Showing papers on "Thermal published in 2006"

Brownian-motion-based convective-conductive model for the effective thermal conductivity of nanofluids

Abstract: Here we show through an order-of-magnitude analysis that the enhancement in the effective thermal conductivity of nanofluids is due mainly to the localized convection caused by the Brownian movement of the nanoparticles. We also introduce a convective-conductive model which accurately captures the effects of particle size, choice of base liquid, thermal interfacial resistance between the particles and liquid, temperature, etc. This model is a combination of the Maxwell-Garnett (MG) conduction model and the convection caused by the Brownian movement of the nanoparficles, and reduces to the MG model for large particle sizes. The model is in good agreement with data on water, ethylene glycol, and oil-based nanofluids, and shows that the lighter the nanoparticles, the greater the convection effect in the liquid, regardless of the thermal conductivity of the nanoparticles.

Numerical simulation of an asymptotically reduced system for rotationally constrained convection

Abstract: For rotationally constrained convection, the Taylor–Proudman theorem enforces an organization of nonlinear flows into tall columnar or compact plume structures. While coherent structures in convection under moderate rotation are exclusively cyclonic, recent experiments for rapid rotation have revealed a transition to equal populations of cyclonic and anticyclonic structures. Direct numerical simulation (DNS) of this regime is expensive, however, and existing simulations have yet to reveal anticyclonic vortical structures. In this paper, we use a reduced system of equations for rotationally constrained convection valid in the asymptotic limit of thin columnar structures and rapid rotation to perform numerical simulation of Rayleigh–B´ enard convection in an infinite layer rotating uniformly about the vertical axis. Visualization indicates the existence of cyclonic and anticyclonic vortical populations for all parameters examined. Moreover, it is found that the flow evolves through three distinct regimes with increasing Rayleigh number (Ra). For small, but supercritical Ra ,t he fl ow is dominated by a cellular system of hot and cold columns spanning the fluid layer. As Ra increases, the number density of these columns decreases, the up- and downdrafts within them strengthen and the columns develop opposite-signed ‘sleeves’ in all fields. The resulting columns are highly efficient at transporting heat across the fluid layer. In the final regime, lateral mixing plays a dominant role in the interior and the columnar structure is destroyed. However, thermal plumes are still injected and rejected from the thermal boundary layers. We identify the latter two regimes with the vortex-grid and geostrophic turbulence regimes, respectively. Within these regimes, we investigate convective heat transport (measured by the Nusselt number), mean temperature profiles, and root-mean-square profiles of the temperature, vertical velocity and vertical vorticity anomalies. For all Prandtl numbers investigated, the mean temperature saturates in a non-isothermal profile as Ra increases owing to intense lateral mixing.

A Simple, Efficient Solution of Flux–Profile Relationships in the Atmospheric Surface Layer

Abstract: This note describes a simple scheme for analytical estimation of the surface-layer similarity functions from state variables. What distinguishes this note from the many previous papers on this topic is that this method is specifically targeted for numerical models in which simplicity and economic execution are critical. In addition, it has been in use in a mesoscale meteorological model for several years. For stable conditions, a very simple scheme is presented that compares well to the iterative solution. The stable scheme includes a very stable regime in which the slope of the stability functions is reduced to permit significant fluxes to occur, which is particularly important for numerical models in which decoupling from the surface can be an important problem. For unstable conditions, simple schemes generalized for varying ratios of aerodynamic roughness to thermal roughness (z0/z0h) are less satisfactory. Therefore, a simple scheme has been empirically derived for a fixed z0/z0h ratio, which represents quasi-laminar sublayer resistance. 1. Background Interactive linkages between state variables at the earth’s surface and in the atmospheric surface layer are essential components of numerical atmospheric models. Surface fluxes of heat, moisture, momentum, and any other modeled quantity (e.g., trace chemical species) are determined by gradients across the surface– atmosphere interface; at the same time, surface fluxes are critical processes determining the time evolution of these gradients. Hence, simultaneous solution of the surface fluxes and the surface layer profiles is required. Atmospheric models typically use surface-layer similarity theory to describe the flux–profile relationships. In accord with this theory, nondimensional profiles are defined for momentum as kz

Satellite thermal IR phenomena associated with some of the major earthquakes in 1999–2003

Abstract: Satellite thermal infrared (TIR) imaging data have recorded short-lived anomalies prior to major earthquakes and associations with fault systems. Others have proposed that these signals originate from electromagnetic phenomena associated with pre-seismic processes, causing enhanced IR emissions, that we are calling TIR anomalies. The purpose of this exploratory study is to verify if TIR anomalies can be found in association with known earthquakes by systematically applying satellite data analysis techniques to imagery recorded prior-to and immediately after large earthquakes. Our approach utilizes both a mapping of surface TIR transient fields from polar orbiting satellites and co-registering geosynchronous weather satellites images. The significance of these observations was explored using data sets of recent worldwide strong earthquakes (1999–2003) and the techniques used to capture the trace of TIR anomalies.

Global nonlinear stability in porous convection with a thermal non-equilibrium model

Abstract: We show that the global nonlinear stability threshold for convection with a thermal non-equilibrium model is exactly the same as the linear instability boundary. This result is shown to hold for the porous medium equations of Darcy, Forchheimer or Brinkman. This optimal result is important because it shows that linearized instability theory has captured completely the physics of the onset of convection. The equivalence of the linear instability and nonlinear stability boundaries is also demonstrated for thermal convection in a non-equilibrium model with the Darcy law, when the layer rotates with a constant angular velocity about an axis in the same direction as gravity.

Convection Heat Transfer and Flow Calculations Suitable for Analytical Modelling of Electric Machines

Abstract: Motor-CAD is a commercial software package dedicated to the optimisation of motor cooling. Its solver is based on thermal lumped circuit analysis. This provides near instantaneous calculations speeds allowing `what-if' scenarios to be run in real time. The user inputs geometric data for the design they wish to simulate using the graphical radial and cross-section editors. Materials to be used in the machine and the cooling type to be modelled (TENV, TEFC, Liquid Cooling, etc) are selected. All thermal parameters such as conduction, radiation and convection thermal resistances are then calculated by the program and the thermal performance calculated. This article concentrates on the formulations used to predict the convection cooling and flow within the machine.

Mixing times in the mantle of the early Earth derived from 2-D and 3-D numerical simulations of convection

Abstract: [1] The Earth's mantle is chemically heterogeneous at all scales, as shown by elemental and isotopic analysis of oceanic basalts. This heterogeneity is continuously destroyed by convective stirring and slow diffusion. It has been argued that 3-D time-dependent convection is less efficient than 2-D convection, except in the presence of a toroidal component. In this study we question this conclusion with numerical simulations and show that mixing in 3-D time-dependent convection is as efficient as in 2-D, and only depends on convective vigor. We compute mixing times of thermal and chemical heterogeneities in the early Earth of 10 and 100 Myrs respectively.

Onset of convection in Soret-driven instability

Abstract: It is common knowledge that light fluids rise while heavy fluids sink in the gravity field. The most obvious case is the isothermal Rayleigh-Taylor instability when a heavy fluid is placed on top of a light one. In the nonisothermal case, while heating from above, the density stratification is stable in a pure liquid. However, unstable density stratification might be established in a binary mixture with a negative Soret effect in the case of heating from above: the heavier liquid is accumulated on the top of the lighter one. Due to the large differences between viscous, thermal, and diffusion times the system has a tendency to fingering buoyant instabilities. At some moment the flow may be initiated. Near the onset of convection the flow pattern has a columnar convective structure: for a relatively low applied temperature difference Delta T the lighter and colder liquid is drawn up in the central part of the cell and the heavier liquid flows down along the walls. For finite size systems the situation is reversed at higher Delta T. Here we present results of three-dimensional direct numerical simulations of heat and mass transfer in a system with a negative Soret effect. While the development of Soret-induced convection is similar for a wide class of liquids: water based mixtures, colloidal, and polymer solutions, the parameters of the chosen system correspond to a realistic binary mixture of water (90%) and isopropanol (10%) enabling comparison of theoretical predictions with planned experimental studies.

Micromachined gas inertial sensor based on convection heat transfer

Abstract: A micromachined gas inertial sensor based on the principle of convection heat transfer is presented in the paper. The configuration of the sensor consists of a small silicon etched cavity, a suspended central heater and four suspended thermistor wires, all of which are assembled and packaged in a hermetic chamber. The sensor has similar configuration with known thermal accelerometers, but is different and novel because it is not only as a dual-axis accelerometer but also as a single-axis gyroscope. Numerical simulations and primary experiments are performed to validate the effectiveness of the sensor.

Sensitivity analysis of thermal performances of flat plate solar air heaters

Abstract: Sensitivity analysis is a mathematical tool, first developed for optimization methods, which aim is to characterize a system response through the variations of its output parameters following modifications imposed on the input parameters of the system. Such an analysis may quickly become laborious when the thermal model under consideration is complex or the number of input parameters is high. In this paper, we develop a mathematical model to analyse the heat exchanges in four different types of solar air collectors. When building this thermal model we show that for each collector, at quasi-steady state, the energy balance equations of the components of the collector cascade into a single first-order non-linear differential equation that is able to predict the thermal behaviour of the collector. Our heat transfer model clearly demonstrates the existence of an important dimensionless parameter, referred to as the thermal performance factor of the collector, that compares the useful thermal energy which can be extracted from the heater to the overall thermal losses of that collector for a given set of input parameters. A sensitivity analysis of our thermal model has been performed for the most significant input parameters such as the incident solar irradiation, the inlet fluid temperature, the air mass flow rate, the depth of the fluid channel, the number and nature of the transparent covers in order to measure the impact of each of these parameters on our model. An important result which can be drawn from this study is that the heat transfer model developed is robust enough to be used for thermal design studies of most known flat plate solar air heaters, but also of flat plate solar water collectors and linear solar concentrators.

Azimuthal winds, convection and dynamo action in the polar regions of planetary cores

Abstract: We investigate azimuthal winds in planetary cores using a thermal convection-driven dynamo. When inertial forces are not negligible in the equation of motion, the inertially driven thermal winds are cyclonic. When the Lorentz forces are strong enough, we find anticyclonic thermal winds as observed in the Earth's polar region from secular variation data. Under strong thermal convection, the azimuthal flow is created by the magnetic mode with one or more coherent, strong upwellings inside the tangent cylinder (TC), offset from the polar axis. We also find that, as the convection in the TC becomes stronger, these vortex plumes shrink in size, consistent with the convection being controlled by the magnetic field. In addition, strong upwellings in the TC could expel magnetic field in its path, creating regions of weak or even reverse flux patches. These patches drift westward, but at a significantly slower angular speed than the rotation about the vortex itself. Calculations with electrically conducting and st...

Land Surface Temperature Retrieval from CBERS-02 IRMSS Thermal Infrared Data and Its Applications in Quantitative Analysis of Urban Heat Island Effect

Abstract: Improving the Jimenez-Munoz and Sobrino's generalized single-channel method for retrieving land surface temperature from remote sensing data(published at Journal of Geophysical Research,2003,108(doi: 10.1029/2003JD003480) according to the thermal channel's characteristics of CBERS- 02 IRMSS and using its thermal infrared remote sensing data to inverse the land surface temperatures of Beijing and Su-Xi-Chang areas.Field measurement data at Lake Qinghai on August 17,2004 was selected as the criterions to validate the inverse results of LST and the outcomes indicated that the improved single-channel LST inversion algorithm was very suitable for CBERS- 02 IRMSS thermal infrared data and with high accuracy inversion results.Based on the LST retrieved from IRMSS thermal infrared data,a new index called urban thermal field variance index was used to quantitatively analyze the urban heat island effect.The conclusions showed that CBERS- 02 IRMSS thermal infrared data can satisfy the requirements of quantitative applications and have a prodigious application potential.

Magnetospheric convection and thermal ions in the dayside outer magnetosphere

Abstract: [1] We have surveyed 3.5 years of Polar Thermal Ion Dynamics Experiment (TIDE) data between 1 January 2000 and 30 June 2003, when Polar apogee paths (sections of orbits with geocentric distances r > 5 RE) were in the dayside outer magnetosphere, to study the spatial distribution of thermal ions and the magnetospheric convection paths of the thermal ions, as a function of interplanetary magnetic field (IMF) orientation. We have found a dawn-dusk asymmetry in the occurrence of detectable thermal ions above the instrument threshold. The occurrence rate was significantly higher at the duskside. The probability of observing thermal ions, particularly at 1300–1600 local time (LT) near the magnetopause, was >50%, compared with <30% at the dawnside. We interpret the thermal ion events as the result of plasmaspheric drainage plumes, as observed by IMAGE spacecraft or geosynchronous orbiters. The episodic appearance of the thermal ions in the outer magnetosphere could be a significant factor for the dynamo process of global magnetospheric convection. The variation of the convection pattern due to the IMF orientation is consistent with equatorward and poleward reconnection scenarios that superimpose dayside convection driven by reconnection on top of the background convection driven by a viscous interaction at the magnetopause, together with the corotation of the magnetospheric plasma with the ionosphere.

Electronically nonadiabatic thermal reactions of organic molecules.

Abstract: This critical review seeks to bring together organic reactions in which thermal generation of electronic excited states plays an important role. The best known such reactions are those producing chemiluminescent products. However, it appears that there may exist at least as many nonadiabatic reactions in which the excited molecules react before they luminesce. An effort is made to understand the efficiency of excited state production. The crucial roles played by reactive intermediates are highlighted.

Heat transfer between a nano-tip and a surface

Abstract: We study quasi-ballistic heat transfer through air between a hot nanometre-scale tip and a sample. The hot tip/surface configuration is widely used to perform non-intrusive confined heating. Using a Monte Carlo simulation, we find that the thermal conductance reaches 0.8 MW m−2 K−1 on the surface under the tip and show the shape of the heat flux density distribution (nanometre-scale thermal spot). These results show that a surface can be efficiently heated locally without contact. The temporal resolution of the heat transfer is a few tens of picoseconds.

A temperature and mass dependent thermal model for fire response prediction of marine composites

Abstract: An accurate assessment of accumulative damage of a composite ship structure subjected to fire is strongly reliant on the accurate characterization of the time dependent temperature distribution within the composite system. Current state-of-the-art analysis tools assume that thermal–mechanical properties of composite structures are independent of temperature change and mass loss. In this paper, a temperature and mass dependent heat diffusion model is developed to characterize the temperature and mass dependent heat conduction, energy consumption resulting from the decomposition, and the energy transfer associated with vaporous migration. The temperature dependent thermal conductivity and specific heat capacity are determined for the composite at a given resin decomposition stage using a recently developed small-scale test apparatus. Given the temperature dependent thermal properties of the fiber and resin materials, the resulting temperature dependent thermal properties of a woven fabric composite are computed from the newly developed thermal–mechanical analysis tool (TMAT). To assess the effect of using single heating rate based mass loss on the composite fire response prediction, the kinetic parameters are obtained from the post-processing of thermogravimetric analysis (TGA) test data conducted at various heating rates. The effects of temperature dependent thermal conductivity, specific heat capacity, and kinetic parameters determined at different heating rates are explored through the application of the temperature and mass dependent fire model to a composite plate subjected to a hydrocarbon fire. The accuracy of the temperature and mass dependent thermal model is demonstrated by comparing its response prediction with available experimental data.

Using the computational fluid dynamics to analyze the thermal sterilization of solid–liquid food mixture in cans

Abstract: Flow pattern, temperature distribution and shapes of the slowest heating zone during heating of solid–liquid food mixture (pineapple slices saturated with its moisture) in a cylindrical can of 84 mm diameter and 82 mm height are predicted. The partial differential equations describing the conservation of mass, momentum and energy are solved numerically using commercial computational fluid dynamics (CFD) software (PHOENICS), which is based on a finite volume method of analysis. Saturated steam at 121 °C is used as a heating medium, where the metal can is heated from all sides. The model liquid is assumed to have constant properties except for the viscosity (temperature dependent) and density (Boussinesq approximation). Two methods of analysis are adopted in the simulation. In one of the methods, the pineapple slices are assumed permeable to natural convection flow in its pores, while in the second method, the pineapple slices are assumed impermeable. The results of the simulations of both cases are very similar. The simulations show, the action of natural convection on the rate of heating, liquid flow pattern and on the shape and movement of the slowest heating zone (SHZ). The SHZ eventually stays in a region that is about 30–35% of the can height from the bottom. Industrial relevance The authors indentified the interesting and obviousely so far neglected topic of natural convection during sterilization heating of solid-liquid food mixtures in containers. The by simulation results obtained suggest that the configuration of the solid in the container can significantly influence the rate of heating. This may also be of significance for other processes such as high hydrostatic pressure treatment or ohmic heating.

Thermal design of high-power LED package and system

Abstract: In this paper we present thermal analysis of three kinds of ceramic package designs for high power LEDs and thermal characterization of high power LED array system. The analysis was made by transient thermal measurement and thermal simulation using the finite volume method (FVM). For the package design, thermal behaviors, as are described in thermal resistance, of the three packaging designs were compared and evaluated as functions of bulk thermal resistance, spreading resistance, and surface roughness. The deviation between the simulated results and measured data were attributed to the different surface roughness in the interfaces between the packaging components. For the system design, the emphasis is placed upon the investigation of junction temperature rise of LED array for a limited range of boundary conditions which include design effect of heat pipe, convection condition, and ambient temperature. It was found out that the measured junction temperatures and thermal resistance of LED array are increased with the input power and ambient temperature and decreased with the air velocity. An analytical thermal model analogous with an equivalent parallel circuit system was proposed and was verified by comparison with experimental data.

Application of the thermal quadrupole method to the propagation of thermal waves in multilayered cylinders

Abstract: Up to now, research in photothermal techniques has been mainly restricted to samples with flat surfaces. In this work the surface temperature oscillation of multilayered cylindrical samples which are heated by a modulated light beam is calculated by using the quadrupole method. Different illumination geometries have been studied. Moreover, the lack of adherence between layers, as well as heat losses at the surface, has been considered in the model. Following this theoretical approach, photothermal techniques can be used for the quantitative thermophysical characterization of cylindrical samples with continuously varying in-depth thermal conductivity.