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NATURAL CONVECTIONIN SHALLOW ENCLOSURES WITH DIFFERENTIALLY HEATED END WALLS AND NONADIABATIC HORIZONTAL WALLS

Description: Numerical studies of laminar natural convection at high Ra numbers in shallow enclosures are reported. In these studies the working fluid is allowed to interact with the horizontal walls. It is shown that even a small amount of heat loss from these walls can lead to a flow structure qualitatively different from the more commonly studied situation where the horizontal walls are adiabatic. This is particularly important in applications where the mass transfer and flow structure are of interest. The results highlight the difficulty in practice of both approximating the adiabatic horizontal wall condition, and interpreting experimental data.
Date: May 1, 1983
Creator: Gadgil, A. & Shiralkar, G.
Partner: UNT Libraries Government Documents Department

Laminar Natural Convection From Isothermal Vertical Cylinders

Description: Laminar natural convection heat transfer from the vertical surface of a cylinder is a classical subject, which has been studied extensively. Furthermore, this subject has generated some recent interest in the literature. In the present investigation, numerical experiments were performed to determine average Nusselt numbers for isothermal vertical cylinders (103 < RaL < 109, 0.5 < L/D <10, and Pr = 0.7) with and without an adiabatic top in a quiescent ambient environment which will allow for plume growth. Results were compared with commonly used correlations and new average Nusselt number correlations are presented. Furthermore, the limit for which the heat transfer results for a vertical flat plate may be used as an approximation for the heat transfer from a vertical cylinder was investigated.
Date: August 2012
Creator: Day, Jerod
Partner: UNT Libraries

Laminar Natural-Convection Flow and Heat Transfer of Fluids With and Without Heat Sources in Channels With Constant Wall Temperatures

Description: Note presenting an analysis of the natural-convection phenomenon, which shows that the flow and heat transfer, in general, are not only functions of the Prandtl and Grashof numbers but also depend on a new dimensionless parameter. If the parameter is not negligibly small, the compression work and frictional heating may appreciably affect this mode of heat transfer. Results regarding the velocity and temperature distributions and heat transfer are provided.
Date: December 1952
Creator: Ostrach, Simon
Partner: UNT Libraries Government Documents Department

Modeling Free Convection Flow of Liquid Hydrogen within a Cylindrical Heat Exchanger Cooled to 14 K

Description: A liquid hydrogen in a absorber for muon cooling requires that up to 300 W be removed from 20 liters of liquid hydrogen. The wall of the container is a heat exchanger between the hydrogen and 14 K helium gas in channels within the wall. The warm liquid hydrogen is circulated down the cylindrical walls of the absorber by free convection. The flow of the hydrogen is studied using FEA methods for two cases and the heat transfer coefficient to the wall is calculated. The first case is when the wall is bare. The second case is when there is a duct some distance inside the cooled wall.
Date: May 8, 2004
Creator: Green, Michael A.; U., Oxford; Yang, S.W.; Green, M.A. & Lau, W.
Partner: UNT Libraries Government Documents Department

Review of Residential Ventilation Technologies

Description: This paper reviews current and potential ventilation technologies for residential buildings in North America and a few in Europe. The major technologies reviewed include a variety of mechanical systems, natural ventilation, and passive ventilation. Key parameters that are related to each system include operating costs, installation costs, ventilation rates, heat recovery potential. It also examines related issues such as infiltration, duct systems, filtration options, noise, and construction issues. This report describes a wide variety of systems currently on the market that can be used to meet ASHRAE standard 62.2. While these systems generally fall into the categories of supply, exhaust or balanced, the specifics of each system are driven by concerns that extend beyond those in the standard and are discussed. Some of these systems go beyond the current standard by providing additional features (such as air distribution or pressurization control). The market will decide the immediate value of such features, but ASHRAE may wish to consider modifications to the standard in the future.
Date: March 1, 2005
Creator: Russell, Marion L.; Sherman, Max H. & Rudd, Armin
Partner: UNT Libraries Government Documents Department

Transient Accident Analysis of the Glovebox System in a Large Process Room

Description: Local transient hydrogen concentrations were evaluated inside a large process room when the hydrogen gas was released by three postulated accident scenarios associated with the process tank leakage and fire leading to a loss of gas confinement. The three cases considered in this work were fire in a room, loss of confinement from a process tank, and loss of confinement coupled with fire event. Based on these accident scenarios in a large and unventilated process room, the modeling calculations of the hydrogen migration were performed to estimate local transient concentrations of hydrogen due to the sudden leakage and release from a glovebox system associated with the process tank. The modeling domain represented the major features of the process room including the principal release or leakage source of gas storage system. The model was benchmarked against the literature results for key phenomena such as natural convection, turbulent behavior, gas mixing due to jet entrainment, and radiation cooling because these phenomena are closely related to the gas driving mechanisms within a large air space of the process room. The modeling results showed that at the corner of the process room, the gas concentrations migrated by the Case 2 and Case 3 scenarios reached the set-point value of high activity alarm in about 13 seconds, while the Case 1 scenario takes about 90 seconds to reach the concentration. The modeling results were used to estimate transient radioactive gas migrations in an enclosed process room installed with high activity alarm monitor when the postulated leakage scenarios are initiated without room ventilation.
Date: January 11, 2008
Creator: Lee, S.
Partner: UNT Libraries Government Documents Department

SIMPLIFIED METHODS FOR COMBINING MECHANICAL VENTILATION AND NATURAL INFILTRATION

Description: During the past ten years, the means of ventilating single-family residences has received considerable attention. In many areas, the use of natural ventilation for infiltration has either come under close scrutiny, or has already been supplanted by mechanical ventilation systems. To evaluate the energy efficiency and ventilation effectiveness of both mechanical and natural ventilation strategies, both complex and simplified infiltration models are used. This paper examines the inaccuracies associated with using simplified models to compare ventilation strategies. Two simplified techniques for combining mechanical ventilation flows to the flows caused by wind and stack effects are examined. The simplified combination techniques are compared with the results obtained with an iterative flow-balance simulation. The flow-balance simulation determines the ventilation by balancing the incoming and outgoing flows under the pressure conditions resulting from the combination of wind effect, stack effect and mechanical ventilation. These comparisons result in three major conclusions: (1) the commonly used flow superposition technique (flow combination in quadrature) provides better estimates of the total flow than does a technique that takes into account measured flow exponents, (2) although flow combination in quadrature overpredicts ventilation when combining wind-induced and stack-induced flows, this is not the case when mechanical ventilation is added to the picture, and (3) a simple correction for the errors caused by the simplified flow superposition technique is not easy to achieve due to the large variations in error that occur with changes in wind direction and individual flow ratios.
Date: January 1, 1985
Creator: Modera, M. & Peterson, F.
Partner: UNT Libraries Government Documents Department

A heat exchanger between forced flow helium gas at 14 to 18 K andliquid hydrogen at 20 K circulated by natural convection

Description: The Muon Ionization Cooling Experiment (MICE) has three 350-mm long liquid hydrogen absorbers to reduce the momentum of 200 MeV muons in all directions. The muons are then re-accelerated in the longitudinal direction by 200 MHz RF cavities. The result is cooled muons with a reduced emittance. The energy from the muons is taken up by the liquid hydrogen in the absorber. The hydrogen in the MICE absorbers is cooled by natural convection to the walls of the absorber that are in turn cooled by helium gas that enters at 14 K. This report describes the MICE liquid hydrogen absorber and the heat exchanger between the liquid hydrogen and the helium gas that flows through passages in the absorber wall.
Date: September 15, 2003
Creator: Green, M. A.; Ishimoto, S.; Lau, W. & Yang, S.
Partner: UNT Libraries Government Documents Department

SCALE ANALYSIS OF CONVECTIVE MELTING WITH INTERNAL HEAT GENERATION

Description: Using a scale analysis approach, we model phase change (melting) for pure materials which generate internal heat for small Stefan numbers (approximately one). The analysis considers conduction in the solid phase and natural convection, driven by internal heat generation, in the liquid regime. The model is applied for a constant surface temperature boundary condition where the melting temperature is greater than the surface temperature in a cylindrical geometry. We show the time scales in which conduction and convection heat transfer dominate.
Date: March 1, 2011
Creator: Crepeau, John
Partner: UNT Libraries Government Documents Department

LOFT Monthly Progress Report for March 1980

Description: During March, evaluation of Test L3-2 continued and preparations began for the next tests, L6-5 and L3-7. Test L6-5 is the first test of the operational transient series and is a loss-of-feedwater incident. Test L3-7 is a small break, similar to L3-2, wherein the emergency coolant flow is adjusted to permit investigation of natural circulation modes. The controlling schedule items involve efforts to add instrumentation to the plant to better characterize system response. Specifically, a new hot-leg penetration will be installed and steam generator water level instrumentation will be improved. For March, costs to date agree very well with budget. Efforts are underway to develop a new baseline program for FY-80 and FY-81 based on a revised test plan and recent budget guidance for FY-81. These efforts are targeted for mid-year review in April.
Date: April 1, 1980
Creator: Kaufman, N. C.
Partner: UNT Libraries Government Documents Department

Experimental Determination of ETS Particle Deposition in a Low Ventilation Room

Description: Deposition on indoor surfaces is an important removal mechanism for tobacco smoke particles. We report measurements of deposition rates of environmental tobacco smoke particles in a room-size chamber. The deposition rates were determined from the changes in measured concentrations by correcting for the effects of coagulation and ventilation. The air flow turbulent intensity parameter was determined independently by measuring the air velocities in the chamber. Particles with diameters smaller than 0.25 {micro}m coagulate to form larger particles of sizes between 0.25-0.5 {micro}m. The effect of coagulation on the particles larger than 0.5 {micro}m was found to be negligible. Comparison between our measurements and calculations using Crump and Seinfeld's theory showed smaller measured deposition rates for particles from 0.1 to 0.3 {micro}m in diameter and greater measured deposition rates for particles larger than 0.6 {micro}m at three mixing intensities. Comparison of Nazaroff and Cass model for natural convection flow showed good agreement with the measurements for particles larger than 0.1 {micro}m in diameter, however, measured deposition rates exceeded model prediction by a factor of approximately four for particles in size range 0.05-0.1 {micro}m diameter. These results were used to predict deposition of sidestream smoke particles on interior surfaces. Calculations predict that in 10 hours after smoking one cigarette, 22% of total sidestream particles by mass will deposit on interior surfaces at 0.03 air change per hour (ACH), 6% will deposit at 0.5 ACH, and 3% will deposit at 1 ACH.
Date: May 1, 1993
Creator: Xu, M.; Nematollahi, M.; Sextro, R.G.; Gadgil, A.J. & Nazaroff, W.W.
Partner: UNT Libraries Government Documents Department

NATURAL CONVECTION IN ROOM GEOMETRIES

Description: Computer programs have been developed to numerically simulate natural convection in room geometries in two and three dimensions. The programs have been validated using published data from the literature, results from a full-scale experiment performed at Massachusetts Institute of Technology, and results from a small-scale experiment reported here. One of the computer programs has been used to study the influence of natural convection on the thermal performance of a single thermal zone in a direct-gain passive solar building. The results indicate that the building heating loads calculated by standard building energy analysis methods may be in error by as much as 50% as a result of their use of common assumptions regarding the convection processes which occur in an enclosure. It is also found that the convective heat transfer coefficients between the air and the enclosure surfaces can be substantially different from the values assumed in the standard building energy analysis methods, and can exhibit significant variations across a given surface.
Date: June 1, 1980
Creator: Gadgil, A.; Bauman, Fred; Kammerud, R. & Ruberg, K.
Partner: UNT Libraries Government Documents Department

Fundamental approach to TRIGA steady-state thermal-hydraulic CHF analysis.

Description: Methods are investigated for predicting the power at which critical heat flux (CHF) occurs in TRIGA reactors that rely on natural convection for primary flow. For a representative TRIGA reactor, two sets of functions are created. For the first set, the General Atomics STAT code and the more widely-used RELAP5-3D code are each employed to obtain reactor flow rate as a function of power. For the second set, the Bernath correlation, the 2006 Groeneveld table, the Hall and Mudawar outlet correlation, and each of the four PG-CHF correlations for rod bundles are used to predict the power at which CHF occurs as a function of channel flow rate. The two sets of functions are combined to yield predictions of the power at which CHF occurs in the reactor. A combination of the RELAP5-3D code and the 2006 Groeneveld table predicts 67% more CHF power than does a combination of the STAT code and the Bernath correlation. Replacing the 2006 Groeneveld table with the Bernath CHF correlation (while using the RELAP5-3D code flow solution) causes the increase to be 23% instead of 67%. Additional RELAP5-3D flow-versus-power solutions obtained from Reference 1 and presented in Appendix B for four specific TRIGA reactors further demonstrates that the Bernath correlation predicts CHF to occur at considerably lower power levels than does the 2006 Groeneveld table. Because of the lack of measured CHF data in the region of interest to TRIGA reactors, none of the CHF correlations considered can be assumed to provide the definitive CHF power. It is recommended, however, to compare the power levels of the potential limiting rods with the power levels at which the Bernath and 2006 Groeneveld CHF correlations predict CHF to occur.
Date: March 30, 2008
Creator: Feldman, E.
Partner: UNT Libraries Government Documents Department

D-Zero Colling Loops

Description: This report provides the necessary sizing for the LN{sub 2} cooling coils in the D-Zero cryostats. Natural convection from finned tubes will be the means of cooling the cryostats and their contents until such time as liquid can be made by condensing. Each cryostat will contain three separate cooling runs. Two runs will be cooldown and the other steady state. These runs will be placed in each cryostat as shown in figure 3, 4a and 4b. By assuming a 100 K tube wall temperature, tube diameter (3/4-inch) and wall thickness (0.065-inch) and the total length of tube (2 x 2940-inch + 1463-inch), the heat transfer can be calculated. Table 1 was calculated from relations in 'Handbook of Heat transfer Fundamentals' for free convection, pages 6-34 through 6-40.
Date: August 10, 1987
Creator: Peterson, T. & Wintercorn, S.
Partner: UNT Libraries Government Documents Department

A Coupled Model for Natural Convection and Condensation in HeatedSubsurface Enclosures Embedded in Fractured Rock

Description: In heated tunnels such as those designated for emplacementof radioactive waste at Yucca Mountain, axial temperature gradients maycause natural convection processes that can significantly influence themoisture conditions in the tunnels and in the surrounding fractured rock.Large-scale convection cells would provide an effective mechanism foraxial vapor transport, driving moisture out of the formation away fromthe heated tunnel section into cool end sections (where no waste isemplaced). To study such processes, we have developed and applied anenhanced version of TOUGH2 (Pruess et al., 1999) adding a new module thatsolves for natural convection in open cavities. The new TOUGH2 simulatorsimultaneously handles (1) the flow and energy transport processes in thefractured rock; (2) the flow and energy transport processes in thecavity; and (3) the heat and mass exchange at the rock-cavity interface.The new module is applied to simulate the future thermal-hydrological(TH) conditions within and near a representative waste emplacement tunnelat Yucca Mountain. Particular focus is on the potential for condensationalong the emplacement section, a possible result of heat outputdifferences between individual waste packages.
Date: April 14, 2006
Creator: Halecky, N.; Birkholzer, J. T.; Webb, S. W.; Peterson, P. F. & Bodvarsson, G. S.
Partner: UNT Libraries Government Documents Department

Flow Visualization of Forced and Natural Convection in Internal Cavities

Description: The general goal of this program is to develop innovative flow visualization methods and reliable predictive techniques for the energy, mass, and momentum transfer in the presence of surface reactions for the passivation treatment operations of SNF elements.
Date: June 1, 2000
Creator: Crepeau, John C.; Clarksean, Randy; McEligot, Donald M. & Guezennec, Yann G.
Partner: UNT Libraries Government Documents Department

CFD Calculation of Internal Natural Convection in the Annulus between Horizontal Concentric Cylinders

Description: The objective of this heat transfer and fluid flow study is to assess the ability of a computational fluid dynamics (CFD) code to reproduce the experimental results, numerical simulation results, and heat transfer correlation equations developed in the literature for natural convection heat transfer within the annulus of horizontal concentric cylinders. In the literature, a variety of heat transfer expressions have been developed to compute average equivalent thermal conductivities. However, the expressions have been primarily developed for very small inner and outer cylinder radii and gap-widths. In this comparative study, interest is primarily focused on large gap widths (on the order of half meter or greater) and large radius ratios. From the steady-state CFD analysis it is found that the concentric cylinder models for the larger geometries compare favorably to the results of the Kuehn and Goldstein correlations in the Rayleigh number range of about 10{sup 5} to 10{sup 8} (a range that encompasses the laminar to turbulent transition). For Rayleigh numbers greater than 10{sup 8}, both numerical simulations and experimental data (from the literature) are consistent and result in slightly lower equivalent thermal conductivities than those obtained from the Kuehn and Goldstein correlations.
Date: October 1, 2002
Creator: N.D. Francis, Jr; Itamura, M.T.; Webb, S.W. & James, D.L.
Partner: UNT Libraries Government Documents Department

Natural convection heat exchangers for solar water heating systems. Techniacl progress report, June 1, 1995--July 31, 1995

Description: The goals of this project are: (1) to develop guidelines for the design and use of thermosyphon side-arm heat exchangers in solar domestic water heating systems, and (2) to establish appropriate modeling and testing criteria for evaluating the performance of systems using this type of heat exchanger. The tasks for the project are as follows: (1) Develop a model of the thermal performance of thermosyphon heat exchangers in solar water heating applications. A test protocol will be developed which minimizes the number of tests required to adequately account for mixed convection effects. The TRNSYS component model will be fully integrated in a system component model and will use data acquired with the specified test protocol. (2) Conduct a fundamental study to establish friction and heat transfer correlations for conditions and geometries typical of thermosyphon heat exchangers in solar systems. Data will be obtained as a function of a buoyancy parameter based on Grashof and Reynolds numbers. The experimental domain will encompass the ranges expected in solar water heating systems.
Date: June 1, 1998
Creator: Davidson, J.H.
Partner: UNT Libraries Government Documents Department

Regular and chaotic flow patterns upon impulsive spin-up of a Rayleigh-Benard convection cell

Description: A cylindrical, completely enclosed Rayleigh-Benard convection cell with radius-to-height ratio {Gamma}={1/2} is subjected to impulsive spin-up about its vertical axis. The authors study produces TLC (thermochromic liquid crystal) temperature measurements and PIV (particle image velocimetry) velocity reconstruction of the transient state between the two regimes of turbulent convection corresponding to the cell at rest and in steady rotation. The most persistent transient feature emerging is a sharply defined ringlike pattern characterized by a decrease in temperature and high azimuthal shear. The latter leads to formation of Kelvin-Helmholz vortices. Initially azimuthally regular, the pattern of these vortices loses its regularity and thus completes the transition to rotating convection state.
Date: October 1, 1997
Creator: Vorobieff, P. & Ecke, R.E.
Partner: UNT Libraries Government Documents Department

Natural convection heat exchangers for solar water heating systems. Technical progress report, September 15, 1996--November 14, 1996

Description: The goals of this project are: (1) to develop guidelines for the design and use of thermosyphon side-arm heat exchangers in solar domestic water heating systems, and (2) to establish appropriate modeling and testing criteria for evaluating the performance of systems using this type of heat exchanger. The tasks for the project are as follows: (1) Develop a model of the thermal performance of thermosyphon heat exchangers in solar water heating applications. A test protocol will be developed which minimizes the number of tests required to adequately account for mixed convection effects. The TRNSYS component model will be fully integrated in a system component model and will use data acquired with the specified test protocol. (2) Conduct a fundamental study to establish friction and heat transfer correlations for conditions and geometries typical of thermosyphon heat exchangers in solar systems. Data will be obtained as a function of a buoyancy parameter based on Grashof and Reynolds numbers. The experimental domain will encompass the ranges expected in solar water heating systems.
Date: June 1, 1998
Creator: Davidson, J.H.
Partner: UNT Libraries Government Documents Department

Natural convection heat transfer within horizontal spent nuclear fuel assemblies

Description: Natural convection heat transfer is experimentally investigated in an enclosed horizontal rod bundle, which characterizes a spent nuclear fuel assembly during dry storage and/or transport conditions. The basic test section consists of a square array of sixty-four stainless steel tubular heaters enclosed within a water-cooled rectangular copper heat exchanger. The heaters are supplied with a uniform power generation per unit length while the surrounding enclosure is maintained at a uniform temperature. The test section resides within a vacuum/pressure chamber in order to subject the assembly to a range of pressure statepoints and various backfill gases. The objective of this experimental study is to obtain convection correlations which can be used in order to easily incorporate convective effects into analytical models of horizontal spent fuel systems, and also to investigate the physical nature of natural convection in enclosed horizontal rod bundles in general. The resulting data consist of: (1) measured temperatures within the assembly as a function of power, pressure, and backfill gas; (2) the relative radiative contribution for the range of observed temperatures; (3) correlations of convective Nusselt number and Rayleigh number for the rod bundle as a whole; and (4) correlations of convective Nusselt number as a function of Rayleigh number for individual rods within the array.
Date: December 1, 1995
Creator: Canaan, R.E.
Partner: UNT Libraries Government Documents Department

Natural convection heat exchangers for solar water heating systems. Technical progress report, August 1, 1995--September 30, 1995

Description: The goals of this project are: (1) to develop guidelines for the design and use of thermosyphon side-arm heat exchangers in solar domestic water heating systems, and (2) to establish appropriate modeling and testing criteria for evaluating the performance of systems using this type of heat exchanger. The tasks for the project are as follows: (1) Develop a model of the thermal performance of thermosyphon heat exchangers in solar water heating applications. A test protocol will be developed which minimizes the number of tests required to adequately account for mixed convection effects. The TRNSYS component model will be fully integrated in a system component model and will use data acquired with the specified test protocol. (2) Conduct a fundamental study to establish friction and heat transfer correlations for conditions and geometries typical of thermosyphon heat exchangers in solar systems. Data will be obtained as a function of a buoyancy parameter based on Grashof and Reynolds numbers. The experimental domain will encompass the ranges expected in solar water heating systems.
Date: June 1, 1998
Creator: Davidson, J.H.
Partner: UNT Libraries Government Documents Department

Numerical Solution of Natural Convection in Eccentric Annuli

Description: The governing equations for transient natural convection in eccentric annular space are solved with two high-order accurate numerical algorithms. The equation set is transformed into bipolar coordinates and split into two one-dimensional equations: finite elements are used in the direction normal to the cylinder surfaces; the pseudospectral technique is used in the azimuthal direction. This report discusses those equations.
Date: September 18, 2001
Creator: Pepper, D.W.
Partner: UNT Libraries Government Documents Department