Heat Release Measurements Research Articles

An Experimental Validation of Heat Release Rate Fluctuation Measurements in Technically Premixed Flames

Understanding the effects of inlet velocity and inlet equivalence ratio fluctuations on heat release rate fluctuations in lean premixed gas turbine combustors is essential for predicting combustor instability characteristics. This information is typically obtained from independent velocity-forced and fuel-forced flame transfer function measurements, where the global chemiluminescence intensity is used as a measure of the flame's overall rate of heat release. Current lean premixed combustors operate in a technically premixed mode where the flame is exposed to both velocity and equivalence ratio fluctuations and, as a result, the chemiluminescence intensity does not provide an accurate measure of the flame's rate of heat release. The objective of this work is to experimentally assess the validity of a technique for measuring heat release rate fluctuations in technically premixed flames based on the linear superposition of fuel-forced and velocity-forced flame transfer function measurements. In the absence of a technique for directly measuring heat release rate fluctuations in technically premixed flames, the heat release rate reconstruction is validated indirectly by comparing measured and reconstructed chemiluminescence intensity fluctuations. The results are reported for a range of operating conditions and forcing frequencies which demonstrate the capabilities and limitations of the heat release rate reconstruction technique. A variation of this technique, referred to as a reverse reconstruction, is also proposed, which does not require a measurement of the fuel-forced flame transfer function. The results obtained using the reverse reconstruction technique are presented and compared to the results from the direct reconstruction technique.

Replenish and relax: explaining logarithmic annealing in ion-implanted c-Si.

We study ion-damaged crystalline silicon by combining nanocalorimetric experiments with an off-lattice kinetic MonteCarlo simulation to identify the atomistic mechanisms responsible for the structural relaxation over long time scales. We relate the logarithmic relaxation, observed in a number of disordered systems, with heat-release measurements. The microscopic mechanism associated with this logarithmic relaxation can be described as a two-step replenish and relax process. As the system relaxes, it reaches deeper energy states with logarithmically growing barriers that need to be unlocked to replenish the heat-releasing events leading to lower-energy configurations.

트레이용 난연 전력 케이블의 화재특성에 관한 실험적 연구

The present study has been conducted to investigate the fire combustion properties and fire behavior of an IEEE-383 qualified flame retardant cable. The reference reaction rate and reference temperature which are commonly used in pyrolysis model of fire propagation process was obtained by the thermo-gravimetric analysis of the cable component materials. The mass fraction of FR-PVC sheath abruptly decreased near temperature range of <TEX>$250{\sim}260^{\circ}C$</TEX> and its maximum reaction rate was about <TEX>$2.58{\times}10^{-3}$</TEX>[1/s]. For the XLPE insulation of the cable, the temperature causing maximum mass fraction change was ranged about <TEX>$380{\sim}390^{\circ}C$</TEX> and it has reached to the maximum reaction rate of <TEX>$5.10{\times}10^{-3}$</TEX>[1/s]. The flame retardant cable was burned by a pilot flame meker buner and the burning behavior of the cable was observed during the fire test. Heat release rate of the flame retardant cable was measured by a laboratory scale oxygen consumption calorimeter and the mass loss rate of the cable was calculated by the measured cable mass during the burning test. The representative value of the effective heat of combustion was evaluated by the total released energy integrated by the measured heat release rate and burned mass. This study can contribute to study the electric cable fire and provide the pyrolysis properties for the computational modeling.

Air pumping action of a plume in a room fire

Air pumping effect of a fire plume to give higher intake rate through vertical openings in a post-flashover room fire will be discussed in this paper. The thermal balance equation was set up with known fire phenomena in a room. The hydrostatic model was applied to study the air intake rate through vertical openings. An equation relating heat release rate to room air temperature rise with empirical constants was then justified by reported experimental data on post-flashover room fire. The heat release rate was measured by the oxygen consumption method in that experiment. The predicted heat release rate from the empirical equation reported in the literature was observed to be proportional to the room air temperature rise as derived from hydrostatics. However, the proportionality constant is lower than the experimental value. A possible explanation is due to neglecting another fire phenomenon on air pumping action of the fire plume in a real room fire. Higher pressure differences across the door would give higher airflow rates across an opening. This would supply more air to give higher heat release rate as observed in the experiment. In this paper, the pressure due to air pumping of the fire plume is taken as a proportion of the hydrostatic pressure due to temperature differences between the upper hot layer and lower cool layer. Comparing the measured heat release rate with the estimated heat release rate due only to hydrostatics will give the air pumping action. The possible increase in heat release rate in a post-flashover fire can then be estimated accordingly.

Simultaneous measurement of localized heat-release with OH/CH2O–LIF imaging and spatially integrated OH∗ chemiluminescence in turbulent swirl flames

In practical flames such as gas turbine combustors, spatially-integrated OH∗ chemiluminescence (CL) is frequently used as a heat release rate (HRR) indicator – which has been questioned by some authors to be restricted to flames of a limited range of equivalence ratios and low Reynolds numbers – while in lab flames the approach of combined detection of OH and H2CO via LIF is an accepted diagnostic technique. Even when using specialized optics with limited acceptance angle the first method is spatially integrating while the second one allows for spatially resolved imaging. In the present work we retrieved simultaneously HRR-based information via both techniques from the same spatial flame volume, i.e., OH∗–CL radiation is collected exclusively from within the light sheet volume cutting through the flame for LIF imaging. Turbulent premixed swirl flames were investigated with a thermal power up to 30kW to shed light on the still unresolved question if correlations exist between signal intensities derived from both methods in turbulent flames. Measurements were performed in methane/air flames with Reynolds numbers between 6900 and 10,000, equivalence ratios between 0.8 and 1.2, and with a replacement of 20vol% of methane by hydrogen. Although scatter plots of HRR vs. CL intensities cluster in certain regions depending on flame conditions, their large scatter shows that correlations are weak, probably caused by flame stretch and curvature. Depending on flame conditions, correlation coefficients to characterize the scatter plots range between 0.45 and 0.81.

반밀폐된 구획에서 발열량 변화에 따른 비정상 화재특성

반밀폐된 구획에서 발열량 변화에 따른 환기부족화재의 비정상 열 및 화학적특성에 관한 실험적 연구가 수행되었다. 이를 위해 실규모 ISO 9705 표준화재실의 출입구 폭이 0.1 m로 축소되었으며, 구획 중앙에 설치된 분사노즐을 통해 헵탄 연료의 유량은 선형적으로 증가되었다. 구획 내부의 온도, 열유속, 화학종농도 그리고 발열량의 연속적 측정이 이루어졌으며, 비정상 열 및 화학적특성을 표현하기 위하여 총괄 당량비의 개념이 적용되었다. 주요 결과로서, 총괄 연소효율과 구획 내부의 국부 연소효율은 시간에 따라 매우 다른 결과를 보이며, 이론과 측정된 발열량을 통한 총괄 당량비 역시 큰 차이를 보이게 된다. 온도, 열유속 및 화학종 농도의 시간에 따른 변화는 총괄 당량비 개념를 통해 표현될 수 있었다. 구획 내부에서 CO의 농도는 총괄 당량비 2.0~3.0의 범위에서 최대값을 보이며, 보다 과농한 조건에서 점차적으로 감소되는 현상이 발견되었다. 추가로 본 실험을 통해 얻어진 반밀폐된 구획화재의 비정상 열 및 화학적 거동은 향후 현실적인 화재 모델링을 위한 검증 데이터로 활용될 수 있을 것으로 기대된다. An experimental study was conducted to investigate the effects of change in heat release rate on unsteady fire characteristics of under-ventilated fire in a semi-closed compartment. A standard doorway width of the full-scale ISO 9705 room was modified to 0.1 m and the flow rate of heptane fuel was increased linearly with time using a spray nozzle located at the center of enclosure. Temperature, heat flux, species concentrations and heat release rate were continuously measured and then global equivalence ratio (GER) concept was adopted to represent the unsteady thermal and chemical characteristics inside the compartment. It was observed that there was a significant difference in unsteady behavior between global and local combustion efficiency, and the GERs predicted by ideal and measured heat release rate were also shown different results in time. The unsteady behaviors of temperature, heat flux and species concentrations were represented well using the GER concept. It was important to note that CO concentration was gradually decreased with the increase in GER after reaching its maximum value in the range of 2.0~3.0 of global equivalence ratio. In addition, the experimental data on unsteady thermal and chemical behaviors obtained in a semi-closed compartment will be usefully used to validate a realistic fire simulation.

Determination of Heat Release Rate Disturbances in Unconfined Flames Based on Fluctuations in the Travel Time of Ultrasonic Waves

The article presents a new method to measure heat release rate disturbances from flames when optical access is limited. The technique is based on the determination of the travel time of ultrasonic waves propagating through the flow. The link between heat release rate and sound travel time disturbances depends on the configuration considered. An expression is established here for the case of unconfined premixed flames driven by buoyancy forces associated with the Kelvin–Helmholtz instability formed by the interaction between accelerating hot burned gases and cold ambient air at rest. The system and the principle used for the determination of the sound travel time are then presented and validated under nonreacting conditions. Effects of the main parameters on the precision of this detection technique are examined experimentally. Measurements in reacting conditions are compared to heat release rate data obtained with another technique based on the chemiluminescence emission. A good agreement is obtained between both signals for the different cases explored demonstrating the sensitivity of the proposed technique.

Simultaneous measurement of localized heat release with OH/CH2O-LIF imaging and spatially integrated OH∗ chemiluminescence in turbulent swirl flames

The in-situ and localized observation of heat release in turbulent flames is important for the validation of computational modeling of turbulent flows with combustion. In the present work we obtain localized information on heat release rate (HRR) by the commonly accepted technique of the simultaneous and single-shot planar imaging of OH and CH2O concentrations by laser-induced fluorescence (LIF). Additionally, we combine this with the simultaneous line-of-sight and temporally resolved chemiluminescence detection of OH∗, spatially integrated within the flame volume, interrogated by the laser sheets used for the HRR imaging technique. The combined diagnostic methods are demonstrated for a swirl-stabilized, premixed turbulent methane/air flame of 30-kW thermal power, and they show the existence of correlations between both HRR-sensitive diagnostic techniques.

Cinematographic OH-PLIF measurements of two interacting turbulent premixed flames with and without acoustic forcing

This paper describes an experimental investigation into the interactions that occur between two lean turbulent premixed flames stabilised on conical bluff-bodies when they are moved closer together. Cinematographic OH-PLIF measurements were acquired to investigate adjacent flame front interactions as a function of flame separation distance ( S). Flame surface density (FSD) and curvature were determined to characterise the unforced flames. Acoustic forcing was then applied to explore the amplitude dependent thermo-acoustic response. Phase-averaged FSD and global heat release measurements in the form of OH ∗ chemiluminescence were obtained for a range of forcing frequencies ( f) and amplitudes ( A) as a function of S. As the flames were brought closer together the adjacent annular jets were found to merge into a single jet structure. This caused adjacent flame fronts to merge above the wake region between the two flames at a location determined by the jet efflux (flame angle) and S. This region of flame–flame interaction we refer to as ‘interacting region’. In the unforced flames, a trend of increasingly negative curvature for decreasing S produced a small net increase in flame surface area via cusp formation. When subjected to acoustic forcing, S-dependent regimes were found in the global heat release response as a function A. The overall trend showed that the occurrence of jet/flame merging reduces the value of A at which non-linear response occurs. In support of previous findings for flames stabilised along shear layers, the phase-averaged FSD showed that the flame dynamics that drive the thermo-acoustic response result from the roll-up of vortices which generate large-scale vortex–flame interactions. Compared with axisymmetric flames, the occurrence of jet merging alters the vortex–flame interactions resulting in an asymmetric contribution to the heat release between the wall and interacting regions. The majority of the heat release was found to occur in the interacting region through the rapid production and destruction of flame surface area. The occurrence of jet merging and large-scale interactions between adjacent flames result in different physical mechanisms that drive the thermo-acoustic response compared with single axisymmetric flames.

Passive control of premixed lifted flame in a dump combustor

This paper reports an experimental investigation of the dynamics of premixed lifted flame inside a dump combustor with tapered exit. The pressure and heat release measurements along with flame visualization were carried out to understand the combustion phenomenon inside the combustion chamber. High levels of combustion driven oscillations in pressure were observed in the lifted flame due to the coupling of the heat release and flow oscillations. The dominant mode of oscillations was seen to be governed by the convection of hot burned gases into the combustion zone due to flow reversal from the exit section. The fluctuations of pressure decreased in the presence of a stabilizing rod due to mistuning of heat release and pressure oscillations resulting in a decrease in their correlation and coupling as the dominant frequency of oscillation in the presence of rod resembled the frequency of vortex shedding from the rod. Consequently, a 4dB decrease in overall sound pressure level could be achieved by passive control in the dump combustor.

Influence of decreased sample size on cone calorimeter results

SUMMARYCone calorimeter sample size sensitivity and the influence of sample size on the results were studied in order to investigate the possibility to use smaller samples than prescribed by the standard. The influence of the position of the sample on the sample holder of the decreased samples was also studied. Samples of birch plywood, industrial two‐component polyurethane adhesive and polypropylene wax were tested. The results from standard size sample experiments, 100 cm 2, and decreased sample size, 50 and 25 cm2, were compared, in order to investigate the sample size influence on the results. The experiments were performed using an external heat flux of 50kW/ m2. The results show differences between the sample types. The measured heat release rates for plywood and polyurethane adhesive indicate that the sample size is possible to down‐scale, while the polypropylene wax results show the opposite. All sample types and sizes show constant results for the effective heat of combustion, indicating down‐scalability. For in‐house testing and screening of materials, downscaling can be used for most sample types. However, results from experiments carried out with decreased sample size should not be compared to results from experiments executed according to the standard procedure. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Precision of the Cone Calorimeter and ICAL Test Methods

Repeatability and reproducibility are typically determined for standard fire test methods on the basis of an interlaboratory test program (round-robin) to define their precision. Significant differences in precision were found in the past between the repeatability and reproducibility of small-scale and intermediate- and large-scale fire test methods that all utilize oxygen calorimetry to measure heat release rate. Repeatability of heat release rate related measurements with small-scale apparatus has been found to be significantly better than the repeatability of the intermediate and large-scale apparatuses. In this paper, the results of two round-robins are compared on the basis of relative repeatability standard deviations and relative reproducibility standard deviations calculated for individual materials for individual test parameters. The first one was a cone calorimeter round-robin and the second was an intermediate-scale calorimeter (ICAL) round-robin. The objective and subjective factors that might have contributed to the differences between the two test methods in both repeatability and reproducibility were analyzed. The most important factors that caused the differences in the ICAL round-robin were higher theoretical uncertainty of the ICAL, inadequate pre-round-robin calibrations, the small numbers of participating laboratories and samples tested, prevalence of fire-retardant-treated materials, and possible failure of some of the laboratories to fully comply with the standard. These factors, especially the fact that the testing procedures and apparatuses in the participating laboratories may not have been in complete accordance with the standards, indicate that the results of the round-robins did not reflect the real precision of the test methods. However, the terminology using the two components of precision, repeatability and reproducibility, is maintained in this paper except where a specific distinction is noted. Recommendations are made on how to improve the results of future fire test round-robins.

Characteristics of cyclic heat release variability in the transition from spark ignition to HCCI in a gasoline engine

We study selected examples of previously published cyclic heat-release measurements from a single-cylinder gasoline engine as stepwise valve timing adjustments were made to shift from spark ignited (SI) combustion to homogeneous charge compression ignition (HCCI). Wavelet analysis of the time series, combined with conventional statistics and multifractal analysis, revealed previously undocumented features in the combustion variability as the shift occurred. In the spark-ignition combustion mode, the heat-release variations were very small in amplitude and exhibited more persistent low-frequency oscillations with intermittent high-frequency bursts. In the HCCI combustion mode, the amplitude of the heat-release variations again was small and involved mainly low-frequency oscillations. At intermediate states between SI and HCCI, a wide range of very large-amplitude oscillations occurred, including both persistent low-frequency periodicities and intermittent high-frequency bursts. It appears from these results that real-time wavelet decomposition of engine cylinder pressure measurements may be useful for on-board tracking of SI–HCCI combustion regime shifts.

Spatial resolution of a chemiluminescence sensor for local heat-release rate and equivalence ratio measurements in a model gas turbine combustor

The spatial resolution of a Chemiluminescence Sensor, based on focused Cassegrain optics, to detect the location of the reaction zone and heat-release rate in a model gas turbine combustor is reported. The sensor measures simultaneously the chemiluminescent intensities from OH* and CH* excited radicals in flames in order to obtain information on the local flame characteristics. The spatial resolution was evaluated by a combined theoretical and experimental study in laminar and turbulent flames and was supported by detailed chemistry calculations, including the chemiluminescent species, of unstrained one-dimensional flames. The experimental study involved simultaneous measurements of chemiluminescence with the sensor and laser-based reaction rate imaging, using the product of OH and CH2O radicals obtained from planar laser-induced fluorescence (PLIF), and OH PLIF for the location of the reaction zone. The study quantified the influence of flame shape and dimensions and the direction of traverse of the focal region of the sensor through the flames on the spatial resolution, thereby identifying the limitations and optimising the applicability of the sensor. The sensor was used to obtain local time-dependent measurements of heat-release and equivalence ratio of a reacting mixture, based on the chemiluminescent intensity ratio of OH*/CH*, in a swirl-stabilised model gas turbine combustor and quantified the degree of air–fuel premixedness, probability of reaction and power spectra of pressure and chemiluminescent intensity fluctuations in two unsteady flames.

CFD Investigation of Large Scale Pallet Stack Fires in Tunnels Protected by Water Mist Systems

A series of full-scale fire suppression tests was conducted at the San Pedro de Anes test tunnel facility near Gijon, Asturias, Spain in February 2006. The fuel was wooden pallets or a mixed load of wood and high density polyethylene pallets. Fire protection was provided by water mist systems in different configurations. Because of facility restrictions, some scenarios of great interest, such as a free burn fire, could not be investigated. However, in order to complement the experimental results, a number of computational fluid dynamics simulations were conducted on a 140 m section of the tunnel facility. The Fire Dynamics Simulator, version 4, was used for the numerical investigation. An algorithm was developed to allow the fire to spread along the top of a series of pallet loads in such a way that the measured heat release rate was reproduced. Verification and validation studies confirmed that the model predicted the measured ventilation speeds and peak temperatures. The agreement between the simulations and the field measurements was very good prior to activation of the water mist. Back-layering was modeled well. After activation of the mist, the simulations predicted a large drop in gas temperatures, and retreat of the back-layer, but under-predicted the thermal cooling by the water mist downstream of the fire. With the suppression system, high temperatures and heat fluxes were limited to the immediate vicinity of the burning pallets. The model was then used to simulate a free burn fire in the tunnel. The simulation demonstrated the catastrophic conditions created by an unsuppressed fire in a tunnel when compared against the thermally managed conditions under suppressed conditions.

On the Adequacy of Chemiluminescence as a Measure for Heat Release in Turbulent Flames With Mixture Gradients

The determination of the heat release in technical flames is commonly done via bandpass filtered chemiluminescence measurements in the wavelength range of OH∗ or CH∗ radicals, which are supposed to be a measure for the heat release rate. However, these indirect heat release measurements are problematic because the measured intensities are the superposition of the desired radical emissions and contributions from the broadband emissions of CO2∗. Furthermore, the chemiluminescence intensities are strongly affected by the local air excess ratio of the flame and the turbulence intensity in the reaction zone. To investigate the influence of these effects on the applicability of chemiluminescence as a measure for the heat release rate in turbulent flames with mixture gradients, a reference method is used, which is based on the first law of thermodynamics. It is shown that although the integral heat release can be correlated with the integral chemiluminescence intensities, the heat release distribution is not properly represented by any signal from OH∗ or CH∗. No reliable information about the spatially resolved heat release can be obtained from chemiluminescence measurements in flames with mixture gradients.

A STUDY ON RERATION BETWEEN HEAT RELEASE RATE AND RADIATIVE HEAT FLUX OF WOOD CRIB BURNING DURING WATER DISCHARGE

Oxygen Consumption Calorimetric Method is generally used for measuring heat release rate (HRR) of combustible items. However, this method might have an uncertain result for gas concentration in case of during water discharged such as sprinkler. In this study, we tried to find out an appropriate and simple method for measuring HRR during water discharged. A relation between HRR in free burning condition, which was calculated by mass loss rate, and radiative heat flux was obtained experimentally. The Mie theory was applied to estimate a radiative attenuation by the absorption of water droplet, because the radiative heat flux during water discharged needs to take it into account. As a result, the radiative attenuation calculated by the theory was verified by experimental data.

Investigation of the flammability of different textile fabrics using micro-scale combustion calorimetry

Evaluating and analyzing the performance of flame retardant (FR) textiles are a critical part of research and development of new FR textiles products by the industry. The testing methods currently used in the industry have significant limitations. Most analytical and testing techniques are not able to measure heat release rate (HRR), the single most important parameter in evaluating the fire hazard of materials. It is difficult to measure HRR of textile fabrics using cone calorimetry because textile fabrics are dimensionally thin samples. The recently developed micro-scale combustion calorimetry (MCC) is able to measure the following flammability parameters for textile using milligram sample sizes: heat release capacity, HRR, temperature at peak heat release rate (PHRR), total heat release and char yield. In this research, we applied MCC to evaluate the flammability of different textile fabrics including cotton, rayon, cellulose acetate, silk, nylon, polyester, polypropylene, acrylic fibers, Nomex and Kevlar. We also studied the cotton fabrics treated with different flame retardants. We found that MCC is able to differentiate small differences in flammability of textile materials treated with flame retardants. We were also be able to calculate the limiting oxygen index (LOI) using the thermal combustion properties of various textile samples measured by the MCC. The calculated LOI data have yielded good agreement with experimental LOI results. Thus, we conclude that MCC is an effective new analytical technique for measuring textile flammability and has great potentials in the research and development of new flame retardants for textiles.

Linear stability analysis of acoustically driven pressure oscillations in a lean premixed gas turbine combustor

The dynamic response of a turbulent premixed flame to acoustic velocity perturbations was experimentally determined in a swirl-stabilized lean-premixed gas turbine combustor. CH* chemiluminescence intensity and the twomicrophone method were used to measure heat release rates and inlet velocity fluctuations, respectively. Using the n-τ formulation, gain and phase of flame transfer functions were incorporated into an analytic thermoacoustic model to predict instability frequencies and modal structures. Self-excited instability measurements were performed to verify eigenfrequencies predicted by the thermoacoustic model. Instability frequency predicted by the model is supported by experimental results. Results show that the self-excited instability frequency of ∼ 220 Hz results from the fact that the flames amplify flow perturbations with f = 150 ∼ 250 Hz. The other instability frequency of ∼ 350 Hz occurs because the whole combustion system has an eigenfrequency corresponding to the ¼-wave eigenmode of the mixing section.

Hypergolic Reactions of TNT

AbstractOne way being considered to destroy trinitrotoluene (TNT) land or surf mines is to exploit its reactivity using darts containing chemicals, which, upon contact with TNT, cause instantaneous decomposition, but not detonation. To determine the best candidates to fill the darts, liquids, specifically amines, which react in a hypergolic fashion with TNT were examined for both the rate of reaction and amount of energy released. Micro‐calorimetry was used to measure heat release while spectroscopy and conventional peak intensity monitoring by chromatography were used to examine the rate of reaction. Calorimetry measurements showed little variation between different amines reacting with TNT (about 110–130 kJ mol−1 TNT). TNT reaction with hydride actually produced more heat than with amines. Further, dinitrotoluene (DNT), which generates substantial heat, did not undergo a hypergolic reaction with amines suggesting that heat release is not the controlling factor for the hypergolic reactions. Rate constants, determined for the loss of TNT in dilute acetonitrile solution, clearly showed distinctions among the amines. The more primary amine functionalities in the amine compound, the faster it destroyed TNT. Hydrides or amine mixtures spiked with hydride decomposed substantially faster than the amines alone. However, a direct correlation between reaction rate and time‐to‐ignition was not observed.