**1. Introduction**

Ship fire accidents often cause serious damage to personnel and other important equipment on board, making ship fires a major threat to ship safety. Among all kinds of ship fire accidents, engine room fire is the main research object at present because of the characteristics of high fire frequency, large fire load, and high difficulty in fire detection and rescue.

In recent years, the problem of smoke control in ship fires has attracted much attention [1–4]. The smoke control of the engine room mainly relies on the mechanical smoke exhaust system and the mechanical air supply system. When fire occurs, the smoke control system is activated, and the original smoke flow of the engine room will change. Therefore, the law of cabin smoke filling under forced ventilation conditions is of great significance for the study of the fire hazard of the engine room.

Previous studies have attracted more attention to the effects of forced ventilation on the development of cabin fires and the law of smoke filling in closed cabins. Many scholars have carried out closed space fires under different forced ventilation conditions, and have studied the effect of forced ventilation on fire parameters such as fire source burning rate [5–8], temperature distribution [9–12], and cabin pressure [13–15]. Alvares [5] found similar fire parameters for two different tested liquid fuels under the same fire intensity and ventilation. Chow and Chan [6] provided more information on the effect of fuel type and found that fire parameters for wood were sensitive to changes in ventilation volume, whereas fire parameters for liquid fuels and polymer materials were less sensitive to changes in ventilation volume. Peatross and Beyler [7] believed that the burning rate was determined by the fuel type and ventilation rate, and the forced ventilation test results showed that the burning rate increased with the increase in ventilation rate. Le [8] analyzed that, in a mechanically ventilated compartment, the mass loss rate of ignition source

**Citation:** Wu, X.; Zhang, Y.; Jia, J.; Chen, X.; Yao, W.; Lu, S. Experimental and Theoretical Analysis of the Smoke Layer Height in the Engine Room under the Forced Air Condition. *Fire* **2023**, *6*, 16. https:// doi.org/10.3390/fire6010016

Academic Editors: Chuangang Fan and Dahai Qi

Received: 4 November 2022 Revised: 27 December 2022 Accepted: 30 December 2022 Published: 4 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). *fire*

<sup>1</sup> State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230026, China

depends on two factors: the oxygen concentration and the blowing effect on the oil pool. In addition, different ventilation configurations, such as ventilation volume, and vent height, can have an impact on combustion characteristics in the cabin. Backovsky [16] studied the effect of ventilation volume and ventilation configuration on the chamber temperature, and the results showed that under the condition of low air inlet position, the ventilation rate was 2–3 times greater than the stoichiometric air volume required for fires (here called well-ventilated fires) produce a two-layer temperature distribution, whereas under-ventilated fires produce a single-layer distribution with a temperature gradient, with poorly ventilated fires having higher temperatures than well-ventilated fires. The high-inlet location perturbs the two-layer temperature distribution of a well-ventilated fire. Zhang [4] found that increasing the height of the air inlet or reducing the height of the air inlet can destroy the formation of the smoke layer. Based on the conservation equation of oxygen concentration, Hayashi [17] considered the oxygen volume exchange between the upper and lower layers due to convection and turbulent diffusion, revealed the conditions for the formation of the smoke layer under forced ventilation, and studied the formation and destruction of smoke layer by CFD. In previous studies on the effect of forced ventilation on cabin fires, there are relatively few studies on the height of the smoke layer under forced ventilation conditions.

Smoke layer height is one of the important parameters in compartment fire research. Existing calculation methods for fire smoke layer height include the classical N-percentage, integral ratio, intra-variance methods and so on. These methods are mostly suitable for the processing of temperature data. Predecessors have carried out a lot of research on the change in smoke layer height in compartment fire [18–20]. Most of these studies are experimental and simulation studies based on the corresponding structures of buildings. For the study of the height of the smoke layer, some common models have been formed, such as the NFPA 92B model [21], the Milke Mowrer model and the Tanaka [22] model. It has been more than 20 years since Zukoski [23] first proposed a simple analytical model of enclosed smoke filling based on energy and mass balance. The Zukoski formula remains the theoretical basis for other enclosed space smoke filling models, including the ASET model developed by Cooper. Li [24] obtained a formula for predicting the smoke layer based on the regional model (there is an obvious layering between the upper hot smoke and the lower cold air) and according to the conservation of energy. Li [25] carried out the natural gas filling experiment and the smoke control experiment in the experimental cabin, and based on the plume theory and the conservation equation, he developed the smoke settlement formula for the filled cabin, and considered the plume rising time. Wang [26] established an explicit smoke-filling model of an enclosed ship engine room through the back analysis of the experimental data in the literature.

It can be seen that there is a lack of theoretical and experimental research on the height of the smoke layer of closed cabin fires under forced ventilation in the current research on engine room fires. Therefore, this paper studies the development of the cabin room fire and the process of smoke settlement under the conditions of different air supply volume and air supply inlet heights through the small-scale engine room fire experiment. the theoretical analysis was carried out to predict the height of the cabin fire smoke layer un-der different air supply conditions based on the zone model, and the prediction results of the model are compared with the experimental results to verify the validity of the theoretical model. The model proposed in this paper can provide a theoretical analysis and prediction method for ship fire risk research.
