*3.2.8 Solution method*

*3.2.4 The physical model*

*Computational domain for analysis.*

**Figure 6.**

**Table 3.** *Grid analysis.*

**86**

the windward walls.

This is the geometry of the area for the simulation. It constitutes the site plan and

the room. The cross-ventilated building model used in this model is showed in **Figure 6**. The computational model was sized 17.4 m 9 m 3 m (length width height). The building height served as the reference length scale H. Two openings with dimension 2.4 m 1.5 m (width height) were installed at the rear end of both

The temperature range of the location is between 22 and 35°C, the outdoor

2.0–6.0 m/s. The lower value of the wind speed was used as the maximum for the

In the CFD set up, the field is subdivided into several grids and the partial differential equations governing a flow field (e.g. velocities, temperature pressure,

In order to analyse fluid flow, flow domains are split into smaller sub domains. The process of obtaining an appropriate mesh is called grid generation (**Figure 7**).

After meshing, the governing equations are then discretised and solved in each of these sub domains. ANSYS Fluent uses the finite volume method for equation

> • Triangular/Tetrahedron • Quadrilateral/Hexaheron

relative humidity is between 35 and 90% and the wind speed is between

etc.) are solved at all points of the field [33, 35] as shown in **Table 3**.

discretisation, which was used to perform the simulations in this study.

2478 7337 Medium Mixed

**Number of nodes Number of elements Smoothing Mesh type**

computational analysis being the worst case scenario.

*Zero-Energy Buildings - New Approaches and Technologies*

*3.2.5 Grid generation of physical model (meshing)*

*3.2.6 Discretise the governing equations*

The solution method used is the SIMPLE Algorithm method. SIMPLE is an acronym for Semi-Implicit Method for Pressure Linked Equations. It is used to couple the pressure and velocity equations.


#### **Table 4.** *Boundary conditions.*

**Figure 8.** *Solar radiation input.*

#### **Figure 9.**

*Pressure outlet (mass flow) input.*

**Figure 10.** *Selection of the point for wind flow input to the model.*

#### *3.2.9 Visualisation*

Results are the objectives of CFD simulation. They reveal the performance of the design. They reveal if the design satisfies or meet its objectives. Results are essential for making informed design decisions as revealed in **Figures 8**–**10**.

#### **3.3 Performance analysis**

Thermal comfort is determined by various factors. These factors include temperature, humidity and evaporative cooling. Using the values for the mass flow rate, the values for the air change per hour of the room can be deduced. Thermal comfort is very important to humans; therefore, using computational fluid dynamics analysis, ventilation performance of a room is determined with or without passive ventilation systems. The main driving force for the ventilation performance is the air change in the room depicted by the air change per hour, ACH.

The model for the performance analysis is written as:

$$ACH = \frac{\\$600m}{\rho \text{fsV}} \tag{9}$$

system (solar chimney) for improving the natural ventilation of a room. These data helped in developing a mathematical model that was used to predict the rate of ventilation (the air change per hour) of a room or space given by Eq. (10):

64 2.25 0.035 1.5 Bansal et al. [36] 50 2.5 0.05 3.46 Maerefat and

27 1.64 0.0607 5.6 Mathur et al. [38] 25 6 0.24 8 Khedari et al. [39] 15 2.5 0.167 7 Yan et al. [40]

*<sup>X</sup>* <sup>¼</sup> *Solar Chimney Size*

Since the various solar chimney models used had different sizes and different room sizes, a means to normalise the data was found by diving the solar chimney volume by the room size. All these data were presented in **Table 5**. This ratio of the solar chimney size to room size was then used to develop a mathematical model

**Figure 11** shows the trend-line graph of the solar chimney in which a model

R2 = 0.8559.

(Eq. (11)) to predict the ventilation rate of the system (**Figure 11**).

equation is obtained from a quadratic fit and given by;

where,

**Table 5.**

**Room size (m3**

**) Solar chimney size (m3**

*DOI: http://dx.doi.org/10.5772/intechopen.92725*

*Computational Analysis of a Lecture Room Ventilation System*

*Parameters of previous researches by various authors.*

and,

**Figure 11.**

**89**

*Thread-line graph of solar chimney model.*

Y ¼ mX þ c (10)

**) Ratio = SCS/RS ACH (1/h) Author**

Haghighi [37]

*Room Size* (10a)

*Y* ¼ ACH ðAir change per hourÞ (10b)

<sup>y</sup> ¼ �190*:*9x<sup>2</sup> <sup>þ</sup> <sup>77</sup>*:*753x <sup>þ</sup> <sup>0</sup>*:*<sup>0625</sup> (11)

where, m = Mass flow rate of air (kg/s). V = Volume of the room (m<sup>3</sup> ).

#### *3.3.1 Solar chimney model*

Using data from various researchers, values which include room size, solar chimney size, etc. have been used to investigate the use of passive ventilation



**Table 5.**

*Parameters of previous researches by various authors.*

system (solar chimney) for improving the natural ventilation of a room. These data helped in developing a mathematical model that was used to predict the rate of ventilation (the air change per hour) of a room or space given by Eq. (10):

$$\mathbf{Y} = \mathbf{m}\mathbf{X} + \mathbf{c} \tag{10}$$

where,

$$X = \frac{\text{Solar Chimney Size}}{\text{Room Size}} \tag{10a}$$

and,

*3.2.9 Visualisation*

**Figure 10.**

**Figure 9.**

*Pressure outlet (mass flow) input.*

where,

**88**

*3.3.1 Solar chimney model*

m = Mass flow rate of air (kg/s). V = Volume of the room (m<sup>3</sup>

**3.3 Performance analysis**

*Selection of the point for wind flow input to the model.*

*Zero-Energy Buildings - New Approaches and Technologies*

Results are the objectives of CFD simulation. They reveal the performance of the design. They reveal if the design satisfies or meet its objectives. Results are essential

Thermal comfort is determined by various factors. These factors include temperature, humidity and evaporative cooling. Using the values for the mass flow rate, the values for the air change per hour of the room can be deduced. Thermal comfort is very important to humans; therefore, using computational fluid dynamics analysis, ventilation performance of a room is determined with or without passive ventilation systems. The main driving force for the ventilation performance is the air

*ACH* <sup>¼</sup> <sup>3600</sup>*<sup>m</sup>*

Using data from various researchers, values which include room size, solar chimney size, etc. have been used to investigate the use of passive ventilation

<sup>ρ</sup>fsV (9)

for making informed design decisions as revealed in **Figures 8**–**10**.

change in the room depicted by the air change per hour, ACH. The model for the performance analysis is written as:

).

$$Y = \text{ACH (Air change per hour)} \tag{10b}$$

Since the various solar chimney models used had different sizes and different room sizes, a means to normalise the data was found by diving the solar chimney volume by the room size. All these data were presented in **Table 5**. This ratio of the solar chimney size to room size was then used to develop a mathematical model (Eq. (11)) to predict the ventilation rate of the system (**Figure 11**).

**Figure 11** shows the trend-line graph of the solar chimney in which a model equation is obtained from a quadratic fit and given by;

$$\mathbf{y} = -190.9\mathbf{x}^2 + 77.753\mathbf{x} + \mathbf{0.0625} \tag{11}$$

$$\mathbf{R}^2 = \mathbf{0.8559}.$$

**Figure 11.** *Thread-line graph of solar chimney model.*


**Table 6.**

*IAQ requirements in classrooms [41].*

#### **3.4 Energy saving model**

This model was developed to reveal the energy saving potential of the passive ventilation system (the solar chimney). Energy saving is described as the level of energy saved and the reduction in cost derived from such saving. The model equations are as follows:

The cost of using mechanical ventilation system is given by,

Total Cost ¼ Installation Cost þ Operation Cost þ Maintenance Cost (12)

Also, the cost of using a passive ventilation system (solar chimney) is given by,

Total Cost ¼ Design Cost þ Installation Cost þ Maintenance Cost (13)

The percentage increase (energy saved) of both ventilation systems is given by,

$$\%Increase = \frac{C\_{Mech} - C\_{\infty}}{C\_{Mech}} \ast 100\% \tag{14}$$

From the initial data, the results obtained fall short of the ASHRAE standard [42, 43], which proposed an average air change per hour of between 4 and 6 in a

*Velocity streamlines of wind in the mechanical lecture room at different inlet velocities (0.05 TRH, 0.5 BRH,*

**Figures 12**–**14** show the velocity streamlines, temperature contour, and velocity

**Figures 12** and **13** reveal that the air velocities at the window sections are higher than in other places within the lecture hall. This is expected, as the airflow past the windows and enters the room, the flow velocity decreases as a result of the sudden expansion of the space in the room and increase in pressure according to Bernoulli's principle. It can also be observed that there are no obvious air movements at the centre and edges of the room. This is due to the initial condition that the inflow is from the side with two windows. This condition would not be pleasant for the occupants at these sections of the room. However, when the inflow comes from the sides with the three windows more sections of the room would have air considerable

**Figure 14** reveals that the air temperature distribution in the whole room is stratified. The lowest temperature appears to be at the side with three windows and highest at the areas with two windows. This is due to the higher convective heat transfer at the side with the three windows coupled with the increase in velocities at this section due to the venture effect. This indicates that areas closest to the three windows are more ventilated as compared to areas closest to that of two windows

**Table 7** shows that the thermal comfort/air quality changes with air mass flow rate. As the mass flow rate increases air quality is also increased which shows that air quality is directly proportional to mass flow rate. At the initial state when the mass flow rate is 2.8 kg/s the air change per hour is 3.10. According to ASHRAE, in public places like the classroom, it must be ventilated in such a way that it has

lecture room with an average wind velocity ranging from 0.12–0.5 m/s.

vector streamlines of the model.

movement.

**91**

**Figure 12.**

*1.0 BLH and 2.0 TRH m/s).*

*4.1.2 Flow and thermal conditions of the lecture hall*

*Computational Analysis of a Lecture Room Ventilation System*

*DOI: http://dx.doi.org/10.5772/intechopen.92725*

and this is a result of different in the number of windows.

where

*CMech* = mechanical ventilation cost. *Csc* = solar chimney cost.

#### **3.5 Comparison against standards**

Indoor air quality (IAQ) requirement is determined by various factors. **Table 6** shows some the acceptable IAQ international standards for classrooms. Therefore, the results obtained were compared with standard to validate the result.

#### **4. Results and discussion**

#### **4.1 Air quality analysis**

#### *4.1.1 Lecture hall*

Using CFD methods, the computational study was successfully performed to obtain the air properties for the lecture room. At the initial condition, where the wind velocity was 0.05 m/s, the air change per hour was found to be 3.1 and this value increases as the wind velocity increases.

*Computational Analysis of a Lecture Room Ventilation System DOI: http://dx.doi.org/10.5772/intechopen.92725*

**Figure 12.**

**3.4 Energy saving model**

*IAQ requirements in classrooms [41].*

UK [Building Bulletin 101

(2006)]

**Table 6.**

**Country [standard] Outdoor air [m<sup>3</sup>**

*Zero-Energy Buildings - New Approaches and Technologies*

tions are as follows:

where

*CMech* = mechanical ventilation cost.

value increases as the wind velocity increases.

*Csc* = solar chimney cost.

**4. Results and discussion**

**4.1 Air quality analysis**

*4.1.1 Lecture hall*

**90**

**3.5 Comparison against standards**

This model was developed to reveal the energy saving potential of the passive ventilation system (the solar chimney). Energy saving is described as the level of energy saved and the reduction in cost derived from such saving. The model equa-

Portugal [RECS (2013)] 600 1250 4.0

Germany [DIN 1946–2 (2005)] 500 1500 3.3 USA [ASHRAE 62.1 (2013)] 558 1080 3.7 Europe [EN 15251 (2007)] 756 550 5.0

Total Cost ¼ Installation Cost þ Operation Cost þ Maintenance Cost (12)

Also, the cost of using a passive ventilation system (solar chimney) is given by,

Total Cost ¼ Design Cost þ Installation Cost þ Maintenance Cost (13)

The percentage increase (energy saved) of both ventilation systems is given by,

*CMech*

Indoor air quality (IAQ) requirement is determined by various factors. **Table 6** shows some the acceptable IAQ international standards for classrooms. Therefore,

Using CFD methods, the computational study was successfully performed to obtain the air properties for the lecture room. At the initial condition, where the wind velocity was 0.05 m/s, the air change per hour was found to be 3.1 and this

∗ 100% (14)

**/h] CO2 concentration [ppm] ACH [h**�**<sup>1</sup>**

450 1500 3.0

**]**

%*Increase* <sup>¼</sup> *CMech* � *Csc*

the results obtained were compared with standard to validate the result.

The cost of using mechanical ventilation system is given by,

*Velocity streamlines of wind in the mechanical lecture room at different inlet velocities (0.05 TRH, 0.5 BRH, 1.0 BLH and 2.0 TRH m/s).*

From the initial data, the results obtained fall short of the ASHRAE standard [42, 43], which proposed an average air change per hour of between 4 and 6 in a lecture room with an average wind velocity ranging from 0.12–0.5 m/s.

**Figures 12**–**14** show the velocity streamlines, temperature contour, and velocity vector streamlines of the model.

#### *4.1.2 Flow and thermal conditions of the lecture hall*

**Figures 12** and **13** reveal that the air velocities at the window sections are higher than in other places within the lecture hall. This is expected, as the airflow past the windows and enters the room, the flow velocity decreases as a result of the sudden expansion of the space in the room and increase in pressure according to Bernoulli's principle. It can also be observed that there are no obvious air movements at the centre and edges of the room. This is due to the initial condition that the inflow is from the side with two windows. This condition would not be pleasant for the occupants at these sections of the room. However, when the inflow comes from the sides with the three windows more sections of the room would have air considerable movement.

**Figure 14** reveals that the air temperature distribution in the whole room is stratified. The lowest temperature appears to be at the side with three windows and highest at the areas with two windows. This is due to the higher convective heat transfer at the side with the three windows coupled with the increase in velocities at this section due to the venture effect. This indicates that areas closest to the three windows are more ventilated as compared to areas closest to that of two windows and this is a result of different in the number of windows.

**Table 7** shows that the thermal comfort/air quality changes with air mass flow rate. As the mass flow rate increases air quality is also increased which shows that air quality is directly proportional to mass flow rate. At the initial state when the mass flow rate is 2.8 kg/s the air change per hour is 3.10. According to ASHRAE, in public places like the classroom, it must be ventilated in such a way that it has

Airflow rate of 7.5 cfm/person. If the class will accommodate 100 students

There are various means of improving the ventilation of the lecture room, they are:

**Wind Velocity (m/s) Mass Flow (kg/s) ACH (1/h)** 0.05 2.8 3.10 0.5 28.68 31.79 1.0 57.36 63.58 1.5 86.04 95.36 2.0 114.72 127.15

It has been revealed by CFD simulations above that indeed the lecture room was inadequately ventilated at low wind speeds. The number/size of openings and introduction of another passive or mechanical system is required to improve the ventilation of the lecture hall. Positioning the ventilation openings such that a pair face each other to induce crossflows, and also positioning openings at the top allowing hot stale air to easily exit the building would create and optimise airflow and circulation pattern of the lecture hall. Windows or vents placed on opposite sides of the building induces natural flow pathway through the structure. Therefore, to improve the indoor air quality of this model, a cross ventilation of the lecture room and addition of topside opening would improve the thermal comfort and IAQ of the lecture hall.

From the Eq. (11), a solar chimney model for the lecture room was developed and the graph shown in **Figure 11** was obtained. **Figure 15** reveals that using passive ventilation system such as a solar chimney with the right size, the indoor air quality of the lecture room can be improved. The graph is parabolic, which implies that some sizes of the solar chimney would work negatively when applied to the lecture

The energy saving potential of using a passive ventilation system was carried out in comparison to using mechanical ventilation system, which is given as follows:

Installation Cost: The installation cost consists of the purchase cost and the fixing cost of the system i.e. fans. The average cost of purchase was found to be ₦5000 per fans and fixing cost was ₦200 per fans. For nine fans present in the room, we have:

), air change per hour begins to decrease with

comfortably, the ventilated facilities must give 750 cfm air quality.

*4.1.3.2 Passive ventilation system (solar chimney) attachment*

hall. After the peak value (100 m<sup>3</sup>

*4.2.2 Mechanical ventilation system's cost analysis*

increasing chimney size.

**4.2 Energy cost analysis**

*4.2.1 Energy saving potential*

**93**

*4.1.3 Room ventilation improvement methods*

*Mass flow rate and corresponding indoor air quality (ACH).*

*Computational Analysis of a Lecture Room Ventilation System*

*DOI: http://dx.doi.org/10.5772/intechopen.92725*

*4.1.3.1 Cross ventilation*

**Table 7.**

**Figure 13.**

*Velocity vector of air in the mechanical lecture room at different inlet velocities (0.05 TRH, 0.5 BRH, 1.0 BLH and 2.0 TRH m/s).*

**Figure 14.** *Temperature contour of air in the mechanical lecture room.*


*Computational Analysis of a Lecture Room Ventilation System DOI: http://dx.doi.org/10.5772/intechopen.92725*

**Table 7.**

**Figure 13.**

**Figure 14.**

**92**

*Temperature contour of air in the mechanical lecture room.*

*and 2.0 TRH m/s).*

*Velocity vector of air in the mechanical lecture room at different inlet velocities (0.05 TRH, 0.5 BRH, 1.0 BLH*

*Zero-Energy Buildings - New Approaches and Technologies*

*Mass flow rate and corresponding indoor air quality (ACH).*

Airflow rate of 7.5 cfm/person. If the class will accommodate 100 students comfortably, the ventilated facilities must give 750 cfm air quality.

## *4.1.3 Room ventilation improvement methods*

There are various means of improving the ventilation of the lecture room, they are:
