Elsevier

Powder Technology

Volume 381, March 2021, Pages 55-67
Powder Technology

Effect of particle collision behavior on heat transfer performance in a down-flow circulating fluidized bed evaporator

https://doi.org/10.1016/j.powtec.2020.12.007Get rights and content

Highlights

  • Fluidized bed heat transfer technology and down-flow evaporator are combined.

  • Polyformaldehyde particles are considered as inert solid particles.

  • Effects of particle collision behavior on heat transfer performance are investigated.

  • Effects of operating parameters on signal power and thermal performance are examined.

  • Signal power increases with the increased amount of added particles and heat flux.

Abstract

The particle collision behavior is investigated to analyze the heat transfer enhancement mechanism in a down-flow circulating fluidized bed evaporator. Water and Polyformaldehyde particles are selected as working media. Collision acceleration signals at varied amount of added particles (0–2%), circulation flow rate (2.15–5.16 m3/h) and heat flux (8–16 kW/m2) are analyzed with kurtosis and power spectral density. Results show that the frequency range of particle collision is 6000–16,000 Hz. Particles provide main energy contribution in multiphase flow collisions. The generation of vapor phase obviously strengthens the collisions of other phases. With increased amount of added particles, the signal power increases, but the heat transfer enhancing factor initially increases and then decreases. Both the heat transfer enhancing factor and signal power increase with increased circulation flow rate. With increased heat flux, the signal power increases but the heat transfer enhancing factor decreases.

Introduction

Vapor–liquid–solid (V–Lsingle bondS) three-phase circulating fluidized bed heat-exchange technology can prevent fouling and enhance heat transfer. Inert solid particles are introduced into heat-exchange device to form a fluidized bed heat exchange state. The shear and destruction of fluidized solid particles to the flow and the heat transfer boundary layer play an important role in reducing thermal resistance and enhancing heat transfer. Meanwhile, the buildup of scale layer on heat-exchange surface is inhibited due to the impact of added particles [[1], [2], [3]]. Many studies on this technology have been conducted [[4], [5], [6], [7], [8], [9]]. And it has been used in chemical industry [10], concentration of traditional Chinese medicine [11], and evaporation of wastewaters [12] and so on.

The interaction between the multiphase flow, especially the solid phase and the heat-transfer wall, has an important impact on the effect of this technology based on the mechanism of heat transfer enhancing and fouling prevention mentioned above. Therefore, several relevant studies on the collision behaviors have been carries out [[13], [14], [15], [16], [17], [18], [19], [20]].

Pronk et al. [13,14] analyzed the collision frequency of particles on the wall in liquid–solid stationary and circulating fluidized bed heat exchangers by using pressure sensors installed the inside wall of the heat exchanger. The anti-scale performance of the heat exchanger was investigated and a correlation was proposed for the collision kinetic energy. Pierre et al. [15] experimentally investigated the effects of solid holdup and gas velocity on the collision frequency and collisional particle pressure in an inverse three-phase fluidized bed by means of a high-frequency-response hydrophone. The results showed that the frequency of the collisions increased with the solid holdup. A direct correlation for estimating the frequency of collisions and the particle pressure as a function of the operating parameters was proposed. Wang [16] conducted the particle-wall collision experiments of spherical and non-spherical glass particles in a gas–solid separation process. Collision velocity was measured by high-speed photography. The effects of particle sphericity and wall roughness were analyzed, and the relationship between impact angles and restitution coefficient was explored. An et al. [17] analyzed the vibration displacement characteristics of the graphite tube in a naturally circulating fluidized bed evaporator by using power spectral density function, which reflected the axial distribution of bubbles and solid particles in the graphite tube. High steam pressure and low solid holdup were recommended for industrial applications. Xu et al. [18] conducted chaos analysis on the vibration acceleration signals and pressure drop signals of the vapor–liquid two-phase flow in a single graphite tube. The relationship between the multi-value phenomenon of the related dimensions and the multi-scale flow behavior was investigated to show the characteristics of low-and-mid frequency signals by using wavelet decomposition and signal reconstruction techniques. Kang et al. [19] analyzed the effect of the collision behaviors between the particles and the wall on the heat transfer characteristics in a liquid-solid circulating fluidized bed heat exchanger by combining numerical simulation and visual observation. Results showed that the heat transfer characteristics were closely related to the hitting frequency of the fluidized particles on tube wall. The particles flowing with water periodically hit the tube wall, broke the thermal boundary layer, and increased the heat transfer rate. Abbasi et al. [20] used accelerometers to record vibration signals generated by particles flowing through the fluidized bed at various superficial gas velocities and particle sizes. They found that different hydrodynamic phenomena such as formation of small bubbles, large bubbles and clusters could be distinguished by using the frequency components of vibration signals.

The data processing methods of collision signals is very important to analyze the mechanism of heat transfer enhancement and fouling prevention. Therefore, accompanied with the investigation on the collision between the fluid and heat-transfer wall by signal analysis, the relevant data processing methods such as statistical analysis [[21], [22], [23]], nonlinear analysis [23,24], and spectrum analysis [23,[25], [26], [27], [28], [29]], have also been developed.

The statistical analysis method is mainly used to analyze fluid motion characteristics, identify different flow patterns, and determine the transitions of flow patterns and so on. The nonlinear analysis methods can be used for statistical and deterministic chaos analysis of signals. The spectrum analysis method can transform the measured signal from the time domain to the frequency domain, reflecting the distribution of signals at different frequencies. In the present study, the statistical analysis and spectrum analysis methods are adopted.

Briens [25] proposed that kurtosis analysis could effectively reflect time series distribution, predict the intensity of particles–wall collision and determine the flow state. PSD could be widely used in the frequency domain analysis of the detection technology for fluidized bed. Shou [28] proposed that energy of the power spectral density function could effectively analyze pressure fluctuations. Sheikhi [29] proposed the non-invasive detection methods for fluid dynamics analysis in a liquid–solid fluidized bed. The method could investigated the internal collision situation by adding an acceleration sensor to outer wall surface.

The above mentioned investigation on collision behavior mainly concentrated on upflow bed. However, few studies are reported on the down-flow bed, which is widely applied in industries such as the evaporation of lithium hydroxide accompanied by serious fouling problem. The preliminary exploration [30] have demonstrated the characteristics of heat transfer enhancement of the down-flow circulating fluidized bed evaporator by combining the circulating fluidized bed heat transfer and fouling prevention technology and evaporation process. Therefore, the present investigation focuses on the effect of particle collision behavior on the heat transfer performance in a down-flow circulating fluidized bed evaporator. The acceleration sensors are used to investigate the collision behaviors. The frequency range of the multiphase flow acting at wall is determined by analyzing the acceleration signal in time and frequency domain. The mechanism of heat transfer enhancement and fouling prevention are revealed by combining the effects of the operating parameters, such as the amount of added particles, circulation flow rate and heat flux. The findings are helpful for the industrial application of the fluidized bed heat transfer and fouling prevention technology.

Section snippets

Experimental apparatuses and procedure

The experiments are conducted in a vapor–liquid–solid three-phase down-flow circulating fluidized bed evaporator, which consists of a heated down-flow bed, an evaporation chamber, a condenser, a particle collector and a data collection system, as illustrated in Fig. 1. The apparatus is made of 304 stainless steel. The heart of tested section is a down-flow bed with a dimension of Ф38 mm × 3 mm and length of 1200 mm, which is heated by the resistance wire. The whole apparatus is insulated by the

Power spectral density analysis

Frequency domain analysis is widely used in complex signal processing. Frequency component and frequency distribution range of collision signals can be acquired with frequency domain analysis. Then the amplitude and energy distribution of frequency component can also be obtained.

Power spectral density (PSD) function is used to convert time series to spectrum. PSD function P(f) is a windowed version of Fast Fourier Transform function, which can reflect the distribution of signals at different

Characteristic collision frequency and signal power of the multiphase flow in the down-flow bed

The characteristic frequency distribution range may be different for different types of collisions in specific situation [14,17,20]. The time series and PSD of acceleration signals are analyzed to acquire the frequency component and energy distribution of collisions for the liquid phase, liquid–solid two-phase, vapor–liquid two-phase and vapor–liquid–solid three-phase flows, respectively.

Conclusions

The effect of particle collision behavior on the heat transfer performance in a vapor–liquid–solid three-phase down-flow circulating fluidized bed evaporator is investigated by analyzing the PSD and kurtosis. The major conclusions are summarized as follows:

The distribution ranges of the collision frequency are 0–2000 Hz and 6000–16,000 Hz for the liquid and solid phases, respectively. The signal power of the solid phase is much greater than that of the liquid phase. The generation of the vapor

Declaration of Competing Interest

None

Acknowledgments

This work is supported by the open foundation of State Key Laboratory of Chemical Engineering (SKL-ChE-18B03) and by the Municipal Science and Technology Commission of Tianjin, China under Contract No. 2009ZCKFGX01900.

Credit authorship contribution statement

Jiang Feng: Conceptualization, Methodology, Supervision, Analysis and Evaluation. Hongyu Wang: Investigation, Software, Experimental testing and calculation, Writing Original Draft, Revising Manuscript. Yi Liu: Investigation, Software, Experimental testing and calculation, Writing Original Draft. Qi Guopeng: Data curation, Review & Editing. Ahmed Esmail Al-Rawni: Investigation. Primrose Nkomazana: Investigation. Li Xiulun: Review & Editing.

All persons above mentioned have made substantial

References (34)

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