## 4 Discussion  
This research into smoke diffusion in curved highway tunnel fires revealed distinct patterns of fire characteristics at human eye height. Compared with straight tunnels, the movement of smoke in curved tunnels exhibits different patterns. The propagation of fire smoke can be categorized into the following two stages: the fire development stage and the one-dimensional longitudinal spreading stage [46]. During the fire development stage, high-temperature smoke generated directly by the fire source undergoes repeated collisions between the inner and outer sidewalls of the tunnel. Consequently, this process results in multiple high-temperature zones between the inner and outer sides. Additionally, CO, which significantly impacts personal evacuation because of the incomplete combustion of the fire source, also exhibits alternating peaks between the inner and outer sides as it spreads with high-temperature smoke. However, subsequent smoke diffusion resembles that in a straight tunnel where visibility is less influenced by smoke temperature due to transmission characteristics but more affected by smoke concentration. Following the initiation of one-dimensional longitudinal smoke diffusion, there is no discernible variation in temperature or visibility across different locations within the tunnel. Nevertheless, the distribution of CO does not entirely align with smoke diffusion. Within high-temperature smoke, distinct zones of alternating CO concentration peaks persist on both the inner and outer side walls during the one-dimensional propagation phase. The results on the distribution of temperature agree well with previous studies [4; 14; 18]. However, it must be pointed out that we further considered two critical indicators, i.e., CO concentration and human visual acuity, when comparing our results to those of older studies. Additionally, the selection of the tunnel curvature radius was not constrained to a minimum value of 2500 m [16]. Based on practical engineering, the longitudinal ventilation velocity was introduced into the simulations to consider the smoke propagation pattern in the actual fire scenario. The results extended beyond the scope of smoke control measures to encompass considerations for the selection of personal evacuation strategies. The limitations of our study should be acknowledged when interpreting these findings. Firstly, it is important to emphasize that the temperature distribution results have been validated by enough experiments [11; 15], ensuring the accuracy of our findings. The distribution of human visibility has also been rigorously substantiated. Nonetheless, there remains a need for experimental verification regarding the distribution pattern of CO concentration in curved highway tunnels. In current numerical simulations, the heat release rate of the fire source is consistently assumed to be a super-fast fire. However, the heat release rate of the fire source exhibits a discernible trend of gradual increase. Moreover, our selection of simulations has limitations. As no completely flat tunnels are unaffected by altitude in actual engineering, our study remains constrained. Variations in air density, humidity, and oxygen content of the tunnel location will also impact the results. Despite its preliminary character, our study clearly indicates the influence of different tunnel curvature radii on fire smoke dispersion and personal evacuation. One important future direction is to explore the combined effects of multiple influencing factors. For example, investigating the impact of altitude effects and chimney effects on the diffusion pattern of fire smoke in a curved tunnel can help elucidate their potential to either enhance or suppress smoke distribution. Another important future direction is a transition curve section between the straight area and the curved area. The influence of the continuously changing curvature radius on smoke distribution is also worth further exploration. A new personal evacuation strategy for curved highway tunnels can be derived from our findings.