Abstract
This study looks to find a suitable turbulence model for calculating pressure losses of ventilation components. In building ventilation, the most relevant Reynolds number range is between 3×104 and 6×105, depending on the duct dimensions and airflow rates. Pressure loss coefficients can increase considerably for some components at Reynolds numbers below 2×105. An initial survey of popular turbulence models was conducted for a selected test case of a bend with such a strong Reynolds number dependence. Most of the turbulence models failed in reproducing this dependence and predicted curve progressions that were too flat and only applicable for higher Reynolds numbers. Viscous effects near walls played an important role in the present simulations. In turbulence modelling, near-wall damping functions are used to account for this influence. A model that implements near-wall modelling is the lag elliptic blending k-ε model. This model gave reasonable predictions for pressure loss coefficients at lower Reynolds numbers. Another example is the low Reynolds number k-ε turbulence model of Wilcox (LRN). The modification uses damping functions and was initially developed for simulating profiles such as aircraft wings. It has not been widely used for internal flows such as air duct flows. Based on selected reference cases, the three closure coefficients of the LRN model were adapted in this work to simulate ventilation components. Improved predictions were obtained with new coefficients (LRNM model). This underlined that low Reynolds number effects are relevant in ventilation ductworks and give first insights for suitable turbulence models for this application. Both the lag elliptic blending model and the modified LRNM model predicted the pressure losses relatively well for the test case where the other tested models failed.
Highlights
1.1 Pressure loss coefficientsTechnical pressure loss data of ductwork components is needed for the design, the optimisation and the energy assessment of ventilation systems
Computational fluid dynamics (CFD) simulations were carried out using Siemens STAR-CCM+ version 15.04 (Siemens 2020)
The wall damping formulation of the LRNM model slightly influenced the prediction of λ in comparison to the SST model
Summary
Technical pressure loss data of ductwork components is needed for the design, the optimisation and the energy assessment of ventilation systems. Measurements of pressure loss coefficients can be time-consuming and costly, especially for large duct dimensions. Ventilation ductworks often are custom designs for which data is lacking (Shao and Riffat 1995). There were many attempts to substitute pressure loss measurements with computational fluid dynamics (CFD) simulations (Sleiti 2013). For this purpose, suitable turbulence models (see Section 1.3) are needed, which are the focus of this study. Pressure losses ΔP for straight ducts and components are usually expressed as non-dimensional pressure loss coefficients λ and ζ according to Eqs. Pressure losses ΔP for straight ducts and components are usually expressed as non-dimensional pressure loss coefficients λ and ζ according to Eqs. (1) and (2) (Idelchick 2008)
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