Showing papers on "Heat transfer published in 2009"

Review on thermal energy storage with phase change materials and applications

Abstract: The use of a latent heat storage system using phase change materials (PCMs) is an effective way of storing thermal energy and has the advantages of high-energy storage density and the isothermal nature of the storage process. PCMs have been widely used in latent heat thermal-storage systems for heat pumps, solar engineering, and spacecraft thermal control applications. The uses of PCMs for heating and cooling applications for buildings have been investigated within the past decade. There are large numbers of PCMs that melt and solidify at a wide range of temperatures, making them attractive in a number of applications. This paper also summarizes the investigation and analysis of the available thermal energy storage systems incorporating PCMs for use in different applications.

Surface phonon polaritons mediated energy transfer between nanoscale gaps.

Abstract: Surface phonon polaritons are electromagnetic waves that propagate along the interfaces of polar dielectrics and exhibit a large local-field enhancement near the interfaces at infrared frequencies. Theoretical calculations show that such surface waves can lead to breakdown of the Planck’s blackbody radiation law in the near field. Here, we experimentally demonstrate that surface phonon polaritons dramatically enhance energy transfer between two surfaces at small gaps by measuring radiation heat transfer between a microsphere and a flat surface down to 30 nm separation. The corresponding heat transfer coefficients at nanoscale gaps are 3 orders of magnitude larger than that of the blackbody radiation limit. The high energy flux can be exploited to develop new radiative cooling and thermophotovoltaic technologies.

Nanowires for enhanced boiling heat transfer.

Abstract: Boiling is a common mechanism for liquid-vapor phase transition and is widely exploited in power generation and refrigeration devices and systems. The efficacy of boiling heat transfer is characterized by two parameters: (a) heat transfer coefficient (HTC) or the thermal conductance; (b) the critical heat flux (CHF) limit that demarcates the transition from high HTC to very low HTC. While increasing the CHF and the HTC has significant impact on system-level energy efficiency, safety, and cost, their values for water and other heat transfer fluids have essentially remained unchanged for many decades. Here we report that the high surface tension forces offered by liquids in nanowire arrays made of Si and Cu can be exploited to increase both the CHF and the HTC by more than 100%.

Experimental and theoretical studies of light-to-heat conversion and collective heating effects in metal nanoparticle solutions.

Abstract: We perform a set of experiments on photoheating in a water droplet containing gold nanoparticles (NPs). Using photocalorimetric methods, we determine efficiency of light-to-heat conversion (η) which turns out to be remarkably close to 1, (0.97 < η < 1.03). Detailed studies reveal a complex character of heat transfer in an optically stimulated droplet. The main mechanism of equilibration is due to convectional flow. Theoretical modeling is performed to describe thermal effects at both nano- and millimeter scales. Theory shows that the collective photoheating is the main mechanism. For a large concentration of NPs and small laser intensity, an averaged temperature increase (at the millimeter scale) is significant (∼7 °C), whereas on the nanometer scale the temperature increase at the surface of a single NP is small (∼0.02 °C). In the opposite regime, that is, a small NP concentration and intense laser irradiation, we find an opposite picture: a temperature increase at the millimeter scale is small (∼0.1 °C)...

Performance enhancement in latent heat thermal storage system: A review

Abstract: Phase change material (PCM) based latent heat thermal storage (LHTS) systems offer a challenging option to be employed as an effective energy storage and retrieval device. The performance of LHTS systems is limited by the poor thermal conductivity of PCMs employed. Successful large-scale utilization of LHTS systems thus depends on the extent to which the performance can be improved. A great deal of work both experimental and theoretical on different performance enhancement techniques has been reported in the literature. This paper reviews the implementation of those techniques in different configurations of LHTS systems. The influence of enhancement techniques on the thermal response of the PCM in terms of phase change rate and amount of latent heat stored/retrieved has been addressed as a main aspect. Issues related to mathematical modeling of LHTS systems employing enhancement techniques are also discussed.

Predicted Efficiency of a Low-Temperature Nanofluid-Based Direct Absorption Solar Collector

Abstract: Due to its renewable and nonpolluting nature, solar energy is often used in applications such as electricity generation, thermal heating, and chemical processing. The most cost-effective solar heaters are of the "flat-plate" type, but these suffer from relatively low efficiency and outlet temperatures. The present study theoretically investigates the feasibility of using a nonconcentrating direct absorption solar collector (DAC) and compares its performance with that of a typical flat-plate collector. Here a nanofluid-a mixture of water and aluminum nanoparticles—is used as the absorbing medium. A two-dimensional heat transfer analysis was developed in which direct sunlight was incident on a thin flowing film of nanofluid. The effects of absorption and scattering within the nanofluid were accounted for. In order to evaluate the temperature profile and intensity distribution within the nanofluid, the energy balance equation and heat transport equation were solved numerically. It was observed that the presence of nanoparticles increases the absorption of incident radiation by more than nine times over that of pure water. According to the results obtained from this study, under similar operating conditions, the efficiency of a DA C using nanofluid as the working fluid is found to be up to 10% higher (on an absolute basis) than that of a flat-plate collector. Generally a DAC using nanofluids as the working fluid performs better than a flat-plate collector, however, much better designed flat-plate collectors might be able to match or outperform a nanofluids based DAC under certain conditions.

A three-dimensional finite element analysis of the temperature field during laser melting of metal powders in additive layer manufacturing

Abstract: Simulating the transient temperature field in additive layer manufacturing (ALM) processes has presented a challenge to many researchers in the field. The transient temperature history is vital for determining the thermal stress distribution and residual stress states in ALM-processed parts that utilise a moving laser heat source. The modelling of the problem involving multiple layers is equally of great importance because the thermal interactions of successive layers affect the temperature gradients, which govern the heat transfer and thermal stress development mechanisms. This paper uses an innovative simulation technique known as element birth and death, in modelling the three-dimensional temperature field in multiple layers in a powder bed. The results indicate that the heated regions undergo rapid thermal cycles that could be associated with commensurate thermal stress cycles. Deposition of successive layers and subsequent laser scanning produces temperature spikes in previous layers. The resultant effect is a steady temperature build-up in the lower layers as the number of layers increases.

Radiative heat transfer at the nanoscale

Abstract: We give a concise introduction into the radiative heat transfer at the nanoscale discussing the contribution of propagating, frustrated and coupled surface modes [1]. Especially, the latter contribution results in a heat flux, which can exceed the heat flux between two black bodies by several orders of magnitude for distances in the nanometer regime [1]. The prediction of such an enormous heat flux enhancement is usually based on Rytov's fluctuational electrodynamics [2] and has been verified in some very recent experiments [3,4,5]. Our aim is to show how the theoretical expression describing the nanoscale heat flux can be interpreted in terms of transmission coefficients and the universal quantum of thermal conductance by means of concepts of mesoscopic physics [6]. Such a formulation allows for studying the fundamental limits of radiative heat transfer [7,8] emphasizing the trade-off between the number of contributing modes and their transmission coefficient. [1] K. Joulain, J.-P. Mulet, F. Marquier, R. Carminati, and J.-J. Greffet, Surface Science Reports 57, 59 (2005). [2] S. M. Rytov, Y. A. Kravtsov, and V. I. Tatarskii, Principles of Statistical Radiophyics (Springer, New York), Vol. 3. (1989). [3] S. Shen, A. Narayanaswamy, and G. Chen, Nano Lett. 9, 2909 (2009). [4] E. Rousseau, A. Siria, G. Jourdan, S. Volz, F. Comin, J. Chevrier, and J.-J. Greffet, Nature Photonics 3, 514 (2009). [5] R. Ottens, V. Quetschke, S. Wise, A. Alemi, R. Lundock, G. Mueller, D. H. Reitze, D. B. Tanner, B. F. Whiting, Phys. Rev. Lett. 107, 014301 (2011). [6] S.-A. Biehs, E. Rousseau, and J.-J. Greffet, Phys. Rev. Lett. 105, 234301 (2010). [7] P. Ben-Abdallah and K. Joulain, Phys. Rev. B 82, 121419 (R) (2010). [8] S. Basu and Z. M. Zhang, J. Appl. Phys. 105, 093535 (2009).

Mixed Convection Boundary-Layer Flow Near the Stagnation Point on a Vertical Surface in a Porous Medium: Brinkman Model with Slip

Abstract: The steady boundary-layer flow near the stagnation point on an impermeable vertical surface with slip that is embedded in a fluid-saturated porous medium is investigated. Using appropriate similarity variables, the governing system of partial differential equations is transformed into a system of ordinary differential equations. This system is then solved numerically. The features of the flow and the heat transfer characteristics for different values of the governing parameters, namely, the Darcy–Brinkman, Γ, mixed convection, λ, and slip, γ, parameters, are analysed and discussed in detail for the cases of assisting and opposing flows. It is found that dual solutions exist for assisting flows, as well as those usually reported in the literature for opposing flows. A stability analysis of the steady flow solutions encountered for different values of the mixed convection parameter λ is performed using a linear temporal stability analysis. This analysis reveals that for γ = 0 (slip absent) and Γ = 1 the lower solution branch is unstable while the upper solution branch is stable.

Natural convection cooling of a localised heat source at the bottom of a nanofluid-filled enclosure

Abstract: This article presents a numerical study of natural convection cooling of a heat source embedded on the bottom wall of an enclosure filled with nanofluids. The top and vertical walls of the enclosure are maintained at a relatively low temperature. The transport equations for a Newtonian fluid are solved numerically with a finite volume approach using the SIMPLE algorithm. The influence of pertinent parameters such as Rayleigh number, location and geometry of the heat source, the type of nanofluid and solid volume fraction of nanoparticles on the cooling performance is studied. The results indicate that adding nanoparticles into pure water improves its cooling performance especially at low Rayleigh numbers. The type of nanoparticles and the length and location of the heat source proved to significantly affect the heat source maximum temperature.

Review of near-field thermal radiation and its application to energy conversion

Abstract: Understanding energy transfer via near-field thermal radiation is critical for the future advances of nanotechnology. Evanescent waves and photon tunneling are responsible for the near-field energy transfer being several orders of magnitude greater than that between two blackbodies. The enhanced energy transfer may be used for improving the performance of energy conversion devices, developing novel nanofabrication techniques, and imaging nanostructures with higher spatial resolution. Near-field heat transfer can be analyzed using fluctuational electrodynamics. This article reviews the fundamentals of near-field radiation and outlines the recent advances in this field. Important results are presented for near-field energy transfer between parallel plates and between multilayered structures. Application of near-field thermal radiation in near-field thermophotovoltaic devices is also discussed. Copyright © 2009 John Wiley & Sons, Ltd.

Temperature-dependent thermal diffusivity of the Earth’s crust and implications for magmatism

Abstract: The rate of heat transfer by conduction is the dominant factor that determines the thermal evolution of planetary crust and lithosphere. Most thermal models of the Earth's crust assume constant values for thermal diffusivity, owing to large experimental uncertainties in measuring this property of rocks at high temperature. Whittington et al. have used recent advances in laser-flash analysis on three different crustal rocks types to show that thermal diffusivity strongly decreases with increasing temperature. They find thermal diffusivity to be about half that commonly assumed at mid-crustal temperatures and therefore conclude that the hot middle and lower crust is a much more effective thermal insulator than previously thought. They also present models of lithospheric thermal evolution during continental collision, and demonstrate that the temperature dependence of rock properties leads to a positive feedback between strain heating in shear zones and more efficient thermal insulation, removing the requirement for unusually high radiogenic heat production to achieve crustal melting temperatures. The thermal evolution of planetary crust and lithosphere is governed by the rate of heat transfer by conduction, which is determined by the rock's thermal diffusivity, usually assumed to remain constant. Alan Whittington and colleagues show that thermal diffusivity in fact decreases strongly with increasing temperature, concluding that the hot middle and lower crust is a much more effective thermal insulator than previously thought; this removes the requirement for unusually high radiogenic heat production to achieve crustal melting temperatures. The thermal evolution of planetary crust and lithosphere is largely governed by the rate of heat transfer by conduction1,2,3. The governing physical properties are thermal diffusivity (κ) and conductivity (k = κρCP), where ρ denotes density and CP denotes specific heat capacity at constant pressure. Although for crustal rocks both κ and k decrease above ambient temperature4,5, most thermal models of the Earth’s lithosphere assume constant values for κ (∼1 mm2 s-1) and/or k (∼3 to 5 W m-1 K-1)6,7 owing to the large experimental uncertainties associated with conventional contact methods at high temperatures. Recent advances in laser-flash analysis8,9 permit accurate (±2 per cent) measurements on minerals and rocks to geologically relevant temperatures10. Here we provide data from laser-flash analysis for three different crustal rock types, showing that κ strongly decreases from 1.5–2.5 mm2 s-1 at ambient conditions, approaching 0.5 mm2 s-1 at mid-crustal temperatures. The latter value is approximately half that commonly assumed, and hot middle to lower crust is therefore a much more effective thermal insulator than previously thought. Above the quartz α–β phase transition, crustal κ is nearly independent of temperature, and similar to that of mantle materials11. Calculated values of k indicate that its negative dependence on temperature is smaller than that of κ, owing to the increase of CP with increasing temperature, but k also diminishes by 50 per cent from the surface to the quartz α–β transition. We present models of lithospheric thermal evolution during continental collision and demonstrate that the temperature dependence of κ and CP leads to positive feedback between strain heating in shear zones and more efficient thermal insulation, removing the requirement for unusually high radiogenic heat production to achieve crustal melting temperatures. Positive feedback between heating, increased thermal insulation and partial melting is predicted to occur in many tectonic settings, and in both the crust and the mantle, facilitating crustal reworking and planetary differentiation12.

Thermal stratification within the water tank

Abstract: To sufficiently store and use high-quality heat energy, thermal stratification is gradually applied in many kinds of energy storage fields such as solar thermal utilization system. Because of the unsteady characteristics of solar radiation, thermal storage becomes very essential in long-term operation of heating load. The wide application of thermal stratification lies in the minimization of the mixing effect by use of the thermal stratification, which is caused by the thermal buoyancy because of the difference of temperature between cold and hot water. According to the review, the conception of thermal stratification allows a wide variety of different design embodiments, which essentially extends the fields of practical application of these devices. In this paper a survey of the various types of thermal stratification tanks and research methods is presented, and reasons of energy storage with efficiency problems related to the applications are introduced and benefits offered by thermal stratification are outlined. The structure designs based on theoretical prediction of thermal-stratified water tank performed at many organizations are introduced and are compared with their experimental results. Finally, the development of the tank with thermal stratification in the future application is predicted.