Yingchun Ji

CFD Modelling of Natural Ventilation: Combined Wind and Buoyancy Forces

Abstract Results of a CFD simulation of the wind-assisted stack ventilation of a single-storey enclosure with high and low-level ventilation openings are presented and compared with both the laboratory measurements and the analytical model of the flow and thermal stratification developed by Hunt and Linden (2001). Comparisons show that close quantitative agreement is obtained between the thermal stratification predicted by the CFD and the analytical model and experimental measurements. A key consideration in the CFD modelling work is how to specify appropriate boundary conditions at the inlet and outlet locations of the enclosure. This paper investigates the use of constant pressure boundary conditions imposed over an opening whose physical area has been reduced to account for the effects of discharge and expansion. The close agreement with the analytical predictions and experimental results gives confidence in this approach and offers guidance on how to model wind-assisted stack ventilation flows using CFD.

Numerical studies of displacement natural ventilation in multi-storey buildings connected to an atrium

This paper describes computational fluid dynamics (CFD) simulations used to investigate displacement natural ventilation in simple multi-storey spaces connected to an atrium. The purpose of the work is to demonstrate the potential of CFD for modelling these airflows using solutions from simple mathematical models and salt bath experiments to provide an indication of the accuracy that can be attained. The storeys are connected to an atrium and air flows into them via top-down-chimneys. The driving force is provided by localised point heat sources on each floor which generate buoyant plumes that entrain the surrounding air and transport warm air upwards forming a warm, stratified layer in each storey. The mathematical models are used to describe the main flow features, such as stratification height, temperature gradient and ventilation flow rate. Results showed that, using the RNG k — ε turbulence model, the predicted airflow patterns, temperature profiles and ventilation flow rates agreed favourably with the mathematical models, demonstrating the potential of using CFD for modelling buoyancy-driven displacement ventilation in multi-storey spaces connected to an atrium. Practical applications: Computer simulation programs have become valuable tools in the building design process, particularly of innovative buildings. This paper looks at the ability of CFD techniques to model buoyancy-driven natural ventilation in simple multi-storey spaces. The methods used in this paper provide a basis for others to use CFD for predicting natural ventilation in more complex, realistic building structures and are useful for both building designers and CFD practitioners.

The role of diffusion on the interface thickness in a ventilated filling box.

We examine the role of diffusivity, whether molecular or turbulent, on the steady-state stratification in a ventilated filling box. The buoyancy-driven displacement ventilation model of Linden et al. (J. Fluid Mech., vol. 212, 1990, p. 309) predicts the formation of a two-layer stratification when a single plume is introduced into an enclosure with vents at the top and bottom. The model assumes that diffusion plays no role in the development of the ambient buoyancy stratification: diffusion is a slow process and the entrainment of ambient fluid into the plume from the diffuse interface will act to thin the interface resulting in a near discontinuity of density between the upper and lower layers. This prediction has been corroborated by small-scale salt bath experiments; however, full-scale measurements in ventilated rooms and complementary numerical simulations suggest an interface that is not sharp but rather smeared out over a finite thickness. For a given plume buoyancy flux, as the cross-sectional area of the enclosure increases the volume of fluid that must be entrained by the plume to maintain a sharp interface also increases. Therefore the balance between the diffusive thickening of the interface and plume-driven thinning favours a thicker interface. Conversely, the interface thickness decreases with increasing source buoyancy flux, although the dependence is relatively weak. Our analysis presents two models for predicting the interface thickness as a function of the enclosure height, base area, composite vent area, plume buoyancy flux and buoyancy diffusivity. Model results are compared with interface thickness measurements based on previously reported data. Positive qualitative and quantitative agreement is observed.

Nodal network and CFD simulation of airflow and heat transfer in double skin facades with blinds

The paper describes a modelling study of heat transfer and buoyancy-driven airflow in double skin facades consisting of a glass outer layer, a control device (venetian blind) and a double-glazed inner skin. The modelling study was based on two approaches — a component-based, lumped parameter simulation which used a public domain, open source differential/algebraic equation solver and a detailed, CFD calculation which included air flow, conduction, convection and radiation. The primary objective of the work was to compare the performance of the simplified model with the output of a rigorous CFD calculation.

CFD modelling of naturally ventilated double-skin facades with venetian blinds.

This study describes computational fluid dynamics (CFD) modelling of naturally ventilated double-skin facades (DSFs) with Venetian blinds inside the facade cavity. The 2D modelling work investigates the coupled convective, conductive and radiative heat transfer through the DSF system. The angles of the Venetian blind can be adjusted and a series of angles (0°, 30°, 45°, 60° and 80°) have been modelled. The modelling results are compared with the measurements from a section of a prototype-facade testing facility and with predictions from a component-based nodal model. Agreement between the three methods is generally good. It is thought that discrepancies in the results are caused by the simplification of the CFD model resulting in less turbulence mixing within the facade cavity. The CFD simulation output suggests that the presence of the Venetian blinds is able to enhance the natural ventilation flow within the facade cavity and significantly reduce the heat gains to the internal environment. It was also found that the convective heat transfer coefficients on the glazing surfaces are insensitive to the blind angles. The work demonstrated the capability of CFD for modelling complicated heat transfer processes through the DSF system and offered some guidance for CFD practitioners who wish to model similar type of flow.

Design summer year weather – outdoor warmth ranking metrics and their numerical verification

The existing methods of selecting design summer year weather data rely on the outdoor dry bulb temperature without considering solar radiation and wind which can impact on indoor thermal conditions. This research sets out to examine the existing outdoor warmth ranking metrics and proposes a new warmth ranking metric (solar air temperature), which takes into account not only dry bulb temperature but also solar and wind conditions. Parametric study was carried out using five typical UK dwelling models, by varying parameters associated with building design and operation, a large model population were generated to statistically determine how well the outdoor warmth ranking metrics correlate the predicted indoor warmth. The outdoor warmth ranking was made for the 20 years source weather data (1976–1995) in London and both CIBSE single temperature criterion and BS EN 15251 adaptive criteria were used to judge overheating in buildings. It is found that the predicted indoor warmth are mostly arbitrary in nature and none of the existing and newly proposed outdoor warmth ranking metrics can strictly correlate. The research also discovers the significant differences between the predicted overheating occurrence and severity in the warmth ranking of weather years. Practical application : The parametric methods described in the paper will facilitate academics and industry researchers to assess design summer year weather generated by various methods. The research also provides guidance on assessing both overheating occurrence and severity in buildings to assist decision making.