Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session R4: Applied Thermodynamics and Heat Transfer |
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Chair: Mahboobe Mahdavi, Gannon University Room: B112 |
Tuesday, November 22, 2016 1:30PM - 1:43PM |
R4.00001: Experimental Analysis of the Effects of Inclination Angle and Working Fluid Amount on the Performance of a Heat Pipe Mahboobe Mahdavi, Saeed Tiari, Songgang Qiu Heat pipes are two-phase heat transfer devices, which operate based on evaporation and condensation of a working fluid inside a sealed container. In the current work, an experimental study was conducted to investigate the performance of a copper-water heat pipe. The performance was evaluated by calculating the corresponding thermal resistance as the ratio of temperature difference between evaporator and condenser to heat input. The effects of inclination angle and the amount of working fluid were studied on the equivalent thermal resistance. The results showed that if the heat pipe is under-filled with the working fluid, energy transferring capacity of the heat pipe decreases dramatically. However, overfilling heat pipe causes over flood and degrades heat pipe performance. The minimum thermal resistances were obtained for the case that 30{\%} of the heat pipe volume was filled with working fluid. It was also found that in gravity-assisted orientations, the inclination angle does not have significant effect on the performance of the heat pipe. However, for gravity-opposed orientations, as the inclination angle increases, the temperature difference between the evaporator and condensation increases and higher thermal resistances are obtained. [Preview Abstract] |
Tuesday, November 22, 2016 1:43PM - 1:56PM |
R4.00002: Neutron Radiography for Determining the Evaporation/Condensation Coefficients of Cryogenic Propellants K. Bellur, E.F. Medici, M. Kulshreshtha, V. Konduru, D. Tyrewala, C.-K. Choi, J.S. Allen, A. Tamilarasan, J.C. Hermanson, J.B. McQuillen, J. Leao, D.S. Hussey, D.L. Jacobson, J. Scherschligt A novel, combined experimental and computational approach was used to determine the accommodation coefficients for liquid hydrogen and liquid methane in aluminum and stainless steel containers. The experimental effort utilized the NIST Neutron Imaging Facility to image the evaporation and condensation of cryogenic, hydrogenated propellants inside metallic containers. The computational effort included a numerical solution of a model for phase change in the contact line and thin film regions as well as a CFD effort for determining the appropriate thermal boundary conditions for the numerical solution of the evaporating and condensing liquid. These three methods in combination allow for extracting the accommodation coefficients from the experimental observations. The condensation and evaporation were controlled by adjusting the system temperature and pressure. The computational thermal model was shown to accurately track the transient thermal response of the test cells. The meniscus shape determination suggests the presence of a finite contact angle, albeit very small, between liquid hydrogen and an aluminum oxide surface. [Preview Abstract] |
Tuesday, November 22, 2016 1:56PM - 2:09PM |
R4.00003: A heat transfer model for slug flow boiling within microchannels Mirco Magnini, John Thome We propose a novel physics-based model for the fluid mechanics and heat transfer associated with slug flow boiling in horizontal circular microchannels, to update the widely used three-zone model for the design of multi-microchannel evaporators. The flow is modelled as the cyclic passage of a liquid slug, an elongated bubble which traps a thin liquid film against the channel wall, and a dry vapor plug. The capillary flow theory, extended to incorporate evaporation effects, is applied to estimate the bubble velocity along the channel. A liquid film thickness prediction method considering bubble proximity effects, which may limit the radial extension of the film, is included. Theoretical heat transfer models accounting for the thermal inertia of the liquid film and for the recirculating flow within the liquid slug are utilized. The heat transfer model is compared to experimental data taken from three independent studies: 833 slug flow boiling data points covering R134a, R245fa and R236fa and channel diameters from 0.4 mm to 1 mm. The new model predicts more than $80\%$ of the database to within $\pm30\%$ and it represents an important step toward a complete physics-based modelling of bubble dynamics and heat transfer within microchannels under evaporating flow conditions. [Preview Abstract] |
Tuesday, November 22, 2016 2:09PM - 2:22PM |
R4.00004: Nonlinear optimization of buoyancy-driven ventilation flow Saleh Nabi, Piyush Grover, C.P. Caulfield We consider the optimization of buoyancy-driven flows governed by Boussinesq equations using the Direct-Adjoint-Looping method. We use incompressible Reynolds-averaged Navier-Stokes (RANS) equations, derive the corresponding adjoint equations and solve the resulting sensitivity equations with respect to inlet conditions. For validation, we solve a series of inverse-design problems, for which we recover known globally optimal solutions. For a displacement ventilation scenario with a line source, the numerical results are compared with analytically obtained optimal inlet conditions available from classical plume theory. Our results show that depending on Archimedes number, defined as the ratio of the inlet Reynolds number to the Rayleigh number associated with the plume, qualitatively different optimal solutions are obtained. For steady and transient plumes, and subject to an enthalpy constraint on the incoming flow, we identify boundary conditions leading to `optimal’ temperature distributions in the occupied zone. [Preview Abstract] |
Tuesday, November 22, 2016 2:22PM - 2:35PM |
R4.00005: Numerical simulation of natural convection in vertical cylinders partially cooled from above Jose Nunez Gonzalez, Alberto Beltran Morales, Sergio Cuevas Steady natural convection in vertical cylinders heated from below and partially cooled from above is studied from a numerical point of view. The governing equations for natural convection are discretized employing a mixed Fourier - Finite volume method using the SIMPLEC algorithm as velocity decoupling strategy. Calculations are performed for constant Prandtl number, Pr=6.667, and Rayleigh number over a range of $10^3 \leq Ra \leq 10^5$ and cooler size $0.125\leq \gamma \leq 1$ and for an aspect ratio (height/diameter) $0.5\leq a \leq 1.25$. Convective complex three-dimensional flow structures are presented. [Preview Abstract] |
Tuesday, November 22, 2016 2:35PM - 2:48PM |
R4.00006: Experimental validation of a solar-chimney power plant model Nima Fathi, Patrick Wayne, Ignacio Trueba Monje, Peter Vorobieff In a solar chimney power plant system (SCPPS), the energy of buoyant hot air is converted to electrical energy. SCPPS includes a collector at ground level covered with a transparent roof. Solar radiation heats the air inside and the ground underneath. There is a tall chimney at the center of the collector, and a turbine located at the base of the chimney. Lack of detailed experimental data for validation is one of the important issues in modeling this type of power plants. We present a small-scale experimental prototype developed to perform validation analysis for modeling and simulation of SCCPS. Detailed velocity measurements are acquired using particle image velocimetry (PIV) at a prescribed Reynolds number. Convection is driven by a temperature-controlled hot plate at the bottom of the prototype. Velocity field data are used to perform validation analysis and measure any mismatch of the experimental results and the CFD data. CFD Code verification is also performed, to assess the uncertainly of the numerical model with respect to our grid and the applied mathematical model. The dimensionless output power of the prototype is calculated and compared with a recent analytical solution and the experimental results. [Preview Abstract] |
Tuesday, November 22, 2016 2:48PM - 3:01PM |
R4.00007: Micro-Scale Thermoacoustics Avshalom Offner, Guy Z. Ramon Thermoacoustic phenomena – conversion of heat to acoustic oscillations - may be harnessed for construction of reliable, practically maintenance-free engines and heat pumps. Specifically, miniaturization of thermoacoustic devices holds great promise for cooling of micro-electronic components. However, as devices size is pushed down to micro-meter scale it is expected that non-negligible slip effects will exist at the solid-fluid interface. Accordingly, new theoretical models for thermoacoustic engines and heat pumps were derived, accounting for a slip boundary condition. These models are essential for the design process of micro-scale thermoacoustic devices that will operate under ultrasonic frequencies. Stability curves for engines - representing the onset of self-sustained oscillations - were calculated with both no-slip and slip boundary conditions, revealing improvement in the performance of engines with slip at the resonance frequency range applicable for micro-scale devices. Maximum achievable temperature differences curves for thermoacoustic heat pumps were calculated, revealing the negative effect of slip on the ability to pump heat up a temperature gradient. [Preview Abstract] |
Tuesday, November 22, 2016 3:01PM - 3:14PM |
R4.00008: Mechanism and Structure of Subsurface Explosions in Granular Media Shuyue Lai, Ryan Houim, Elaine Oran Numerical simulations of explosions in granular media were performed with an unsteady multidimensional fully compressible model, which solves two sets of coupled Euler equations, one for the gas and one for the granular medium. An explosive charge, buried in the granular medium, is modeled by a pocket of high-pressure and high-temperature gas. The initial conditions were determined based on an estimate of~subsurface conditions on a comet. A series of simulations were performed in which the charge was buried at 3 m and 1.5 m and the particle volume fractions and the coefficient of restitution varied in the ranges 0.25 to 0.45 and 0 to 1, respectively. The simulations show the process of granular shock formation and propagation as a blast wave is created during the explosion. The blast wave initiates the particle motion and the particles accumulate to form a granular shock. The granular shock, in turn, produces a weak gas shock following it. There is a power law that relates the granular-shock radius to the explosion time: R \textasciitilde t$^{\mathrm{0.4}}$, which is consistent with the results found by G. I. Taylor for 3-D spherical shock waves. The exponent of the power law remains at 0.4 regardless of the volume fraction and the elasticity of the granular material. For denser granular flows, the intergranular stress becomes stronger, and so the granular shock propagates at a higher velocity. [Preview Abstract] |
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