Hasil untuk "Heat"

L. J. Lara, M. Rostagno

Simple Summary Due to the common occurrence of environmental stressors worldwide, many studies have investigated the detrimental effects of heat stress on poultry production. It has been shown that heat stress negatively affects the welfare and productivity of broilers and laying hens. However, further research is still needed to improve the knowledge of basic mechanisms associated to the negative effects of heat stress in poultry, as well as to develop effective interventions. Abstract Understanding and controlling environmental conditions is crucial to successful poultry production and welfare. Heat stress is one of the most important environmental stressors challenging poultry production worldwide. The detrimental effects of heat stress on broilers and laying hens range from reduced growth and egg production to decreased poultry and egg quality and safety. Moreover, the negative impact of heat stress on poultry welfare has recently attracted increasing public awareness and concern. Much information has been published on the effects of heat stress on productivity and immune response in poultry. However, our knowledge of basic mechanisms associated to the reported effects, as well as related to poultry behavior and welfare under heat stress conditions is in fact scarce. Intervention strategies to deal with heat stress conditions have been the focus of many published studies. Nevertheless, effectiveness of most of the interventions has been variable or inconsistent. This review focuses on the scientific evidence available on the importance and impact of heat stress in poultry production, with emphasis on broilers and laying hens.

S. Perkins, L. Alexander

M. Maldovan

D. Connolly, H. Lund, B. Mathiesen et al.

S. Kotak, J. Larkindale, Ung Lee et al.

M. S. A. Rahaman, A. Ismail, A. Mustafa

Abstract Developing carbon fiber from polyacrylonitrile (PAN) based fiber is generally subjected to three processes namely stabilization, carbonization, and graphitization under controlled conditions. The PAN fiber is first stretched and simultaneously oxidized in a temperature range of 200–300 °C. This treatment converts thermoplastic PAN to a non-plastic cyclic or a ladder compound. After oxidation, the fibers are carbonized at about 1000 °C in inert atmosphere which is usually nitrogen. Then, in order to improve the ordering and orientation of the crystallites in the direction of the fiber axis, the fiber must be heated at about 1500–3000 °C until the polymer contains 92–100%. High temperature process generally leads to higher modulus fibers which expel impurities in the chain as volatile by-products. During heating treatment, the fiber shrinks in diameter, builds the structure into a large structure and upgrades the strength by removing the initial nitrogen content of PAN precursor and the timing of nitrogen. With better-controlled condition, the strength of the fiber can achieve up to 400 GPa after this pyrolysis process.

V. Arpaci, A. Salamet, S. Kao et al.

H. Martin

Sarit K. Das, Stephen U. S. Choi, Hrishikesh E. Patel

B. S. Petukhov

Imen Belhadj Slimen, T. Najar, A. Ghram et al.

Khosrow Ebrahimi, G. Jones, A. Fleischer

T. Frölicher, J. Sarmiento, D. Paynter et al.

Xinyu Chen, C. Kang, M. O’Malley et al.

Allison M. King, T. MacRae

J. Périard, S. Racinais, M. Sawka

Jacob Velazquez, Pawel Keblinski, Jeffrey Moran

Liquid coolants containing conductive nanoparticles (nanofluids) have been widely studied over the past 30 years but have seen limited adoption in real-world cooling applications. The ability of passive nanoparticles to enhance heat transfer in liquids is fundamentally limited because the nanoparticles cannot move on their own relative to the bulk fluid, and thus generate negligible convective enhancements in heat transport. In this work, we present experimental evidence that micron-scale self-propelled particles, which convert chemical energy into autonomous motion, enhance convective heat transfer in liquids. We quantified this enhancement by measuring the convective heat transfer coefficient in a pool of suspension heated from below. The enhancements associated with self-propulsion are most pronounced at low heating powers (Rayleigh number of 10,000), in which case the heat transfer coefficient can be over 100 percent higher in the self-propelled case compared to the same particles without propulsion. This work provides a proof-of-concept demonstration that active particles can enhance heat transfer in liquids, motivating the development of "Active Heat Transfer Fluids" for various cooling applications.

Sergei D. Liazhkov

We consider unsteady ballistic heat transport in a semi-infinite Hooke chain with a free end and an arbitrary heat source. An analytical description of the evolution of the kinetic temperature is proposed in both discrete (exact) and continuum (approximate) formulations. The continualization of the discrete solution for kinetic temperature is performed through a large-time asymptotic estimate of the fundamental solution of the dynamical problem for the instantly perturbed conservative semi-infinite chain at the fronts of the incident and reflected thermal waves. By analyzing the continuum solution, we observe that any instantaneous heat supply (i.e., a heat pulse) results in the anti-localization of the reflected thermal wave. We demonstrate that sudden point heat supply leads to a transition to a non-equilibrium steady state, which, unexpectedly, may exist even in the non-dissipative case. The results of this paper are expected to provide insight into the continuum description of nanoscale heat transport.

Tom Joshi-Hartley, Matthew K. Browning, Laura K. Currie et al.

Models of astrophysical convection, such as mixing length theory, typically assume that the heat transport is independent of microphysical diffusivities. Such 'diffusion-free' behaviour is, however, not observed in numerical simulations employing standard fixed-flux or fixed-temperature boundary conditions, except possibly in extreme parameter regimes that are computationally expensive to achieve. Recent numerical and experimental work has suggested that internally heated and cooled convection can exhibit diffusion-free scalings in more numerically accessible regimes. Here, we present direct numerical simulations of 2.5D Cartesian rotating thermal convection driven by an internal heating and cooling function. The use of distributed heating and cooling functions alleviates sharp thermal boundary layers that would otherwise be present, allowing the flows to be simulated with modest computational resources. We show that for high Rossby numbers this set-up recovers mixing length theory scalings for the heat transport. The velocity amplitudes, in contrast, are observed to display diffusion-limited scalings. By comparing against boundary driven rotating convection, we show that internally heated cases have a larger fraction of their thermal dissipation occuring in the bulk of the fluid. We suggest this is connected to the increased convective efficiency observed in these cases. Our results indicate that 2.5D internally heated convection can be used as a computationally inexpensive test-bed to investigate some aspects of diffusion-free heat transport.

Vincent Beauchamps, Theo Vanneau, Cyprien Bourrilhon et al.

Objective This study aimed to examine the evolution of vigilance, sleepiness and electrophysiological markers of arousal in healthy subjects exposed to moderate hyperthermia, during habitual or restricted sleep conditions.Methods Twelve healthy males (30.4 ± 7.3 yr) completed two experimental crossover sessions in a bioclimatic chamber, consisting of sequential exposure to a thermoneutral condition (TCORE = 37.0 ± 0.2 °C) then an hyperthermic condition (TCORE = 38.3 ± 0.2 °C). Sessions followed either an habitual night of sleep (>6h time in bed, TIB) or sleep restriction (<3h TIB). A 10-minute psychomotor vigilance task (PVT) and a sleepiness scale were administered under thermoneutrality and hyperthermia conditions, immediately after recording a one-minute eyes-closed resting state electroencephalogram (EEG). This allowed for the calculation of individual alpha frequency (IAF), relative spectral powers (alpha, theta and beta bands) and theta-to-alpha ratio in frontal and parieto-occipital territories.Results Moderate hyperthermia induced an increase in PVT speed and a decrease in sleepiness. This arousal response was associated with an increase of IAF, a reduction in frontal theta power and an increase in frontal alpha power, leading to a decrease in the theta/alpha ratio. In contrast, sleep restriction induced the opposite effect on PVT performances and sleepiness, as well as for EEG parameters (without influence on IAF). No significant interaction was observed for all parameters.Conclusion Sleep restriction and moderate hyperthermia induced opposite effects in our model with limited time exposure to heat. This confirms that heat can help with arousing under certain conditions, although this needs to be confirmed by further studies.