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Heat transfer

About: Heat transfer is a research topic. Over the lifetime, 181795 publications have been published within this topic receiving 2923586 citations. The topic is also known as: heat exchange.


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Journal ArticleDOI
TL;DR: In this article, an equation of phonon radiative transfer (EPRT) was developed which shows the correct limiting behavior for both purely ballistic and diffusive transport, and the solution of the EPRT for diamond thin films not only produces wall temperature jumps under ballistic transport but shows markedly different transient response from that of the Fourier law and the hyperbolic heat equation.
Abstract: Ballistic and diffusive phonon transport under small time and spatial scales are important in fast‐switching electronic devices and pulsed‐laser processing of materials. The Fourier law represents only diffusive transport and yields an infinite speed for heat waves. Although the hyperbolic heat equation involves a finite heat wave speed, it cannot model ballistic phonon transport in short spatial scales, which under steady state follows the Casimir limit of phonon radiation. An equation of phonon radiative transfer (EPRT) is developed which shows the correct limiting behavior for both purely ballistic and diffusive transport. The solution of the EPRT for diamond thin films not only produces wall temperature jumps under ballistic transport but shows markedly different transient response from that of the Fourier law and the hyperbolic heat equation even for predominantly diffusive transport. For sudden temperature rise at one film boundary, the results show that the Fourier law and the hyperbolic heat equat...

392 citations

Journal ArticleDOI
TL;DR: In this paper, liquid metal microdroplets are incorporated into a soft elastomer to achieve an unprecedented combination of metal-like thermal conductivity, an elastic compliance similar to soft biological tissue, and a unique thermal-mechanical coupling that exploits the deformability of the LM inclusions to create thermally conductive pathways in situ.
Abstract: Soft dielectric materials typically exhibit poor heat transfer properties due to the dynamics of phonon transport, which constrain thermal conductivity (k) to decrease monotonically with decreasing elastic modulus (E). This thermal-mechanical trade-off is limiting for wearable computing, soft robotics, and other emerging applications that require materials with both high thermal conductivity and low mechanical stiffness. Here, we overcome this constraint with an electrically insulating composite that exhibits an unprecedented combination of metal-like thermal conductivity, an elastic compliance similar to soft biological tissue (Young's modulus 600% strain). By incorporating liquid metal (LM) microdroplets into a soft elastomer, we achieve a ∼25× increase in thermal conductivity (4.7 ± 0.2 W⋅m-1⋅K-1) over the base polymer (0.20 ± 0.01 W⋅m-1·K-1) under stress-free conditions and a ∼50× increase (9.8 ± 0.8 W⋅m-1·K-1) when strained. This exceptional combination of thermal and mechanical properties is enabled by a unique thermal-mechanical coupling that exploits the deformability of the LM inclusions to create thermally conductive pathways in situ. Moreover, these materials offer possibilities for passive heat exchange in stretchable electronics and bioinspired robotics, which we demonstrate through the rapid heat dissipation of an elastomer-mounted extreme high-power LED lamp and a swimming soft robot.

392 citations

Journal ArticleDOI
TL;DR: In this paper, a three-dimensional non-isothermal model is developed to account rigorously for various heat generation mechanisms, including irreversible heat due to electrochemical reactions, entropic heat, and Joule heating arising from the electrolyte ionic resistance.

391 citations

Journal ArticleDOI
TL;DR: In this paper, a computational fluid dynamics multiphase model of a proton exchange membrane (PEM! fuel cell) was presented, which accounts for three-dimensional transport processes including phase change and heat transfer, and includes the gas-diffusion layers and gas flow channels for both anode and cathode, as well as a cooling channel.
Abstract: A computational fluid dynamics multiphase model of a proton-exchange membrane ~PEM! fuel cell is presented. The model accounts for three-dimensional transport processes including phase change and heat transfer, and includes the gas-diffusion layers ~GDL! and gas flow channels for both anode and cathode, as well as a cooling channel. Transport of liquid water inside the gas-diffusion layers is modeled using viscous forces and capillary pressure terms. The physics of phase change is accounted for by prescribing local evaporation as a function of the undersaturation and liquid water concentration. Simulations have been performed for fully humidified gases entering the cell. The results show that different competing mechanisms lead to phase change at both anode and cathode sides of the fuel cell. The predicted amount of liquid water depends strongly on the prescribed material properties, particularly the hydraulic permeability of the GDL. Analysis of the simulations at a current density of 1.2 A/cm 2 show that both condensation and evaporation take place within the cathode GDL, whereas condensation prevails throughout the anode, except near the inlet. The three-dimensional distribution of the reactants and products is evident, particularly under the land areas. For the conditions investigated in this paper, the liquid water saturation does not exceed 10% at either anode or cathode side, and increases nonlinearly with current density. The operation of proton-exchange membrane ~PEM! fuel cells depends not only on the effective distribution of air and hydrogen, but also on the maintenance of an adequate cell operating temperature and fully humidified conditions in the membrane. The fully humidified state of the membrane is crucial to ensuring good ionic conductivity and is achieved by judicious water management. Water content is determined by the balance between various water transport mechanisms and water production. The water transport mechanisms are electro-osmotic drag of water ~i.e., motion of water molecules attaching to protons migrating through the membrane from anode to cathode!; back diffusion from the cathode ~due to nonuniform concentration!; and diffusion and convection to/from the air and hydrogen gas streams. Water production depends on the electric current density and phase change. Without control, an imbalance between production and removal rates of water can occur. This can result in either dehydration of the membrane, or flooding of the electrodes, which are both detrimental to performance. A common water management technique relies on the humidification of the air and hydrogen gas streams. At higher current densities, the excess product water is removed by convection via the air stream, and the rate of removal is controlled by adjusting moisture content in concert with pressure drop and temperature in the flow channels. Thermal management is also required to remove the heat produced by the electrochemical reaction in order to prevent drying out of the membrane, which in turn can result not only in reduced performance but also in eventual rupture of the membrane. Thermal management, which is performed via forced convection cooling in larger stacks, is also essential for the control of the water evaporation or condensation rates. The operation of a fuel cell and the resulting water and heat distributions depend on numerous transport phenomena including charge-transport and multicomponent, multiphase flow, and heat transfer in porous media. The complexity and interaction of these processes and the difficulty in making detailed in situ measurements have prompted the development of a number of numerical models. The theoretical framework was laid out in early one-dimensional numerical models of the membrane-electrode. 1-3 A quasi-twodimensional model based on concentrated solution theory was also proposed by Newman and Fuller, 4 and a full two-dimensional model including flow channels but no electrodes was also presented by Nguyen and White. 5 This model was refined in a number of subsequent studies to account for the porous electrodes and interdigitated

390 citations

Journal ArticleDOI
TL;DR: In this paper, the effects of the squeeze number, the nanofluid volume fraction and Eckert number and δ on Nusselt number were investigated, and the results showed that Nussellt number has a direct relationship with nanoparticle volume fraction, δ, the squeeze and EKN when two plates are separated but it has reverse relationship with the squeeze when two plate are squeezed.

389 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
20235,737
202210,641
20217,860
20208,182
20198,826
20188,737