CFD in buildings

[1] In the past few years, CFD has been playing an increasingly important role in building design, following its continuing development for over a quarter of a century.

The information provided by CFD can be used to analyse the impact of building exhausts to the environment, to predict smoke and fire risks in buildings, to quantify indoor environment quality, and to design natural ventilation systems.

Recently CFD finds very wide application in different areas of science and engineering; some examples are:[2] Previously, most building-related issues such as ventilation analysis, wind loading, wind environment etc.

CFD can resolve all of the above-mentioned issues in a relatively short time period, and it is more economical as well as being a stronger approach than the older one (experimental).

CFD can examine the effectiveness and efficiency of various heating ventilation and air conditioning (HVAC) systems by easily changing the type and location of different components, supply air conditions and system control schedules.

Furthermore, CFD helps in developing passive heating/cooling/ventilation strategies (e.g. natural ventilation) by modelling and optimizing building site-plans and indoor layouts.

[4] Globally, the building sector is the source of approximately 40% of total energy consumption.

In order to reduce heat losses from buildings, CFD analysis can be done for the optimum configuration of composite walls, roof and floor.

In buildings, for heat transfer analysis, the scalar function ф in equation (1) is replaced by Temperature (T), diffusion coefficient Γ is replaced by thermal conductivity k and the source term

In buildings the heat transfer analysis can be done for all parts of buildings (walls, roof and floor) in following two ways The steady state thermal analysis consist the following type of governing differential equations.

Case-1: Transient heat conduction For this case the governing differential equation (GDE) (1) becomes as follows: Case- 2: Transient heat conduction (no heat generation) For this case the governing differential equation (GDE) (1) becomes as follows: Case-3: Transient heat conduction (no heat generation and no convection) For this case the governing differential equation (GDE) (1) becomes as follows: We can solve these above mentioned governing differential equation (GDE) equations using CFD technique.

CFD finds an important role in regulating the indoor air parameters to predict the ventilation performance in buildings.

[8] These air parameters are crucial for designing a comfortable indoor as well as a good integration of the building in the outdoor environment.

[9] Recently ventilation and its related fields has becomes a great part of wind engineering.

A ventilation study can be done using wind tunnel investigation (experimentally) or by CFD modeling (theoretically).

In present era, due to development of a lot of CFD software and other building performance simulation software, it has become easier to assess the possibility of natural/forced ventilation system in a building.

The data obtained either experimental or numerically is useful in two ways:[10] Earlier, the choice of dwelling location was dependent on the need for water, so most developments started in valley areas.

In our present era, due to advancements in science and technology, it has become easier to select the building orientation, site and location based on local geographical and environmental conditions.

In the selection of building site and location, wind loading plays an important role.

Architects and wind engineers are often asked to look over the design (orientation, site, location and gaps between the surrounding buildings) in the formative planning stage of construction.

[10] By using CFD analysis, it is possible to find the suitable information (local wind velocity, convective coefficients, and solar radiation intensity) for optimal orientation, site and location selection of buildings.

CFD technique finds the solution by following ways: Consider a building having a plane wall with thickness L, heat generation e and constant thermal conductivity k. The wall is subdivided into M equal regions of thickness

The FDM technique presumes that temperature varies linearly in walls (shown in figure-3).

To obtain the solution for exterior nodes we have to apply the boundary conditions (as applicable), which are as follows.

or when radiation and convection heat transfer coefficient are combined, above equation becomes as follows; 5.

Note: For interior side of wall we can apply the suitable boundary condition from above (as applicable), in that case

The finite difference solution of transient heat conduction requires discretization in time in addition to space, as shown in figure-6.

The nodal points and volume elements for the transient FDM formulation of 1-D conduction in a plane wall exist as shown in the figure-7.

) is available and absorptivity-transmissivity constant K is known, the relation for temperature is obtained as follows; Note: the thermal analysis for the roof and floor of a building can be done in same way, as discussed for walls.\\

Figure-1 (a): Flow around a building (collection of air at height and delivering it at ground level)
Figure-1 (b): Flow around a building (center of the front face )
Figure-2:the nodal points and volume elements for the finite difference formulation of 1-D conduction in a plane wall
Figure-3:Linear temperature variation in finite difference formulation
Figure-4: Schematic for the FDM formulation of combined convective and radiation on the left boundary of a plane wall
Figure-5: Schematic for the FDM of the interface boundary condition for two mediums A and B having perfect thermal contact
Figure-6: FDM fotirmulation of time dependent problem involves discrete points in time as well as in space
Figure-7:The nodal points and volume elements for the transient FDM formulation of 1-D conduction in a plane wall