David Etheridge

Natural Ventilation of Buildings Theory Measurement and Design

Among policy makers in many countries there is seemingly an almost unstoppable demand to require homes, schools and offices to be hermetically sealed and mechanically ventilated. The reasoning is one of control. As the thermal insulation properties of buildings improve, ventilation accounts for an everincreasing proportion of the total thermal energy loss. Current thinking suggests that, by incorporating mechanical systems, a substantial amount of ventilation losses can be recovered through heat recovery. It is a very interesting time, therefore, to publish a textbook on natural ventilation. This highlights, perhaps, that for many climates, a significant body of experts are questioning current convention. Indeed it is interesting to note that the introduction to this book states that “Natural ventilation is increasingly considered a prerequisite for sustainable buildings.” In essence this statement emphasises the apparently huge chasm that exists between opposing views towards the meaning and implementation of sustainability.

Unsteady flow effects due to fluctuating wind pressures in natural ventilation design—instantaneous flow rates

Abstract The paper presents the results of a study into the effects of unsteady wind pressures on the mean flow rates in certain types of purpose-designed naturally ventilated buildings. The study used nondimensional parameters and the results should therefore cover a wide range of conditions and should be of general application. It is concluded that unsteady effects are restricted to a relatively narrow band of conditions. These conditions have been quantified in terms of nondimensional parameters. In particular a simple procedure for calculating the mean flow rates when the unsteady effects are large has been derived. In its simplest form the procedure does not require knowledge of the instantaneous wind pressures.

Nondimensional methods for natural ventilation design

The paper deals with the use of nondimensional graphs for designing the envelopes of naturally ventilated buildings. It is argued that such graphs offer several benefits over conventional procedures i.e. generality, ease and speed of use, wide application and accuracy. The graphs can be generated from theoretical models or from experimental data obtained from a direct measurement of ventilation rates in a wind tunnel model. Examples of graphs are given which cover conventional design conditions and, perhaps more importantly, off-design conditions. The off-design conditions include effects which would normally require a great deal of effort to calculate for a particular building e.g. the effects of adventitious leakage and wind turbulence. With the graphs described a designer can obtain a quick indication of whether such effects are important.

A Comparison of CFD and Full-scale Measurements for Analysis of Natural Ventilation

Abstract CFD modelling techniques have been used to simulate the coupled external and internal flow in a cubic building with two dominant openings. CFD predictions of the time-averaged cross ventilation flow rates have been validated against full-scale experimental data under various weather conditions in England. RANS model predictions proved reliable when wind directions were near normal to the vent openings. However, when the fluctuating ventilation rate exceeded the mean flow, RANS models were incapable of predicting the total ventilation rate. Improved results are expected by applying more sophisticated turbulence models, such as LES or weighted quasi-steady approximations.

Natural Ventilation through Large Openings - Measurements at Model Scale and Envelope Flow Theory

Abstract An overview is given of the current position regarding the use of wind tunnel modelling and envelope flow theory for determining natural ventilation through large openings. The overview is, to a large extent, a personal one and is illustrated primarily by recent research carried out in Nottingham, some of which has yet to be published in full. Attention is focused on the following uses of wind tunnels:- direct measurement of time-average ventilation rate; measurement of instantaneous flows in stacks; determination of discharge coefficients for theoretical envelope flow models; assessment of steady and unsteady envelope flow models. These are believed to be the more important areas for research and are reflected in the recent work carried out by the author and his colleagues.

Direct wind tunnel modelling of natural ventilation for design purposes

The paper describes an investigation into the feasibility of natural ventilation design by means of direct wind tunnel modelling of ventilation rates. The investigation covers the three cases encountered in practice, i.e. wind alone, buoyancy alone and wind and buoyancy combined. The direct determination of ventilation rates from a wind tunnel model is potentially a very powerful tool for natural ventilation design of nondomestic buildings, where the design aim is to specify the size and position of purpose-provided openings. It offers more accuracy in the determination of wind effects than either the indirect use of wind tunnels (where pressure coefficients are measured for use in a mathematical model) or the salt-bath technique. From an analysis of experimental and theoretical results it is concluded that the direct technique is feasible for a wide range of building sizes. Some desirable improvements to the technique have also been identified. Further work is also required to clarify why the observed values of discharge coefficients are larger than might be expected.

Leakage measurements using unsteady techniques with particular reference to large buildings

The paper describes investigations of unsteady techniques for measuring leakage of buildings. The main investigation is theoretical. The theory used accounts for compressibility and inertia and is based on a quadratic flow equation. It is therefore an advance over what has been used in the few previous investigations. In particular, it allows uncertainties due to inertia to be assessed. The investigations cover three unsteady techniques i.e., the AC and the pulse techniques and a novel DC/AC technique. It is concluded that for most buildings, the conventional steady DC technique is preferable. However, for large buildings, the DC technique becomes increasingly inaccurate and an alternative technique is required. A novel form of pulse technique is proposed and theoretical and experimental investigations are presented which indicate that it has the potential to provide a simple and quick way of measuring the leakage of large buildings.

A NOTE ON CRACK FLOW EQUATIONS FOR VENTILATION MODELLING

Abstract A recent theoretical justification for the power law flow equation is considered. It is shown that this justification also indicates that the quadratic flow equation is preferable to the power law.

Determining the adventitious leakage of buildings at low pressure. Part 2: pulse technique

Part 1 of this paper examined the uncertainties (errors) inherent in the determination of the low-pressure leakage of a building envelope. It was shown that a technique for direct measurement of Q4 could reduce the uncertainty by a factor of three or more. In Part 2, a technique is described that enables such measurements to be made. There are three key features that lead to a successful technique. The first is to measure Dp a short time before and after the pulse, thereby allowing wind effects to be largely eliminated. The second is to choose the shape and duration of the pulse, such that a period of quasi-steady flow is obtained. The third is to minimise the variation of Dp during the quasi-steady period, such that envelope flexing is not a problem. Examples of measurements are presented that demonstrate these key features. Further evidence of the suitability of the technique is provided by demonstrating its ability to determine a known change of opening. Comparisons are also made between the new technique, the conventional steady technique, and a theoretical model. It is concluded that the new technique allows the low-pressure leakage of a building to be determined to an accuracy that has not previously been possible.

Location of air intakes to avoid contamination of indoor air: a wind tunnel investigation

Abstract The location of air intakes is of prime importance in buildings that are situated in close proximity to busy urban roads. If intakes are placed where the concentration of traffic pollution is high then indoor air concentrations can reach similarly high levels. This paper presents the findings from a wind tunnel investigation into the dispersion of a simulated traffic pollutant in a 1:100 scale model. The concentrations at different points on a building in the model are measured and a comparison with full-scale data is made.