Cheol-Hong Hwang

The Effect of Residence Time and Heat Loss on NOx Formation Characteristics in the Downstream Region of CH4/Air Premixed Flame

In this study, the NOx formation characteristics of one-dimensional /Air premixed flame using detailed-kinetic chemistry are examined numerically. The combustor length and the amount of heat loss are varied to investigate the effect of residence time and heat loss on the NOx formation in a post-flame region. In the flame region, NO is mainly produced by the Prompt NO mechanism including O-intermediate NO mechanism over all equivalence ratios. However, thermal NO mechanism is more important than Prompt NO mechanism in the post-flame region. In the case of adiabatic condition, the increase of combustor length causes the remarkable increase of NO emission at the exit due to the increase of residence time. On the other hand, NO reaches the equilibrium state in the vicinity of flame region, considering radiation and conduction heat losses. Furthermore the NO, in the case of

NOx Emission Characteristics of Dimethyl Ether/Air Nonpremixed Flames

The NOx emission characteristics of DME in laminar coaxial jet and counterflow nonpremixed flames were investigated using experimental and numerical approaches, respectively. The flame structure and NOx emission of DME were compared with those of and . The DME flame was calculated using the Kaiser`s mechanism, while the and flames were calculated using the mechanism. These mechanisms were combined with the modified Miller-Bowman mechanism for the analysis of NOx. Experimental results show in coaxial jet flame that DME flame has the characteristics of partial premixed flame and the flame length decreases up to 1/3 than that of in the same condition of fuel mass flowrate. Then, the NOx emission of DME decreases to 40% approximately, comparing with that of . In the calculated results of counterflow nonpremixed flame, DME flame shows the decreases up to 50% approximately than those of and flames when the equivalent fuels are consumed per unit mass and time. Although the overall NOx reaction path of DME is similar with other hydrocarbon fuels, it can be identified that DME flame has a distinct NO reduction mechanism due to the reburning NO chemistry in fuel rich region. From these results, we can conclude that the different NOx emission characteristics of DME flame with other hydrocarbon fuels are attributed to not the temperature increase and the activation of NO reactions due to O atom in DME fuel but the rapid processes of pyrolysis/oxidation.

Numerical Studies on Combustion Characteristics of a Hybrid Catalytic Combustor

The combustion characteristics of the hybrid catalytic(catalytic+thermal) combustor with a lean methane-air mixture on platinum catalyst were investigated numerically using a 2-D boundary layer model with detailed homogeneous and heterogeneous chemistries. for the more accurate calculations, the actual surface site density of monolith coated with platinum was decided by the comparison with experimental data. It was found that the homogeneous reactions in the monolith had little effect on the change of temperature profile, methane conversion rate and light off location. However, the radicals such as OH and CO were produced rapidly at exit by homogeneous reactions. The effect of operation conditions such as equivalence ratio, temperature, velocity, pressure and diameter of the monolith channel at the entrance were studied. In thermal combustor, the production of NO was more dominant than that of NO due to the relative importance of the reaction N+O(+M)→NO(+M). Finally the productions of CO and NOx by amount of methane addition were studied.

A Study on the Development of a Low-cost Device for Measuring the Optical Smoke Density

A low-cost device using the light-extinction method was developed to measure the optical smoke density in various fire experiments in the present study. The relative measurement accuracy of low-cost device was evaluated through the comparison of optical density measured by a high-cost standard device consisting of He-Ne laser, photo detector and various optical components. The low-cost device was composed of laser module, photocell and acrylic board. From the experiments using a smoke generator can be easily adjusted the smoke concentration, it was found that the low-cost device could measure the smoke density within the range of ± 10%, compared to the standard device. In addition, the reliability of low-cost device was also confirmed in the experiment using a polyethylene flame. Finally, it is expected that the lowcost device developed with real-time measurement and simple installation for measuring the smoke density will be used instead of the high-cost standard device.

Measurement of the Device Properties of Fixed Temperature Heat Detectors for the Fire Modeling

성능위주 소방설계(PBD)의 과정에서 화재 및 피난모델링의 신뢰성을 확보하기 위해서는 화재감지기 모델의 높은 예측성능이 필수적으로 요구된다. 본 연구의 목적은 대표적인 화재모델인 FDS에 적용되고 있는 정온식 열감지기의 정확한작동 개시시간을 예측하기 위한 입력정보를 측정하는 것이다. 이를 위해 화재감지기의 장치특성을 측정할 수 있는 FireDetector Evaluator (FED)가 사용되었으며, 국내에서 적용되는 2개의 써미스터 방식(Thermistor types)과 1개의 바이메탈 방식(Bimetal type)의 스포트형 정온식 열감지기가 연구대상으로 고려되었다. 화재모델링을 위해 요구되는 감지기의작동 개시온도 및 반응시간지수(Response Time Index)가 측정되었으며, 이때 RTI는 감지기의 설치위치를 고려하여 천장제트기류(Ceiling jet flow)와 수직제트기류(Vertical jet flow)에 대하여 측정되었다. 측정된 장치물성을 이용한 화재모델링 결과는 실규모 구획화재실험 결과와 비교·검증되었다. 그 결과 수치적으로 예측된 감지기의 작동 개시시간은 실험결과를 적절히 잘 예측함을 확인하였다.ABSTRACTThe high predictive performance of fire detector models is essentially needed to assure the reliability of fire and evacua-tion modeling in the process of Performance-Based fire safety Design (PBD). The main objective of the present study isto measure input information in order to predictive the accurate activation time of fixed temperature heat detectorsadopted in Fire Dynamics Simulator (FDS) as a representative fire model. To end this, Fire Detector Evaluator (FDE)which could be measured the device properties of detector was used, and the spot-type fixed temperature heat detectors oftwo thermistor types and one bimetal type were considered as research objectors. Activation temperature and ResponseTime Index (RTI) of detectors required for the fire modeling were measured, and then the RTI was measured for ceilingjet flow and vertical jet flow in consideration of the install location of detectors. The results of fire modeling using mea-sured device properties were compared and validated with the experimental results of full-scale compartment fires. It wasconfirmed that, in result, the numerically predicted activation time of detector showed reasonable agreement with themeasured activation time.

Performance Evaluation of Large Eddy Simulation for Recirculating and Swirling Flows

The objective of this study is to evaluate the efficiency and the prediction accuracy of developed large eddy simulation (LES) program for complex turbulent flows, such as recirculating and swirling flows. To save the computational cost, a Beowulf cluster system consisting 16 processors was constructed. The flows in backward-facing step and dump combustor were examined as representative recirculating and swirling flows. Firstly, a direct numerical simulation (DNS) for laminar backward-facing step flows was previously conducted to validate the overall performance of program. Then LES was carried out for turbulent backward-facing step flows. The results of laminar flow showed a qualitative and quantitative agreement between simulations and experiments. The simulations of the turbulent flow also showed reasonable results. Secondly, LES results for non-swirling and swirling flows in a dump combustor were compared with the results of Reynolds-averaged Navier-Stokes (RANS) using standard model. The results show that LES has a better performance in predicting the mean axial and azimuthal velocities, comer recirculation zone (CRZ) and center toroidal recirculation zone (CTRZ) than those of RANS. Finally, it was examined the capability of LES for the description of unsteady phenomena.

An Experimental Study on the Applicability of Plate Thermometer in Steady and Unsteady-State Fire Conditions

The applicability of plate thermometer (PT), which feature simple installation and low cost, was experimentally examined in steady and unsteady-state fire conditions. An infrared radiation heater and a square burner with C3H8 as fuel were used as heat sources. The relative measurement accuracy of the PT was evaluated by comparing measurements made using a Gardon-type heat flux meter. From a practical point of view and in terms of measurement accuracy, the optimal size and thickness of the PT in steady and unsteady-state fire conditions were 100 mm and 0.6 mm, respectively. These results can be explained by the conductive heat losses and thermal inertia of the PT for different sizes and thicknesses. It can be also concluded that measurements of heat flux using the PT in conditions of faster fire growth rate than slow require considerable attention.

Measurement of the Device Properties of Photoelectric Smoke Detector for the Fire Modeling

화재모델링 결과를 이용한 신뢰성이 확보된 피난안전설계를 위해서는 화재감지기 모델들의 높은 예측성능이 필수적으로 요구된다. 본 연구의 목적은 대표적인 화재모델인 FDS에 적용된 광전식 연기감지기의 정확한 작동 개시시간을 예측하기 위한 입력정보를 측정하는 것이다. 이를 위해 화재감지기의 장치특성을 측정할 수 있는 fire detector evaluator(FDE)가 사용되었으며, 스포트형 광전식 연기감지기에 대한 Heskestad 및 Cleary 모델의 입력정보가 얻어졌다. 또한 일반적으로 사용되는 FDS의 기본 값과 측정된 값이 적용된 연기감지기의 작동 개시시간을 정량적으로 비교하였다. 주요결과로써, Heskestad 모델은 Cleary 모델에 비해 광전식 연기감지기의 작동 개시시간 예측에 부정확한 결과를 초래할수 있다. 또한 본 연구에서 검토된 광전식 연기감지기의 장치물성은 FDS에 적용된 기본 값과 매우 명확한 차이를 가지며, 장치물성의 변화에 따라 연기감지기 작동 개시시간 또한 매우 큰 차이가 보였다.ABSTRACTThe high predictive performance of fire detector models is essentially required for the reliable design of evacuationsafety using the fire modeling. The main objective of the present study is to measure input information in order to predictthe accurate activation time of photoelectric smoke detector adopted in fire dynamics simulator (FDS) recognized a repre-sentative fire model. To end this, the fire detector evaluator (FDE) which could be measured the device properties ofdetector was used, and the input information of Heskestad and Cleary’s models was obtained for a spot-type photoelectricsmoke detector. In addition, the activation times of smoke detector predicted using default values into FDS and measuredvalues in the present study were quantitatively compared. As a result, the Heskestad model could result in an inaccuratethe activation time of photoelectric smoke detector compared to the Cleary model. In addition, there was a distinct differ-ence between the default values used into FDS and the measured values in terms of device properties of smoke detector,and thus the activation time also showed a significant difference.Keywords : Photoelectric smoke detector, Fire modeling, Fire Dynamics Simulator (FDS), Performance Based Design(PBD)

Effect of Radiation Models on the Suppression Limits in Counterflow Methane/Air Diffusion Flames

Effect of radiation models on the suppression limits in counterflow CH /air diffusion flame was numerically investigated with fundamental experiments for the numerical validation. N and CO were considered as extinguishing agents. The differences in extinguishing concentration between OTM and SNB radiation models which have different accuracy levels were examined. As a result, there is no considerable difference in extinguishing concentration for the N dilution as the radiation models with different accuracy levels were used. As the CO having strong radiative effect was diluted in the low strain flames, however, the radiation model with high predictive accuracy such as SNB should be used. In particular, the CO dilution in fuel stream leads to the significant difference in extinguishing concentration between OTM and SNB models. Therefore, it is necessary that the radiation model should be reasonably chosen with the consideration of numerical accuracy and computational time for the prediction of extinguishing concentration.

Effects of the Geometry and Location of an Vertical Opening on the Fire Characteristics in the Under-Ventilated Compartment Fire

To investigate numerically the effects of geometry and location of vertical opening on the thermal and chemical fire characteristics in full-scale under-ventilated compartment fires, the ventilation factor () to estimate a theoretical maximum inflow of ambient air and the mass loss rate in a heptane pool fire were fixed for all cases. It was shown that variations in door geometry affected significantly the change in thermal and chemical characteristics inside the compartment. Variations in window location resulted in the complex change in additional fire characteristics including the fire duration time and recirculating flow structure. These results were analyzed in details by the multi-dimensional flow and fire characteristics including the vent flow and fuel/air mixing phenomena.