Pollutant dispersion in a large indoor space. Part 2: Computational fluid dynamics predictions and comparison with a scale model experiment for isothermal flow

This paper reports on an investigation of the adequacy of computational fluid dynamics (CFD), using a standard Reynolds Averaged Navier-Stokes (RANS) model, for predicting dispersion of neutrally buoyant gas in a large indoor space. We used CFD to predict pollutant (dye) concentration distribution in a water-filled scale model of an atrium with a continuous pollutant source in the absence of furniture and occupants. Predictions from the RANS formulation are comparable with an ensemble average of independent identical experiments. Model results were compared with pollutant concentration data in a horizontal plane from experiments in a scale model atrium. Predictions were made for steady-state (fully developed) and transient (developing) pollutant concentrations. Agreement between CFD predictions and ensemble averaged experimental measurements is quantified using the ratios of CFD-predicted and experimentally measured dye concentration at a large number of points in the measurement plane. Agreement is considered good if these ratios fall between 0.5 and 2.0 at all points in the plane. The standard k-epsilon two-equation turbulence model obtains this level of agreement and predicts pollutant arrival time to the measurement plane within a few seconds. These results suggest that this modeling approach is adequate for predicting isothermal pollutant transport in a large room with simple geometry. PRACTICAL IMPLICATIONS: CFD modeling of pollutant transport is becoming increasingly common but high quality comparisons between CFD and experiment remain rare. Our results provide such a comparison. We demonstrate that the standard k-epsilon model provides good predictions for both transient and fully developed pollutant concentrations for an isothermal large space where furnishings are unimportant. This model is less computationally intensive than a large eddy simulation or low Reynolds number k-epsilon model.     

On the suitability of steady RANS CFD for forced mixing ventilation at transitional slot Reynolds numbers

Accurate prediction of ventilation flow is of primary importance for designing a healthy, comfortable, and energy‐efficient indoor environment. Since the 1970s, the use of computational fluid dynamics (CFD) has increased tremendously, and nowadays, it is one of the primary methods to assess ventilation flow in buildings. The most commonly used numerical approach consists of solving the steady Reynolds‐averaged Navier–Stokes (RANS) equations with a turbulence model to provide closure. This article presents a detailed validation study of steady RANS for isothermal forced mixing ventilation of a cubical enclosure driven by a transitional wall jet. The validation is performed using particle image velocimetry (PIV) measurements for slot Reynolds numbers of 1000 and 2500. Results obtained with the renormalization group (RNG) k‐ε model, a low‐Reynolds k‐ε model, the shear stress transport (SST) k‐ω model, and a Reynolds stress model (RSM) are compared with detailed experimental data. In general, the RNG k‐ε model shows the weakest performance, whereas the low‐Re k‐ε model shows the best agreement with the measurements. In addition, the influence of the turbulence model on the predicted air exchange efficiency in the cubical enclosure is analyzed, indicating differences up to 44% for this particular case.     

On the comparison of the ventilation performance of street canyons of different aspect ratios and Richardson number

In this paper, the ventilation performances of (1) isothermal street canyons of different building-height-to-street-width (aspect) ratios (h/b) and (2) a ground-heated street canyon of h/b=1 at different Richardson numbers (Ri) are examined numerically by solving the Reynolds-averaged Navier-Stokes (RANS) equations with the use of the Renormalization Group (RNG) k-ɛ turbulence model. The mean () and turbulent (ACH’) air exchange rates (ACH) are calculated by the eddy-viscosity model instead of the turbulence kinetic energy (TKE) used elsewhere. For the isothermal street canyons, the ACH’ is found to account for 90% of the total ACH for 0.5 ≤ h/b ≤ 2. Similar to the previous large-eddy simulation (LES) and k-ɛ turbulence model, the magnitudes and shapes of the roof-level profiles of mean and fluctuating vertical winds are close to each other for different h/b. This suggests that turbulent mixing is important for the ventilation of isothermal street canyons. For the ground-heated street canyon, both the mean wind and turbulence are strengthened as illustrated by the increasing and ACH’ with decreasing Ri. A secondary recirculation is developed at the ground-level windward corner that pushes the primary recirculation upward and enhances and ACH as well.