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An experimental investigation of wall-bed heat transfer and flow characteristics in a swirling fluidized bed reactor
Applied Thermal Engineering 155 (2019) 501–507
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
An experimental investigation of wall-bed heat transfer and flow characteristics in a swirling fluidized bed reactor Mohamed Hamam M. Tawfika, Mohamed Refaat Diabb, Hamada Mohmed Abdelmotalibb, a b
T ⁎
Elminya High Institute for Engineering and Technology, Minia 61111, Egypt Dept. of Mechanical Power and Energy Engineering, Faculty of Engineering, Minia University, Minia 61111, Egypt
A R T I C LE I N FO
A B S T R A C T
Keywords: Annular-blade distributor Bed pressure drop Distributor pressure drop Heat transfer coefficient Immersed heater Swirling fluidized bed
The present work investigates the heat transfer between the immersed heater and bed and hydrodynamics of the bed in swirling fluidized bed. The heat transfer process and bed hydrodynamics characteristics for swirling fluidized bed reactor (SFBR) were compared to those of conventional fluidized bed reactor (CFBR). Then the influence of particle diameter on the bed flow characteristics and heat transfer process in SFBR were studied. The heat transfer coefficient was calculated at different superficial air velocities, radial positions, and bed heights. Compared to a CFBR, the bed pressure drop and distributor pressure drop for SFBR were lower, that characterizes swirling bed reactor. The heat transfer coefficient of SFBR was higher than the corresponding of CFBR at lower portion of the bed and it was lower at the upper portion of the bed for all radial positions and for all air superficial velocities. Increasing particle diameter in SFBR resulted in decreasing both the heat transfer coefficient and bed pressure drop.
1. Introduction In the fluidized bed, solid bed particles are moved like fluid via fluidized agent liquid or gas. These reactors have many apparent advantages like intensive solid-fluid mixing, controllable and uniform temperature, intensive mass and heat transfer rates, the flexibility of fuel and gas emission control. Due to these advantages, theses reactors become the most efficient technology used in many applications such as combustion, coating, pyrolysis, drying, and particle segregation [1–4]. In the CFBR primary air enters the distributor which may be perforated or porous plate located at the reactor bottom. However, this type of reactors has some limitations, for instance, solid particles elutriation, channeling, particle size limitation and slugging [5]. Swirling fluidized bed reactor (SFBR) is considered as one of efficient technology was developed to overcome different limitations and drawbacks involved in CFBR. A swirling motion in these reactors is generated using either tangential gas entry via nozzles or axial gas entry via annular blade distributor. In SFBR the gas enters the bed with velocity having two components, the tangential one which causes the swirl motion of bed and the vertical component that causes the fluidization. In the current study, the SFBR with annular blade distributor was used. This type of distributor consists of blades which provides lateral momentum, that is very useful for different applications [5–7]. Studying reactors heat transfer and bed hydrodynamics have been ⁎
given more attention in the last period due to their importance for good design and work of these reactors. The immersed surface-to-bed heat transfer is considered very essential because of its industrial applications, so it is given more attention. Most of those results are conducted in CFBs. Al-Dahhan and Taofeeq [8] studied the influence of the tube size on the heat transfer process in a fluidized bed with an internal diameter of 0.14 m. The experiments were achieved at various air superficial velocities, radial position and static heights for two tube sizes of 0.0127 and 0.0254 m. The heat transfer coefficient rose with increasing tube diameter for all studied different boundary conditions. A correlation was predicted based on dimensionless analysis and there was a well agreement between calculated and experimental results. In another study by Al-Dahhan and Taofeeq [9], the effect of using a bundle of vertical tubes on the hydrodynamics and heat transfer was studied. The coefficient of heat transfer was estimated using advanced non-intensive heat transfer probe, while both gas-hold up and bubble frequency were measured using sophisticated optical fiber probe techniques. Glass beads were chosen as bed material at different gas inlet velocities, radial positions, and axial heights. The results demonstrated that increasing both bubble frequency and local void fraction resulted in improving the heat transfer process. A new correlation taking in the account the effect of gas-hold up, the frequency of bubble and other parameters was established. Both experimental and correlation results were in good agreement. The heat transfer between the lignite particles
Corresponding author. E-mail address: [email protected] (H. Mohmed Abdelmotalib).
https://doi.org/10.1016/j.applthermaleng.2019.04.022 Received 17 February 2019; Received in revised form 28 March 2019; Accepted 4 April 2019 Available online 05 April 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.
Applied Thermal Engineering 155 (2019) 501–507
M.H.M. Tawfik, et al.
transfer was estimated using the following equation.
and the immersed surface was studied by Yang et al. [10] using a distributor with inclined slotted and surface renewable theory. The influence of immersed object diameter, particle diameter and velocity of inlet gas on the coefficient of heat transfer was studied. The obtained results were compared with CFBR. Using a distributor with inclined slotted enhanced the heat transfer compared to a traditional reactor. A correlation for estimating heat transfer coefficient was established using the experimental results and surface renewable theory. There was a good agreement between experimental and predicted results when neglecting the radiative heat transfer. In addition to the previous studies, there are also other studies investigate the heat transfer and fluidization behavior are conducted in the conventional circulating fluidized bed reactor such as [11–13]. Compared to CFBR, studies of heat transfer and bed flow characteristics in SFBR are still very limited. Most of these studies focus on studying the bed flow characteristics in swirling fluidized bed. Kaewklum et al. [5] investigate the bed hydrodynamics in air-sand SFBR using two methods of swirl generation, four-nozzle system, and annular-spiral air distributor. The pressure drop was measured at various air superficial velocities for three static heights. The results illustrated that there were four regimes in bed for two-swirl generators. The mathematical model for expecting velocity at minimum fluidization and corresponding pressure drop was developed and there was a similar trend for two air injection systems. Sreenivasan and Raghavan [7] studied the hydrodynamics of bed in SFBR using a distributor with annular blade and inclined gas injection. The study indicated that there are three or sometimes four operation regions in SFBR depending on the weight of the used bed. The bed pressure drop rose with increasing gas inlet velocity, unlike CFBR. An approximation model was established to predict both mean angular velocity and pressure drop at certain gas flow rates and a good agreement between thermotical and experimental results were obtained. The uncertainty analysis was also performed on the model indicated that the proposed model can satisfactorily describe the fluidized regime in SFB. Kumar and Murthy [14] studied experimentally the minimum swirl velocity in SFBR under different conditions. A minimum swirl velocity empirical correlation was established. The results explained the possibility of an existing stable swirling region of the bed. Also, there was an upper limit of static bed height at which stable swirling of the whole bed isn’t possible. The minimum swirl velocities were higher than the minimum fluidizing velocity used in CFBR. Reviewing literature shows that most studies related to the heat transfer process of the fluidized bed are achieved in a conventional fluidized bed. Most of the studies on swirling fluidized bed reactors concern only about the bed hydrodynamics characteristics. Despite, this type of reactors is used in many applications involve heat transfer, there are no studies concerning the heat transfer between the immersed surface and swirl fluidized bed. Therefore, the main goal of this study is investigating the gas-solid behavior and wall-bed heat transfer at different inlet gas velocities in a swirl fluidized bed reactor and making a comparison with the conventional type. Then, the effects of particle diameter on bed flow and heat transfer in swirling fluidized bed were studied.
h=
Q As (Ts − Tb)
(1)
where Ts and Tb are the time-averaged surface and bed temperatures, respectively. As is the heat transfer surface area. Q is the input heat transfer. The tubular heater with length and diameter of 0.3 and 0.016 m, respectively, was used in this study. The coefficient of heat transfer was calculated at two heights (Z = 0.05, and Z = 0.1 m) and four radial positions (r/D = 0.1, 0.3, 0.7, 0.9 from left wall) while the heater is located at the reactor center (at r/D = 0.5). Three pressure taps were used to measure pressure drops connected with a digital manometer (Comark C9503/IS). First one (P1) was found at a height of 0.01 m under the air inlet while the other two taps (P2 and P3) were found at heights of 0.01 and 0.49 m above the air inlet. The pressure drop of the distributor was measured using pressure taps P1 and P2 while the bed pressure drop was measured via taps P2 and P3 as shown in Fig. 1. The experiments were performed three times for a period from 20 to 30 min and the averaged values of measured parameters were used in the calculations and results analysis. In this study sand particles with a mean diameter of 1.5 mm was used as bed material fluidized by air. In this study two types of gas distributor were used, for CFBR a metallic mesh with a hole size of 0.12 mm was used as a distributor. While for SFBR a distributor annular-blade with covered by the same mesh to prevent sand particles from fall down was used. Fig. 2 shows the geometric details of annular-blade distributor used to generate swirl motion in SFBR. The annular blade distributor composed of seven blades located at a distance of 1.2 cm on the circumference of the distributor and fixed at an angle of 45° to the horizontal. The blades are welded to a circular hub with a diameter of 3.2 cm and a height of 1.6 cm. The blade was curved and had a trapezoidal cross-sectional area with dimensions of (2.2 × 3.2 cm). The uncertainty analysis was performed for the heat transfer coefficient using Mcclintok and Kline [15] method. As given by Eq. (1) the heat transfer coefficient is a function of input heat flux and temperature. Therefore, the uncertainty of heat transfer coefficient relies on the thermocouple accuracy ( ± 0.4 °C) and the wattmeter accuracy ( ± 5 W). The estimated uncertainty was within 6%. 3. Results and discussion 3.1. Comparison between SFBR and CFBR In this section, both bed hydrodynamics and heat transfer of CFBR and of SFBR for the particle size of 1.5 mm were compared. Flow dynamics of a bed are very essential to understand the behavior of a system and then to determine the optimum design and operating parameters of fluidized bed reactors. In this work, the bed hydrodynamics includes velocity at minimum fluidization, and pressure drop across distributor and bed are studied for CFBR and SFBR. The term minimum fluidization velocity is used to indicate the minimum flow rate required to reserve a certain amount of bed material in a fluidized state. In some application, it is required to calculate this minimum flow rate to prevent fluid from consumption and hence decreasing the required power. Therefore, it is important to estimate this parameter for CFBR and SFBR. In this study, correlations for estimating velocity at minimum fluidizing reported in Chitester at al. [16] and Wen et al. [17] were used. Fig. 3 illustrates the calculated minimum fluidization velocity compared to experimental value. The figure shows a fairly good agreement between the experimental values and that calculated by Wen et al. correlation. Fig. 4 shows the pressure drop of the distributor for both CFBR and SFBR at various air superficial velocities. The pressure drop of the distributor is used to explain the loss of kinetic energy for the empty bed reactors. For two types of reactors, the distributor pressure drop
2. Experimental procedure The test rig schematic diagram used in the current study is shown in Fig. 1. The fluidized bed unit consisted of a cylindrical column made from stainless steel with total height and diameter of 0.5 and 0.1 m, respectively. Air was supplied into the system using an air blower and pipe connected with a control valve. Air velocity was measured via a nozzle meter by measuring the pressure drop across the nozzle using water U-tube manometer. Thermocouples type K was chosen to measure temperatures of the surface and bed. All thermocouples were connected to a data logger (Midi logger GL820) and temperatures are taken and stored every ten seconds. The local coefficient of heat 502
Applied Thermal Engineering 155 (2019) 501–507
M.H.M. Tawfik, et al.
Fig. 1. The test rig schematic diagram used the experiments.
Fig. 4. Effect of air superficial velocity on the pressure drop of the distributor for CFBR and SFBR.
Fig. 2. The annular-blade distributor used in this study.
Fig. 5. Effect of air superficial velocity on bed pressure drop for CFBR and SFBR.
consumed by the fluidized bed system. Another important parameter to be considered in the fluidized bed design is a bed pressure drop. Fig. 5 illustrates the bed pressure drop versus air superficial velocity for CFBR and SFBR. The bed pressure drop for CFBR exhibits the typical trend. The bed pressure drop reaches its maximum value at minimum fluidizing velocity and the then exhibits constant value. While for SFBR the bed pressure drop is gradually increased with increasing superficial gas velocity. In SFBR, there is an
Fig. 3. The calculated minimum fluidizing velocity against the experimental value.
increased with increasing air inlet velocity which is typical for fluidized bed distributor. Moreover, the distributor pressure drop is lower for SFBR than CFBR that characterizes the SFBR because lowering values of distributor pressure drop results in decreasing the total energy 503
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M.H.M. Tawfik, et al.
Fig. 6. Effect of air superficial velocity on the local heat transfer coefficient at different radial positions at Z = 0.05 m for CFBR and SFBR.
additional downward centrifugal force besides the particles weight. This force resulted from the swirl motion of particles and increased as inlet gas velocity increased, therefore, the pressure drop increased with increasing inlet air velocity. The same behavior was observed in some studies related to SFBR such as [7]. Moreover, The figure shows that the bed pressure drop for SFBR is lower than that for CFBR. This can attribute to the CFBR characterizes with bubbling behavior where the gas bubbles are formed increasing the void among solid particles leading to bed expansion and increasing its height which means increases the weigh acting on the distributor area and hence increases the drag force requires for fluidization and hence increasing the bed pressure drop. But for SFBR bubbling behavior disappears. The same trend was found by Batch and Raghavan [18]. The time-averaged local heat transfer coefficient for both CFBR and SFBR at the different radial position and a height of 0.05 m is given by Fig. 6. The figure illustrates that the heat transfer coefficient increases with increasing air inlet velocity for both CFBR and SFBR. Increasing inlet air velocity increases the turbulent motion and hence improves the heat transfer process. Also, increasing inlet air velocity resulted in decreasing surface temperature as shown in Fig. 7. This may be due to the cooling effect of coming air which leads to increasing the heat transfer coefficient as given in Eq. (1). The figure indicated that the heat transfer coefficient of SFBR was higher than that of CFBR. At the lower height, the bed particles are in fully swirl state due to the vertical component of velocity which results in excellent gas-particles mixing and increasing heat transfer. In addition, as shown in Fig. 7 the surface temperature of SFBR is lower than that of CFBR and then the heat transfer coefficient of SFBR was higher than that of CFBR. Fig. 8 shows also the effect of superficial gas velocity on the heat
Fig. 7. Effect of superficial gas velocity on surface temperature at Z = 0.05 m for CFBR and SFBR.
transfer coefficient at a height of 0.1 m. Increasing air inlet velocity increased the heat transfer coefficient at all radial positions. It is noticed that at the upper height the heat transfer coefficient of CFBR is higher than that of SFBR at all radial positions. As mentioned by Kumar and Murthy [14] when the fluid emerges from exit the fluid has only tangential components which responsible for swirl so the bed particles are in fully swirl state at lower bed regions. With increasing height, particles move up taking a spiral path near the walls, the bed is in swirl state but the tangential component tends to decrease. At a certain height of the bed, the tangential component dies out and bed particles move up. Therefore, the solid particles near to the air inlet are in completely swirl motion while at the upper region the swirl action may disappear to bubbling, slugging fluidization which leading to 504
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Fig. 8. Local heat transfer coefficient versus superficial gas velocity at different radial positions at Z = 0.1 m for CFBR and SFBR.
decreasing heat transfer process. The heat transfer coefficient was also calculated at two other bed heights 0.025 m and 0.2 m. The axial distribution of heat transfer coefficient for CFBR and SFBR at air superficial velocity of 1.8 m/s and radial position (r/D) of 0.1 is shown in Fig. 9. For two types of reactors, the heat transfer coefficient decreased with increasing bed height. Moreover, the heat transfer coefficient of SFBR is higher than those of CFBR at lower portion of bed (at bed height smaller than 0.1 m) and this agrees with finding in Figs. 6 and 8. 3.2. Effect of bed particle size In this section, the effects of bed particle size on hydrodynamics and Fig. 10. The calculated and experimental values for minimum fluidization velocity for dp = 0.9 and 1.5 mm.
heat transfer in SFBR were studied. In this study, two particles size of 0.9 mm and 1.5 mm were used. The previous results given for particle size of 1.5 mm are compared to those of particles size of 0.9 mm in the following sections. Fig. 10 shows the minimum fluidizing velocity calculated using Wen et al. [17] correlation compared to the experimental value. The figure shows well agreement between the experiments and correlation results for different particle size. The minimum fluidizing velocity for small particles was lower than that of large particles because their weight is lighter and can be easily fluidized. Fig. 11 shows the bed pressure drop against air inlet velocity for two different particle sizes. The bed pressure drop rose with increasing air inlet velocity for two particle sizes and this a typical trend in SFBR as mentioned in Fig. 5. The bed pressure drop increased with the reduction
Fig. 9. The axial distribution of the heat transfer coefficient at r/D = 0.1 and U = 1.8 m/s for CFBR and SFBR. 505
Applied Thermal Engineering 155 (2019) 501–507
M.H.M. Tawfik, et al.
in increasing surface area and improving gas-solid mixing as air void among particles decreases, all these parameters lead to improving the heat transfer coefficient. This behavior was similar for all other radial positions. The local heat transfer coefficient versus inlet air velocity for two particles size is shown in Fig. 13. The heat transfer coefficient was calculated at the center of the reactors at two heights of 0.05 and 0.1 m. For two different sizes, the heat transfer in the lower height of 0.05 m is higher than that in the upper height of 0.1 m. As mentioned in Fig. 8 the bed particles in lower part were in complete swirl state and then improve the heat transfer process and with increasing bed height the swirl action decrease or disappear results decreasing heat transfer process. The figure shows also the heat transfer of small particles is higher than that of large particles. Fig. 11. The bed pressure drop versus superficial gas velocity for various particle sizes.
4. Conclusion Swirling fluidized bed reactors have given more attention in the last decades due to its apparent advantages that overcome the drawbacks of the conventional fluidized bed reactor. The present work studied experimentally the fluidization behavior and the heat transfer between the immersed heater and bed in SFBR. The heat transfer process was studied at different air superficial velocities, radial positions, and bed heights. First, the comparison between SFBR and CFBR were performed, then the effect of particle diameter on heat transfer and bed hydrodynamics in SFBR was carried out. The pressure drop across the air distributor and bed for SFBR were lower than those of CFBR and this characterizes SFBR. The heat transfer coefficient for SFBR was higher than that of CFBR at lower bed regime but at the upper regime, it was lower. This behavior can be interpreted as at lower bed heights the bed particles were at fully swirl state which improving heat transfer process and with increasing bed height the swirl action gradually decreased. The change of particle size affected the heat transfer and bed flow characteristics in SFBR. The bed pressure drop increased with a reduction in particle size. The heat transfer coefficient increased with the reduction of particle size. Also, it is found that the heat transfer coefficient was higher in lower bed height than that of upper bed heights for two different particle sizes.
Fig. 12. The heat transfer coefficient versus superficial gas velocity for dp = 0.9 and 1.5 mm.
Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.applthermaleng.2019.04.022. References [1] D. Kunii, O. Levenspiel, Fluidization Engineering, second ed., ButterworthHeinemann, Boston, 1991. [2] Subhajit Dutta, Chanchal Loha, Pradip Kumar Chatterjee b, Anup Kumar Sadhukhan, Parthapratim Gupta, Numerical investigation of gas-particle hydrodynamics in a vortex chamber fluidized bed, Adv. Powder Technol. 29 (12) (2018) 3357–3367. [3] E.J. Anthony, Fluidized bed combustion of alternative solid fuels; status, successes and problems of technology, Prog Energy Combust Sci. 21 (1995) 239–268. [4] J.P. Jacobs, The future of fluidized-bed combustion, Chem. Eng. Sci. 54 (22) (1999) 5559–5563. [5] Rachadaporn Kaewklum, Vladimir I. Kuprianov, Peter L. Douglas, Hydrodynamics of air–sand flow in a conical swirling fluidized bed: A comparative study between tangential and axial air entries, Energy Convers. Manage. 50 (2009) 2999–3006. [6] Mohd Faizal1, Suzairin Md Seri, Sh. Ezamuddin1, Vijay R. Raghavan, Numerical investigation of air flow distribution in a swirling fluidized bed, Adv. Mater. Res. 499 (2012) 132–137. [7] Binod Sreenivasan, Vijay R. Raghavan, Hydrodynamics of a swirling fluidized bed, Chem. Eng. Process. 41 (2002) 99–106. [8] Haidar Taofeeq, Muthanna Al-Dahhana, Investigation of the effect of vertical immersed tube diameter on heat transfer in a gas-solid fluidized bed, Int. J. Therm. Sci. 135 (2019) 546–558. [9] Haidar Taofeeq, Muthanna Al-Dahhan, Heat transfer and hydrodynamics in a gassolid fluidized bed with vertical immersed internals, Int. J. Heat Mass Transf. 122 (2018) 229–251. [10] Ning Yang, Yunlong Zhou, Tianyu Qi, The heat transfer between an immersed
Fig. 13. The radial distribution of heat transfer coefficient at different heights for different particle sizes.
of particle size. Smaller particle size results in increasing surface area and enhancing swirling motion which results in increasing centrifugal weight so larger drag force was needed to keep both fluidization and swirl in the bed which leads to increasing bed pressure drop. The same observation was reported by Mohideen et al. [19]. The impact of particle diameter on the heat transfer coefficient is given by Fig. 12. The heat transfer coefficient was measured at a distance of 0.02 m from the heater at a height of 0.05 m. As indicated by the figure, reduction in particle diameter results in increasing the heat transfer coefficient. This fact due to a reduction in particle size results
506
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[11]
[12]
[13]
[14]
swirled fluidized bed, Chem. Eng. Process. 49 (2010) 1095–1100. [15] S.J. Kline, F.A. McClintock, Describing uncertainties in single sample experiments, Mech. Eng. (1953) 3–8. [16] D.C. Chitester, R.M. Kornosky, L.S. Fan, Characteristics of fluidization at high pressure, Chem. Eng. Sci. 39 (1984) (1984) 253–261. [17] C.Y. Wen, Y.H. Yu, Mechanics of fluidization, Chem. Eng. Progress Symp. Ser. 62 (1966) 100–111. [18] M.F.M. Batcha, V.R. Raghavan, Experimental studies on a swirling fluidized bed with annular distributor, J. Appl. Sci. 11 (11) (2011) 1980–1986. [19] Mohd Faizal Mohideen, Suzairin Md Seri, Vijay R. Raghavan, Fluidization of geldart type-D particles in a swirling fluidized bed, Appl. Mech. Mater. 110 (116) (2012) 3720–3727.
surface of moving lignite and small particles in a fluidized bed equipped with an inclined slotted distributor, Exp. Therm. Fluid Sci. 92 (2018) 366–374. A. Blaszczuk, W. Nowak, J. Krzywanski, Effect of bed particle size on heat transfer between fluidized bed of group B particles and vertical rifled tubes, Powder Technol. 316 (2017) (2017) 111–122. J. Krzywanski, M. Wesolowska, A. Blaszczuk, A. Majchrzak, M. Komorowski, W. Nowak, The non-iterative estimation of bed-to-wall heat transfer coefficient in a CFBC by fuzzy logic methods, Procedia Eng. 157 (2016) 66–71. K.K. Win, W. Nowak, H. Matsuda, M. Hasatani, Z. Bis, J. Krzywański, W. Gajewski, Transport velocity of coarse particles in multi – solid fluidized bed, J. Chem. Eng. Jpn. 28 (5) (1995) 535–540. S. Harish Kumar, D.V.R. Murthy, Minimum superficial fluid velocity in a gas-solid
507
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
An experimental investigation of wall-bed heat transfer and flow characteristics in a swirling fluidized bed reactor Mohamed Hamam M. Tawfika, Mohamed Refaat Diabb, Hamada Mohmed Abdelmotalibb, a b
T ⁎
Elminya High Institute for Engineering and Technology, Minia 61111, Egypt Dept. of Mechanical Power and Energy Engineering, Faculty of Engineering, Minia University, Minia 61111, Egypt
A R T I C LE I N FO
A B S T R A C T
Keywords: Annular-blade distributor Bed pressure drop Distributor pressure drop Heat transfer coefficient Immersed heater Swirling fluidized bed
The present work investigates the heat transfer between the immersed heater and bed and hydrodynamics of the bed in swirling fluidized bed. The heat transfer process and bed hydrodynamics characteristics for swirling fluidized bed reactor (SFBR) were compared to those of conventional fluidized bed reactor (CFBR). Then the influence of particle diameter on the bed flow characteristics and heat transfer process in SFBR were studied. The heat transfer coefficient was calculated at different superficial air velocities, radial positions, and bed heights. Compared to a CFBR, the bed pressure drop and distributor pressure drop for SFBR were lower, that characterizes swirling bed reactor. The heat transfer coefficient of SFBR was higher than the corresponding of CFBR at lower portion of the bed and it was lower at the upper portion of the bed for all radial positions and for all air superficial velocities. Increasing particle diameter in SFBR resulted in decreasing both the heat transfer coefficient and bed pressure drop.
1. Introduction In the fluidized bed, solid bed particles are moved like fluid via fluidized agent liquid or gas. These reactors have many apparent advantages like intensive solid-fluid mixing, controllable and uniform temperature, intensive mass and heat transfer rates, the flexibility of fuel and gas emission control. Due to these advantages, theses reactors become the most efficient technology used in many applications such as combustion, coating, pyrolysis, drying, and particle segregation [1–4]. In the CFBR primary air enters the distributor which may be perforated or porous plate located at the reactor bottom. However, this type of reactors has some limitations, for instance, solid particles elutriation, channeling, particle size limitation and slugging [5]. Swirling fluidized bed reactor (SFBR) is considered as one of efficient technology was developed to overcome different limitations and drawbacks involved in CFBR. A swirling motion in these reactors is generated using either tangential gas entry via nozzles or axial gas entry via annular blade distributor. In SFBR the gas enters the bed with velocity having two components, the tangential one which causes the swirl motion of bed and the vertical component that causes the fluidization. In the current study, the SFBR with annular blade distributor was used. This type of distributor consists of blades which provides lateral momentum, that is very useful for different applications [5–7]. Studying reactors heat transfer and bed hydrodynamics have been ⁎
given more attention in the last period due to their importance for good design and work of these reactors. The immersed surface-to-bed heat transfer is considered very essential because of its industrial applications, so it is given more attention. Most of those results are conducted in CFBs. Al-Dahhan and Taofeeq [8] studied the influence of the tube size on the heat transfer process in a fluidized bed with an internal diameter of 0.14 m. The experiments were achieved at various air superficial velocities, radial position and static heights for two tube sizes of 0.0127 and 0.0254 m. The heat transfer coefficient rose with increasing tube diameter for all studied different boundary conditions. A correlation was predicted based on dimensionless analysis and there was a well agreement between calculated and experimental results. In another study by Al-Dahhan and Taofeeq [9], the effect of using a bundle of vertical tubes on the hydrodynamics and heat transfer was studied. The coefficient of heat transfer was estimated using advanced non-intensive heat transfer probe, while both gas-hold up and bubble frequency were measured using sophisticated optical fiber probe techniques. Glass beads were chosen as bed material at different gas inlet velocities, radial positions, and axial heights. The results demonstrated that increasing both bubble frequency and local void fraction resulted in improving the heat transfer process. A new correlation taking in the account the effect of gas-hold up, the frequency of bubble and other parameters was established. Both experimental and correlation results were in good agreement. The heat transfer between the lignite particles
Corresponding author. E-mail address: [email protected] (H. Mohmed Abdelmotalib).
https://doi.org/10.1016/j.applthermaleng.2019.04.022 Received 17 February 2019; Received in revised form 28 March 2019; Accepted 4 April 2019 Available online 05 April 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.
Applied Thermal Engineering 155 (2019) 501–507
M.H.M. Tawfik, et al.
transfer was estimated using the following equation.
and the immersed surface was studied by Yang et al. [10] using a distributor with inclined slotted and surface renewable theory. The influence of immersed object diameter, particle diameter and velocity of inlet gas on the coefficient of heat transfer was studied. The obtained results were compared with CFBR. Using a distributor with inclined slotted enhanced the heat transfer compared to a traditional reactor. A correlation for estimating heat transfer coefficient was established using the experimental results and surface renewable theory. There was a good agreement between experimental and predicted results when neglecting the radiative heat transfer. In addition to the previous studies, there are also other studies investigate the heat transfer and fluidization behavior are conducted in the conventional circulating fluidized bed reactor such as [11–13]. Compared to CFBR, studies of heat transfer and bed flow characteristics in SFBR are still very limited. Most of these studies focus on studying the bed flow characteristics in swirling fluidized bed. Kaewklum et al. [5] investigate the bed hydrodynamics in air-sand SFBR using two methods of swirl generation, four-nozzle system, and annular-spiral air distributor. The pressure drop was measured at various air superficial velocities for three static heights. The results illustrated that there were four regimes in bed for two-swirl generators. The mathematical model for expecting velocity at minimum fluidization and corresponding pressure drop was developed and there was a similar trend for two air injection systems. Sreenivasan and Raghavan [7] studied the hydrodynamics of bed in SFBR using a distributor with annular blade and inclined gas injection. The study indicated that there are three or sometimes four operation regions in SFBR depending on the weight of the used bed. The bed pressure drop rose with increasing gas inlet velocity, unlike CFBR. An approximation model was established to predict both mean angular velocity and pressure drop at certain gas flow rates and a good agreement between thermotical and experimental results were obtained. The uncertainty analysis was also performed on the model indicated that the proposed model can satisfactorily describe the fluidized regime in SFB. Kumar and Murthy [14] studied experimentally the minimum swirl velocity in SFBR under different conditions. A minimum swirl velocity empirical correlation was established. The results explained the possibility of an existing stable swirling region of the bed. Also, there was an upper limit of static bed height at which stable swirling of the whole bed isn’t possible. The minimum swirl velocities were higher than the minimum fluidizing velocity used in CFBR. Reviewing literature shows that most studies related to the heat transfer process of the fluidized bed are achieved in a conventional fluidized bed. Most of the studies on swirling fluidized bed reactors concern only about the bed hydrodynamics characteristics. Despite, this type of reactors is used in many applications involve heat transfer, there are no studies concerning the heat transfer between the immersed surface and swirl fluidized bed. Therefore, the main goal of this study is investigating the gas-solid behavior and wall-bed heat transfer at different inlet gas velocities in a swirl fluidized bed reactor and making a comparison with the conventional type. Then, the effects of particle diameter on bed flow and heat transfer in swirling fluidized bed were studied.
h=
Q As (Ts − Tb)
(1)
where Ts and Tb are the time-averaged surface and bed temperatures, respectively. As is the heat transfer surface area. Q is the input heat transfer. The tubular heater with length and diameter of 0.3 and 0.016 m, respectively, was used in this study. The coefficient of heat transfer was calculated at two heights (Z = 0.05, and Z = 0.1 m) and four radial positions (r/D = 0.1, 0.3, 0.7, 0.9 from left wall) while the heater is located at the reactor center (at r/D = 0.5). Three pressure taps were used to measure pressure drops connected with a digital manometer (Comark C9503/IS). First one (P1) was found at a height of 0.01 m under the air inlet while the other two taps (P2 and P3) were found at heights of 0.01 and 0.49 m above the air inlet. The pressure drop of the distributor was measured using pressure taps P1 and P2 while the bed pressure drop was measured via taps P2 and P3 as shown in Fig. 1. The experiments were performed three times for a period from 20 to 30 min and the averaged values of measured parameters were used in the calculations and results analysis. In this study sand particles with a mean diameter of 1.5 mm was used as bed material fluidized by air. In this study two types of gas distributor were used, for CFBR a metallic mesh with a hole size of 0.12 mm was used as a distributor. While for SFBR a distributor annular-blade with covered by the same mesh to prevent sand particles from fall down was used. Fig. 2 shows the geometric details of annular-blade distributor used to generate swirl motion in SFBR. The annular blade distributor composed of seven blades located at a distance of 1.2 cm on the circumference of the distributor and fixed at an angle of 45° to the horizontal. The blades are welded to a circular hub with a diameter of 3.2 cm and a height of 1.6 cm. The blade was curved and had a trapezoidal cross-sectional area with dimensions of (2.2 × 3.2 cm). The uncertainty analysis was performed for the heat transfer coefficient using Mcclintok and Kline [15] method. As given by Eq. (1) the heat transfer coefficient is a function of input heat flux and temperature. Therefore, the uncertainty of heat transfer coefficient relies on the thermocouple accuracy ( ± 0.4 °C) and the wattmeter accuracy ( ± 5 W). The estimated uncertainty was within 6%. 3. Results and discussion 3.1. Comparison between SFBR and CFBR In this section, both bed hydrodynamics and heat transfer of CFBR and of SFBR for the particle size of 1.5 mm were compared. Flow dynamics of a bed are very essential to understand the behavior of a system and then to determine the optimum design and operating parameters of fluidized bed reactors. In this work, the bed hydrodynamics includes velocity at minimum fluidization, and pressure drop across distributor and bed are studied for CFBR and SFBR. The term minimum fluidization velocity is used to indicate the minimum flow rate required to reserve a certain amount of bed material in a fluidized state. In some application, it is required to calculate this minimum flow rate to prevent fluid from consumption and hence decreasing the required power. Therefore, it is important to estimate this parameter for CFBR and SFBR. In this study, correlations for estimating velocity at minimum fluidizing reported in Chitester at al. [16] and Wen et al. [17] were used. Fig. 3 illustrates the calculated minimum fluidization velocity compared to experimental value. The figure shows a fairly good agreement between the experimental values and that calculated by Wen et al. correlation. Fig. 4 shows the pressure drop of the distributor for both CFBR and SFBR at various air superficial velocities. The pressure drop of the distributor is used to explain the loss of kinetic energy for the empty bed reactors. For two types of reactors, the distributor pressure drop
2. Experimental procedure The test rig schematic diagram used in the current study is shown in Fig. 1. The fluidized bed unit consisted of a cylindrical column made from stainless steel with total height and diameter of 0.5 and 0.1 m, respectively. Air was supplied into the system using an air blower and pipe connected with a control valve. Air velocity was measured via a nozzle meter by measuring the pressure drop across the nozzle using water U-tube manometer. Thermocouples type K was chosen to measure temperatures of the surface and bed. All thermocouples were connected to a data logger (Midi logger GL820) and temperatures are taken and stored every ten seconds. The local coefficient of heat 502
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Fig. 1. The test rig schematic diagram used the experiments.
Fig. 4. Effect of air superficial velocity on the pressure drop of the distributor for CFBR and SFBR.
Fig. 2. The annular-blade distributor used in this study.
Fig. 5. Effect of air superficial velocity on bed pressure drop for CFBR and SFBR.
consumed by the fluidized bed system. Another important parameter to be considered in the fluidized bed design is a bed pressure drop. Fig. 5 illustrates the bed pressure drop versus air superficial velocity for CFBR and SFBR. The bed pressure drop for CFBR exhibits the typical trend. The bed pressure drop reaches its maximum value at minimum fluidizing velocity and the then exhibits constant value. While for SFBR the bed pressure drop is gradually increased with increasing superficial gas velocity. In SFBR, there is an
Fig. 3. The calculated minimum fluidizing velocity against the experimental value.
increased with increasing air inlet velocity which is typical for fluidized bed distributor. Moreover, the distributor pressure drop is lower for SFBR than CFBR that characterizes the SFBR because lowering values of distributor pressure drop results in decreasing the total energy 503
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Fig. 6. Effect of air superficial velocity on the local heat transfer coefficient at different radial positions at Z = 0.05 m for CFBR and SFBR.
additional downward centrifugal force besides the particles weight. This force resulted from the swirl motion of particles and increased as inlet gas velocity increased, therefore, the pressure drop increased with increasing inlet air velocity. The same behavior was observed in some studies related to SFBR such as [7]. Moreover, The figure shows that the bed pressure drop for SFBR is lower than that for CFBR. This can attribute to the CFBR characterizes with bubbling behavior where the gas bubbles are formed increasing the void among solid particles leading to bed expansion and increasing its height which means increases the weigh acting on the distributor area and hence increases the drag force requires for fluidization and hence increasing the bed pressure drop. But for SFBR bubbling behavior disappears. The same trend was found by Batch and Raghavan [18]. The time-averaged local heat transfer coefficient for both CFBR and SFBR at the different radial position and a height of 0.05 m is given by Fig. 6. The figure illustrates that the heat transfer coefficient increases with increasing air inlet velocity for both CFBR and SFBR. Increasing inlet air velocity increases the turbulent motion and hence improves the heat transfer process. Also, increasing inlet air velocity resulted in decreasing surface temperature as shown in Fig. 7. This may be due to the cooling effect of coming air which leads to increasing the heat transfer coefficient as given in Eq. (1). The figure indicated that the heat transfer coefficient of SFBR was higher than that of CFBR. At the lower height, the bed particles are in fully swirl state due to the vertical component of velocity which results in excellent gas-particles mixing and increasing heat transfer. In addition, as shown in Fig. 7 the surface temperature of SFBR is lower than that of CFBR and then the heat transfer coefficient of SFBR was higher than that of CFBR. Fig. 8 shows also the effect of superficial gas velocity on the heat
Fig. 7. Effect of superficial gas velocity on surface temperature at Z = 0.05 m for CFBR and SFBR.
transfer coefficient at a height of 0.1 m. Increasing air inlet velocity increased the heat transfer coefficient at all radial positions. It is noticed that at the upper height the heat transfer coefficient of CFBR is higher than that of SFBR at all radial positions. As mentioned by Kumar and Murthy [14] when the fluid emerges from exit the fluid has only tangential components which responsible for swirl so the bed particles are in fully swirl state at lower bed regions. With increasing height, particles move up taking a spiral path near the walls, the bed is in swirl state but the tangential component tends to decrease. At a certain height of the bed, the tangential component dies out and bed particles move up. Therefore, the solid particles near to the air inlet are in completely swirl motion while at the upper region the swirl action may disappear to bubbling, slugging fluidization which leading to 504
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Fig. 8. Local heat transfer coefficient versus superficial gas velocity at different radial positions at Z = 0.1 m for CFBR and SFBR.
decreasing heat transfer process. The heat transfer coefficient was also calculated at two other bed heights 0.025 m and 0.2 m. The axial distribution of heat transfer coefficient for CFBR and SFBR at air superficial velocity of 1.8 m/s and radial position (r/D) of 0.1 is shown in Fig. 9. For two types of reactors, the heat transfer coefficient decreased with increasing bed height. Moreover, the heat transfer coefficient of SFBR is higher than those of CFBR at lower portion of bed (at bed height smaller than 0.1 m) and this agrees with finding in Figs. 6 and 8. 3.2. Effect of bed particle size In this section, the effects of bed particle size on hydrodynamics and Fig. 10. The calculated and experimental values for minimum fluidization velocity for dp = 0.9 and 1.5 mm.
heat transfer in SFBR were studied. In this study, two particles size of 0.9 mm and 1.5 mm were used. The previous results given for particle size of 1.5 mm are compared to those of particles size of 0.9 mm in the following sections. Fig. 10 shows the minimum fluidizing velocity calculated using Wen et al. [17] correlation compared to the experimental value. The figure shows well agreement between the experiments and correlation results for different particle size. The minimum fluidizing velocity for small particles was lower than that of large particles because their weight is lighter and can be easily fluidized. Fig. 11 shows the bed pressure drop against air inlet velocity for two different particle sizes. The bed pressure drop rose with increasing air inlet velocity for two particle sizes and this a typical trend in SFBR as mentioned in Fig. 5. The bed pressure drop increased with the reduction
Fig. 9. The axial distribution of the heat transfer coefficient at r/D = 0.1 and U = 1.8 m/s for CFBR and SFBR. 505
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in increasing surface area and improving gas-solid mixing as air void among particles decreases, all these parameters lead to improving the heat transfer coefficient. This behavior was similar for all other radial positions. The local heat transfer coefficient versus inlet air velocity for two particles size is shown in Fig. 13. The heat transfer coefficient was calculated at the center of the reactors at two heights of 0.05 and 0.1 m. For two different sizes, the heat transfer in the lower height of 0.05 m is higher than that in the upper height of 0.1 m. As mentioned in Fig. 8 the bed particles in lower part were in complete swirl state and then improve the heat transfer process and with increasing bed height the swirl action decrease or disappear results decreasing heat transfer process. The figure shows also the heat transfer of small particles is higher than that of large particles. Fig. 11. The bed pressure drop versus superficial gas velocity for various particle sizes.
4. Conclusion Swirling fluidized bed reactors have given more attention in the last decades due to its apparent advantages that overcome the drawbacks of the conventional fluidized bed reactor. The present work studied experimentally the fluidization behavior and the heat transfer between the immersed heater and bed in SFBR. The heat transfer process was studied at different air superficial velocities, radial positions, and bed heights. First, the comparison between SFBR and CFBR were performed, then the effect of particle diameter on heat transfer and bed hydrodynamics in SFBR was carried out. The pressure drop across the air distributor and bed for SFBR were lower than those of CFBR and this characterizes SFBR. The heat transfer coefficient for SFBR was higher than that of CFBR at lower bed regime but at the upper regime, it was lower. This behavior can be interpreted as at lower bed heights the bed particles were at fully swirl state which improving heat transfer process and with increasing bed height the swirl action gradually decreased. The change of particle size affected the heat transfer and bed flow characteristics in SFBR. The bed pressure drop increased with a reduction in particle size. The heat transfer coefficient increased with the reduction of particle size. Also, it is found that the heat transfer coefficient was higher in lower bed height than that of upper bed heights for two different particle sizes.
Fig. 12. The heat transfer coefficient versus superficial gas velocity for dp = 0.9 and 1.5 mm.
Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.applthermaleng.2019.04.022. References [1] D. Kunii, O. Levenspiel, Fluidization Engineering, second ed., ButterworthHeinemann, Boston, 1991. [2] Subhajit Dutta, Chanchal Loha, Pradip Kumar Chatterjee b, Anup Kumar Sadhukhan, Parthapratim Gupta, Numerical investigation of gas-particle hydrodynamics in a vortex chamber fluidized bed, Adv. Powder Technol. 29 (12) (2018) 3357–3367. [3] E.J. Anthony, Fluidized bed combustion of alternative solid fuels; status, successes and problems of technology, Prog Energy Combust Sci. 21 (1995) 239–268. [4] J.P. Jacobs, The future of fluidized-bed combustion, Chem. Eng. Sci. 54 (22) (1999) 5559–5563. [5] Rachadaporn Kaewklum, Vladimir I. Kuprianov, Peter L. Douglas, Hydrodynamics of air–sand flow in a conical swirling fluidized bed: A comparative study between tangential and axial air entries, Energy Convers. Manage. 50 (2009) 2999–3006. [6] Mohd Faizal1, Suzairin Md Seri, Sh. Ezamuddin1, Vijay R. Raghavan, Numerical investigation of air flow distribution in a swirling fluidized bed, Adv. Mater. Res. 499 (2012) 132–137. [7] Binod Sreenivasan, Vijay R. Raghavan, Hydrodynamics of a swirling fluidized bed, Chem. Eng. Process. 41 (2002) 99–106. [8] Haidar Taofeeq, Muthanna Al-Dahhana, Investigation of the effect of vertical immersed tube diameter on heat transfer in a gas-solid fluidized bed, Int. J. Therm. Sci. 135 (2019) 546–558. [9] Haidar Taofeeq, Muthanna Al-Dahhan, Heat transfer and hydrodynamics in a gassolid fluidized bed with vertical immersed internals, Int. J. Heat Mass Transf. 122 (2018) 229–251. [10] Ning Yang, Yunlong Zhou, Tianyu Qi, The heat transfer between an immersed
Fig. 13. The radial distribution of heat transfer coefficient at different heights for different particle sizes.
of particle size. Smaller particle size results in increasing surface area and enhancing swirling motion which results in increasing centrifugal weight so larger drag force was needed to keep both fluidization and swirl in the bed which leads to increasing bed pressure drop. The same observation was reported by Mohideen et al. [19]. The impact of particle diameter on the heat transfer coefficient is given by Fig. 12. The heat transfer coefficient was measured at a distance of 0.02 m from the heater at a height of 0.05 m. As indicated by the figure, reduction in particle diameter results in increasing the heat transfer coefficient. This fact due to a reduction in particle size results
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