## 1 Introduction  
Because they represent a good compromise between speed and accuracy, fire zone models are always widely used by the research community and fire safety engineers. Compared to more sophisticated models, such as CFD (computational fluid dynamics) models, the computational run times of zone models are significantly shorter, on the order of a minute, or even less. In addition, they require little memory and few data, which makes them still particularly attractive.  
Zone models may be classified based on the number of zones in each compartment: one-zone, two-zone, and multi-zone models [1; 2]. One-zone models are widely used in the analysis of post-flashover fires, as well as smoke transport in the compartments separate from the fire room (e.g., COMPF2 [3] or OZone [4]). In a two-zone model, the compartment is divided into two uniform well-mixed zones: a hot upper smoke layer and a cold lower layer, separated by a planar interface. They solve conservation equations between the two zones and use empirical relationships to describe phenomena such as fire plume, flame height, air entrainment, ceiling jet, or flows at vents. The multi-zone model is an extension of the two-zone model as the room volume is divided into an arbitrary number of horizontal layers, in which the physical properties (e.g., temperature or gas concentrations) are assumed to be uniform. They were developed to better predict the vertical distributions of temperature and gas concentrations in the fire room. Due to the assumptions on which they are based, two- and multi-zone models exhibit drawbacks or have limitations when applied to spaces of complex geometry or large size, but as mentioned before they give satisfactory results at lower cost and, because of their capabilities, they can be used for pre- and post-flashover fire modeling. Examples of such models developed since the 2000's are CFAST [5], MAGIC [6], BRI\({}_{2002}\)[7], B-ISK [8; 9], and BRANZFIRE [10] (among others).  
In existing two-zone models, the pressure is assumed to be uniform within a compartment and the momentum of the smoke interface is ignored, which avoids the time step imposed by acoustic waves (Courant condition). However, for under-ventilated fire scenarios, this strategy can lead to total room involvement by the smoke layer, which is not observed experimentally [11].  
Some zone models, such as CFAST, include the ability to track multiples fires in the compartment, but these fires are treated as totally separate entities, with no interaction of the plumes or radiative exchanges between fires [5]. This approach allows multiple fires to be handled when they are far enough apart.  
In an attempt to address these issues, a two-zone computational model is developed and validated to determine the fire environment inside a compartment where different fuels, close to each other, are burning simultaneously. The model includes a momentum equation to improve the accuracy of tracking the smoke layer interface and a new concept of surrogate fuel molecule to mimic multiple fuel combustion, as well as the effects of the ceiling jet on the convective heat transfer at the enclosure walls.  
This paper is organized as follows. Section 2 deals with the mathematical basis of the model. Section 3 describes the experimental setup and the ten multiple fire scenarios that have been conducted in the IUSTI facility, varying the fuels involved and the confinement level of the enclosure. Section 4 provides a comparative analysis between the model results and the experimental data to evaluate the performance of the model developed.