Showing papers on "Thermal published in 1990"

Transition to hard turbulence in thermal convection at infinite Prandtl number

Abstract: Direct numerical simulations of two‐dimensional high Rayleigh (Ra) number, base‐heated thermal convection in large aspect‐ratio boxes are presented for infinite Prandtl number fluids, as applied to the Earth’s mantle. A transition is characterized in the flow structures in the neighborhood of Ra between 107 and 108. These high Ra flows consist of large‐scale cells with strong intermittent, boundary‐layer instabilities. For Ra exceeding 107 it is found that the heat‐transfer mechanism changes from one characterized by mushroom‐like plumes to one consisting of disconnected ascending instabilities, which do not carry with them all the thermal anomaly from the bottom boundary layer. Plume–plume collisions become much more prominent in high Ra situations and have a tendency of generating a pulse‐like behavior in the fixed plume. This type of instability represents a distinct mode of heat transfer in the hard turbulent regime. Predictions of this model can be used to address certain issues concerning the mode of time‐dependent convection in the Earth’s mantle.

Sheared boundary layers in turbulent Rayleigh-Bénard convection.

Abstract: Thermal boundary layers in turbulent Rayleigh-B\'enard convection are studied experimentally using a novel system in which the convecting fluid is sheared from below with a flowing layer of mercury. Oscillatory shear substantially alters the spatial structure and frequency of the eruptions, with minimal effect on the heat flux (5%). The temperature probability distribution function (PDF) just above the lower boundary layer changes from Gaussian to exponential, without significant changes in the interior PDF. Implications for theories of ``hard'' turbulence are discussed.

A methodology for building thermal dynamics studies and control applications

Abstract: A methodology is presented for a unified approach to building thermal control studies and building energy analysis. The same detailed building thermal models are used for energy calculations and for thermal control studies. Laplace transfer functions for the building are obtained by means of thermal network models that include both distributed parameter elements, such as thermal mass, and lumped elements, such as the room air thermal capacitance. For detailed models for which an analytical solution is not feasible, the s-domain transfer functions are obtained through a modified least squares polynomial fit to the discrete frequency responses. Laplace transfer functions are also used for HVAC system and control components. Transient thermal control studies are performed by means of a numerical Laplace transform inversion technique. Typical studies with the methodology led to the following results: A separation of building thermal dynamics into short-term and long-term dynamics for convective loads begins at frequencies of about 35 cycles per day (period 41 minutes), with the room air thermal capacitance being important only for the short-term, high-frequency thermal dynamics. Air temperature sensor time constants are shown to have a substantial effect on room temperature response to setpoint changes, such as an increase of 50% inmore » the settling time (with P-1 control) when the sensor time constant is increased from 30 s to 60 s.« less

Thermal X-Ray Emission from Ion-Beam-Heated Matter

Abstract: The conversion of ion beam energy into thermal x-ray radiation by means of stretched cylindrical plasma volumes is studied by analytic modeling and numerical simulation. The analysis is restricted to one-dimensional radiation hydrodynamics. Scaling relations for different materials are derived. Due to equation-of-state properties, high-Z material turns out superior for achieving high conversion efficiency. The paper provides practical answers for thermal x-ray generation with intense heavy ion beams. Deposition powers in the order of 1016 W/g are required for applications to ion beam inertial confinement fusion.

Real-time thermal model for an oil-immersed, forced-air cooled transformer

Abstract: A thermal model for predicting the real-time temperatures of an oil-immersed, forced-air cooled transformer is presented. The model consists of a set of three differential equations that are formulated into a computer program. Equations that can be solved for the average real-time temperatures of the core and coil assembly, oil, and tank are developed. Expressions for the convection heat transfer coefficients at all internal oil-solid interfaces, equations for calculation of the incident solar radiation, and correlations for the convective cooling rate from the external tank and cooling tubes are discussed in detail. The thermal model can predict the real-time transformer temperatures for any variation in electrical loading and for any type of ambient thermal environment. Since this model is more detailed than the ANSI/NEMA equations, the results can be expected to predict the transformer temperatures more accurately. The model yielded conservative estimates of the component temperatures when compared with temperatures measured on a distribution transformer. While this test does not verify the accuracy of the model for all types of transformers under all types of loading conditions, it does give initial credibility to the accuracy of the equations and their applicability to transformer heating. >

Issues concerning shock temperature measurements of iron and other metals

Abstract: Determining the melting temperature of iron at the Earth's inner and outer boundary near 3 Mbar is an important problem in geophysics. A knowledge of the pressure, volume, temperature, and internal energy of solid and molten iron at megabar pressures and several thousand degrees provides a critical constraint for modeling the Earth's core. An optical method measuring the thermal radiation from the thin metal film deposited on an optically transparent window material is described. Issues which need to be addressed to obtain accurate thermodynamic shock temperatures of metals from the thermal radiation measured are discussed. These issues include characteristics of metal films, thermal conduction occurring at the metal/window interface, optical and thermal properties of metal and window material, and characteristics of the radiation measured.

Thermal behavior of composite bridges

Abstract: This paper presents the analytical results obtained from a parametric study on the thermal behavior of three types of composite bridge structures subjected to solar radiation, namely, a plate-girder, a single-cell box, and a two-cell box-girder concrete-steel composite bridge. A series of transient temperature distributions and the thermal stresses for a few selected temperature cases corresponding to a given geographic location and assumed environmental conditions are computed. The results of this study confirm that a steady-state thermal condition never exists within a bridge structure, in that ambient air temperature and intensity of solar radiation are both time-dependent. The effects of convection coefficient, slab overhang, and air temperature inside an enclosed box on the temperature distributions of the bridge deck are examined and discussed.

On solar radiation-driven surface transport of sodium atoms at Mercury

Abstract: The ballistic motion of the exospheric sodium atoms on the surface Mercury is modeled, taking into account the solar radiation pressure acceleration and partial surface thermal accommodation. The Monte Carlo simulations show that there should be a significant degree of limb brightening as well as brightness enhancement over the poles. To maintain the observed sodium optical emission, a surface production rate on the order of 5-9 x 10 to the 24th atoms/s is needed. It is also found that, under the present set of assumptions, a reasonable agreement can be reached between theoretical results and ground-based measurements for the dependence of the disk-averaged abundance of the sodium atoms on the solar radiation pressure acceleration. If the low-altitude portion of the planetary surface is shielded from the magnetospheric convective electric field, the effective loss rate of the sodium atoms via photoionization and magnetospheric pickup may be reduced to about 2 x 10 to the 24th atoms/s, with the polar regions acting as the main area of ion outflows. 14 refs.

The Initial Phase of Transient Thermal Stresses Due to General Boundary Thermal Loads in Orthotropic Hollow Cylinders

Abstract: The stresses and displacements are obtained using the Hankel asymptotic expansions for the Bessel functions of the first and second kind. The material properties are assumed to be independent of temperature. A constant applied temperature at the one surface and convection into a medium at a different temperature at the other surface is studied

Unsteady Laminar Forced Convection in Ducts With Periodic Variation of Inlet Temperature

Abstract: A theoretical and experimental study of laminar forced convection in the thermal entrance region of a rectangular duct, subjected to a sinusoidally varying inlet temperature, is presented. A general boundary condition of the fifth kind that accounts for both external convection and wall thermal capacitance effects is considered, and an analytical solution is obtained through extending the generalized integral transform technique. The variations of amplitudes and pahse lags of centerline and bulk temperatures are determined as functions of modified Biot number, fluid-to-wall thermal capacitance ratio, and dimensionaless inlet frequency. An apparatus has been designed, built, and used for the experimental study to provide validation of the mathematical modeling employed. Good agreement is obtained when the nonuniform sinusoidally varying inlet temperature profile obtained by experiments is incorporated into the theoretical model.

Method and apparatus for the rapid thermal processing of transfusion fluid

Abstract: The temperature of bags of blood or blood components is varied by directing a heated or refrigerated flow of air through vents (138) of pairs of opposing ducts (130A, 130B) longitudinally spaced across gaps to present thermal processing chambers (136) into which the bags are received. The vents are centrally located on vertical tubes (132) laterally separated by vertical air return slots (135), and act to direct the airflow substantially perpendicular and with turbulence against the bags. In a described thermal processing unit (110), the spacings of the vents and tubes are chosen to match the corresponding spacing of vertical tubular chambers (62) of a preferred bag (10) with which the unit (110) can be used. A central system automatically regulates airflow temperature in accordance with bag internal core temperature measurements taken externally of the bags by a sensor (140) having a sensing tip (142) that is pressed inwardly into bag flexible sheet material (11, 12) when airflow is present, and retracted when airflow is stopped.

Mixed convection heat transfer from thermal sources mounted on horizontal and vertical surfaces

Abstract: An experimental study is carried out on the fundamental aspects of the conjugate, mixed convective heat transfer from two finite width heat sources, which are of negligible thickness, have a uniform heat flux input at the surface, and are located on a flat plate in the horizontal or the vertical orientation. The heat sources are wide in the transverse direction and, therefore, a two-dimensional flow circumstance is simulated

Response of apparent mass to thermal gradients

Abstract: The change of the apparent mass of 20-g masses and tubes that are not in thermal equilibrium with ambient air has been observed. Buoyancy, adsorption and convection influences are discussed. Quantitative comparisons show that, under such conditions, it is predominantly free convection forces which change the apparent mass.

On possible origins of relatively short-term variations in the solar structure

Abstract: The general large-scale redistribution of magnetic field over the solar cycle is possible associated with an overall variation of thermal structure of the convection zone, which modulates not only the total luminosity but also the latitudinal distribution of radiative flux, thereby modifying the irradiance of the Earth. Whether the cause of this variation lies within the convection zone or is more deeply seated is still an open question.

Influences of atmospheric conditions and air mass on the ratio of ultraviolet to total solar radiation

Abstract: The technology to detoxify hazardous wastes using ultraviolet (UV) solar radiation is being investigated by the DOE/SERI Solar Thermal Technology Program. One of the elements of the technology evaluation is the assessment and characterization of UV solar radiation resources available for detoxification processes. This report describes the major atmospheric variables that determine the amount of UV solar radiation at the earth's surface, and how the ratio of UV-to-total solar radiation varies with atmospheric conditions. These ratios are calculated from broadband and spectral solar radiation measurements acquired at SERI, and obtained from the literature on modeled and measured UV solar radiation. The following sections discuss the atmospheric effects on UV solar radiation and provide UV-to-total solar radiation ratios from published studies, as well as measured values from SERI's data. A summary and conclusions are also given.

Cooling and quasi-static contraction of the primitive solar nebula after gas accretion

Abstract: The evolution of the primitive solar nebula in the quasi-static contraction phase where the nebula cools down toward the thermal steady state is studied. The solar irradiation onto the nebula keeps the surface temperature constant, so that the convective ozone retreats from the surface as the nebula cools. Thus if thermal convection is the only source of turbulence, convection will quiet down in an early time of the cooling. Afterward, the nebula evolves toward an isothermal structure in a time scale of 1000 yr. The cooling rates in the vicinity of the midplate at 1 AU are 0.003 K/hr at T(c) = 1000 K and 3 x 10 to the -5th K/hr at T(c) = 300 K for the standard model. If some turbulence exists irrespective of convection, convection may continue for sufficiently strong turbulent heating. 39 refs.

Effect of different earth surface treatments on the thermal performance of a room coupled to an earth-air tunnel

Abstract: The thermal performance of a normal size room (4 m × 3 m × 3 m) connected to an idealized earth-air tunnel has been analysed interms of heating degree-hours (base 18°C) and cooling degree-hours (base 30°C). The effects of various earth-air tunnel parameters on the heating degree-hours and cooling degree-hours have been studied. The effects of various treatments of the earth surface over the tunnel have been analysed for three different climates (namely hot-dry, hot-humid and cold-dry) prevalant in India. The earth-surface treatments for hot-dry and hot-humid climates include shading and wetting of the earth surface, and for the cold-dry climate, the surface treatments considered are blackening and glazing of the earth surface.

Mathematical models of scraped-surface heat exchangers in relation to food sterilization†

Abstract: This paper provides a review of the mathematical models to predict heat transfer coefficient in both batch and continuous systems with/without particulates and those related to the thermal process calculation. Heat transfer coefficient based on penetration theory, dimensional analysis and empirical models are also presented. In addition, residence time distribution of product in a scraped-surface heat exchanger (SSHE) and a holding tube with/without particulates is discussed.