International Journal of Ventilation ISSN 1473-3315 Volume 6 No 3 ________________________________________________________________________________________________________________________
Book Review Computational Fluid Dynamics in Ventilation Design REHVA Guidebook No 10 Peter V Nielsen (editor), Francis Allard, Hazim B Aw bi, Lars Davidson and Alois Schälin
Publisher REHVA: Federation of European Heating and Air-Conditioning Associations, ISBN 2-9600468-9-7
Computational Fluid Dynamics in Ventilation Design is a new title in the REHVA guidebook series. This guidebook is written for people who need to use and discuss results based on CFD predictions, and it gives insight into the subject for those who are not used to working with CFD. It is also written for users working with CFD who have to be more aware of how this numerical method is applied in the area of ventilation. The guidebook has, for example, chapters that are very important for CFD quality control in general and for the quality control of ventilation related problems in particular.
CFD and also with measurements in rooms for validating the predictions. The turbulence model is a particularly important aspect of CFD. It is obvious that room airflow will be turbulent because of geometry and practical velocity levels, but it will not always be a fully developed turbulent flow. Some of the widely used models are discussed such as the k-ε model, the SST model and the Reynolds Stress model. The Large Eddy simulation is also briefly mentioned. The chapter on Numerical method illustrates the structure of a CFD program and it demonstrates much of the experience a user will have in using a commercial program. Most of this chapter is based on one-dimensional theory. The use of one-dimensional analysis makes it possible to understand, by hand calculation, many concepts and issues such as order of accuracy, necessary number of grid points, oscillations in the solution, iterations, divergence, etc. This is demonstrated with a convection-diffusion equation which is solved with the use of different schemes at different velocity levels.
A large number of CFD predictions are made nowadays and it is often difficult to judge the quality level of these predictions. The guidebook introduces rules for good quality prediction work. It is the purpose of the guidebook to improve the technical level of CFD work in ventilation. The book contains the following main chapters: • • • • • • • • •
Mathematical background Turbulence models Numerical methods Boundary conditions Quality control CFD combined with other prediction models Application of CFD codes in building design Case studies Benchmark tests
The chapters Mathematical background and Turbulence models give a short introduction to the theory behind the methods used in CFD modelling. The fundamental transport equations are discussed with emphasis on ventilation applications. Descriptions are given for two-dimensional geometry to simplify the concepts. A user of CFD predictions must have some knowledge of fluid mechanics. It is important to understand conditions such as laminar flow, turbulent flow, steady flow, time dependent flow etc. both in connection with
The chapter on Boundary conditions is especially important in the ventilation area. Often the flow in a room is determined by small details in the diffuser design. This means that a numerical prediction method should be able to handle small details in dimensions of one or two millimetres, as well as dimensions of several metres. This wide range of the geometry necessitates a large number of cells in the numerical scheme, which increases the prediction cost and computing time to a rather high level. The problem is overcome by applying different simplifications such as simplified boundary conditions, box method, prescribed velocity method or momentum method. Continuous development of computational capacity and speed will undoubtedly make the direct methods with local grid refinements or multigrid solution possible. This is illustrated in Figure 1 in which a diffuser consists of 12 small slots which can be adjusted to different flow directions.
________________________________________________________________________________________________________________________
291
________________________________________________________________________________________________________________________
Figure 1. A direct simulation of a diffuser consisting of 12 small slots which can be adjusted to different flow directions. Development of computer capacity will make direct simulations of diffusers possible in the future.
This chapter also discusses other boundary conditions such as the surface boundary which is important for heat transfer predictions, free boundary, plane of symmetry, air exit opening and obstacle boundary. The chapter on Quality Control is one of the more important chapters in this guidebook. Quality control consists of four major steps: to recognize possible error sources, to check for them in the simulation, to estimate the accuracy of the simulations, and to improve the simulation whenever possible. Two of the many examples given in the guidebook are shown here.
There is a strong reduction in computing cost by working with two-dimensional flow instead of three-dimensional flow but is it possible in all situations where the boundary conditions are “twodimensional”? Figure 2 shows a long hall with a shed roof. There are complaints about strong downdraught, but a two-dimensional prediction is not able to show this effect. It is necessary to use a three-dimensional transient approach to predict the downdraught. For each set of problems a grid independence study should be performed. Figure 3 shows an example of such a study, where the same case (a transient fire
Figure 2. Hall with shed roof, two-dimensional prediction and transient three-dimensional prediction. ________________________________________________________________________________________________________________________
292
International Journal of Ventilation ISSN 1473-3315 Volume 6 No 3 ________________________________________________________________________________________________________________________
Figure 3. Simulation of a transient fire with 50,000 and 200,000 cells in the solution domain.
simulation) is run for various grids from coarse to fine, and with homogeneous grids and other grids with mixtures of prisms, tetra- and hexahedral cells. The latter performs best. Figure 3 shows the temperature distribution for 50,000 cells and 20,000 cells. The use of a CFD program in connection with other programs is discussed in the chapter CFD combined with other prediction models. Of highest interest and importance to ventilation design are the following: coupling of airflow and multi-zone dynamic thermal simulation where, especially, energy storage is an important issue; coupling of airflow, moisture and energy transport through walls; coupling of airflow and multi-zone flow simulation, where the zonal flow simulation also handles the transport of additional components such as contaminants (e.g. smoke, CO2, odours, moisture); and, lastly, coupling of the airflow and emission from building materials. The possibilities of applying CFD for simulating airflow in a building are discussed in the chapter
Application of CFD codes in building design. A number of areas such as: room air movement, concentration distribution, emission from materials, thermal comfort assessment, ventilation effectiveness prediction and smoke management can be evaluated by a CFD program from the conceptual design to the preliminary design and right through to the final detail design stages. The chapter, Case studies, shows different practical applications of predictions made at different stages in the initial design, detail design and commissioning phases. In particular, four different air distribution systems are studied by CFD and compared to measurements. The four systems are mixing ventilation with a wall-mounted diffuser, vertical ventilation, displacement ventilation, and mixing ventilation generated by a ceiling mounted radial diffuser. All systems are designed to handle the same load in the same room. Figure 4 shows the results for vertical ventilation. The final chapter is a small chapter discussing different types of benchmark tests. A benchmark test can be used for new beginners in CFD to obtain a fast insight into different problems for the prediction of ventilation and to obtain an initial experience by comparing CFD outputs.
REHVA - the Federation of European Heating and Air-Conditioning Associations represents more than 110000 building engineers in 29 European countries. REHVA Guidebooks are appreciated for their solid scientific quality and are also easily understood by practitioners. All REHVA Guidebooks can be ordered online on www.rehva.eu Figure 4. Velocity distribution in the centre plane of an office room ventilated by a ceiling mounted textile terminal giving vertical ventilation.
Peter V. Nielsen
________________________________________________________________________________________________________________________
293
________________________________________________________________________________________________________________________
International Journal of Ventilation - Notes to Authors The International Journal of Ventilation aims to accept and publish papers of the highest quality. Papers submitted to the journal should fulfil one or more of the following categories : •
•
•
• • •
•
•
•
Introduce a novel idea concerned with the development or application of ventilation; Present a validated case study demonstrating the performance of a ventilation strategy; Studies on ventilation needs and solutions for specific building types including: offices, dwellings, schools, hospitals, parking garages, urban buildings and recreational buildings; Numerical methods for application in design; Practical measurement techniques; Related issues in which the impact of ventilation plays an important role (e.g. the interaction of ventilation with indoor air quality, health and comfort); Energy issues related to ventilation (e.g. low energy systems, ventilation heating and cooling loss); Driving forces (weather data, fan performance etc).
•
•
•
•
Submission Details
Papers should not exceed 4MB in size and should be submitted, in electronic form (e.g. MSWord, WordPerfect, Star/Open Office or ‘txt’), by email, to:
[email protected]. It is essential that papers comply with the preparation instructions presented below.
Preparation Instructions
Conclusions: This should summarise the essential results and benefits of the work. This Section should be in the region of 200 to 300 words in length. References: Within the body of the text references should be made by first author (et al) and date, e.g. (Waters et al 1999). In the reference section, references should be presented in alphabetical order in the following format: Waters JR, Simons MW and Grazebrook J. (1999) “Air distribution and air quality in a large open space”, Building Serv. Eng Res Technol. 20 pp195 – 200. Acknowledgements (Optional): To be included, necessary, to acknowledge funders and co-workers etc.
as
Appendix (Optional): An appendix may be included, especially to outline a complex numerical procedure that may be of value to researchers but would upset the flow of the paper. Figures: These should be referenced in the text as Figure 1, Figure 2 etc; Figures must be presented, in the size in which they are to be published , in electronic format. For each paper, there is a maximum of 5 pages available for figures (A4 or Letter size). Authors may submit figures in 1 page, half page and quarter page format subject to this 5-page quota. The submitted figures must be legible (line thickness, style etc) and the smallest print font, at final size, should not be less than 8pt. Colour should be avoided. Colour graphics must translate into unique grey scale.
Size of Paper •
A maximum of 12 A4 or Letter format pages should be submitted based on: • •
•
11pt Times Roman font (approx 4500 – 6000 words); Figures submitted in final print size (see layout of submitted papers); Tables submitted in final print size and fully formatted (see layout of submitted papers).
Papers submitted in ‘txt’ format must also be sent as a hard copy.
Review Process An aim of the journal is to publish papers as rapidly as possible. To achieve this, papers will undergo a two-stage review, i.e:
Exceptionally, longer papers may be accepted subject to content and space availability. Layout of Submitted Papers
Flexibility in presentation is permitted subject to the following: •
Numbered headings with a maximum of 2 levels e.g.
Pre Review: This process is aimed at filtering papers that do not fit within the specified format. It is essential that papers be submitted to the correct format (especially in relation to overall size and legibility of figures and tables). Authors of papers rejected at this stage will be encouraged to resubmit. Peer Review: After screening, papers will be sent for peer review. This review will be based on technical content, accuracy and appropriateness for the journal. This process will sort papers according to:
1. …….. 1.1 ……… •
•
Tables: These must be submitted, fully formatted, in the size in which they are to be published.
Abstract: This should not exceed 150 words and should explain both the objectives of the work and outline the results and conclusions. This section should end with a selection of key words.
• • •
Introduction: This should introduce and explain the concept of the work. It should also explain the structure of the paper and outline its uniqueness;
Accepted with none or only minor revision; Accepted subject to substantial revision; Rejected (major errors, duplicated work or does not fit the ethos of the journal).
Fast Tracking: Papers deemed by reviewers to be of high quality and requiring no more than minor revision will be fast-tracked into publication.
Author Copyright and Declaration: Please visit www.ijovent.org.uk/AuthorCopyright for copyright and declaration details. ________________________________________________________________________________________________________________________
294
