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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 ever-increasing 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.
In this book the author, David Etheridge, does not dwell too much on the political arguments. In Chapter 1, however, he sets out to make the case for natural ventilation as well as highlighting its limitations. He tells us that natural ventilation is an experience that has been gained over the centuries, that it requires no electrical energy to operate and that there is evidence that occupants of buildings prefer to have control over their environment rather than being completely isolated from it. The main disadvantage listed is one of providing for cooling, especially in hot, humid climates. However he stresses that this problem too can be overcome by using alternative low energy cooling systems.
David Etheridge also touches upon the subject of airtightness by pointing out that Sir Isaac Newton almost certainly relied on infiltrating air to enable safe combustion from the giant fire places situated at both ends of his home. Equally the lack of combustion air in the Antarctic mission of Scott, Shackleton and Byrd resulted in them almost succumbing to carbon monoxide poisoning. The point being made was that, in the past, adventitious openings had an important role to play in securing safety.
While the first chapter of this book gives an interesting and largely descriptive insight into natural ventilation, the main purpose of the book is to provide a detailed presentation of the governing equations and theory of wind and buoyancy driven airflow. The resultant equations are then studied in order to explain some of the phenomena of natural ventilation that are observed in practice.
Following from the introductory chapter, Chapter 2 commences with a presentation of the Navier-Stokes Equations for unsteady flow. These are then developed to take into account the characteristics of airflow in buildings including wind driven and buoyancy driven flow. Other fundamentals are also introduced including the concepts of laminar, transitional and turbulent flows. In addition, definitions relating to ventilation rate and various boundary conditions are presented.
Chapter 3 provides a comprehensive analysis of airflow through openings. Aspects cover opening type and size, the impact of wind and buoyancy forces, impinging flow direction and the relevant flow equations. The concept of cross-flow ventilation is also introduced.
Chapter 4 introduces 'conventional' steady flow models. These ignore external pressure fluctuations but form the backbone of current single-zone and multi-zone ventilation modelling techniques. Various solutions are analysed for wind driven and buoyancy (stack driven) configurations and the results explained. The relevance of this type of model to design is also emphasised, especially in relation to determining opening areas and configurations to achieve the required ventilation flow rates.
In practice, flow conditions are unsteady rather than steady. In other words airflow can be influenced by pressure and turbulent fluctuations. In the case of single-sided ventilation such fluctuations may be the prime driver of airflow while, in the case of multiple openings, fluctuations can precipitate a change in flow direction. The theory and consequences of unsteady flow are presented in Chapter 5. This particularly looks at the parameters that can influence flow reversal and provides guidance on designing to reduce risk.
Once air enters a space it will move and mix within the zone before eventually leaving. This indoor airflow pattern has important consequences for indoor air quality and thermal comfort. The theory and practice of indoor airflow is presented in Chapter 6. Various solution techniques are introduced.
Complementing indoor airflow behaviour is the impact of flow pattern on the dilution and removal of indoor pollutants. From the point of view of health, the distribution of air as well as the amount of fresh air provided is critical. The complex relationship between airflow and contaminant transport is described in Chapter 7. Again, modelling techniques are introduced and examples considered. From a practical aspect, basic natural ventilation designs based on occupant sources such as carbon dioxide concentration are considered. The time that air, once it has entered a space, remains in the space, and the characteristic way in which air propagates or mixes are described by the age of air and ventilation efficiency respectively. These concepts are described in Chapter 8. From the design point of view, optimising these parameters helps to secure the freshness of the air.
Increasingly, computational fluid dynamics (CFD) is being used to predict flow fields and associated ventilation parameters. This technique is mentioned throughout the book but is covered in detail in Chapter 9. Particular consideration is given to the choice of turbulence approximations and the comparison of CFD results with measurements. In relation to comparison tests, it is concluded that such tests have led to the improvement of CFD models. It is further concluded that, in the longer term, any computational benefits of current steady zonal ventilation models will probably be outweighed by the more rigorous approach offered by CFD.
Chapters 10 and 11 are devoted to measurement techniques. Often the wind regime is measured using wind tunnels while the monitoring of basic ventilation configurations is undertaken using scale wind tunnel or salt bath models. These techniques are covered in detail in Chapter 10. Full scale measurements are described in Chapter 11 and include air leakage testing by building pressurisation and measurement of ambient ventilation rate using tracer gas.
To complete the book, Chapter 12 is devoted to design procedures. Various generic design configurations are considered and the theoretical concepts outlined in the book are applied. Here the author concludes that CFD will increasingly be used in design and that simple dynamic thermal models should be used to understand the interaction of thermal mass. Also, the importance of a control strategy is stressed to accommodate the variability in natural driving forces.
In conclusion this is a substantially numerical book which is very much aimed at the student of building services engineering. However, in addition to the extensive use of equations, a good descriptive account of the theory is presented. Hence this book should also have wider appeal to anyone requiring a good background knowledge of the theory and application of natural ventilation.