This study is part of the preliminary experimental investigations designed to assess the feasibility of using supercritical carbon dioxide (sCO2) in the vicinity of its critical point for thermal management applications. In the present study, sCO2 was used as a working fluid in a diffusion bonded 316/316L stainless steel test section having staggered micro pin fin array flow passages of hydraulic diameter 679 µm (0.679 mm). The test section was subjected to a single wall non-uniform heat flux boundary condition and was operated in a horizontal orientation. The primary objective was to characterize the heat transfer performance of sCO2 as it flows through the staggered pin fin array for experimental conditions that span its critical and pseudocritical point. Data analysis methods employing 2-D and 3-D heat transfer models of the test section were used to calculate the average heat transfer coefficients for a given set of experimental conditions. Experiments were conducted by varying the inlet temperature (18 ≤ T_in ≤ 50 °C) and for fixed mass flux (300 kg m-2 s-1), heat flux (40 W cm-2), and reduced pressure (1.1). Experimental data were also compared against the predictions of a correlation proposed for single phase flows in microchannel staggered diamond pin arrays. The correlation predicted the data within 4.3 % when the ratio, T_Bulk/T_PC exceeded 1. It was also found that the enhancement in the heat transfer, a result of employing staggered pin array flow geometry instead of microchannels, carries a commensurate penalty in pressure drop.
more »
« less
LES Simulation of turbulent supercritical CO2 heat transfer in microchannels
Although supercritical CO2 (sCO2) heat transfer has been employed in industrial process since the 1960s, the underlying transport phenomenon in high-flux microscale geometries, as could be employed in concentrating solar receivers, is poorly understood. To date, nearly all experimental studies and simulations of supercritical convective heat transfer have focused on large diameter vertical channel and tube bundle flows, which may differ dramatically from microscale supercritical convection. Computational studies have primarily employed Reynolds averaged (RANS) turbulence modeling approaches, which may not capture effects from the sharply varying property trends of supercritical fluids. In this study, large eddy simulation (LES) turbulence modeling techniques are employed to study heat transfer characteristics of sCO2 in microscale heat exchangers. The simulation geometry consists of a microchannel of 750×737
μm cross-section and 5 mm length, heated from all four sides. Simulation cases are evaluated at reduced pressure P_r = 1.1, mass flux G = 1000 kg/m^2-s, heat flux q'' = 1.7 − 8.9 W/cm^2 , and varying inlet temperature: 20 − 100℃. Computational results reveal thermal transport mechanisms specific to microscale sCO2 flows. Results have been compared with available supercritical convection correlations to identify the most applicable heat transfer models for
engineering of microchannel sCO2 heat exchangers.
more »
« less
- Award ID(s):
- 1604538
- NSF-PAR ID:
- 10074126
- Date Published:
- Journal Name:
- 6th International Supercritical CO2 Power Cycles Symposium
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
more » « less
SUMMARY We present investigations of rapidly rotating convection in a thick spherical shell geometry relevant to planetary cores, comparing results from quasi-geostrophic (QG), 3-D and hybrid QG-3D models. The 170 reported calculations span Ekman numbers, Ek, between 10−4 and 10−10, Rayleigh numbers, Ra, between 2 and 150 times supercritical and Prandtl numbers, Pr, between 10 and 10−2. The default boundary conditions are no-slip at both the ICB and the CMB for the velocity field, with fixed temperatures at the ICB and the CMB. Cases driven by both homogeneous and inhomogeneous CMB heat flux patterns are also explored, the latter including lateral variations, as measured by Q*, the peak-to-peak amplitude of the pattern divided by its mean, taking values up to 5. The QG model is based on the open-source pizza code. We extend this in a hybrid approach to include the temperature field on a 3-D grid. In general, we find convection is dominated by zonal jets at mid-depths in the shell, with thermal Rossby waves prominent close to the outer boundary when the driving is weaker. For the thick spherical shell geometry studied here the hybrid method is best suited for studying convection at modest forcing, $Ra \le 10 \, Ra_c$ when Pr = 1, and departs from the 3-D model results at higher Ra, displaying systematically lower heat transport characterized by lower Nusselt and Reynolds numbers. We find that the lack of equatorially-antisymmetric motions and z-correlations between temperature and velocity in the buoyancy force contributes to the weaker flows in the hybrid formulation. On the other hand, the QG models yield broadly similar results to the 3-D models, for the specific aspect ratio and range of Rayleigh numbers explored here. We cannot point to major disagreements between these two data sets at Pr ≥ 0.1, with the QG model effectively more strongly driven than the hybrid case due to its cylindrically averaged thermal boundary conditions. When Pr is decreased, the range of agreement between the hybrid and 3-D models expands, for example up to $Ra \le 15 \, Ra_c$ at Pr = 0.1, indicating the hybrid method may be better suited to study convection in the low Pr regime. We thus observe a transition between two regimes: (i) at Pr ≥ 0.1 the QG and 3-D models agree in the studied range of Ra/Rac while the hybrid model fails when $Ra\gt 15\, Ra_c$ and (ii) at Pr = 0.01 the QG and 3-D models disagree for $Ra\gt 10\, Ra_c$ while the hybrid and 3-D models agree fairly well up to $Ra \sim 20\, Ra_c$. Models that include laterally varying heat flux at the outer boundary reproduce regional convection patterns that compare well with those found in similarly forced 3-D models. Previously proposed scaling laws for rapidly rotating convection are tested; our simulations are overall well described by a triple balance between Coriolis, inertia and Archimedean forces with the length-scale of the convection following the diffusion-free Rhines-scaling. The magnitude of Pr affects the number and the size of the jets with larger structures obtained at lower Pr. Higher velocities and lower heat transport are seen on decreasing Pr with the scaling behaviour of the convective velocity displaying a strong dependence on Pr. This study is an intermediate step towards a hybrid model of core convection also including 3-D magnetic effects.
-
In the present investigation, high resolution large eddy simulations (LES) of sCO2 vertical upward flows in microchannels are performed at high mass fluxes (G = 1000 kg/m^2-s) and moderate heat fluxes (q'' = 1.6 − 8.7 W/cm^2), and flow inlet temperature in the range of T = 20 − 100 ℃ to predict sCO2 heat transfer coefficients inside and outside pseudocritical region. Results are compared with our prior computational study of horizontal microchannels at similar thermophysical conditions to determine the effect of channel orientation on possible enhancement or deterioration of heat transfer. Results are also compared with available empirical supercritical heat transfer correlations to assess their applicability at these working conditions.more » « less
-
more » « less
Abstract Forest canopies play a critical role in affecting momentum and scalar transfer. Although there have been recent advances in numerical simulations of turbulent flows and scalar transfer across plant canopies and the atmosphere interface, few models have incorporated all important physical and physiological processes in subcanopy layers. Here we describe and evaluate an advanced multiple‐layer canopy module (MCANOPY), which is developed based largely on the Community Land Model version 4.5 and then coupled with the Weather Research and Forecasting model with large‐eddy simulations (WRF‐LES). The MCANOPY includes a suite of subcanopy processes, including radiation transfer, photosynthesis, canopy layer energy balance, momentum drag, and heat, water vapor, and CO2exchange between canopy layers and the canopy atmosphere. Numerical schemes for heat and water transport in soil, ground surface energy balance, and soil respiration are also included. Both the stand‐alone MCANOPY and the coupled system (the WRF‐LES‐MCANOPY) are evaluated against data measured in the Canopy Horizontal Array Turbulence Study field experiment. The MCANOPY performs reasonably well in reproducing vertical profiles of mean and turbulent flows as well as second‐order statistical quantities including heat and scalar fluxes within the canopy under unstable stability conditions. The coupled WRF‐LES‐MCANOPY captures major features of canopy edge flows under both neutral and unstable conditions. Limitations of the MCANOPY are discussed for our further work. Our results suggest that our model can be a promising modeling system for a variety of applications to study canopy flows and scalar transport (e.g., CO2).
-
Optimization of mixing in microfluidic devices is a popular application of computational fluid dynamics software packages, such as COMSOL Multiphysics, with an increasing number of studies being published on the topic. On one hand, the laminar nature of the flow and lack of turbulence in this type of devices can enable very accurate numerical modeling of the fluid motion and reactant/particle distribution, even in complex channel geometries. On the other hand, the same laminar nature of the flow, makes mixing, which is fundamental to the functionality of any microfluidic reactor or assay system, hard to achieve, as it forces reliance on the slow molecular diffusion, rather than on turbulence. This in turn forces designers of microfluidic systems to develop a broad set of strategies to enable mixing on the microscale, targeted to the specific applications of interest. In this context, numerical modeling can enable efficient exploration of a large set of parameters affecting mixing, such as geometrical characteristics and flow rates, to identify optimal designs. However, it has to be noted that even very performant mixing topologies, such as the use of groove-ridge surface features, require multiple mixing units. This in turn requires very high resolution meshing, in particular when looking for solutions for the convection-diffusion equation governing the reactant or chemical species distribution. For the typical length of microfluidic mixing channels, analyzed using finite element analysis, this becomes computationally challenging due to the large number of elements that need to be handled. In this work we describe a methodology using the COMSOL Computational Fluid Dynamics and Chemical Reaction Engineering modules, in which large geometries are split in subunits. The Navier-Stokes and convection-diffusion equations, are then solved in each subunit separately, with the solutions obtained being transferred between them to map the flow field and concentration through the entire geometry of the channel. As validation, the model is tested against data from mixers using periodic systems of groove-ridge features in order to engineer transversal mixing flows, showing a high degree of correlation with the experimental results. It is also shown that the methodology can be extended to long mixing channels that lack periodicity and in which each geometrical mixing subunit is distinct.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Conference Paper:
- The DOI is not currently available.
