Convective Heat Release Rate Research Articles

Experimental study on the heat flux profile upon the building facade of the window-ejected fire plume

The window-ejected fire plume with the excess fuel released outside the opening poses great threat to the building facade due to the hazardous heat flux. This paper investigates experimentally the temperature inside the compartment, convective- and excess- heat release rate, facade flame height and the heat flux profile upon the building facade of the under-ventilated window-ejected fire plume. A set of experiments are carried out with a compartment-facade model consisting of a 0.40 m reduced cubic compartment and a vertical facade. An opening with various sizes is set at the center of the sidewall of the compartment. The temperature inside the compartment, convective- and excess- heat release rate, facade flame height and the heat flux profile upon the building facade are measured and quantified by thermocouples, camera and heat flux gauge array upon the building facade of the under-ventilated window-ejected fire plume for various openings and heat release rates. The center line heat flux could be correlated well with the non-dimensional height normalized by the flame height involving the correction factor. A global model on the heat flux profile upon the building facade incorporating the aspect ratio of the opening, flame height and the convective heat release rate, is developed to describe the transverse heat flux evolution in a general dimensionless form, and the proposed model correlates the experimental data well. The obtained experimental data and proposed model provide a basic understanding of the heat/thermal impact on the facade of the window-ejected fire plume due to excess fuel diffusion combustion.

Experimental study on temperature profile in long-narrow compartment fire with multiple lateral openings

The openings of a compartment under fire condition are very important to exhaust the hot smoke. However, the related impacts on the indoor environment are still not clear so far to guide future designs. Experiments were then conducted using a model long-narrow compartment with multiple lateral openings. The temperature inside the compartment and the temperature profile underneath the ceiling were investigated and compared with the predict ions of those previous models. Experimental results show that the lateral opening conditions mainly affect the temperature near the fire source, and this effect does not follow a single pattern with the change of the number of lateral openings. Keeping the openings near the fire source closed and symmetrically opening the openings away from the fire source can expand the fire hazard, while asymmetrically opening the openings on the side of the fire source can effectively reduce the temperature caused by the fire. The classic models for predicting the maximum temperature rise based on the confined space with open ends underestimate the experimental data in this work. An empirical model considering various fire sizes was developed to predict the temperature decay profile underneath the ceiling. The critical dimensionless distance and the attenuation index of the attenuation zone are both related to the convective heat release rate, showing power and linear functions, respectively. These findings and results can provide reference for the fire risk assessment of a long-narrow compartment with multiple lateral openings.

Dynamics of spray and swirling flame under acoustic oscillations : A joint experimental and LES investigation

The dynamics of spray swirling flames is investigated by combining experiments on a single sector generic combustor and large eddy simulations of the same configuration. Measurements and calculations correspond to a self-sustained limit cycle operation where combustion coupled by an axial quarter wave acoustic mode induces large amplitude oscillations of pressure in the system. A detailed analysis of the mechanisms controlling the process is carried out first by comparing the measured and calculated spray and flame dynamics. Considering in a second stage that the spray and flame are compact with respect to the acoustic wavelength the analysis can be simplified by defining state variables that are obtained by taking averages over the combustor cross section and representing the behavior of these average quantities as a function of the axial coordinate and time. This reveals a first region in which essentially convective processes prevail. The convective heat release rate then couples further downstream with the pressure field giving rise to positive Rayleigh source terms which feed energy in the axial acoustic mode. In the convective region, the swirl number features oscillations around its mean value with an impact on the flow aerodynamics and flame radial displacement. Fluctuations in the fuel flow rate are initiated at the injector exhaust and likewise convected downstream. The total mass flow rate that exhibits strong convective disturbances is dominated further downstream by the acoustic motion. This information provides new insights on the convective-acoustic coupling that controls the heat release rate disturbances and reveals the time delays governing the combustion oscillation process.

A simple correlation for predicting the rise time of fire-induced smoke flow in a tunnel with longitudinal ventilation

The rise time of the smoke flow fronts is an important parameter for fire detection. However, very limited literatures, if not none, have been available to report the predictive correlation or data for the rise time of fire-induced smoke flow in a longitudinally ventilated tunnel. In this study, theoretical analyses were conducted to deduce a simple mathematical correlation considering the effect of longitudinal crosswind disturbance. Computational fluid dynamics (CFD) with large eddy simulation (LES) method was performed to simulate the rising smoke flow with various heat release rates, source-ceiling heights, and wind velocities. The results show that the rise time of smoke flow in the presence of longitudinal crosswind was much larger than that in the absence of longitudinal crosswind, which may be attributed to the enhanced air entrainment that decelerated the rising smoke flow. The numerical results then served as the validation data for the proposed correlation, which confirmed that the smoke flow rise time varied as the −1/2 power of the convective heat release rate, 1/2 power of the ambient wind velocity, and 4/3 power of the height above the fire source. The proposed correlation was found to be capable of predicting the rise time of smoke flow in both windy and windless environments. The findings of this paper contribute to an improved understanding of rising smoke propagation in the tunnel under windy condition. It would also be beneficial to the design of fire detection system for the longitudinally ventilated tunnels.

Time-resolved fire heat release rate under a ceiling based on ceiling layer measurements

Measurement of time-resolved fire heat release rate (HRR) is important to characterize fire growth and suppression. Under certain conditions, such as large fire tests conducted under a ceiling with sprinkler protection, quantification of a time-resolved HRR is still a challenging problem. This work attempts to develop a new methodology of estimating convective and chemical HRRs for a fire under a ceiling. The instantaneous measurements of gas temperature, velocity and species concentration in the ceiling layer are used to calculate the HRRs. To verify the proposed method, two heptane pan fire tests were conducted under a ceiling and the chemical HRR was calculated from the fuel mass loss data. The results demonstrate that the proposed method can provide a time-resolved chemical HRR that is comparable to that obtained from the fuel mass loss rate. The fraction of convective HRR to chemical HRR also agrees well with the test data obtained under a calorimeter. The application to rack-storage fires under a ceiling with sprinkler suppression shows that the proposed method can provide the fire growth in the free-burn period and consistent trends for the fire development with water application.

Large Eddy Simulations of a Set of Experiments with Water Spray-Hot Air Jet Plume Interactions

Large eddy simulations of water spray-hot air jet plume interactions, as obtained with FireFOAM 2.2.x, are presented. Three hot air jet plumes, with thermal powers of 1.6, 2.1 and 2.6 kW, are examined, interacting with a water spray with discharge rate of 0.084 lpm. A systematic comparison between simulations and experiments involving only the hot air jet plumes, the water spray alone and the combination of the two has been performed in order to evaluate the predictive capabilities of FireFOAM. Overall, the code is capable of predicting well the mean values of the hot air jet plumes but deviations are evident for the rms values. Discrepancies in the predictions of the volume fluxes in the near-field for the water spray alone case are observed if the experimentally reported injection angle is used. Improvements are observed if the injection angle is modified based on the experimentally reported data in the near-field. The interactions between the hot air jet plumes and water sprays, are characterized by the location of the interaction region. The interaction boundary moves up from the base of the plume by increasing the convective heat release rates. The simulation results follow the experimental trend but deviate up to 26% due to the differences in the predicted hot air jet plumes and spray characteristics.

An Efficacy Evaluation of Water Mist Protection Against Solid Combustible Fires in Open Environment

CEN and NFPA standards have accepted water mist for protecting solid combustible fires more challenging than light hazard fires in open environment, but with little data in the public domain to support such applications. Therefore, a series of exploratory experiments was conducted to evaluate the efficacy of fire suppression in open environment by water mist sprays for a rack storage of corrugated cardboard cartons on wood pallets (Fire A), and for a palletized storage of cartoned plastics stacked on wood pallets (Fire B). Both are in the accepted hazard categories by both standards. Representative water mist sprays were employed to provide water application densities of 4.1 mm/min and 6.1 mm/min for Fire A, and 8.1 mm/min for Fire B. These sprays had downward thrust forces ranging from 18 N to 73 N per spray, and volume median droplet diameters from 75 µm to 345 µm. A standard ½-in. pendent sprinkler was also employed to provide the suppression performance references for these two fire hazards. In each experiment, four open sprinklers or nozzles were centered 1.7 m above the fuel array with nozzle spacing ranging from 2.6 × 2.6 m to 3 × 3 m. All experiments were conducted under a 20-MW calorimeter with no ceiling presence. Water discharge in each experiment was started at a designated fire convective heat release rate of 1000 kW. The experiments indicated that, for the same application density, not all water mist sprays having different characteristics would lead to a comparable fire suppression result. Therefore, each water mist system should be tested individually for the same intended protection. Furthermore, the results showed that when water mist protection was effective, it required application densities comparable to those of sprinkler protection to provide comparable fire suppression results in open environment under the present experimental conditions.

Driving force for preventing smoke backlayering in downhill tunnel fires using forced longitudinal ventilation

The critical ventilation velocity plays an important role in smoke control in longitudinally ventilated tunnel fires. The magnitude of the critical velocity has been widely studied. In the present study, we carried out theoretical analyses and numerical simulations to investigate the driving force necessary for achieving the critical velocity in downhill tunnels. Theoretical models suggest that buoyant source location has a considerable effect on the condition for achieving the critical velocity. For a constant convective heat release rate of the fire, the driving force necessary for achieving the critical velocity increases with the height difference between the fire source and the outlet of the smoke. According to FDS simulation results, with the increase of the height difference between the fire source and the smoke outlet, the pressure loss induced by the stack effect increases significantly; however, the variation in the ventilation critical velocity is insignificant. Therefore, the variation in the driving force is mainly because of the stack effect rather than the variation in the critical velocity. In addition, the effects of the fire occurrence on the longitudinal ventilation velocity and pressure distribution in the tunnel are also numerically studied. If the pressure rise of the jet-fan keeps constant, the longitudinal velocity declines remarkably after the fire occurrence as a result of the pressure loss induced by the fire and the pressure loss induced by the stack effect. The phenomenon presented in the paper has implications for both tunnel ventilation and smoke control in inclined tunnels.

An experimental study on vertical temperature profile of facade fire plume ejected from compartment with an opening subjected to external wind normal to facade

This paper investigated the vertical temperature profile of facade fire plume ejected from compartment with an opening subjected to external wind normal to the facade, which has not been quantified in the literature. Experiments were carried out using a cubic compartment (dimensions: 0.4 m) with an opening attached by a vertical facade. The vertical temperature profile in the facade fire plume under different ventilation conditions (opening geometries) and wind speeds was measured by thermocouple arrays consisting of 42 K-type thermocouples installed along the facade wall. It was found that the temperature at a given height decreased with increasing wind speed, as the air entrainment into the plume was stronger than that without wind. Then, a correlation was proposed based on physical analysis on the variation of total convective heat release rate (virtual origin) and air entrainment (correlation coefficient) into the facade fire plume, to describe the front wind effect on the vertical temperature profile. Experimental data for different opening geometries (ventilation factor) and wind speeds can be well correlated by the proposed correlation. The new findings and the proposed correlation provide a fundamental base for understanding basically the facade fire behavior with an opening under external wind normal to facade.

Analysis on ventilation pressure of fire area in longitudinal ventilation of underground tunnel

In order to solve the problem of ventilation pressure loss in the fire area under the fire condition, the wind pressure loss model of the fire area is established based on the thermodynamic equilibrium relation. The semi-empirical calculation formula is obtained by using the model experiment and CFD simulation. The validity of the formula is verified. The results show that the ventilation pressure loss in the fire zone is proportional to the convective heat release rate at the critical velocity, which is inversely proportional to the upstream ventilation velocity and the tunnel cross-sectional area. The proposed formula is consistent with the law of the tunnel fire test fitting formula that results are close, in contrast, the advantage lies in a clear theoretical basis and ventilation velocity values. The resistance of road tunnel ventilation system is calculated accurately and reliably, and then an effective emergency ventilation operation program is developed. It is necessary to consider the fire zone ventilation pressure loss. The proposed ventilation pressure loss formula can be used for design calculation after thorough verification.

Temperature and velocity distributions from numerical simulations of ceiling jets under unconfined, inclined ceilings

Numerical simulations of ceiling jets under unconfined, inclined ceilings were conducted with the open-source code FireFOAM. A range of ceiling inclinations, 0–30° was considered with a 14kW convective heat release rate (HRR) heptane fire used as the plume source, and the ceiling mid-point clearance from the top of the 0.228m diameter burner kept fixed at 0.89m. The predicted temperature and velocity in the developing ceiling jets were compared against the experimental data and empirical correlations. Temperature and velocity predictions on the elevated side of the ceiling are in general agreement with experimental data. Flow reversal in the lower side of the ceiling was predicted with good confidence, and comparison with experimental data was found to be reasonable. Following existing convention in the literature, the predicted results were non-dimensionalized using the convective HRR, ceiling height and radial distance from the ceiling mid-point. Comparison of the non-dimensional data on the elevated ceiling side showed better agreement for temperature against the correlation, whereas predicted velocity data showed a wider spread around the correlation values.

Inverse design of underfloor heating power rates and air-supply temperature for an aircraft cabin

Thermal boundary conditions in commercial airliner cabins are crucial for creating a comfortable cabin environment. Cabin temperature distributions depend on the thermo-fluid boundary conditions of the boundary walls and the air supply. This paper proposed a combined inverse-forward model for designers to determine the total underfloor heating rates and the air-supply temperature in an aircraft cabin. The contribution ratio of indoor climate (CRI) is applied to describe the cause–effect relationship between the boundary wall convective heat release rates and the resulting temperature rise at certain points, which can be cast into a matrix. The solution contains three sub-models: (i) regularized inversion of the cause–effect matrix with the target cabin air temperatures as the known input, which solves the convective heat rates of the underfloor heaters and the air-supply temperature, (ii) solution of underfloor heater's surface temperatures based on Newton's law of cooling, and (iii) computation of the radiative heat rates. The above model was used to determine the total underfloor heating rates and air-supply temperature in a single-aisle aircraft cabin. The design targets are to create an average temperature of 24 °C near the upper human body and 26 °C at the ankle level. An experimental test was conducted in a simplified half section of an aircraft cabin for model validation. The results show that the proposed methodology is able to provide the total underfloor heating rates and air-supply temperature in good agreement with the measurement data and forward-simulation boundary conditions.

헵탄의 화원 직경 변화를 고려한 대형콘칼로리미터의 발열량 측정 결과 분석

The present study has been conducted to analyse the effects of various pool diameters on the measurement of heat release rate (HRR) of heptane fire using large scale cone calorimeter (LSC). The burning rate which is the major parameter for HRR compared with the previous model suggested by A. Hamins. The combustion efficiency for heptane by oxygen consumption method is about 91%, which is almost same with the previous results of 92% suggested by J. Gore. The convective HRR by enthalpy consumption method was 54% lower than HRR by oxygen consumption method. This results are practical use for establishing the reliability of heat release rate for fire experiment.

Characterization of interactions between hot air plumes and water sprays for sprinkler protection

One important aspect of the complex sprinkler protection process is the interaction between the water spray and the fire plume. In order to provide suitable data for the development and validation of a LES-based fire protection models, such as FireFOAM, a series of small-scale experiments were conducted to examine the interaction of hot air plumes and water sprays through combined gas–liquid velocity and droplet size measurements. Laser-based particle image velocimetry (PIV) was used to acquire the spatially-resolved velocity data; and a shadow imaging system (SIS) was used to measure the water droplet size and volume flux. Hot air plumes with three convective heat release rates (1.6, 2.1 and 2.6kW) were selected to interact with a water spray at a discharge rate of 0.084Lpm. The velocity field of the hot air plume and ceiling flow with/without water spray, the droplet size and volume flux of water spray with/without hot air plume were measured. The interaction between the hot air plume and water spray was characterized by the location of the interaction boundary with the momentum ratio of the hot air to that of the spray. The results showed that that the momentum ratio and the evaporation effect due to hot air on the water droplets played a significant role to change the interaction structure and the ceiling layer pattern.

Large-Scale Water Spray and Water Mist Fire Suppression System Tests for the Protection of Ro–Ro Cargo Decks on Ships

This paper summarises a series of large-scale fire suppression tests conducted to simulate a fire in the trailer of a heavy goods freight truck on a roll-on roll-off (ro–ro) cargo deck. The tests were conducted with a traditional deluge water spray system as well as a deluge high-pressure water mist system. Parameters such as the water discharge density, the system operating pressure, the nozzle K-factor and whether the fire was fully exposed to the water spray or shielded were varied. The total and convective heat release rate of the fire was measured in order to determine the fire suppression and fire control capabilities of the tested systems. Test results indicate that a water discharge density of at least 10 mm/min is necessary to provide fire suppression of a fire in a heavy goods freight truck, whilst 5 mm/min would provide fire control. Furthermore, the test results indicate that a high-pressure water mist system would require higher flow rates as compared to a traditional water spray system in order to provide fire control.

Simple Analysis of Flame Dynamics via Flexible Convected Disturbance Models

DOI: 10.2514/1.B34405 Flame sheet modeling is a common approach for the determination of flame transfer functions for prediction and modeling of thermoacoustic combustion instabilities. The dynamics of the flame-sheet model for simple flame geometries can be shown to be equivalent to a basic model of convective disturbances interacting with a steady heat release region. This framework shows that the flame transfer functions predicted by linearized flame-sheet models are the Fourier transform of the steady heat release rate profile for the flamesheet geometry transformed into a Lagrangian convective time reference frame. This result is significant relative to existing flame-sheet modeling approaches in allowing the prediction of dynamic behaviors on the basis of steady information only. Multiple perturbations on the flame can be treated simply via superposition of individual perturbations. Analysis of results from these convective disturbance models illuminates the existence of two independent length scales governing the flame transfer function dynamics. Magnitude is governed by the tip-to-tail length of the flame, whereas phase is governed by the heat release rate profile center of mass calculated from the disturbance origin. The convective disturbanceapproachshowspromiseinitspotentialtoderive flametransferfunctionpredictionsfromasteady flame heat release rate profile.

Sprinkler protection of non-storage occupancies with high ceiling clearance

Five large-scale fire tests were conducted to investigate the effects of ceiling clearance, sprinkler type and sprinkler discharge rate on sprinkler fire control performance of a non-storage high-ceiling-clearance scenario. The fire scenario selected for this study is to simulate fire loads of either sale booths or exhibition booths in non-storage occupancies under a high ceiling. Three ceiling heights were employed: 12, 16 and 18m. Two Large Orifice (LO) sprinklers, an Extra Large Orifice (ELO) sprinkler and a K363 Control Mode Specific Application (CMSA) sprinkler were used. All the sprinklers tested are pendent sprinklers with a nominal temperature rating of 70°C. The sprinklers were installed at 3m×3m spacing. The effectiveness of fire control of the sprinklers depends on ceiling height, sprinkler spray pattern, water drop size (as related to orifice size and discharge pressure), and sprinkler discharge rate. The test results show that the standard-response LO sprinklers at a discharge rate of 141L/min per sprinkler did not control the test fire under a 12 m high ceiling and sprinkler skipping occurred. For a ceiling height of 16 m, the quick-response LO sprinklers discharging at a rate of 182L/min per sprinkler were not effective in controlling the test fire and sprinkler skipping also occurred; however, the standard-response ELO sprinkler discharging at a rate of 209L/min was marginally effective in controlling the test fire. For a ceiling height of 18 m, the K363 CMSA sprinklers discharging at a rate of 363L/min per sprinkler either controlled or suppressed the test fire, because the sprinkler generates considerably large water drops and provides sufficient high discharge rate. The test series has demonstrated that water drop size is an important factor for achieving fire control or suppression of the high-ceiling-clearance fire scenario. With regard to the fire size at sprinkler actuation, the convective heat release rate of the fire is estimated from the gas temperature measured at ceiling level directly over ignition by using a plume correlation. Using a sprinkler with a RTI of 130 (m⁎s)1/2 under the 18 m high ceiling, the fire size at first sprinkler actuation was estimated to be 7-8 MW. The test results provide a technical basis for developing sprinkler installation rules for non-storage high-ceiling-clearance occupancies in the revised Chinese National Code of Designs for Sprinkler Systems.

Heat Lost of Smoke Layer at Quasi-Steady State of Mechanical Exhaust

Under mechanical extraction in a fire room, a quasi-steady state will be eventually formed based on a two-layer zone concept. Heat loss through solid boundaries is a key factor which determines the temperature of smoke layer at quasi-steady state and will be discussed in this paper. A simplified model on heat loss of smoke layer is proposed. Calculation is performed and results are plotted. It is indicated that the ratio of heat loss through solid boundaries to the convective part of fire power, χ, varies under different exhaust rates and heat release rates of the fire. Heat loss of smoke layer through solid boundaries should be considered and calculated using heat transfer theory, instead of taking as a constant part of convective heat release rates of the fire.

CFD study of relation between ventilation velocity and smoke backlayering distance in large closed car parks

When fire occurs in a car park, smoke is often extracted by means of a horizontal ventilation system. To design this system, a necessary extraction velocity is needed, in order to guarantee a certain smoke clearance in the car park. In the present paper, a large number of car park CFD simulations are performed for use as ‘numerical experiments’. Four parameters are varied (convective heat release rate per unit area, fire source area, car park height and car park width). From the parameter variation, three formulae are proposed for the calculation of the required extraction velocity of hot smoke, corresponding to a certain smoke backlayering distance. These formulae are only valid for large closed car parks with flat ceiling and uni-directional smoke and ventilation flow pattern. Agreement with full-scale car park experiments confirms the expressions for the studied configuration.

A Numerical Study on the Estimation of Heat Release Rate Based on the Flow Field through the Doorway

The present study has been conducted to examine the possibility of estimation of heat release rate using doorway flow in the real structure fire. As a preliminary study to quantify the heat release rate based on the flow field through the opening, a series of CFD calculations for the ISO-9705 room has been performed and compared with experiments. Prior to the full numerical simulation, the grid independence test was performed to optimize the mesh size, and then the doorway flow calculated by the CFD analysis is compared with PIV and bi-directional probe measurements. The chemical and convective heat release rate was obtained by integration of individual cell values through the doorway and the chemical heat release rate by the CFD model was matched well with the nominal heat release rate within 8 %. The convective heat release rate through the doorway shows reasonable agreement with the nominal heat release rate having maximum discrepancy of 20 % when considering the heat loss effect. In this matter, this work improves our understanding of heat release rate measurement by doorway flow.