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Performance analysis of a novel W-type radiant tube
Applied Thermal Engineering 152 (2019) 482–489
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Research Paper
Performance analysis of a novel W-type radiant tube Qian Xu lidi Jiaf
a,b
a,⁎
c,⁎
d
a
T a
e
, Junxiao Feng , Jingzhi Zhou , Chong Ding , Ziqi Bai , huanbao Fan , Yong Zang ,
a
School of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, China Beijing Key Laboratory of Energy Conservation and Emission Reduction for Metallurgical Industry, Beijing 100083, China Institute of Engineering Thermophysics, Chinese Academy of Science, Beijing 100190, China d School of Mathematics, Taiyuan University of Technology, Taiyuan 030024, China e School of Mechanical Engineering, University of Science and Technology Beijing, Beijing, China f ANGANG Steel Company Limited, Liaoning, China b c
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
W-type radiant tube with a • Afluenovel gas circulation structure was developed.
effects of nozzle diameter on the • The N-WRT performance were analyzed and discussed.
effects of nozzle location on the N• The WRT performance were analyzed and discussed.
effects of circulating tube dia• The meter on the N-WRT performance were discussed.
gas flow, temperature distribu• The tion, and NOx emission of the N-WRT were analyzed.
A R T I C LE I N FO
A B S T R A C T
Keywords: Radiant tube NOx emission Temperature uniformity Thermal efficiency Circulation ratio
In this study, based on the structural characteristics of the traditional W-type radiation tube, a novel W-type radiant tube (N-WRT) with a flue gas circulation structure was developed to improve the heating uniformity of the radiator tube and the heating efficiency of the workpiece. New three-dimensional computational fluid dynamics modeling of the N-WRT was conducted to assess the heat transfer and combustion phenomena. In addition, the effects of nozzle diameter, nozzle location, and circulating tube diameter (all of which affect the main performance of the N-WRT) on the gas flow, temperature distribution, and nitrogen oxide (NOx) emission were analyzed and discussed in detail. Moreover, the amount of heat transfer, ratio of circulating flue gas, surface temperature distribution, thermal efficiency, and NOx concentration of the N-WRT based on different nozzle characteristics were compared. The results showed that the circulation ratio of flue gas increased from 0.47 at D60 mm to 2.2 at D28 mm. The NOx emission decreased most significantly from 100 ppm at D60 mm to 40 ppm at D50 mm. When the circulating tube diameter changed from 180 to 120 mm, the gas velocity at the junction decreased by 12%.
⁎
Corresponding authors. E-mail addresses: [email protected] (Q. Xu), [email protected] (J. Zhou).
https://doi.org/10.1016/j.applthermaleng.2019.02.097 Received 8 January 2019; Received in revised form 12 February 2019; Accepted 18 February 2019 Available online 19 February 2019 1359-4311/ © 2019 Published by Elsevier Ltd.
Applied Thermal Engineering 152 (2019) 482–489
Q. Xu, et al.
1. Introduction With the progress of industrialization and economic development, ecological environment has become increasingly prominent [1–3], especially in steel and chemical industry. Metal heat treatment can change the microstructure or chemical composition of a workpiece surface without changing the shape of the workpiece itself [4]. Metal heat treatment is necessary to change the mechanical, physical, and chemical properties of the workpiece and ultimately meet industrial needs, and thus it is one of the most important processes in mechanical manufacturing [5]. The most important heating device in the heat treatment furnace is the gas radiation tube, and it plays a major role in the industrial heat treatment process [6]. Research on radiation tubes began in the 1930s with straight-tube radiation tubes [7]. With the increasing demand for heat treatment, U-type and W-type radiation tubes [8,9], as well as P- type and O-type radiation tubes with flue gas recycling, gradually appeared beginning in the 1950s. Radiation tubes with different structures have different characteristics and applications [10–12]. For example, U-type radiation tubes have high thermal efficiency but a significant temperature difference along the axial direction and high NOx emission [13,14]. O-type radiation tubes are widely used in cold-rolled steel plate annealing furnace, but they are difficult to replace. Currently, W-type radiation tubes are most commonly used in industry, but the temperature difference along the axial direction is considerable, and the production of NOx is high [15,16]. With the development of radiation tubes, an increasing amount of attention has been given to their circulating structures [17,18]. Because of the cyclic structure of the radiation tube, the circulating flue gas not only can reduce the wall temperature difference of a radiation tube but it can also dilute the oxygen concentration in a high-temperature zone as well as reduce the combustion center temperature and inhibit the formation of thermodynamic NOx, thus reducing the emission of NOx [19,20]. The circulating structure has become a new development direction for radiation tubes because of its advantages of high-temperature uniformity and low pollution [21]. Therefore, based on the research of traditional W-type radiation tubes (T-WRT), our research team developed a new type of W-type radiation tube (N-WRT) with a flue gas circulation structure to solve the problems of temperature uniformity and NOx emission. In addition, by comparing the T-WRT and our N-WRT with respect to the flow field, temperature field, thermal efficiency, and NOx emission, we found that the N-WRT has considerable advantages in terms of temperature uniformity and NOx emission control. However, the performance of the N-WRT is influenced by the structural characteristics of the tube and burner [22,23]. The burner is the core component of the radiant tube. The diameter of the burner nozzle directly affects the flame shape and its stability. A negative pressure condition at the nozzle is required for the N-WRT, so that the flue gas in the circulating tube can be entrained to promote the circulating flow of the entire system. Therefore, the gas velocity at the nozzle must be strictly controlled, which then affects the pressure difference and amount of circulating flue gas, thus altering the circulation ratio, pressure, and temperature distributions in the flow field as well as the generation and emission of NOx [24]. The diameter of the circulating tube directly affects the amount of circulating flue gas and the distributions of temperature and pressure. The circulating flue gas in the N-WRT can dilute the oxygen content in the high-temperature combustion zone, reduce the flame center temperature, and inhibit the generation of NOx. In addition, the nozzle positions will form different locations of the low-pressure zone at the nozzle, thus affecting the entrainment and circulation of flue gas. In summary, the diameter of the burner nozzle, the location characteristics of the burner, and the diameter of the circulating tube, all of which affect the main performance of the N-WRT, are analyzed and discussed in detail in this study.
Fig. 1. The overall structure of the N-WRT.
2. Numerical calculation model for the N-WRT The structure of the N-WRT is based on that of the T-WRT. The general structure of the N-WRT is shown in Fig. 1. For the design of the nozzle, to ensure that the negative pressure generated by the gas in the flame center can eject the gas from the circulating tube and return it to the combustion center, the outlet structure of the combustion chamber is altered to produce a gradually shrinking nozzle, the specific structure of which is shown by the dotted red lines in Fig. 1. For the design of the circulating structure, a circulating tube of flue gas is added between the first and fourth straight tube sections of the T-WRT, as shown in Fig. 1(9), which can direct a portion of the flue gas to the flame combustion center. The circulating tube can dilute the oxygen content. In addition, the high-temperature circulating flue gas can reduce the temperature difference and local hot spots on the tube wall, improve the uniformity of temperature distribution, and reduce the stress concentration caused by local hot spots. The geometry, material properties, model validation and main features of the burner under study have been deeply described in detail in previous works [25,26]. The governing equations are solved by using the software Ansys Fluent 16.0 and Gambit 3.0 in Think Station D30 with 4 dual-core CPU and 64 GB RAM. For the grid generation, based on the three-dimensional symmetry of the N-WRT structural characteristics, half of the structure is selected to establish the finite element model to speed up the numerical calculation. The block partitioning grid method is adopted for mesh generation. The central tube, elbow, and straight tube of the N-WRT are divided by the structural mesh. Because the structure of the burner is complex, the local mesh is refined for the burner, and a tetrahedral mesh is adopted. The specific mesh generation is shown in Fig. 2. The continuity equation, and energy equation, the k-e turbulence model, eddy-dissipation (ED) combustion model, and discrete ordinates (DO) radiation model are used, respectively [27–30]. The specific boundary conditions are listed in Table 1. 3. Numerical simulation results and discussion 3.1. Effects of the nozzle diameter on the N-WRT’s performance For comparative analysis, five typical nozzles with different nozzle diameters of D60, 50, 40, 36, and 28 mm were selected. The effects of the nozzle diameter on the flow field, temperature field, thermal efficiency, and NOx emission of the N-WRT were studied in depth. 3.1.1. Analysis of the flow field and NOx emission Table 2 (Row: 1-3) shows the changes in gas velocity, circulation ratio of flue gas, and NOx concentration at the outlet of the N-WRT based on different nozzle diameters. When the nozzle diameter increased from D60 mm to D28 mm, the velocity at the nozzle increased, and the change rate of velocity increased significantly. The circulation 483
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Fig. 2. The specific mesh generation of the N-WRT.
ratio of flue gas in the radiant tube also increased rapidly from 0.47 at D60 mm to 2.2 at D28 mm. This indicates that the negative pressure at the nozzle increased as the nozzle diameter decreased. This caused a greater amount of flue gas to be entrained to participate in secondary combustion. When the air volume was constant, reducing the nozzle diameter can considerably change the velocity of the gas flow at the nozzle and improve the circulation ratio of flue gas. In addition, when the nozzle diameter decreased, the emission of NOx at the outlet decreased considerably. Particularly in the initial stage (i.e., in the range of D50 to D60 mm), the nozzle velocity changed in the range of 50–90 m/s, and the emission of NOx from the radiant tube decreased considerably from 100 to 40 ppm. However, when the NOx emission at the outlet was less than 40 ppm and the nozzle diameter continued to decrease, the effect of the nozzle diameter on the NOx concentration at the outlet became increasingly weaker. Fig. 3 shows the gas pressure distribution on the symmetrical plane of the N-WRT with different nozzle diameters. It can be seen that when the diameter of the nozzle decreased, the overall pressure distribution range of the gas in the radiant tube increased and the minimum value of negative pressure changed from −700 to −1050 Pa. In particular, the negative pressure value of the gas at the nozzle also increased, which enabled a greater amount of gas in the circulating tube to enter the combustion main tube to participate in reburning. It also helped to reduce the temperature of the combustion center zone and improve the uniformity of the wall temperature.
Table 2(Row:4–9) shows the other performance parameters of the N-WRT with different nozzle diameters. The table shows that when the nozzle diameter decreased, the minimum wall temperature of the radiant tube increased gradually from 1226 to 1239 K, and the maximum wall temperature decreased from 1411 to 1396 K, which decreased the wall temperature difference of the radiant tube from 185 to 157 K. After the diameter of the nozzle was reduced to 36 mm, the temperature difference in the wall of the radiant tube remained nearly unchanged, but the overall thermal efficiency of the radiant tube decreased. The shortened nozzle diameter increased the average flow velocity of gas in the tube and increased the flue gas temperature of the exhaust radiant tube, which resulted in flue gas loss. However, this loss can be recovered by improving the heat exchanger or regenerative burner, thus avoiding a reduction in thermal efficiency.
3.1.2. Analysis of the temperature field Fig. 4 shows the gas temperature distribution profile on a symmetrical plane of the radiant tube with different nozzle diameters. The figure shows that when the nozzle diameter gradually decreased, the high-temperature zone of combustion gradually decreased, whereas the center temperature of the combustion flame did not change noticeably. However, the temperature gradient of the gas in the radiant tube decreased and the temperature distribution became increasingly uniform. Particularly when the diameter of the nozzle was 60 mm, the temperatures at the tail end and in the circulating section of the radiant tube were in the range of 1160–1260 K, whereas the temperature of the fourth straight tube was in the range of 1260–1370 K. When the diameter of the nozzle was 36 mm, the temperatures of these two sections were in the range of 1260–1360 K, which was nearly the same as that of the fourth straight tube. This trend of gas temperature distribution also affected the wall temperature distribution of the radiant tube, thus reducing the wall temperature difference.
3.2.1. Analysis of the flow field and NOx emission Table 3(Row:1–3) shows the changes in gas velocity, circulation ratio, and NOx concentration at the outlet of the N-WRT with different nozzle locations. The table shows that when the nozzle was located at the circulating central line (x = 0 mm), the gas velocity near the radiant nozzle was the highest, which was approximately 20 m/s greater than that of the other nozzle position conditions. This shows that when the burner nozzle was located at the circulating central line, the gas velocity increased effectively to control the flue gas. In addition, as Table 3 shows, when the nozzle moved inward along the combustion main tube, the high-temperature zone moved backwards, and the dilution effect of the circulating flue gas on the oxygen concentration in the high-temperature zone was weakened. As a result, the flame center temperature was higher, and the NOx emission increased. The gas pressure distribution in the N-WRT based on different nozzle locations is shown in Fig. 6. When the nozzle moved inward along the combustion main tube, the range of the high-pressure zone of
3.2. Effects of nozzle location on the N-WRT’s performance An analysis of the effects of the nozzle location on the flow field, temperature field, thermal efficiency, and NOx emission of the N-WRT was conducted. The central line of the flue gas circulating tube was set to x = 0 mm, which is shown in Fig. 5. To ensure a sufficient amount of circulating flue gas, the nozzle should be located within the intersection range of the circulating and first straight tubes. Therefore, with the nozzle located at x = 100, x = 45, x = 0, x = −50, and x = −90 mm, numerical simulations of the N-WRT were conducted.
Table 1 The boundary conditions of the radiant tube. Boundary Inlet
Fuel Air
Outlet Wall
Flue gas wall function method without slip
Furnace temperature
Type
Value
Mass flow rate Mass flow rate Excess air coefficient Pressure Convection and Radiation
3.21 × 10−3 kg/s 6.06 × 10−2 kg/s 1.1 −600 Pa
Constant temperature
484
α = 9 W/(m2 K), ε = 0.85 1223 K
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Table 2 The performance parameters of the N-WRT under different nozzle diameters.
(1) (2) (3) (4) (5) (6) (7) (8) (9)
Project
D60 mm
D50 mm
D40 mm
D36 mm
D28 mm
Gas velocity (m/s) Circulation ratio NOx concentration (ppm) Surface temperature/max (K) Surface temperature/min (K) Average temperature (K) Temperature difference (K) Heat transfer (kW) Thermal efficiency (%)
61 0.47 98.04 1411 1226 1286 185 125.6 78.5
88 0.80 29.50 1402 1228 1286 174 124.8 78.0
138 1.26 14.81 1395 1234 1286 161 123.8 77.4
171 1.51 6.00 1393 1236 1286 157 123.4 77.1
280 2.20 4.04 1396 1239 1286 157 122.6 76.6
−50, 0, 45, and 100 mm. The high temperature central areas were located at the axial positions of 600, 700, 900, 1000, and 1100 mm. When the nozzle was located on the left side of the circulating center line (x = 0), the gas temperature distribution in the range of 0–1100 mm along the axial direction decreased by approximately 5–40 K when the nozzle moved inward. When the nozzle was located on the right side of the circulating center line (x = 0 mm), the gas temperature distribution in the range of 0–1100 mm along the axial direction decreased by approximately 50–200 K when the nozzle moved inward. In the range of 1100–1600 mm along the axial direction, the gas temperature was clearly higher than that on the left side of the circulating center. The gas temperature at the nozzle position of x = 45 mm was the highest and decreased when the nozzle moved inward. Table 3(Row:4–10) shows the other performance parameters of the N-WRT with different nozzle locations. The table shows that when the nozzle moved inward away from the burner, the minimum wall temperature of the radiant tube decreased gradually from 1234 to 1210 K, whereas the maximum wall temperature was lowest at the circulating center and increased when the nozzle moved toward both sides. However, the fluctuation in temperature was extremely small (approximately 3 K). Because the position of the nozzle moved inward, the hightemperature zone of the flame moved backward, causing the wall temperature of the radiant tube near the end of the burner to decrease. Therefore, the wall temperature difference of the radiant tube increased from 158 to 181 K. In addition, the flue gas that returned from the circulating tube was dispersed after reaching the main combustion tube. As the combustion zone moved backward, the oxygen content in the combustion center was diluted only after the circulating flue gas flowed for a certain distance. This in turn reduced the dilution effect of the circulating flue gas on the oxygen content in the high-temperature zone. Thus, the flame center temperature increased, resulting in an increase in the production
gas in the tube decreased, whereas the range of the low-pressure zone at the burner nozzle increased, resulting in a greater amount of flue gas from the circulating tube to flow into the combustion main tube and participate in flue gas recirculation combustion. When the nozzle moved inward, the circulation ratio increased, but after the position of the circulating central line changed (i.e., x = 0 mm), the range decreased gradually. This indicates that during actual operation, to improve the circulation ratio, the nozzle should be properly moved inward. However, the position of the circulating central line should not be exceeded to prevent circulating flue gas with high temperature from flowing into the combustor and shortening the service life of the burner. Fig. 7 shows oxygen distribution along the axial direction with different nozzle locations. The figure shows that in the region of 100–800 mm along the axial direction, the “turning” phenomenon caused by the decreased distribution of oxygen derived from the circulating flue gas appears in all curves. When the nozzle was located on the left side of the circulating central line (x = 0 mm), the bending degree was greater. By contrast, when the nozzle was located at the right side of the circulating central line (x = 0 mm), the bending degree was smaller. This indicates that when the nozzle is moved inward, the dilution effect of the circulating flue gas on oxygen concentration decreased, which was not conducive to reducing the production of NOx. In the region of 800–1000 mm along the axial direction where the gas temperature distribution was higher, the oxygen content was lower. This indicates that most of the oxygen was mainly used for combustion, whereas a small part of the oxygen was used to generate NOx or other oxides. 3.2.2. Analysis of the temperature field Fig. 8 shows gas temperature distribution along the axial direction in the main combustion tube based on different nozzle locations. Fig. 8 shows that when the nozzle moved inward, the corresponding hightemperature zone moved inward. The nozzles were located at x = −90,
Fig. 3. The gas pressure distribution of the N-WRT with different nozzle diameters. 485
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Fig. 4. The gas temperature distribution of the N-WRT with different nozzle diameters.
velocity at the nozzle fluctuated only by approximately 1 m/s. However, the gas velocity at the confluence surface decreased dramatically with an increase in the diameter of the circulating tube. When the diameter of the circulating tube was decreased from 180 to 120 mm, the gas velocity at the junction decreased by 12%. By contrast, the gas velocity at the nozzle remained unchanged, and the velocity gap between the circulating flue gas and gas at the nozzle increased. This caused a greater amount of flue gas to flow back into the combustion main tube, thus increasing the amount of circulation flue gas. Fig. 10 shows the relationship between the gas circulation ratio and the diameter of the circulating tube. The figure shows that the circulation ratio increased with the increase in the circulating tube diameter, thus verifying the previous inference concerning the increase in flue gas circulation. The change of the diameter of the circulating tube directly affected the velocity of the gas flow and flue gas circulation ratio. It also indirectly affected the production of NOx. The large amount of circulating flue gas diluted the oxygen content in the high-temperature zone of combustion and reduced the peak value of the flame temperature, thus reducing the generation of NOx. Fig. 11 shows the variations in fuel gas mass fraction along the axis direction with different circulating tube diameters. The figure shows that the distribution of the fuel gas within the four structures was nearly the same, indicating that the change in the circulating tube diameter had no obvious effect on the distribution of gas mass fraction. Fig. 12 shows the maximum temperature of the flame combustion zone with different circulating tube diameters. The peak value of flame combustion decreased with an increase in the diameter of the circulating tube. When the diameter of the circulating tube was 120 mm, the peak value of flame was 2300 K, while when the diameter of the circulating tube was 180 mm, the peak value of flame reached 2260 K, and decreased approximately 40 K. Fig. 13 shows the variations in NOx concentrations at the outlet of the N-WRT based on different diameters of the circulating tube. When the diameter of the
Fig. 5. The different nozzle positions along the central line of the flue gas circulating tube.
of NOx. Simultaneously, because the nozzle moved inward, the moving distance of high-speed gas that was emitted from the nozzle decreased, and the exhaust temperature increased from 1276 to 1289 K. The heat loss in the flue gas increased with the increase in exhaust temperature, and the heat efficiency of the radiant tube decreased from 77 to 76.6%. 3.3. Effects of the circulating tube diameter on the N-WRT’s performance For a comparative analysis, four typical circulating tubes with different tube diameters of 120, 140, 160, and 180 mm were selected, respectively. Simulations to determine the effects of nozzle diameter on the flow field, temperature field, thermal efficiency, and NOx emission of the N-WRT were conducted and analyzed. 3.3.1. Analysis of the flow field and NOx emission The gas velocities at the burner nozzle and the junction of the circulating and main combustion tubes are shown in Fig. 9. When the diameter of the circulating tube changed from 120 to 180 mm, the gas Table 3 The performance parameters of the N-WRT under different nozzle locations.
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
Project
X = −90 mm
X = −50 mm
X = 0 mm
X = 45 mm
X = 100 mm
Gas velocity (m/s) Circulation ratio NOx concentration (ppm) Surface temperature /max (K) Surface temperature /min (K) Average temperature (K) Temperature difference (K) Heat transfer (kW) Exhaust temperature (K) Thermal efficiency (%)
128 1.26 7.10 1392 1234 1286 158 123.8 1276 77.4
130 1.36 7.81 1392 1229 1286 163 123.6 1280 77.3
148 1.58 14.65 1389 1222 1286 167 123.0 1284 76.9
130 1.62 28.49 1391 1215 1286 176 123.2 1285 77.0
128 1.67 63.65 1391 1210 1286 181 122.5 1289 76.6
486
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Fig. 6. The gas pressure distribution of the N-WRT with different nozzle locations.
Fig. 8. The gas temperature distribution along the axial direction with different nozzle locations.
Fig. 7. The oxygen distribution along the axial direction with different nozzle locations.
circulating tube did not change the trend of temperature distribution. With an increase in the diameter of the circulating tube, the gas temperature distribution of the radiant tube remained unchanged in the area of 150–300 mm in the axial distance, whereas the trend in gas temperature variation for the radiant tube began to change in the region of 300–700 mm. When the diameter of the circulating tube increased from 120 to 180 mm, the gas temperature at the same location decreased by approximately 10–25 K. In the region of 700–2000 mm along the axial direction, the drop in gas temperature caused by the diameter of the circulating tube was maintained at approximately 30 K. In particular, when the diameter of the circulating tube was in the range of 140–160 mm, the gas temperature maintained a relatively uniform distribution. With the increase in the diameter of the circulating tube, the temperature difference of the main combustion tube decreased and
circulating tube increased from 120 to 180 mm, the concentration of NOx at the outlet decreased from 62.4 to 45 ppm. 3.3.2. Analysis of the temperature field With an increased amount of circulating flue gas, a greater amount of high-temperature gas participates in secondary combustion, which not only can reduce the oxygen concentration in the flame combustion zone but can also reduce the peak value of the flame and the generation of NOx. It can also improve the wall temperature uniformity of the radiant tube. Fig. 14 shows the variations in gas temperature along the gas flow direction of the main combustion tube with different circulating tube diameters. The figure shows that the change of the diameter of the 487
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Fig. 9. The gas velocities at the burner nozzle and the junction.
Fig. 12. The peak value of flame combustion with different circulating tube diameters.
Fig. 10. The variation curve of the gas circulating ratio with different circulating diameters. Fig. 13. The variation curve of NOx concentration with different circulating tube diameters.
Fig. 11. The variation curve of fuel gas mass fraction with different circulating tube diameters. Fig. 14. The variation curves of gas temperature with different circulating tube diameters.
the uniformity of the gas temperature increased as a result of the influence of the temperature distribution. Table 4 shows the performance parameters of the N-WRT based on different circulating tube diameters. The table shows that when the diameter of the circulating tube increased from 60 to 90 mm, the heat
transfer surface area of the circulating tube increased, and thus the surface heat transfer was enhanced. The maximum wall temperature of the radiant tube decreased, whereas the corresponding minimum wall 488
Applied Thermal Engineering 152 (2019) 482–489
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Table 4 The performance parameters of the N-WRT under different circulating tube diameters.
(1) (2) (3) (4) (5) (6) (7)
Project
D = 120 mm
D = 140 mm
D = 160 mm
D = 180 mm
Surface temperature /max (K) Surface temperature /min (K) Average temperature (K) Temperature difference (K) Temperature nonuniformity coefficient Heat transfer (kW) Thermal efficiency (%)
1403 1228 1287 175 0.1247 124.5 77.8
1401 1229 1286 172 0.1227 124.7 77.9
1398 1229 1286 169 0.1209 124.9 78.1
1396 1230 1285 166 0.1189 125.0 78.1
temperature increased slightly, which inevitably improved the uniformity of the radiant tube wall temperature. The wall temperature difference decreased from 175 to 166 K, the surface temperature nonuniformity coefficient increased from 0.1247 to 0.1189, and the thermal efficiency of the radiant tube increased from 77.8 to 78.1%.
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4. Conclusion The following summarize the findings of this study. (1) When the nozzle diameter increased from D60 to D50 mm, the nozzle velocity changed in the range of 50–90 m/s, and the emission of NOx from the radiant tube decreased significantly from 100 to 40 ppm. When the nozzle diameter increased from D60 to D36 mm, the minimum wall temperature of the radiant tube increased gradually from 1226 to 1239 K, and the maximum wall temperature decreased from 1411 to 1396 K. (2) When the nozzle was located at x = 0 mm, the gas velocity near the radiant nozzle was the highest, which was approximately 20 m/s greater than when the nozzle was in other locations. (3) When the nozzle moved inward, the dilution effect of the circulating flue gas on oxygen concentration decreased, which was not conducive to reducing the production of NOx. The gas temperature at the nozzle position of x = 45 mm was the highest and decreased when the nozzle moved inward. (4) When the diameter of the circulating tube changed from 180 to 120 mm, the gas velocity at the junction decreased by 12%, whereas that at the nozzle remained unchanged. In addition, the velocity gap between the circulating flue gas and the gas at the nozzle increased. Acknowledgments The authors gratefully acknowledge the National key Research and Development Program of China under Grant No. 2017YFC0210303. And this work is supported by the Fundamental Research Funds for the Central Universities of China (FRF-BD-18-015A) and the State Key Laboratory of Technologies in Space Cryogenic Propellants of China (SKLTSCP1702). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.applthermaleng.2019.02.097. References [1] Z. Liu, Y. Liu, B.-J. He, W. Xu, G. Jin, X. Zhang, Application and suitability analysis of the key technologies in nearly zero energy buildings in China, Renew. Sustain. Energy Rev. 101 (2019) 329–345. [2] X. She, L. Cong, B. Nie, G. Leng, H. Peng, Y. Chen, X. Zhang, T. Wen, H. Yang, Y. Luo, Energy-efficient and -economic technologies for air conditioning with vapor compression refrigeration: a comprehensive review, Appl. Energy 232 (2018) 157–186. [3] P. Cui, M. Yu, Z. Liu, Z. Zhu, S. Yang, Energy, exergy, and economic (3E) analyses and multi-objective optimization of a cascade absorption refrigeration system for
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Performance analysis of a novel W-type radiant tube Qian Xu lidi Jiaf
a,b
a,⁎
c,⁎
d
a
T a
e
, Junxiao Feng , Jingzhi Zhou , Chong Ding , Ziqi Bai , huanbao Fan , Yong Zang ,
a
School of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, China Beijing Key Laboratory of Energy Conservation and Emission Reduction for Metallurgical Industry, Beijing 100083, China Institute of Engineering Thermophysics, Chinese Academy of Science, Beijing 100190, China d School of Mathematics, Taiyuan University of Technology, Taiyuan 030024, China e School of Mechanical Engineering, University of Science and Technology Beijing, Beijing, China f ANGANG Steel Company Limited, Liaoning, China b c
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
W-type radiant tube with a • Afluenovel gas circulation structure was developed.
effects of nozzle diameter on the • The N-WRT performance were analyzed and discussed.
effects of nozzle location on the N• The WRT performance were analyzed and discussed.
effects of circulating tube dia• The meter on the N-WRT performance were discussed.
gas flow, temperature distribu• The tion, and NOx emission of the N-WRT were analyzed.
A R T I C LE I N FO
A B S T R A C T
Keywords: Radiant tube NOx emission Temperature uniformity Thermal efficiency Circulation ratio
In this study, based on the structural characteristics of the traditional W-type radiation tube, a novel W-type radiant tube (N-WRT) with a flue gas circulation structure was developed to improve the heating uniformity of the radiator tube and the heating efficiency of the workpiece. New three-dimensional computational fluid dynamics modeling of the N-WRT was conducted to assess the heat transfer and combustion phenomena. In addition, the effects of nozzle diameter, nozzle location, and circulating tube diameter (all of which affect the main performance of the N-WRT) on the gas flow, temperature distribution, and nitrogen oxide (NOx) emission were analyzed and discussed in detail. Moreover, the amount of heat transfer, ratio of circulating flue gas, surface temperature distribution, thermal efficiency, and NOx concentration of the N-WRT based on different nozzle characteristics were compared. The results showed that the circulation ratio of flue gas increased from 0.47 at D60 mm to 2.2 at D28 mm. The NOx emission decreased most significantly from 100 ppm at D60 mm to 40 ppm at D50 mm. When the circulating tube diameter changed from 180 to 120 mm, the gas velocity at the junction decreased by 12%.
⁎
Corresponding authors. E-mail addresses: [email protected] (Q. Xu), [email protected] (J. Zhou).
https://doi.org/10.1016/j.applthermaleng.2019.02.097 Received 8 January 2019; Received in revised form 12 February 2019; Accepted 18 February 2019 Available online 19 February 2019 1359-4311/ © 2019 Published by Elsevier Ltd.
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1. Introduction With the progress of industrialization and economic development, ecological environment has become increasingly prominent [1–3], especially in steel and chemical industry. Metal heat treatment can change the microstructure or chemical composition of a workpiece surface without changing the shape of the workpiece itself [4]. Metal heat treatment is necessary to change the mechanical, physical, and chemical properties of the workpiece and ultimately meet industrial needs, and thus it is one of the most important processes in mechanical manufacturing [5]. The most important heating device in the heat treatment furnace is the gas radiation tube, and it plays a major role in the industrial heat treatment process [6]. Research on radiation tubes began in the 1930s with straight-tube radiation tubes [7]. With the increasing demand for heat treatment, U-type and W-type radiation tubes [8,9], as well as P- type and O-type radiation tubes with flue gas recycling, gradually appeared beginning in the 1950s. Radiation tubes with different structures have different characteristics and applications [10–12]. For example, U-type radiation tubes have high thermal efficiency but a significant temperature difference along the axial direction and high NOx emission [13,14]. O-type radiation tubes are widely used in cold-rolled steel plate annealing furnace, but they are difficult to replace. Currently, W-type radiation tubes are most commonly used in industry, but the temperature difference along the axial direction is considerable, and the production of NOx is high [15,16]. With the development of radiation tubes, an increasing amount of attention has been given to their circulating structures [17,18]. Because of the cyclic structure of the radiation tube, the circulating flue gas not only can reduce the wall temperature difference of a radiation tube but it can also dilute the oxygen concentration in a high-temperature zone as well as reduce the combustion center temperature and inhibit the formation of thermodynamic NOx, thus reducing the emission of NOx [19,20]. The circulating structure has become a new development direction for radiation tubes because of its advantages of high-temperature uniformity and low pollution [21]. Therefore, based on the research of traditional W-type radiation tubes (T-WRT), our research team developed a new type of W-type radiation tube (N-WRT) with a flue gas circulation structure to solve the problems of temperature uniformity and NOx emission. In addition, by comparing the T-WRT and our N-WRT with respect to the flow field, temperature field, thermal efficiency, and NOx emission, we found that the N-WRT has considerable advantages in terms of temperature uniformity and NOx emission control. However, the performance of the N-WRT is influenced by the structural characteristics of the tube and burner [22,23]. The burner is the core component of the radiant tube. The diameter of the burner nozzle directly affects the flame shape and its stability. A negative pressure condition at the nozzle is required for the N-WRT, so that the flue gas in the circulating tube can be entrained to promote the circulating flow of the entire system. Therefore, the gas velocity at the nozzle must be strictly controlled, which then affects the pressure difference and amount of circulating flue gas, thus altering the circulation ratio, pressure, and temperature distributions in the flow field as well as the generation and emission of NOx [24]. The diameter of the circulating tube directly affects the amount of circulating flue gas and the distributions of temperature and pressure. The circulating flue gas in the N-WRT can dilute the oxygen content in the high-temperature combustion zone, reduce the flame center temperature, and inhibit the generation of NOx. In addition, the nozzle positions will form different locations of the low-pressure zone at the nozzle, thus affecting the entrainment and circulation of flue gas. In summary, the diameter of the burner nozzle, the location characteristics of the burner, and the diameter of the circulating tube, all of which affect the main performance of the N-WRT, are analyzed and discussed in detail in this study.
Fig. 1. The overall structure of the N-WRT.
2. Numerical calculation model for the N-WRT The structure of the N-WRT is based on that of the T-WRT. The general structure of the N-WRT is shown in Fig. 1. For the design of the nozzle, to ensure that the negative pressure generated by the gas in the flame center can eject the gas from the circulating tube and return it to the combustion center, the outlet structure of the combustion chamber is altered to produce a gradually shrinking nozzle, the specific structure of which is shown by the dotted red lines in Fig. 1. For the design of the circulating structure, a circulating tube of flue gas is added between the first and fourth straight tube sections of the T-WRT, as shown in Fig. 1(9), which can direct a portion of the flue gas to the flame combustion center. The circulating tube can dilute the oxygen content. In addition, the high-temperature circulating flue gas can reduce the temperature difference and local hot spots on the tube wall, improve the uniformity of temperature distribution, and reduce the stress concentration caused by local hot spots. The geometry, material properties, model validation and main features of the burner under study have been deeply described in detail in previous works [25,26]. The governing equations are solved by using the software Ansys Fluent 16.0 and Gambit 3.0 in Think Station D30 with 4 dual-core CPU and 64 GB RAM. For the grid generation, based on the three-dimensional symmetry of the N-WRT structural characteristics, half of the structure is selected to establish the finite element model to speed up the numerical calculation. The block partitioning grid method is adopted for mesh generation. The central tube, elbow, and straight tube of the N-WRT are divided by the structural mesh. Because the structure of the burner is complex, the local mesh is refined for the burner, and a tetrahedral mesh is adopted. The specific mesh generation is shown in Fig. 2. The continuity equation, and energy equation, the k-e turbulence model, eddy-dissipation (ED) combustion model, and discrete ordinates (DO) radiation model are used, respectively [27–30]. The specific boundary conditions are listed in Table 1. 3. Numerical simulation results and discussion 3.1. Effects of the nozzle diameter on the N-WRT’s performance For comparative analysis, five typical nozzles with different nozzle diameters of D60, 50, 40, 36, and 28 mm were selected. The effects of the nozzle diameter on the flow field, temperature field, thermal efficiency, and NOx emission of the N-WRT were studied in depth. 3.1.1. Analysis of the flow field and NOx emission Table 2 (Row: 1-3) shows the changes in gas velocity, circulation ratio of flue gas, and NOx concentration at the outlet of the N-WRT based on different nozzle diameters. When the nozzle diameter increased from D60 mm to D28 mm, the velocity at the nozzle increased, and the change rate of velocity increased significantly. The circulation 483
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Fig. 2. The specific mesh generation of the N-WRT.
ratio of flue gas in the radiant tube also increased rapidly from 0.47 at D60 mm to 2.2 at D28 mm. This indicates that the negative pressure at the nozzle increased as the nozzle diameter decreased. This caused a greater amount of flue gas to be entrained to participate in secondary combustion. When the air volume was constant, reducing the nozzle diameter can considerably change the velocity of the gas flow at the nozzle and improve the circulation ratio of flue gas. In addition, when the nozzle diameter decreased, the emission of NOx at the outlet decreased considerably. Particularly in the initial stage (i.e., in the range of D50 to D60 mm), the nozzle velocity changed in the range of 50–90 m/s, and the emission of NOx from the radiant tube decreased considerably from 100 to 40 ppm. However, when the NOx emission at the outlet was less than 40 ppm and the nozzle diameter continued to decrease, the effect of the nozzle diameter on the NOx concentration at the outlet became increasingly weaker. Fig. 3 shows the gas pressure distribution on the symmetrical plane of the N-WRT with different nozzle diameters. It can be seen that when the diameter of the nozzle decreased, the overall pressure distribution range of the gas in the radiant tube increased and the minimum value of negative pressure changed from −700 to −1050 Pa. In particular, the negative pressure value of the gas at the nozzle also increased, which enabled a greater amount of gas in the circulating tube to enter the combustion main tube to participate in reburning. It also helped to reduce the temperature of the combustion center zone and improve the uniformity of the wall temperature.
Table 2(Row:4–9) shows the other performance parameters of the N-WRT with different nozzle diameters. The table shows that when the nozzle diameter decreased, the minimum wall temperature of the radiant tube increased gradually from 1226 to 1239 K, and the maximum wall temperature decreased from 1411 to 1396 K, which decreased the wall temperature difference of the radiant tube from 185 to 157 K. After the diameter of the nozzle was reduced to 36 mm, the temperature difference in the wall of the radiant tube remained nearly unchanged, but the overall thermal efficiency of the radiant tube decreased. The shortened nozzle diameter increased the average flow velocity of gas in the tube and increased the flue gas temperature of the exhaust radiant tube, which resulted in flue gas loss. However, this loss can be recovered by improving the heat exchanger or regenerative burner, thus avoiding a reduction in thermal efficiency.
3.1.2. Analysis of the temperature field Fig. 4 shows the gas temperature distribution profile on a symmetrical plane of the radiant tube with different nozzle diameters. The figure shows that when the nozzle diameter gradually decreased, the high-temperature zone of combustion gradually decreased, whereas the center temperature of the combustion flame did not change noticeably. However, the temperature gradient of the gas in the radiant tube decreased and the temperature distribution became increasingly uniform. Particularly when the diameter of the nozzle was 60 mm, the temperatures at the tail end and in the circulating section of the radiant tube were in the range of 1160–1260 K, whereas the temperature of the fourth straight tube was in the range of 1260–1370 K. When the diameter of the nozzle was 36 mm, the temperatures of these two sections were in the range of 1260–1360 K, which was nearly the same as that of the fourth straight tube. This trend of gas temperature distribution also affected the wall temperature distribution of the radiant tube, thus reducing the wall temperature difference.
3.2.1. Analysis of the flow field and NOx emission Table 3(Row:1–3) shows the changes in gas velocity, circulation ratio, and NOx concentration at the outlet of the N-WRT with different nozzle locations. The table shows that when the nozzle was located at the circulating central line (x = 0 mm), the gas velocity near the radiant nozzle was the highest, which was approximately 20 m/s greater than that of the other nozzle position conditions. This shows that when the burner nozzle was located at the circulating central line, the gas velocity increased effectively to control the flue gas. In addition, as Table 3 shows, when the nozzle moved inward along the combustion main tube, the high-temperature zone moved backwards, and the dilution effect of the circulating flue gas on the oxygen concentration in the high-temperature zone was weakened. As a result, the flame center temperature was higher, and the NOx emission increased. The gas pressure distribution in the N-WRT based on different nozzle locations is shown in Fig. 6. When the nozzle moved inward along the combustion main tube, the range of the high-pressure zone of
3.2. Effects of nozzle location on the N-WRT’s performance An analysis of the effects of the nozzle location on the flow field, temperature field, thermal efficiency, and NOx emission of the N-WRT was conducted. The central line of the flue gas circulating tube was set to x = 0 mm, which is shown in Fig. 5. To ensure a sufficient amount of circulating flue gas, the nozzle should be located within the intersection range of the circulating and first straight tubes. Therefore, with the nozzle located at x = 100, x = 45, x = 0, x = −50, and x = −90 mm, numerical simulations of the N-WRT were conducted.
Table 1 The boundary conditions of the radiant tube. Boundary Inlet
Fuel Air
Outlet Wall
Flue gas wall function method without slip
Furnace temperature
Type
Value
Mass flow rate Mass flow rate Excess air coefficient Pressure Convection and Radiation
3.21 × 10−3 kg/s 6.06 × 10−2 kg/s 1.1 −600 Pa
Constant temperature
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α = 9 W/(m2 K), ε = 0.85 1223 K
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Table 2 The performance parameters of the N-WRT under different nozzle diameters.
(1) (2) (3) (4) (5) (6) (7) (8) (9)
Project
D60 mm
D50 mm
D40 mm
D36 mm
D28 mm
Gas velocity (m/s) Circulation ratio NOx concentration (ppm) Surface temperature/max (K) Surface temperature/min (K) Average temperature (K) Temperature difference (K) Heat transfer (kW) Thermal efficiency (%)
61 0.47 98.04 1411 1226 1286 185 125.6 78.5
88 0.80 29.50 1402 1228 1286 174 124.8 78.0
138 1.26 14.81 1395 1234 1286 161 123.8 77.4
171 1.51 6.00 1393 1236 1286 157 123.4 77.1
280 2.20 4.04 1396 1239 1286 157 122.6 76.6
−50, 0, 45, and 100 mm. The high temperature central areas were located at the axial positions of 600, 700, 900, 1000, and 1100 mm. When the nozzle was located on the left side of the circulating center line (x = 0), the gas temperature distribution in the range of 0–1100 mm along the axial direction decreased by approximately 5–40 K when the nozzle moved inward. When the nozzle was located on the right side of the circulating center line (x = 0 mm), the gas temperature distribution in the range of 0–1100 mm along the axial direction decreased by approximately 50–200 K when the nozzle moved inward. In the range of 1100–1600 mm along the axial direction, the gas temperature was clearly higher than that on the left side of the circulating center. The gas temperature at the nozzle position of x = 45 mm was the highest and decreased when the nozzle moved inward. Table 3(Row:4–10) shows the other performance parameters of the N-WRT with different nozzle locations. The table shows that when the nozzle moved inward away from the burner, the minimum wall temperature of the radiant tube decreased gradually from 1234 to 1210 K, whereas the maximum wall temperature was lowest at the circulating center and increased when the nozzle moved toward both sides. However, the fluctuation in temperature was extremely small (approximately 3 K). Because the position of the nozzle moved inward, the hightemperature zone of the flame moved backward, causing the wall temperature of the radiant tube near the end of the burner to decrease. Therefore, the wall temperature difference of the radiant tube increased from 158 to 181 K. In addition, the flue gas that returned from the circulating tube was dispersed after reaching the main combustion tube. As the combustion zone moved backward, the oxygen content in the combustion center was diluted only after the circulating flue gas flowed for a certain distance. This in turn reduced the dilution effect of the circulating flue gas on the oxygen content in the high-temperature zone. Thus, the flame center temperature increased, resulting in an increase in the production
gas in the tube decreased, whereas the range of the low-pressure zone at the burner nozzle increased, resulting in a greater amount of flue gas from the circulating tube to flow into the combustion main tube and participate in flue gas recirculation combustion. When the nozzle moved inward, the circulation ratio increased, but after the position of the circulating central line changed (i.e., x = 0 mm), the range decreased gradually. This indicates that during actual operation, to improve the circulation ratio, the nozzle should be properly moved inward. However, the position of the circulating central line should not be exceeded to prevent circulating flue gas with high temperature from flowing into the combustor and shortening the service life of the burner. Fig. 7 shows oxygen distribution along the axial direction with different nozzle locations. The figure shows that in the region of 100–800 mm along the axial direction, the “turning” phenomenon caused by the decreased distribution of oxygen derived from the circulating flue gas appears in all curves. When the nozzle was located on the left side of the circulating central line (x = 0 mm), the bending degree was greater. By contrast, when the nozzle was located at the right side of the circulating central line (x = 0 mm), the bending degree was smaller. This indicates that when the nozzle is moved inward, the dilution effect of the circulating flue gas on oxygen concentration decreased, which was not conducive to reducing the production of NOx. In the region of 800–1000 mm along the axial direction where the gas temperature distribution was higher, the oxygen content was lower. This indicates that most of the oxygen was mainly used for combustion, whereas a small part of the oxygen was used to generate NOx or other oxides. 3.2.2. Analysis of the temperature field Fig. 8 shows gas temperature distribution along the axial direction in the main combustion tube based on different nozzle locations. Fig. 8 shows that when the nozzle moved inward, the corresponding hightemperature zone moved inward. The nozzles were located at x = −90,
Fig. 3. The gas pressure distribution of the N-WRT with different nozzle diameters. 485
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Fig. 4. The gas temperature distribution of the N-WRT with different nozzle diameters.
velocity at the nozzle fluctuated only by approximately 1 m/s. However, the gas velocity at the confluence surface decreased dramatically with an increase in the diameter of the circulating tube. When the diameter of the circulating tube was decreased from 180 to 120 mm, the gas velocity at the junction decreased by 12%. By contrast, the gas velocity at the nozzle remained unchanged, and the velocity gap between the circulating flue gas and gas at the nozzle increased. This caused a greater amount of flue gas to flow back into the combustion main tube, thus increasing the amount of circulation flue gas. Fig. 10 shows the relationship between the gas circulation ratio and the diameter of the circulating tube. The figure shows that the circulation ratio increased with the increase in the circulating tube diameter, thus verifying the previous inference concerning the increase in flue gas circulation. The change of the diameter of the circulating tube directly affected the velocity of the gas flow and flue gas circulation ratio. It also indirectly affected the production of NOx. The large amount of circulating flue gas diluted the oxygen content in the high-temperature zone of combustion and reduced the peak value of the flame temperature, thus reducing the generation of NOx. Fig. 11 shows the variations in fuel gas mass fraction along the axis direction with different circulating tube diameters. The figure shows that the distribution of the fuel gas within the four structures was nearly the same, indicating that the change in the circulating tube diameter had no obvious effect on the distribution of gas mass fraction. Fig. 12 shows the maximum temperature of the flame combustion zone with different circulating tube diameters. The peak value of flame combustion decreased with an increase in the diameter of the circulating tube. When the diameter of the circulating tube was 120 mm, the peak value of flame was 2300 K, while when the diameter of the circulating tube was 180 mm, the peak value of flame reached 2260 K, and decreased approximately 40 K. Fig. 13 shows the variations in NOx concentrations at the outlet of the N-WRT based on different diameters of the circulating tube. When the diameter of the
Fig. 5. The different nozzle positions along the central line of the flue gas circulating tube.
of NOx. Simultaneously, because the nozzle moved inward, the moving distance of high-speed gas that was emitted from the nozzle decreased, and the exhaust temperature increased from 1276 to 1289 K. The heat loss in the flue gas increased with the increase in exhaust temperature, and the heat efficiency of the radiant tube decreased from 77 to 76.6%. 3.3. Effects of the circulating tube diameter on the N-WRT’s performance For a comparative analysis, four typical circulating tubes with different tube diameters of 120, 140, 160, and 180 mm were selected, respectively. Simulations to determine the effects of nozzle diameter on the flow field, temperature field, thermal efficiency, and NOx emission of the N-WRT were conducted and analyzed. 3.3.1. Analysis of the flow field and NOx emission The gas velocities at the burner nozzle and the junction of the circulating and main combustion tubes are shown in Fig. 9. When the diameter of the circulating tube changed from 120 to 180 mm, the gas Table 3 The performance parameters of the N-WRT under different nozzle locations.
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
Project
X = −90 mm
X = −50 mm
X = 0 mm
X = 45 mm
X = 100 mm
Gas velocity (m/s) Circulation ratio NOx concentration (ppm) Surface temperature /max (K) Surface temperature /min (K) Average temperature (K) Temperature difference (K) Heat transfer (kW) Exhaust temperature (K) Thermal efficiency (%)
128 1.26 7.10 1392 1234 1286 158 123.8 1276 77.4
130 1.36 7.81 1392 1229 1286 163 123.6 1280 77.3
148 1.58 14.65 1389 1222 1286 167 123.0 1284 76.9
130 1.62 28.49 1391 1215 1286 176 123.2 1285 77.0
128 1.67 63.65 1391 1210 1286 181 122.5 1289 76.6
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Fig. 6. The gas pressure distribution of the N-WRT with different nozzle locations.
Fig. 8. The gas temperature distribution along the axial direction with different nozzle locations.
Fig. 7. The oxygen distribution along the axial direction with different nozzle locations.
circulating tube did not change the trend of temperature distribution. With an increase in the diameter of the circulating tube, the gas temperature distribution of the radiant tube remained unchanged in the area of 150–300 mm in the axial distance, whereas the trend in gas temperature variation for the radiant tube began to change in the region of 300–700 mm. When the diameter of the circulating tube increased from 120 to 180 mm, the gas temperature at the same location decreased by approximately 10–25 K. In the region of 700–2000 mm along the axial direction, the drop in gas temperature caused by the diameter of the circulating tube was maintained at approximately 30 K. In particular, when the diameter of the circulating tube was in the range of 140–160 mm, the gas temperature maintained a relatively uniform distribution. With the increase in the diameter of the circulating tube, the temperature difference of the main combustion tube decreased and
circulating tube increased from 120 to 180 mm, the concentration of NOx at the outlet decreased from 62.4 to 45 ppm. 3.3.2. Analysis of the temperature field With an increased amount of circulating flue gas, a greater amount of high-temperature gas participates in secondary combustion, which not only can reduce the oxygen concentration in the flame combustion zone but can also reduce the peak value of the flame and the generation of NOx. It can also improve the wall temperature uniformity of the radiant tube. Fig. 14 shows the variations in gas temperature along the gas flow direction of the main combustion tube with different circulating tube diameters. The figure shows that the change of the diameter of the 487
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Fig. 9. The gas velocities at the burner nozzle and the junction.
Fig. 12. The peak value of flame combustion with different circulating tube diameters.
Fig. 10. The variation curve of the gas circulating ratio with different circulating diameters. Fig. 13. The variation curve of NOx concentration with different circulating tube diameters.
Fig. 11. The variation curve of fuel gas mass fraction with different circulating tube diameters. Fig. 14. The variation curves of gas temperature with different circulating tube diameters.
the uniformity of the gas temperature increased as a result of the influence of the temperature distribution. Table 4 shows the performance parameters of the N-WRT based on different circulating tube diameters. The table shows that when the diameter of the circulating tube increased from 60 to 90 mm, the heat
transfer surface area of the circulating tube increased, and thus the surface heat transfer was enhanced. The maximum wall temperature of the radiant tube decreased, whereas the corresponding minimum wall 488
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Table 4 The performance parameters of the N-WRT under different circulating tube diameters.
(1) (2) (3) (4) (5) (6) (7)
Project
D = 120 mm
D = 140 mm
D = 160 mm
D = 180 mm
Surface temperature /max (K) Surface temperature /min (K) Average temperature (K) Temperature difference (K) Temperature nonuniformity coefficient Heat transfer (kW) Thermal efficiency (%)
1403 1228 1287 175 0.1247 124.5 77.8
1401 1229 1286 172 0.1227 124.7 77.9
1398 1229 1286 169 0.1209 124.9 78.1
1396 1230 1285 166 0.1189 125.0 78.1
temperature increased slightly, which inevitably improved the uniformity of the radiant tube wall temperature. The wall temperature difference decreased from 175 to 166 K, the surface temperature nonuniformity coefficient increased from 0.1247 to 0.1189, and the thermal efficiency of the radiant tube increased from 77.8 to 78.1%.
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4. Conclusion The following summarize the findings of this study. (1) When the nozzle diameter increased from D60 to D50 mm, the nozzle velocity changed in the range of 50–90 m/s, and the emission of NOx from the radiant tube decreased significantly from 100 to 40 ppm. When the nozzle diameter increased from D60 to D36 mm, the minimum wall temperature of the radiant tube increased gradually from 1226 to 1239 K, and the maximum wall temperature decreased from 1411 to 1396 K. (2) When the nozzle was located at x = 0 mm, the gas velocity near the radiant nozzle was the highest, which was approximately 20 m/s greater than when the nozzle was in other locations. (3) When the nozzle moved inward, the dilution effect of the circulating flue gas on oxygen concentration decreased, which was not conducive to reducing the production of NOx. The gas temperature at the nozzle position of x = 45 mm was the highest and decreased when the nozzle moved inward. (4) When the diameter of the circulating tube changed from 180 to 120 mm, the gas velocity at the junction decreased by 12%, whereas that at the nozzle remained unchanged. In addition, the velocity gap between the circulating flue gas and the gas at the nozzle increased. Acknowledgments The authors gratefully acknowledge the National key Research and Development Program of China under Grant No. 2017YFC0210303. And this work is supported by the Fundamental Research Funds for the Central Universities of China (FRF-BD-18-015A) and the State Key Laboratory of Technologies in Space Cryogenic Propellants of China (SKLTSCP1702). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.applthermaleng.2019.02.097. References [1] Z. Liu, Y. Liu, B.-J. He, W. Xu, G. Jin, X. Zhang, Application and suitability analysis of the key technologies in nearly zero energy buildings in China, Renew. Sustain. Energy Rev. 101 (2019) 329–345. [2] X. She, L. Cong, B. Nie, G. Leng, H. Peng, Y. Chen, X. Zhang, T. Wen, H. Yang, Y. Luo, Energy-efficient and -economic technologies for air conditioning with vapor compression refrigeration: a comprehensive review, Appl. Energy 232 (2018) 157–186. [3] P. Cui, M. Yu, Z. Liu, Z. Zhu, S. Yang, Energy, exergy, and economic (3E) analyses and multi-objective optimization of a cascade absorption refrigeration system for
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