Transient characteristics of fine powder flows within fluidized dense phase pneumatic conveying systems

Powder Technology 343 (2019) 629–643

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Transient characteristics of fine powder flows within fluidized dense phase pneumatic conveying systems Yassin Alkassar a,⁎, Vijay K. Agarwal a, Niranjana Behera b, Mark G. Jones c, R.K. Pandey d a

Industrial Tribology, Machine Dynamic and Maintenance Engineering Centre, IIT Delhi, New Delhi 110016, India School of Mechanical Engineering (SMEC), VIT University, Vellore 632014, India Centre for Bulk Solids and Particulate Technologies, School of Engineering, University of Newcastle, 2308, Australia d Department of Mechanical Engineering, I.I.T. Delhi, New Delhi 110016, India b c

a r t i c l e

i n f o

Article history: Received 19 July 2018 Received in revised form 15 November 2018 Accepted 18 November 2018 Available online 22 November 2018 Keywords: Fluidized dense phase Pulsatile Wavelet Pulse structures

a b s t r a c t The objective of this paper is to report the experimental findings related to the pressure fluctuations in the fluidized dense phase pneumatic conveying of fine powders in the pipeline. Investigation has also been carried out to understand the relation between pressure pulse characteristics and specific power consumption and pneumatic conveying parameters. In addition, the transition in mode of flow along the downstream of flow of fine powders has been assessed and discussed. The wavelet analysis (Daubechies db4 wavelet) of signals of air pulses revealed that the moving bed flow possesses the transient feature and pulsatile phenomenon characterized by multiple amplitudes and random frequencies. Variations of pulse structures along the length of pipeline have been observed due to occurring of frequent aeration and de-aeration of dunes during the conveying. Substantial variations in pulse structures have also been found with different types of bulk materials. Pulse velocities of air between two data points are found higher at high solid loading ratios leading to low voidage in case of dense phase flow. © 2018 Elsevier B.V. All rights reserved.

1. Introduction Dense phase pneumatic conveying is a preferred choice for conveying bulk materials in industries/plants at low conveying air velocity. It has many advantages over dilute phase mode of conveying. In dense phase flow, the erosive wear of components and particle attrition are much lower due to low conveying velocity [1]. In addition, specific power consumption required to convey the bulk materials is lower in dense phase flow [2]. Depending on the particle properties, the dense phase flow occurs in two modes. One is the dense phase plug or slug flow in which the granular materials occupy the full bore separated by air gaps, and the other is fluidized dense phase flow (i.e. moving bed flow) in which the fine powders flow in two layers [3]. The fine powders in fluidized dense phase flow as wave or dune types of motion in slower dense phase layer flowing at the bottom of the pipe and as suspended in a rapid dilute phase layer flowing over the dense layer in the upper section of the pipe. However, this kind of transportation is more dynamic [4], pulsatile flow [5] and discontinuous [6], and a strong interaction between air and solid phases is observed [7]. In addition, the friction between the moving dense layer and pipe wall induce a retarding force on the flow of materials; shear force introduced on top of material by ⁎ Corresponding author. E-mail address: [email protected] (Y. Alkassar).

https://doi.org/10.1016/j.powtec.2018.11.081 0032-5910/© 2018 Elsevier B.V. All rights reserved.

relative high gas velocity in dilute layer, and flow of air through the materials [8] make the mechanism of flow more complex. The successful design of pneumatic conveying system is based on determining the important parameters such as pressure drop and air and solid mass flow rates. Typically, two methods have been used to model fluidized dense phase flows: scale-up method [9–11] and empirical two-phase method [12–14]. In scale-up method, the conveying characteristics obtained from pilot scale pneumatic conveying rigs are scaled to that actual test pipeline of different geometries. The ratio of mass flow rate of material from test to the pilot is inversely proportional to the pipe lengths and proportional to pipeline cross sectional area [15]. With regard to empirical two-phase method, the total pressure drop across the pipe is contributed to gas phase and solid phase separately. The pressure drop due to gas phase is usually calculated using DarcyWeisbahch's equation. The pressure drop due to solid phase used a term called solid friction factor which has been investigated and correlated to non-dimensional parameters such as solids loading ratio, gas and solid Froud number, air to particle density ratio, pipe diameter to mean particle diameter ratio, etc. [16]. These approaches have usually used steady state data, average value of parameters and assuming that particle velocity is equal to gas velocity. The predicted results provided by these approaches prove to be inaccurate compared to those of data collected from actual plants. Thus, a deeper insight into the flow of bulk materials and gas-solid interaction during the conveying is

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required. However, recent research has shown that the flow mechanism of fine powders in dense phase is highly transient in nature and displays pulsatile phenomenon [17]. Studying the pressure fluctuation signals in gas-solid fluidized bed provides a quantitative way to understand the hydrodynamic behaviour and mechanism of flow. The pressure fluctuation signal acquired from fluidized bed is decomposed into a global pressure fluctuation signal, which is characterized by fast compression waves and a local pressure fluctuation signal which is characterized by slow compression wave [18]. Pressure fluctuations are generally generated in fluidized bed systems due to bubble motion [19], bubble/jet formation/splitting [20], bubble eruption at bed surface [21], compressibility of gas in the chamber [22] and fluidized particle [23]. A full review about the effect of each of these factors has been presented by Bi [24]. Pressure wave from each of these sources propagates through the particulate media and gets attenuated [25] or amplified [26] as studied from simultaneous multipoint reading of pressure from the dynamic wave [27]. The nature of pressure wave attenuation has been studied by many authors. There are two mechanisms which play a major role in attenuation of pressure wave: inter-particle collision and relative movement between particle and interstitial gas [28]. In addition, pressure wave could be attenuated due to three factors as assumed by Bi et al. [29]. These factors were relaxation of the particles, particleparticle contact and particle-gas relative movement which caused viscous damping of fluctuation. Similar factors are also responsible for pressure fluctuations in pneumatic conveying. The analysis of pressure fluctuation signals generated in fluidized dense phase pneumatic conveying system has been studied through power spectrum density [30].It is observed that dominate frequency for fluidized dense phase flow was within the 0–3 Hz range. However, Fourier analysis decomposes a signal to complex exponential functions of different frequencies which is not suitable for non-stationary signals [31] generated in fluidized dense phase pneumatic. Hence, wavelet technique has been growingly utilized in gas-solid flow for better assessing the flow regimes [32–34]. In wavelet technique, the timebased signal comprises of breaking up a signal into low-frequency component and high-frequency component [35] which provides capability to deal with time-frequency localization [36]. Li et al. [37] investigated the pressure fluctuation generated in dense phase pneumatic conveying system by performing wavelet multiresolution analysis in order to study the features of flow of 3.5 mm Polyethylene pellets in accelerated and developed regions. Pahk and Klinzing [38] applied wavelet analysis to decompose the original signal into five levels. The level one estimated that the gas turbulence and amplitude of the power spectrum was small whereas the other levels evaluated the gas-particle interaction. Few researchers have employed the wavelet technique to study and understand the transient phenomenon in fluidized dense phase flow of fine powders. Williams et al. [39] studied the pulsatile gas flow behavior in fluidized dense flow by addressing pressure fluctuating signal to wavelet analysis. The changes in pulse amplitude extracted from filtered fluctuating signal along the pipeline length have been analyzed and concluded that the gas expansion was an essential factor in the increase of the pulse amplitude as the flow proceed downstream. In addition, Behera et al. [40] analyzed variation of the transient parameters along the length of conveying pipeline after preforming Daubechies db4 wavelet analysis to eliminate high frequency components. Variation in aeration rate and de-aeration rate of the bed of material along the pipeline were observed by varying in transient parameters. The location of changing from dense to dilute phases was also determined through pulse slope ratio analysis. The objective of this work is to study the behavior of pulse structures associated in fluidized dense phase pneumatic conveying. Moreover, the variation of pulse structures along the length of conveying pipeline is conducted in order to assess the flow mechanism of fine powder in fluidized dense phase flow, and hence predict the location of transition of the mode of flow. Furthermore, the correlations between the conveying parameters and power consumption parameters with pulse structure are carried out.

2. Experimental program The following section provides details about the experimental test of fluidized dense phase pneumatic conveying. 2.1. Test procedure Fluidized dense phase pneumatic conveying experiments were carried out with fly ash (d50 = 14.91 μm, ρp = 2096 kg/m3, ρbl = 724 kg/m3) and alumina (d50 = 37.11 μm, ρp = 3810 kg/m3, ρbl = 988 kg/m3) through a 173-m pipeline of 53 mm diameter as seen in Fig. 1. Initially, the blow tank of top discharge filled with a certain amount of bulk material and then the pinch valve was closed. The compressed air from primary line was provided to fluidize the materials inside the blow tank, and then data acquisition was turned on. After fluidization, the vent valve was closed and the secondary air valve was opened. Then the pinch valve was opened to start the conveying process. The tests were performed within the range of ratio of mass flow rate of material to mass flow rate of air (i.e. solid loading ratio) between 12–50 and 26–57 for fly ash and alumina respectively and total pressure drop per unit length in a range of 0.965–1.878 kPa/m and 1.33– 1.79 kPa/m for fly ash and alumina respectively. Solid loading ratio or phase density (m*), is the ratio of the mass flow rate of material conveyed to the mass flow rate of air used to convey the material. In pneumatic conveying, it is used to describe nature of flow (i.e. dense or dilute). It is a non-dimensional parameter which is independent of conveying air pressure and hence does not vary along the length of the pipeline. The plant was equipped with two pressure transmitters: one was mounted near the feeding point for measuring secondary air pressure, and another was placed in a blow tank for measuring primary air pressure. In addition, pressure transducers were mounted along the pipeline for measuring pressure drop. Pressure signals were being recorded at a frequency between 80–100 Hz satisfying Nyquist sampling-frequency theorem. It states that the minimum sampling frequency of signal should be more than double the maximum frequency in the signal for avoiding aliasing. Based on our work [30], The powers were considered negligible beyond 3 Hz. So, the minimum sampling frequency based of Nyquist's criteria is 3*2 = 6 Hz. Thus, 80–100 Hz of sampling frequency proved more than sufficient for the analyses of the data. Finally, the materials lost and gained from feeder and receiver respectively were measured via a series of load cells. Fly ash and alumina conveying materials have been classified as Geldart A type of material to be transported in fluidized dense phase as described in the Geldart fluidization chart [41]. They have good aeration properties. The minimum fluidization velocity (Umf) for fly ash is 0.14 m/s, and for alumina is 0.085 m/s. 2.2. Test analysis The conveying cycle of fluidized dense phase pneumatic conveying is comprised of three stages. The first stage was pre-pressurization process in which the air-solid mixture in the blow tank pressurized to a specific pressure value. The second stage was a steady state process in which materials started conveying from blow tank in steady state mode associated with steady state value of pressure. The pressure signal data of sensors placed along the pipeline during the steady state period were considered for the investigation of the structure of pulsatile pressure fluctuations within the pipeline. At the end of each experiment, the amount of material in blow tank reached a critical level which was associated with more considerable pressure fluctuations indicating the emptying cycle. Fig. 2 represents the conveying cycle of dense phase pneumatic conveying. The captured raw pressure signals consisted of low and high frequency components. These signals contained high frequency components which were unwanted noise components eliminated by filtering

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Fig. 1. Schematic of 173 m pipeline of pneumatic conveying setup.

the signals. Wavelet technique is widely used to filter the noise from sharp signal structure of pressure fluctuation occurred in dense phase pneumatic conveying. It decomposes the original signal into different scales and positions of mother wavelet. The Daubechies db4 wavelet with 4th level of signal decomposition was activated in wavelet analysis for the elimination of the high frequency components from the raw

signal. Fig. 3 shows the levels of break up of pressure signal executed in wavelet Toolbox of MATLAB16b. The decomposition has shown the signal being gradually cut up five parts with the high frequencies being removed from level d1 to d4 and finally a4 being considered as low frequency approximation of the pressure trace. Subsequently, the level a4 was analyzed to set the pressure fluctuation features.

Fig. 2. Conveying cycle of dense phase pneumatic conveying.

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Fig. 3. 4th level wavelet decomposition of pressure signal at T1 using Daubechies db4 wavelet.

3. Features of pressure traces As stated previously, the high frequencies content in the pressure signals were removed by using Daubechies db4 wavelet. Fig. 4 shows the a4 level of filtered signal over a specified period. According to it, it is observed that the flow of fine powder, in dense phase pneumatic conveying, represented pulsatile characteristics. Ultimately, the identification of pulses within the flow (i.e. peaks and valley) were found. This was implemented using MATLAB intrinsic function. As a result, a pulse was then determined as part starting from one valley/peak point to the next. The characteristics of gas pulses are: pulse growth amplitude, pulse growth duration, pulse decay amplitude and pulse decay duration. In addition, the inverse of pulse time is defined as pulse frequency, which is considered as another parameter. Whereas the pulse growth segment performs the superficial fluidization process of fine powders, the pulse decay part represents the superficial de-aeration process of fine powders. So: • Pulse amplitude: The vertical pressure difference between the valley and the peak of this part; • Pulse duration: The horizontal time difference between the valley and peak of this part.

The pulse amplitude is categorized into pulse growth and pulse decay. In the pulse growth amplitude, the pulse increases up to a certain level of pressure due to expansion of bed representing fluidization of solids layer (dunes). During fluidization process, the air penetrates the solid layer and interacts with particles causing aeration and particles concentration (dunes) decreases. In case of fluidized bed, this will improve the quality of fluidization by eliminating gas channeling and reduction in bubble size. Similarly, during the pulse decay amplitude, the pulse down to certain level of pressure with a decrement in height of bed representing the deaeration of solids layer. During deaeration process, the gas escapes from the dunes (solid layer) causing the particles to settle down and hence concentration increases. In addition, particle-particle interactions are also enhanced. The pulse duration refers to the total time for superficial fluidization/de-aeration process. 4. Pulse Structures analyses along the length of conveying pipeline The pulse growth and decay (amplitude and duration) parts for flows in the experiments were obtained by applying the pulse characterizations on all flow. Averaged amplitude and duration in each test during steady-state process were calculated and correlated to experimental parameters.

Fig. 4. Transient parameters of pulsatile pressure fluctuations at transducer location T1.

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The mode of flow in fluidized dense phase may not continue the same at all segments along the long pipes with uniform pipe bore. This means that the flow starting with dense phase at the inlet of pipeline may either remain in dense phase along the downstream of flow or change to dilute phase at a specific location. However, the flow that starting with dilute phase at the beginning of pipeline will continue in dilute mode till the end of pipeline. It is important to locate the transition of mode of flow at every cross-sectional pipeline. For this purpose, changes in Pulse structures parameters along the length of pipeline are calculated and analyzed for studying the behavior of pressure fluctuation. Three pressure transmitters mounted on the top of pipeline at different locations along the pipeline (T1 at 27.29 m; T15 at 100,42 m; and T17 at 129.11 m) were considered for investigation. Three various cases of experiments data of fly ash conveying material [case 1: m* = 38.7, v = 3.9 m/s (case 1 represents the flow of bulk material in dense phase along the pipeline); case 2: m* = 25.9, v = 8.1 m/s (case 2 represents the changes in mode of flow along the pipeline); case 3: m* = 14.4, v = 10 m/s (case 3 represents the flow of material in dilute phase along the pipe)] and three different cases for alumina conveying material [case 1: m* = 47.8, v = 4.76 m/s (denser phase flow along the pipeline); case 2: m* = 33.7, v = 5.47 m/s (moderate dense flow with changing from dense to dilute phase at location 129.11 m); case 3: m* = 29.8, v = 6.88 m/s (less dense flow with changing from dense to dilute phase at location 100.42 m)] were plotted for the aim of comparisons.

4.1. Pulse amplitude Averaged pulse amplitude for the growth and decay segments are plotted against the length of pipeline in fluidized dense pneumatic conveying system. Fig. 5 shows changes in pressure pulse amplitude along the length of pipeline for experiments data of fly ash. It is observed, during the flow of bulk material in dense phase mode such as Case1 and Case 2, that the pressure pulse amplitude increases as the fly ash flows from upstream to downstream of the pipeline. The reason for such an increase along the pipeline is a rise in aeration rate of bed material as the fly ash is conveyed form blow tank to the receiver. The location of maximum aeration rate may be taking place at a distance over 130 m. However, since the flow of fly ash in case 3 starts in dilute phase, there is a considerable reduction in pulse amplitude until a distance of 100 m because of damping of gas turbulence by suspended particles. Although the flow of fly ash in case 3 is dilute phase, the pulse amplitude increases beyond

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a 100 m distance as the powders become fully suspended flow due to greater gas expansion. The pressure pulse amplitude of the three different experiments results, of alumina flowing in fluidized dense phase along the pipeline, are plotted in Fig. 6. It is seen that the pressure pulse amplitude increases with an increase in distance from the inlet of pipeline. This is due to dunes being progressively aerated along the pipeline. By comparing Fig. 5 and Fig. 6, it is observed that the trend of pressure fluctuation along the length of pipeline is same for both materials in case of dense phase. In addition, the pressure fluctuation amplitude is higher for alumina compared to fly ash. This is because alumina has high particle density. Similar observation is reported in a study by Nosrati et al. [19]. The values of pressure pulse amplitude for all data points along the length of pipeline for fly ash and alumina are provided in Appendices. It can be revealed in Fig. A1 and Fig. A2 that the value of pulse amplitude decreases quite sharp just after bend, and then, the values increase along the pipeline. The reason for such a decrease is because particles are subjected to deceleration which dampen turbulence and hence reduced the amplitude. 4.2. Pulse time ratio The pulse time is categorized into a time of pulse growth and time of pulse decay. In the pulse growth duration, the pulse increases up to a particular level of pressure with increment in pressure amplitude. Similarly, during the pulse decay duration, the pulse downs to a certain level pressure with a decrement in pressure amplitude. The time of pulse growth is the time interval that gas has taken to aerate the bed of material (superficial fluidization), and the time of pulse decay is the time interval that gas has taken to de-aerate the material (de-aeration process). So, studying the pulse time ratio (time of pulse growth/time of pulse decay) provides an idea about the rate of aeration/de-aeration of the bed of materials. The variations of pulse time ratio for different cases of fly ash along the pipeline length are plotted in Fig. 7. In case of dense phase flow like case 1, the pulse time ratio gradually decreases with an increase in a distance from the inlet, and this indicates a reduction in time of pulse growth, and hence high aeration rate, as the fly ash is conveyed along the downstream of flow. In case of less dense phase flow such as case 2, the value of pulse time ratio initially reduces to a specific location of 100 m because faster aeration rate of dunes is maintained as compared to de-aeration rate of dunes which points to duration of fluidize of solid layer is less. However, in the second region beyond

Fig. 5. Results of pressure fluctuation amplitude for fly ash along the pipeline length based on wavelet analysis.

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Fig. 6. Results of pressure fluctuation amplitude for alumina along the pipeline length based on wavelet analysis.

100 m, the pulse time ratio starts increasing which refers to increase in time of aeration the dunes. In other word, in the second region, the dune structure breaks down and dilute phase is more prevalent. Since the flow of fly ash in case 3 starts in dilute phase at the beginning of pipeline, the pulse time ratio increases along the pipeline because the materials are continuously drawn from the base of pipeline due to pulse growth. The changes in pulse time ratio for different flow conditions of alumina along the length of the pipeline are plotted in Fig. 8. Here, three different cases of various dense phases flow are analyzed for investigation. In the condition of denser phase flow such as case 1, the pulse time ratio progressively reduces as the flow of alumina drawn to the end of the pipeline. This is due to less rate of de-aeration of deposited layer. In case of moderate dense phase flow as case 2, the pulse time ratio also falls down with an increase in a distance of pipeline from the inlet. This is also due to the increase in time of de-aerating the dunes, and hence the duration of de-aeration of materials is longer. The main difference between case 1 and case 2 is the pulse time ratio beyond the distance 100 m; in case 2, the pulse time ratio is reduced in a little manner compared to the reduction in case 1 indicating that the mode of flow will be converting to dilute phase nearby the location of 130 m. In case of less dense flow such as case 3, compared to case 1

and 2, it is observed that the pulse time ratio downs to a certain distance and then starts going up in the second region beyond the distance of 100 m. The explanation for this is that the dense phase becomes less prevalent in the second region. 4.3. Pulse slope ratio It is observed form the investigation of above Pulse structures (i.e. amplitude and duration) that any changes in superficial fluidization / de-aeration process of dunes led to a variation in these parameters. To further investigate the change of mode of flow along the pipeline length, Pulse slope ratio is considered an analysis parameter and was defined as follows: Pulse slope ratio ¼

Pulse growth amplitude Pulse decay amplitude = Time of pulse growth Time of pulse decay

Fig. 9 presents the variation of pulse slope ratio along the flow direction of fly ash. For case 1 which represents the dense phase flow, the pulse slope ratio mainly increases as the flow proceeds from the inlet to the outlet of the pipeline indicating no change in mode of flow. This is because of continuous aeration of the bed of material. So, in case 1

Fig. 7. Results of pressure fluctuation time ratio for fly ash along the pipeline length based on wavelet analysis.

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Fig. 8. Results of pressure fluctuation time ratio for alumina along the pipeline length based on wavelet analysis.

the flow starts with dense phase flow from blow tank and continues in the same mode along the pipeline. However, for case 2, the pulse slope ratio initially goes up to a specific distance of 100 m and then gradually decreases. The increase in pulse slope ratio prior to 100 m is due to the domination of de-fluidization process of dunes but it decreases beyond the second location of transmitter of 100 m due to decrease in aeration rate, and hence domination of fluidization process of dunes. This indicates that the flow of fly ash starts with dense phase at the start of the pipeline and continues in the same mode until location of 100 m, while in the second region the flow begins to change from dense to dilute phase. This also confirms from experimental data that the superficial air velocity at the second and third locations was 14.7 m/s and 17.9 m/s respectively. In this pipeline system, once the mode of flow shifts to dilute phase, it cannot return back to its dense phase mode because of uniform pipe diameter throughout the whole pipeline length. The flow mode would have been converted into dense phase if the pipeline was stepped at the location of flow mode transition. With regard to the condition of flow starting in a range of dilute phase (with pick up velocity 10 m/s) at the start of the pipeline such as in case 3, the pulse slope ratio gradually decreases along the length of the pipeline. The flow of fly ash becomes fully dilute at the location of 130 m with superficial gas velocity 21 m/s.

Fig. 10 shows three different cases of alumina with variations of pulse slope ratio along the conveying pipeline. In case of denser and moderate dense phase flow such as case 1 and case 2, the analysis revealed that the pulse slope ratio progressively increases as the alumina is conveyed along the pipeline, and hence gradually aeration of dunes. However, in case 2 the rate of increase in pulse slope ratio, at a location beyond the distance of 100 m, is less in comparison with that of case 1. This indicates that the mode of flow in case 2 will be changing at a distance near to 130 m. In the condition where the less dense phase flow, such as in case 3, the pulse slope ratio starts increasing up to a certain location of 100 m and then starts to reduce as the flow proceeds to the receiver. So, de-fluidization process is prevailed in prior 100 m and fluidization process is dominated in the second region. Therefore, in this case, the transition of mode of flow from dense to dilute phase occurs close to the location of 100 m. In a similar manner, the dilute phase could be converted to dense phase if a proper stepped pipe was provided at the location of transition. 4.4. Pulse frequency Each signal has many pulses over a period of time and the number of appearance of pulses per second can be defined as pulse frequency. The

Fig. 9. Results of pressure pulse slope ratio for fly ash along the pipeline length based on wavelet analysis.

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Fig. 10. Results of pressure pulse slope ratio for alumina along the pipeline length based on wavelet analysis.

following section discusses variations of pulse frequency along the pipeline for both fly ash and alumina conveying materials. The plot of changes the pulse frequency of fly ash as the flow proceeds downstream is shown in Fig. 11. In the condition of dense phase flow such as case 1 and case 2, it is noticed that the pulse frequency goes in reduction along the pipeline since the bed of material continues to aerate or increase in aeration rate as the fly ash is forwarded from upstream to downstream through the pipeline. However, in the condition of dilute phase like case 3, the pulse frequency also decreases as the flow becomes more smooth and stable. Fig. 12 presents changes in the frequency of pressure fluctuations of alumina across the length of the pipeline. Since the dunes repeatedly aerate along the length of pipeline in case of dense phase flow, the pulse frequency goes down. It can be seen that the increase in solid loading ratio leads to an increase in pulse frequency. This suggests that the number of pulses increases with an increase in solid loading ratio.

As the flow proceeds from the inlet to the outlet of the pipeline, the gas pressure is decreasing, and hence its velocity increases. So, the kinetic energy of air increases and subsequently the pulse growth occurs with a corresponding increase in pulse amplitude. Fig. 13 presents a reduction in pressure pulse amplitude with an increase in solid loading ratio for both fly ash and alumina conveying materials. This observation is similar to that reported by Musmarra et al. [26] who observed larger pressure amplitude, the higher fluidization velocity. In addition, at high solids loading ratio and hence more particle-particle interaction occurs, the pressure amplitude damped by energy loss from interparticle interaction. With the same solid loading ratio, lower pulse amplitude occurs for fly ash conveying material as compared to alumina conveying material. This is because fly ash has a lower bulk density compared to alumina.

5. Effect of conveying parameters on pulse structures

5.2. Influence of solids loading ratio on pulse slope ratio

The pulse structures are affected by experimental parameters including air mass flow rate and solid mass flow rate. This section aims at investigating the relation between pulse structures and solid loading ratio at the location of pressure sensor T1.

Solid concentration of fine powders plays a crucial role in the expansion and collapse of the bed of material during aeration and de-aeration process. Fig. 14 shows the plot between the pulse slope ratio and solids loading ratio. It is observed that as an increase in solids loading ratio, the

5.1. Impact of the solid loading ratio on pressure pulse amplitude

Fig. 11. Results from wavelet-based analysis of pulse frequency for fly ash along the pipeline length.

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Fig. 12. Results from wavelet-based analysis of pulse frequency for alumina along the pipeline length.

Fig. 13. Variation pressure pulse amplitude with solid loading ratio for fly ash and alumina conveying materials.

Fig. 14. The effect of solid loading ratio on pulse slope ratio for fly ash and alumina conveying materials.

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pulse slope ratio decreases for both fly ash and alumina conveying materials. This is because the expansion of the bed of materials at high solids loading ratio is not too much, and hence the pulse growth is not considerable. Therefore, the pulse slope ratio is significantly lower at high solids loading ratio rather than at low solids loading ration. However, the expansion of bed of materials at low solids loading ratio is relatively higher compared to that at high solids loading ratio and, thus, representing higher magnitudes of pulse slope ratio. Another important feature revealed is the effect of bulk materials density on the magnitudes of the pulse slope ratio.Where the bulk material which has a higher bulk density such as alumina presents higher values of pulse slope ratio compared to lower bulk density material like fly ash. 5.3. Effect of solid loading ratios on non-dimensional pulse frequency for fly ash and alumina conveying materials It is observed from Figs. 11 and 12 that there is no difference in pulse frequency variation with kind of flow of fine powders along the length of the pipeline. So, Non-dimensional frequency is defined as follows is considered for further investigation. This parameter is similar to Strouhal number, which is used to analyzing unsteady, oscillating flow problem. Non−dimensional frequency Pulse frequency  distance of location of transmitter ¼ Local superficial gas velocity Fig. 15 shows the relation between non-dimensional frequency and solid loading ratio for both fly ash and alumina conveying materials. The non-dimensional frequency decreases with a decrease in solid loading ratio for both fly ash and alumina conveying materials. Higher values of non-dimensional frequency occur when the pulse frequency is high and superficial gas velocity is low. This suggests that at high values of non-dimensional frequency, the material is subjected to more repeated driving forces in order to a rise in the rate of mass flow of conveying materials, and hence higher solid loading ratio. These repeated driving forces of gas on the conveying materials are responsible for appearing the pressure pulses in the captured signal of pressure. The nondimensional frequency values for dense phase flow is found to be nearly equal to or greater than 2.5. That is to say, where the flow is dense, such value is considered a transit one if the non-dimensional frequency values are above it. In addition, non-dimensional frequency values in less dense phase flow and dilute flow in fly ash are higher than denser phase flow such as alumina because the duration of superficial

aeration/de-aeration process of the bed of material is longer in alumina (denser). 6. The Influence of pulse structures on power consumption This section attempts to find a relation between pulse structures and specific power consumption for different experimental data of fly ash and alumina conveying materials. Specific power consumption is defined as power needed to convey a unit mass of material for a given pressure drop. It provides a simple method of comparison for different conveying flow conditions. The specific power consumption is given as follows for isothermal model [42]. Specific power consumption ¼

  1 pin 202V o ln ms pout

where specific power consumption has a unit kw/ kg/s when: Pin pressure at the inlet (Pa). Pout pressure at the outlet (Pa). ms Solids mass flow rate (kg/s). V0 Volumetric flow rate of free air (m3/s). The analysis has been carried out for different flow conditions of fly ash and alumina conveying materials at the location of pressure sensor T1. 6.1. Effect of specific power consumption on non-dimensional frequency for fly ash and alumina conveying materials Fig. 16 shows a reduction in non-dimensional frequency with an increase in specific power consumption for both fine powders. The increase in non-dimensional frequency at lower specific power consumption indicates a high in pulses frequency and a low in local superficial gas velocity at particular location for both fine powders. At high pulses frequency, the materials are subjected to more repeated driving forces for possible conveying, and hence an increase in the solids mass flow rate. Here repeated driving force means the force causing the expansion of bed and causing the pulse peaks in the pressure signal. It is considered as force due to flow of air bubbles within the bed of materials which caused by turbulent motion of the gas. Similar force has been observed in fluidized bed in a study by Bi et al. [43]. In addition, at low superficial gas velocity, the supply air needs a minimum quantity of kinetic air energy for conveying the fine powders. So, specific power consumption which is used effectively in the conveying process

Fig. 15. Variation of non-dimensional pulse frequency with solid loading ratio for fly ash and alumina conveying materials.

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Fig. 16. The correlation between the specific power consumption with non-dimensional frequency.

reduces. Moreover, the magnitudes of specific power consumption are lower for denser phase flow such as alumina as compared to less dense phase flow and dilute phase flow in fly ash for the same nondimensional frequency. So, non-dimensional frequency parameter can also be utilized as a boundary for distinguishing the dense and dilute mode of flow.

6.2. Effect of specific power consumption on pulse slope ratio for fly ash and alumina conveying materials The optimum value of specific power consumption occurs when the conveying process requires a minimum total pressure drop with a high solid loading ratio. Lower specific power consumption exists for a dense phase flow compared to dilute phase flow. Fig. 17 shows the relation between specific power consumption with pulse slope ratio. It is seen that the pulse slope ratio goes up with an increase in specific power consumption for both fly ash and alumina conveying materials. At high solid loading ratio and hence minimum specific power consumption, the pulse slope ratio reduces for both fly ash and alumina. This is because the bed expansion is not too much. So, transporting the materials at high solid loading ratio with low aeration rate gives minimum specific power consumption. Also, the specific power consumption is

lower for case denser phase flow like alumina compared to dense phase flow and dilute flow in fly ash. 7. Pressure wave velocity The pressure signal of two consecutive transmitters can be used for the calculation of pulse velocity. Fig. 18 shows the pressure pulse fluctuation signals at two consecutive transmitters. It is observed that at each sensor location, there is a fluctuation period (t(T3)) and time delay between two signals. Based on the distance between two transmitters and time delay, the pulse velocity can be calculated by using the following relation: pulse velocity ¼

distance between two sensors time delay

The time delay between two transmitter signals was determined by means of cross correlation. Table 1 summarizes the result of pulse velocity. It is seen that at high solids loading ratio, and hence low voidage, the pulse velocity is higher than that of low loading ratio. Similar observation has been reported in studies of Musmarra et al. [27], Bi et al. [29], and Davidson et al. [44].

Fig. 17. Effect of specific power consumption on pulse slope ratio of fine powders.

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Fig. 18. pressure pulse fluctuation at transducer T1 and T3 along the pipeline.

Table 1 pulse velocity for different conveying conditions. Material

Cases

Pulse velocity (m/s) T1-T3

T13-T15

T17-T19

Fly ash

m* = 38.7, v = 3.9 m/s m* = 25.9, v = 8.1 m/s m* = 14.4, v = 10 m/s

22.2 12.5 12.9

9.17 8.7 23.35

7.41 9.3 29.38

Alumina

m* = 47.8, v = 4.76 m/s m* = 33.7, v = 5.47 m/s m* = 29.8, v = 6.88 m/s

24.62 14.66 15.56

10.26 13.9 9.28

11.5 10.76 24.8

8. Conclusion Daubechies db4 wavelet analysis was applied to analyze the pressure fluctuation behavior of gas stream in dilute layer flowing above the dunes. The construction of gas pressure signal showed that the flow of fine powders in fluidized dense phase was not in a steady state (i.e. pulsatile motion). From wavelet analysis, crucial information about variation of pulse structures along the length of the pipeline was analyzed which indicated a repeated change in aeration and de-aeration rate of dune in the dense layer during the conveying process. The location of transition in the mode of flow from dense to dilute phase was determined by studying the change in pulse slope ratio as well as pulse time ratio. The results of

Fig. A1. Variation of pressure pulse amplitude for fly ash along the pipeline length.

non-dimensional frequency showed that there was a minimum value of which the flow changed from dilute to dense phase flow. It is also noted that the amplitudes of pulse structures, as well as pulse slop ratio, were influenced by solid loading ratio, and they varied with the type of bulk materials. The relation between non-dimensional frequency and solid loading ratio showed that the pulse frequency increased with an increase in solids loading ratio due to more frequent driving forces subjected on the surface of dune. Moreover, the variations of pulse slop ratio and non-dimensional frequency with respect to specific power consumption were also analyzed, these variations suggested that at high solids loading ratio the expand of the bed of materials was not too high indicating a lower aeration rate of the dune with lower specific power consumption. Finally, the pulse velocities obtained by cross correlating the signals from two consecutive transmitters were high at high solids loading ratio. Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Acknowledgments The author would like to thank the Centre for Bulk Solids and Particulate Technologies, University of Newcastle, Australia for providing necessary support for conducting the experimental work.

Y. Alkassar et al. / Powder Technology 343 (2019) 629–643

Fig. A2. Variation of pressure pulse amplitude for alumina along the pipeline length.

Appendix A Fig. A1 and Fig. A2 show variation of pressure pulse amplitude for fly ash and alumina along the pipeline length.

Fig. A1. Variation of pressure pulse amplitude for fly ash along the pipeline length.

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Fig. A2. Variation of pressure pulse amplitude for alumina along the pipeline length.

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Y. Alkassar et al. / Powder Technology 343 (2019) 629–643 [38] J.B. Pahk, G.E. Klinzing, Assessing flow regimes from pressure fluctuations in pneumatic conveying of polymer pellets, Part. Sci. Technol. 26 (2008) 247–256, https:// doi.org/10.1080/02726350802028926. [39] K.C. Williams, M.G. Jones, A.A. Cenna, Characterization of the gas pulse frequency, amplitude and velocity in non-steady dense phase pneumatic conveying of powders, Particuology 6 (2008) 301–306, https://doi.org/10.1016/j.partic.2008.03.007. [40] N. Behera, V.K. Agarwal, M.G. Jones, K.C. Williams, Transient parameter analysis of fluidized dense phase conveying, Powder Technol. 217 (2012) 261–268, https:// doi.org/10.1016/j.powtec.2011.10.036. [41] M.G. Jones, K.C. Williams, Predicting the mode of flow in pneumatic conveying systems-a review, Particuology 6 (2008) 289–300, https://doi.org/10.1016/j.partic. 2008.05.003. [42] N. Behera, V.K. Agarwal, M.G. Jones, K.C. Williams, Parameters affecting power consumption in pneumatic conveying of fine particles, Bulk Solids Handl. 31 (2011) 336–340. [43] H. Bi, A. Chen, Pressure fluctuations in gas-solids fluidized beds, China Particuology 1 (2003) 139–144, https://doi.org/10.1016/S1672-2515(07)60130-4. [44] R. Roy, J.F. Davidson, Similarity between Gas-Fluidized Beds at Elevated Temperature and Pressure, Fluid. VI, Eng. Found. New York, 1989 293–300. Y. Alkassar obtained Master of Technology (M.Tech) degree in machine dynamic & analysis from National Institute of Technology Rourkela, India in 2016 and honored with first position in his batch. He is currently pursuing Ph.D. in ITMMEC at IIT Delhi, India. His research interest is in material handling systems such as pneumatic conveying system. His Ph.D research is with cooperation between the University of Newcastle, Australia and IIT Delhi, India.

643 N. Behera graduated in 1998 from VSSUT University, Burla in India with a first lass degree in Mechanical Engineering. He first worked as a lecturer in MITS, Rayagada, Orissa, India. He undertook M.Tech programme at IIT, Kharagpur with specialization Mechanical system Design in 2004. After M.Tech he worked in JITM, Paralakhemundi, Orissa, India and ITER, Bhubaneswar, Orissa, India. He obtained PhD degree from IIT Delhi in the year 2013. In 2009 he visited University of Newcastle, Australia for six months to carry out experimental work on pneumatic conveying. Presently he is working as Associate Professor at VIT University, Vellore (India).

M. G. Jones obtained both Bachelors and PhD from Thames Polytechnic (England). Presently he is Head of the School of Engineering and Director of Centre for Bulk Solids and Particulate Technologies. His specific area of interest is in the field of Bulk Material Handling and Pneumatic Conveying which spans more than 20 years. He has worked with many multi-national companies on industrial projects.

R. K. Pandey obtained his bachelors and master degrees for NIT Allahabad and Ph.D. from IIT BHU, India. Presently he is professor in department of Mechanical Engineering IIT Delhi, India. His Research area is machine design, bearing & lubricant, Tribological elements design. V. K. Agarwal obtained both Masters and PhD degrees from IIT Delhi, India. His PhD work was related to the study of bend erosion in pneumatic conveying. Presently he is Professor at ITMMEC, IIT Delhi. His principal area of research is in the field of Pneumatic Conveying of Bulk Materials. A pneumatic conveying pilot plant has been set up by him to investigate design and operational problems in pneumatic conveying.