Showing papers on "Heat transfer published in 2002"

Physics of Shock Waves and High-Temperature Hydrodynamic Phenomena

Abstract: : Contents: Elements of gas dynamics and classical theory of shock waves; thermal radiation and radiant heat exchange in a medium; thermodynamic properties of gases at high temperatures; shock tubes; absorption and emission of radiation in gases at high temperatures; speed of relaxation processes in gases; structure of front of shock waves in gases; physico-chemical kinetics in hydrodynamic processes; light phenomena in shock waves and during strong explosion in air; thermal waves; shock waves in solids; certain self-similar processes in gas dynamics.

The Surface Energy Balance System (SEBS) for estimation of turbulent heat fluxes

Abstract: A Surface Energy Balance System (SEBS) is proposed for the estimation of atmospheric turbulent fluxes and evaporative fraction using satellite earth observation data, in combination with meteorological information at proper scales SEBS consists of: a set of tools for the determination of the land surface physical parameters, such as albedo, emissivity, temperature, vegetation coverage etc, from spectral reflectance and radiance measurements; a model for the determination of the roughness length for heat transfer; and a new formulation for the determination of the evaporative fraction on the basis of energy balance at limiting cases Four experimental data sets are used to assess the reliabilities of SEBS Based on these case studies, SEBS has proven to be capable to estimate turbulent heat fluxes and evaporative fraction at various scales with acceptable accuracy The uncertainties in the estimated heat fluxes are comparable to in-situ measurement uncertainties Keywords: Surface energy balance, turbulent heat flux, evaporation, remote sensing

Thermal Energy Storage: Systems and Applications

Abstract: List of Contributors.Acknowledgements.Preface.General Introductory Aspects for Thermal Engineering. Energy Storage Systems. Thermal Energy Storage (TES) Methods. Thermal Energy Storage and Environmental Impact. Thermal Energy Storage and Energy Savings. Heat Transfer and Stratification in Sensible Heat Storage Systems. Modeling of Latent Heat Storage Systems. Heat Transfer with Phase Change in Simple and Complex Geometries. Thermodynamic Optimization of Thermal Energy Storage Systems. Energy and Exergy Analyses of Thermal Energy Storage Systems. Thermal Energy Storage Case Studies.Appendix A -- Conversion Factors.Appendix B -- Thermophysical Properties.Appendix C -- Glossary.Subject Index.

Fundamental issues related to flow boiling in minichannels and microchannels

Abstract: Flow boiling in small hydraulic diameter channels is becoming increasingly important in many diverse applications. The previous studies addressing the effects of the channel size on the flow patterns, and heat transfer and pressure drop performance are reviewed in the present paper. The fundamental questions related to the presence of nucleate boiling and characteristics of flow boiling in microchannels and minichannels in comparison to that in the conventional channel sizes (3 mm and above) are addressed. Also, the effect of heat exchanger configuration—single-channel and multichannel—on the heat transfer and pressure drop performance is reviewed. The areas for future research are identified.

Dropwise condensation theory and experiment: A review

Abstract: The paper reviews progress in dropwise condensation research from 1930 to the present. Particular attention is given to heat transfer measurements, theory, transition and effects of surface material. Although it has been known since the 1930s that heat transfer coefficients for dropwise condensation of steam are much higher than those for film condensation, there were, until the 1960s, wide discrepancies between the results of different investigators. Subsequently, more accurate measurements have shown good consistency and the mechanism and theory of the dropwise condensation have become better understood. There has been considerable controversy over the magnitude of the so-called ‘constriction resistance’ and the effect of the surface thermal conductivity on the heat transfer coefficient. The balance of evidence suggests that this is only significant at very low heat fluxes and for very small condensing surfaces. Measurements have also been made with sufficiently high cooling intensities to cover...

Thermometry and Thermal Transport in Micro/Nanoscale Solid-State Devices and Structures

Abstract: We review recent advances in experimental methods for high spatial-resolution and high time-resolution thermometry, and the application of these and related methods for measurements of thermal transport in low-dimensional structures. Scanning thermal microscopy (SThM) achieves lateral resolutions of 50 nm and a measurement bandwidth of 100 kHz: SThM has been used to characterize differences in energy dissipation in single-wall and multi-wall carbon nanotubes. Picosecond thermoreflectance enables ultrahigh time-resolution in thermal diffusion experiments and characterization of heat flow across interfaces between materials; the thermal conductance G of interfaces between dissimilar materials spans a relatively small range, 20

Heat transfer in stagnation-point flow towards a stretching sheet

Abstract: Steady two-dimensional stagnation-point flow of an incompressible viscous fluid over a flat deformable sheet is investigated when the sheet is stretched in its own plane with a velocity proportional to the distance from the stagnation-point. It is shown that for a fluid of small kinematic viscosity, a boundary layer is formed when the stretching velocity is less than the free stream velocity and an inverted boundary layer is formed when the stretching velocity exceeds the free stream velocity. Temperature distribution in the boundary layer is found when the surface is held at constant temperature and surface heat flux is determined.

Fundamentals of exergy analysis, entropy generation minimization, and the generation of flow architecture

Abstract: This paper outlines the fundamentals of the methods of exergy analysis and entropy generation minimization (or thermodynamic optimization—the minimization of exergy destruction). The paper begins with a review of the concept of irreversibility, entropy generation, or exergy destruction. Examples illustrate the accounting for exergy flows and accumulation in closed systems, open systems, heat transfer processes, and power and refrigeration plants. The proportionality between exergy destruction and entropy generation sends the designer in search of improved thermodynamic performance subject to finite-size constraints and specified environmental conditions. Examples are drawn from energy storage systems for sensible heat and latent heat, solar energy, and the generation of maximum power in a power plant model with finite heat transfer surface inventory. It is shown that the physical structure (geometric configuration, topology) of the system springs out of the process of global thermodynamic optimization subject to global constraints. This principle generates structure not only in engineering but also in physics and biology (constructal theory). Copyright © 2002 John Wiley & Sons, Ltd.

Modeling of glow discharge-induced fluid dynamics

Abstract: Modeling of fluid dynamics and the associated heat transfer induced by plasma between two parallel electrodes is investigated. In particular, we consider a capacitvely coupled radio frequency discharge plasma generator, where the plasma is generated on the surface of a dielectric circuit board with electrode strips on the top and bottom. The electrodes have a thickness of 100 μm, which is comparable to the height of the boundary layer. The regime considered is that the electron component is in the non-equilibrium state, and the plasma is nonthermal. Overall, due to the ion and large fluid particle interaction, the pressure is higher in the downstream of the electrode, causing the velocity structure to resemble that of a wall jet. Parameters related to the electrode operation, including the voltage, frequency, and free stream speed are varied to investigate the characteristics of the plasma-induced flow. Consistent with the experimental observation, the model shows a clear dependence of the induced jet velocity on the applied voltage and frequency. The heat flux exhibited a similar dependence on the strength of the plasma. The present plasma-induced flow concept can be useful for thermal management and active flow control.

Convective heat transfer and flow characteristics of Cu-water nanofluid

Abstract: An experimental system is built to investigate convective heat transfer and flow characteristics of the nanofluid in a tube. Both the convective heat transfer coefficient and friction factor of Cu-water nanofluid for the laminar and turbulent flow are measured. The effects of such factors as the volume fraction of suspended nanoparticles and the Reynolds number on the heat transfer and flow characteristics are discussed in detail. The experimental results show that the suspended nanoparticles remarkably increase the convective heat transfer coefficient of the base fluid and show that the friction factor of the sample nanofluid with the low volume fraction of nanoparticles is almost not changed. Compared with the base fluid, for example, the convective heat transfer coefficient is increased about 60% for the nanofluid with 2.0 vol% Cu nanoparticles at the same Reynolds number. Considering the factors affecting the convective heat transfer coefficient of the nanofluid, a new convective heat transfer correlation for nanofluid under single-phase flows in tubes is established. Comparison between the experimental data and the calculated results indicate that the correlation describes correctly the energy transport of the nanofluid.

Enhanced radiative heat transfer at nanometric distances

Abstract: We study in this article the radiative heat transfer between two semi-infinite bodies at subwavelength scale. We show that this transfer can be enhanced by several orders of magnitude when the surfaces support resonant surface waves. In these conditions, we show that the transfer is almost monochromatic.

Reversible quantum Brownian heat engines for electrons

Abstract: Brownian heat engines use local temperature gradients in asymmetric potentials to move particles against an external force. The energy efficiency of such machines is generally limited by irreversible heat flow carried by particles that make contact with different heat baths. Here we show that, by using a suitably chosen energy filter, electrons can be transferred reversibly between reservoirs that have different temperatures and electrochemical potentials. We apply this result to propose heat engines based on mesoscopic semiconductor ratchets, which can quasistatically operate arbitrarily close to Carnot efficiency.

Onset of Nucleate Boiling and Active Nucleation Site Density During Subcooled Flow Boiling

Abstract: The partitioning of the heat flux supplied at the wall is one of the key issues that needs to be resolved if one is to model subcooled flow boiling accurately. The first step in studying wall heat flux partitioning is to account for the various heat transfer mechanisms involved and to know the location at which the onset of nucleate boiling (ONB) occurs. Active nucleation site density data is required to account for the energy carried away by the bubbles departing from the wall. Subcooled flow boiling experiments were conducted using a flat plate copper surface and a nine-rod (zircalloy-4) bundle. The location of ONB during the experiments was determined from visual observations as well as from the thermocouple output. From the data obtained it is found that the heat flux and wall superheat required for inception are dependent on flow rate, liquid subcooling, and contact angle. The existing correlations for ONB underpredict the wall superheat at ONB in most cases. A correlation for predicting the wall superheat and wall heat flux at ONB has been developed from the data obtained in this study and that reported in the literature. Experimental data are within630 percent of that predicted from the correlation. Active nucleation site density was determined by manually counting the individual sites in pictures obtained using a CCD camera. Correlations for nucleation site density, which are independent of flow rate and liquid subcooling, but dependent on contact angle have been developed for two ranges of wall superheat—one below 15°C and another above 15°C. @DOI: 10.1115/1.1471522#

Prediction of Two-Phase Pressure Gradients of Refrigerants in Horizontal Tubes

Abstract: Two-phase pressure drop data were obtained for evaporation in two horizontal test sections of 10.92 and 12.00 mm diameter for five refrigerants (R-134a, R-123, R-402A, R-404A and R-502) over mass velocities from 100 to 500 kg/m2 s and vapor qualities from 0.04 to 1.0. These data have then been compared against seven two-phase frictional pressure drop prediction methods. Overall, the method by Muller-Steinhagen and Heck (Muller-Steinhagen H, Heck K. A simple friction pressure drop correlation for two-phase flow in pipes. Chem. Eng. Process 1986;20:297–308) and that by Gronnerud (Gronnerud R. Investigation of liquid hold-up, flow-resistance and heat transfer in circulation type evaporators, part IV: two-phase flow resistance in boiling refrigerants. Annexe 1972-1, Bull. de l'Inst. du Froid, 1979) were found to provide the most accurate predictions while the widely quoted method of Friedel (Friedel L. Improved friction drop correlations for horizontal and vertical two-phase pipe flow. European Two-phase Flow Group Meeting, paper E2; June 1979; Ispra, Italy) gave the third best results. The data were also classified by two-phase flow pattern using the Kattan-Thome-Favrat (Kattan N, Thome JR, Favrat D. Flow boiling in horizontal tubes. Part 1: development of a diabatic two-phase flow pattern map. J. Heat Transfer 1998;120:140–7; Kattan N, Thome JR, Favrat D. Flow boiling in horizontal tubes. Part 2; new heat transfer data for five refrigerants. J Heat Transfer 1998;120:148–55; Kattan N, Thome JR, Favrat D. Flow boiling in horizontal tubes. Part 3: development of a new heat transfer model based on flow patterns. J. Heat Transfer 1998;120:156–65) flow pattern map. The best available method for annular flow was that of Muller-Steinhagen and Heck. For intermittent flow and stratified-wavy flow, the best method in both cases was that of Gronnerud. It was observed that the peak in the two-phase frictional pressure gradient at high vapor qualities coincided with the onset of dryout in the annular flow regime.

Measurements and modeling of two-phase flow in microchannels with nearly constant heat flux boundary conditions

Abstract: Two-phase forced convective flow in microchannels is promising for the cooling of integrated circuits. There has been limited research on boiling flow in channels with dimensions below 100 /spl mu/m, in which bubble formation and flow regimes can differ from those in larger channels. This work develops single and multi-channel experimental structures using plasma-etched silicon with pyrex glass cover, which allow uniform heating and spatially-resolved thermometry and provide optical access for visualization of boiling regimes. Boiling was observed with less than 5/spl deg/C of super-heating in rectangular channels with hydraulic diameters between 25 and 60 /spl mu/m. The channel wall widths are below 350 /spl mu/m, which minimizes solid conduction and reduces variations in the heat flux boundary condition. Pressure drop and wall temperature distribution data are consistent with predictions accounting for solid conduction and homogeneous two-phase convection.