Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session NJ: Convection and Buoyancy-Driven Flows IV |
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Chair: William George, Chalmers University of Technology Room: Salt Palace Convention Center 250 D |
Tuesday, November 20, 2007 11:35AM - 11:48AM |
NJ.00001: Thermal Diffusivity of Turbulent Convection J.J. Niemela, K.R. Sreenivasan Measurement of the damping at the cell mid-height of a sinusoidal temperature perturbation, superimposed on the bottom plate of a convection cell, yields an effective thermal diffusivity of turbulent convection, up to Rayleigh numbers $Ra = 1x10^{13}$ in cylindrical cells of aspect ratios 1 and 4, using low temperature helium gas. The ability to achieve very high Ra using helium gas, combined with the novel use of this measurement technique, allows us to estimate the height of a ``Nusselt layer" adjacent to the horizontal boundaries for which the thermal resistivity is non-negligible. The remainder of the fluid layer constitutes a turbulent core region whose thickness is in good agreement with the calculation of the mean thermal gradient in recent numerical simulations. [Preview Abstract] |
Tuesday, November 20, 2007 11:48AM - 12:01PM |
NJ.00002: Reconsidering turbulent natural convection in a differentially heated vertical channel Abolfazl Shiri, William K. George The turbulent natural convection boundary layer next to heated vertical surfaces was originally considered by George and Capp (IJHMT 1979). By recognizing the existence of a constant heat flux layer, but the absence of a constant stress layer (due to gravity), they postulated the existence of a buoyant sublayer in which the buoyancy flux from the wall, g$\beta $F$_{o}$, was the crucial parameter. Using it they postulated the existence of inner and outer scales for temperature and velocity. Asymptotic matching led to cube root and inverse cube root profiles for the mean velocity and temperature profiles respectively, and Nu $\sim $ C(Pr) Ra$^{1/3}$ for the wall heat transfer. The theory has generally been found to be in excellent agreement with numerous studies over the past three decades for the temperature and heat transfer relations, but the velocity scaling has always been problematical. This work reconsiders the velocity scaling, and shows the GC result to be inconsistent with the momentum integral. We argue instead that the proper inner velocity should be u$_{i}$= (g$\beta \Delta $T$_{w}$ h)$^{1/2}$, where h is the channel width. This is in turn proportional to the friction velocity, hence sets the friction law. [Preview Abstract] |
Tuesday, November 20, 2007 12:01PM - 12:14PM |
NJ.00003: Natural Convection in Interconnected Systems: PIV Measurements Ramon Ramirez-Tijerina, Carlos Ivan Rivera-Solorio, Alejandro Garcia-Cuellar The fluid dynamics in a configuration of interconnected systems was experimentally studied. The configuration consists of one system where the fluid is heated with a power source and another system where the heat is dissipated. The fluid circulates between the two systems due to the buoyancy forces presented. This mechanics of natural convection is important due to their application in the cooling of power oil transformer, electrical devices and electronic components. Particle Image Velocimetry (PIV) equipment was employed to obtain measurements of fluid velocities. The effect in the flow patterns due to the variation of the configuration and conditions of the interconnected systems are investigated. Conclusions obtained from the analysis of the experimental data are presented. [Preview Abstract] |
Tuesday, November 20, 2007 12:14PM - 12:27PM |
NJ.00004: Numerical Analysis of Turbulent Natural Convection In A Cavity Mohamed Omri, Nicolas Galanis CFD codes are used extensively to analyse complex flow fields with heat and/or mass transfer, chemical reactions, etc{\ldots}. It is therefore necessary to continuously compare their predictions with experimental values in order to test their validity and eventually improve them. In this work, numerical predictions of turbulent natural convection in a square differentially heated cavity are analysed. Results are confronted to the detailed experimental data of [2] and [1] obtained with a Rayleigh number of 1.5x10$^{9}$. The purpose of this study is to evaluate the capacity of second order models to reproduce mean and fluctuating quantities. Thus, we first analyse mean velocities and mean temperature profiles. Then particular attention is given to turbulent quantities. Also, we compare the local Nusselt number along the four walls with the corresponding experimental values. Moreover five different grids are used (50x50, 100x100x100, 150x150, 200x200 and 300x300) to analyse grid-sensitivity. \newline \newline [1] Ampofo F.; Karayiannis T.G. (2003). Experimental benchmark data for turbulent natural convection in an air filled square cavity. Int. J. Heat and Mass Transfer. \newline [2] Tian Y.S.; Karayiannis T.G. (2000) Low turbulence natural convection in an air filled square cavity. Int. J. of Heat and Mass Transfer. [Preview Abstract] |
Tuesday, November 20, 2007 12:27PM - 12:40PM |
NJ.00005: The role of heat source area on the transition from displacement to mixing flow in natural ventilation Nigel Kaye, Gary Hunt We present a theoretical model for the role of heat source area on the transition from displacement to mixing flow for a naturally ventilated room. We examine the relationship between the existing standard models for natural ventilation of a room with floor and ceiling level vents (Linden {\it{et al.}} 1990 and Gladstone \& Woods 2001). We show that the uniform heat distribution model of Gladstone \& Woods is the limit of an infinite number of localized heat sources based on the Linden {\it{et al.}} model. We then examine the transition from localized to distributed heat source behaviour as a function of the horizontal extent of the heat source. Our model is based on recent measurements of the plume flow above large area heat sources that suggests the flow rate increases linearly with height. The flow transition is a function of the room vent area scaled on the ceiling height squared and the ratio of the ceiling height to heat source radius. As the heat source radius increases there is a rapid transition from displacement ventilation, driven by localized heat sources, toward mixing ventilation, driven by distributed heat sources. This transition is independent of the floor area of the room implying that for displacement ventilation to be established the heat source(s) need to be small compared to the vertical, as well as horizontal scale of the room. [Preview Abstract] |
Tuesday, November 20, 2007 12:40PM - 12:53PM |
NJ.00006: ABSTRACT WITHDRAWN |
Tuesday, November 20, 2007 12:53PM - 1:06PM |
NJ.00007: Numerical Simulations of Unsteady Natural Convection in Interconnected Systems Carlos Ivan Rivera-Solorio, Ramon Ramirez-Tijerina Numerical simulations are performed to study the process of unsteady natural convection in a configuration formed by two interconnected systems. In this configuration, one of the systems has a heat source that increases the temperature of the fluid. By natural convection, this fluid moves to a second system, which works as a radiator. The fluid cools off and descends to return to the first system. The process studied applies to oil heaters, power oil transformers, electrical devices and electronic equipment. The evolution of the velocities and temperature fields of the fluid are analyzed for different configurations and operating conditions of the interconnected systems. The effect in the time response of the heat transfer process is studied for the conditions considered. Conclusions drawn from the numerical results are presented. [Preview Abstract] |
Tuesday, November 20, 2007 1:06PM - 1:19PM |
NJ.00008: The effects of unsteady boundary conditions on heat flux for flow past a circular cylinder Izhak Givoni, Tait Pottebaum Heat transfer equipment is usually designed for specific, steady boundary and inflow conditions. In practice, systems typically deviate from the design conditions and fluctuate, resulting in an unsteady system. Models of heat transfer that incorporate fluctuations would allow for the design of equipment targeted to the actual conditions. In order to create such a model, understanding the non-linear relationship between fluctuations in the boundary conditions and fluctuations in the heat flux is critical. To reveal the details of the non-linear relationship, experiments were conducted in a water tunnel for flow past a heated cylinder. A cylinder was selected because the separation point of the boundary layer is sensitive to fluctuations in surface temperature, allowing non-linear effects to occur. Results will be presented from experiments in which random and sinusoidal temperature fluctuations were imposed on 1/3 of the cylinder surface while 2/3 of the surface was held at constant temperature. The observed relationship between heat flux and temperature fluctuations will be presented in the form of PDF's and PSD's. [Preview Abstract] |
Tuesday, November 20, 2007 1:19PM - 1:32PM |
NJ.00009: Numerical study on the heat transfer in a horizontal annulus with rib-mounted on outer cylinder Gyudeok Hwang, Seong-Gu Baek This study deals with the three--dimensional numerical simulation for laminar mixed convection of air in a horizontal concentric annulus between a heated, rotating inner cylinder and a riblet-mounted outer cylinder. The riblet with a triangluar cross-section is attached on the inner surface of the outer cylinder in azimuthal direction, which is the main streamwise direction. The height and the width of the riblet is about 0.1\% of the distance(H) between the two cylinders and the distance is set to be the same of the radius of the inner cylinder. The heat transfer in a horizontal annulus has been studied experimentally and numerically by many researchers in recent decades. So far as authors know, the research on the effect of the small structures on the annular cylinder is very few. The riblet structure near the wall is widely utilized to many applications because it plays role to reduce the drag in a turbulent boundary layer. It increases the drag in a laminar boundary layer. It is interesting point to note that the heat transfer is enhanced for turbulent flows and deteriorated for laminar flows, which was the results of the Direct Numerical Simulation. We will examine the effect of the riblets on the heat transfer in annular cylinders. Numerical results are obtained for the Reynolds number Re$_H$, ranging from 10 to 10$^3$, the Rayleigh number Ra, ranging up to 10$^6$. [Preview Abstract] |
Tuesday, November 20, 2007 1:32PM - 1:45PM |
NJ.00010: Spatially localized states in natural doubly diffusive convection Alain Bergeon, Edgar Knobloch Numerical continuation is used to compute a large multiplicity of stable spatially localized steady states in doubly diffusive convection in a vertical slot driven by imposed horizontal temperature and concentration gradients. The calculations focus on the so-called opposing case, in which the imposed horizontal thermal and solutal gradients are in balance. No-slip boundary conditions are used at the sides; periodic boundary conditions with large spatial period are used in the vertical direction. The results demonstrate the presence of homoclinic snaking in this system, and can be interpreted in terms of a pinning region in parameter space. The dynamics outside of this region are studied using direct numerical simulation. The behavior of the system resembles that recently identified in the cubic-quintic Swift-Hohenberg equation [1]. \vskip0.1in \noindent [1] J. Burke and E. Knobloch, Phys. Lett. A 360, 681--688 (2007). [Preview Abstract] |
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