Showing papers on "Heat transfer published in 1999"

A twin fluid-bed reactor for removal of CO2 from combustion processes

Abstract: A new process is proposed for CO2 removal from flue gas using the reaction CaO + CO2 ↔ CaCO3. This process consists of two fluidized bed reactors connected by solid transportation lines. In one reactor (absorber), CO2 in the flue gas is captured by CaO at 873 K and the produced CaCO3 is transported to another reactor (regenerator), in which CaCO3 is decomposed to CaO at 1223 K. The produced CaO is transported to the absorber again. The heat of decomposition in the regenerator is supplied by feeding coal and pure oxygen, thus the flue gas from the regenerator is high purity CO2 (>95%, dry base). In this work, a conceptual study is conducted for material balance, heat balance, power generation, and power consumption for O2 production and CO2 liquefaction (compression). Also, a kinetic study of CaO+CO2 → CaCO3 was conducted to design the absorber. The required bed height of the absorber was calculated by use of a bubbling fluidized bed model. The bed height was found to be determined not by the chemical reaction rate but by the arrangement of heat transfer tubes for heat recovery.

Dynamics and Heat Transfer Associated With a Single Bubble During Nucleate Boiling on a Horizontal Surface

Abstract: In this study, a complete numerical simulation of a growing and departing bubble on a horizontal surface has been performed. A finite difference scheme is used to solve the equations governing conservation of mass, momentum, and energy in the vapor-liquid layers. The vapor-liquid interface is captured by a level set method which is modified to include the influence of phase change at the liquid-vapor interphase. The disjoining pressure effect is included in the numerical analysis to account for heat transfer through the liquid microlayer. From the numerical simulation, the location where the vapor-liquid interface contacts the wall is observed to expand and then retract as the bubble grows and departs. The effect of static contact angle and wall superheat on bubble dynamics has been quantified. The bubble growth predicted from numerical analysis has been found to compare well with the experimental data reported in the literature and that obtained in this work

Operating Characteristics of Loop Heat Pipes

Abstract: The loop heat pipe (LHP) was invented in Russia in the early 1980’s. It is a two-phase heat transfer device that utilizes the evaporation and condensation of a working fluid to transfer heat, and the capillary forces developed in fine porous wicks to circulate the fluid. The LHP is known for its high pumping capability and robust operation because it uses finepored metal wicks and the integral evaporator/hydroaccumulator design. It has gained rapid acceptance in recent years as a thermal control device in space applications. This paper presents an overview of the LHP operation. The physical processes and the thermal-hydraulic behaviors of the LHP are first described. Operating characteristics as functions of various parameters including the heat load, sink temperature, ambient temperature, and elevation are presented. Peculiar behaviors in LHP operation such as temperature hysteresis and temperature overshoot during start-up are explained. Issues related to multiple -evaporator LHP’s are also addressed.

New Insights into the Ocean Heat Budget Closure Problem from Analysis of the SOC Air–Sea Flux Climatology

Abstract: Results from an analysis of the Southampton Oceanography Centre (SOC) global air–sea heat flux climatology, which has been calculated using in situ weather reports from voluntary observing ships covering the period 1980–93, are presented. Systematic errors in the fluxes arising from differences in observing procedure have been quantified and corrected; the magnitude of these errors is up to 15 W m−2 with strong seasonal and regional variations. Despite these corrections, closure of the ocean heat budget is not obtained as the global mean net heat flux is an oceanic gain of 30 W m−2. The validity of closing the heat budget by global scaling of the flux components is assessed by comparison of the SOC flux fields with Woods Hole Oceanographic Institute research buoy measurements. The level of agreement between the two is found to vary from one site to another. Thus, closure of the ocean heat budget requires regional adjustments to the flux components in order to avoid significant biases in the adjus...

New thermal wave aspects on burn evaluation of skin subjected to instantaneous heating

Abstract: Comparative studies on the well-known Pennes' equation and the newly developed thermal wave model of bioheat transfer (TWMBT) were performed to investigate the wave like behaviors of bioheat transfer occurred in thermal injury of biological bodies. The one-dimensional TWMBT in a finite medium was solved using separation of variables and the analytical solution showed distinctive wave behaviors of bioheat transfer in skin subjected to instantaneous heating. The finite difference method was used to simulate and study practical problems involved in burn injuries in which skin was stratified as three layers with various thermal physical properties. Deviations between the TWMBT and the traditional Pennes' equation imply that, for high flux heating with extremely short duration (i.e., flash fire), the TWMBT which accounts for finite thermal wave propagation may provide realistic predictions on burn evaluation. A general heat flux criterion has been established to determine when the thermal wave propagation dominates the principal heat transfer process and the TWMBT can be used for tissue temperature prediction and burn evaluation. A preliminary interpretation on the mechanisms of the wave like behaviors of heat transfer in living tissues was conducted. The application of thermal wave theory can also be possibly extended to other medical problems which involve instantaneous heating or cooling.

A short time step response factor model for vertical ground loop heat exchangers

Abstract: The short-term behavior of ground-coupled heat pump systems is important for design of ground loop heat exchangers, energy analysis of ground source heat pump systems, and design of hybrid ground source systems. This paper describes the development of short time-step temperature response factors for vertical ground loop heat exchangers as used in ground-coupled heat pump systems. The short time-step response factors allow for a direct evaluation of system energy consumption and electrical demand in hourly or shorter time intervals. The development of the temperature response factors is based on an analytically validated, transient two-dimensional implicit finite volume model designed for the simulation of heat transfer over a vertical U-tube ground heat exchanger. The short time-step response factors are implemented as part of a component model for TRNSYS and an example application is provided based on an actual building.

A Macroscopic Turbulence Model for Flow in a Porous Medium

Abstract: A complete set of macroscopic two-equation turbulence model equations has been established for analyzing turbulent flow and heat transfer within porous media. The volume-averaged transport equations for the mass, momentum, energy, turbulence kinetic energy and its dissipation rate were derived by spatially averaging the Reynolds-averaged set of the governing equations. The additional terms representing production and dissipation of turbulence kinetic energy are modeled introducing two unknown model constants, which are determined from a numerical experiment using a spatially periodic array. In order to investigate the validity of the present macroscopic turbulence model, a macroscopically unidirectional turbulent flow through an infinite array of square rods is considered from both micro- and macroscopic-views

Cryogenic heat transfer

Abstract: Presents applied heat transfer principles in the range of extremely low temperatures. The specific features of heat transfer at cryogenic temperatures, such as variable properties, near critical convection, and Kapitza resistance, are described. This book includes many example problems, in each section, that help to illustrate the applications of t

Construction of the adjoint MIT ocean general circulation model and application to Atlantic heat transport sensitivity

Abstract: We first describe the principles and practical considerations behind the computer generation of the adjoint to the Massachusetts Institute of Technology ocean general circulation model (GCM) using R. Giering's software tool Tangent-Linear and Adjoint Model Compiler (TAMC). The TAMC's recipe for (FORTRAN-) line-by-line generation of adjoint code is explained by interpreting an adjoint model strictly as the operator that gives the sensitivity of the output of a model to its input. Then, the sensitivity of 1993 annual mean heat transport across 29oN in the Atlantic, to the hydrography on January 1, 1993, is calculated from a global solution of the GCM. The "kinematic sensitivity" to initial temperature variations is isolated, showing how the latter would influence heat transport if they did not affect the density and hence the flow. Over 1 year the heat transport at 29oN is influenced kinematically from regions up to 20 o upstream in the western boundary current and up to 5 o upstream in the interior. In contrast, the dynamical influences of initial temperature (and salinity) perturbations spread from as far as the rim of the Labrador Sea to the 29oN section along the western boundary. The sensitivities calculated with the adjoint compare excellently to those from a perturbation calculation with the dynamical model. Perturbations in initial interior salinity influence meridional overturning and heat transport when they have propagated to the western boundary and can thus influence the integrated east-west density difference. Our results support the notion that boundary monitoring of meridional mass and heat transports is feasible.

Non-Newtonian flow in the process industries : fundamentals and engineering applications

Abstract: Non-Newtonian fluid behaviour Rheometry for non-Newtonian fluids Flow in pipes and conduits of non-circular cross-sections Flow of multi-phase mixtures in pipes Particulate systems Heat transfer characteristics of non-Newtonian fluids in pipes Momentum, heat and mass transfer in boundary layers Liquid mixing.

White layers and thermal modeling of hard turned surfaces

Abstract: White layers in hard turned surfaces are identified, characterized and measured as a function of tool flank wear and cutting speed. White layer depth progressively increases with flank wear. It also increases with speed, but approaches an asymptote. A thermal model based on Jaeger's moving heat source problems (J.C. Jaeger, Moving source of heat and the temperature at sliding contacts, in: Proceedings of the Royal Society, NSW, vol. 56, pp. 203–224) is applied to simulate the temperature field in machined surfaces and to estimate white layer depth in terms of the penetration depth for a given critical temperature. The analysis shows good agreement with the trend in experimental results. White layer formation seems to be dominantly a thermal process involving phase transformation of the steel, possibly plastic strain activated; flank wear land rubbing may be a primary heat source for white layer formation. A strong material dependence of surface alteration is also observed.

A transient two-dimensional finite volume model for the simulation of vertical U-tube ground heat exchangers

Abstract: The ability to predict both the long-term and short-term behavior of ground loop heat exchangers is critical to the design and energy analysis of ground source heat pump systems. A numerical model for the simulation of transient heat transfer in vertical ground loop heat exchangers is presented. The model is based on a two-dimensional fully implicit finite volume formulation. Numerical grids have been generated for different pipe sizes, shank spacing and borehole sizes using an automated parametric grid generation algorithm. The numerical method and grid generation techniques have been validated against an analytical model. The model has been developed with two main purposes in mind. The first application is used in a parameter estimation technique used to find the borehole thermal properties from short time scale test data. The second application is the calculation of nondimensional temperature response factors for short time scales that can be used in annual energy simulation.

Mixing and Energetics of the Oceanic Thermohaline Circulation

Abstract: Using an idealized tube model and scaling analysis, the physics supporting the oceanic thermohaline circulation is examined. Thermal circulation in the tube model can be classified into two categories. When the cooling source is at a level higher than that of the heating source, the thermal circulation is friction-controlled; thus, mixing is not important in determining the circulation rate. When the cooling source is at a level lower than that of the heating source, the circulation is mixing controlled; thus, weak (strong) mixing will lead to weak (strong) thermal circulation. Within realistic parameter regimes the thermohaline circulation requires external sources of mechanical energy to support mixing in order to maintain the basic stratification. Thus, the oceanic circulation is only a heat conveyor belt, not a heat engine. Simple scaling shows that the meridional mass and heat fluxes are linearly proportional to the energy supplied to mixing. The rate of tidal dissipation in the open oceans (excluding the shallow marginal seas) is about 0.9‐1.3 (31012 W); the rate of potential energy generated by geothermal heating is estimated to be 0.5 3 1012 W. Accordingly, the global-mean rate of mixing inferred from oceanic climatological data is about 0.22 3 1024 m2 s21. Using a primitive equation model, numerical experiments based on a fixed energy source for mixing have been carried out in order to test the scaling law. In comparison with models under fixed rate of mixing, a model under a fixed energy for mixing is less sensitive to changes in the forcing conditions due to climatic changes. Under a surface relaxation condition for temperature and standard parameters, the model is well within the region of Hopf bifurcation, so decadal variability is expected.

Thermal conductivity and diffusivity of wood.

Abstract: Transient simultaneous measurements of thermal conductivity and diffusivity of Swedish wood have been performed with the plane source technique on oven-dry hardwood (birch) samples at room temperature and at 100 °C. The influences of temperature, density, porosity and anisotropy on thermal conduction were investigated. The measurements were done in longitudinal (parallel to the grain) and transverse (intermediate between radial and tangential) directions. As the temperature increased from 20 to 100 °C, the thermal conductivity of each sample increased slightly for both longitudinal and transverse directions. The effect of density and porosity on the thermal conductivity may be related to the presence of other scattering mechanisms such as voids and cell boundaries. It seems that the dominant mechanism of heat transfer across the cell lumina in these types of wood is the heat conduction through the voids. An attempt was made to explain the behaviour of the effective thermal conductivity by adopting a model based on the ratio between heat conduction in parallel and serial layers of gas, liquid, and solid phases.

Novel thermal properties of nanostructured materials.

Abstract: A new class of heat transfer fluids, termed nanofluids, has been developed by suspending nanocrystalline particles in liquids. Due to the orders-of-magnitude larger thermal conductivities of solids compared to those of liquids such as water, significantly enhanced thermal properties are obtained with nanofluids. For example, an approximately 20% improvement in effective thermal conductivity is observed when 5 vol.% CuO nanoparticles are added to water. Even more importantly, the heat transfer coefficient of water under dynamic flow conditions is increased more than 15% with the addition of less than 1 vol.% CuO particles. The use of nanofluids could impact many industrial sectors, including transportation, energy supply and production, electronics, textiles, and paper production by, for example, decreasing pumping power needs or reducing heat exchanger sizes. In contrast to the enhancement in effective thermal transport rates that is obtained when nanoparticles are suspended in fluids, nanocrystalline coatings are expected to exhibit reduced thermal conductivities compared to coarse-grained coatings. Reduced thermal conductivities are predicted to arise because of a reduction in the mean free path of phonons due to presence of grain boundaries. This behavior, combined with improved mechanical properties, makes nanostructured zirconia coatings excellent candidates for future applications as thermal barriers. Yttria-stabilizedmore » zirconia (YSZ) thin films are being produced by metal-organic chemical vapor deposition techniques. Preliminary results have indicated that the thermal conductivity is reduced by approximately a factor-of-two at room temperature in 10 nm grain-sized YSZ compared to coarse-grained or single crystal YSZ.« less

Impact of nonlinear heat transfer on temperature control in regional hyperthermia

Abstract: We describe an optimization process specially designed for regional hyperthermia of deep-seated tumors in order to achieve desired steady-state temperature distributions. A nonlinear three-dimensional heat transfer model based on temperature-dependent blood perfusion is applied to predict the temperature. Using linearly implicit methods in time and adaptive multilevel finite elements in space, we are able to integrate efficiently the instationary nonlinear heat equation with high accuracy. Optimal heating is obtained by minimizing an integral objective function which measures the distance between desired and model predicted temperatures. A sequence of minima is calculated from successively improved constant-rate perfusion models employing a damped Newton method in an inner iteration. We compare temperature distributions for two individual patients calculated on coarse and fine spatial grids and present numerical results of optimizations for a Sigma 60 Applicator of the BSD 2000 Hyperthermia System.

Flat Miniature Heat Pipes With Micro Capillary Grooves

Abstract: Flat miniature heat pipes (FMHP's) are shown to be very promising in the cooling of electronic component systems. This investigation presents a detailed experimental and theoretical analysis on maximum heat transfer capabilities of two copper-water FMHP's with diagonal trapezoidal micro capillary grooves and one copper-water FMHP with axial rectangular micro capillary grooves. Maximum heat flux on the evaporator wall of the 120-mm long axial grooved heat pipe, with a vapor channel cross-sectional area of approximately 1.5 x 12 mm 2 and rectangular grooves of dimensions 0.20 mm wide by 0.42 mm deep, exceeded 90 W/cm 2 in the horizontal orientation and 150 W/cm 2 in the vertical orientation. Theoretical prediction of the capillary limitation in the horizontal orientation agreed reasonably well with the experimental data.