Direct Heat Transfer Research Articles

Copper fiber wick with scaly fins fabricated by multi-tooth cutting for directional heat transfer

Wicks with directional liquid transport properties are the decisive factor in fabricating high-performance directional heat transfer devices. Aiming at the lack of wicks that can maintain directional liquid transport performance after high-temperature processes, sintered copper fiber wicks (SCFWs) with scaly fins were prepared. The copper fibers with scaly fins were fabricated by multi-tooth cutting and the additional driving force of scaly fins on ethanol was used to achieve directional capillary flow. The effects of scaly fin direction, sintering parameters, and porosity on the capillary performance of SCFWs were studied. The results show that the direction of scaly fins can adjust the liquid flow direction. The capillary performance parameter of SCFW3, with the same fin direction as the flow direction of ethanol, is 65.5% higher than that of SCFW1 with the opposite fin direction. Sintering temperature and holding time affect the formation of sintered necks and micropores, which affects the capillary performances of SCFWs. The effects of sintered necks on the capillary performances of SCFWs are more obvious than micropores on copper fibers. In addition, the relatively low porosity in the range of 70–80% helps improve the capillary performance of SCFWs. The SCFWs provide a feasible method for the fabrication of phase change heat transfer devices with directional heat transfer performances.

Flexible, multimodal device for measurement of body temperature, core temperature, thermal conductivity and water content

Body core temperature is an important physiological indicator for self-health management and medical diagnosis. However, existing devices always fails to achieve continuous monitoring of core body temperature due to their invasive or motion-restricted measurement principles. Here, a wearable flexible device which can continuously monitor the core body temperature was developed. The flexible device integrated with fourteen temperature sensors and one thermal conductivity sensor on the polydimethylsiloxane substrate can be conformally attached to the human skin. With the wearable data processing module and wireless communication module, the continuous monitoring of the core body temperature for 24 h and the portable monitoring of the skin thermal conductivity were realized using this device. Owing to the annular distribution design of the temperature sensor and the directional heat transfer design of the thermal conductivity sensor, this device is comparable in accuracy and stability compared to standard instruments that require invasive or motion-restricted measurements.

The optimization of phase change heat transfer model for direct contact condensation heat transfer simulation based on Bayesian inversion method

Direct contact condensation heat transfer between steam and water is widely applied across various industries. The utilization of phase change models significantly influences the precision of numerical simulation outcomes in this domain. This study presents numerical simulations of direct contact condensation between steam and liquid films generated by nozzles. Employing Bayesian inversion method, this research conducts optimizations on the Lee and Schrage models and examines the impact of varying treatments of interface area density. Additionally, it proposes enhancements to these methodologies. The findings of this study are expected to foster a more profound comprehension of phase change models and their role in numerical simulations.

Thermal-Hydraulic Network Model for steady-state thermal analysis and design of power transformer magnetic cores

The safe operation of power transformers is related to their internal temperatures, which increase due to the heat generated in their active part. The core is subjected to hysteresis, eddy current and stray losses, which lead to its heating and the temperature rise. This paper presents a TH model for determining temperatures along the magnetic cores of power transformers. The inclusion of the transformer’s magnetic core in thermal modeling is of paramount importance due to its significant impact on the overall thermal performance and reliability of the transformer. Traditional thermal models predominantly focus on the oil flow and windings, overlooking the core’s contribution to heat generation and dissipation. Based on the constructive and operational data, the model determines the complete oil flow and temperature distributions over the columns, yokes, and joints by iteratively solving an algorithm composed of hydraulic and thermal resistance networks. The constructive characteristics, as the stacking sheets forming different steps and the anisotropy for heat transfer in different directions, are considered. The analytical results are compared to experimental measurement using an optical fiber sensor allocated in the core of a prototype transformer, indicating that the model can satisfactorily predict the hydraulic and thermal behaviors of magnetic cores.

Clinical Efficacy and Safety of a Thermomechanical Fractional Injury Device for Neck Rejuvenation.

Neck rejuvenation has consistently become a popular cosmetic procedure. While various treatment modalities have been used, a novel fractional thermomechanical skin rejuvenation system was recently developed to create dermal coagulation through direct heat transfer with subsequent neocollagenesis. A prospective clinical trial evaluated the efficacy and safety of a thermomechanical fractional injury device (Tixel 2, Novoxel, Netanya, Israel) for neck rhytides. Subjects with moderate to severe neck rhytides were enrolled for 4 monthly treatments. Twenty-six subjects were enrolled and completed all study visits. The mean age was 58.4 years, and 100.0% were women. Fitzpatrick skin types I to IV were included. For Fitzpatrick Wrinkle Classification System (FWCS), the mean baseline score was 6.3. As per investigator, there was a mean 1.5-grade improvement in FWCS at 1-month follow-up (p < .00001) and 1.4-grade improvement in FWCS at 3-month follow-up (p < .00001). For physician Global Aesthetic Improvement Scale, all subjects (100%) had improvement at both 1- and 3-month follow-up visits. There were no severe adverse events, and subjects experienced minimal pain. A novel thermomechanical fractional injury device is effective and safe for the treatment of neck rhytides.

Simulating film boiling with sharp interface and direct calculation of mass transfer to investigate the hydrodynamic and thermal transport phenomena near the interface

The current study presents a computational fluid dynamics (CFD) model for film boiling using two fluids, including liquid nitrogen. Ansys Fluent was customized using user-defined functions (UDFs) to accurately model thermal and fluid dynamic interactions. The UDFs accommodate mass transfer at the liquid-vapor interface, maintaining a sharp interface by accounting for the saturation temperature. Direct heat and mass transfer calculations are implemented rather than relying on empirical correlations. Mesh analysis was performed with grid sizes of 615, 410, 307, and 123 μm, with the 307 μm grid selected for its agreement with average Nusselt number values and accurate bubble shape and height. Verification using the Stefan problem showed a maximum error of 0.45 % for interface displacement and 0.1 % for temperature distribution. Validation against the Berenson correlation demonstrated good agreement in Nusselt number values. Qualitative validation of the developed method is performed using the shapes of the growing bubble. The results obtained show that the sharp interface is preserved throughout the simulation, with temperature contours showing significant variations during bubble growth and departure. Local heat transfer coefficient analysis identified peak values of 2170 W/m²-K at locations of minimal film thickness. Velocity vector analysis revealed velocities of 2 m/s in the vapor phase, influencing bubble shape during growth and departure. The study highlights the impact of thermal film thickness on heat transfer, with the highest Nusselt number of 5.69 observed at a film thickness of 0.5 mm. The results demonstrate the application of VOF sharp interface tracking and direct calculation of interfacial conditions utilizing customized Ansys Fluent in the modeling of film boiling as a way to understand the fundamental aspects of the fluid and thermal behavior.

A new reactor with porous baffle for thermochemical heat storage: Design and performance analysis

The Ca(OH)2/CaO reaction system presents broad development prospects for thermochemical heat storage. However, the poor thermal conductivity of heat storage material and the complex internal flow limit the performance of the reactor. To improve the performance of the reactor, this work designs a new improved fixed bed reactor with baffle. The comparison with and without baffle, as well as the effects of operation conditions and structural parameters of the improved reactor on heat storage and release performance, are numerically investigated. The results show that the improved reactor can enhance heat transfer and flow inside the reactor, resulting in performance improvement. After baffle installation, there is a 64% decrease in the pressure drop, a 29.14% reduction in the heat storage time, and a 35.45% reduction in the heat release time. Besides that, the improved reactor can maintain the high temperature for longer at the outlet during heat release. The orthogonal test design analysis reveals that the inlet velocity of direct heat transfer fluid has the greatest impact on heat storage, while the inlet velocity of indirect heat transfer fluid has the smallest impact, and the same applies to heat release. The thermal conductivity and permeability of the porous baffle have minor impacts, but the air distribution chamber has a noticeable influence. This work can guide the optimization design and operation adjustment of the reactor.

6E analysis and particle swarm optimization of a novel ultra-high temperature solar cogeneration system fusing thermochemical energy storage and multistage direct heat transfer

The reshaping of the energy development model depends on the penetration of renewable energy on the input side and the substitution of electrical energy on the supply side. Therefore, this research proposes a novel solar cogeneration system in which the Brayton cycle possesses triple pressure and can be flexibly coupled directly to each subsystem under supercriticality without intermediate exchangers. Firstly, a series of research tasks are executed sequentially, including model establishment, cycle potential comparison, operating parameter analysis, and multi-objective particle swarm optimization. Secondly, the exergy flow rate, consumption rate, and 6E (energy, exergy, exergoeconomic, exergoenvironmental, emergoeconomic, and emergoenvironmental) performances of each subsystem and subcomponent are thoroughly discussed. Lastly, the operating performance, economic benefits, and regional characteristics of each subsystem and the cogeneration system are explored. After optimization, the exergy efficiency, average advanced exergy-emergy factor, and advanced exergy-emergy impact difference of the system are 22.72 %, 56.17 %, and 23.97 %. Compared with similar research, the energy efficiency of the system is improved by 8.2 %, which can repay the capital cost in 8.83 years, and the economic benefit reaches $5,503,086 for the entire service year. The system embodies excellent 6E performance, aligns with the goals of reshaping the energy development model, and provides guidelines for the development of ultra-high temperature solar cogeneration.

The copper matrix composite with "sandwich" structure and three-dimensional networks, showcasing significantly improved thermal conducting performance

Copper matrix composites can be used to address the issue of heat dissipation in electronic devices. Nevertheless, the thermal conductivity (TC) of carbon fiber (CF)/copper (Cu) composites falls significantly below expectations. This is primarily attributed to the challenge of achieving a well-ordered arrangement of CF within the matrix, coupled with the issue of low interfacial bonding strength. A highly effective and uncomplicated approach to fabricate copper matrix composites featuring a "sandwich structure" of Cu-CF-Cu is introduced in this paper. This configuration, achieved through structural design facilitated by hot-press sintering and vacuum suction filtration, exhibited a three-dimensional graphene-like networks (3D-GLNs) and meticulously ordered CF layers over long distances. More importantly, in comparison to pure copper (350 W m−1 K−1), the laminated composite with 40 vol% CF of the CF layer had an in-plane TC of 510 W m−1 K−1, which was approximately 1.5 times greater than that of pure copper. Additionally, the coefficient of thermal expansion (CTE) of the laminated composite with 45 vol% was 9.98 ppm/K. The in-plane TC of the composite was comparable to that of the graphene/Cu composites with the same content. This underscores the advantages of the structural design, even in cases where the TC of the reinforced phase differs by a factor of two. The aforementioned findings demonstrate that this methodology for developing a unique "sandwich" structure can effectively enhance in-plane TC in CF/metal composites, presenting potential for utilization in electronic packaging requiring efficient directional heat transfer.

Vertically aligned montmorillonite aerogel–encapsulated polyethylene glycol with directional heat transfer paths for efficient solar thermal energy harvesting and storage

Vertically aligned montmorillonite aerogel–encapsulated polyethylene glycol with directional heat transfer paths for efficient solar thermal energy harvesting and storage

Thermo-economic optimization of an innovative integration of thermal energy storage and supercritical CO2 cycle using artificial intelligence techniques

This research delves into the integration of Thermal Energy Storage (TES) and Supercritical Carbon Dioxide (s-CO2) in an innovative Energy Recycling System (ERS) that aims to improve overall system efficiency. The combination of TES and s-CO2 is a promising solution to address modern energy challenges and promote a sustainable and efficient energy future. For this research, the TES system is based on a packed bed of stones (PBS). This approach has several benefits, such as the use of inexpensive and readily available materials for storage, a broad range of operating temperatures, allowing for direct heat transfer, and the ability to achieve high maximum temperatures. The simulation methodology for TES is based on Schumann's approach, and energy and exergy analysis are used for thermodynamic modeling. Additionally, levelized costs are employed for the economic modeling of the system. Through rigorous thermos-economic modeling and simulation, operational parameters are optimized using genetic algorithms and neural network techniques to balance thermodynamic efficiency, energy storage, and economic feasibility. The findings reveal exceptional energy and exergy efficiencies, exceeding expectations, and suggest the ERS, with grid-scale energy storage, as a cost-effective solution. The simulations demonstrate remarkable energy and exergy efficiencies of 92.15% and 49.66%, respectively, with levelized costs of 104.69 €/MWh for heat, 135.20 €/MWh for electricity, and 65.7€/MWh for storage.

Experimental optimization of butterfly-shaped film cooling hole configuration with a 30 degree injection angle

Film cooling is one of the external cooling techniques that is widely applied to the hot-gas path components of a gas turbine, forming a thin film of a coolant on the surface to prevent the direct convective heat transfer between combustion gas and components. The film cooling effectiveness (FCE) is significantly influenced by the geometry of the injection hole. Therefore, it is important to adjust the shape parameters in order to derive the optimal hole configuration under given flow conditions. In a previous study, a butterfly-shaped film cooling hole configuration (BSHC), which combines the advantages of fan-shaped holes and anti-vortex holes, was proposed. In this study, an investigation into the optimal shape design of the BSHC was performed using the design of experiments. The forward expansion angle, lateral expansion angle, and twisted angle were defined as design variables. Before the optimization, the effect of each design variable on local and averaged FCE was analysed. Generally, a high FCE could be obtained when the forward expansion angle was small within a range of 7⁰ - 15⁰. However, at high blowing ratios, high effectiveness was also observed even with a large forward expansion angle. Both the lateral expansion angle and the twisted angle had high FCE near the centre point, 11⁰ and 20⁰, respectively. The response surface of the overall averaged FCE was predicted using the Kriging model. The optimized BSHC under a density ratio of 2.0 and blowing ratio of 2.0 was determined and named OPT BSHC. The OPT BSHC showed 8.6 % and 10.5 % higher FCE respectively, compared to two types of the optimized fan-shaped holes that were optimized under the same conditions.

Self-sustained and Insulated Radiative/Evaporative Cooler for Daytime Subambient Passive Cooling.

Passive cooling technologies are one of the promising solutions to the global energy crisis due to no consumption of fossil fuels during operation. However, the existing radiative and evaporative coolers still have problems achieving daytime subambient cooling while maintaining evaporation over the long term. Here, we propose a self-sustained and insulated radiative/evaporative cooler (SIREC), which consists of a porous polyethylene film (P-PE) at the top, an air layer in the middle, and poly(vinyl alcohol) hydrogel with lithium bromide (PLH) at the bottom. In particular, the P-PE shows high solar reflectance (R̅solar = 0.91) and long-wave infrared transmittance (τ̅LWIR = 0.92), which reflects sunlight while enhancing the direct radiative heat transfer between outer space and PLH (ε̅LWIR = 0.96) for sky radiative cooling. In addition, the desirable vapor permeability (579 s m-1) of the P-PE also results in good compatibility with PLH for evaporative cooling (EC). Moreover, the PLH's ability to harvest atmospheric water at night provides self-sustainment for daytime EC. The air layer between P-PE and PLH further enhances the subambient cooling performance of the SIREC. These findings indicate promising prospects for the integration of passive cooling technologies.

A Comparative Radiative Property Evaluation of Sintered Bauxite and AMS4003 Ceramic Particles

The radiative properties of sintered bauxite (ACCUCAST ID80) and AMS4003 particles were measured and compared to assess their performances as direct absorption and heat transfer media for particle-based concentrated solar power (CSP) plants. Reflectance measurements were performed over the spectral range 0.2–20 µm and used to calculate solar and thermal emission properties of the particles. In addition, reflectance was measured as a function of temperature up to 1000 °C for the spectral range 1–20 µm. The solar absorptance of AMS4003 was greater than that of ACCUCAST both before and after thermal cycling of the materials at 1000 °C, although thermal cycling was found to substantially decrease the solar absorptance of both materials. The thermal emittance of AMS4003 was also greater than that of ACCUCAST at all temperatures tested. Finally, the radiaitve properties measured in this study were used to estimate absorber efficiencies as a way to compare the particles in terms of their performance in a solar receiver. AMS4003 yielded greater efficiencies than ACCUCAST, suggesting its potential as a material for CSP.

Experimental and computational investigation of heat transfer during quenching of semi-solid aluminum plates under hot cracking condition

The present work attempts to study the cooling characteristics of 7050 aluminium alloy under semi-solid conditions during the jet impingement quenching. An aluminium plate heated to 485℃ (10% liquid fraction) is prone to hot cracks, causing large fluctuations in the temperature data. The 2-D direct heat transfer problem is formulated with the finite element technique, and moving boundary conditions are applied to simulate the quenching process. The heat flux is estimated from the boiling curve parameters (Qmax, TDNB, TLeid) by matching the simulated and experimental temperature data. The cracks are incorporated as the interface elements in the finite element domain along with the phase change model to simulatethe heat transfer phenomena. The influence of phase change and hot cracks on heat transfer is studied in detail with static and dynamic cracking conditions. The parity plot reveals that experimental and simulation results are in good agreement for uncracked plates, with an error range of ± 3% and R2 = 0.9987. Phase change incorporation improves the simulation results for cracked plates, reducing the error range from ± 10% (without phase change) to ± 5% (with phase change) along with R2 values of 0.9929 and 0.9863, respectively. The Boiling curve parameters for the cracked plate are found to be Qmax: 7.2 MW/m2, TDNB: 190℃, and TLeid: 450℃. Further, the findings show that the crack originates in the impingement zone of the quenched side of the plate.

Oriented bouncing of droplets with a small Weber number on inclined one-dimensional nanoforests.

Asymmetric superhydrophobic structures with anisotropic wettability can achieve directional bouncing of droplets and thus can have applications in directional self-cleaning, liquid transportation, and heat transfer. To achieve convenient large-scale preparation of asymmetric superhydrophobic surfaces, inclined nanoforests are prepared in this work using a technique of competitive ablation polymerization, which allows the control of the inclined angles, diameters, and heights of the nanostructures. In this study, such asymmetric structures with the smallest dimension (230 nm diameter) known are achieved by a simple etching method to guide droplet unidirectional bouncing. With such nanoforests, the mechanism of droplet bouncing on their surface is investigated, and controllable droplet bouncing over a long distance is achieved using droplets with a low Weber number. The proposed structure has a promising future in directional self-cleaning, liquid transportation and heat transfer.

Thermal Modeling With Surrogate Model-Based Optimization of Direct Oil Cooling Heat Transfer Coefficient for HEV Motor

Thermal Modeling With Surrogate Model-Based Optimization of Direct Oil Cooling Heat Transfer Coefficient for HEV Motor

Experimental set-up for the validation of phase change models in case of direct and inverse heat transfer problems

Experimental set-up for the validation of phase change models in case of direct and inverse heat transfer problems

Regulating nanoscale directional heat transfer with Janus nanoparticles.

Janus nanoparticles (JNPs) with heterogeneous compositions or interfacial properties can exhibit directional heating upon external excitation with optical or magnetic energy. This directional heating may be harnessed for new nanotechnology and biomedical applications. However, it remains unclear how the JNP properties (size, interface) and laser excitation method (pulsed vs. continuous) regulate the directional heating. Here, we developed a numerical framework to analyze the asymmetric thermal transport in JNP heating under photothermal stimulation. We found that JNP-induced temperature contrast, defined as the ratio of temperature increase on the opposite sides in the surrounding medium, is highest for smaller JNPs and when a low thermal resistance coating covers a minor fraction of JNP surface. Notably, we discovered up to 20-fold enhancement of the temperature contrast based on thermal confinement under pulsed heating compared with continuous heating. This work brings new insights to maximize the asymmetric thermal responses for JNP heating.

Comprehensive evaluation of thermally conductive functional layer on snow-melting performance for electric heating bridge system

The most commonly used chemical salt agents for melting ice and snow can lead to the corrosion of the steel structure of the bridge deck, as well as potential pollution of nearby rivers and soil through infiltration and surface runoff. The heating bridge system with electric cable is a promising environmentally friendly technology for snow removal. However, the unique suspension and multi-layer characteristics of the bridge pavement make it challenging to efficiently transfer the heat generated by the electric heating bridge system to the pavement surface. To enhance snow-melting efficiency, a novel directional heat transfer electric heating bridge system with a thermally conductive functional layer (EHBS-TCFL) has been introduced. Initially, a series of tests, including thermal conductivity tests, compressive tests, and bending tests, were performed on TCFL with different mix ratios. Then the TCFL with the most favorable mix ratios was determined based on the fuzzy mathematics method. Subsequently, the heating and snow-melting performance of EHBS-TCFL was evaluated. The results indicated that the thermally conductive functional layer, which includes reduced iron powder with a water-cement ratio of 0.53 and an iron powder-cement ratio of 2, represents the optimal solution for achieving exceptional thermal conductivity, flexural strength, and compressive strength. Furthermore, the presence of TCFL positively impacts the enhancement and uniformity of surface temperatures, leading to a 12.7% increase in total energy utilization efficiency. This study offers significant potential for the application of snow-melting on bridge decks in cold regions.