Cross Ventilation in a Building

The design team was requested to design three mid-rise buildings for a residential building development in Shanghai (Figure 11).

Since wind around the buildings is the driving force in cross ventilation, this investigation involves the simulation of indoor and outdoor airflow by CFD. In order to study the impact of surrounding buildings, the computational domain for outdoor airflow should be sufficiently large (e.g., an area of tens of thousands to a million square meters). Due to the limitation in current computer capacity and speed, the grid size used cannot be very small (it can be a few meters). On the other hand, the grid size for indoor airflow simulation should be small enough (in terms of a few centimeters) for one to see the details. Therefore, the indoor and outdoor airflow should be separately simulated. For natural ventilation design, the outdoor airflow simulation can provide flow information as boundary conditions for the indoor airflow simulation. Zhai et al (2000) have discussed a few methods to provide the flow information.

For simplicity, this investigation used a CFD program to calculate the pressure difference around the buildings and uses it as the boundary conditions for indoor airflow simulation. Ideally, the calculation should be performed for different wind directions under various wind speeds in a period suitable for natural ventilation, such as summer. Figure 12 illustrates the pressure distribution under the prevailing wind direction (southeast) and speed (3 m/s). In order to correctly take the impact of the surrounding buildings into account, the computational domain is much larger than the one shown in the figure. Clearly, the pressure difference is the highest between the northern and southern façades. It is also interesting to note that the highest pressure difference is neither at the top floor nor at the bottom floor, but somewhere near the top, as shown in Figure 12b.

Figure 11 Architectural elevation and plans of final design for Shanghai Taidong Residential Quarter

Figure 12 Pressure distribution around the buildings proposed (shown circled) for Shanghai Taidong Residential Quarter, due to prevailing winds, from the southeast at 3 m/s (dark gray - high pressure, light gray - low pressure): (a) is the plan (north is up) and (b) is the section looking west (see color version of Figure 12b in chapter 12, Figure 34)

Figure 12 Pressure distribution around the buildings proposed (shown circled) for Shanghai Taidong Residential Quarter, due to prevailing winds, from the southeast at 3 m/s (dark gray - high pressure, light gray - low pressure): (a) is the plan (north is up) and (b) is the section looking west (see color version of Figure 12b in chapter 12, Figure 34)

Figure 13 Building floor plan - unit G (the unit farthest to the right) was analyzed for interior CFD studies

By working together with the architects, the design team evaluated the ventilation performance for the buildings. Unit G in the middle building was used (Figure 13) as an example to illustrate the evaluation of cross-ventilation design.

With the unit layout in Figure 13, a CFD model can be established, as shown in Figure 14a. With the pressure distribution from Figure 12, the CFD program can calculate the distributions of airflow, air temperature, relative humidity, predicted percentage dissatisfied (PPD), and the mean age of air, as shown in Figure 14. CFD uses the humidity ratio and air temperature to determine the relative humidity. The PPD is determined by using the air velocity, temperature, humidity ratio, and environmental temperature. The results shown in Figure 14 are with an outside air temperature of 24°C and a relative humidity of 70 percent.

The computed results by CFD indicate that the maximum air velocity in the unit is less than one meter per second - a comfortable value for cross ventilation. The air exchange rate varies from 16 ACH on the first floor to a maximum of 40 air changes per hour (ACH) two-thirds up the height of the building. With the air exchange rate of 16 ACH, the indoor air temperature increases less than 1°K, although there are heat sources in the unit. The relative humidity is around 65 to 70 percent, a value close to that of the outdoors. Since the air exchange rate is high, the mean age of air is less than 120 seconds. Therefore, the air quality would be very good when outdoor air quality is high.

Since the air exchange rate is a very important parameter in cross ventilation design, this investigation indicates the design to be very successful. However, the wind is not always at the prevailing speed and direction, and the outdoor air temperature varies over time. A more complete evaluation of the design should be combined with an energy analysis of the building, as described in chapter 5. Carrilho da Graga et al (2002) have shown how to combine the information from flow and energy analysis for such a building. The paper also emphasizes the importance in using different control strategies. For example, in Shanghai it is more appropriate to use night cooling and minimum daytime ventilation to achieve an indoor air temperature lower than that of the outdoors. This is superior to ventilating buildings twenty-four hours a day. See Chapter 12, Case Study Three- Shanghai Taidong Residential Quarter for more information.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

Get My Free Ebook


Post a comment