Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session Q13: Convection and Buoyancy-driven Flows: Heat Transfer and Forced Convection |
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Chair: Chenguang Zhang, MIT Room: 304 |
Tuesday, November 26, 2019 7:45AM - 7:58AM |
Q13.00001: Heat transport by baroclinic acoustic streaming Jacques Abdul-Massih, Guillaume Michel, Christopher White, Greg Chini Recently, Chini \emph{et al.} [\emph{J. Fluid Mech.}, Vol. 744 (2014)] and Michel \& Chini [\emph{J. Fluid Mech.}, Vol. 858 (2019)] demonstrated that strong acoustic streaming flows can be generated in gases subjected to an imposed cross-channel temperature gradient. In contrast with classic Rayleigh streaming, standing acoustic waves of $\mathit{O}(\epsilon)$ amplitude acquire vorticity owing to baroclinic torques acting throughout the domain rather than via viscous torques acting in Stokes boundary layers. More significantly, these baroclinically-driven streaming flows have a magnitude that is $\mathit{O}(\epsilon)$, i.e. comparable to that of the sound waves, leading to fully two-way wave/mean-flow coupling. The present investigation extends these earlier studies by relaxing the restriction to small aspect-ratio domains, thereby enabling the (forced) heat transport across the channel to be quantified as a function of aspect ratio. This extension requires the numerical solution of a two-dimensional eigenvalue problem for the sound-wave frequency and mode structure. Nevertheless, the resulting computations are orders of magnitude faster than DNS of the compressible Navier-Stokes equations. The prospect for using baroclinic acoustic streaming as a cooling technology is evaluated. [Preview Abstract] |
Tuesday, November 26, 2019 7:58AM - 8:11AM |
Q13.00002: Experiments of Flow Boiling of R245fa in a Horizontal Pipe Using Particle Image Velocimetry Hannah Moran, Victor Voulgaropoulos, Dimitri Zogg, Omar Matar, Christos Markides A bespoke facility is used to measure flow boiling of the refrigerant R245fa in a 12.7-mm diameter stainless steel pipe, to which uniform heating of up to 135 kW/m$^2$ may be applied. A range of measurement techniques are utilised in the facility to obtain results over a range of flow rates and heat fluxes. Differential pressure measurements are taken, whilst thermocouples at the tube walls allow the calculation of the heat transfer coefficient, and flow visualisation is accomplished using high-speed imaging. The results are then compared to other experimental work and to correlations in the literature. In addition, laser-based measurements, such as particle image velocimetry (PIV), are performed, providing detailed spatially- and temporally-resolved information on the velocity and turbulence characteristics in the liquid phase. The measurements provide new insight into the hydrodynamic and thermal interactions in these flows and help to build a comprehensive picture of the phenomena involved in the boiling process. [Preview Abstract] |
Tuesday, November 26, 2019 8:11AM - 8:24AM |
Q13.00003: Vibratory control of heat transfer in fluid in containers with elastic boundaries Nikolai Kozlov An experimental study is carried on of heat transfer in a viscous fluid enclosed in cylindrical and spherical containers with vibrating elastic boundaries. The container boundary is thermally stabilized by pumping a liquid around it. Two cases are considered: the fluid bulk is heated while the boundary is cooled and the boundary is set at a constant higher temperature. Vibrations generate steady streaming in the container, intensifying the heat transfer. The former is studied using PIV method, the latter -- by evaluation of temperature difference between the fluids in the container and the thermostatic bath. The dimensionless velocity of steady flows increases with the pulsation Reynolds number, $Re_p$. The steady flows are in competition with the free convection. The domain of dimensionless parameters (Rayleigh number, $Ra$, and $Re_p$) where the free convection can be neglected is found. For this domain, the analysis of heat transfer by steady streaming as a function of the dimensionless vibration frequency is done. The obtained results can be applied for the assessment of flow structure and heat-mass transfer in oscillating elastic containers of various scale, for example liquid drops. [Preview Abstract] |
Tuesday, November 26, 2019 8:24AM - 8:37AM |
Q13.00004: DNS of thermal channel flow for $Re_\tau=5000$ Francisco Alcantara Avila, Sergio Hoyas Calvo A new DNS of a thermal channel flow has been conducted at $Re_\tau=5000$. The thermal field has been considered as a passive scalar and the Mixed Boundary Condition is employed. The Prandtl number of air, $0.71$, has been used. A large enough computational box of dimensions $2\pi h$ x $2 h$ x $\pi h$ has been set to obtain accurate statistics. The mesh used had a total of $6144$ x $1285$ x $6144$ $\approx$ 5e10 points. The simulation has run on 2048 cores of the supercomputer MareNostrum. The logarithmic region of the mean temperature profile is starting to be properly developed with a von Karman constant of $\kappa_t = 0.444$. Maxima of the temperature intensities increase and move towards the wall with respect to other cases with lower Reynolds numbers. A power function has been obtained to calculate the Nusselt number as a function of Reynolds, for $Pr = 0.71$. Turbulent Prandtl number does not show remarkable differences with the ones obtained for lower Reynolds and it keeps being close to $1$ near the wall. Finally, turbulent budgets for heat fluxes, temperature variance and its dissipation rate have been calculated. Scaling of all terms is analysed and discussed. [Preview Abstract] |
Tuesday, November 26, 2019 8:37AM - 8:50AM |
Q13.00005: Toroidal-helical pipe as a passive mixing and heat transfer device in laminar flows Chenguang Zhang, Krishnaswamy Nandakumar The secondary flow of Dean vortex pair is a long-known mechanism for the enhancement of mixing and heat transfer, with example uses including circular or spiral pipes. We present a novel design of a toroidal helical pipe, which follows a three-dimensional toroidal helix (an analytical curve smoothly winding around a torus) as its pipe axis. Unlike the circle or straight helix which has constant curvatures, the toroidal helix has a spatially oscillating curvature. This unique feature causes the classic separated Dean vortex pair to couple, periodically re-orient, shrink and grow within each turn of the pipe. The complex flow pattern spans the entire cross-section and enhances mixing via advection. Using heat transfer as an example, the Nusselt number in the toroidal helical pipe varies periodically, with mean values several times higher than a straight pipe even at moderate Reynolds numbers. [Preview Abstract] |
Tuesday, November 26, 2019 8:50AM - 9:03AM |
Q13.00006: An Assessment of the Reynolds Analogy in Predicting Heat Transfer in Turbulent Flows of Low Prandtl Numbers Matthias Ziefuss, Amirfarhang Mehdizadeh Heat transfer modeling plays a major role in design and optimization of modern and efficient cooling systems. However, currently available models suffer from a fundamental shortcoming: their development is based on the general notion that an accurate prediction of the flow field will guarantee an appropriate prediction of the thermal field, known as the Reynolds Analogy. This analogy works reasonably well when applied to fluids with a Prandtl number around unity to obtain first order statistics. Concerning fluids with non-unity Prandtl number, there is no comprehensive assessment available. Thus, this investigation presents an introductory assessment of the capability of the Reynolds Analogy when applied to turbulent shear flows of fluids with small Prandtl number. The assessment includes steady and unsteady state simulations. In case of steady state simulations, it turns out that the Reynolds Analogy is not able to predict the mean temperature at an acceptable level of accuracy, while second order statistics are severely mispredicted. In case of unsteady simulations, it is shown that the Reynolds Analogy cannot be considered as an appropriate sub-grid scale model as it fails to feature basic properties of a reliable sub-grid scale model. [Preview Abstract] |
Tuesday, November 26, 2019 9:03AM - 9:16AM |
Q13.00007: Investigating the Impact of Variable Properties for Cryogenic Helium in a Heat Transfer in a Microchannel Choah Shin, Michael Martin, Shashank Yellapantula, Marc Henry de Frahan, Ray Grout Cryogenic helium in liquid, vapor, and supercritical phases is used for cooling in a range of applications, including quantum computers, superconducting magnets, and infrared sensors used in astronomical measurements. Properties such as density, viscosity, thermal conductivity, and specific heat vary significantly near the critical point (5.2 K and 227.46 kPa). We have simulated laminar flow in a heated square microchannel with widths ranging from 25 to 100 microns under supercritical conditions. We perform the simulations using \textit{PeleC}, a computational fluid dynamics code with adaptive mesh refinement, embedded boundaries, and complex fluid equations of state. At temperatures between 30 and 60 K, the Soave-Redlich-Kwong equation of state is used to calculate thermodynamic properties. A temperature and pressure sensitive model is also used for transport properties. As the temperature changes due to wall heating, we observe the variations of properties in the fluid due to these models, and measure the impact on flow structure and channel heat transfer. [Preview Abstract] |
Tuesday, November 26, 2019 9:16AM - 9:29AM |
Q13.00008: Flow field and heat transfer characteristics in a model solar PV farm James McNeal, Andrew Glick, Naseem Ali, Juliaan Bossuyt, Gerald Recktenwald, Marc Calaf, Raúl Bayoán Cal Large scale solar farms supply an increasing amount of the world's electricity supply. However, high operation temperatures can strongly reduce efficiency and panel lifetime, negatively affecting the levelized cost of energy. In this work, the convective heat transfer coefficient for a utility-scale solar farm is studied using thermal and particle image velocimetry measurements in a scaled wind tunnel experiment. The results confirm the applicability of the scaled experimental setup to studies of large solar arrays. Further, the velocity measurements indicate the complex flow structure within the solar array, governed by wakes directed upwards due to the orientation of the solar panels. [Preview Abstract] |
Tuesday, November 26, 2019 9:29AM - 9:42AM |
Q13.00009: Temperature reduction through system level flow enhancement via model solar PV farm wind tunnel experiments Andrew Glick, Juliaan Bossuyt, Naseem Ali, Marc Calaf, Ra\'{u}l Bayo\'{a}n Cal To further facilitate the reduction in cost of photovoltaic energy, new approaches to limit module temperature increase in natural ambient conditions should be explored. Thus far only approaches based at the individual panel level have been investigated, while the more complex, systems approach remains unexplored. Here, we perform the first wind tunnel scaled solar farm experiments to investigate the potential for temperature reduction through system level flow enhancement. Results indicate that significant changes in the convective heat transfer coefficient are possible, based on wind direction, wind speed, and module inclination. We show that 30-45\% increases in convection are possible through an array-flow informed approach to layout design, leading to a potential overall power increase of $\sim 5\%$ and decrease of solar panel degradation by +0.3\%/year. Previous models demonstrating the sensitivity to convection are validated through the wind tunnel results, and a new conceptual framework is provided that can lead to new means for solar PV array optimization. [Preview Abstract] |
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