Cyclone-jet Combustor Research Articles

OH-PLIF/ステレオPIVによる火炎形状と流れ場の同時計測

It is difficult to understand turbulent combustion, because the combustion process is very complex and unsteady. One of the useful parameters for discussion of the unsteady flame behaviors is the flame stretch rate, which is evaluated by local flame curvature and strain rate. In this study, we conducted simultaneous OH-PLIF/Stereo PIV measurements of turbulent premixed flames. Flame shape and flow field were obtained. Then, local flame curvature and strain rate were discussed. The turbulent flame was established by a cyclone-jet combustor for propane/air mixtures. The main jet velocity was Um = 10, 30 m/s, and the equivalence ratio was Φm = 0.75, 0.90. The equivalence ratio of the cyclone jet was the same as that of the main jet, but the velocity of the cyclone jet was the constant of 10 m/s. It was found that the variation of flame curvature was large when the main jet velocity was high. Difference between curvature convex to unburned gas and curvature convex to burned gas was relatively small. Frequency of larger strain rate was high when the main jet velocity was large and equivalence ratio was small. It is suggested that the local flame extinction could be caused by the large strain rate because the local flame extinction occurs more frequently. At all conditions, compared with the positive strain rate, the frequency of negative strain rate was high.

312 旋回噴流燃焼器を用いた水素火炎の保炎に関する研究(OS5-3 燃焼技術の基礎と応用研究(3),OS5 燃焼技術の基礎と応用研究)

312 旋回噴流燃焼器を用いた水素火炎の保炎に関する研究(OS5-3 燃焼技術の基礎と応用研究(3),OS5 燃焼技術の基礎と応用研究)

Effect of Swirl Flow on Flame Structure and Stability in Turbulent Combustion

In general, a turbulent flame is observed in most practical combustors. As for the premixed combustion, by controlling the equivalence ratio of fuel and air, it is possible to suppress the emission of harmful substances such as NOx. However, the turbulent combustion process is very complex. Since there is unsteady flame and flow field interaction, it is very difficult to understand the combustion phenomena. To handle such a complex situation, the numerical cost may be too large. Then, by experiments, it is important to investigate turbulent combustion phenomena by laser diagnostics. In this study, we conducted a PIV/OH-PLIF simultaneous measurement of turbulent premixed flames. To focus on the effect of swirl flow on flame structure and stability, we used a cyclone-jet combustor. The extinction limit as well as CO and NOx emissions was experimentally obtained. By changing the flow condition of cyclone nozzles, we discussed the flow field with turbulence and local flame extinction.

Burning velocity and OH concentration in premixed combustion

In the present paper, we have simulated laminar premixed flames in several types of flow configuration. The counter-flow flame, 1D flame in uniform flow, and 2D flame on a slot burner have been considered. The fuel is propane, and detailed chemistry has been used. We have obtained burning velocity in these flames to examine the response of peak OH concentration in flame zone to burning velocity (SL). In case of counter-flow flame, peak OH concentration is decreased at the larger stretch rate. At the same time, the burning velocity is decreased. The peak OH concentration of 1D flame in uniform flow is decreased at lower equivalence ratio, with smaller burning velocity. It is interesting to note that, for both flames as well as 2D flame, the same linear relationship is observed between two parameters of peak OH and SL. Thus, consistent with earlier findings of turbulent flames, peak OH concentration can be a good measure of burning velocity. In the experiments, we have estimated local burning velocity in turbulent combustion with a cyclone-jet combustor. The slot burner has been used for calibration. It is found that the peak OH concentration in the flame becomes smaller with an increase of turbulence. The local burning velocity is monotonically decreased at higher turbulence. In some cases, its value is almost zero, which could be local extinction. When the fuel is propane, Markstein number is positive for lean mixture. In that case, the burning velocity is decreased due to the stretch effect, which is well in accordance with the fact that the turbulence always suppresses the burning velocity in our measurements.

Soot and NOx Measurement in a Cyclone-Jet Combustor and Numerical Simulation on Flame Structure

In this study, we measured soot and NOx in a cyclone-jet combustor. A laser induced incandescence (LII) technique was applied to obtain instantaneous soot concentration. As for NOx emission, premixed and non-premixed combustion were examined with changing equivalence ratio. To discuss the flame structure and flame characteristics in detail, numerical simulation on a jet diffusion flame was conducted. The location of ignition point was moved to discuss the effect of mixing between fuel (propane) and air in combustor. The flame index was obtained to estimate the premixed and non-premixed combustion region. The mixedness was also examined to distinguish the region of lean or rich premixed mixture. Results show that, when the air volume rate is increased, EINOx in non-premixed combustion is reduced to the level of premixed combustion. The soot is not detected when the equivalence ratio is less than 2. Based on numerical simulation, the degree of propane and air mixing is increased when the location of the ignition point is moved downstream. Resultantly, the local equivalence ratio is decreased with lower flame temperature. Hence, the low level of NOx emission in a cyclone-jet combustor is explained by this partial premixing effect of fuel and air.

2202 旋回噴流燃焼器による低NOxに及ぼす混合の影響(G06-3 熱工学(3) 燃焼(1),21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

As a means for a stable premixed combustion, there is a so-called cyclone combustor, which consists of a cylindrical chamber and fuel nozzles installed tangentially on the side wall. In this combustor an extremely stable flame can be obtained in the swirl flow, formed along the inner wall of the combustor. The authors utilized this combustor as a flame holder, to burn a high velocity jet flowing axially in the central part, and named this new combustor a cyclone-jet combustor. In the present study, the excellent flame stability is shown for the cyclone-jet combustor and, comparing among premixed, non-premixed and partially premixed flame, the low NOx combustion characteristics was experimentally examined for this combustor.

Flow Field of Turbulent Premixed Combustion in a Cyclone-Jet Combustor

The flow field in highly turbulent premixed combustion has been examined, using a cyclone-jet combustor. The shear-strain rate is obtained by PIV system. Results show that, in combustion field, mean axial velocity does not decay along center axis and RMS fluctuation velocity is much lower, compared with those in cold flow. The reaction zone detected by an ion probe well corresponds to region of large shear-strain rate. In combustion, turbulence is small around reaction zone to have the undiminished jet velocity, which leads to an increase in length of potential core, although the flame generated turbulence is observed through flow expansion.

Flame Image and Flame Structure of Turbulent Premixed Flames Using Simultaneous OH-HCHO PLIF Technique

In this study, using simultaneous OH-HCHO PLIF (Planar Laser-Induced Fluorescence) technique, we have obtained instantaneous two-dimensional flame images to investigate highly turbulent premixed combustion in a cyclone-jet combustor. On the phase diagram, conditions of Um<15m/s for φm=0.75 and Um<20m/s for φm=0.90 belong to the flamelet regime, and those of Um>20m/s for φm=0.75 and Um=30m/s for φm=0.90 belong to thin reaction zones regime.In combustion field, HCHO plays an important role at relatively earlier stage of reaction and there is HCHO region mostly in preheat zone at the temperature range of 400 K to 1 400 K. On the other hand, OH exists in reaction zone or burned gas. From simultaneous images of these two species, we have discussed the local flame structure, based on flame length and number of flame regions. When the exit velocity is increased, more cusps appear on the flame front, with larger flame length. Interestingly, it is observed that the flame front is disconnected in some images, corresponding to the local flame extinction.

1816 旋回噴流燃焼器を用いたすすとNO_xの同時低減(J14-2 環境モニタリングと負荷低減のための熱流体・環境工学の展開(2),J14 環境モニタリングと負荷低減のための熱流体・環境工学の展開)

1816 旋回噴流燃焼器を用いたすすとNO_xの同時低減(J14-2 環境モニタリングと負荷低減のための熱流体・環境工学の展開(2),J14 環境モニタリングと負荷低減のための熱流体・環境工学の展開)

Premixing Combustion of Fuel Sprays by the Cyclone-Jet Combustor and NOx Reduction

As a means for a stable premixed combustion, there is a so-called cyclone combustor, which consists of a cylindrical chamber and fuel nozzles installed tangentially on the side wall. In this combustor an extremely stable flame can be obtained in the swirl flow, formed along the inner wall of the combustor. The authors utilized this combustor as a flame holder, to burn a high velocity jet flowing axially in the central part, and named this combustor a cyclone-jet combustor. In general, it is difficult for the low NOx combustion to occur in spray combustion. In the present study, experiments on the spray combustion by the cyclone-jet combustor were carried out, and comparing the co-axial fuel spray and fuel gas jet flames. Kerosene and propane were used for fuel. With the air flow rate changed, the effect of the premixing on the NOx formation was examined. It was shown that the increase of the air flow rate reduces the NOx emission, and the shape of the spray flames is almost the same as the gas flames. The results suggest that the premixing after the process of the injection of fuel becomes very effective for NOx reduction in spray combustion.

The Influence of the Preheated Air on Low NOx Combustion Using the Cyclone-Jet Combustor

As a means for a stable premixed combustion, there is a so-called cyclone combustor, which consists of a cylindrical chamber and fuel nozzles installed tangentially on the side wall. In this combustor an extremely stable flame can be obtained in the swirl flow, formed along the inner wall of the combustor. The authors utilized this combustor as a flame holder, to burn a high velocity jet flowing axially in the central part, and named this combustor a cyclone-jet combustor. In the present study, experiments on the air preheated by the cyclone-jet combustor were carried out. The maximum air temperature is 700 K, and the propane was used for fuel. With the air flow rate changed, the effects of the turbulence on the NOx formation were examined in non-premixed and premixed flames. It was shown that the increase of the air temperature increases the NOx emission, while the increase of the turbulence reduces the NOx, emission. The results suggest that the air preheated combustion using this combustor becomes very effective for the NOx reduction and the low fuel consumption, particularly under the strong turbulence.

Persistence of Laminar Flamelet Structure Under Highly Turbulent Combustion

We have investigated the premixed flame structure in highly turbulent flow with a cyclone-jet combustor. Based on the turbulent properties determined by Slot-Correlation method, the condition of Um 20m/s belongs to the distributed reaction zone regime on the combustion diagram. Also, we have quantitatively estimated the reaction zone thickness, using the probability of reaction zone existing. Results show that the dependence of reaction zone thickness on equivalence ratio is very similar to those of the experimental values by Yamaoka and Tsuji or the Zeldovich thickness. When the exit velocity is increased, the reaction zone thickness is almost constant for Ka>1. Hence, the persistence of laminar flamelet structure is observed, even when the Kolmogorov scale is smaller than reaction zone thickness. It could be concluded that the reaction region remains undisturbed with thin reaction zone under highly turbulent conditions. These results are useful for modeling turbulent combustion.

A Study on Highly Turbulent Premixed Flames in a Cyclone-Jet Combustor. 3rd Report. Measurement of Turbulence Scale in Combustion Field by Slot-Correlation Method.

A Study on Highly Turbulent Premixed Flames in a Cyclone-Jet Combustor. 3rd Report. Measurement of Turbulence Scale in Combustion Field by Slot-Correlation Method.

Flow Field and Structure of Highly Turbulent Premixed Flames.

To examine the premixed flame structure in highly turbulent flow, by a cyclone-jet combustor, we have examined the flow field both in cold flow and in combustion field using LDV system. The turbulence scale has been obtained by Slot-Correlation method. The experiments have been conducted from the flamelet regime (Um=10 m/s) to the distributed reaction zone regime (Um=30 m/s) in combustion diagram. Results show that, in combustion filed, the mean axial velocity does not decrease as much and rms fluctuation velocity is lower along the center axis, compared with those in cold flow. It could be explained with the flow expansion in combustion. That is, the turbulence transport is reduced due to the existence of low-density gas in shear layer at the flame front, which leads to a longer potential core in combustion field than that in cold flow. Flame generated turbulence is observed in radial velocity distribution. The integral length scale becomes larger at the flame zone due to the flow expansion.

Low NOx Combustion by a Cyclone-Jet Combustor

For achieving a stable premixed combustion, there is a device termed the cyclone combustor, which consists of a cylindrical chamber and fuel nozzles installed tangentially on the sidewall. In this combustor an extremely stable flame can be obtained in the swirl flow, formed along the inner wall of the combustor. The authors utilized this combustor as a flame holder, to burn a high velocity jet flowing axially in the central part, and termed this new combustor a cyclone--jet combustor. In the present study, an excellent flame stability is shown for the cyclone-jet combustor and, by comparing premixed, non-premixed and partially premixed flames, the low NO x combustion characteristics were experimentally examined for this combustor.

A Study on Highly Turbulent Premixed Flames in a Cyclone-Jet Combustor. 2nd Report. Investigation on Flame Microstructure with an Electrostatic Probe.

To examine flame structure in turbulent flow, we have established highly tubulent flames of a propane-air mixture in a cyclone-jet combustor, and have measured ion current by an electrostatic probe. Based on the measured flow field by a hot wire anemometer, we have obtained turbulence characteristics in the non-reacting flow, and have plotted them on the Peter's phase-diagram to classify the micro-structure of turbulent flames. From the ion current characteristics and tomographic images of flames, we have discussed the flame structure and turbulence. Results show that, as the exit velocity is relatively low, a wrinkled laminar flame is formed. As the exit velocity is further increased, the flame wrinkling is intensified to increase the fluctuation in wave of ion current. When the velocity exceeds 30m/s, the PDF of ion current is no longer that of a wrinkled laminar flame, which shows that, with turbulence, one continuous flame sheet cannot be sustained, and the destruction of laminar flame structure occurs, in which small reacting eddies in an intermediate stage of combustion are embedded. It appears that the flame structure changes from the wrinkled laminar flame to those with distribution reaction zone.

Low NOx Combustion by a Cyclone-Jet Combustor.

As a means for a stable premixed combustion, there is a so-called cyclone combustor, which consists of a cylindrical chamber and fuel nozzles installed tangentially on the side wall. In this combustor an extremely stable flame can be obtained in the swirl flow, formed along the inner wall of the combustor. The authors utilized this combustor as a flame holder, to burn a high velocity jet flowing axially in the central part, and named this new combustor a cyclone-jet combustor. In the present study, the excellent flame stability is shown for the cyclone-jet combustor and, comparing among premixed, non-premixed and partially premixed flame, the low NOx combustion characteristics was experimentally examined for this combustor.

A Study on Highly Turbulent Premixed Flames in a Cyclone-Jet Combustor. 1st Report. Visualization of Flame Structure with Tomographic Images.

By using a cyclone-jet combustor, we have investigated premixed flames in a highly turbulent flow over a wide range of turbulent properties, with u'/SL exceeding 10 in a stationary jet flame. With Mie scattering, we obtained tomographic images to visualize the flame structure. We have investigated the velocity field in cold flow with a hot wire anemometer. Results show that, as the turbulence is relatively low, the scale of the turbulence is large and one flame sheet wrinkles, which shows that a typical wrinkled laminar flame is formed. As the exit velocity is further increased, the flame wrinkling is increased, but the size of its wrinkles becomes small with many cusps of the flame front. In some cases, the burned gas may exist in the unburned gas, and vice versa. When the velocity exceeds 30 m/s, one continuous flame sheet cannot be sustained, and the destruction of laminar flame structure occurs, in which small reacting eddies in an intermediate stage of combustion may be embedded. These results are supported by the fact in previous experiments that the probability of intermediate temperature between those of unburned and burned gases becomes high and the PDF of temperature is no longer bimodal. Thus, it appears that, a flame structure changes to be that with distributed reaction zone as the intensity of turbulence is increased.