Heating Power Density Research Articles

Simultaneous thermal camouflage and radiative cooling for ultrahigh-temperature objects using inversely designed hierarchical metamaterial

Abstract Sophisticated infrared detection technology, operating through atmospheric transmission windows (usually between 3 and 5 μm and 8–13 μm), can detect an object by capturing its emitted thermal radiation, posing a threat to the survival of targeted objects. As per Wien’s displacement law, the shift of peak wavelength towards shorter wavelengths as blackbody temperature rises, underscores the significance of the 3–5 μm range for ultra-high temperature objects (e.g., at 400 °C), emphasizing the crucial need to control this radiation for the objects’ viability. Additionally, effective heat management is essential for ensuring the consistent operation of these ultrahot entities. In this study, based on a database with high-temperature resist materials, we introduced a material-informatics-based framework aimed at achieving the inverse design of simultaneous thermal camouflage (low emittance in the 3–5 μm range) and radiative cooling (high emittance in the non-atmospheric window 5–8 μm range) tailored for ultrahigh-temperature objects. Utilizing the transfer matrix method to calculate spectral properties and employing the particle swarm optimization algorithm, two optimized multilayer structures with desired spectral characteristics are obtained. The resulted structures demonstrate effective infrared camouflage at temperatures up to 250 °C and 500 °C, achieving reductions of 86.7 % and 63.7 % in the infrared signal, respectively. At equivalent heating power densities applied to the structure and aluminum, structure 1 demonstrates a temperature reduction of 29.4 °C at 0.75 W/cm2, while structure 2 attains a temperature reduction of 57.5 °C at 1.50 W/cm2 compared to aluminum, showcasing enhanced radiative cooling effects. This approach paves the way for attenuating infrared signals from ultrahigh-temperature objects and effectively managing their thermal conditions.

Bubble Interaction and Heat Transfer Characteristics of Microchannel Flow Boiling With Single and Multiple Cavities

Abstract Flow boiling in microchannels can effectively address the challenges of high power density heat dissipation in electronic devices. However, the intricate bubble dynamics during the two-phase flow in microchannel necessitates understanding the characteristics of complex bubble hydrodynamics. In this study, we perform 2D numerical simulations of flow boiling using the Cahn-Hilliard phase-field method for a 200-µm width microchannel with single and multiple cavities in COMSOL Multiphysics (V5.3). The numerical model successfully captures bubble dynamics, encompassing vapor embryo generation, bubble growth, departure, coalescence, sliding, and stable vapor plug formation. The heat transfer mechanism inside the microchannel is dominated by bubble nucleation and thin-film evaporation. Elevated wall superheats in a single nucleation cavity, and increased mass flux facilitates higher bubble departure frequency and heat transfer performance. Temporal pressure fluctuations are observed inside microchannels in multiple cavities due to bubble coalescence, departure, and subsequent nucleation. Increasing the nucleating cavities from 2 to 5 within the microchannel while maintaining consistent cavity spacing of 100 µm has resulted in nearly 32% enhancement in heat transfer performance. This study offers valuable findings that can help improve the thermal management of electronic devices.

Predictive thermal performance analysis of T-wall based adsorption thermal battery for solar building heating

Moisture-based adsorption thermal battery (ATB) holds great potential for addressing energy storage and utilization challenges. In this work, a proof-of-concept solar harvesting building envelope using the Trombe-wall (T-wall) based ATB design is proposed and investigated, featuring a developed composite sorbent as the porous wall for effective heat storage and utilization. To demonstrate the feasibility of employing the ATB-based building envelope for day-and-night space heating, a 3-dimensional simulation model of the ATB wall is meticulously designed and comprehensively studied. Parametric analyses of various working conditions, including examining the effects of solar radiation intensity, air temperature, air humidity, and airflow velocity on the heat charging and discharging performances of the ATB wall, are conducted using the numerical model. Simulation results indicate that, under a solar irradiance level of 700 W m−2 during the daytime, an average output air temperature of 42.4 °C and an average heating power density of 2.5 kW m−3 are achieved. Extending the heat charging time to 8 h and 12 h significantly improves the desorption efficiency, which is able to reach 47.2% and 75.0%, respectively. In terms of heat discharging performances, various working conditions investigated in the ATB wall model will lead to different thermal output performances.

Highly oriented BN-based TIMs with high through-plane thermal conductivity and low compression modulus.

In the pursuit of effective thermal management for electronic devices, it is crucial to develop insulation thermal interface materials (TIMs) that exhibit exceptional through-plane thermal conductivity, low thermal resistance, and minimal compression modulus. Boron nitride (BN), given its outstanding thermal conduction and insulation properties, has garnered significant attention as a potential material for this purpose. However, previously reported BN-based composites have consistently demonstrated through-plane thermal conductivity below 10 W m-1 K-1 and high compression modulus, whilst also presenting challenges in terms of mass production. In this study, low molecular weight polydimethylsiloxane (PDMS) and large-size BN were utilized as the foundational materials. Utilizing a rolling-curing integrated apparatus, we successfully accomplished the continuous preparation of large-sized, high-adhesion BN films. Subsequent implementation of stacking, cold pressing, and vertical cutting techniques enabled the attainment of a remarkable BN-based TIM, characterized by an unprecedented through-plane thermal conductivity of up to 12.11 W m-1 K-1, remarkably low compression modulus (55 kPa), and total effective thermal resistance (0.16 °C in2 W-1, 50 Psi). During the TIMs performance evaluation, our TIMs demonstrated superior heat dissipation capabilities compared with commercial TIMs. At a heating power density of 40 W cm-2, the steady-state temperature of the ceramic heating element was found to be 7 °C lower than that of the commercial TIMs. This pioneering feat not only contributes valuable technical insights for the development of high-performance insulating TIMs but also establishes a solid foundation for widespread implementation in thermal management applications across a range of electronic devices.

Evaluation and selection of eutectic salts combined with metal foams for applications in high-temperature latent heat thermal energy storage

This study presents a systematic process for selecting eutectic salts for use in latent heat thermal energy storage (LHTES), and provides experimental evidence regarding their performance. One primary goal was to identify the most suitable eutectic salt for use with metallic foams over a temperature range of 450 °C–500 °C. Twenty one eutectic salts were preselected for comparison. The performance and cost-effectiveness of each were evaluated based on the Ragone relation, enabling a fair comparison of each salt's heat storage capacity and power density relative to cost. As a result, MgCl-NaCl, CaCl2-NaCl, and FLiNaK were selected as the most promising candidates. Subsequently, experiments were performed to demonstrate the compatibility of the selected salts with metal foams while the copper and aluminum foams were immersed in the salts at 550 °C for 120 h. Combining C10100 (copper alloy) foam with CaCl2-NaCl was observed to generate the least surface corrosion due to oxidation. The charge/discharge performance and thermal stability of CaCl2-NaCl, both with and without the copper foam, were also tested to confirm the feasibility of this combination. The capsule with the copper foam showed enhanced performance (a 13 % reduced discharge rate) in comparison to the capsule without copper foam. The melting point of the CaCl2-NaCl remained unchanged for 300-h duration of the cyclic melting/solidification experiments, and no thermal stability issues were observed.

Nanobrick Wall Multilayer Thin Films with High Dielectric Breakdown Strength.

Current thermally conductive and electrically insulating insulation systems are struggling to meet the needs of modern electronics due to increasing heat generation and power densities. Little research has focused on creating insulation systems that excel at both dissipating heat and withstanding high voltages (i.e., have both high thermal conductivity and a high breakdown strength). Herein, a polyelectrolyte-based multilayer nanocomposite is demonstrated to be a thermally conductive high-voltage insulation. Through inclusion of both boehmite and vermiculite clay, the breakdown strength of the nanocomposite was increased by ≈115%. It was also found that this unique nanocomposite has an increase in its breakdown strength, modulus, and hydrophobicity when exposed to elevated temperatures. This readily scalable insulation exhibits a remarkable combination of breakdown strength (250 kV/mm) and thermal conductivity (0.16 W m-1 K-1) for a polyelectrolyte-based nanocomposite. This dual clay insulation is a step toward meeting the needs of the next generation of high-performance insulation systems.

High quality factor infrared notch filter with compact electromagnetically induced transparency metamaterial structure

A high quality factor infrared notch filter with compact electromagnetically induced transparency (EIT) structure is demonstrated theoretically and experimentally. The design process is carried out by using flowchart and finite-difference time-domain numerical method. The numerical quality factors of the notch filter are 2634 and 2462 at incident angles of 0° and 5° respectively. Heat power density distributions of the filter are also calculated to interpret the physical mechanism. A filter sample is fabricated and measured to verify the theoretical results.

Experimental study of the edge radial electric field in different drift configurations and its role in the access to H-mode at ASDEX Upgrade

The formation of the equilibrium radial electric field (Er) has been studied experimentally at ASDEX Upgrade (AUG) in L-modes of “favorable” (ion ∇ B-drift toward primary X-point) and “unfavorable” (ion ∇ B-drift away from primary X-point) drift configurations, in view of its impact on H-mode access, which changes with drift configurations. Edge electron and ion kinetic profiles and impurity velocity and mean-field Er profiles across the separatrix are investigated, employing new and improved measurement techniques. The experimental results are compared to local neoclassical theory as well as to a simple 1D scrape-off layer (SOL) model. It is found that in L-modes of matched heating power and plasma density, the upstream SOL Er and the main ion pressure gradient in the plasma edge are the same for either drift configurations, whereas the Er well in the confined plasma is shallower in unfavorable compared to the favorable drift configuration. The contributions of toroidal and poloidal main ion flows to Er, which are inferred from local neoclassical theory and the experiment, cannot account for these observed differences. Furthermore, it is found that in the L-mode, the intrinsic toroidal edge rotation decreases with increasing collisionality and it is co-current in the banana-plateau regime for all different drift configurations at AUG. This gives rise to a possible interaction of parallel Pfirsch–Schlüter flows in the SOL with the confined plasma. Thus, the different H-mode power threshold for the two drift configurations cannot be explained in the same way at AUG as suggested by LaBombard et al. [Phys. Plasmas 12, 056111 (2005)] for Alcator C-Mod. Finally, comparisons of Er profiles in favorable and unfavorable drift configurations at the respective confinement transitions show that also the Er gradients are all different, which indirectly indicates a different type or strength of the characteristic edge turbulence in the two drift configurations.

A Hierarchically Structured Graphene/Ag Nanowires Paper as Thermal Interface Material

With the increase in heat power density in modern integrating electronics, thermal interface materials (TIM) that can efficiently fill the gaps between the heat source and heat sinks and enhance heat dissipation are urgently needed owing to their high thermal conductivity and excellent mechanical durability. Among all the emerged TIMs, graphene-based TIMs have attracted increasing attention because of the ultrahigh intrinsic thermal conductivity of graphene nanosheets. Despite extensive efforts, developing high-performance graphene-based papers with high through-plane thermal conductivity remains challenging despite their high in-plane thermal conductivity. In this study, a novel strategy for enhancing the through-plane thermal conductivity of graphene papers by in situ depositing AgNWs on graphene sheets (IGAP) was proposed, which could boost the through-plane thermal conductivity of the graphene paper up to 7.48 W m-1 K-1 under packaging conditions. In the TIM performance test under actual and simulated operating conditions, our IGAP exhibits strongly enhanced heat dissipation performance compared to the commercial thermal pads. We envision that our IGAP as a TIM has great potential for boosting the development of next-generation integrating circuit electronics.

Two-dimensional simple structured ultranarrow-band metamaterial perfect absorber with dielectric nanocylindrical array

A two-dimensional (2D) simple structured ultranarrow-band metamaterial perfect absorber (MPA) with nanocylindrical array is studied both theoretically and experimentally. Using a dielectric 2D cylindrical array as the top grating layer of the MPA, an ultranarrow nonpolarizing absorption peak at normal incidence is obtained. Furthermore, we use the same dielectric material to simplify the topology of the ultranarrow-band MPA, and the number of film layers in the MPA is reduced from 3 to 1. By optimizing its structural parameters and metal substrate, the highest ultranarrow nonpolarizing absorption peak that approaches 1 at normal incidence is obtained. To analyze the absorption physical mechanism, the heat power density distributions of the optimized MPA are simulated to see where the incident light is absorbed. Moreover, numerical simulation results show that the main transverse-electric absorption peak of the optimized MPA is insensitive to the incident angle, and the main transverse magnetic absorption peak is sensitive to the incident angle. Finally, we fabricate an MPA sample with dielectric nanocylindrical array and measure its absorption spectra to make a further comparison. The experimental data are consistent with the theoretical ones. This method might be helpful to reduce the manufacture difficulty and cost of the ultranarrow-band MPAs and also to promote the development of applications of the ultranarrow-band MPAs.

Simple structured ultranarrow‐band metamaterial perfect absorber with dielectric‐dielectric‐metal configuration

AbstractA simple structured ultranarrow‐band metamaterial perfect absorber (MPA) with dielectric‐dielectric‐metal configuration is studied both theoretically and experimentally. By optimizing the dielectric connectivity layer and metal substrate, the optimized simple structured ultranarrow‐band MPA is obtained. Theoretical simulation results show that the MPA exhibits single ultranarrow absorption peak at normal incidence and double ultranarrow absorption peaks at oblique incidence. The heat power density distributions of the optimized MPA are also calculated to depict the absorption mechanism. At last, we fabricate a MPA sample and measure its absorption spectra to make a further comparison. The experimental data are consistent with the theoretical ones. This novel method might be helpful to reduce the manufacture difficulty and cost of the ultranarrow‐band MPAs, and also to promote the development of applications of the ultranarrow‐band MPAs.

Post treatment of cold sprayed coatings using high-energy infrared radiation: First comprehensive study on structure-property correlation

It is a well-known fact that cold sprayed coatings need post treatment to achieve properties close to their bulk counterparts even when the coatings are obtained using high pressure cold spray. Most often post treatments are conventional furnace heat treatments primarily to relieve stresses and improve splat bonding. However, this methodology cannot be used all the time given the dimensional constraints of a furnace. Use of in tandem heating or mobile heating tools can greatly enhance the commercial potential of cold spray provided the heating tools are cheaper and easily available. In the present study, one such heating tool viz., high energy infrared emitters are used to post treat cold sprayed copper based coatings. A comprehensive study exploring the influence of heating power density and exposure duration on coating properties and microstructure has been taken up in the present work. Furnace heat treatment studies and post treatment of CuAl alloy coatings has also been studied for comparison. Pure Cu coatings respond exceedingly well to the mobile post treatment in terms of amelioration in coating microstructure and properties comparable to and better than furnace. An analysis of the contribution from different microstructural features to the coating properties especially electrical conductivity have been made. In contrast, CuAl alloy coatings respond less favourably to both infrared and furnace post treatments. A discussion towards this anomalous behavior is also provided with the help of the microstructural observations.

Analysis on ignition kernel formation in a quiescent mixture: Different characteristic time scales and critical heating powers

Flame ignition is one of the most fundamental and ubiquitous problems in the field of combustion. Flame ignition in a quiescent flammable mixture consists of two phases: ignition kernel formation and subsequent transition to self-sustained expanding spherical flame. Most of previous studies focused on the second phase while little attention was paid to the first phase. In this work, theoretical analysis is conducted for ignition kernel formation induced by external heating within finite domain and finite duration. The ignition kernel formation consists of three stages, i.e., onset of thermal runaway, generation of reaction front at the center, and arrival of reaction front at the edge of heating domain. The characteristic time scales for these three stages are evaluated. Good agreement between theoretical analysis and transient simulation has been obtained. The delay times for the generation of reaction front at the center and its propagation to the edge of heating domain decrease rapidly with the external heating power density; and the delay time for thermal runaway is the primary component in the total time for ignition kernel formation. Moreover, a minimum power density for external heating below which thermal runaway cannot occur is determined. Since chemical reaction is self-sustained by releasing heat, two additional threshold values of external heating power densities are determined respectively corresponding to spontaneous generation of reaction front at the center and its subsequent propagation across the edge of heating domain. This study provides useful insights into ignition kernel formation occurring in premixed flame ignition.

Influence of Electrical Heating Metal Mesh and Power Density on Resistance Welding of Carbon Fiber/PEEK Composite.

An experimental investigation on the resistance welding of carbon-fiber-reinforced polyetheretherketone (PEEK) composite laminate using three types of stainless steel (SS) meshes with different sizes and electrical resistances as heating elements is reported. The objective of this study is to determine the influence of the metal mesh on the welding process and performance at different power densities ranging from 29 to 82 kW/m2. Resistance welding equipment is used to monitor the temperature and displacement along the thickness of the laminate. The results show that the power density determines the welding time and heat concentration. A large power density results in a short welding time, but also increases the temperature gradient at the joining interface (almost 50 °C) and causes an obvious deformation of a contraction of more than 0.1 mm along the thickness of the laminate. A SS mesh with low resistance has a strong welding capability, i.e., a high welding efficiency under low power density. A lap shear strength of approximately 35 MPa can be obtained with the appropriate power density. The shear strength is affected by the bonding between the metal mesh and polymer, the metal mesh load bearing, and the metal mesh size.

Propeller-integrated airfoil heater system for small multirotor drones in icing environments: Anti-icing feasibility study

Atmospheric icing is one of the most common and hazardous environmental challenges for aircraft operations. So far, only large passenger aircraft, a few general aviation aircraft, and specialized rotorcraft are equipped with ice protection systems because of their additional energy consumption and weight. The emerging markets of small electric aircraft like Urban Air Mobility and Unmanned Aerial Vehicles for transportation, logistics, search and rescue, and other specialized operations will require efficient, lightweight and all-electric ice protection systems in order to allow operations in ice and snow conditions. In this work, an electrothermal ice protection system with an average heat power density of 1Wcm−2 was designed and developed for use on small multirotor drones. Commercially available propeller blades with diameters of 33cm were retrofitted with lightweight multilayer heating foils independently powered by super-capacitors. The heated propellers were tested in close-to-real icing conditions in a small-scale icing wind tunnel. Thrust, rotational speed, and power consumption of a non-heated reference propeller and a heated propeller were compared. In glaze ice conditions at air temperatures > −5°C the thrust of the non-heated propeller fluctuated between 60 and 90% with frequent cycles of ice accretion and shedding for the entire duration of the test. The heated propeller maintained a thrust of >95% in the same icing conditions. In rime ice conditions at air temperatures < −8°C the heating was not sufficient and the thrust of the non-heated and heated propellers similarly decreased over time. Heat flux calculations along the span of the propeller were made for different icing conditions and compared to the heat power density distribution of the tested ice protection system.

Comprehensive investigations on printability and thermal performance of cementitious material incorporated with PCM under various conditions

Energy consumed in building and construction accounts for 40% of the total end-use energy. To achieve the energy reduction in this area, conventional construction approaches need to be reshaped. To overcome existing challenges, this study explores the use of 3D cementitious material printing (3DCMP) and investigates the printability of these cementitious materials incorporated with the phase change material (PCM). To realize the newly developed composite cementitious materials for real-world applications, we examinate the benefits of utilizing them for building’s energy consumption reduction. From our study, we demonstrate, for the first time, the successful fabrication of the PCM composite wall by 3DCMP method. The enthalpy porosity method is then proposed to study the time-dependent thermal performance of the composite wall by modelling the encapsulated PCM and cementitious material as porous and medium, respectively. Our results show the proposed model is reliable in predicting the PCM composite wall thermal performance and demonstrate the composite wall has the potential of smoothing and reducing energy consumption by the building. From our investigation, it is determined that the PCM melting temperature should be chosen based on heating time and heat power density. Additionally, the total cost of precasting a wall by conventional methods with a dimension of 4 m × 0.12 m × 2.8 m (L × W × H) is 28.4% higher than the printing counterpart. Furthermore, the performance enhancements resulted in approximately 30% savings in building energy consumption during the day using plain walls infused with 3% PCM. This study not only demonstrates the potential of fabricating enhanced building materials through the utilization of additive manufacturing technique, but it also provides the guidelines to design PCM composite walls under various operating conditions.

Impact of liquid coolant subcooling on boiling heat transfer and dryout in heat-generating porous media

The present article discusses the impact of liquid coolant subcooling on multiphase fluid flow and boiling heat transfer in porous media with internal heat generation. Although extremely relevant and important, only limited studies are available in the open literature on the effects of coolant subcooling in heat-generating porous media and hence, this requires a detailed analysis. The present analysis is carried out using a validated computational model of multiphase fluid flow through clear fluid and porous media considering the relevant fluid transport, heat transfer and mass transfer phenomena. Results suggest that the qualitative nature of boiling heat transfer from the heat-generating porous body, with subcooled coolants, remain similar to that observed with a saturated coolant. Quantitative differences are, however, observed as a result of solid–liquid convective heat transfer, and competing mechanisms of boiling and condensation heat transfer, when subcooled liquid coolant is present. It is observed that a substantially larger heating power density is required - as coolant subcooling is increased - for achieving the same temperature rise in the porous body. Dryout power density at different coolant subcooling is also observed to follow a similar trend. The thermal energy removal limit is, hence, observed to be substantially enhanced in presence of subcooled coolants. Further, the dryout region within the porous body is observed to gradually shift towards its interior as the coolant subcooling is increased.

Rotation-induced significant modulation of near-field radiative heat transfer between hyperbolic nanoparticles

Modulation of near-field radiative heat transfer (NFRHT) with rotated anisotropic hBN and α-MoO3 nanoparticles (NPs) are investigated. The spectral heat power, total heat power and electric field energy density are calculated at different particle orientations. Numerical results show that the modulation factor of the α-MoO3 NPs could be up to ∼12,000 with particle radius of 40 nm at a gap distance of 200 nm, which is ∼ 10-fold larger than the state of the art. Excitation of localized hyperbolic phonon polaritons (LHPPs) in different particle orientations allow for the excellent modulation. The modulation factor of the hBN NPs is 5.9 due to the insensitivity of the LHPPs in the type II hyperbolic band to the particle orientations. This work might be helpful in designing a high performance near-field radiative modulator in particle systems.

Plasmonic nanoscale temperature shaping on a single titanium nitride nanostructure.

Arbitrary shaping of temperature fields at the nanometre scale is an important goal in nanotechnology; however, this is challenging because of the diffusive nature of heat transfer. In the present work, we numerically demonstrated that spatial shaping of nanoscale temperature fields can be achieved by plasmonic heating of a single titanium nitride (TiN) nanostructure. A key feature of TiN is its low thermal conductivity (kTiN = 29 [W m-1 K-1]) compared with ordinary plasmonic metals such as Au (kAu = 314 [W m-1 K-1]). When the localised surface plasmon resonance of a metal nanostructure is excited, the light intensity is converted to heat power density in the nanostructure via the Joule heating effect. For a gold nanoparticle, non-uniform spatial distributions of the heat power density will disappear because of the high thermal conductivity of Au; the nanoparticle surface will be entirely isothermal. In contrast, the spatial distributions of the heat power density can be clearly transcribed into temperature fields on a TiN nanostructure because the heat dissipation is suppressed. In fact, we revealed that highly localised temperature distributions can be selectively controlled around the TiN nanostructure at a spatial resolution of several tens of nanometres depending on the excitation wavelength. The present results indicate that arbitrary temperature shaping at the nanometre scale can be achieved by designing the heat power density in TiN nanostructures for plasmonic heating, leading to unconventional thermofluidics and thermal chemical biology.

Thermophysical and mechanical properties of active membranes for photoacoustic generators of forced acoustic oscillations

Thermophysical and mechanical properties of active membranes for photoacoustic generators of forced acoustic oscillations are studied. Dependencies of photoacoustic generators output characteristics versus active membranes for forced acoustic oscillations are found out for different heat power densities within the structures studied. Keywords: Photoacoustic generator, forced acoustic oscillations, linear oscillations, full width at half maximum, active membrane.