Flame Tilt Research Articles

Numerical Simulation of an Isolated N-Heptane Pool Fire

Refineries are industrial complexes of great economic importance which are located close to major cities. A pool fire accident that can occur from an oil leak combined with wind can result in disastrous consequences for such an industry. This study investigates the characteristics of an isolated n-heptane square pool fire of 36 m2 under the influence of a cross wind. The pool fire characteristics are numerically studied using open-source Computational Fluid Dynamics (CFD) software, such as FireFoam (v4.1) and Fire Dynamic Simulator (FDS) (version 6.9.0). The turbulent flow field and the fire characteristics were simulated with the LES Method. The crucial parameters of the pool fire, such as (a) the temperature and velocity fields, (b) the flame length and height, (c) the surface emissive power, and (d) the flame tilt angles, were computed. Comparisons against experimental data for both small and large-area pool fires from the literature were made successfully. The flame tilt angle is shown to correlate very well with the reciprocal of the Richardson number, which was approximated within a multiplication constant to the Froude number. Thus, both the reciprocal Richardson number and Froude number can be used for correlating the flame tilt angle. It is shown that both of these numbers are used to correlate the tilt angle of experimental pool fires with effective diameters from a fraction of a meter to approximately 16 m, and wind speeds up to 7 m/s. The goodness of a linear fit based on the sum of the residual squares is 0.91.

Experimental study of flame morphology in slope buildings under the influence of aspect ratio and ambient wind

Slope structures are commonly used in buildings, but such slope structures can exacerbate the rate of fire spread, complicate building fire safety and suppression, and lead to injuries, deaths, and property damage. Particularly, the thermal hazards posed by ambient wind on sloped fires remain unquantified. In this study, a propane burner was used to simulate the flame morphology behavior of a sloped building fire. 480 tests were conducted with varying aspect ratios (1∼8) of the fire source, different ambient winds (0 m/s∼3.5 m/s), and slope inclination angles (0°∼40°). Results demonstrate that the flame length initially increases and then decreases with an increment in ambient wind velocity. Moreover, with an increased aspect ratio of the fire source (length-to-width ratio), the turning point at which the flame length peaks shift to occur at a lower wind velocity threshold. The tilt angle and height of the flame change (respectively increases and decreases) significantly at lower ambient wind speeds and they tend to asymptotically approach respective constant values. As the ambient wind is further promoted. Existing physical models of flame morphology do not consider the combined effects of fire source aspect ratio, slope inclination angle, and ambient wind. A new slope entrainment restriction factor (EF) was proposed based on an analysis of the upward and leeward entrainment volumes. Using this factor, new models for flame morphology (including flame length, flame tilt angle, and flame height) were established. The accuracy and generalizability of the new flame models have been validated using reference data.

Forecasting the flame geometry and ground received radiative heat fluxes of two parallel adjacent fires of hydrocarbon fuel with crosswinds

Radiative heat transfer in fires is a critical parameter and a key aspect of gas storage and fire prevention design. In this study, the flame geometric scale characteristics and ground-received radiative heat fluxes from two parallel adjacent hydrocarbon fuel fires with crosswinds were examined. The separation distance between the two parallel flames, crosswind speed, and heat release rates were considered. The experimental results indicated that the flame tilt angle of the two parallel flames increased with increasing crosswinds. The horizontal flame length scale of the two adjacent parallel flames first increased with the crosswind until a critical point was reached, followed by a fluctuation with a further increase in the wind speed. To further quantify this flame phenomenon, a physical model of the flame interaction between two parallel fires under the effects of crosswinds was proposed. The model analyzed major factors affecting the geometrical parameters of the flame, including the inertial force caused by the crosswind, thermal buoyancy from flame combustion, and the interaction and separation distance between the two fires. Additionally, correlations between the flame tilt angle and the horizontal flame length scale of the two adjacent flames were proposed. It was also demonstrated that the ground-received downstream radiative heat fluxes were the dominant parameters determining flame spread. Based on the assumption of the flame geometry of two adjacent parallel fires, the viewing factor was revised, and the relationship between the position of the radiant point and the horizontal length of the flame was considered in the flame model. Furthermore, based on the updated viewing factor of two parallel adjacent flames, a prediction model of ground-received radiative heat fluxes was proposed, which successfully estimated the variation of ground-received downstream radiative heat flux profiles along the centerline of the two parallel flames under crosswind conditions. This estimation is crucial for future fire research, particularly cross-wind-related experiments.

Experimental study on the effect of longitudinal ventilation on spill fire characteristics in sloped tunnels

A series of spill fire tests were carried out in a reduced-scale tunnel (6 m × 0.6 m × 0.45 m), considering different discharge rates (60–140 ml/min), ventilation rates (0∼1.26 m/s), and tunnel slopes (3 %, 5 %, 7 %). Some crucial characteristic parameters during the quasi-steady combustion stage of tunnel spill fires were analyzed, including the diffusion shape of the liquid fuel, mass burning rate per unit area, flame morphology and plume temperature, and the critical ventilation velocity. The results show that as the tunnel slope increases, the diffusion shape of the liquid fuel transitions from circular to an ellipse shape and then to rectangular or linear shape. Moreover, the mass burning rate per unit area decreases with increasing ventilation rates at low ventilation (V < 0.5 m/s), while it remains almost constant at high ventilation rates (V ≥ 0.5 m/s). This could be attributed to variations in flame tilt behavior under different ventilation conditions. Predictive models for the flame tilt angle and the vertical temperature distribution of tunnel spill fires under the effect of ventilation were further established and verified. Furthermore, the critical ventilation velocity of tunnel spill fires remains essentially unchanged as the discharge rate (or fire source power) increases. This phenomenon was explained by analyzing the resistance of the spill fire plume and the buoyancy generated by the smoke temperature. This work could serve as a critical reference for assessing disaster risks and executing emergency rescue operations during tunnel spill fires.

Full-scale experimental study on combustion characteristics and smoke temperature of double-source fires in different tunnels

Through a series of full-scale fire tests conducted in three different tunnels, the common characteristics of combustion and smoke temperature of double pool fires in actual operation tunnel environments are investigated. The evolution process of the coupling effects of shielding entrainment and thermal feedback enhancement is revealed. It is found that the ventilation state and the fire source spacing can alter the dominant relationship between the shielding entrainment effect and the thermal feedback enhancement effect, resulting in different evolution patterns of combustion characteristics and smoke flow patterns of double pool fires in tunnels. The heat release rate increases by 4 times when the double pool fire flames merge under the longitudinal ventilation compared with the double pool fire flames without fusion under the natural ventilation. The flame tilt angles of double pool fires were determined through the image processing method, and the prediction model of the flame inclination angle of downstream fire sources was established, considering fire source spacing. At the same time, the characteristic of temperature measured by Fiber Bragg Grating in double-source fire scenarios was investigated, developing a model to predict the longitudinal temperature rise distribution. It is found that the maximum temperature rise prediction model based on steady-state fire has poor applicability in the weak-plume fire scenario of full-scale tunnels. The analysis of the smoke flow patterns shows that the two-stage ventilation is more suitable for smoke control compared to full longitudinal ventilation in tunnel double-source fire accidents, especially for ensuring the safety of individuals trapped in the area between the two fire sources.

Experimental study and new-proposed characterization of burning rate and flame geometry of gasoline pool fires with different aspect ratios

This study investigated the burning rate and the flame geometry of gasoline pool fires under crosswinds in the context of bridge vehicle fires. A 1:4 scale bridge fire platform was established. A total of 45 fire experiments were conducted on 9 gasoline pool with 1–2.2 aspect ratios under 0–2 m/s cross-wind. The influence mechanism of aspect ratio n on the characteristic parameters of pool fires was investigated. The results showed that the burning rate of gasoline pool ranges from 0.028 kg‧m−2‧s−1 to 0.064 kg‧m−2‧s−1, which increases with wind speed and pool size. When the wind speed exceeds 1 m/s, the title angle of the flame on the windward side is greater than 45° and is less affected by pool size. The wall heat conduction at wind downstream increases with aspect ratio when placing the longer side facing to cross flow. The correction factor Fr·n can be used to describe the competition between the air entrainment and the buoyancy in gasoline pool fire. A general model based on aspect ratio is proposed to quantify the burning rate and flame tilt characteristics of gasoline fires. The reliability and applicability of the model are compared with the existing experimental data.

Experimental study on fire behaviours of double fires with asymmetric heat release rates in the tunnel with natural ventilation

This study experimentally investigates fire behaviors of double fires with asymmetric heat release rates in a naturally ventilated tunnel including flame merging and longitudinal ceiling gas temperature distribution. The comparison between double fires with symmetric and asymmetric heat release rates was also made. The results show that the flame merging possibility increases as the heat release rate on one side increases. The ratio of heat release rates between double fire sources still influences the flame merging possibility under a given total heat release rate. The ceiling gas temperature distribution is related to the flame merging state. As the fire separation distance increases from zero, the position of peak temperature is closer to the fire source with a larger heat release rate due to a smaller flame tilt angle. For scenarios with mean flame merged, the ceiling gas temperature distribution on both sides is equal. For scenarios with mean flame not merged, the ceiling gas temperature on the side with a larger heat release rate is much higher. Finally, different empirical models are established to predict the excess ceiling gas temperature attenuation, considering different flame merging states and smoke spread regions.

STUDY ON THE INFLUENCE OF MULTI-FREQUENCY NOISE ON THE COMBUSTION CHARACTERISTICS OF POOL FIRES IN SHIP ENGINE ROOMS

A large amount of low- and medium-frequency noise can be found in the engine room of a ship. However, during the development of a fire, vibrations of the air in the engine room at different frequencies can be caused by noise disturbances, and the flow field distribution in the flame zone will be changed, which will affect the combustion characteristics of the pool fire. In this paper, an n-heptane pool with a diameter of 6 cm in a confined space of 1500 mm &amp;times; 1500 mm &amp;times; 1000 mm is used. The effects of noise of 75 dB, 90 dB, 105 dB, and 112 dB at 250 Hz, 700 Hz, and 1000 Hz and the noise of the engine room on the combustion behavior of the pool fire are studied experimentally. The results of the analysis of the variation in fuel mass, flame height, and flame tilt show that the multi-frequency noise significantly affects the combustion characteristics of the pool fire in a confined space. Under the perturbation of noise waves, the fuel mass flux of the pool is larger than that of the pool fire when it burns freely, and the fuel mass flux is exponentially nonlinearly related to the noise pressure. In general, the flame height gradually decreases with an increasing noise pressure in the engine room. Additionally, a new coupling relationship between the flame height and the noise pressure is established based on the noise motion equation, and it is found that there is a negative exponential between the noise pressure and the flame height. In addition, the flame can tilt under the action of the air particle displacement caused by the noise of the engine room. The noise pressure field formed in the confined space has a restraining effect on the pool fire, and the flame tilt angle gradually decreases with an increasing noise pressure.

Analysis of the Energy Process of a Radiant Tube for Heating a Flat Material Using a CFD Model

In Reducing energy consumption and increasing energy efficiency have long been very important issues affecting all sectors of energy consumption, including the industrial sector. Industrial furnaces have a significant energy consumption that needs to be optimised, but before that an analysis of the energy situation is necessary. This article discusses the energy state analysis of a burner system, which was performed using CFD model. Specifically, the analysis of the combustion process in a radiant tube, for which a 3D model of this tube was created. The analysis also addressed the subsequent heat transfer from the surface of the radiant tube in the power range of 50-109 kW. Input parameters and model conditions were defined based on the parameters of the real burner system. The CFD modelling performed showed that as the fuel consumption increased, the radiant tube power increased and at the same time the flue gas temperature also increased. The heat flux from the tube surface was slightly different on both sides of the tube, which was due to the flame tilt inside the radiant tube.

An Experimental Study of Pool Fire Characteristics under the Effects of Cross Winds and Baffles

The pool fires that occur behind obstructions in a ventilated environment are very different from other wind-blown pool fires. The pool fire formed by fuel leakage in an engine nacelle is a typical example of a pool fire influenced by cross winds and baffles. Mastering the combustion characteristics of this type of fire is of great significance for fire prevention and control. In this study, the burning rate, flame length, and flame tilt angle of heptane pool fires behind a baffle under different cross wind velocities (ranging from 0 to 5 m/s) were experimentally investigated. Square pool fires with dimension of 8 cm and 12 cm with baffle height from 4 to 12 cm and different distances between fire and baffle (0, 20, 30 cm) were tested in a wind tunnel. The experimental results show that the burning rate increases with the increase in cross wind velocity for each baffle height. As wind velocity exceeds 2 m/s, the burning rate first decreases and then increases with the increase in baffle height. The flame length initially increases and then decreases with increasing wind velocity. The upper flame tilt angle is mainly affected by the cross wind, while the bottom flame tilt angle is influenced by the combined effects of cross wind velocity, baffle height, and distance between baffle and flame. The empirical correlations under different distances between baffle and flame, with wind velocity and baffle height accounted for, are then proposed for the dimensionless heat release rate and the flame length of heptane pool fires.

Effect of the Cross-Section of a Porous Burner on the Combustion Stability Limit of Premixed Oxy-Methane Flames.

This study presents the idea of using a porous media burner to improve the oxy-methane combustion reaction rate and broaden the stability limit. Numerical studies on the premixed combustion of CH4/O2/CO2 in a two-layer porous medium burner using a two-dimensional symmetrical volume-average model with the skeleton mechanism based on OpenFOAM. The combustion characteristics of burners with variable cross-sectional (VC) and straight cylindrical (SC) structures were compared, including stable range, temperature field, thermal cycle efficiency, and CO emissions. It is confirmed that the cross-sectional structure is effective for broadening the stable range, and the broadening rate is more than 4 times. As more heat is lost from the outlet due to the increased cross-section, the solid-phase temperature of VC is lower than that of SC. As a result, the flame temperature of VC will be about 200 K lower than that of SC under the condition of insulated walls. It also leads to a reduction of about 2% in thermal cycle efficiency compared with SC. Meanwhile, it is shown that the VC structure slightly increases CO emissions at low thermal power but is smaller than that of SC at high power. When the wall heat loss of the burner is considered, the VC structure is still effective for improving the stable range. In addition, the VC structure slightly affects flame tilt and temperature uniformity at the burner outlet when the thermal power is large. It is confirmed that the VC structure is effective for increasing the power adjustment range and reducing pollutant emissions at high power.

Heat transfer mechanism and emergency operating speed of moving train fires in a metro tunnel

The primary strategy for a burning train in metro tunnels is to drive it out of the tunnel, or to the station or safe area ahead. However, the heat transfer mechanism and the emergency operating speed of moving train fires in tunnels have not been addressed. In this study, a 1:10 train-tunnel model was designed to conduct a series of moving model experiments. The results show that with the increasing train speed in tunnels, the flame height decreases, the flame width and length first increase and then decrease, and the flame tilt angle increases. The maximum temperature at the train top increases by 81.7 % when the train speed increases from 2 to 6 m/s. A piecewise model is proposed to predict the maximum temperature at the train top. The maximum temperature rise at the tunnel ceiling is 17.4 K and exponentially decreases with the train speed, posing a limited threat to the tunnel structure. The convective heat flux received by the train accounts for 82.1 %–94.4 %. The heat transfer between the train, tunnel and flame is codetermined by the train-tunnel aerodynamic effect and fuel-rich zone. The emergency operating speed of the train is suggested to be 22.8 km/h when the heat release rate is 7.5 MW. This study innovatively reveals the combustion behavior of moving train fires in a metro tunnel and proposes guidelines for determining the emergency operating speed of burning trains. The results can provide references for the fire safety design and running capacity improvement of metro trains.

Experimental study on the mass burning rate and flame tilt angle of annular pool fires under cross air flow

Annular pool fire is a common type of accident that occurs in the early stages of a liquid fuel leakage incident. It often results in a large pool fire that can be challenging to control if not properly managed. This paper investigates the evolution of the mass burning rate and flame tilt angle of medium-scale annular pool fires (0.3 m ∼ 0.6 m) with different inner and outer diameter ratios (Din/Dout) under cross air flow speeds ranging from 0 to 5 m/s through experiments. It has been found that the mass burning rate of an annular pool fire is positively correlated with the cross air flow. With a maximum error of about 20%, the new burning rate model very closely matched the data from the literature. With an increase in cross air flow, the flame tilt angle gradually increases. Under the same cross air flow rate, the flame tilt angle decreases as the Din/Dout ratio of the annular pool increases. In this paper, a prediction model for flame tilt angle is established, considering the influence of the inner and outer diameters of the annular pool. And after comparing with reference data, it was found the maximum error is about 15%.

Experimental study on the morphology of turbulent diffusion flames with different aspect ratios under slope conditions

In this paper, the morphology of turbulent diffusion flames with varying aspect ratios under slope conditions was investigated. A simulated fire source using propane gas with an area of 80 cm2 was utilized. The influences of slope angle and aspect ratio of the fire source on the flame tilt angle (between the plume centerline and the slope), flame length, and flame height were systematically studied. The aspect ratio of the fire source (L/W) was changed from 1 to 8, and the slope angle was changed from 0° to 40°. Results indicate that the presence of a slope causes asymmetrical air entrainment on both sides of the fire plume, causing a pressure difference that leads to a decrease in the flame tilt angle as the slope angle increases. Additionally, the flame length increases monotonically with the slope angle due to the enhanced component of thermal buoyancy along the slope. Under the same slope condition, the flame tilt angle and flame height (perpendicular to the slope) decrease as the aspect ratio increases. This phenomenon is attributed to the increased entrainment restriction effect of the slope on the fire plume. New models for calculating flame tilt angle, flame length, and flame height, which consider the slope angle and aspect ratio of the fire source were established by using the equivalent perimeter of plume entrainment. To further verify the reliability of the newly established models, reference data with wide heat release rate (6.15 kW–231 kW), different slope inclination angle (0°–80°), pool size (equivalent diameter: 9 cm–44.1 cm), and different fuels (propane, ethylene) were used to against with the predicted values of our correlations. It is found that the maximum error of all correlations is within 15–20%, their universality and accuracy are further verified.

General models of buoyant sidewall flame morphology affected by crossflows on the inclined surface based on the mirror approach

This paper investigates the evolution of sidewall flame morphology on inclined surfaces subjected to crossflows. Experiments were conducted with 125 different experimental conditions using propane as the fuel. The study focuses on the sidewall flame tilt angle, flame length, and flame attachment length. The sidewall flame tilt angle is smaller than that of the non-sidewall flame since the boundary layer on the sidewall surface exerts horizontal friction on the flame. The sidewalls constrain the air entrainment of the flame causing the sidewall flame length to be greater than the non-sidewall flame length. The relationship between the sidewall flame attachment length and the non-sidewall flame attachment length is related to the extent of flame tilting and the friction in the vertical direction to which the flame is subjected. The mirror approach was utilized to quantify the impact of sidewalls on flames. General correlations were proposed for the sidewall flame tilt angle, flame length, and flame attachment length, by analyzing the forces acting on the flame. These prediction models were found to be in good agreement when compared to non-sidewall flame data from previous studies.

The effect of terrain-influenced winds on fire spread in QUIC-Fire

In this manuscript, we describe the implementation of the terrain-following version of QUIC-URB into QUIC-Fire and a demonstration of the impacts of terrain-influenced winds on QUIC-Fire-simulated fire spread. No changes to the underlying QUIC-Fire fire spread algorithm were made other than what was required to correctly account for the inclusion of terrain. This paper summarizes simulations used to understand how the QUIC-URB terrain-influenced winds affect upslope fire behavior without additional changes to the QUIC-Fire fire spread algorithm. Previously published FIRETEC results are compared to simulation results from the modified QUIC-Fire (incorporating terrain-influenced winds) that use the same topographies and fuels. QUIC-Fire results showed overall similar behaviors in terms of how the topographies affected fire shapes and trends in spread rates. Due to the terrain-following version of QUIC-URB being unable to generate flow separations at the crest of hills, fire spread rates in these regions across all non-flat topographies were over-predicted when compared to FIRETEC. Lateral fire growth showed similar trends with FIRETEC between topographies but did not capture the increase in spread due to a diagonal interface between grassland and forested fuel regions in the test domain. These simulations suggest possible refinements that are necessary to improve QUIC-Fire and thus guide ongoing efforts related to: how flame tilt angle is accounted for, the incorporation of non-local drag effects, and the inclusion of the wake-eddy parameterizations that are used in QUIC-URB.

An experimental study on the evolution of flame length and tilt angle under the joint effect of tunnel curved ceiling restriction and longitudinal ventilation

The model about flame length and fuel mass loss rate in this paper was developed based on the flame morphology with different longitudinal ventilation speeds when the flame was confined by the tunnel curved ceiling restriction. Secondly, after analyzing the experimental data, it was found that the air entrainment velocity was proportional to the 0.22nd power of longitudinal ventilation speed. Thirdly, the product of flame length and air entrainment velocity was proportional to the fuel mass loss rate, and the above models could better describe the characteristic evolution behaviors of flame length. Furthermore, the relation between fuel mass loss rate and longitudinal ventilation speed was established. Finally, this paper broadened the applicability of other scholars' flame tilt angle models, and found that the flame tilt angle is smaller when the flame is attached to the wall at the same wind speed. It is obvious that the above results supplement the classical tunnel-fire theory, which can be further applied to prevention and control of tunnel fire, and ultimately ensure building safety.

Experimental study on prediction models of diffusion flame geometry in moving fires

Flame geometry is a fundamental problem in assessing the thermal hazard during the transport process of hazardous chemicals, while the existing literature mainly focuses on stationary fires, with little understanding of flame behavior in moving fires. The objective of this study is to address the air entrainment mechanism of moving fires and derive the characterization models of relevant flame parameters. Dimensional analysis was used to determine the main factors influencing flame geometry in moving fires. A 1:10 scale burning car was then designed, and a series of moving model experiments were conducted to investigate the flame geometry. The results show that the flame height decreases monotonically with the increasing moving velocity, while the flame tilt angle gradually increases to a constant value. The variation of the flame width and the flame length includes three regimes: the decreasing regime, the transition regime, and the increasing regime. The evolution mechanism of the flame geometry can be attributed to the competition between the buoyancy and the inertia forces, which forms the bilateral, unilateral, and enhanced unilateral entrainment modes. Dimensionless correlations are provided to describe the flame geometries. Comparison between previous wind-blown flame models and the experimental data suggests that considerable error exists when studying moving fires by using wind tunnel experiments, despite their applicability to relevant aerodynamics studies.

Influence of sidewall restriction on flame characteristics of double fires in a channel

In this paper, the influences of sidewall restriction on flame characteristics of double fires in a channel were studied. Two typical fire scenarios (center fire and wall fire), different heat release rates and burner separation distances were considered. The results show that ΔT = 300 K can be used as the boundary temperature of the flame. Further, the merging phenomenon is more likely to occur for wall fires, and the critical flame merging separation distances (Sc) of center and wall fires are around 0.13∼0.16 and 0.21, respectively. The flame tilt angle decreases with the increasing fire separation distance, and it is not sensitive to HRR for both center and wall fires. The flame tilt angle of wall fire is slightly larger than that of center fire under a given fire separation distance. Finally, two prediction models are proposed to estimate the normalized effective flame lengths of impinging and non-impinging flames for the center and wall fires in a channel. This study will provide a basis for exploring the interaction mechanisms of double fires under restricted boundary conditions.

Experimental study on flame tilt angles of two unequal fires in a longitudinal ventilated tunnel

In historical tunnel fire accidents causing major casualties and property losses, the special fire phenomenon of multiple fires burning was usually accompanied. Other than the longitudinal ventilation in tunnel, the interaction burning of multiple fires will also make the flame tilt. Flame tilt angle is an important parameter to characterize the magnitude of acting forces on fires and it determines the possibility of ignition and flame spread. Former studies were mainly focused on multiple equal fires with same sizes, very limited attention has been paid on multiple unequal fires. Due to the different air entrainment mechanisms, the proposed correlations for flame tilt angle of multiple equal fires might be unsuitable for the multiple unequal fires. This paper presents an experimental study on flame tilt angles of two unequal square gaseous fires of propane in a 1/10 scale tunnel with longitudinal ventilation. Two square burners with different side lengths were located parallel and along the longitudinal centerline of tunnel. The effects of heat release rate of each burner, the burner edge spacing and the ventilation velocity were explored. Using the image processing method, the flame heights and tilt angles of upstream fire and downstream fire were determined. The results showed that due to the shielding effect of upstream fire acting on the downstream fire, the flame height of downstream fire is larger than the upstream fire, while the flame tilt angle of downstream fire is lower than the upstream fire. In addition, the flame heights of upstream and downstream fires are reduced with increasing the ventilation velocity, while the flame height of downstream fire is weakly affected by the separation distance. As the ventilation velocity increases, both the upstream and downstream flame tilt angles increase first and then maintain unchanged. The separation distance has little influence on the upstream flame tilt angle, while it affects the downstream fire more. Among the impact factors, the longitudinal ventilation plays a leading role of the flame tilt behaviors of two fires in tunnel. Based on the force analysis, it is found that the pressure thrust between two fires is much smaller than the buoyancy force and ventilation force. As a result, different correlations for predicting the flame tilt angles of two unequal fires in tunnel are developed based on the coupling effect of buoyancy force and ventilation force of each fire.