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An experimental investigation for cold-state flow field of regenerative heating annular furnace
Applied Thermal Engineering 29 (2009) 3426–3430
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
An experimental investigation for cold-state flow field of regenerative heating annular furnace Liang-ming Pan a,*, Hong-chun Ji a, Shu-ming Cheng b, Cheng-bo Wu c, Hai-quan Yong b a
School of Power Engineering, Chongqing University, Chongqing 400044, China CISDI Industrial Furnace Lt. Co. of Chongqing, Chongqing 400012, China c School of Science of Material and Engineering, Chongqing University, Chongqing 400044, China b
a r t i c l e
i n f o
Article history: Received 6 October 2008 Accepted 27 May 2009 Available online 6 June 2009 Keywords: HiTAC Air–gas dual regenerative combusting technology Annular furnace PIV technology Velocity profile
a b s t r a c t According to modeling theory, a cold-state model with 1:10 scale was established for a regenerative heating annular furnace. Following to the PIV (Particle Image Velocimetry) testing method by using tracing particles, a high-speed camera was adopted to measure the velocity distribution in the furnace. The experimental results found that the airflow from the burners could not be sucked by the first opposite burner on the other side wall; and that closing the soaking section results to farther suction location, which prolongs gas residence time in the furnace, and it also avoids the rich-fueled smoke gas short circuit which usually occurs in the regenerative heating furnace. The velocity profiles have a great variance in the chamber. With farther location to the nozzle, velocity profiles are more even. At a certain angle of the air jet, there are two vortexes close to the burner. With higher flow rate, it is easier for the airflow to reach the opposite chamber wall. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction High Temperature Air Combustion (HiTAC) is a promising technology for energy savings and emissions reduction [1–6]. It has been developed to meet the demand of various symmetrical heating situations since 1980s [7]. The main feature of this technology is to preheat the air/gas fuel to a specific temperature previously to the fuel combustion. Therefore, this technology has the potential to use the lower BTU gas derived from blast furnace or gasification process of coal and wastes. The breakthrough of HiTAC technology is that it has the potential to overcome the limitations of conventional combustion and allows engineers to finally meet this longstanding imperative [8]. This technology is based on utilizing the sensible heat of flue gas by using regenerative materials which heat up the combustion air/gas fuel to a temperature even higher than 1000 °C while reducing the exhaust temperature to less than 200 °C. Therefore, the utmost heat utilization could be reached by using this technology. Furthermore, combustion will be more stable at low equivalence ratios and the NOx emission is even less than 50 ppm when gaseous fuels are burnt [9]. Up to now, HiTAC technology has been successfully applied to a number of industries including iron and steel [2,4], power generation [10] and chemical industry for coal gasification [11]. However, the applications are only lim* Corresponding author. Tel.: +86 23 65111297; fax: +86 23 65102473. E-mail address: [email protected] (L. Pan). 1359-4311/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2009.05.022
ited to the facilities where the heat loads are symmetrical. It is of great importance to assess whether and how we can implement the HiTAC technology to the asymmetrical heat load situation, such as annular furnace and other similar applications. Traditionally, the heat input at the soaking zone is characterized by flat flame burning on the roof of the furnace and variable flame burner at the heating zone with high flue gas speed. When this technology was considered in an asymmetrical heat load situation, it is impossible to form flat flame on the roof due to the configuration characteristic. Besides that, it is still unknown that whether the flow of the heated burning air can be well organized to form a favorable flow field for slab or rod heating. Rafidi and Blasiak [2] found that with highly reduced temperature fluctuations, turbulent intensity and combustion intensity, the HiTAC flame has a larger reaction zone than the conventional flame. Therefore, the investigation of flow field in the furnace with regenerative burner has become essential to optimize the design. So far, there is no report to show that HiTAC has been used in annular furnace, and the fundamental investigation relating to HiTAC in the asymmetrical heat load situation has not been even started yet. In this paper, a model downscaled to 1:10 and provided with optical access, as shown in Fig. 1, was established to experimentally study flow field and pressure fluctuation characteristics within furnace. The inner diameter of the model is 3.07 m, the height of the model is 0.27 m, and the outer diameter is 4.13 m. The furnace based on present work has been put in operation at Chengdu Steel Co. Lt.
L.-m. Pan et al. / Applied Thermal Engineering 29 (2009) 3426–3430 Table 1 Operating conditions of the experiments. Run no.
Location
Infeeding (m3/h)
Exsuction (m3/h)
1 2 3 4 5
No. 1 heating zone No. 2 heating zone No. 3 heating zone Holding zone Heat recovery zone
992.4 992.4 1212.5 1735.1 0
1002.9 1002.9 1225.3 0 1731.2
Converted pressure (Pa) 2.16 1.33 0.22 0.18
3427
(i) Ensure maximum heat transfer intensity under possible heating process. (ii) According to the heating technical requirements, keep the cross-sectional temperature evenness in the furnace by flue gas motion control. (iii) Ensure slight positive furnace pressure to prevent ambient air from entering the furnace, and at same time ensure evenly distribution of pressure along the furnace length to avoid too much hot flue gas overflowing the furnace.
Note: the above pressure measurement points are located at the center top of corresponding zone.
3. Fluid flow measurement and distribution analysis 2. Basic requirements for gas motion in annular furnace 3.1. Experimental method and data processing Because of the switch-over process, the flow field in the regenerative furnace is different than that in channel or in smoke flue. How to organize the gas motion in a furnace mainly depends on kinetic energy of jet flow from burners. However, the primary resistance of fluid flow is the mixing process between the fuel and preheated air. And a portion of energy is consumed by the recirculation process of flue gas induced by jet flow entrainment. In order to keep positive pressure in a furnace, a portion of kinetic energy is transformed to pressure energy. All the processes mentioned above generate uneven furnace pressure and velocity distributions. Fluid flow in the furnace is mainly controlled by three streams (i.e., free stream, intercrossed free stream and limited stream). Among these three streams, the free stream is the dominant one in the furnace, and it depends on the upstream flow of the furnace. At current situation the upstream flow is the gas load input to the soaking zone. The intercrossed free stream is formed by the preheated air and the fuel blasted into furnace through burners, which determines the local flow patterns near burners. For the asymmetrical flow, it would become a quite complicated process. And the third limited stream, which is very important to spread out in the furnace and ensure the flame stiffness and stability, is generated by the intercrossed stream blasted along the furnace wall, The basic requirements for flue gas flow in the furnace are as follows:
So far, there are three popular experimental methods in the cold aerodynamic field, i.e., (i) flexible ribbon visualization, (ii) particles/smoke visualization; (iii) quantitative data measurement. The first two methods are qualitative method used to visually inspect the flow field, but the third one is a quantitative experimental method by using apparatuses to exam the details of velocity and pressure distributions in the field. In the present paper, both the visualization and quantitative methods are adopted to get better understanding about the cold aerodynamics field of the annular furnace using HiTAC technology. In present work, velocity was measured by an in-house developed two-dimensional approx-PIV system, which comprises light tracking particles, optical light sheet, computer-connected highspeed camera and the image post-processing software. The schematic configuration is depicted at Fig. 2. During the experiments, a fixed camera was set to record the images with a constant interval. With the help of the reference scale, travel distance of a specific particle could be measured using the two consecutive images. Thus, a continuous velocity distribution is obtained. 3.2. Testing apparatus and tracking particles A Redlake HK100 high-speed camera system was adopted to record the flow field. This camera can take pictures at 100,000 fps (frame per second), and is equipped with 2 Giga flash memory and 1000 M Base-T computer-connecting cable. At present work, the speed of the camera has been set to 500 fps, so the time intervals between images are 2 ms. The tracer particles are even diam-
No.3 Heating Zone Sacking Zone
No.2 Heating Zone
Preheating section
No.1 Heating Zone
Fig. 1. Model of annular furnace for cold-state simulation.
Fig. 2. Schematic configuration of a self constructed two-dimensional approx-PIV system.
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3.3. Experimental scheme and data processing At the central location of each zone, a hole was drilled on the nozzle tube to feed in tracer particles. By using a concave mirror halogen light sheet, a horizontal slice of the furnace was illuminated whose thickness is approximate 5 cm. The high-speed camera was vertically mounted above the furnace to take continuous pictures of the track particles. At present work, three operating conditions were investigated, i.e. load at 100%, 70% and 50%, the operating conditions of the experiments of the load at 100% is shown at Table 1. One actual picture is shown as Fig. 3. Before running, a ruler was placed on the furnace to scale the pictures. The pictures taken by the high-speed camera were analyzed with Photoshop CS software to get two-dimensional velocity vector distribution of every individual particle in the field of view. The velocity vector of every point was input into origin 7.5 software to plot the vector graph of the flow field in the furnace. Fig. 3. Image of tracing particles at 70% load of no. 3 heating zone.
4. Experimental results analysis and discussion
eter foam granules with averaged diameter within 2–3 mm. The infeeding and exsuction flow rate was measured by Pitot tube with error of 2.5%, (square root at 1.24%).
To facility the measurement, the outer circle burners were burning and inner circle burners were sucking. To visualize the flow field, the light sheet was located just a little above the burners. As shown in Fig. 4a in the full load situation, the velocity of
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Y /mm
b 1000
500 400 300 200
outer circle
500 400 300
particle jet
inner circle
inner circle
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outer circle
4 m/s
100
100
particle jet
4 m/s
0
0 0
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at Rated Load
c
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100 0
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at 70% Rated Load
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X /mm
X /mm
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Y /mm
a 1000
0
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300
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X / mm
at 50% Rated Load Fig. 4. Velocity vector graph of no. 1 heating zone.
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L.-m. Pan et al. / Applied Thermal Engineering 29 (2009) 3426–3430
the flow from the nozzle of no. 1 heating zone could reach as high as 30 m/s. As showed in the vector graph, a great amount of particles were concentrated in the area of inner circle of the furnace, and this may be due to the high velocity infeeding flow. It is hard for particles to diffuse to the center area of the furnace. According to the figure, it is quite clear that a vortex exists at the downstream of every nozzle, which may enhance the mixing of fuel gas and preheated air. Although most of the tracking particles moved to the inner circle of the furnace, the tracking particles did not enter the first opposite sucking burner as showed in the velocity vector graph. Thus, it implies that by adopting an appropriate switch-over strategy, the short circuit could be effectively avoided to prevent the fuel from being sucked into the opposite sucking burner, which results in the fracture of refractory material and the decreasing of overall thermal efficiency. The velocity vector distribution of the horizontal cross-section at 70% and 50% rated flow are shown at Fig. 4b and c. At the 70% load, the maximum velocity at the outlet of infeeding burner can reach 25 m/s. As shown at Fig. 4b, a vortex-shape cavity exists at the downstream of the outlet of the infeeding burner. It can be concluded that there was a large-scale vortex at that position. The vortex was resulted from asymmetrical distribution of burners along the two side walls. The vortex prolonged the residence time of the flue gas, which will result in higher velocity flue gas flow to enhance convective heat transfer [12], complete combustion in low oxygen environment and lower combustion temperature [13]. Furthermore, when at 70% load the furnace was still filled with high-speed fresh flue gas, which would meet the demand of temperature uniformity for steel heating. Moreover, it can reduce the temperature concentration and greatly decrease the production of NOx because of the flue gas reflux. At 50% load, the maximum velocity at the outlet of infeeding burner can reach 16 m/s. As shown in Fig. 4c, after particles were spewed out from the outer burner, it could not immediately flow into the inner circle of the furnace, but slowly spread out along the furnace chamber, and the moving direction would change to that of the mainstream very quickly. A vortex still exists at the downstream of the outer burner. It can be deduced from the velocity vector graph that the flame is still stiff enough to overspread the entire furnace chamber at 50% load. If a portion of the burners is turned off, the flue gas circulation would be organized more efficiently to heat steel piece well. But under the low load condition, the inadequate reflux flow will result in a certain temperature difference because the flame stiffness is not strong enough. In the real situation, to ensure the temperature evenness of the furnace, it is necessary to avoid burners running in the low load condition as far as possible, but should adopt to stop a portion of burners. This
a
3429
problem is not too serious at the first two heating zones, thus the strategy to adjust burner flow still can be adopted to organize heating process. Comparing among the above three operating conditions, although higher flow rate can form higher velocity profile distribution at the furnace cross-section, but according to furnace operation principles, flue gas residence time is also an important factor to be considered which impacts on steel heating efficiency and combustion efficiency, designers want both higher flow speed and longer flue gas residence time. According to the results shown on Fig. 4, it is clear that the optimized load for burners is above 70% rated load. Although there is a large vortex when the burner works at 50% rated load, but the flue gas velocity is too low to get high heat transfer coefficient and uniform temperature distributed flue gas thereafter. Furthermore, as shown on Fig. 4, because the annular shape of the furnace, the flow spewing out from burner has very strong interaction with the flow that comes from upstream, a very strong secondary flow was formed at the downstream location, close to the burner. It is a key feature for the regenerative combustion annular furnace. Generally speaking, the higher the flow rate is, the easier the flow arrived at the other side wall and was sucked out. But the suction does not happen at the first opposite burner. Smaller flow rate results in longer residence time and further downstream sucking location. Generally, the sucking location is the second or the third opposite burner. Moreover, for three different heating zones, we can know from the experiment that the closer infeeding burners are to the soaking section, the more difficult for the fuel gas is to be sucked by the opposite sucking burner. This implies that the flue gas produced by soaking section almost entirely is sucked out in the heating zones and could not arrive at tail flue. The flue gas generated at the present heating zone would be sucked out from the next heating zones. Because of this, it could consumedly prolong the resident time of fresh flue gas in the furnace, and the combustion short circuit would unlikely occur. So it is favorable to perform combustion under high temperature. The velocity profile of cross-section at 10 cm downstream of a burner is shown in Fig. 5. It reveals that because of being too close to the burner, along the path of the gas flowing transversely the cross-section, the violent gas movement could not fill the whole furnace chamber, and the velocity fluctuation is very big. At 30 cm downstream from the burner jet, the speed fluctuation is more weak, which shows that gas and air have been mixed completely, and the velocity fluctuates within 6–9 m/s. At 50 cm downstream from the burner jet, the velocity profile becomes very even, and the velocity fluctuates within 7–8 m/s. In this situation, the flue gas movement could be efficiently organized. Through the comparison among different working conditions and heating zones, it can be concluded that at farther downstream
b
Fig. 5. Velocity profiles at radial cross-section of no. 1 heating zone.
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from the burner jet there is more even velocity profiles and more mild speed fluctuation, which would be favorable to form even temperature field and stable pressure field; smaller flow rate also can form more even temperature field, and the place to attain stability would be closer to the upstream burner than that of larger flow rate. Moreover, the velocity distribution would become more uniform with smaller flow rate, which can effectively eliminate temperature difference of the heated steel piece along the length direction.
flue gas and brings out the complete combustion in low oxygen environment and the lower combustion temperature. Acknowledgements The authors are grateful for the support of the research funding from the National Science Foundation of China (No. 50406012) and CISDI Co. Lmt., and also the technical supports from Mr. Zhengrong Zhang and prof. Jiaofen Pan.
5. Summary
References
According to the cold-state experimental results for the regenerative heating annular furnace, the conclusions could be drawn as follows:
[1] A.K. Gupta, Thermal characteristics of gaseous fuel flames using high temperature air, Journal of Engineering for Gas Turbines and Power 126 (1) (2004) 9–19. [2] N. Rafidi, W. Blasiak, Heat transfer characteristics of HiTAC heating furnace using regenerative burners, Applied Thermal Engineering 26 (16) (2006) 2027– 2034. [3] T. Zhu, D. Chen, H. Zhang, et al., Numerical simulation of NOx emission during high temperature air combustion for burning low heat value gas, Proceedings of the International Conference on Energy and the Environment 2 (2003) 1030–1035. [4] T. Ishii, Numerical simulations of highly preheated air combustion in an industrial furnace, Journal of Energy Resource Technology 120 (1998) 276–284. [5] R. Tanaka. High temperature air combustion advanced strategic technology originated in Japan, Technical Note 900420, 1996, p. 4. [6] T. Hasegawa, R. Tanaka, High temperature air combustion – revolution in combustion technology – part I: new findings on high temperature air combustion, JSME International Journal, Series B 41 (4) (1998) 1079–1084. [7] Keith J. Stokes, Recovery of heat from flue gas, US patent 4299561, 1981. [8] H. Tsuji, A.K. Gupta, T. Hasegawa, M. Katsuri, et al., High Temperature Air Combustion: From Energy Conservation to Pollution Reduction, CRC Press, New York, 2003. [9] M. Katsuki, T. Hasegawa, The science and technology of combustion in highly preheated air, Symposium (International) on Combustion 27 (2) (1998) 3135– 3146. [10] Roman Weber, John P. Smart, Willem vd Kamp, On the (MILD) combustion of gaseous, liquid, and solid fuels in high temperature preheated air, Proceedings of the Combustion Institute 30 (2) (2005) 2623–2629. [11] S. Sugiyama, N. Suzuki, Y. Kato, et al., Gasification performance of coals using high temperature air, Energy 30 (2–4) (2005) 399–413. [12] M.T. Glinkov, G.M. Glinkov, A General Theory of Furnaces, Mir Publishers, Moscow, 1980. [13] K.S. Shanmukharadhya, K.G. Sudhakar, Experimental and numerical investigation of vortex-induced flame propagation in a biomass furnace with tangential over fire registers, The Canadian Journal of Chemical Engineering 86 (1) (2008) 43–52.
(1)
Generally speaking, the infeeding flow from the burner would not be sucked in by the first opposite suction. According to the flow rate, it would be sucked out by the second or third suction. The closer infeeding burners to the soaking section, the more difficult for the fuel gas to be sucked by opposite suction; and the flue gas produced at soaking section almost completely is sucked out in the heating zones and could not arrive at the tail flue. The flue gas generated at the present heating zone would be sucked out from next heating zones. It can consumedly prolong the resident time of fresh flue gas, and the combustion short circuit would unlikely occurs, so it is favorable to perform combustion under high temperature. (2) From the outer furnace wall to the inner circle, the velocity changed remarkably, and the furnace chamber could not be wholly filled with moving mixture, so the velocity fluctuation is very strong. Farther downstream from the burner jet there are more even velocity profiles and more mild speed fluctuation, which would be favorable to form a even temperature field and a stable pressure field. (3) Because all of the infeeding burners supply gas with certain angle, it always results in a pair of vortexes at both sides of the burners. The vortex prolongs the residence time of the
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
An experimental investigation for cold-state flow field of regenerative heating annular furnace Liang-ming Pan a,*, Hong-chun Ji a, Shu-ming Cheng b, Cheng-bo Wu c, Hai-quan Yong b a
School of Power Engineering, Chongqing University, Chongqing 400044, China CISDI Industrial Furnace Lt. Co. of Chongqing, Chongqing 400012, China c School of Science of Material and Engineering, Chongqing University, Chongqing 400044, China b
a r t i c l e
i n f o
Article history: Received 6 October 2008 Accepted 27 May 2009 Available online 6 June 2009 Keywords: HiTAC Air–gas dual regenerative combusting technology Annular furnace PIV technology Velocity profile
a b s t r a c t According to modeling theory, a cold-state model with 1:10 scale was established for a regenerative heating annular furnace. Following to the PIV (Particle Image Velocimetry) testing method by using tracing particles, a high-speed camera was adopted to measure the velocity distribution in the furnace. The experimental results found that the airflow from the burners could not be sucked by the first opposite burner on the other side wall; and that closing the soaking section results to farther suction location, which prolongs gas residence time in the furnace, and it also avoids the rich-fueled smoke gas short circuit which usually occurs in the regenerative heating furnace. The velocity profiles have a great variance in the chamber. With farther location to the nozzle, velocity profiles are more even. At a certain angle of the air jet, there are two vortexes close to the burner. With higher flow rate, it is easier for the airflow to reach the opposite chamber wall. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction High Temperature Air Combustion (HiTAC) is a promising technology for energy savings and emissions reduction [1–6]. It has been developed to meet the demand of various symmetrical heating situations since 1980s [7]. The main feature of this technology is to preheat the air/gas fuel to a specific temperature previously to the fuel combustion. Therefore, this technology has the potential to use the lower BTU gas derived from blast furnace or gasification process of coal and wastes. The breakthrough of HiTAC technology is that it has the potential to overcome the limitations of conventional combustion and allows engineers to finally meet this longstanding imperative [8]. This technology is based on utilizing the sensible heat of flue gas by using regenerative materials which heat up the combustion air/gas fuel to a temperature even higher than 1000 °C while reducing the exhaust temperature to less than 200 °C. Therefore, the utmost heat utilization could be reached by using this technology. Furthermore, combustion will be more stable at low equivalence ratios and the NOx emission is even less than 50 ppm when gaseous fuels are burnt [9]. Up to now, HiTAC technology has been successfully applied to a number of industries including iron and steel [2,4], power generation [10] and chemical industry for coal gasification [11]. However, the applications are only lim* Corresponding author. Tel.: +86 23 65111297; fax: +86 23 65102473. E-mail address: [email protected] (L. Pan). 1359-4311/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2009.05.022
ited to the facilities where the heat loads are symmetrical. It is of great importance to assess whether and how we can implement the HiTAC technology to the asymmetrical heat load situation, such as annular furnace and other similar applications. Traditionally, the heat input at the soaking zone is characterized by flat flame burning on the roof of the furnace and variable flame burner at the heating zone with high flue gas speed. When this technology was considered in an asymmetrical heat load situation, it is impossible to form flat flame on the roof due to the configuration characteristic. Besides that, it is still unknown that whether the flow of the heated burning air can be well organized to form a favorable flow field for slab or rod heating. Rafidi and Blasiak [2] found that with highly reduced temperature fluctuations, turbulent intensity and combustion intensity, the HiTAC flame has a larger reaction zone than the conventional flame. Therefore, the investigation of flow field in the furnace with regenerative burner has become essential to optimize the design. So far, there is no report to show that HiTAC has been used in annular furnace, and the fundamental investigation relating to HiTAC in the asymmetrical heat load situation has not been even started yet. In this paper, a model downscaled to 1:10 and provided with optical access, as shown in Fig. 1, was established to experimentally study flow field and pressure fluctuation characteristics within furnace. The inner diameter of the model is 3.07 m, the height of the model is 0.27 m, and the outer diameter is 4.13 m. The furnace based on present work has been put in operation at Chengdu Steel Co. Lt.
L.-m. Pan et al. / Applied Thermal Engineering 29 (2009) 3426–3430 Table 1 Operating conditions of the experiments. Run no.
Location
Infeeding (m3/h)
Exsuction (m3/h)
1 2 3 4 5
No. 1 heating zone No. 2 heating zone No. 3 heating zone Holding zone Heat recovery zone
992.4 992.4 1212.5 1735.1 0
1002.9 1002.9 1225.3 0 1731.2
Converted pressure (Pa) 2.16 1.33 0.22 0.18
3427
(i) Ensure maximum heat transfer intensity under possible heating process. (ii) According to the heating technical requirements, keep the cross-sectional temperature evenness in the furnace by flue gas motion control. (iii) Ensure slight positive furnace pressure to prevent ambient air from entering the furnace, and at same time ensure evenly distribution of pressure along the furnace length to avoid too much hot flue gas overflowing the furnace.
Note: the above pressure measurement points are located at the center top of corresponding zone.
3. Fluid flow measurement and distribution analysis 2. Basic requirements for gas motion in annular furnace 3.1. Experimental method and data processing Because of the switch-over process, the flow field in the regenerative furnace is different than that in channel or in smoke flue. How to organize the gas motion in a furnace mainly depends on kinetic energy of jet flow from burners. However, the primary resistance of fluid flow is the mixing process between the fuel and preheated air. And a portion of energy is consumed by the recirculation process of flue gas induced by jet flow entrainment. In order to keep positive pressure in a furnace, a portion of kinetic energy is transformed to pressure energy. All the processes mentioned above generate uneven furnace pressure and velocity distributions. Fluid flow in the furnace is mainly controlled by three streams (i.e., free stream, intercrossed free stream and limited stream). Among these three streams, the free stream is the dominant one in the furnace, and it depends on the upstream flow of the furnace. At current situation the upstream flow is the gas load input to the soaking zone. The intercrossed free stream is formed by the preheated air and the fuel blasted into furnace through burners, which determines the local flow patterns near burners. For the asymmetrical flow, it would become a quite complicated process. And the third limited stream, which is very important to spread out in the furnace and ensure the flame stiffness and stability, is generated by the intercrossed stream blasted along the furnace wall, The basic requirements for flue gas flow in the furnace are as follows:
So far, there are three popular experimental methods in the cold aerodynamic field, i.e., (i) flexible ribbon visualization, (ii) particles/smoke visualization; (iii) quantitative data measurement. The first two methods are qualitative method used to visually inspect the flow field, but the third one is a quantitative experimental method by using apparatuses to exam the details of velocity and pressure distributions in the field. In the present paper, both the visualization and quantitative methods are adopted to get better understanding about the cold aerodynamics field of the annular furnace using HiTAC technology. In present work, velocity was measured by an in-house developed two-dimensional approx-PIV system, which comprises light tracking particles, optical light sheet, computer-connected highspeed camera and the image post-processing software. The schematic configuration is depicted at Fig. 2. During the experiments, a fixed camera was set to record the images with a constant interval. With the help of the reference scale, travel distance of a specific particle could be measured using the two consecutive images. Thus, a continuous velocity distribution is obtained. 3.2. Testing apparatus and tracking particles A Redlake HK100 high-speed camera system was adopted to record the flow field. This camera can take pictures at 100,000 fps (frame per second), and is equipped with 2 Giga flash memory and 1000 M Base-T computer-connecting cable. At present work, the speed of the camera has been set to 500 fps, so the time intervals between images are 2 ms. The tracer particles are even diam-
No.3 Heating Zone Sacking Zone
No.2 Heating Zone
Preheating section
No.1 Heating Zone
Fig. 1. Model of annular furnace for cold-state simulation.
Fig. 2. Schematic configuration of a self constructed two-dimensional approx-PIV system.
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3.3. Experimental scheme and data processing At the central location of each zone, a hole was drilled on the nozzle tube to feed in tracer particles. By using a concave mirror halogen light sheet, a horizontal slice of the furnace was illuminated whose thickness is approximate 5 cm. The high-speed camera was vertically mounted above the furnace to take continuous pictures of the track particles. At present work, three operating conditions were investigated, i.e. load at 100%, 70% and 50%, the operating conditions of the experiments of the load at 100% is shown at Table 1. One actual picture is shown as Fig. 3. Before running, a ruler was placed on the furnace to scale the pictures. The pictures taken by the high-speed camera were analyzed with Photoshop CS software to get two-dimensional velocity vector distribution of every individual particle in the field of view. The velocity vector of every point was input into origin 7.5 software to plot the vector graph of the flow field in the furnace. Fig. 3. Image of tracing particles at 70% load of no. 3 heating zone.
4. Experimental results analysis and discussion
eter foam granules with averaged diameter within 2–3 mm. The infeeding and exsuction flow rate was measured by Pitot tube with error of 2.5%, (square root at 1.24%).
To facility the measurement, the outer circle burners were burning and inner circle burners were sucking. To visualize the flow field, the light sheet was located just a little above the burners. As shown in Fig. 4a in the full load situation, the velocity of
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Y /mm
b 1000
500 400 300 200
outer circle
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particle jet
inner circle
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4 m/s
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100
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0 0
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at 70% Rated Load
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at 50% Rated Load Fig. 4. Velocity vector graph of no. 1 heating zone.
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the flow from the nozzle of no. 1 heating zone could reach as high as 30 m/s. As showed in the vector graph, a great amount of particles were concentrated in the area of inner circle of the furnace, and this may be due to the high velocity infeeding flow. It is hard for particles to diffuse to the center area of the furnace. According to the figure, it is quite clear that a vortex exists at the downstream of every nozzle, which may enhance the mixing of fuel gas and preheated air. Although most of the tracking particles moved to the inner circle of the furnace, the tracking particles did not enter the first opposite sucking burner as showed in the velocity vector graph. Thus, it implies that by adopting an appropriate switch-over strategy, the short circuit could be effectively avoided to prevent the fuel from being sucked into the opposite sucking burner, which results in the fracture of refractory material and the decreasing of overall thermal efficiency. The velocity vector distribution of the horizontal cross-section at 70% and 50% rated flow are shown at Fig. 4b and c. At the 70% load, the maximum velocity at the outlet of infeeding burner can reach 25 m/s. As shown at Fig. 4b, a vortex-shape cavity exists at the downstream of the outlet of the infeeding burner. It can be concluded that there was a large-scale vortex at that position. The vortex was resulted from asymmetrical distribution of burners along the two side walls. The vortex prolonged the residence time of the flue gas, which will result in higher velocity flue gas flow to enhance convective heat transfer [12], complete combustion in low oxygen environment and lower combustion temperature [13]. Furthermore, when at 70% load the furnace was still filled with high-speed fresh flue gas, which would meet the demand of temperature uniformity for steel heating. Moreover, it can reduce the temperature concentration and greatly decrease the production of NOx because of the flue gas reflux. At 50% load, the maximum velocity at the outlet of infeeding burner can reach 16 m/s. As shown in Fig. 4c, after particles were spewed out from the outer burner, it could not immediately flow into the inner circle of the furnace, but slowly spread out along the furnace chamber, and the moving direction would change to that of the mainstream very quickly. A vortex still exists at the downstream of the outer burner. It can be deduced from the velocity vector graph that the flame is still stiff enough to overspread the entire furnace chamber at 50% load. If a portion of the burners is turned off, the flue gas circulation would be organized more efficiently to heat steel piece well. But under the low load condition, the inadequate reflux flow will result in a certain temperature difference because the flame stiffness is not strong enough. In the real situation, to ensure the temperature evenness of the furnace, it is necessary to avoid burners running in the low load condition as far as possible, but should adopt to stop a portion of burners. This
a
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problem is not too serious at the first two heating zones, thus the strategy to adjust burner flow still can be adopted to organize heating process. Comparing among the above three operating conditions, although higher flow rate can form higher velocity profile distribution at the furnace cross-section, but according to furnace operation principles, flue gas residence time is also an important factor to be considered which impacts on steel heating efficiency and combustion efficiency, designers want both higher flow speed and longer flue gas residence time. According to the results shown on Fig. 4, it is clear that the optimized load for burners is above 70% rated load. Although there is a large vortex when the burner works at 50% rated load, but the flue gas velocity is too low to get high heat transfer coefficient and uniform temperature distributed flue gas thereafter. Furthermore, as shown on Fig. 4, because the annular shape of the furnace, the flow spewing out from burner has very strong interaction with the flow that comes from upstream, a very strong secondary flow was formed at the downstream location, close to the burner. It is a key feature for the regenerative combustion annular furnace. Generally speaking, the higher the flow rate is, the easier the flow arrived at the other side wall and was sucked out. But the suction does not happen at the first opposite burner. Smaller flow rate results in longer residence time and further downstream sucking location. Generally, the sucking location is the second or the third opposite burner. Moreover, for three different heating zones, we can know from the experiment that the closer infeeding burners are to the soaking section, the more difficult for the fuel gas is to be sucked by the opposite sucking burner. This implies that the flue gas produced by soaking section almost entirely is sucked out in the heating zones and could not arrive at tail flue. The flue gas generated at the present heating zone would be sucked out from the next heating zones. Because of this, it could consumedly prolong the resident time of fresh flue gas in the furnace, and the combustion short circuit would unlikely occur. So it is favorable to perform combustion under high temperature. The velocity profile of cross-section at 10 cm downstream of a burner is shown in Fig. 5. It reveals that because of being too close to the burner, along the path of the gas flowing transversely the cross-section, the violent gas movement could not fill the whole furnace chamber, and the velocity fluctuation is very big. At 30 cm downstream from the burner jet, the speed fluctuation is more weak, which shows that gas and air have been mixed completely, and the velocity fluctuates within 6–9 m/s. At 50 cm downstream from the burner jet, the velocity profile becomes very even, and the velocity fluctuates within 7–8 m/s. In this situation, the flue gas movement could be efficiently organized. Through the comparison among different working conditions and heating zones, it can be concluded that at farther downstream
b
Fig. 5. Velocity profiles at radial cross-section of no. 1 heating zone.
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from the burner jet there is more even velocity profiles and more mild speed fluctuation, which would be favorable to form even temperature field and stable pressure field; smaller flow rate also can form more even temperature field, and the place to attain stability would be closer to the upstream burner than that of larger flow rate. Moreover, the velocity distribution would become more uniform with smaller flow rate, which can effectively eliminate temperature difference of the heated steel piece along the length direction.
flue gas and brings out the complete combustion in low oxygen environment and the lower combustion temperature. Acknowledgements The authors are grateful for the support of the research funding from the National Science Foundation of China (No. 50406012) and CISDI Co. Lmt., and also the technical supports from Mr. Zhengrong Zhang and prof. Jiaofen Pan.
5. Summary
References
According to the cold-state experimental results for the regenerative heating annular furnace, the conclusions could be drawn as follows:
[1] A.K. Gupta, Thermal characteristics of gaseous fuel flames using high temperature air, Journal of Engineering for Gas Turbines and Power 126 (1) (2004) 9–19. [2] N. Rafidi, W. Blasiak, Heat transfer characteristics of HiTAC heating furnace using regenerative burners, Applied Thermal Engineering 26 (16) (2006) 2027– 2034. [3] T. Zhu, D. Chen, H. Zhang, et al., Numerical simulation of NOx emission during high temperature air combustion for burning low heat value gas, Proceedings of the International Conference on Energy and the Environment 2 (2003) 1030–1035. [4] T. Ishii, Numerical simulations of highly preheated air combustion in an industrial furnace, Journal of Energy Resource Technology 120 (1998) 276–284. [5] R. Tanaka. High temperature air combustion advanced strategic technology originated in Japan, Technical Note 900420, 1996, p. 4. [6] T. Hasegawa, R. Tanaka, High temperature air combustion – revolution in combustion technology – part I: new findings on high temperature air combustion, JSME International Journal, Series B 41 (4) (1998) 1079–1084. [7] Keith J. Stokes, Recovery of heat from flue gas, US patent 4299561, 1981. [8] H. Tsuji, A.K. Gupta, T. Hasegawa, M. Katsuri, et al., High Temperature Air Combustion: From Energy Conservation to Pollution Reduction, CRC Press, New York, 2003. [9] M. Katsuki, T. Hasegawa, The science and technology of combustion in highly preheated air, Symposium (International) on Combustion 27 (2) (1998) 3135– 3146. [10] Roman Weber, John P. Smart, Willem vd Kamp, On the (MILD) combustion of gaseous, liquid, and solid fuels in high temperature preheated air, Proceedings of the Combustion Institute 30 (2) (2005) 2623–2629. [11] S. Sugiyama, N. Suzuki, Y. Kato, et al., Gasification performance of coals using high temperature air, Energy 30 (2–4) (2005) 399–413. [12] M.T. Glinkov, G.M. Glinkov, A General Theory of Furnaces, Mir Publishers, Moscow, 1980. [13] K.S. Shanmukharadhya, K.G. Sudhakar, Experimental and numerical investigation of vortex-induced flame propagation in a biomass furnace with tangential over fire registers, The Canadian Journal of Chemical Engineering 86 (1) (2008) 43–52.
(1)
Generally speaking, the infeeding flow from the burner would not be sucked in by the first opposite suction. According to the flow rate, it would be sucked out by the second or third suction. The closer infeeding burners to the soaking section, the more difficult for the fuel gas to be sucked by opposite suction; and the flue gas produced at soaking section almost completely is sucked out in the heating zones and could not arrive at the tail flue. The flue gas generated at the present heating zone would be sucked out from next heating zones. It can consumedly prolong the resident time of fresh flue gas, and the combustion short circuit would unlikely occurs, so it is favorable to perform combustion under high temperature. (2) From the outer furnace wall to the inner circle, the velocity changed remarkably, and the furnace chamber could not be wholly filled with moving mixture, so the velocity fluctuation is very strong. Farther downstream from the burner jet there are more even velocity profiles and more mild speed fluctuation, which would be favorable to form a even temperature field and a stable pressure field. (3) Because all of the infeeding burners supply gas with certain angle, it always results in a pair of vortexes at both sides of the burners. The vortex prolongs the residence time of the