## 3 Results and Discussion
### Effects of Fire Locations  
In the previous section, the influence of the fire HRR was evaluated, whereas in this section an analysis of the effect of the fire location is presented. In an actual accident case, the location of the fire source may be random due to the location of atria, meaning piled up flammable debris cannot be predicted; thus, the location of the smoke is also random, which has a significant influence on the flow of smoke. Moreover, changes in the location of the fire source lead to changes in the neutral surface. Therefore, it is necessary to study the influence of the fire location on the smoke flow and temperature distribution in the stairwell. To control the variables, the doors of the compartment of each stairwell were opened to form a stairwell model with laterally uniform openings. In addition, the simulation configurations of the fire sources were varied, as they were positioned on Floors 1, 5, 10, and 15. To understand the flow state of the thermal smoke, the velocity vector fields at x = 8.5 m in the stairwell and atria room at different fire HRRs are presented in Figure 8, which were compiled using _Tecplot_ software [35; 36; 37]. As can be seen from Figure 8, the entire flow field of the 21-floor stairwell was divided into three parts (I, II, III) according to the direction of airflow. Part I includes all the outlet airflow, part III includes all the inlet airflow, and part II includes the lift, which is referred to as turbulent airflow, wherein the airflow can be either inlet or outlet flow. By changing the fire location from lower to higher heights in the simulation process, a regularity was found, in that the direction of the airflow in the openings below the fire location was considered as the \"inlet\". Hence, the amount of air entrainment was greater at a higher fire location. For example, if the fire is on Floor 1 (Figure 8a), fresh air can only enter through a single opening; however, if the fire is on Floor 15 (Figure 8d), the air entrainment can be formed by air from openings on Floors 1-14. However, the air from openings below is non-uniform. As can be seen from Figure 8d, the area near the fire location had a higher air entrainment capacity with high intensity speed vector arrows, i.e., the air flow was more turbulent near the fire source. Moreover, from the data of the smoke flow direction, irrespective of the fire location, the smoke flow direction of the top three floors (Floors 19, 20, and 21) was \"outlet\". Therefore, the height of fire location has no influence of the quantity of the \"outlet openings\"; however, it may have an influence on the degree of airflow disorder. With an increase in the height of the fire location, there was an increase in the intensity of the airflow in the opening on Floor 21, as indicated by the more emphasized and disorderly arrows shown in the opening area. Hence, it was briefly concluded that with an increase in the height of the fire location, there is an increase in the number of inlet openings (Part III), and the number of outlet openings (Part I) remain constant; thus, there is a decrease in the number of turbulent openings (Part II). These regular patterns (applicable when the fire is on Floors 1-18) can be expressed as follows:  
\[\left\{ \begin{array}{l}N_{outlet=3}\\ N_{\text{turbulent}}=21-3-N_{\text{inlet}}=18-N_{\text{inlet}}\\ N_{inlet}=N_{fire}-1\end{array} \right. \tag{6}\]  
where \(N_{outlet}\) is the number of openings through which the direction of the airflow is outward, \(N_{outlet}\) is the number of openings through which the direction of the airflow is inward, \(N_{turbulent}\) is the number of openings through which the direction of the airflow is neither outward or inward, and \(N_{fire}\) is the floor on which the fire source is present. As mentioned above, Figure 9 describes the simulation results in a stairwell with laterally uniform openings.