###### Abstract  
According to recent UN reports, it is estimated that more than one billion people live in informal settlements globally, exposing them to a large potential fire risk. In previous research, it was found that the main fire spread mechanism between dwellings is the external flaming (plume) and radiative heat fluxes from the vertical openings at the dwelling of origin to the surroundings. In this paper, an experimental and numerical study was conducted to quantify the effect of adding horizontal roof openings to the design of informal settlement dwellings to reduce the fire spread risk by decreasing the length of flames and radiation from the external plumes at the vertical openings. In total, 19 quarter scale ISO-9705 compartment fire experiments were conducted using an identical fuel load (80 MJ/m\({}^{2}\)) of polypropylene and were used to validate a physical computational fluid dynamics model for future studies. Five different total horizontal openings areas (0.0025, 0.01, 0.04, 0.09, and 0.16 m\({}^{2}\)) were investigated using two horizontal openings designs: (1) four square openings at the four corners of the compartment and (2) one slot cut at the middle of the compartment. It was found that adding horizontal openings decreased the average heat flux measured at the door by up to 65% and 69% for corner and slot cases, respectively. Heat flux reductions were achieved at opening areas as low as 0.01 m\({}^{2}\) for slot cases, whereas reductions were only achieved at areas of at least 0.09 m\({}^{2}\) for corner cases. The Computational Fluid Dynamics (CFD) model was validated using the experimental results. It successfully captured the main fire dynamics within the compartment in addition to the values of the external radiative heat flux. Further, a new empirical ventilation factor was generated to describe the flow field through both openings configurations which showed strong coupling with the inlet mass of fresh air to the compartment. compartment fire; horizontal opening; external plume; thermally thin; flashover; urban fire spread; risk; CFD +
Footnote †: journal: Applied Sciences
any available materials to build their homes, which do not comply with formal building codes leaving them vulnerable to hazards such as fire. For example in Western Cape, South Africa, steel sheets and timber are the most used materials for building dwellings [3]. In the last few years, many destructive fires took place within informal settlements around the globe: Dhaka, Bangladesh in 2019 [4]; Imizamo Yethu, South Africa in 2017 [5]; and Bahay Toro, Philippines in 2011 [6]. Informal settlement dwellings (ISDs) are unique in a few aspects compared to formal compartments/dwellings. For example, the walls/boundaries are usually made out of thermally thin (e.g., steel sheets) or combustible materials (e.g., timber); for insulation, the walls are usually internally lined with combustible linings (e.g., cardboard); and due to poor construction, there are usually gaps/leakages in the dwellings' boundaries (walls and roof) [3]. Therefore, most current understandings of compartment fire dynamics must be adapted to take into account these features. As shown in Figure 1, fire spread in IS is also unique due to the high dwelling proximity in these settlements, the presence of combustibles between the dwellings, high fuel loads which affects the size of the external plumes, and the effect of vegetation and topography (e.g., wind and land slope) [3; 7]. Fire development within Informal Settlements Dwellings (ISDs) and fire spread within the ISs are currently under the focus of intensive research. Since 2017, there have been many experimental and numerical studies to fill the gap of knowledge for these unique compartments and fire spread techniques. In summary, these studies have included (1) bench scale material tests ([PERSON] et al. [8]); (2) small scale experiments determining heat transfer and the heat release rate needed for flashover in thermally thin and thick boundaries ([PERSON] et al.