Soumya Suddha Mallick

An Investigation into Modeling of Solids Friction for Dense-Phase Pneumatic Conveying of Powders

This article presents results from an investigation into the modeling of solids friction factor for fluidized dense-phase pneumatic conveying of powders. A fundamental design approach was pursued by employing “straight pipe” and “back calculation” techniques for modeling and using two types of power function formats. The “straight pipe” models were found to be unexpectedly different depending on the selected location of pressure-measuring tapping points (even for the same product). An attempt to explain this variation by studying the “straight pipe” conveying characteristics suggested significant changes in flow mechanisms along the pipe. The derived models were evaluated for scaleup accuracy and stability by predicting for larger and longer pipes. The results showed significant variations in predictions. One format of power function model was found to result in more stable predictions than the other. Possible explanations for the causes of such variations are provided. Physical observations of the flow phenomena of dense-phase conveying for different powders showed the products were mostly conveyed as a dense non-suspension liquid-type-layer along the bottom of the pipe. This mechanism does not seem to be correctly represented by the existing design approach of using a Froude number term in the solids friction factor models, thus initiating a search for suitable alternative dimensionless grouping(s) that can adequately represent the non-suspension flow phenomena. In this study, Steady-state conveying data of three different powders conveyed in various pipes (diameter/lengths) were used for the purpose of modeling and scaleup investigations.

Modeling Solids Friction for Dense-Phase Pneumatic Conveying of Powders

This article results from an ongoing investigation aimed at developing a new validated test-design procedure for the accurate prediction of pressure drop for dense-phase pneumatic conveying of powders. Models for combined pressure drop coefficient (“K”) for solids-gas mixture were derived using the concept of “suspension density” by using the steady-state “straight pipe” pressure drop data between two different tapping locations of the same pipe and also for two different diameter pipes. It was observed that the derived models were different depending on the location of tapping points (for the same pipe) and selected pipe diameters. The derived models were then evaluated by predicting the pressure drop for pipelines with various diameters or lengths (69 mm I.D. × 168 m, 105 mm I.D. × 168 m, 69 mm I.D. × 554 m) for the conveying of power station fly ash. A comparison between the predicted pneumatic conveying characteristics (PCC) and the experimental plots showed that the models resulted in significant over-predictions. In the second part of the article, the “system” approach of scaleup was evaluated. “Total” pipeline pressure drop characteristics for test-rig pipelines were scaled up to predict the PCC for larger/longer pipes. It was found that the “system” approach generally resulted in grossly inaccurate predictions. It was concluded that further studies are needed for a better understanding of the solids-gas flow mechanism under dense-phase conditions.

Modeling Thermal Conductivity for Alumina-Water Nanofluids

This article presents results of an ongoing investigation into the modeling of thermal conductivity for Alumina-Water nanofluids. In spite of having the promise of being an improved heat transfer medium, fundamental understanding and modeling of important thermo-physical properties of nanofluids (such as thermal conductivity) have remained a difficult task due to the possible influence of several particle and base-fluid properties on the behavior of nanofluids. The existing theories to explain the phenomenon of thermal conductivity augmentation have provided different and sometimes contrasting mechanisms. In this study, seven existing theoretical models for thermal conductivity of nanofluids have been evaluated for their accuracy by comparing the predicted versus experimental data for a wide range of test conditions. The existing models were found to provide inaccuracies (over/underpredictions) in the range of 3 to 58%. A new model has been developed using dimensionless analysis, which includes Prandtl number and a new dimensionless number that is a ratio of Reynolds number to the square root of Brinkman number for particle and fluids. The new model has been found to generally predict the thermal conductivity ratio (nanofluids to base fluids) within 5% accuracy range.

Investigating Straight-Pipe Pneumatic Conveying Characteristics for Fluidized Dense-Phase Pneumatic Conveying

This article presents results from an investigation into the pneumatic conveying characteristics (PCC) for horizontal straight-pipe sections for fluidized dense-phase pneumatic conveying of powders. Two fine powders (median particle diameter: 30 and 55 µm; particle density: 2300 and 1600 kg m−3; loose-poured bulk density: 700 and 620 kg m−3) were conveyed through 69 mm I.D. × 168 m, 69 mm I.D. × 148 m, 105 mm I.D. × 168 m and 69 mm I.D. × 554 m pipelines for a wide range of air and solids flow rates. Straight-pipe pneumatic conveying characteristics obtained from two sets of pressure tappings installed at two different locations in each pipeline have shown that the trends and relatively magnitudes of the pressure drops can be significantly different depending on product, pipeline diameter and length and location of tapping point in the pipeline (indicating a possible change in transport mechanism along the flow direction). The corresponding models for solids friction factor were also found to be different. There was no distinct pressure minimum curve (PMC) in any of the straight-pipe PCC, indicating a gradual change in flow transition (change in flow mechanism from dense to dilute phase). For total pipeline conveying characteristics, the shapes of the PCC curves and the location of the PMC were found to be significantly influenced by pipeline layout (e.g., location and number of bends) and not entirely by the dense-to-dilute-phase transition of flow mechanism. Seven existing models and a new empirically developed model for PMC for straight pipes have been evaluated against experimental data.

On improving scale-up procedures for dense-phase pneumatic conveying of powders

This article presents results of an ongoing effort toward improving the modeling and scale-up procedures for the dense-phase pneumatic conveying of fine powders through pipes. Two new approaches are employed in this study. One approach, derived by modifying an existing reliable dilute-phase model to make it suitable for the dense-phase, has resulted in relatively stable predictions for diameter and length scale-up for two types of fly ash, ESP dust, pulverized brown coal and fly ash/cement mixture. Although some over-predictions still remain for the cases of diameter scale-up, there seems to be a substantial relative improvement in the overall accuracy of predictions (compared to the existing design methods). Another method has been derived using the concept of “two-layer” slurry flow modeling (suspension flow occurring on top of a non-suspension moving layer), and this has also resulted in similar improvements. Although the “two-layer” technique is believed to be more representative of the actual flow conditions under dense-phase conveying, the simpler “modified” method appears to be adequate for practical design purposes.

Evaluation of Scaleup Procedures Using “System” Approach for Pneumatic Conveying of Powders

This article presents results from an ongoing research effort aimed towards developing a validated scaleup procedure for pressure drop for the dense-phase pneumatic conveying of powders. Two existing/popular forms of the “system” approach for scaling up of diameter were evaluated. The validity of the current technique for length scaleup using a “system” approach was also examined. The existing method showed good potential for dilute-phase flow, but resulted in appreciable under-predictions when predicting for dense-phase flow. The effect of bends on the accuracy of the method was also investigated. In this study, steady-state conveying data of four different powders conveyed in various pipes (diameter/lengths) were used for the purpose of scaleup investigations.

Prediction and optimization of nanoclusters-based thermal conductivity of nanofluids: Application of Box–Behnken design (BBD)

ABSTRACT The existing models to predict the thermal conductivity of nanofluids are based on single particle diameter, whereas, in actual solutions, nanoparticles mostly exist in a cluster form. Experiments are carried out to observe the effects of various surfactants on stability, nanocluster formation, and thermal conductivity of Al2O3–H2O nanofluid, which is found to be improved considerably with SDS surfactant. The prolonged sonication was not adequate to break the clusters of Al2O3 nanoparticles, into an average size of less than 163 nm, indicating the tendency of Al2O3 nanoparticles to remain in the form of clusters instead of individual nanoparticles of primary size of 20 nm. Response surface methodology has been employed to design and optimize the experimental strategy by taking volumetric concentration, temperature, and surfactant amount as the contributing factors. The developed model has been validated against the experimental data and the existing models with an accuracy level of ± 8% in the former case. Analysis reveals about the formation of nanoclusters and enhancement in thermal conductivity. The results confirmed that the model can predict thermal conductivity enhancement with an accuracy level of R square value of the order of 0.9766.

Modeling Dense-Phase Pneumatic Conveying of Powders Using Suspension Density

This article presents results of an investigation into the modeling of pressure drop in horizontal straight pipe section for fluidized dense-phase pneumatic conveying of powders. Suspension density and superficial air velocity have been used to model pressure drop for two-phase solids-gas flow. Two applicable models formats (developed by other researchers using two different definitions of suspension density) were used to represent the pressure drop due to solids-gas flow through straight pipe sections. Models were generated based on the test data of conveying power-station fly ash and electrostatic precipitator (ESP) dust (median particle diameter: 30 and 7 µm; particle density: 2300 and 3637 kg m−3; loose-poured bulk density: 700 and 610 kg m−3, respectively) through a relatively short length of a smaller diameter pipeline. The developed models were evaluated for their scale-up accuracy and stability by using them to predict the total pipeline pressure drop (with appropriate bend model) for 69 mm I.D. × 168 m; 105 mm I.D. × 168 m and 69 mm I.D. × 554 m pipes and comparing the predicted versus with experimental data. Results show that both the models with suspension density and air velocity generally provide relatively better prediction compared to the conventional use of solids loading ratio and Froude number. For fly ash, the two formats result in considerable different predictions, whereas they provide relatively similar results for ESP dust.

Remarkably Improved Dispersion Stability and Thermal Conductivity of WO3–H2O Suspension by SiO2 Coating

The long term dispersion stability for an improved thermal conductivity is a challenging issue that needs to be solved for heat transfer applications. Hence, this research investigated that a thin layer of SiO2 coating (2-5 nm) over WO3 nanostructures (SiO2@WO3) of different shapes exhibit superior dispersion (0.01%) stability for longer duration (∼3 days) as evident by steady zeta potential (-30 ↔ -60.70 mV), no significant change in particle-size (139 ↔ 147 nm) distribution, density (1.001 ↔ 0.988 g/cm3) and refractive index (1.335 ↔ 1.332) etc., are indicator for colloidal stability relative to bare WO3 nanoparticles and bulk SiO2 aqueous suspension which quickly settles down within 1-2 hours after 30 min sonication at 23 °C. Thin Si-OH layer over WO3 surface imparts superior hydrophilicity, larger surface area for effective solute-solvent (SiO2@WO3-H2O) interaction for improved colloidal stability showing no sedimentation and color change of SiO2@WO3 dispersion (0.01%) even after 3 days due to repulsive interaction between negatively charged Si-O- particles. Thereby, thermal conductivity is found to be quite stable (0.631 ↔ 0.618 W/m K) up to 3 days, whereas aqueous suspension of bare WO3 and SiO2 particles quickly settle down and thermal conductivity rapidly decreased to a value of 0.584 W/m K for de-ionized water further indicates the significance of SiO2 coating. Depending on the thickness of SiO2 layer and volume fraction of SiO2@WO3, a maximum of 8-10% increment of thermal conductivity was achieved where anisotropic WO3 displayed always more (∼5%) thermal conductivity than typical spherical nanoparticles.

An investigation into the transition of flow mechanism during fluidized dense-phase pneumatic conveying of fine powders

ABSTRACT This paper presents results from an investigation into the changes in flow mechanism of pneumatic conveying of fine powders from dilute to fluidized dense-phase. Pressure fluctuations have been analyzed using power spectral density (PSD), Hilbert–Huang transformation (HHT), and wavelet transformation. Pressure fluctuations were obtained from conveying trials performed for two different types of powders, i.e., fly ash (median particle diameter: 30 µm; particle density: 2300 kg m−3; loose-poured bulk density: 700 kg m−3) and white powder (median particle diameter: 55 µm; particle density: 1600 kg m−3; loose-poured bulk density: 620 kg m−3). Power spectra obtained from PSD analysis, energy distribution of intrinsic mode functions obtained from HHT transformation, and wavelet scalogram show unique features related to the changing flow mechanism along the length of the pipeline. Results indicate variation from low frequency to higher frequency components in the signal, along the flow direction. Higher frequency components and wide range of frequencies in the signal obtained at pipeline exit might have resulted from increased level of interactions among solid particles, carrier gas, and pipe wall as compared to the power spectra with single dominant frequency (which corresponds to probable periodic dune formation) at pipeline entry region.