Hasil untuk "Heat"

G. Baffou, R. Quidant

Huijuan Chen, D. Goswami, E. Stefanakos

J. Rossnagel, S. Dawkins, K. N. Tolazzi et al.

Making a teeny tiny engine Steam locomotives, cars, and the drinking bird toy all convert heat into useful work as it cycles between two reservoirs at different temperatures. Usually, the working substance where the heat-work conversion occurs is a liquid or a gas, consisting of many molecules. Roβnagel et al. have made a working substance of a single calcium ion in a tapered ion trap. A laser-cooling beam plays the part of a cold reservoir for the calcium ion, and in turn, electric field noise acts as a hot reservoir. Science, this issue p. 325 A calcium ion held in a tapered trap is used as the working substance of a tiny thermodynamic engine. Heat engines convert thermal energy into mechanical work and generally involve a large number of particles. We report the experimental realization of a single-atom heat engine. An ion is confined in a linear Paul trap with tapered geometry and driven thermally by coupling it alternately to hot and cold reservoirs. The output power of the engine is used to drive a harmonic oscillation. From direct measurements of the ion dynamics, we were able to determine the thermodynamic cycles for various temperature differences of the reservoirs. We then used these cycles to evaluate the power P and efficiency η of the engine, obtaining values up to P = 3.4 × 10–22 joules per second and η = 0.28%, consistent with analytical estimations. Our results demonstrate that thermal machines can be reduced to the limit of single atoms.

A. Hsu, C. Murphy, C. Kenyon

C. Stein, S. Stein

A. Bejan, A. Kraus

C. Kadzere, M. Murphy, N. Silanikove et al.

A. Green, P. Naghdi

Marc Alexa, Michael M. Kazhdan, Jian Sun et al.

R. Teskey, T. Wertin, I. Bauweraerts et al.

B. Orr, A. Akbarzadeh, M. Mochizuki et al.

The internal combustion engine (ICE) does not efficiently convert chemical energy into mechanical energy. A majority of this energy is dissipated as heat in the exhaust and coolant. Rather than directly improving the efficiency of the engine, efforts are being made to improve the efficiency of the engine indirectly by using a waste heat recovery system. Two promising technologies that were found to be useful for this purpose were thermoelectric generators (TEGs) and heat pipes. Both TEGs and heat pipes are solid state, passive, silent, scalable and durable. The use of heat pipes can potentially reduce the thermal resistance and pressure losses in the system as well as temperature regulation of the TEGs and increased design flexibility. TEGs do have limitations such as low temperature limits and relatively low efficiency. Heat pipes do have limitations such as maximum rates of heat transfer and temperature limits. When used in conjunction, these technologies have the potential to create a completely solid state and passive waste heat recovery system.

Jason Glaser, J. Lemery, B. Rajagopalan et al.

Chunxiang Qian, Wenxiang Du, Yudong Xie et al.

With the growing demand for large-scale infrastructure development in China—such as deep-sea, deep-underground, and urban subsurface projects—combined with the widespread use of general-purpose raw materials, there is an urgent need for more precise crack control technologies in concrete. This need stems from the imperative to reduce unnecessary material consumption and environmental impact caused by excessive safety margins. To address this, a set of governing equations that account for the mutual feedback between temperature and humidity was first proposed. A non-constant form of the diffusion coefficient was introduced, alongside latent heat terms and unsteady-state heat source terms, to establish a hygrothermal coupling model. This model was further enhanced by incorporating the effects of creep relaxation, reinforcement constraint, structural restraint, and thermal conduction characteristics of formwork, thereby forming a comprehensive multi-field coupling evaluation framework that encompasses the temperature field, moisture content field, strain field, and cracking index field. Subsequently, the proposed theoretical framework was applied to representative engineering scenarios, including large-scale concrete foundation slabs, bridge bearing platforms, large-area long-span side walls and prefabricated tunnel segments. The accuracy and reliability of the model were validated through comparisons between simulation results and field-monitored data. The results demonstrate that this method effectively overcomes the technical limitations of traditional concrete crack prediction models, particularly those relying on constant parameter assumptions and decoupled field interactions. It offers a practical and robust approach for engineering applications, providing a novel perspective for precision crack control in concrete and contributing to the broader goals of sustainability and resource efficiency.

Kotoko Kodama, Yasuhiro Hasegawa

Abstract All key thermoelectric properties of thermoelectric materials (i.e., resistivity ρ, dimensionless figure of merit zT, thermal conductivity κ TE , and Seebeck coefficient S) were measured at 300 K using time-domain impedance spectroscopy (TDIS). This method accounted for heat leakage through lead wires connected to a bismuth–telluride-based Π-shaped thermoelectric module. The values of ρ and zT, without considering heat leakage, were rapidly determined in the frequency domain using impedance measurements with alternating current and in the time domain through transient-response measurements with direct current. The thermal conductance ratio K lead /K TE , which represents the relationship between the thermal conductance of the thermoelectric material K TE and that of the lead wires causing heat leakage K lead , was also evaluated. The effective dimensionless figure of merit zT eff was estimated at various K lead /K TE ratios to assess the influence of heat leakage. At 300 K, the estimated thermoelectric parameters were ρ = 10.19 μΩm, zT = 0.8645 ± 0.0003, κ TE  = 1.259 ± 0.003 W/mK, and |S| = 192.3 ± 0.4 μV/K. The results indicate that for accurate determination of all thermoelectric parameters using TDIS, the condition K lead /K TE  > 0.1 is required when performing K lead /K TE -dependent experiments on the modules with zT = 0.8645.

Takumi Watanabe, Shiho Kurosaka, Yuriko Namatame et al.

Heat-killed <i>Lactiplantibacillus plantarum</i> SNK12 (SNK), isolated from a traditional Japanese fermented food, has been suggested to influence sleep quality, but human data on sleep improvement with heat-killed lactic acid bacteria (postbiotics) remain limited. We conducted a randomized controlled trial to test whether heat-killed SNK (≥1 × 10¹¹ cells/day for 4 weeks) improves sleep quality and alters stress-related immune and neuroendocrine biomarkers. Healthy adults received SNK or a placebo for 4 weeks. The primary outcome was the Oguri–Shirakawa–Azumi Sleep Inventory MA version (OSA-MA) factor “Sleepiness on Rising”; secondary outcomes were other OSA-MA factors and the stress-related biomarkers salivary cortisol and plasma tumor necrosis factor-α (TNF-α). Compared with placebo, SNK improved Sleepiness on Rising (<i>p</i> = 0.032) and Initiation and Maintenance of Sleep (<i>p</i> = 0.010). Salivary cortisol (<i>p</i> = 0.016) and plasma TNF-α (<i>p</i> = 0.037) were also lower with SNK, and no safety concerns emerged. These concomitant changes in subjective sleep indices and stress-related biomarkers are consistent with modulation of hypothalamic–pituitary–adrenal axis activity and inflammatory pathways along the gut–brain axis. SNK may, therefore, represent a practical postbiotic option to support sleep quality.

Odumuyiwa A. Odumosu, Hongying Li, Tianyou Wang et al.

Flow boiling in microchannel heat sinks is an efficient way to dissipate high heat flux by utilizing the large surface-to-volume ratio and high latent heat. Previous studies of boiling heat transfer in microchannels mainly consider the fluid flow in channels only, but often neglect the conjugate effects of the heat conduction in the solid wall, which becomes important for microchannels because of the comparable sizes of the flow channel and the solid wall. In the present study, the effects of conjugate heat transfer on bubble growth during flow boiling in microchannels are examined by numerical simulation. The results indicate that the bubble growth is non-uniform for different bottom wall thicknesses or different solid materials even with the same heat flux at the wall. As the bottom wall thickness increases, the bubble growth rate increases because of the heat conduction in the solid wall along the channel direction. The increased bubble size also increases the perturbation to the flow field, and enhances the thermal convection between the fluid and the wall. For different solid materials, the high-thermal-diffusivity material possesses a higher heat transfer performance because of the quick diffusion of thermal energy from the heat source to the solid-fluid interface.

Michiel van den Berg, Katie Gittins

We obtain monotonicity and convexity results for the heat content of domains in Riemannian manifolds and in Euclidean space subject to various initial temperature conditions. We introduce the notion of a strictly decreasing temperature set, and show that it is a sufficient condition to ensure monotone heat content. In addition, in Euclidean space, we construct a domain and an initial condition for which the heat content is not monotone, as well as a domain and an initial condition for which the heat content is monotone but not convex.

Tea Wick Barsten, Emilie Sunde, Øyvind Thomassen et al.

Abstract Background Accidental hypothermia is associated with increased morbidity and mortality and poses a significant challenge for both professional and volunteer rescue services in prehospital settings. This study investigated the methods and equipment available to treat patients with cold stress or accidental hypothermia before reaching hospital in Norway. Methods We surveyed 156 respondents representing 708 units from both the professional and volunteer Norwegian prehospital chain of care between 2023 and 2024. Professional services included national ground ambulances, boat ambulances, national fixed wing and helicopter air ambulance services, search and rescue helicopter services, and urban search and rescue services. Volunteer services included Norwegian People’s Aid and the Norwegian Red Cross Search and Rescue Corps. The survey queried the availability of active warming equipment, passive insulation materials, thermometers for detecting hypothermia, and preferred sites for temperature measurements. The study also investigated whether there has been a development in available equipment compared to a similar study conducted in 2013. Results The survey achieved a response rate of 70.5%. Chemical heat pads were the most frequently used type of equipment for active external warming and were the only equipment used by volunteer rescue services. All services possessed equipment for passive external warming, with duvets, space blankets and wool- or cotton blankets being the most commonly available. Thermometers for detecting hypothermia were found in 86.3% of professional rescue services and 15% of volunteer units. Almost all respondents reported consistent equipment setups year-round. Conclusion All Norwegian prehospital services, both professional and volunteer, reported having equipment available for active and passive external warming. Thermometers for detecting hypothermia were reported by all professional services. The most notable change in the equipment available to treat patients with prehospital cold stress and accidental hypothermia in Norway was the increased availability of active external rewarming equipment in 2024 compared with that in 2013.

Yajing Tian, Yuyang Wang, Shasha Yin et al.

U-tube steam generators (UTSGs) are crucial in nuclear power plants, serving as the interface between the primary and secondary coolant loops. UTSGs ensure efficient heat exchange, operational stability, and safety, directly impacting the plant’s efficiency and reliability. Existing UTSG models have fixed structures, which can only be used when certain parameters are given as model input. Such constraints hinder their ability to accommodate the diverse operating conditions, where input and output parameters can vary significantly. To address this challenge, we propose a machine learning-based method for developing a high-degree-of-freedom UTSG thermal model. The most notable feature of this approach is its capacity to flexibly interchange input and output parameters. By adopting comprehensive parameter sensitivity analysis, the most efficient method for training dataset generation is determined. Leveraging a lightweight machine learning method, the prediction accuracy for all UTSG parameters is improved to within 2.1%. The flexibility of the proposed machine learning approach ensures that the UTSG model can accommodate any type of parameter input without extensive reconfiguration of the model structure, thereby enhancing its applicability and robustness in real-world applications.

T. Rehman, H. Ali

Abstract Owing to enormously high surface area and high thermal conductivity, copper foam based heat sinks for electronic cooling are investigated in this paper. Copper foam1 with porosity 0.95 and pore density 15 pores per inch and copper foam2 with 0.97 porosity and pore density 35 pores per inch are used to investigate the performance of heat sinks filled with phase change material (PCM). Various configurations of heat sink with PCM volume fractions 0.0, 0.6, 0.7 and 0.8 are investigated under heat load of 8–24 W to figure out the optimum performance of the heat sink. Experimental results revealed that base temperature of the heat sink is reduced as the volume fraction of PCM is increased. Anyhow, discharging process is not affected significantly. Furthermore, copper foam1 (0.95 porosity) exhibited better heat transfer both in charging and discharging as compared to that of copper foam2 (0.97 porosity). Maximum temperature reduction of 9.81% was found for copper foam1/PCM at 8 W and PCM volume fraction of 0.8 when it is compared with copper foam2/PCM composite. For the same porosity, maximum reduction in base temperature was observed for 0.8 volume fraction of PCM at 16 W heat input. Finally, it is concluded that copper foam1/PCM composite impregnated with 0.8 volume fraction is an optimized configuration of heat sink.