Daesik Kim

Effect of Flame Structure on the Flame Transfer Function in a Premixed Gas Turbine Combustor

The flame transfer function in a premixed gas turbine combustor is experimentally determined. The fuel (natural gas) is premixed with air upstream of a choked inlet to the combustor. Therefore, the input to the flame transfer function is the imposed velocity fluctuations of the fuel/air mixture without equivalence ratio fluctuations. The inlet-velocity fluctuations are achieved by a variable-speed siren over the forcing frequency of 75-280 Hz and measured using a hot-wire anemometer at the inlet to the combustor. The output function (heat release) is determined using chemiluminescence measurement from the whole flame. Flame images are recorded to understand how the flame structure plays a role in the global heat release response of flame to the inlet-velocity perturbation. The results show that the gain and phase of the flame transfer function depend on flame structure as well as the frequency and magnitude of inlet-velocity modulation and can be generalized in terms of the relative length scale of flame to convection length scale of inlet-velocity perturbation, which is represented by a Strouhal number. Nonlinear flame response is characterized by a periodic vortex shedding from shear layer, and the nonlinearity occurs at lower magnitude of inlet-velocity fluctuation as the modulation frequency increases. However, for a given modulation frequency, the flame structure does not affect the magnitude of inlet-velocity fluctuation at which the nonlinear flame response starts to appear.

Effect of the Fuel Injection Strategy on Diesel Particulate Filter Regeneration in a Single-Cylinder Diesel Engine

This research was supported by the Center for Environmentally Friendly Vehicle (CEFV) as a Global-Top Project of Ministry of Environment, Korea (KMOE).

Effects of Fuel Composition on Flame Transfer Function in Lean Premixed Combustor

Abstract Flame transfer function is used to determine the relationship between flow fluctuations and heat release perturbations in alean premixed gas turbine combustor. The characteristics of flame transfer function are known to depend greatly on flamegeometries in addition to other various flow conditions. However, it is not easy to experimentally measure the flame transferfunction under various actual combustor operating conditions in terms of time and cost. The current research tries to modelthe flame transfer function using CFD(Computational Fluid Dynamics). From the results, it is shown that the calculatedsteady flame geometry can be exactly captured with consideration of the wall heat transfer and radiations. Also, unsteadyanalysis results show the close characteristics of the flame transfer function to the measured one in both gain and phase. 1. 서론 기존의 연소기는 연소실에 연료와 공기를 각각 직접분사하여 연소시키는 확산 화염 방식을 사용하였다. 이때문에 (1-5)연소실 내에서 국부적으로 농후한 당량비 영역이 생성되어 많은 Thermal NOx가 발생하게 된다. 하지만 전 세계적으로 점점 강화되고 있는 배출 가스 규제로 인하여 연료소비율을 낮추고 NOx 배출을 줄이기 위하여 희박 예혼합 연소기에 대한 많은 연구가 진행 되고 있다. 희박 예혼합 연소는 당량비를 희박 가연한계로유지하면서, 연료와 공기를 연소실 이전에서 완전 혼합한 균일한 당량비의 혼합기를 연소실로 분사하는 것이다. 사용되는 연료의 양이 적고, 연소실 각 구간에서 균일한 당량비로 혼합기가 분포하기 때문에 연소 온도가낮아져 Thermal NOx의 발생이 줄어들게 된다 . 그러나 희박 예혼합 연소기는 운전이 희박 가연 한계에서 이루어지기 때문에 화염이 매우 불안정해지게 된다. 화염이 당량비와 입구 속도와 같은 열역학적 상태량의 작은 외부 유동 변화에도 민감하게 반응하여, 결국열발생률 섭동을 일으킨다. 열발생률 변동은 연소실 내부의 압력 변화에 영향을 미치고, 이러한 압력 섭동은상류로 피드백 되어 속도 및 당량비의 진폭을 가진시킨다. 이러한 현상을 연소 불안정(Combustion Instability)라고 하며 이 현상이 일어나게 되면 과도한 압력 변동으로 인하여 소음을 발생시키고, 연소기 내부부품, 더나아가 가스터빈 시스템 일부의 손상으로 이어지게 된다

Thermoacoustic Analysis Model for Combustion Instability Prediction - Part 1 : Linear Instability Analysis

․ Kyu Tae Kim**ABSTRACT For predicting eigenfrequency and initial growth rate of comb ustion instabilities in lean premixed gas turbine combustor, linear thermoacoustic analysis model was developed in the current paper. A model combustor was selected for the model validation, which has well-defined inlet and outlet conditions and a relatively simple geometry, compared to the combustor in the previous works. Analytical linear equations for thermoacoustic waves were derived for a given combustion system. It was found that the prediction results showed a good agreement w ith the measurements, even though there was underestimation for instability frequencies. This und erestimation was more obvious for a longer flame (i.e. wider temperature distribution) than for a shorter flame.초 록 가스터빈 희박 예혼합 연소기에서 발생하는 연소 불안정의 고유 주파수 및 초기 성장률의 예측을 위하여 선형 열음향 해석 모델이 소개되었다. 모델 검증을 위하여 입출구 조건이 잘 정의되고, 상대적으로 이전 연구 결과에서 적용된 연소기에 비하여 구조가 간단한 모델 연소기가 선정되었다. 정의된 연소기에서 음향 해석을 위한 선형 관계식이 유도되었고, 이를 통하여 선형 안정성 해석 방안이 제시되었다. 해석 결과 연소 불안정의 특성에 대한 전체적인 변화 경향은 성공적으로 예측하였으나, 주파수의 절대값에 있어서는 실제 실험 결과보다 다소 작은 값을 예측하는 것으로 나타났다. 이러한 주파수의 예측 오차는 짧은 화염보다는 긴 화염에서 더욱 두드러지는 것으로 나타났다.Key Words: Linear Thermoacoustic Model(선형열음향모델), Combustion Instability(연소불안정), Flame Transfer Function(화염전달함수), Lean Premixed Combustor(희박예혼합연소기)NomenclatureAlphabets

Introduction to Flame Transfer Function in Lean Premixed Gas Turbine Combustor

Lean premixed gas turbine combustors were successful in meeting current NOx emission regulations. However, these combustors have been found to be susceptible to combustion instability. In this study, general mechanisms for combustion dynamics and instabilities in lean premixed gas turbine combustors are introduced. In addition, the flame transfer functions in the combustor are experimentally determined. The inputs to the flame transfer function are the imposed velocity fluctuations of the mixture. The key results of the measurements are reviewed.

Improved Thermoacoustic Model Considering Heat Release Distribution

* School of Mechanical and Automotive Engineering, Gangneung-Wonju Nat’l Univ.,** GE Global Research Center (Received November 28, 2013 ; Revised March 12, 2014 ; Accepted March 18, 2014)Key Words: Combustion Instability(연소 불안정), Thermoacoustic Model(열음향 모델), Flame Location(화염 위치), Heat Release Distribution(열분포)초록: 가스터빈 희박 예혼합 연소기에서 발생하는 연소 불안정 현상을 예측하기 위하여 열음향 해석 기법이 폭넓게 사용되고 있다. 그러나 이러한 모델들은 전체 연소기 시스템과 화염의 형상을 과도하게 단순화함으로써, 모델 정확도에 한계를 드러내왔다. 본 연구에서는 두 가지 측면에서 열음향 모델의 정확도를 개선하고자 하는 노력을 시도하였다. 우선 화염의 위치를 연소기 입구가 아닌 실제 계측 결과를 반영할 수 있도록 열음향 모델을 수정하였으며, 두 번째로는 열 발생 위치가 얇은 화염의 형태가 아닌, 실제 화염의 형상과 같이 열 분포 현상을 반영할 수 있도록 하였다. 모델 수정 결과 기존의 열음향 모델 대비, 불안정 현상의 성장률을 예측하는데 있어서 오차가 줄어드는 것으로 나타났다. Abstract: Thermoacoustic (TA) models have been widely used to predict combustion instability characteristics in a gas turbine lean premixed combustor. However, these techniques have shown some limitations in improving the model accuracy related to an over-simplification of the combustion system and flame geometry. Efforts were made in the current study to improve the limitations of the TA models. One strategy was to modify the actual flame location in the model, and another was to consider the heat release distribution through the flames. The modified TA model results show better accuracy in predicting the growth rate of instabilities compared with the previous results.

Review on the Gas Turbine Combustor Sizing Methodologies using Fuel Atomization and Evaporation Characteristics

Abstract The current paper reviews the main characteristics and the operating principles of major fuel atomizers used for gas turbinecombustors, including various empirical SMD equations for each atomizers. We have summarized various methodologies forevaluation of the combustion efficiency and for combustor sizing from the selected SMD data. It is found that the combustorsizing as well as the combustion efficiency are totally dependent upon the SMD calculation resutls, which means that specialcares should be taken in choosing the SMD empirical equations. 기호설명 σ : 표면장력 [N/m]µ : 점성계수 [m 2 /s]: 질량유량 [kg/s]P : 압력 [Pa]SMD : Sauter Mean Diameter [m]ρ : 밀도 [kg/m 3 ]t : 유막두께 [m]θ : 연소 파라미터θ s : 분무각 [deg]u r : 상대속도 [m/s]d 0 : 오리피스 출구 직경 [m]C : 실험상수η c : 연소효율V : 부피 [m 3 ]t res : 액적 잔류 시간 [sec]k : 열전도도 [J/msK]C p : 정압비열 [J/kgK]B : 질량 전달 상수f : 질량 분율T : 온도 [K]λ eff : 증발 상수 [mm 2 /s]Subscripts3: 연소기 입구c : 연소기a : 공기f : 연료m· Recieved: 02 Jun 2014, Recieved in revised form: 05 Sep2014, Accepted: 06 Sep 2014)

Effect of Fuel Injection Strategy on DPF Regeneration in Single Cylinder Diesel Engine

Stringent emission regulations (e.g., Euro-6) force automotive manufacturers to equip DPF (diesel particulate filter) on diesel cars. Generally, post injection is used as a method to regenerate DPF. However, it is known that post injection deteriorates specific fuel consumption and causes oil dilution for some operating conditions. Thus, an injection strategy for regeneration becomes one of key technologies for diesel powertrain equipped with a DPF.This paper presents correlations between fuel injection strategy and exhaust gas temperature for DPF regeneration. Experimental apparatus consists of a single cylinder diesel engine, a DC dynamometer, an emission test bench, and an engine control system. In the present study, post injection timing covers from 40 deg aTDC to 110 deg aTDC and double post injection was considered. In addition, effects of injection pressures were investigated. The engine load was varied from low-load to mid-load and fuel amount of post injection was increased up to 10mg/stk.Oil dilution during fuel injection and combustion processes were estimated by diesel loss measured by comparing two global equivalences ratios; one is measured from Lambda sensor installed at exhaust port, the other one is estimated from intake air mass and injected fuel mass. In the present study, the differences in global equivalence ratios were mainly caused from oil dilution during post injection.The experimental results of the present study suggest an optimal engine operating conditions including fuel injection strategy to get appropriate exhaust gas temperature for DPF regeneration. Experimental results of exhaust gas temperature distributions for various engine operating conditions were summarized.In addition, it was revealed that amounts of oil dilution were reduced by splitting post injection (i.e., double post injection). Effects of injection pressure on exhaust gas temperature were dependent on combustion phasing and injection strategies.Copyright

Flame Response Modeling for Lean Premixed Combustors Using CFD

A qualitative and quantitative analysis on flame dynamics is required to model combustion instability characteristics in gas turbine lean premixed combustors. The current paper shows the flame transfer function modeling results using CFD(Computational Fluid Dynamics) techniques for the flame dynamics study. It is generally known that flame shapes determine the basic characteristics of the flame transfer function. The comparisons of the modeled flame shapes with the measured ones were made using the optimized heat transfer conditions. Modeling results of the flame transfer function show the close behaviors to the measured data with a reasonable accuracy if the flame geometry can be exactly captured. 이 논문은 년도 대한기계학회 강원지회 춘계 § 2014 학술대회 강원대 발표논문임 (2014. 5. 16., ) . Corresponding Author, dkim@gwnu.ac.kr 2014 The Korean Society of Mechanical Engineers C

INTRODUCTION TO GAS TURBINE COMBUSTION ISSUES FOR COMBINED CYCLE COGENERATION POWER PLANT

Combined cycle cogeneration power (CCCP) generation is the cleanest and most efficient way for fossil fuel power generation. A CCCP system combines the technologies of gas turbines and steam turbines to produce electricity more efficiently than can be done using either of these technologies separately. Lean premixed combustion is regarded as the state-of-the-art technology for stationary gas turbines for its superior efficiency and especially low NOx emission. In spite of a high fuel efficiency and low NOx emission, the lean premixed combustor has a severe technical issue called “combustion instability”, feedback results of heat release oscillations and pressure wave perturbations in the lean premixed combustor. In this paper, the concept of the combustion instability and its basic mechanisms were explained, and as one of the method to predict and control the instability in the gas turbine combustor, flame transfer function could be effectively used to understand flame dynamics.