Building Ventilation

The State of the Art

Mat Santamouris, Peter Wouters, Mat Santamouris, Peter Wouters

• Éditeur: Routledge
• Année: 2006
• ISBN:
  9781136570711
• Édition: 1
• Sujet: Architecture
• Sous-sujet: Architecture General

Ensuring optimum ventilation performance is a vital part of building design. Prepared by recognized experts from Europe and the US, and published in association with the International Energy Agency's Air Infiltration and Ventilation Centre (AIVC), this authoritative work provides organized, classified and evaluated information on advances in the key areas of building ventilation, relevant to all building types. Complexities in airflow behaviour, climatic influences, occupancy patterns and pollutant emission characteristics make selecting the most appropriate ventilation strategy especially difficult. Recognizing such complexities, the editors bring together expertise on each key issue. From components to computer tools, this book offers detailed coverage on design, analysis and performance, and is an important and comprehensive publication in this field.Building Ventilation will be an invaluable reference for professionals in the building services industry, architects, researchers (including postgraduate students) studying building service engineering and HVAC, and anyone with a role in energy-efficient building design.

1

Natural Ventilation in the Urban Environment

Francis Allard and Cristian Ghiaus

Introduction

The invention of boilers and chillers made total indoor climate control technologically possible regardless of outdoor conditions, building architecture and use. Buildings became, as Le Corbusier portended during the 1930s, of the same type: hermetically closed and controlled at a constant temperature in all climates (Mahdavi and Kumar, 1996). This energy-intensive solution is supported by the belief that maintaining constant conditions ensures comfort and satisfaction. By applying this approach, however, the building industry failed in many cases to satisfy the comfort needs of occupants. Different studies claim that as many as 43 per cent of occupants are dissatisfied with heating, ventilating and air conditioning (HVAC), and 56 to 89 per cent of government workers regard HVAC as a problem in Europe and the US (Lomonaco and Miller, 1996; DiLouie, 2002). The tendency for the HVAC industry is, then, to keep indoor temperature at a constant value that will dissatisfy the least number of people without affecting productivity since the cost of salaries is 8 to 13 times the cost of building operation.

The trend towards a greater control of the indoor environment is accentuated by the development of building management systems (BMS). BMS control the indoor temperature within narrow limits in the hope that occupants’ complaints will be reduced. To test this hypothesis, let us assume that a female wearing a skirt because of the corporate dress code or current fashion sits at her desk with bare legs. She shares the same thermal comfort zone with a male colleague in a business suit. A comfort evaluation with a predicted mean vote (PMV) index will show that there is no temperature that will satisfy both people and that the offset between preferred temperatures will be 3° Celsius (C). In the absence of individual temperature control, the compromise is to control the building at a constant value based on average clothing (clo) and metabolic rate (met). If clo = 0.7 (average winter/summer) and met = 1.2 (office activity), the temperature should be 24.1° C in order to obtain a PMV index of 0. This temperature will dissatisfy both the woman and the man from the previous example (Fountain et al, 1996).

Keeping the indoor temperature at a constant value has a high investment cost and is energy intensive, with implications for resource consumption and environmental impact. Important energy savings can be obtained if the building has a larger range in which it can run freely. This saving can be augmented if ventilation is used for cooling. In fact, field studies show that people accept a larger range of temperature variation in naturally ventilated buildings than in air-conditioned ones (de Dear et al, 1997; Brager and de Dear 1998, 2000).

The European project URBVENT: Natural Ventilation in Urban Areas studied the potential for energy savings when ventilation is used instead of air conditioning and the alteration of this potential by the urban environment. A synthesis of the main findings of this project is given in the following section. First, general aspects about modelling and strategies for natural ventilation in the urban climate are reviewed. Results of the URBVENT project are then presented. A method for estimating the energy savings for cooling by using ventilation is introduced. Although the potential savings may be important, they are affected by the influence of the urban environment through reduced wind velocity, urban heat island, noise and pollution. These changes, which were studied experimentally in the project, are synthesized along with their results in simple algorithms.

The Physics of natural ventilation

Eddy, turbulent and mean description of flow

Airflow is described mathematically through a set of differential equations for mass, momentum and energy conservation based on the solution of the transport equation:

where φ, Г_φ and S_φ (for the k-∊ model of turbulence) are given in Table 1.1, No general integral of these equations has yet been found and numerical solution for an arbitrary three-dimensional unsteady motion requires the use of a supercomputer.

Turbulent flow is one of the unsolved problems of classical physics. Despite more than 100 years of research, we still lack a complete understanding of turbulent flow. Nevertheless, the principal physical features of turbulent flow, especially with regard to engineering applications, are by now well determined.

Turbulent flow is distorted in patterns of great complexity containing both coarse and fine features. The flow is said to contain eddies, regions of swirling flow that, for a time, retain their identities as they drift with the flow, but which ultimately break up into smaller eddies. The velocity field of a turbulent flow can be regarded as the superposition of a large number of eddies of various sizes, the largest being limited by the transverse dimension of the flow and the smallest being those that are rapidly damped out by viscous forces. Mathematical analyses of steady laminar viscous flows show that infinitesimal disturbances to the flow can grow exponentially with time whenever the Reynolds number is sufficiently large. Under these conditions, the flow is unstable and cannot remain steady under practical circumstances because there are always some flow disturbances that may grow spontaneously. The most rapidly growing disturbances are those whose size is comparable to the transverse dimension of the flow. These disturbances grow to form the largest eddies, with a velocity amplitude of generally 10 per cent of the average flow speed. These large eddies are themselves unstable, breaking down into smaller eddies and being replaced by new large eddies that are continually being generated.

The generation and break-up of eddies provides a mechanism for converting the energy of the mean flow into the random energy of molecules by viscous dissipation in the smallest eddies. Compared to laminar flow, turbulent flow is like a short circuit in the flow field; it increases the rate at which energy is lost. As a result, turbulent flow produces higher drag forces and pressure losses than a laminar flow would under the same flow conditions.

When the unsteadiness of the flow is not an overwhelming feature, but rather a small disturbance of the average flow, the velocity field can be expressed as the sum of a mean val...