and Spatial Distribution of Charged Dust on the Moon Surface

The levitation of charged dust, which may cause serious hazards to astronauts and lunar rovers, has been one of the most significant challenges in lunar exploration. Here we simulate lunar sheath potentials in different solar wind conditions and solar zenith angles (SZAs) on the lunar surface by the particle-in-cell method. The simulated potentials exhibit two types of distributions as a function of height, depending on the SZAs. For SZA ∼ 0°–70°, the nonmonotonic distribution with positive surface potential dominates in the photoelectron sheath. For SZA >∼81°, the monotonic distribution with negative surface potential is observed in the plasma sheath. With the calculated potentials and the assumption that the dust radius distribution exponentially decreases, we further investigate spatial distributions of the dust levitated above the surface. It is found that number density of the levitating lunar dust is enhanced at the terminator (SZA ∼ 81°) in the plasma sheath. In the photoelectron sheath it gradually decreases as the SZA increases from 0° to 70°. Further calculations of the potential and the derived electrostatic field suggest that the dust spatial distributions can be influenced by the bulk velocity, number density, and temperature of the solar wind. Those findings deepen our understanding of lunar surface charging and the mechanism of lunar dust levitation, which can provide technical support for lunar explorations.

Thermodynamic analysis of a parabolic trough power plant integrating supercritical carbon dioxide Brayton cycle and direct contact membrane distillation

This research advances current efforts to develop 3rd gen concentrated solar power (CSP) systems, addressing current research and practical implementation gaps. It focuses on key challenges identified by the academic community: advanced power cycles and utilising waste heat effectively to enhance efficiency. This research introduced a novel system comprising a direct parabolic trough with supercritical carbon dioxide (sCO2) Brayton cycle, and a Direct Contact Membrane Distillation (DCMD) desalination system, aiming to overcome these barriers. The methodology entailed developing a mathematical model and conducting thermodynamic analyses to evaluate system efficiencies, the impact on water production, and the number of DCMD units needed under various conditions. The results reveal that both thermal and exergetic efficiencies decline with decreasing direct normal irradiance across all pressure ratios with the optimal efficiency observed at pressure ratios of approximately 3.1 to 3.2 with exergetic efficiency of around 0.383. Higher bulk temperature differences increased water production and reduced need for direct contact membrane distillation units. However, an initial decline and subsequent rise in water production at bulk temperature differences of 10 °C and 20 °C indicates that feed temperature alone does not determine water production capacity. The existence of an inverse correlation between the cycle’s efficiency and water production prompted the introduction of a unifying coefficient, the Integrated Performance Coefficient (IPC), peaking at 50.32 between pressure ratios of 3.1 and 3.2. High bulk feed temperatures combined with low bulk permeate temperatures enhance water production but also increase its vulnerability to variations in direct normal irradiance. Conversely, increased bulk permeate temperatures lessen the effect of direct normal irradiance changes on water production, albeit with a reduced output.

Numerical study on natural convection heat transfer of supercritical CO2 in an infinite space

A numerical study of natural convection heat transfer from horizontal heating wire to supercritical carbon dioxide (sCO2) in an infinite space was performed. The qw ∼ ∆T curves of sCO2 natural convection heat transfer were calculated in a wide range of parameters, where initial bulk temperature (Tib) and operating pressure (P) varied in the range: 275 K < Tib < 345 K and 7.5 MPa < P < 11 MPa, respectively. Then, we reported from the viewpoint of pseudo-phase transition for supercritical fluids, the characteristics and mechanism of sCO2 natural convection heat transfer were revealed through comparing with qw ∼ ∆T curve of subcritical pool boiling. The similarity between film-like heat transfer at Tw > Tpc for supercritical pressure and film boiling at Tw > Tsat for subcritical pressure was demonstrated. The influence of operating parameters such as Tw, Tib, and P on sCO2 heat transfer was also discussed. For supercritical film-like heat transfer, thermal resistance of film-like which reflects the effect of gas-like film thickness and its thermal conductivity on heat transfer was proposed. Expectedly, the variation of thermal resistance explained successfully the difference of sCO2 heat transfer under different operating parameters. The investigation is of paramount importance in comprehension of sCO2 heat transfer mechanism for designing heat exchanger component in the advanced sCO2 application.

Responses of physico-chemical properties and CH4 adsorption/desorption behaviors of diverse rank coal matrices to microwave radiation

To assess feasibility of microwave radiation in CH4 production, temperature rise of coals under microwave field and resultant alterations in physico-chemical properties were addressed. Results indicated that the temperature rise of coals is dominated by mineral compositions. The bulk temperature of coals containing strong microwave-absorbing minerals such as pyrite, smectite, and kaolinite rises markedly. The elevated temperature breaks alkyl side chains and aromatic rings of coals. Moreover, the total oxygen-containing functional groups decrease by radiation. Generally, the radiation reduces the micro- and mesopores of coals. Besides, the pore surface of low-rank coals after radiation becomes smooth, whereas that of medium- and high-rank coals becomes rough. Finally, the radiation decreases CH4 maximum adsorption capacity by 6.74–36.69%. The radiation also diminishes CH4 adsorption/desorption hysteresis for low-rank coals, while an opposite trend exists in medium- and high-rank coals. Such variations rely on pore structure responses to microwave. Overall, microwave radiation could promote CH4 recovery.

Effects of operating parameters on single-stage ultrasonic separation of ethanol from its aqueous solutions

The effects of operating parameters on separation of ethanol from its aqueous solutions using ultrasonic atomization were investigated in a single-stage process. With an experimental setup similar to those used in previous work, we measured the degree of enrichment and the combined molar collection rate of ethanol and water for a single combination of the ultrasound power and frequency at 126 combinations of bulk liquid depth, composition, and temperature; carrier-gas inlet/outlet elevation and flow rate; and collection temperature. Enrichment depends strongly on bulk liquid composition, reaching a maximum at an ethanol mole fraction below 0.1, with the maximum enrichment depending on bulk liquid depth and inlet/outlet elevation. The collection rate depends strongly on inlet/outlet elevation and bulk liquid pool depth, and for high ethanol mole fractions is nearly independent of bulk liquid composition. Enrichment and collection rate increase with increasing bulk liquid temperature and decreasing collection temperature. The fact that increasing the carrier-gas flow rate increases the collection rate and decreases enrichment is interpreted in terms of size-selective transport of drops from the ultrasonically-generated fountain where they are atomized, to the cold trap where they are collected. The results are discussed in terms of operating parameters, transport phenomena (including evaporation and gravitational settling of atomized drops), the ultrasonically-induced fountain, and previously reported drop-size distributions. The duration of the experiments, conducted in batch, was such that the results should be relevant to both batch and continuous-flow operation. The work provides guidance and critical data for process scale-up.

Microwave heating-assisted chemical looping dry reforming of methane

Chemical looping dry reforming of methane (CLDRM) offers a sustainable pathway to convert methane (CH4) and carbon dioxide (CO2) to syngas, i.e., a mixture of hydrogen (H2) and carbon monoxide (CO). However, carbon deposition caused by high gas temperature remains a crucial challenge for the CLDRM. We proposed microwave (MW) heating-assisted CLDRM to leverage the selective heating feature of MWs to create a thermal gradient between solid and gas phases. This thermal gradient, i.e., a higher solid temperature than the gas temperature, can promote desired reduction and oxidation (redox) reactions, while suppressing undesired gas phase reactions, in particular, CH4 decomposition. At bulk temperature of 800 °C, the MW heating-assisted CLDRM achieved a maximum CH4 conversion of 97% and H2/CO ratio of around 2. Compared to the conventionally heated CLDRM, the MW heating increased the redox extents of magnetite (the oxygen carrier) by 2.5 and 1.5 times, respectively, and suppressed carbon deposition.

Electrode wettability and capacitance of electrical double layer capacitor: a classical density functional theory study

We suggest a coarse-grained water model for use in classical density functional theory (cDFT) to describe aqueous inorganic salt solutions that act as working electrolytes in electrical double-layer capacitors (EDLCs) with electrodes comprising two face-to-face doped carbon monolayers. Focus of the cDFT calculations lies on the influence of solvent electrode wettability (SEW) on capacitance and energy storage behaviors, while also considering its interaction with factors like electrolyte bulk concentration, pore size, electrode voltage, and temperature. New phenomena are disclosed theoretically. Remarkably, this study challenges the traditional notion that energy storage is consistently boosted by enhancing the electrode’s ionophobicity. Contrarily, the SEW effect reduces energy storage below the standard aqueous electrochemical window voltage (around 1.2 V) and only enhances the energy storage as the voltage surpasses a certain threshold up to the optimal window voltage (2 V–2.5 V). Furthermore, a non-monotonic SEW effect on energy storage is demonstrated under appropriate conditions, shedding new light on the complex relationship between ionophobicity and energy storage. Moreover, the present coarse-grained water model enables the prediction of the experimentally observed inverse relationship between temperature and capacitance. In contrast, the widely used electrolyte primitive model predicts the existence of a maximum value. The decisive factor for the impact of SEW on capacitance and energy storage is identified as congestion within the electrode pore, while other factors contribute by affecting this congestion. The present research offers valuable insights, highlighting the significance of SEW in the innovative and strategic design of aqueous inorganic EDLC devices.

Turbulent drag reduction with streamwise-travelling waves in the compressible regime

The ability of streamwise-travelling waves of spanwise velocity to reduce the turbulent skin-friction drag is assessed in the compressible regime. Direct numerical simulations are carried out to compare drag reduction in subsonic, transonic and supersonic channel flows. Compressibility improves the benefits of the travelling waves, in a way that depends on the control parameters: drag reduction becomes larger than the incompressible one for small frequencies and wavenumbers. However, the improvement depends on the specific procedure employed for comparison. When the Mach number is varied and, at the same time, wall friction is changed by the control, the bulk temperature in the flow can either evolve freely in time until the aerodynamic heating balances the heat flux at the walls, or be constrained such that a fixed percentage of kinetic energy is transformed into thermal energy. Physical arguments suggest that, in the present context, the latter approach should be preferred. This provides a test condition in which the wall-normal temperature profile more realistically mimics that in an external flow, and also leads to a much better scaling of the results, over both the Mach number and the control parameters. Under this comparison, drag reduction is only marginally improved by compressibility.

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A comprehensive investigation of the use of freeze concentration appro aches for the concentration of fish protein hydrolysates

Fish protein hydrolysates (FPH) are inherently unstable in their liquid form, necessitating either freezing or dewatering for stabilization. Gentle methods such as freeze concentration can be used to remove water, this can be achieved by freezing water in solution by decreasing the bulk temperature below freezing point and separating pure ice crystals from concentrated solution. This approach serves as an alternative to techniques like evaporation and reverse osmosis for concentrating solutions that have high water content, significant nutritional value, and thermolabile compounds. This is crucial as many bioactive compounds degrade when exposed to elevated temperatures. Another notable advantage of this technology is its potential to reduce energy consumption by up to 40% when integrated into the FPH drying process. Although this technology is currently industrialized primarily for juices, it can achieve concentrations of up to 60°Brix and manage viscosities up to 400 mPa.s. Numerous studies have been dedicated to enhancing design and processes, leading to a 35% reduction in the system's capital cost and a 20% reduction in energy consumption. Moreover, freeze concentration can synergize with other concentration techniques, creating more efficient hybrid processes. This review aims to introduce freeze concentration as a superior option for preserving fish protein hydrolysates, enhancing their stability, and maintaining their nutritional and bioactive qualities.

Seasonal changes of ammonia-oxidizing bacterial communities during tropical forest restoration

In this study, we used a high-throughput sequencing technology to survey the dry-wet seasonal change characteristics of soil ammonia-oxidizing bacteria (AOB) communities in the three restoration stages [i.e., Mallotus paniculatus community (early stage), Millettia leptobotrya community (middle stage), and Syzygium oblatum community (later stage)] of Xishuangbanna tropical forest ecosystems. We analyzed the effects of soil physicochemical characteristics on AOB community composition and diversity during tropical forest restoration. The results showed that tropical forest restoration significantly affected the relative abundance of dominant AOB phyla and their dry-wet seasonal variation. The maximum relative abundance of Proteobacteria (71.3%) was found in the early recovery stage, while that of Actinobacteria was found in the late recovery stage (1.0%). The abundances of Proteobacteria and Actinobacteria had the maximum ranges of dry-wet seasonal variation in the early and late stages, respectively. The abundance of dominant AOB genera and its dry-wet seasonal variation varied across tropical forest restoration stages. The maximum average relative abundance of Nitrosospira and Nitrosomonas in the late recovery stage was 66.2% and 1.5%, respectively. In contrast, the abundance of Nitrosovibrio reached its maximum (25.6%) in the early recovery stage. The maximum dry-wet seasonal variation in relative abundance of Nitrosospira and Nitrosomonas occurred in the early recovery stage, while that of Nitrosovibrio occurred in the middle recovery stage. The Chao1, Shannon, and Simpson diversity indices of AOB communities increased along the restoration stages, which were significantly higher in the wet season than in the dry season. The results of canonical correspondence analysis showed that soil easily oxidized carbon was the main factor controlling AOB community diversity and Actinobacteria abundance. Soil bulk density and temperature were the main factors affecting Proteobacteria abundance. Soil pH, microbial biomass carbon, water content, ammonium nitrogen, bulk density, and temperature were the main factors controlling the abundances of Nitrosospira, Nitrosomonas, and Nitrosovibrio. Therefore, tropical forest restoration can regulate the change of relative abundance of dominant AOB taxa via mediating the changes of soil temperature, bulk density, and readily oxidized carbon, leading to an increase in soil AOB community diversity.

Supercritical heat transfer of Novec 649 refrigerant in a minichannel with various inclination angles: Horizontal to vertically downward flows

The study of organic fluids under supercritical conditions has been less extensive than that of supercritical carbon dioxide and water. In addition, most studies on supercritical heat transfer have focused on horizontal or vertical channels, with limited research on inclination angles. Therefore, the focus of this study was to investigate the supercritical heat transfer of organic fluid (Novec 649) in a miniaturized channel across various inclination angles. The experimental findings reveal that the heat transfer coefficient (HTC) increases with an increase in the fluid's bulk temperature at a given mass flux and with an increase in the mass flux at a given bulk temperature. Notably, the HTC in horizontal flow channels marginally surpasses that in non-horizontal channels. The buoyancy effect, found to be non-dominant in a studied channel, does not lead to heat transfer deterioration. In vertical flow channels, under the experimental conditions, the acceleration parameters exceeded the threshold values, suggesting a potential significance of the acceleration effect. Particularly at higher mass fluxes, the acceleration parameter peaked as the film temperature approached the fluid's pseudocritical value. We further developed the supercritical heat transfer correlation by introducing an inclination angle parameter, achieving a mean absolute error of 12.7 %.

Computed entropy spectra for grain boundary segregation in polycrystals

Grain boundary solute segregation influences most bulk material properties, and understanding solute thermodynamics at grain boundaries is critical for engineering them. However, the vast grain boundary space in polycrystals is challenging to evaluate due to its size, especially for the intrinsically hard-to-compute segregation excess entropy. Here data science methods are used to generate a database of site-wise grain boundary segregation entropy spectra for 155 dilute binary alloys within the harmonic approximation. The spectral framework allows scale bridging between the calculated atomistic site-wise energy-entropy spectra and macroscopic segregation entropy estimates. The results affirm that macroscopic averaging is not sufficient: a spectral treatment of grain boundary segregation is needed to accurately model bulk temperature dependence of grain boundary solute segregation. The calculated spectral entropy database and thermodynamic framework can be applied for both understanding segregation experiments and alloy design exercises, paving the way to a finite-temperature grain boundary genome.

Measurement and comparison of transient thermal boundary layer of a Local-Heated Mini-Channel under sub- and super-critical conditions

This study is focused on experimentally measuring the transient thermal boundary layer following a sudden heat addition to turbulent flow boundary in supercritical mini-channels. Interferometric diagnostics were performed using a visualization channel to capture the time-evolution boundary layer. The measured pressure drop shows good agreement with numerical iterations according to the friction laws. Transient scenarios have been extracted from the raw interferograms to facilitate quantitative comparison of boundary conditions, such as bulk temperature and heat flux: (1) Localized heat accumulation and a slowly thermal response have been observed in subcritical state; (2) Relatively high density of 1.2 kg/m3 and low temperature changes of 0.008 K are observed in cases with relatively shorter critical distances to the critical point, attributed to the sharp increase in heat capacity and thermal expansion coefficients. (3) A relatively fast thermal convective field is established at larger heat fluxes due to the rapid density shifts of 1.2 kg/m3 and associated flow acceleration in the boundary layer.

Effect of Strip-Fin Height on Jet Impingement Heat Transfer in a Rectangular Channel at Two Jet-to-Target Surface Spacings

Abstract An experimental investigation of heat transfer performance in a rectangular impingement channel featuring staggered strip-fins was completed. Four configurations were considered to study the effects of varying the strip-fin height (H/d = 1.5 and 2.75) at two jet-to-target surface spacings (z/d = 3 and 6) on the heat transfer, pressure loss, and crossflow magnitude for a long impingement channel with in-line, 4 × 12 impinging jets. Also, the effect of the reference temperature choice, either jet inlet temperature or local bulk temperature, for calculating the local heat transfer coefficients was considered. The regionally averaged heat transfer coefficients were measured at seven Reynolds numbers, based on the jet diameter (10k–70k) utilizing the copper plate experimental method. The empirical correlations were expressed for the area averaged Nusselt number estimation of impingement channels with strip-fin or pin-fin roughness elements. The results showed that the long strip-fin channel with z/d = 3 provided the best thermal performance. The discharge coefficients are similar for all configurations between Rejet = 10k and 50k. The results are compared with the impingement channels with conventional pin-fins. They show that strip-fin channels provide lower pressure drop with marginally better heat transfer coefficients compared to the conventional pin-fin channels. However, when the channel weight is considered, strip-fins would increase the roughness material volume more than the conventional pin-fins.

Power Loss and Damage Behavior of Gears Operating Under Loss of Lubrication

AbstractGeared turbofans need to withstand loss of lubrication due to inevitable off-design conditions. Additionally, the loss of lubrication specifications that have to be fulfilled are getting stricter. For a loss of lubrication event, today's geared turbofans are equipped with secondary oil supply systems. Their omission would reduce space and weight and, consequently, carbon emissions. However, this requires geared transmissions to withstand loss of lubrication events. In order to enable this, knowledge of the power loss and damage behavior of gears under loss of lubrication is required first. In this study, power loss and bulk temperature measurements of test gears under loss of lubrication on an FZG gear efficiency test rig are presented. Hertzian pressures in the pitch point up to 1723 N/mm2 and circumferential speeds up to 20 m/s were considered. The experimental results show the characteristics of increasing power loss and bulk temperatures under loss of lubrication depending on load and speed. At moderate operating conditions, no damage occurs within 20 s under loss of lubrication, whereas a load increase results in slight scuffing, and a speed increase results in severe scuffing. Oil centrifugation has a strong effect on the loss of lubrication performance. Additional experiments under reduced quantity lubrication demonstrate the impact of the remaining oil on the survivability of gears facing the loss of lubrication.

Variations in gas temperature and average velocity during laminar-turbulent transition in high-speed gas flow through microtubes

The primary objective of this study is to investigate variations in gas temperature and average velocity during the laminar-turbulent transition of compressible flow. This investigation encompasses an exploration of the effect of Mach number on transitional Reynolds number. Experiments were conducted utilizing a pair of adiabatic stainless steel microtubes with a diameter (D) of 131.6 μm. The first microtube was dedicated to measuring local pressure to obtain friction factor, local Mach number, and bulk temperature. The second microtube was employed for measuring local wall temperature. Additionally, a stainless steel microtube with a diameter (D) of 124 μm and a rectangular microchannel with a hydraulic diameter (Dh) of 99.36 μm were incorporated in the study. Friction factor, Mach number, and bulk temperature were determined by measuring mass flow rate and local pressure near the outlet. The measured mass flow rate exhibited a slight increase that plateaued during the laminar-turbulent transition as the Reynolds number increased. In laminar flow, an increase in the Reynolds number resulted in an increase of the Mach number and a reduction in bulk temperature. Conversely, during the laminar-turbulent transition, the Mach number either remained constant or decreased, and the bulk temperature increased due to the conversion of kinetic energy to thermal energy. Experimental validation of this phenomenon was achieved by measuring the wall temperature of an adiabatic microtube.

The Effect of Uneven Metal Foam Distribution on Solar Compound Parabolic Trough Collector Receiver Thermal Performance

Solar energy is a key player among other renewable energies that reduce greenhouse gases and replace conventional fuel, i.e., to solve global warming and fossil fuel descending issues. However, the thermal solar systems’ performance should be enhanced to cope with intermittent solar radiation. Metal foam can be used as an enhancer in solar collectors’ receivers. However, metal foams are associated with pressure drop. To benefit from the metal foam as a thermal enhancer and overcome the pressure drop issue, the present study numerically and experimentally investigates a novel Compound Parabolic Solar Collector (CPC) receiver with innovative uneven metal foam inserts of varying thickness. Two pores per inch (PPI) Cu-foam inserts, PPI10 and PPI20, were tested. These inserts were strategically placed at three different positions along the receiver, with thicknesses of 3 cm, 2 cm, and 1 cm starting from the inlet side. In the experimental part, three tubular receivers were tested, empty, i.e., without metal foam, inserted with Cu-foam of PPI10, and inserted with Cu-foam of PPI20. The experiments were conducted from 09:00 AM to 04:00 PM. The investigation involved water volume flow rates from 0.2 to 0.6 l/min. The numerical part included solving the governing equations, i.e., mass, momentum, and energy conservation, simulating conditions similar to the experiments. The Brinckman model described the fluid flow through the metal foam. The thermal performance of the CPC system was evaluated using the Nusselt number (Nu), thermal efficiency, water bulk temperature, and water outlet-inlet temperature difference. Inserting Cu-foam of PPI20 resulted in the hourly maximum Nu and thermal efficiency compared to the empty and PPI10 cases. Experimentally, the hourly maximum Nu was 8.9, 8.4, and 7.9 for PPI20, PPI10, and empty receivers, respectively, at 0.6 l/min. The average thermal efficiency was 88.3, 85, and 81.9 for PPI20, PPI10, and empty receivers, respectively, at 0.6 l/min. As for the outlet-inlet water temperature difference, the highest values were at 0.2 l/min. Again, PPI20 recorded the best results, i.e., 23.3 K, 21.5 K, and 20.2 K for PPI20, PPI10, and empty cases, respectively.

Experimental Analysis of the Improvement of Heat Transfer in Tube Heat Exchangers via Passive Flow Generators Using Wire Coil Inserts

Abstract Due to their high performance and low-cost demands, internally treated tube heat exchanger surfaces are one of the passive heat transfer enhancements that have caught the industry&amp;#39;s attention. At bulk temperatures of 30 &amp;#176;C, an experiment for the insertion of 1 mm and 0.5 mm wire coils with a constant pitch length of 8 mm was carried out in this study. The results on the improvement of heat transfer, including the velocity profile, Nusselt number, friction factor, and thermal enhancement efficiency, were significant. Based on a lower surface temperature recorded beyond the uncertainty value, the results demonstrated an improvement in heat transfer for smaller diameters of wire coil inserts. Interestingly, this improvement is concentrated at low Reynolds numbers, indicating that there may be a point at which an increase in wire thickness does not necessarily result in an equivalent improvement in heat transfer. For both wire thicknesses, a Nusselt number increase of up to 5 times was observed. The friction factor penalty, however, varies depending on the wire thickness, with a higher magnitude (3.2-fold increase) obtained for 1mm as opposed to a 1.8-fold increase for the lower counterpart. This distinction results in the 0.5 mm coil insert gaining better overall performance with an average of 2.2 for the thermal performance ratio, further solidifying the advantage of this technique for enhancing heat transfer in conduits.