Showing papers on "Heat transfer published in 1996"

Bulk parameterization of air‐sea fluxes for Tropical Ocean‐Global Atmosphere Coupled‐Ocean Atmosphere Response Experiment

Abstract: This paper describes the various physical processes relating near-surface atmospheric and oceanographic bulk variables ; their relationship to the surface fluxes of momentum, sensible heat, and latent heat ; and their expression in a bulk flux algorithm. The algorithm follows the standard Monin-Obukhov similarity approach for near-surface meteorological measurements but includes separate models for the ocean's cool skin and the diurnal warm layer, which are used to derive true skin temperature from the bulk temperature measured at some depth near the surface. The basic structure is an outgrowth of the Liu-Katsaros-Businger [Liu et al., 1979] method, with modifications to include a different specification of the roughness/stress relationship, a gustiness velocity to account for the additional flux induced by boundary layer scale variability, and profile functions obeying the convective limit. Additionally, we have considered the contributions of the sensible heat carried by precipitation and the requirement that the net dry mass flux be zero (the so-called Webb correction [Webb et al., 1980]). The algorithm has been tuned to fit measurements made on the R/V Moana Wave in the three different cruise legs made during the Coupled Ocean-Atmosphere Response Experiment. These measurements yielded 1622 fifty-min averages of fluxes and bulk variables in the wind speed range from 0.5 to 10 m s -1 . The analysis gives statistically reliable values for the Charnock [1955] constant (a = 0.011) and the gustiness parameter (β = 1.25). An overall mean value for the latent heat flux, neutral bulk-transfer coefficient was 1.11 x 10 -3 , declining slightly with increasing wind speed. Mean values for the sensible and latent heat fluxes were 9.1 and 103.5 W m -2 ; mean values for the Webb and rain heat fluxes were 2.5 and 4.5 W m -2 . Accounting for all factors, the net surface heat transfer to the ocean was 17.9 ± 10 W m -2 .

Entropy generation minimization: The new thermodynamics of finite-size devices and finite-time processes

Abstract: Entropy generation minimization (finite time thermodynamics, or thermodynamic optimization) is the method that combines into simple models the most basic concepts of heat transfer, fluid mechanics, and thermodynamics. These simple models are used in the optimization of real (irreversible) devices and processes, subject to finite‐size and finite‐time constraints. The review traces the development and adoption of the method in several sectors of mainstream thermal engineering and science: cryogenics, heat transfer, education, storage systems, solar power plants, nuclear and fossil power plants, and refrigerators. Emphasis is placed on the fundamental and technological importance of the optimization method and its results, the pedagogical merits of the method, and the chronological development of the field.

Cool-skin and warm-layer effects on sea surface temperature

Abstract: To obtain bulk surface flux estimates approaching the ±10 W m−2 accuracy desired for the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (COARE) program, bulk water temperature data from ships and buoys must be corrected for cool-skin and diurnal warm-layer effects. In this paper we describe two simple scaling models to estimate these corrections. The cool-skin model is based on the standard Saunders [1967] treatment, including the effects of solar radiation absorption, modified to include both shear-driven and convectively driven turbulence through their relative contributions to the near-surface turbulent kinetic energy dissipation rate. Shear and convective effects are comparable at a wind speed of about 2.5 m s−1. For the R/V Moana Wave COARE data collected in the tropical western Pacific, the model gives an average cool skin of 0.30 K at night and an average local noon value of 0.18 K. The warm-layer model is based on a single-layer scaling version of a model by Price et al. [1986]. In this model, once solar heating of the ocean exceeds the combined cooling by turbulent scalar heat transfer and net longwave radiation, then the main body of the mixed layer is cut off from its source of turbulence. Thereafter, surface inputs of heat and momentum are confined to a depth DT that is determined by the subsequent integrals of the heat and momentum. The model assumes linear profiles of temperature-induced and surface-stress-induced current in this “warm layer.” The model is shown to describe the peak afternoon warming and diurnal cycle of the warming quite accurately, on average, with a choice of a critical Richardson number of 0.65. For a clear day with a 10-m wind speed of 1 m s−1, the peak afternoon warming is about 3.8 K with a warm-layer depth of 0.7 m, decreasing to about 0.2 K and 19 m at a wind speed of 7 m s−1. For an average over 70 days sampled during COARE, the cool skin increases the average atmospheric heat input to the ocean by about 11 W m−2; the warm layer decreases it by about 4 W m−2 (but the effect can be 50 W m−2 at midday).

Radiation effect on mixed convection along a vertical plate with uniform surface temperature

Abstract: This paper investigates the effect of radiation on the forced and free convection flow of an optically dense viscous incompressible fluid along a heated vertical flat plate with uniform free stream and uniform surface temperature with Rosseland diffusion approximation. With appropriate transformations, the boundary layer equations governing the flow are reduced to local nonsimilarity equations valid in the forced convection regime as well as in the free convection regime. A group of transformation is, also, introduced to reduce the boundary layer equations to a set of local nonsimilarity equations valid in both the forced and free convection regimes. Solutions of the governing equations are obtained by employing the implicit finite difference methods together with Keller box scheme and are expressed in terms of local shear stress and local rate of heat transfer for a range of values of the pertinent parameters.

Basic heat transfer

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Nonlocal and Nonequilibrium Heat Conduction in the Vicinity of Nanoparticles

Abstract: Heat transfer around nanometer-scale particles plays an important role in a number of contemporary technologies such as nanofabrication and diagnosis The prevailing method for modeling thermal phenomena involving nanoparticles is based on the Fourier heat conduction theory This work questions the applicability of the Fourier heat conduction theory to these cases and answers the question by solving the Boltzmann transport equation The solution approaches the prediction of the Fourier law when the particle radius is much larger than the heat-carrier mean free path of the host medium In the opposite limit, however, the heat transfer rate from the particle is significantly smaller, and thus the particle temperature rise is much larger than the prediction of the Fourier conduction theory The differences are attributed to the nonlocal and nonequilibrium nature of the heat transfer processes around nanoparticles This work also establishes a criterion to determine the applicability of the Fourier heat conduction theory and constructs a simple approximate expression for calculating the effective thermal conductivity of the host medium around a nanoparticle Possible experimental evidence is discussed

Introduction to thermodynamics and heat transfer

Abstract: Intro to Thermodynamics and Heat Transfer 2e 1 Introduction and Overview Part 1 Thermodynamics 2 Introduction and Basic Concepts 3 Energy, Energy Transfer, and General Energy Analysis 4 Properties of Pure Substances 5 Energy Analysis of Closed Systems 6 Mass and Energy Analysis of Control Volumes 7 The Second Law of Thermodynamics 8 Entropy Part 2 Heat Transfer 9 Mechanisms of Heat Transfer 10 Steady Heat Conduction 11 Transient Heat Conduction 12 External Forced Convection 13 Internal Forced Convection 14 Natural Convection 15 Radiation Heat Transfer 16 Heat Exchangers Appendix 1 Property Tables and Charts (SI Units) Appendix 2 Property Tables and Charts (English Units)

A theoretical approach to predict the performance of chevron-type plate heat exchangers

Abstract: Manufacturers of plate and frame heat exchangers nowadays mainly offer plates with chevron (or herringbone) corrugation patterns. The inclination angleof the crests and furrows of that sinusoidal pattern relative to the main flow direction has been shown to be the most important design parameter with respect to fluid friction and heat transfer. Two kinds of flow may exist in the gap between two plates (pressed together with the chevron pattern of the second plate turned into the opposite direction): the crossing flow of small substreams following the furrows of the first and the second plate, respectively, over the whole width of the corrugation pattern, dominating at lower inclination angles (lower pressure drop); and the wavy longitudinal flow between two vertical rows of contact points, prevailing at highangles (high pressure drop). The combined effects of the longer flow paths along the furrows, the crossing of the substreams, flow reversal at the edges of the chevron pattern, and the competition between crossing and longitudinal flow are taken into account to derive a relatively simple but physically reasonable equation for the friction factor ξ as a function of the angleand the Reynolds number Re. Heat-transfer coefficients are then obtained from a theoretical equation for developing thermal boundary layers in fully developed laminar or turbulent channel flow — the generalized Leveque equation — predicting heat-transfer coefficients as being proportional to (ξ·Re2)1/3. It is shown, by comparison, that this prediction is in good agreement with experimental observations quoted in the literature.

The Finite Element Method in Heat Transfer Analysis

Abstract: Conduction Heat Transfer and Formulation. Linear Steady State Problems. Time Stepping Methods for Heat Transfer. Non--Linear Heat Conduction Analysis. Phase Change Problems----Solidification and Melting. Convective Heat Transfer. Nomenclature. Index.

Combustion in a Porous Medium-Advances and Applications

Abstract: In this paper research and development work of the authors is described, which led to the design of a new combustor-heat exchanger system based on the combustion in porous media. Combustion in inert porous media is possible if the Peclet-number is high enough ( > 65), so that quenching of the flame inside the pores is prohibited. The heat transfer from the combustion zone, to the porous medium itself is very effective because of the very large surface between them. The combustion temperature can be controlled through the porous medium temperature. Prompt and thermal NOx formation, which is temperature dependent, can be controlled by appropriate cooling of the combustion zone. The heat transfer mechanisms in the combustor are discussed and new designs of porous materials are proposed, which allow a high power density and a better control of the temperature level in the combustion zone. The possible application field and the expected benefits of this combustion technique are discussed.

Darryl E. Metzger Memorial Session Paper: Experimental Heat Transfer and Friction in Channels Roughened With Angled, V-Shaped, and Discrete Ribs on Two Opposite Walls

Abstract: Experimental investigations have shown that the enhancement in heat transfer coefficients for air flow in a channel roughened with angled ribs is on the average higher than that roughened with 90 deg ribs of the same geometry. Secondary flows generated by the angled ribs are believed to be responsible for these higher heat transfer coefficients. In an effort basically to double the area of high heat transfer coefficients, the angled rib is broken at the center to form a V-shaped rib, and tests are conducted to investigate the resulting heat transfer coefficients and friction factors. Three different square rib geometries, corresponding to blockage ratios of 0.083, 0.125, and 0.167, with a fixed pitch-to-height ratio of 10, mounted on two opposite walls of a square channel in a staggered configuration, are tested in a stationary channel for 5,000 < Re < 30,000. Heat transfer coefficients, friction factors, and thermal performances are compared with those of 90 deg, 45 deg, and discrete angled ribs. The V-shaped ribs are tested for both pointing upstream and downstream of the main flow. Test results show that: (a) 90 deg ribs represent the lowest thermal performance, based on the same pumping power, and is essentially themore » same for the 2:1 change in blockage ratio, (b) low-blockage-ratio (E/D{sub h} = 0.083) V-shaped ribs pointing downstream produced the highest heat transfer enhancement and friction factors. Among all other geometries with blockage ratios of 0.125 and 0.167, 45 deg ribs showed the highest heat transfer enhancements with friction factors less than those of V-shaped ribs, (c) thermal performance of 45 deg ribs and the lowest blockage discrete ribs are among the highest of the geometries tested in this investigation, and (d) discrete angled ribs, although inferior to 45 deg and V-shaped ribs, produce much higher heat transfer coefficients and lower friction factors compared to 90 deg ribs.« less

The Effect of Periodic Ribs on the Local Aerodynamic and Heat Transfer Performance of a Straight Cooling Channel

Abstract: The local aerodynamic and heat transfer performance were measured in a rib-roughened square duct as a function of the rib pitch to height ratio. The blockage ratio of these square obstacles was 10 or 20 percent depending on whether they were placed on one single (1s) or on two opposite walls (2s). The Reynolds number, based on the channel mean velocity and hydraulic diameter, was fixed at 30,000. The aerodynamic description of the flow field was based on local pressure distributions along the ribbed and adjacent smooth walls as well as on two-dimensional LDV explorations in the channel symmetry plane and in two planes parallel to the ribbed wall(s). Local heat transfer distributions were obtained on the floor, between the ribs, and on the adjacent smooth side wall. Averaged parameters, such as friction factor and averaged heat transfer enhancement factor, were calculated from the local results and compared to correlations given in literature. This contribution showed that simple correlations derived from the law of the wall similarity and from the Reynolds analogy could not be applied for the present rib height-to-channel hydraulic diameter ratio (e/D h = 0.1). The strong secondary flows resulted in a three-dimensional flow field with high gradients in the local heat transfer distributions on the smooth side walls.

Mechanisms of Heat Exchange: Biophysics and Physiology

Abstract: The sections in this article are: 1 Human Heat Balance Equation 2 Independent Variables Affecting the Thermal Environment 2.1 Ambient Temperature 2.2 Dew Point Temperature and Ambient Vapor Pressure 2.3 Air and Fluid Movement 2.4 Mean Radiant Temperature and Effective Radiant Field 2.5 Clothing Insulation 2.6 Barometric Pressure 3 Peripheral Factors to Heat Exchange 3.1 Mean Skin Temperature 3.2 Skin Wettedness 3.3 Body Heat Storage and Rate of Change of Mean Body Temperature 3.4 Metabolic Energy 4 Sensible Heat Exchange by Radiation and Convection 4.1 Operative Temperature 4.2 Clothing Properties Effective in Sensible Heat Exchange 5 Radiation Exchange 5.1 Mean Radiant Temperature and Effective Radiant Field 6 Convective Heat Exchange 6.1 Heat Transfer Theory 6.2 Measurement of the Convective Heat Transfer Coefficient 6.3 Effect of Altitude (Barometric Pressure) on Convective Heat Loss 7 Evaporative Heat Exchange 7.1 Direct Measurement of Evaporative Heat Loss 8 Psychrometrics of the Human Heat Balance Equation 8.1 Lewis Relation: Interpretation of Wet-Bulb Temperature and Enthalpy 8.2 Generalization to Energy Transfer between Humans and the Environment 8.3 Enthalpy of the Human Environment 8.4 Enthalpy and the Rational Indices of the Human Environment 9 Pierce Two-Node Model of Thermoregulation 9.1 The Passive State 9.2 The Control System 9.3 Initial Warm and Cold Signals 9.4 Control of Skin Blood Flow 9.5 Control of the Whole-Body Sweating Drive 9.6 Control of the Skin Shell Properties 9.7 Control of Shivering 10 Heat Loss Factors in Warm and Cold Environments: Physiology 10.1 Heat and Mass Transfer from the Body to the Environment 10.2 Body Motion 10.3 Thermal Aspects 10.4 Thermoregulatory Control Relative to Heat Loss Factors 10.5 Physiological Responses of Heat Loss by Thermal Radiation and Adaptive Response 11 Heat Loss in Special Environments 11.1 Hypobaric Environments 11.2 Hyperbaric Environments 11.3 Aquatic Environments 11.4 Heat Exchange in Spacecraft

Natural convection in the annulus between concentric horizontal circular and square cylinders

Abstract: Numerical solutions are presented for natural convection heat transfer between a heated horizontal cylinder placed concentrically inside a square enclosure. Three different aspect ratios (R/L 0.1, 0.2, and 0.3), and four different Rayleigh numbers (Ra = 10, 10, 10, and 10), are considered. The governing elliptic conservation equations are solved in a boundary-fitted coordinate system using a control volumebased numerical procedure. Results are displayed in the form of streamlines, isotherms, maximum stream function estimates, and local and average normalized Nusselt number values. At constant enclosure aspect ratio, the total heat transfer increases with increasing Rayleigh number. For constant Rayleigh number values, convection contribution to the total heat transfer decreases with increasing values of R/L. For convection-dominated flows, the average Nusselt number correlation is expressed as Nu = 0.92Ra(R/ L)*. Generated results are in good agreement with previously published experimental and numerical data.

Kinetic and Heat Transfer Control in the Slow and Flash Pyrolysis of Solids

Abstract: The coupled effects of particle size and external heating conditions (reactor heating rate and final temperature) on cellulose pyrolysis are investigated by means of a computer model accounting for all main transport phenomena, variable thermophysical properties and primary, and secondary reaction processes. The dynamics of particle conversion are predicted, and final product distributions are favorably compared with experimental measurements. A map is constructed, in terms of particle size as a function of the reactor temperature, to identify the transition from a kinetically controlled conversion to a heat transfer controlled conversion (thermally thin and thermally thick regimes) and from flash to slow-conventional pyrolysis. Conditions for maximizing oil, gas, or char yields are also discussed.

On the thermal evolution of the Earth's core

Abstract: The Earth's magnetic field is sustained by dynamo action in the fluid outer core. The energy sources available to the geodynamo are well established, but their relative importance remains uncertain. We focus on the issue of thermal versus compositional convection, which is inextricably coupled to the evolution of the core as the Earth cools. To investigate the effect of the various physical processes on this evolution, we develop models based on conservation of energy and the assumption that the core is well mixed by vigorous convection. We depart from previous numerical studies by developing an analytical model. The simple algebraic form of the solution affords insight into both the evolution of the core and the energy budget of the geodynamo. We also present a numerical model to compare with the quantitative predictions of our analytical model and find that the differences between the two are negligible. An important conclusion of this study is that thermal convection can contribute significantly to the geodynamo. In fact, a modest heat flux in excess of that conducted down the adiabatic gradient is sufficient to power the geodynamo, even in the absence of compositional convection and latent-heat release. The relative contributions of thermal and compositional convection to the dynamo are largely determined by the magnitude of the heat flux from the core and the inner-core radius. For a plausible current-day heat flux of Q = 3.0 × 1012 W and the current inner-core radius, we find that compositional convection is responsible for approximately two thirds of the ohmic dissipation in the core and thermal convection for the remaining one third. The proportion of ohmic dissipation produced by thermal convection increases to 45% with an increase in Q to 6.0 × 1012 W. In the early Earth, when the inner core was smaller and the heat flux probably greater than the present values, thermal convection would have been the dominant energy source for the dynamo. We also calculate the history of inner-core growth as a function of the heat flux. For example, the inner core would have grown to its present size in 2.8 × 109 years if the average heat flux was Q = 4.0 × 1012 W. The model does not require the heat flux to be constant.

Advances in Numerical Heat Transfer

Abstract: 1.High-Perfomance Computing for Fluid Flow and Heat Transfer 2.Unstructured Finite Volume Methods for Multi-Mode Heat Transfer 3.SpectralElement Methods for Unsteady Fluid Flow and Heat Transfer in Complex Geometries:Methodology and Applications 4.Finite-Volume Method for Radiation Heat Transfer 5.Boundary Element Methods for Heat Conduction 6.Molecular Dynamics Method for Microscale Heat Transfer 7.Numerical Methods in Microscale Heat Transfer:Modeling of Phase-Change and Laser Interactions with Materials 8.Current Status of the Use of Parallel Computing in Turbulent Reacting Flows:Computations Involving Sprays, Scalar Monte Carlo Probability Density Function and Unstructured Grids 9.Overview of Current Computational Studies of Heat Transfer in Porous Media and Their Applications-Forced Convection and Multiphase Heat Transfer 10.Overview of Current Computational Studies of Heat Transfer in Porous Media and Their Applications-Natural and Mixed Convection 11.Recent Progress and Some Challenges in Thermal Modeling of Electronic Systems 12.Index

A Particle Numerical Model for Wall Film Dynamics in Port-Injected Engines

Abstract: To help predict hydrocarbon emissions during cold-start conditions the authors are developing a numerical model for the dynamics and vaporization of the liquid wall films formed in port-injected spark-ignition engines and incorporating this model in the KIVA-3 code for complex geometries. This paper summarizes the current status of the project and presents illustrative example calculations. The dynamics of the wall film is influenced by interactions with the impinging spray, the wall, and the gas flow near the wall. The spray influences the film through mass, tangential momentum, and energy addition. The wall affects the film through the no-slip boundary condition and heat transfer. The gas alters film dynamics through tangential stresses and heat and mass transfer in the gas boundary layers above the films. New wall functions are given to predict transport in the boundary layers above the vaporizing films. It is assumed the films are sufficiently thin that film flow is laminar and that liquid inertial forces are negligible. Because liquid Prandtl numbers are typically about then, unsteady heating of the film should be important and is accounted for by the model. The thin film approximation breaks down near sharp corners, where an inertial separation criterion is used. A particle numerical method is used for the wall film. This has the advantages of compatibility with the KIVA-3 spray model and of very accurate calculation of convective transport of the film. The authors have incorporated the wall film model into KIVA-3, and the resulting combined model can be used to simulate the coupled port and cylinder flows in modern spark-ignition engines. They give examples by comparing computed fuel distributions with closed- and open-valve injection during the intake and compression strokes of a generic two-valve engine.

Natural Convection as a Heat Engine: A Theory for CAPE

Abstract: On many planets there is a continuous heat supply to the surface and a continuous emission of infrared radiation to space by the atmosphere. Since the heat source is located at higher pressure than the heat sink, the system is capable of doing mechanical work. Atmospheric convection is a natural heat engine that might operate in this system. Based on the heat engine framework, a simple theory is presented for atmospheric convection that predicts the buoyancy, the vertical velocity, and the fractional area covered by either dry or moist convection in a state of statistical equilibrium. During one cycle of the convective heat engine, heat is taken from the surface layer (the hot source) and a portion of it is rejected to the free troposphere (the cold sink) from where it is radiated to space. The balance is transformed into mechanical work. The mechanical work is expended in the maintenance of the convective motions against mechanical dissipation. Ultimately, the energy dissipated by mechanical friction is transformed into heat. Then, a fraction of the dissipated energy is radiated to space while the remaining portion is recycled by the convecting air parcels. Increases in the fraction of energy dissipated at warmer temperature, at the expense of decreases in the fraction of energy dissipated at colder temperatures, lead to increases in the apparent efficiency of the convective heat engine. The volume integral of the work produced by the convective heat engine gives a measure of the statistical equilibrium amount of convective available potential energy (CAPE) that must be present in the planet's atmosphere so that the convective motions can be maintained against viscous dissipation. This integral is a fundamental global number qualifying the state of the planet in statistical equilibrium conditions. For the earth's present climate, the heat engine framework predicts a CAPE value of the order of 1000 J kg^−1 for the tropical atmosphere. This value is in agreement with observations. It also follows from our results that the total amount of CAPE present in a convecting atmosphere should increase with increases in the global surface temperature (or the atmosphere's opacity to infrared radiation).

The intertube falling film: Part 1 - Flow characteristics, mode transitions, and hysteresis

Abstract: When a liquid film falls from one horizontal tube to another below it, the flow may take the form of discrete droplets, jets, or a continuous sheet ; the mode plays an important role in the wetting and heat transfer characteristics of the film. Experiments are reported that explore viscous, surface tension, inertial, and gravitational effects on the falling-film mode transitions. New flow classifications, a novel flow regime map, and unambiguous transition criteria for each of the mode transitions are provided. This research is part of an overall study of horizontal-tube, falling-film flow and heat transfer, and the results may have important implications on the design and operation of falling-film heat exchangers.

Thermo‐hydro‐mechanical analysis of partially saturated porous materials

Abstract: Presents a fully coupled numerical model to simulate the slow transient phenomena involving heat and mass transfer in deforming partially saturated porous materials. Makes use of the modified effective stress concept together with the capillary pressure relationship. Examines phase changes (evaporation‐condensation(, heat transfer through conduction and convection, as well as latent heat transfer. The governing equations in terms of gas pressure, capillary pressure, temperature and displacements are coupled non‐linear differential equations and are discretized by the finite element method in space and by finite differences in the time domain. The model is further validated with respect to a documented experiment on partially saturated soil behaviour, and the effects of two‐phase flow, as compared to the one‐phase flow solution, are analysed. Two other examples involving drying of a concrete wall and thermoelastic consolidation of partially saturated clay demonstrate the importance of proper physical modelling and of appropriate choice of the boundary conditions.