Heat Transport Research Articles

Regression modeling of Bödewadt slip flow dynamics involving Reiner-Rivlin nanofluid based on a modified Buongiorno approach

Regression models are useful in analyzing rotational flows as they enable accurate predictions of wall shear and heat transfer coefficient. In addition, Bödewadt flow is of paramount importance in fluid dynamics of rotating systems such as turbomachinery and geophysical flows. Moreover, nanofluid’s enhanced heat transfer properties can improve cooling efficiency in applications involving turbines and electronic systems. This study delves into the Bödewadt boundary layer flow of a Reiner-Rivlin fluid containing nanoparticles over a stationary porous disk under slip conditions. The two-phase Buongiorno model is employed, incorporating temperature-dependent diffusion coefficients for enhanced accuracy. To facilitate numerical simulations, the transport equations are converted into an ordinary differential system comprising four unknowns. In the present work, a highly reliable Keller-Box methodology is adopted which agrees very well with the MATLAB built-in program ‘bvp4c’. The computed 2-D and 3-D streamlines vividly capture the Bödewadt flow scenario with Reiner-Rivlin nanofluid. The principle aim to investigate the impact of non-Newtonian behaviour and slip on the flow pattern, while also examining the behavior of temperature/concentration field for nanoparticle working fluids. As thermophoretic diffusion increases, the thermal boundary layer thickens considerably, leading to a notable decrease in the cooling rate of the disk. In contrast, Brownian diffusion has only a minimal impact on the heat transport. In addition, wall suction effect is observed to significantly boost the disk’s cooling rate, though at the expanse of increasing skin friction coefficients. This study introduces linear and quadratic regression models designed to precisely predict both the surface drag and disk cooling rate, which are crucial factors in engineering processes.

Multiscale modeling of heat and mass transfer in dry snow: influence of the condensation coefficient and comparison with experiments

Abstract. Temperature gradient metamorphism in dry snow is driven by heat and water vapor transfer through snow, which includes conduction/diffusion processes in both air and ice phases, as well as sublimation and deposition at the ice–air interface. The latter processes are driven by the condensation coefficient α, a poorly constrained parameter in the literature. In the present paper, we use an upscaling method to derive heat and mass transfer models at the snow layer scale for values of α in the range 10−10 to 1. A transition α value arises, of the order of 10−4, for typical snow microstructures (characteristic length ∼ 0.5 mm), such that the vapor transport is limited by sublimation–deposition below that value and by diffusion above it. Accordingly, different macroscopic models with specific domains of validity with respect to α values are derived. A comprehensive evaluation of the models is presented by comparison with three experimental datasets, as well as with pore-scale simulations using a simplified microstructure. The models reproduce the two main features of the experiments: the non-linear temperature profiles, with enhanced values in the center of the snow layer, and the mass transfer, with an abrupt basal mass loss. However, both features are underestimated overall by the models when compared to the experimental data. We investigate possible causes of these discrepancies and suggest potential improvements for the modeling of heat and mass transport in dry snow.

Heat and mass transfer visualisation of an exothermic acid-base microfluidic reactor using infrared thermospectroscopy

ABSTRACT Here, a microfluidic reactor that is transparent in the mid-wave infrared spectrum with an effective path length of 38 µm is developed for characterisation via operando Fourier-transform Infrared (FTIR) thermospectroscopy. This imaging technique couples an infrared (IR) camera with an FTIR spectrometer to quantify heat and mass transfer in the exothermic acid-base reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH). The absorbance fields of the reaction are used to simultaneously derive the molar concentration fields of the reactants (HCl and NaOH) and products (sodium chloride [NaCl]) at the microscale through a linear inverse method. The heat generated at the reaction interface is visualised through a thermal field captured from the measured proper emission. Concentration and thermal fields are then compared to an advection-diffusion model for validation. The results proposed in this work present a refined contactless calorimetry process for characterising the heat and mass transport in an acid-base reaction. Further refinement of this work will enable accurate and non-invasive measurements for the enthalpy of reaction in chemical reactions.

The nanoparticles aggregation aspects on the chemically reactive unsteady flow of alumina-water based nanofluid: A Keller box approach with applications of wavelet physics inspired neural networks

The present study explores the unsteady flow of a nanoliquid past a stretching cylinder with the consequence of heat source/sink and chemical reaction. Additionally, the effect of nanoparticle aggregation, convective boundary conditions, and magnetic field on the liquid flow is taken into consideration. Utilizing similarity variables, the modeled equations are transformed into dimensionless ordinary differential equations (ODEs). Further, the obtained ODEs are numerically solved by using the Keller box method. Moreover, the physics-informed neural network (PINN) is applied to analyze the flow, heat, and mass transport features. Graphical illustrations are used to display the influence of various parameters on the velocity, concentration, and temperature profiles for aggregation and without aggregation cases. As the value of the magnetic parameter increases, the temperature and concentration profile upsurge, but the reverse trend can be seen in the velocity profile. The concentration and temperature profiles rise as the unsteadiness parameter increases, but the velocity profile declines. The concentration, velocity, and temperature profiles are strengthened by an improvement in the curvature parameter value. The intensification in the values of the chemical reaction parameter declines the concentration.

Tokamak edge-SOL turbulence in H-mode conditions simulated with a global, electromagnetic, transcollisional drift-fluid model

Abstract The design of commercially feasible magnetic confinement fusion reactors strongly relies on the reduced turbulent transport in the plasma edge during operation in the high confinement mode (H-mode). We present first global turbulence simulations of the ASDEX Upgrade tokamak edge and scrape-off layer in ITER baseline H-mode conditions. Reasonable agreement with the experiment is obtained for outboard mid-plane measurements of plasma density, electron and ion temperature, as well as the radial electric field. The radial heat transport is underpredicted by roughly 1/3. These results were obtained with the GRILLIX code implementing a transcollisional, electromagnetic, global drift-fluid plasma model, coupled to diffusive neutrals. The transcollisional extensions include neoclassical corrections for the ion viscosity, as well as either a Landau-fluid or free-streaming limited model for the parallel heat conduction. Electromagnetic fluctuations are found to play a critical role in H-mode conditions. We investigate the structure of the significant E × B flow shear, finding both neoclassical components as well as zonal flows. But unlike in L-mode, geodesic acoustic modes are not observed. The turbulence mode structure is mostly that of drift-Alfvén waves. However, in the upper part of the pedestal, it is very weak and overshadowed by neoclassical transport. At the pedestal foot, on the other hand, we find instead the (electromagnetic) kinetic ballooning mode, most clearly just inside the separatrix. Our results pave the way towards predictive simulations of fusion reactors.

Flow and heat transfer insights into a chemically reactive micropolar Williamson ternary hybrid nanofluid with cross-diffusion theory

Abstract The need for efficient nanotechnology has led to unexpected developments. Conserving continuous thermal propagation is essential in many industrial and thermal systems because it improves the efficiency of thermal engineering engines and machinery. Therefore, a promising platform to increase thermal power energy is the hybridization of magnetic nanoparticles in a heat-supporting, non-Newtonian fluid. In light of the above applications, a mathematical model is established to analyze the variable fluid features of the thermally radiative and chemically reactive flow of a micropolar Williamson ternary hybrid nanofluid with electromagnetohydrodynamic and electroosmosis forces on a porous stretching surface. Stratification boundary conditions and variable fluid properties were used to analyze the thermal and solutal behavior of the fluid flow. Furthermore, to measure the disorder of the flow system, entropy generation was considered by the impact of Joule heating and viscous dissipation. To develop the numerical scheme BVP4C in MATLAB, we first converted the mathematical flow model into two ordinary differential equations using a suitable transformation. The graphical and numerical results were determined against several parameters of a ternary hybrid nanofluid ( MWCNT , A l 2 O 3 , SiC ) ({\rm{MWCNT}},\hspace{0.25em}{\rm{A}}{{\rm{l}}}_{2}{{\rm{O}}}_{3},\hspace{0.25em}{\rm{SiC}}) and unary nanofluid ( A l 2 O 3 ) ({\rm{A}}{{\rm{l}}}_{2}{{\rm{O}}}_{3}) . The results indicate that the heat transfer rate is more prominent in the ternary hybrid nanofluid than in the unary nanofluid because the addition of nanofluids to the base fluid is used to improve the heat transport rate. It can be seen from the figures that a greater estimation of the magnetic and electric field parameters improves the fluid velocity because, for low values of M ≤ 1 M\le 1 , the aiding force is dominant compared to the retarding force, which results in an improvement in the velocity profile. Furthermore, the entropy generation rate increases for higher values of the Brinkman number and temperature ratio parameter because more heat is produced due to the greater values of Br {\rm{Br}} .

Polymer‐Sorbent Direct Air Capture Contactors with Complex Geometries 3D‐Printed via Templated Phase Inversion

AbstractLow‐cost direct air capture (DAC) systems require efficient gas–solid contactors. These contactors provide rapid rates of heat and mass transport with minimal pressure drops. Triply periodic minimal surfaces (TPMS) are a class of geometries that are shown to have heat transfer properties that exceed those of other geometries, and these benefits are expected to manifest in mass transfer as well due to the analogies between heat and mass transfer. However, creating TPMS contactors is difficult using conventional manufacturing techniques such as injection molding. Non‐solvent‐induced phase separation (NIPS) of a polymeric ink containing the adsorbent is one way to construct adsorption contactors with excellent mass transport rates, but contactors have to date only been fabricated in simple geometries (e.g., films, fibers). This study demonstrates that NIPS of polymeric inks occurs within a simultaneously‐dissolving, water‐soluble, 3D‐printed template. This templated phase inversion (TPI) technique can create seemingly unlimited macroscopic architectures, including TPMS contactors, using a variety of potential DAC adsorbents, including zeolite, silica, activated carbon, and metal–organic frameworks. SEM, micro‐computed tomography, and nitrogen adsorption experiments reveal the microscopic pore structures and macroscopic geometries of the resulting sorbent contactors. TPMS DAC contactors with poly(ethyleneimine)/silica adsorbents are shown to have CO2 capture performances that rival or exceed other contactor geometries.

Impacts of Atlantic meridional overturning circulation weakening on Arctic amplification

Enhanced warming of the Arctic region relative to the rest of the globe, known as Arctic amplification, is caused by a variety of diverse factors, many of which are influenced by the Atlantic meridional overturning circulation (AMOC). Here, we quantify the role of AMOC changes in Arctic amplification throughout the twenty-first century by comparing two suites of climate model simulations under the same climate change scenario but with two different AMOC states: one with a weakened AMOC and another with a steady AMOC. We find that a weakened AMOC can reduce annual mean Arctic warming by 2 °C by the end of the century. A primary contributor to this reduction in warming is surface albedo feedback, related to a smaller sea ice loss due to AMOC slowdown. Another major contributor is the changes in ocean heat uptake. The weakened AMOC and its associated anomalous ocean heat transport divergence lead to increased ocean heat uptake and surface cooling. These two factors are inextricably linked on seasonal timescales, and their relative importance for Arctic amplification can vary by season. The weakened AMOC can also abate Arctic warming via lapse rate feedback, creating marked cooling from the surface to lower-to-mid troposphere while resulting in relatively weaker cooling in the upper troposphere. Additionally, the weakened AMOC increases the low-level cloud fraction over the North Atlantic warming hole, causing significant cooling there via shortwave (sw) cloud feedback despite the overall effect of sw cloud feedback being a slight warming of the average temperature over the Arctic.

Effect of heat production on MHD natural convection transport of nanofluid flow via a vertical uniform perforated plate: An unsteady analysis

This article statistically investigates the impact of heat production on the unstable MHD natural convection transport of nanofluid flow via a perforated sheet. The ordinary differential equations (ODEs) are derived from the partial differential equations (PDEs) by using of the similarity transformation. The dimensionless ordinary differential equations (ODEs) may be numerically resolved with the help of the MATLAB ODE45 tool and the finite difference method (FDM) along with the shooting strategy. Four innovative water-based nanofluids such as TiO2, Cu, Al2O3, and Ag are considered nanoparticles. The numerical results have been explained for the role of numerous non-dimensional numbers or parameters such as heat generation or absorption (Q), nanoparticle volume fraction (φ), Dufour number (Df), Prandtl number (Pr), magnetic force parameter (M), Soret number (Sr), and Schmidt number (Sc) on the fluid flow, and heat and mass transfer rates. The fluid temperature drops but velocity is enhanced for higher amounts of Q. Copper nanoparticle volume fraction up to 4 % shows a rise in temperature, concentration, and velocity curves. Heat transfer rate ( − θ′(0)) diminishes by about 124 %, while the values of f′(0) promote by approximately 26 % owing to an increase in the values of Q (heat generation) from 1.0 to 2.0. The value of ( − θ′(0)) increases by 49 %, but f′0 decreases by 15 % due to a rise in Q (heat absorption) from -3.0 to -10.0. The local skin friction coefficient (f′(0)) diminishes by about 65.21 % due to an increase in the values of the magnetic force parameter (M) from 0.5 to 3.5 whereas the rate of heat and mass transfer remain unchanged. As φ increased from 0.01 to 0.04, the local skin friction coefficient (f′(0)) exhibited a 36 % increase, while the heat transport rate (θ′(0)) decreased around by 10 %. In conclusion, a comparison was made between our findings and those of the published research. The comparison indicates a high degree of consistency.

Remote Forcing for a Circulation Pattern Favorable to Surface Melt over the Ross Ice Shelf

Abstract The Ross Ice Shelf (RIS) experiences surface melt events in summer, which could accelerate ice loss and destabilize the ice sheet in a warming world. This study links the interannual variability of RIS surface melt to the northerly wind anomaly over the Ross Sea sector, which is established in association with the quasigeostrophic barotropic Rossby wave trains from the tropical Pacific and subtropical Australia toward West Antarctica. Atmospheric general circulation model experiments suggest that these Rossby wave trains are regulated by El Niño–related sea surface temperature (SST) anomalies in the tropical central–eastern Pacific and atmospheric heating anomalies over western Australia. El Niño provides an important forcing of the atmospheric circulation anomalies over the Ross Sea via inducing a Rossby wave train, and most surface melt events over the RIS happen during El Niño years. In addition, the anomalous atmospheric heating over western Australia, which is independent of El Niño, is another important forcing that triggers a Rossby wave train extending from subtropical Australia to the Ross Sea. The northerly flow toward the Ross Sea induces strong poleward moisture and heat transport, which further contributes to surface melt over the RIS. Significance Statement During austral summer, surface melt occurs over the Ross Ice Shelf, accelerates ice loss, and poses ice-sheet destabilization risks in a warming world. The northerly wind anomaly over the Ross sector provides strong poleward heat and moisture transport and is favorable for the surface melt. This wind anomaly is influenced by two remote forcings, El Niño and heating anomaly over western Australia, through generating Rossby wave trains from the tropics and subtropical Australia. This study reveals a previously unexplored relationship that atmospheric heating over western Australia influences large-scale circulation, contributing to surface melt.

Optimizing the geometry of tube-in-tube Thermosyphon heat transport device from instability Perspective: A computational study

Since the Fukushima disaster, there has been a significant surge in the adoption of infinite time passive decay heat removal systems. In this context, the thermosyphon heat transport device (THTD) has distinct advantages over natural circulation loops (NCLs). Previous studies proved that the THTD experiences flow instability characterized by recirculating loops under specific geometric and operational conditions. However, identifying the critical value for these parameters remains unresolved. Therefore, the present study deals with the geometric optimization of THTD through computational (CFD) simulations.Initially, the CFD model is validated with earlier experimental data, which resulted in a minimal deviation of −2 % to 5 %. The parameters for geometric optimization of THTD are height of the system (H), the center-line elevation difference between the source and the sink (Δz), heater length (Lh), cooler length (Lc), hot leg length (Lhl), and area ratio (AR) between hot leg and cold leg, under the heater power range of 100 W to 1000 W. The impact of these geometric and operational variations on flow instability is evaluated by observing the presence of recirculation loops, which are identified by analyzing velocity vectors and contours. The key findings of the present study are: flow instability in the form of a single recirculating loop sweeping radially along the cross-section of the annulus, thereby leading to radial flow maldistribution; a center-line elevation greater than 0.8 m eliminates the flow instabilities; interchanging heating and cooling lengths for a given center-line elevation doesn’t alter the criteria for instability; Heater power of 100 W leads to instability irrespective of center-line elevation; the optimal cooling length can be considered as 5 % to 25 % of the total height of the THTD, whereas the optimal hot leg length can be considered as 25 % to 45 % of the total height; and the HTF flow rate increases with a rise in the area ratio until AR=2, after which a flow anomaly occurs, rendering the system unstable. In addition to these aspects, this study also suggests optimal geometric features of THTD to reduce instability and attain perfect unidirectional flow. By integrating findings from current research and past studies, the design methodology for conventional THTD is comprehensively presented to the scientific community.

Preferential enhancement of convective heat transfer over drag via near-wall turbulence manipulation using spanwise wall oscillations

This study investigates the manipulation of convective heat transfer through spanwise wall oscillations in a turbulent channel flow. Direct numerical simulations are performed at Reτ=180 and Pr=1.The primary focus of this work is to explore the heat transfer response to oscillation parameters that promote drag increase, a regime that has received limited attention. By adopting an extended oscillation period (T+=500) and amplitude (W+=30), which have been reported to enhance drag, a remarkable dissimilarity between momentum and heat transport emerges. Under these conditions, the convective heat transfer undergoes a substantial 15% intensification, while the drag increases by a comparatively moderate 7.7%, effectively breaking the Reynolds analogy. To elucidate the physical mechanisms responsible for this dissimilar behaviour, a comprehensive statistical analysis is conducted. The control effect on the near-wall streaks and the associated mixing of momentum and heat is investigated by examining the energy distribution across scales and wall-normal locations. This analysis provides valuable insights into the control’s impact on the turbulent structures. Furthermore, the correlation between wall-normal velocity fluctuations and both streamwise velocity and temperature fluctuations is scrutinized to understand the modification of sweep and ejection events, which drive the transport of momentum and heat. The Fukagata–Iwamoto–Kasagi (FIK) identity is employed to identify the contributing factors to the changes in drag and heat transfer. The analysis highlights the importance of the pressure term in the streamwise velocity equation and the linearity of the temperature equation. Further investigation is necessary to fully unravel the complex mechanisms governing the decoupling of heat and momentum transport. The results of this study underscore the potential of using unconventional spanwise wall oscillations parameters to preferentially enhance convective heat transfer while minimizing the associated drag penalty.

XCT images-based modeling for elucidating electrochemical inert phase-dependent multiscale electrode kinetic behaviors

The electrochemically inert phase (EIP) within the electrode is formed by the combination of carbon black and binder, characterized by a highly heterogeneous spatial distribution and structural morphology. This heterogeneity impairs electrode performance under varying discharge rates and temperatures. In this study, X-ray computed tomography is employed to image the original electrode. The resulting grayscale images are digitally processed to reveal the two-dimensional (2D) and three-dimensional (3D) features of the EIP morphology, thereby providing a design window to understand the influence of EIP morphology. The physical structure characterization indicates that, compared to bulk-like EIP (B-EIP), film-like EIP (F-EIP) tends to cover the active surface of the active material (AM), exacerbating the heterogeneity of the lithiation reactions within the electrode. The reduction in the AM active surface area correlates the state of lithiation (SOL) of particles with their active specific surface area (αa). At low temperatures, the temperature sensitivity of electrodes intensifies the risk of αa loss, promoting heterogeneous reaction transfer between particles. The Biot number (Bi) is used to evaluate the competing mechanisms between interfacial reactions and bulk diffusion within particles. In electrodes with a high EIP content, the reaction rate at the particle/electrolyte interface is accelerated, causing particle performance to be dominated by interfacial reactions. Finally, the electrochemistry and heat generation from the particle-level to the electrode-level in thicker electrodes are examined. It demonstrates that ion diffusion in the pore controls interfacial reactions and heat transport behavior. Aggregated high-flux reactions and localized hot spots exacerbate the heterogeneous degradation and side reactions within the electrode.

Can temperature be a low-cost tracer for modelling water age distributions in a karst catchment?

To investigate the feasibility of using temperature for tracking rainfall-runoff processes in karst catchments, this study developed a tracer-aided conceptual model using temperature as a tracer by coupling water and heat transport processes at the catchment scale. The model was calibrated and validated using hourly hydrometeorological and temperature data from a 1.25 km2 karst catchment in south-western China. The results showed that the model was able to capture the water flux and temperature dynamics of different landscape units in the karst catchment. Utilizing this framework, the model delineated the flux age distribution within different landscape units, as well as the overall water transit times through the catchment. The average flux ages were determined to be approximately 80 days for the hillslope unit, 452 days for the slow flow system, and 260 days for the fast flow regime within the depression areas. These estimations align broadly with those acquired using stable isotopes as tracers. Comparative analysis revealed that the flux age distributions derived from both temperature and isotopic tracers exhibited analogous patterns at the catchment outlet and across the hillslope compartments. However, the simulations based on temperature hinted at a heightened proportion of exceedingly young and decidedly old water in the outflow, alluding to a potential overestimation of these extreme age classes by the temperature-tracer model. From the temperature-simulated transit time distribution, about 31 % of the precipitation entering during the study period have left the catchment within 3 years, and a notable proportion of rain water was either stored in the aquifer or lost through evapotranspiration. The general characteristics of the transit time distribution simulated using temperature was similar with that simulated using isotopes, though a higher proportion of precipitation being drained by fast flows was inferred from the transit time distribution simulated using temperature. Collectively, our study demonstrated that temperature can serve as a cost-effective tracer for modelling of water age distributions and associated hydrological processes in karst catchments.

Heat transport in SiCOH thin films: an experimental and molecular dynamics study

Carbon-doped silicon dioxide (SiCOH) film is currently regarded as one of the most promising low-k materials in the integrated circuits (ICs) industry for advanced technology nodes. However, there have been limited studies on the thermal properties of SiCOH compared to its electrical and mechanical properties. In this study, we investigate the thermal conductivity of SiCOH thin films through molecular dynamics simulations (MD) and experimental characterizations. Our findings indicate that the size effect on thermal conductivity at 300 K is negligible when the thickness of SiCOH film is less than 20 nm. Additionally, we observe a contrasting temperature dependence law for the thermal conductivity of SiCOH thin films compared to crystal SiO2 thin films. Furthermore, we demonstrate a significant decrease in thermal conductivity with increasing porosity in SiCOH films; specifically, an increase in porosity from 5.35% to 42.77% results in a 60% reduction in thermal conductivity. Moreover, we validate our simulation results by characterizing the thermal conductivity of SiCOH using 3 ω method.

Irida-graphene phonon thermal transport via non-equilibrium molecular dynamics simulations.

Recently, a new 2D carbon allotrope called Irida-Graphene (Irida-G) was proposed, and its reliable stability has been previously predicted. Irida-G is a flat sheet topologically arranged into 3-6-8 carbon rings exhibiting metallic and non-magnetic properties. In this study, we investigated the thermal transport properties of Irida-G using classical reactive molecular dynamics simulations. The findings indicate that Irida-G has an intrinsic thermal conductivity of approximately 215 W mK-1 at room temperature, significantly lower than that of pristine graphene. This decrease is due to characteristic phonon scattering within Irida-G's porous structure. Additionally, the phonon group velocities and vibrational density of states for Irida-G were analyzed, revealing reduced average phonon group velocities compared to graphene. The thermal conductivity of Irida-G is isotropic and shows significant size effects, transitioning from ballistic to diffusive heat transport regimes as the system length increases. These results suggest that while Irida-G has lower thermal conductivity than graphene, it still holds potential for specific thermal management applications, sharing characteristics with other two-dimensional materials.

Theory and Measurement of Heat Transport in Solids: How Rigidity and Spectral Properties Govern Behavior.

Models of heat transport in solids, being based on idealized elastic collisions of gas molecules, are flawed because heat and mass diffuse independently in solids but together in gas. To better understand heat transfer, an analytical, theoretical approach is combined with data from laser flash analysis, which is the most accurate method available. Dimensional analysis of Fourier's heat equation shows that thermal diffusivity (D) depends on length-scale, which has been confirmed experimentally for metallic, semiconducting, and electrically insulating solids. A radiative diffusion model reproduces measured thermal conductivity (K = DρcP = D × density × specific heat) for thick solids from ~0 to >1200 K using idealized spectra represented by 2-4 parameters. Heat diffusion at laboratory temperatures (conduction) proceeds by absorption and re-emission of infrared light, which explains why heat flows into, through, and out of a material. Because heat added to matter performs work, thermal expansivity is proportional to ρcP/Young's modulus (i.e., rigidity or strength), which is confirmed experimentally over wide temperature ranges. Greater uptake of applied heat (e.g., cP generally increasing with T or at certain phase transitions) reduces the amount of heat that can flow through the solid, but because K = DρcP, the rate (D) must decrease to compensate. Laser flash analysis data confirm this proposal. Transport properties thus depend on heat uptake, which is controlled by the interaction of light with the material under the conditions of interest. This new finding supports a radiative diffusion mechanism for heat transport and explains behavior from ~0 K to above melting.

Hygroscopic hydrogel-based cooling system for photovoltaic panels: An experimental and numerical study

Thermal management is an essential aspect of photovoltaic (PV) system design because of the negative effects of high temperatures on the efficiency of PV panels. The use of hygroscopic hydrogels for the evaporative cooling of PV panels is an emerging technique that has attracted attention owing to the high latent heat and recyclability of these materials. However, a comprehensive understanding of the heat and mass transport mechanisms in hygroscopic hydrogels is necessary. Factors that would ensure the long-term performance of PV panel/hydrogel (PV/HG) systems under real-world conditions are still poorly understood, and the intrinsic properties of hydrogels and the effect thereof on their cooling performance have hitherto remained unexplored. Herein, we propose a mathematical model for hydrogel-cooled PV panels to study the heat and mass transfer mechanisms of PV/HG systems in practical environments. The model is used to examine the impact of the thermal conductivity, diffusion coefficient, and thickness of the hydrogel on the performance of the PV/HG system. Unlike previously reported approaches to enhance the efficiency, in this study, the external heat transfer resistance and internal mass transfer resistance were identified as the primary controlling factors. Enhancement of the thermal conductivity of the hydrogel is shown to have a limited effect on the performance, whereas improving the water diffusion within the hydrogel significantly boosts the system efficiency. Optimally, the thickness of the hydrogel must be such that it balances the heat and mass transfer distances with the available water volume. During the summer in Guangzhou, China, a 6-mm thick layer of hydrogel lowered the temperature of a PV cell by up to 7.5 °C to increase the daily power generation by 2.1%. Analysis of the nationwide situation showed that hydrogels have substantial potential for lowering the peak PV panel temperatures (by as much as 19.3 °C) to mitigate fluctuations in the output power. Our findings offer valuable insights into the design of PV/HG systems towards improving their performance.

Directional crystallization of a two-phase region with a mixed conductive–convective heat and mass transport

Directional crystallization of a two-phase region with a mixed conductive–convective heat and mass transport

An Integrated Experimental and Modeling Approach for Assessing High-Temperature Decomposition Kinetics of Explosives.

We present a new integrated experimental and modeling effort that assesses the intrinsic sensitivity of energetic materials based on their reaction rates. The High Explosive Initiation Time (HEIT) experiment has been developed to provide a rapid assessment of the high-temperature reaction kinetics for the chemical decomposition of explosive materials. This effort is supported theoretically by quantum molecular dynamics (QMD) simulations that depict how different explosives can have vastly different adiabatic induction times at the same temperature. In this work, the ranking of explosive initiation properties between the HEIT experiment and QMD simulations is identical for six different energetic materials, even though they contain a variety of functional groups. We have also determined that the Arrhenius kinetics obtained by QMD simulations for homogeneous explosions connect remarkably well with those obtained from much longer duration one-dimensional time-to-explosion (ODTX) measurements. Kinetic Monte Carlo simulations have been developed to model the coupled heat transport and chemistry of the HEIT experiment to explicitly connect the experimental results with the Arrhenius rates for homogeneous explosions. These results confirm that ignition in the HEIT experiment is heterogeneous, where reactions start at the needle wall and propagate inward at a rate controlled by the thermal diffusivity and energy release. Overall, this work provides the first cohesive experimental and first-principles modeling effort to assess reaction kinetics of explosive chemical decomposition in the subshock regime and will be useful in predictive models needed for safety assessments.