Session R06: CFD: Applications II

A General Approach for Predicting Convective Heat Transfer Coefficients in Turbulent Systems using Large Eddy Simulations

A generalized approach for predicting turbulent convective heat transfer coefficients using large eddy simulation is presented. This model is based on semi-empirical transport theory, which links the local convective heat transfer rate to the local energy dissipation rate within the near-wall boundary layer.  We apply these approaches to agitated tanks, pipe flow systems, cylinders in crossflow, and tube bundles. We validate the generality and reliability of the theoretical model against expectations from experimentally derived empirical design correlations.

We assess the generality of this approach by considering various industrial systems, including agitated vessels, pipe flow systems, cylinders in crossflow, and tube bundle systems. We assess its correctness by comparing predictions for each system at multiple operating conditions to expectations from experimentally parameterized design correlations. We demonstrate that this first-principles approach can consistently predict heat transfer rates with an accuracy comparable to the empirical correlations. Although attention must be given to simulation resolution, the procedure requires no parameter tuning or adjustment between system topology, operating conditions, or scale. The agreement between theory and experiment is noteworthy, considering the simplicity of the implementation and the simulation calculation speed.

The findings from this investigation present three key implications: first, as a complement to physical testing, this modeling approach can be used to reliably predict local and overall heat transfer rates in silico using first-principles simulations. Second, because the approach directly appeals to the Navier-Stokes equations, it provides mechanistic insight into the link between operating conditions, fluid flow, and convection. Third, because heat transfer is modeled in tandem and on-the-fly with the underlying fluid dynamics, the combined effects of convection and advection can be investigated directly. In this sense, the approach can be used to perform first-principles multi-physics simulations that make a priori predictions of convective heat transfer rates and thermal transport through turbulent flows.   

Numerical investigation of the flow in scroll compressor: consideration of newly enforced refrigerant

We numerically investigated the three-dimensional flow field in a scroll compressor with bypass valves under different pressure ratios (between inlet and discharge) of 2.1 and 3.4. Restricting the use of refrigerants with a GWP (global warming potential) above 750 for split air conditioners by EU regulations, it is required to analyze the variation caused by the alternative refrigerant. In the same context, we consider R454B and R452B, as alternative refrigerant of conventional R410A. We apply the SDM (structured dynamic mesh) to simulate the shape change due to the movement of the orbiting scroll. During the opening or closing of a valve, rapid fluctuations in velocity occur near the valve, and there is a pressure drop in the flank gap region. However, these local flow characteristics have little effect on the global pressure distribution within the chamber, resulting in a relatively uniform pressure distribution. Nonetheless, these local flows can affect the performance of the compressor and the functioning of lubricating oil during actual compressor operation. We will further discuss not only the performance differences of the compressor under each operating condition, but also on the local flow characteristics.   

Parametric Study of Solid-Solid and Solid-Liquid Phase Change Materials for a Pin-Finned Heat Sink in Avionics Thermal Control.

This paper reports a comparative investigation of solid-solid phase change material (SS-PCM) and solid-liquid phase change material (SL-PCM) for cooling of electronics in avionics applications.In this study, the thermal performance of a pin-finned heat sink containing these two PCMs is examined. A Polyethylene Glycol-based Form-stable SS-PCM and a commercial organic-based SL-PCM named A24 are selected for this study. The two PCMs are chosen to have nearly the same thermo-physical properties and operate at a similar temperature range within the scope of the given application. The characterization is carried out for both PCMs for their thermal properties, such as latent heat, phase transition characteristics, specific heat, thermal conductivity, and thermal degradation. A numerical model is developed for the transient heat transfer study of the two PCMs under the same heat input rates and charging-discharging time. Parameters such as the mass of the PCM, response time, temperature variation, and thermal degradation are the criteria for evaluating both PCMs. It is found that a higher temperature is obtained for SS-PCM, compared to SL-PCM, after the heating cycle. A better thermal response is achieved by the considered SS-PCM compared to the chosen SL-PCM. Finally, this study aids the selection of SS-PCM and SL-PCM for a particular thermal storage-based application.   

Effective thermal management of Lithium-Ion Batteries with inorganic PCMs and mini-channel cold plate

The share of electric mobility in passenger vehicle sector is increasing rapidly. Neat electric vehicles work with battery, controller and electric motor as its essential components.Among different types of batteries, Lithium-Ion batteries have gained popularity for electric vehicle application owing to its important characteristics such as high energy and power density and nominal voltage. However, the functioning of these batteries is hindered by temperature under high discharge rates and high ambient temperatures, causing thermal runaway and related catastrophic issues. Many thermal management approaches have been in vogue to bring down the battery temperature to threshold limit. Among various Phase Change Materials[PCMs], inorganic type being non-flammable offer high thermal conductivity and thus tending provide better cooling. An attempt is made to implement inorganic PCMs with different thickness under high discharge rate conditions. A computational study, with ANSYS 21 R, is aimed at to achieve better thermal management both with inorganic PCMs and mini-channel cold plate. The numerical work is performed on a single cell, module and a battery pack with LiFePO4 as chosen battery cathode material.It is observed combination of PCM and cold plate could bring down the peak battery temperature to a safer limit.   

Numerical Simulation of Ventilation System in Engine Room under Ship Construction

Manual welding operations are performed with human efforts inside the ship, when constructing an LNG carrier. Workers for welding inside an engine room are directly exposed to welding fume, by which its long time exposure may lead to serious health issue. In this background, it is important to improve the air quality in the working space by ventilating air using multiple fans. The placement of ventilation fans on site has been empirically determined, but it has yet to investigate if such the fan arrangement is optimal or not. In this study, we introduce a computational fluid dynamics (CFD) model that simulates the interior of a ship engine room using a commercial software ANSYS Fluent. A numerical quantity, i.e., the age of air, is computed using a transport equation with velocity vector field obtained from incompressible Navier-Stokes equations. As a parametric study, we explore various conditions of the fans such as the number, location and angles for a room structure surrounded by wall for each deck. The current study can suggest an efficient arrangement of ventilation fans inside an engine room under construction, which is possibly used for assessing air quality in similar interior models.   

Rotating Turbulent Convection in a Spherical Shell With a Hybrid Discrete Exterior Calculus and Finite Difference Method

A hybrid discrete exterior calculus and finite difference discretization of turbulent convective flow in a spherical shell is developed as a paradigm for solar convective flows. An anelastic approximation is used, and our previous formulation (Jagad, P., Khan, H., & Parsani, M. (2023). Anelastic thermal convection in spherical shells using hybrid discrete exterior calculus and finite difference method. Bulletin of the American Physical Society) is extended to include rotation, i.e., the governing equations are considered in a rotating frame of reference. The operators and variables are decomposed into surface and radial components and they are approximated using DEC and FD discretizations, respectively. Several validation cases are presented. Moreover, formation of convective patterns is studied.   

Efficient Levitation in Vertical Wind Tunnels: A Novel Design for Stable Liquid Droplet Studies

The efficient study of liquid droplets, ranging from micrometers to a few centimeters, through levitation, is often hindered by limitations in conventional designs. This is primarily due to continuous droplet deformation in the test section, which is further aggravated by the reaching critical Weber velocity for breakup before reaching the terminal velocity, particularly for larger droplets (diameter > capillary number). Additionally, as the droplet size increases, deviations from the spherical (0,0) mode become more prominent due to reduced surface tension effects. This results in continuous transitions to other energy-minimizing modes. Here, we propose a novel test section configuration for stabilizing large droplets in a vertical wind tunnel to overcome these challenges. The unconventional design introduces a diverging cross-section to introduce a decreasing vertical velocity with height and a symmetric airfoil rotated about its chord (spheroid) is proposed to make a velocity bucket keep the droplets levitating in the center of the test section. This offers flexibility for experimental researchers to customize the levitation (tunnel) profile for studying droplets of various sizes, ultimately enhancing the efficiency of droplet investigations. Numerical tools are employed to analyze longitudinal and lateral velocity variations in the test section, comparing the concepts for minimal turbulence intensity.