Showing papers on "Heat transfer coefficient published in 2007"
TL;DR: In this paper, stable aqueous TiO2 nanofluids with different particle sizes and concentrations were formulated and measured for their static thermal conductivity and rheological behaviour.
889 citations
TL;DR: In this paper, a laminar flow forced convection heat transfer of Al2O3/water nanofluid inside a circular tube with constant wall temperature was investigated experimentally.
766 citations
TL;DR: In this article, the authors investigated the behavior and heat transfer enhancement of a particular nanofluid, Al2O3 nanoparticle-water mixture, flowing inside a closed system that is destined for cooling of microprocessors or other electronic components.
575 citations
TL;DR: In this article, the authors explored the micro-channel cooling benefits of water-based nanofluids containing small concentrations of Al 2 O 3, showing that the high thermal conductivity of nanoparticles is shown to enhance the single-phase heat transfer coefficient, especially for laminar flow.
567 citations
TL;DR: In this paper, a literature review is presented to compare different cooling technologies currently in development in research laboratories that are competing to solve the challenge of cooling the next generation of high heat flux computer chips.
Abstract: The purpose of this literature review is to compare different cooling technologies currently in development in research laboratories that are competing to solve the challenge of cooling the next generation of high heat flux computer chips. Today, most development efforts are focused on three technologies: liquid cooling in copper or silicon micro-geometry heat dissipation elements, impingement of liquid jets directly on the silicon surface of the chip, and two-phase flow boiling in copper heat dissipation elements or plates with numerous microchannels. The principal challenge is to dissipate the high heat fluxes (current objective is 300 W/cm2) while maintaining the chip temperature below the targeted temperature of 85°C, while of second importance is how to predict the heat transfer coefficients and pressure drops of the cooling process. In this study, the state of the art of these three technologies from recent experimental articles (since 2003) is analyzed and a comparison of the respective merits and ...
511 citations
TL;DR: In this article, the effects of the volume fraction, the size of nanoparticles, and the average temperature of nanofluids on natural convective instability and heat transfer characteristics of water-based Al2O3 nanoparticles in a rectangular cavity heated from below are theoretically analyzed.
350 citations
01 Jan 2007
TL;DR: In this paper, a two-dimensional numerical model of spiral counterflow heat recirculating combustors was developed including the effects of temperature-dependent gas and solid properties, viscous flow, surface-to-surface radiative heat transfer, heat conduction within the solid structure, one-step chemical reaction and heat loss from the combustor to its surroundings.
Abstract: A two-dimensional numerical model of spiral counterflow heat recirculating combustors was developed including the effects of temperature-dependent gas and solid properties, viscous flow, surface-to-surface radiative heat transfer, heat conduction within the solid structure, one-step chemical reaction and heat loss from the combustor to its surroundings. A simplified model of heat loss in the 3rd dimension was implemented and found to provide satisfactory representation of such losses at greatly reduced computational cost compared to fully three-dimensional models. The model predicts broad reaction zones with structure decidedly different from conventional premixed flames. Extinction limits were determined over a wide range of Reynolds numbers (2 500, modeling of turbulent flow and transport was required to obtain such agreement. Heat conduction along the heat exchanger wall has a major impact extinction limits; the wall thermal conductivity providing the broadest limits is actually less than that of air. Radiative heat transfer between walls was found to have an effect similar to that of heat conduction along the wall. In addition to weak-burning extinction limits, strong-burning limits in which the reaction zone moves out of the combustor center toward the inlet were also predicted by the numerical model, in agreement with experiments. It is concluded that several physical processes including radiative transfer, turbulence and wall heat conduction strongly affect the performance of heat-recirculating combustors, but the relative importance of such effects is strongly dependent on Re.
289 citations
TL;DR: In this paper, a new microchannel heat exchanger with S-shaped fins was developed using the three-dimensional computational fluid dynamics (3D CFD) FLUENT code, which provided 6-7 times lower pressure drop while maintaining heat transfer performance that was almost equivalent to that of a conventional MCHE with zigzag fins.
244 citations
TL;DR: In this article, the conical-ring turbulators and twisted-tape swirl generators were used for heat transfer, friction factor and enhancement efficiency in a circular tube fitted with conical ring turbulators.
243 citations
TL;DR: In this paper, the authors show that the presence of nanoparticles enhances thermal conduction under macroscopically static conditions mainly due to nanoparticle structuring / networking, while the natural convective heat transfer coefficient decreases systematically with increasing nanoparticle concentration, and the deterioration is partially attributed to the high viscosity of nanofluids.
Abstract: This paper summarises some of our recent work on the heat transfer of nanofluids (dilute liquid suspensions of nanoparticles). It covers heat conduction, convective heat transfer under both natural and forced flow conditions, and boiling heat transfer in the nucleate regime. The results show that, despite considerable data scattering, the presence of nanoparticles enhances thermal conduction under macroscopically static conditions mainly due to nanoparticle structuring / networking. The natural convective heat transfer coefficient is observed to decrease systematically with increasing nanoparticle concentration, and the deterioration is partially attributed to the high viscosity of nanofluids. However, either enhancement or deterioration of convective heat transfer is observed under the forced flow conditions and particle migration is suggested to be an important mechanism. The results also show that the boiling heat transfer is enhanced in the nucleate regime for both alumina and titania nanofluids, and the enhancement is more sensitive to the concentration change for TiO(2) nanofluids. It is concluded that there is still some way to go before we can tailor-make nanofluids for any targeted applications.
233 citations
TL;DR: In this article, the effect of peristaltic wave amplitude on the flow and heat transfer in a vertical porous annular region between two concentric tubes was studied. But the authors focused on the interaction between peristalsis with heat transfer for the flow of a viscous fluid.
Abstract: In this paper, we study the interaction of peristalsis with heat transfer for the flow of a viscous fluid in a vertical porous annular region between two concentric tubes. Long wavelength approximation (that is, the wavelength of the peristaltic wave is large in comparison with the radius of the tube) is used to linearise the governing equations. Using the perturbation method, the solutions are obtained for the velocity and the temperature fields. Also, the closed form expressions are derived for the pressure–flow relationship and the heat transfer at the wall. The effect of pressure drop on flux is observed to be almost negligible for peristaltic waves of large amplitude; however, the mean flux is found to increase by 10–12% as the free convection parameter increases from 1 to 2. Also, the heat transfer at the wall is affected significantly by the amplitude of the peristaltic wave. This warrants further study on the effects of peristalsis on the flow and heat transfer characteristics.
TL;DR: The steady laminar flow and heat transfer of a second grade fluid over a radially stretching sheet is considered in this article, where axisymmetric flow is induced due to linear stretching of a sheet.
TL;DR: In this article, the relative magnitudes of the local heat transfer coefficients are compared to the fluctuating components and to the mean and root-mean-square local velocity components, and it has been shown that at low nozzle to surface spacings (<2 diameters) secondary peaks in the radial heat transfer distributions are due to an abrupt increase in turbulence in the wall jet.
TL;DR: In this article, the authors present a systematic experimental method of studying the heat transfer behavior of buoyancy-driven nanofluids, where the authors estimate the range of volume fractions that results in maximum thermal enhancement and the impact of volume fraction on Nusselt number.
Abstract: This paper presents a systematic experimental method of studying the heat transfer behavior of buoyancy-driven nanofluids. The presence of nanoparticles in buoyancy-driven flows affects the thermophysical properties of the fluid and consequently alters the rate of heat transfer. The focus of this paper is to estimate the range of volume fractions that results in maximum thermal enhancement and the impact of volume fraction on Nusselt number. The test cell for the nanofluid is a two-dimensional rectangular enclosure with differentially heated vertical walls and adiabatic horizontal walls filled with 27 nm Al 2 O 3 -H 2 O nanofluid. Simulations were performed to measure the transient and steady-state thermal response of nanofluid to imposed isothermal condition. The volume fraction is varied between 0% and 8%. It is observed that the trend of the temporal and spatial evolution of temperature profile for the nanofluid mimics that of the carrier fluid. Hence, the behaviors of both fluids are similar. Results shows that for small volume fraction, 0.2 ≤ O≤2% the presence of the nanoparticles does not impede the free convective heat transfer, rather it augments the rate of heat transfer. However, for large volume fraction O>2%, the convective heat transfer coefficient declines due to reduction in the Rayleigh number caused by increase in kinematic viscosity. Also, an empirical correlation for Nu o as a function of O and Ra has been developed, and it is observed that the nanoparticle enhances heat transfer rate even at a small volume fraction.
TL;DR: In this paper, an experimental study measuring the axial heat transfer distributions and the pressure drop coefficients of the tube fitted with a broken twisted tape of twist ratio 1, 1.5, 2, 2.5 or ∞ is performed in the Re range of 1000-40,000.
TL;DR: In this paper, a Chen-type correlation for flow boiling heat transfer of R-134a in horizontal tubes was modified taking into account the effect of tube diameter, and the proposed experimental expressions to predict both the dryout quality and the post-dryout heat transfer coefficient could also be applied to a wide range of tube diameters (0.5-11mm-ID).
TL;DR: In this paper, an experimental study was performed to understand the nucleate boiling heat transfer of water-CuO nanoparticles suspension (nanofluids) at different operating pressures and different nanoparticle mass concentrations.
TL;DR: In this paper, two different models from the literature are used to express these properties in terms of particle loading and they lead to very different qualitative and quantitative results in two types of problems.
TL;DR: In this paper, the authors demonstrate the ability of ionic winds to decrease the wall temperature substantially in the presence of a bulk flow over a flat plate, corresponding to local enhancement of the heat transfer coefficient by more than twofold.
Abstract: Ionic wind engines can be integrated onto surfaces to provide enhanced local cooling. Air ions generated by field-emitted electrons or a corona discharge are pulled by an electric field and exchange momentum with neutral air molecules, causing air flow. In the presence of a bulk flow, ionic winds distort the boundary layer, increasing heat transfer from the wall. Experiments demonstrate the ability of ionic winds to decrease the wall temperature substantially in the presence of a bulk flow over a flat plate, corresponding to local enhancement of the heat transfer coefficient by more than twofold. Multiphysics simulations of the corona and flow describe the ability of the ionic wind to distort a bulk flow boundary layer and confirm the experimentally observed heat transfer enhancement trends.
TL;DR: In this article, a dispersion model is used to account for the presence of nanoparticles in a Laminar-flow convective heat transfer of nanofluid in a circular tube with constant wall temperature boundary condition.
Abstract: Laminar-flow convective heat transfer of nanofluid in a circular tube with constant wall temperature boundary condition is investigated numerically. A dispersion model is used to account for the presence of nanoparticles. Numerical predictions are in agreement with experimental results obtained in our laboratory for different particles in different sizes. Results clearly show that addition of nanoparticles to base liquid produces considerable enhancement of heat transfer. Heat transfer coefficients increase with nanoparticle concentration. Decreasing nanoparticles size at a specific concentration increases heat transfer coefficients.
TL;DR: In this paper, a microscale three-dimensional (3D) urban energy balance model, TUF-3D, is developed to predict urban surface temperatures for a variety of surface geometries and properties, weather conditions, and solar angles.
Abstract: A microscale three-dimensional (3-D) urban energy balance model, Temperatures of Urban Facets in 3-D (TUF-3D), is developed to predict urban surface temperatures for a variety of surface geometries and properties, weather conditions, and solar angles. The surface is composed of plane-parallel facets: roofs, walls, and streets, which are further sub-divided into identical square patches, resulting in a 3-D raster-type model geometry. The model code is structured into radiation, conduction and convection sub-models. The radiation sub-model uses the radiosity approach and accounts for multiple reflections and shading of direct solar radiation. Conduction is solved by finite differencing of the heat conduction equation, and convection is modelled by empirically relating patch heat transfer coefficients to the momentum forcing and the building morphology. The radiation and conduction sub-models are tested individually against measurements, and the complete model is tested against full-scale urban surface temperature and energy balance observations. Modelled surface temperatures perform well at both the facet-average and the sub-facet scales given the precision of the observations and the uncertainties in the model inputs. The model has several potential applications, such as the calculation of radiative loads, and the investigation of effective thermal anisotropy (when combined with a sensor-view model).
TL;DR: In this article, a generalized transport model including memory and nonlocal effects is proposed to describe the transition of heat transport from the diffusive regime to the ballistic regime, where the authors describe in a single equation the behavior of conductivity in terms of the system size and a reduction in the limit flux through nanoscale devices.
Abstract: The authors discuss a generalized transport model including memory and nonlocal effects, which aims to describe the transition of heat transport from the diffusive regime to the ballistic regime. By using an effective thermal conductivity depending on the Knudsen number, they describe in a single equation the behavior of conductivity in terms of the system size and a reduction in the limit flux through nanoscale devices.
TL;DR: In this article, analytical solutions of highly non-linear momentum equation and confluent hypergeometric similarity solution of heat transfer equations are obtained and the effect of various parameters like visco-elastic parameter, Eckert number, Prandtl number, and non-uniform heat source/sink parameter on temperature distribution are analyzed and effect of all these parameters on wall temperature gradient and wall temperature are tabulated and discussed.
TL;DR: In this paper, the performance of the cycle is studied by means of a method introduced in previous papers which consists of assessing the goodness of a calculation method by looking at representative variables such as the evaporation or the condensation temperature depending on the case evaluated.
Abstract: This paper studies refrigeration cycles in which plate heat exchangers are used as either evaporators or condensers. The performance of the cycle is studied by means of a method introduced in previous papers which consists of assessing the goodness of a calculation method by looking at representative variables such as the evaporation or the condensation temperature depending on the case evaluated. This procedure is also used to compare several heat transfer coefficients in the refrigerant side. As in previous works the models of all the cycle components are considered together with the heat exchanger models in such a way that the system of equations they provide is solved by means of a Newton–Raphson algorithm. Calculated and measured values of the evaporation and the condensation temperatures are also compared. The experimental results correspond to the same air-to-water heat pump studied in other papers and they have been obtained by using refrigerants R-22 and R-290.
TL;DR: In this article, the effect of grit geometry on the heat transfer coefficient and friction factor in a rectilinear rectangular duct of an aspect ratio 10:1 was investigated, where the top wall surface is made rough with metal ribs of circular cross section in staggered manner to form defined grid.
TL;DR: In this paper, the authors used a finite volume method (FVM) implemented on a Cartesian grid system in the range as 10 ≤ Re ≤ 45 and 0.7 ≤ Pr ≤ 400.
Abstract: Forced convection heat transfer from an unconfined circular cylinder in the steady cross-flow regime has been studied using a finite volume method (FVM) implemented on a Cartesian grid system in the range as 10 ≤ Re ≤ 45 and 0.7 ≤ Pr ≤ 400. The numerical results are used to develop simple correlations for Nusselt number as a function of the pertinent dimensionless variables. In addition to average Nusselt number, the effects of Re, Pr and thermal boundary conditions on the temperature field near the cylinder and on the local Nusselt number distributions have also been presented to provide further physical insights into the nature of the flow. The rate of heat transfer increases with an increase in the Reynolds and/or Prandtl numbers. The uniform heat flux condition always shows higher value of heat transfer coefficient than the constant wall temperature at the surface of the cylinder for the same Reynolds and Prandtl numbers. The maximum difference between the two values is around 15–20%.
TL;DR: In this paper, the authors investigated the natural convection heat transfer of nano-fluids in a laminar external boundary layer from the integral formalism approach and found that the sensitivity to the viscosity model played a key role in the heat transfer behavior.
TL;DR: In this paper, latent heat of fusion, melting and freezing points, and temperature and concentration-dependent viscosity data were presented, and the impact of using enhanced surface tubing in combination with MPCM slurry under constant heat flux and turbulent conditions.
TL;DR: In this paper, the anti-vortex film cooling hole concept has been modeled computationally for a single row of 30 degree angled holes on a flat surface using the 3D Navier-Stokes solver Glenn-HT.
Abstract: A novel turbine film cooling hole shape has been conceived and designed at NASA Glenn Research Center. This “anti-vortex” design is unique in that it requires only easily machinable round holes, unlike shaped film cooling holes and other advanced concepts. The hole design is intended to counteract the detrimental vorticity associated with standard circular cross-section film cooling holes. This vorticity typically entrains hot freestream gas and is associated with jet separation from the turbine blade surface. The anti-vortex film cooling hole concept has been modeled computationally for a single row of 30 degree angled holes on a flat surface using the 3D Navier-Stokes solver Glenn-HT. A blowing ratio of 1.0 and density ratios of 1.05 and 2.0 are studied. Both film effectiveness and heat transfer coefficient values are computed and compared to standard round hole cases for the same blowing rates. A net heat flux reduction is also determined using both the film effectiveness and heat transfer coefficient values to ascertain the overall effectiveness of the concept. An improvement in film effectiveness of about 0.2 and in net heat flux reduction of about 0.2 is demonstrated for the anti-vortex concept compared to the standard round hole for both blowing ratios. Detailed flow visualization shows that as expected, the design counteracts the detrimental vorticity of the round hole flow, allowing it to remain attached to the surface.
TL;DR: In this paper, the effect of carbon nanotubes (CNTs) on nucleate boiling heat transfer is investigated, and two halocarbon refrigerants of R123 and R134a for building chillers were used as working fluids and 1.0% of CNTs were added to them to examine the heat transfer enhancement with CNT.