Per Heiselberg

An experimental investigation of a solar chimney model with uniform wall heat flux

Abstract Experiments were carried out using an experimental solar chimney model with uniform heat flux on one chimney wall with a variable chimney gap-to-height ratio between 1:15 and 2:5 and different heat flux and inclination angles. Results showed that a maximum airflow rate was achieved at an inclination angle around 45° for a 200 mm gap and 1.5 m high chimney, and the airflow rate is about 45% higher than that for a vertical chimney at otherwise identical conditions. It was found that the prediction method available in the literature can substantially overpredict the airflow rate for the chimney geometry investigated in this work, especially for vertical chimneys with large gaps. The main reason for the overprediction of airflow rate was shown due to the underestimation of the pressure losses at the chimney outlet by using loss coefficients obtained for normal forced flows.

Characteristics of Airflow from Open Windows

Abstract In natural ventilation systems fresh air is often provided through opening of windows. However, the knowledge of the performance of windows is rather limited, especially with regard to their impact on thermal comfort and draught risk in the occupied zone. This paper describes and summarizes the results of a series of laboratory measurements that is performed on two different window types to determine the characteristics of the air flow in rooms. It is shown that the air flow can be described by traditional theory for jets and stratified flow and semi-empirical flow element models are developed for estimation of thermal comfort parameters in the occupied zone.

The airborne transmission of infection between flats in high-rise residential buildings: Tracer gas simulation

Abstract Airborne transmission of infectious respiratory diseases in indoor environments has drawn our attention for decades, and this issue is revitalized with the outbreak of severe acute respiratory syndrome (SARS). One of the concerns is that there may be multiple transmission routes across households in high-rise residential buildings, one of which is the natural ventilative airflow through open windows between flats, caused by buoyancy effects. Our early on-site measurement using tracer gases confirmed qualitatively and quantitatively that the re-entry of the exhaust-polluted air from the window of the lower floor into the adjacent upper floor is a fact. This study presents the modeling of this cascade effect using computational fluid dynamics (CFD) technique. It is found that the presence of the pollutants generated in the lower floor is generally lower in the immediate upper floor by two orders of magnitude, but the risk of infection calculated by the Wells–Riley equation is only around one order of magnitude lower. It is found that, with single-side open-window conditions, wind blowing perpendicularly to the building may either reinforce or suppress the upward transport, depending on the wind speed. High-speed winds can restrain the convective transfer of heat and mass between flats, functioning like an air curtain. Despite the complexities of the air flow involved, it is clear that this transmission route should be taken into account in infection control.

Load matching and grid interaction of net zero energy buildings

Net Zero Energy Building” has become a prominent wording to describe the synergy of energy efficient building and renewable energy utilization to reach a balanced energy budget over a yearly cycle. Taking into account the energy exchange with a grid infrastructure overcomes the limitations of seasonal energy storage on-site. Even though the wording “Net Zero Energy Building” focuses on the annual energy balance, large differences may occur between solution sets in the amount of grid interaction needed to reach the goal. The paper reports on the analysis of example buildings concerning the load matching and grid interaction. Indices to describe both issues are proposed and foreseen as part of a harmonized definition framework. The work is part of subtask A of the IEA SHCP Task40/ECBCS Annex 52: “Towards Net Zero Energy Solar Buildings”.

Some examples of solution multiplicity in natural ventilation

Abstract This paper shows that under certain conditions, multiple solutions for the flow rate exist in a natural ventilation system, induced by the non-linear interaction between buoyancy and wind forces. Under certain physical simplifications, the system is governed in steady state by a non-linear algebraic equation or a system of equations. Three examples are given here: a single-zone building with two openings, a channel with two end openings, and a two-zone building with two openings in each zone. Analytical and numerical solutions are presented. It is shown that in all three cases the flow rate exhibits hysteresis. These results have significant implications for multi-zone modelling of natural ventilation and smoke spread in buildings. An experimental investigation using a small-scale model in a water tunnel confirms that two steady-state solutions exist for a single-zone building.

Analysis Methods for Natural and Hybrid Ventilation: A Critical Literature Review and Recent Developments

Abstract This paper is based on the results of Subtask B of the HybVent Project. It presents a critical review of the literature and recent developments in analysis methods for natural and hybrid ventilation in buildings. Ideally, analytical methods aimed at modelling hybrid ventilation systems should incorporate at least three key aspects i.e: the natural ventilation mode, the mechanical ventilation mode and the control strategy. Depending on the input parameters used within the control strategy, thermal comfort and indoor air quality related parameters may also need to be predicted. Available design methods for hybrid ventilation range from simple analytical and empirical methods through to multi-zone and computational fluid dynamics (CFD) techniques. When simulating the performance of hybrid ventilation throughout an entire year of operation, a fully integrated combined multi-zone and thermal model provides the most promising approach. In addition to detailing the available simulation techniques and their applications, this paper also reviews challenging issues such as the existence of multiple solutions to air flow analysis. It is suggested that more research and development is needed in the areas of multiple solutions and the uncontrolled divergence of transient multi-zone simulations.

Evaluation of the Indoor Environment of Comfort Houses: Qualitative and Quantitative Approaches

Currently, the built environment accounts for about 40% of the total energy consumed in the European Union and this level is continuing to increase. This high level of energy consumption is partially a result of attempts to create an optimum indoor environment for the occupants of buildings. New buildings and renovation projects on existing buildings need to improve their energy performance while maintaining a good and healthy indoor environment. One way of saving energy is to build passive houses. This paper presents the results of a case study of some of the first certified passive houses in Denmark, called Comfort Houses. The indoor environment is evaluated using both quantitative measurements of these houses and qualitative interviews with the occupants about their experiences of the indoor environment. The combination of these two sets of knowledge leads to a more complete and holistic picture of the indoor environment. It is shown that attention needs to be paid to the objectives in the design process, the documentation of the designs and on the occupant’s lifestyle and behaviour. Finally, it is important to educate the occupants on how to live in an appropriate manner to achieve a comfortable indoor environment.

Impact of Open Windows on Room Air Flow and Thermal Comfort

Abstract In the design of natural ventilation systems, there is a wide range of possibilities with regard to the selection of window type and the positioning of windows in the facade. Each window type has unique characteristics, which affect air flow and thermal comfort conditions in the occupied zone. A combination of various window types and façade locations in a room should be capable of improving thermal comfort and minimising draught risk. This paper describes the results of laboratory investigations in a mock-up of an office space with the purpose of investigating the impact of different opening strategies on thermal comfort conditions in the occupied zone. The results show that different window opening strategies result in quite different air flow and thermal comfort conditions. The conditions are a result of a multivariable impact, and thus detailed descriptions of the flows involved are quite complex.

Draught risk from cold vertical surfaces

Abstract Cold, natural convective flows from glazed facades are in winter time often the cause of thermal discomfort and there is a need for research to improve design methods. The objective of the research is to develop expressions for the air flow beyond the floor area, which influences the thermal comfort in the occupied zone. Expressions for the rate of decrement of the maximum velocity with distance from the surface and for the maximum temperature difference between the cold air flow along the floor and the rest of the occupied zone have been developed. The percentage of dissatisfied persons in the occupied zone, due to cold downdraught from a vertical surface, has been estimated.

Evaluation of Discharge Coefficients for Window Openings in Wind Driven Natural Ventilation

Abstract This paper describes the classical approach for calculation of wind driven airflow through large openings in buildings and discusses the fulfilment of the limiting assumptions. It is demonstrated that the limiting assumptions are not fulfilled for large openings in buildings for cross ventilation, and therefore, the classical approach is not appropriate for prediction of airflow through large openings in buildings in the cross ventilation case. Using the approach for real openings and estimating the discharge coefficient for window openings has also not been very successful. The discharge coefficient cannot be regarded as a constant and it is very difficult to estimate correct values resulting in less accuracy of prediction of natural ventilation.