Isothermal Enclosure Research Articles

Implications for experimental design from numerical modeling of natural convective heat transfer

ABSTRACT This paper discusses the important role that numerical modelling can play in providing detailed information on the structure of natural convective heat flows. This information, (velocity, temperature, streamline and heat flux fields throughout the entire flow), is generally unavailable from experimental studies due to limitations in the experimental techniques currently available. Computational results are presented and compared with experimental measurements for heated plates inside an isothermal enclosure, over a range of Rayleigh number, plate spacing, plate inclination and plate position. The paper also describes some potential dangers and misleading results that can emerge when computing these flows. Difficulties arising when performing experimental measurements on physical models of real equipment are also described.

Influence of air infiltration on airflow in a ventilated isothermal two-zone enclosure

Influences of air infiltration on the airflow and contaminant distribution in a ventilated isothermal two-zone enclosure are investigated by numerical simulation. The direction of infiltration airflow is considered to be in the opposite direction of the room ventilation air. The enclusure is divided into two zones, zone A and zone B, by a partition with a door opening. The ventilation air flows into zone A (namely ventilition zone) through a rectangular supply opening near the floor on the end-wall of the zone and leaves the enclosure through a ceiling-mounted exhaust opening in zone B. The infiltration air flows uniformly into zone B (namely infiltration zone) through the other end-wall, and leaves through the exhaust. Three different values of air infiltration rate are considered. In the analysis of the combined influence of door-opening position and infiltration on the air movement, the door opening is placed at three positions, namely, y D/ W = 0.167, 0.5 and 0.833 respectively. The airflow is considered to be three-dimensional and fully turbulent. The predicted results indicate that a local exhaust will effectively limit the transportation of the contaminant from the infiltration zone into the ventilation zone, and the effect of infiltration on its downstream zone could be negligibly small, if the door opening and the exhaust are properly placed.

The effects of exchange gas temperature and pressure on the beta-layering process in solid deuterium-tritium fusion fuel

It has recently been shown that when solid tritium is confined in an isothermal enclosure, self-heating due to beta decay drives a net sublimation of material from thick, warmer layers to thin, cooler ones, ultimately resulting in layer thickness uniformity. We have observed this process of “beta-layering” in a 50-50 D-T mixture in both cylindrical and spherical enclosures at temperatures from 19.6 K, down to 11.6 K. The measured time constants are found to depend on the 3 He content as suggested by recent theoretical predictions. When using an enclosure having low thermal conductivity, the ultimate layer uniformity is found to be a strong function of the exchange gas pressure. This is due to the presence of thermal convection in the exchange gas and consequent temperature anisotropy at the solid layer surface.

The effects of exchange gas temperature and pressure on the beta-layering process in solid deuterium-tritium fusion fuel

It has recently been shown that when solid tritium is confined in an isothermal enclosure, self-heating due to beta decay drives a net sublimation of material from thick, warmer layers to thin, cooler ones, ultimately resulting in layer thickness uniformity. We have observed this process of “beta-layering” in a 50-50 D-T mixture in both cylindrical and spherical enclosures at temperatures from 19.6 K, down to 11.6 K. The measured time constants are found to depend on the 3He content as suggested by recent theoretical predictions. When using an enclosure having low thermal conductivity, the ultimate layer uniformity is found to be a strong function of the exchange gas pressure. This is due to the presence of thermal convection in the exchange gas and consequent temperature anisotropy at the solid layer surface.

Natural convection in an enclosure heated from below with a conductive horizontal partition

This paper investigates the phenomenon of natural convection in an enclosure heated from below, fitted with a completely horizontal conductive partition. The horizontal partition is parallel to the two horizontal isothermal enclosure walls, which are maintained at different temperatures, while the vertical walls of the enclosure are adiabatic. The governing equations for stream function, vorticity, and energy are solved with the aid of the cubic spline collocation method. Parametric studies of the effects of the partition on the fluid flow and temperature fields in the enclosure have been performed. Numerical results indicate that the effects of the conductivity ratio of the partition and the aspect ratio of the enclosure on the heat-transfer rate are not significant. However, the location of the partition affects the fluid flow and heat-transfer rate profoundly. Numerical results are also compared with the results obtained from a flow visualization experiment. The streamline distribution obtained from numerical result is shown to be in good agreement with the experimental results.

Heat Transfer Analysis of Digital Transmission Equipment with Horizontally Arranged Printed Circuit Boards

Printed circuit boards (PCBs) may be arranged horizontally in many situations, as for example in some digital transmission equipment being used in Brazil. Such pieces of equipment, known as slim racks, have a high aspect ratio (height/width) and are assembled side by side and back to back. The present work deals with the steady-state thermal dissipation in the so-called thermal unit of the slim rack made of the PCB, the magnetic shield above it and the bounding walls constituting an enclosure. The heat transfer in each thermal unit was investigated by assuming isothermal enclosure surfaces with uniform radiosities. Once the thermal paths are identified, an electric circuit analogy is employed to obtain the overall thermal resistance network. Most of the thermal resistances are temperature-dependent, so the associated nonlinear algebraic equations can be solved only by an iterative method. An experimental investigation was carried out in a module of the slim rack with simplified boundary conditions in orde...

The transfer of heat by natural convection between bodies and their enclosures

Natural convection heat transfer between concentrically located isothermal spherical, cylindrical and cubical inner bodies and their isothermal cubical enclosure was experimentally investigated. Comparisons were made with the existing data for these same inner bodies and a spherical enclosure. In addition, temperature distributions and flow visualization data were obtained for most of those geometries. Comparisons made between the spherical and cubical enclosure, for the same inner body types, showed that the cubical enclosure resulted in a larger Nusselt number for a given Rayleigh number and inner body size. Heat transfer data taken with a non-isothennal inner body showed that the isothermality conditions on the inner body could be relaxed. All of the heat transfer data was correlated with an average deviation of less than 14%. Overall, the effect of the enclosure shape was small so long as the appropriate length scale is employed.

Band Radiation within Diffuse-Walled Enclosures—Part I: Exact Solutions for Simple Enclosures

Exact radiative transfer solutions for an isothermal medium confined within an isothermal enclosure are given for the planar, cylindrical and spherical configurations. The emphasis is upon band radiation and diffuse reflections. Specular reflections are also considered because solutions are easily obtained and are of interest for comparative purposes as well as intrinsic value.

Water states and water gates in osmotic processes, and the inoperative concept of molfraction of water.

An historical account is given of concepts regarding the mechanism of osmosis and imbibition, starting with Lord Kelvin's gravitational column, where he pointed out that a capillary standing in a dish of water within an isothermal enclosure must have a lowered vapor pressure at its elevated meniscus so as to match that emanating from the surface in the dish, otherwise distillation would violate the Second law. A brilliant sequence to this simple idea followed through Poynting, Arrhenius, Noyes and culminated with Hulett, who in 1901 formulated the "solvent tension theory" of osmosis, stating in essence that the thermal motion of the solute molecules by impact with the free solvent surface put the solvent under tension. This lowers the vapor pressure and thereby also its freezing point. Perrin, in famous experiments on Brownian motion, demonstrated solute-solvent independence within a solution and further support came through Herzfeld, Mysels and Duclaux. We measured negative pressures in salt-free sap of mangroves and other plants matching the osmotic pressure in the leaf cells. A series of measurements on magnetic and gravitational effects on osmotic pressure likewise bore out the tension theory. The fashionable "water concentration theory" is left experimentally contradicted and in violation of the Second law.

An air refractometer for interference length metrology

A refractometer for the measurement of the refractive index of the surrounding air is described, in which interference fringes are produced between two beams, one propagated in air, the other in vacuum. The accuracy aimed at is one part in 108, corresponding to the accuracy of the best vacuum wavelength standards used in the length measurement of mechanical standards. The refractometer is placed with the comparator inside an airtight isothermal enclosure and is operated from outside.

Thermodynamic Measurements by Means of Simulated Isothermal Enclosures

A theory is worked out for a simulated isothermal enclosure created by irradiating an object with a beam of radiant flux from a reference blackbody and comparing the blackbody radiant flux with that reflected, transmitted, and emitted by the object. By modifying Kirchhoff’s law to conform to the new geometry, conditions of isothermal balance or imbalance between blackbody and object can be determined, thus enabling temperature measurement or control to be effected. Also, measurements of emissivity, reflectance, and transmittance can be made.

The thermodynamic coupling between heat and mass transfer in free convection with helium injection

Measurements are described of local heat transfer by free convection under steady state conditions around a 2-in o.d. circular cylinder mounted with its axis horizontal in an isothermal large enclosure, at several levels of wall temperature ranging from about 20°F below ambient, to about 80°F above ambient. Helium was injected through the porous wall of the cylinder into the boundary layer at a uniform rate per unit area of outside cylinder surface, in the range 1.00–7.50 lb m/h ft 2. Analysis of the results showed that when the convective heat flux is zero, the wall temperature, in this case termed the adiabatic wall temperature, was higher than ambient temperature by up to 190°F, depending upon the location around the cylinder, and the helium injection rate. This phenomenon, so far unreported in free convection, is similar to the findings in references [1, 2], and hence is due to the diffusion thermo-effect. The local heat-transfer coefficient was defined in terms of the adiabatic wall temperature, and determined to be essentially independent of the temperature difference between wall and ambient. At the lower stagnation point, its value increased rapidly with injection, but then levelled off to an almost constant value when the injection rate exceeded about 2 lb m/h ft 2. At a given injection rate, the heat-transfer coefficient decreased from a maximum value at the lower stagnation point to a minimum value at the upper stagnation point. At a given location on the cylinder circumference, the adiabatic wall temperature increased steadily as the injection rate increased. At a given injection rate, the adiabatic wall temperature increased from a minimum value at the lower stagnation point to a maximum value at the upper stagnation point. The body forces at work in the binary boundary layer are discussed in their importance on heat transfer.

Analysis of molecular flow in an isothermal enclosure: K. D. Carlson, et al., J. Chem. Phys., 38 (9), 1 May 1963, 2064–2074

Analysis of molecular flow in an isothermal enclosure: K. D. Carlson, et al., J. Chem. Phys., 38 (9), 1 May 1963, 2064–2074

Analysis of Molecular Flow in an Isothermal Enclosure

Previously presented analyses of the molecular flow of saturated vapor from a condensed phase in an isothermal enclosure have introduced simplifying approximations early in the treatments. Consequently functions and subtle inconsistencies have appeared which can be recognized, identified, and understood only by returning to the original formulation. A more desirable procedure is to pursue rigorously as far as possible the problem before introducing approximations. The presently described analysis differs from the others in the following respects: (1) The precise system described and the fundamental assumptions employed are defined initially. (2) For a cylindrical (and a generalized) enclosure it is shown that the form p=pe(1—Δ) with pe equal to the equilibrium pressure and Δ a function of the parameters of the cell is a convenient one for the purpose of effecting approximations. (3) The exact physical system corresponding to the classical equation, pe=p[1+(S0/S1)], in which S0 is the area of an orifice and S1 is the evaporating area (both on a spherical surface), has been found to be a spherical enclosure for which Δ can be easily calculated. (4) The single approximation leading to the formulation of Whitman and of Motzfeldt has been found and Δ has been evaluated. An intercomparison is given.

Molecular Composition of Alkali Halide Vapors

An analysis of the velocity distribution of the molecules which escape through an ideal aperture from an isothermal enclosure has been made for ten alkali halides (CsCl, CsBr, RbCl, KCl, KI, NaF, NaCl, NaI, LiCl, and LiBr). It is found that there is a significant abundance of dimers in all cases except those of the cesium salts, and that an observable concentration of trimers occurs for NaF, LiCl and LiBr. Observation of the relative abundances of the polymers as the temperature of the effusing gas and the pressure within the source is varied permits the determination of the energy of dissociation of the dimer into two monomers and of the trimer into a monomer and dimer.