Sand Bed Material Research Articles

Hydrodynamic Modeling and Comprehensive Assessment of Pier Scour Depth and Rate Induced by Wood Debris Accumulation

This study mainly investigates the impact of debris accumulation on scour depth and scour hole characteristics around bridge piers. Through controlled experiments with uniform sand bed material, the influence of various debris shapes (high wedge, low wedge, triangle yield, rectangular, triangle bow, and half-cylinder), upstream debris length, downstream debris extension, and debris thickness on scour depth and scour hole area and volume around the cylindrical pier were analyzed. The findings revealed that the shape and location of debris in the water column upstream of piers are key factors that determine the depth of scour, with high wedge shapes inducing the deepest scour and potentially the largest scour hole, particularly when positioned close to the pier and fully submerged. Scenarios in which triangle bow debris was submerged at full depth upstream of the pier closely resembled situations devoid of debris. Conversely, debris extension downstream of the pier was found to reduce local scour depth while concurrently enlarging the dimensions of the scour hole. The existing scour prediction equations tend to overestimate scour depth in scenarios involving debris, particularly when applying effective and equivalent pier width. This discrepancy arises because these equations were originally developed to predict scour depth around piers in the absence of debris. In response, a refined model for predicting scour induced by debris was proposed, integrating factors such as upstream debris length, downstream extension, obstruction percentage, and debris shape factor. This model demonstrated strong agreement with experimental data within the scope of this study and underwent further validation using additional experimental datasets from other research endeavors. In conclusion, this experimental study advances the comprehension of scour processes around cylindrical bridge piers, providing valuable insights into the role of debris characteristics and positioning.

Artificial intelligence prediction and thermodynamic analysis on particle agglomeration/defluidization during waste incineration: Effect of sand bed composition and agglomeration indices

In the incineration process, agglomeration is caused by the combustion of wastes containing adhesion materials such as alkalis and earth alkalis. Alkali oxides not only have a low melting point but also can react with Si to form low-melting-point eutectics. Most of the researches focused on the inlet materials, however, the properties of sand bed materials are also playing an important role on defluidization process. A comprehensive analysis of sand compositions on the prediction of defluidization tendency is carried out. Variation of sand properties before and after fluidized bed operation is estimated by XRF characterization, and the effect of particle size distribution is also in consideration. Experimental results indicated that defluidization time decreases with the increasing particle size of all sand sources. Higher temperature promotes agglomeration/defluidization. According to the agglomeration indices calculation, indicator Al + Ca + Mg/(Na + K + Fe) can be used to estimate the defluidization tendency before fluidized bed operation for particle sizes below 770 μm. In contrast, for a particle size of 920 μm, Al + Ca + Mg + Fe /(Na + K) can be utilized to qualitatively characterize the defluidization time. On the other hand, the Support Vector Regression (SVR) model was the best model for predicting the defluidization time (R2 of 93.40% and MSE of 3470). Causal analysis shows particle size, K2O, and Na2O were the top three most important factors that affect the particle agglomeration. The results are helpful to the evaluation of the agglomeration tendency of sand bed materials before using fluidized bed waste incineration agglomeration in terms of qualitative analysis and quantitative analysis.

Scour depth downstream sharp-crested weir

Weirs are hydraulic obstructions constructed across an open-channel flow causing a rise in water which is useful to convert water to the channels upstream as well as using these structures as a measuring device. There are different types, and a sharp-crested weir is one of them. Local scour downstream hydraulic structures are one of the most critical problems affecting their safety and stability. Many studies conducted to prevent these facilities from collapsing, but the effect of adding a solid apron downstream weir still need deep studies. The current study included a survey of scour reduction downstream sharp-crested weir using a solid apron consisting of three different lengths, four different discharges, and sand bed material to calculate the maximum depth and length of scour hole when using an apron solid. The results proved that the depth and length of the scour hole are reduced to 19.3% and 27.7%, respectively, when an apron solid is used. Two empirical equations were found to determine the maximum depth and length of the hole with a coefficient of determination are 0.892 and 0.804, respectively.

Experimental Study of Rice Husk Fluidization Without a Sand Bed Material on a Bubbling Fluidized Bed Gasifier

This study aimed to determine the effect of rice husk fluidization and variation in the equivalence ratio of bubbling fluidized bed gasifiers without sand bed materials. It also aimed to improve the fluidization quality by reducing the diameter of rice husks. Therefore, the bulk density increases, whereas voidage decreases, both of which are the main parameters for improving the quality of fluidization in solid particles. Experiments were carried out at a velocity of 0.82 m/s, by varying the equivalent ratios ranging from 0.20 to 0.35, and analyzing the syngas composition, cold gas and carbon conversion efficiencies, lower heating value, and temperature distribution. An equivalence ratio of 0.30 was obtained for a bubbling fluidized gasifier with syngas compositions of 7.415%, 15.674%, 3.071%, 17.839%, and 56.031% for H2, CO, CH4, CO2, and N2, respectively. Under these conditions, we obtained cold gas and carbon conversion efficiencies and a lower heating value of 31.340%, 37.120%, and 3.881 MJ/Nm3, respectively.

Effectiveness of bed additives in abating agglomeration during biomass air/oxy combustion in a fluidised bed combustor

In this study, molochite (calcined kaolin) and dolomite with particle sizes (ca.1 mm) similar to those of the sand bed materials of industrial-scale fluidised beds, rather than powders which were normally used by previous studies, were investigated as the bed additives to counter agglomeration with the combustion of wheat straw and miscanthus pellets in a 20 kWth bubbling fluidised bed combustor under both conventional air combustion and oxy-fuel combustion conditions. The performance of the additives in abating agglomeration was compared to that achieved with another agglomeration countermeasure, lime addition to fuel. Dolomite was found to be superior to molochite in terms of prolonging defluidisation time when firing wheat straw. When firing miscanthus, similar effects of these two bed additives on defluidisation time were observed. The most significant improvement to the combustion performance was achieved by lime addition to fuel for both fuels. Moreover, the oxy-fuel combustion atmosphere showed a great impact on the effectiveness of dolomite but little influence on the effectiveness of molochite. The agglomeration samples were analysed by CAMSIZER, XRD and SEM/EDX and the results were used to derive the possible anti-agglomeration mechanisms of the additives under both air and oxy combustion conditions.

The Variation of Scour Depth near Vertical and Inclined Cylindrical Bridge Piers: An Experimental Study

The stability of bridge piers against scouring phenomena require to estimate the scour depth carefully at different hydraulic condition to ensure safe design. In this study, laboratory experiments were carried out to examine and compare the scour depth for vertical and inclined cylindrical piers with downstream direction under clear water scour conditions. The measured scour depth in front of three inclined configurations (ϴ = 10°, 16° and 22°) for single cylindrical pier was compared to that observed at vertical cylindrical pier (ϴ = 0°). The experiments were conducted at four flow intensities (V/VC=0.495, 0.714, 0.824 and 0.989) for sand bed material of d50=0.72 mm. The obtained results revealed and confirmed the inverse relationship between the measured scour depth and the angle of inclination of pier, whereas the maximum and the minimum scour depth was observed at ϴ = 0°, and ϴ = 22° respectively. The results of this study showed that the scour depth near threshold conditions (V/VC=0.989) can be reduced approximately by (5%, 10%, and 15%) for inclined piers with (ϴ = 10°, 16° and 22°) respectively. Furthermore, by assessment of five existing formulas, new modified empirical formula has been proposed for scour depth estimation of inclined piers with ± 9% error margins.

Using nanomaterial to minimize the local scour downstream of sluice gate

Prediction of scour depths is an important aspect related to the overall stability of hydraulic structures. In this research, small-scale laboratory experiments were carried out to explore the effectiveness of mixing different percentages of Nanomaterial (Silica fume) to non-cohesive (alluvial) movable bed soil to develop mixture of bed material with significant characteristics to minimize the local scour caused by high-velocity jets downstream of a sluice gate. The experiments were executed employing 9 jet Froude numbers ranged from 1.88 to 2.81 associated with 3 heights for gate opening, and 3 tailwater depths. The bed material has 5 different percentages of Silica fume mixed by weight and 2 types of mixing with and without water were used. Ninety-nine experiments were conducted, out of which 9 experiments were done using fine sand as a bed material for the referenced cases. Graphical relationships were presented for maximum scouring and silting depths for different hydraulic conditions and bed materials. This research demonstrated that the type of mix (dry-mix or wet-mix) had a significant influence on the geometry of scouring and silting for the same flow conditions and the mixed percentage ratio of Silica fume. For dry-mix, the scouring and silting depths increased as the percentage ratio of Silica fume increased. While for wet-mix, the scouring and silting depths decreased as the percentage ratio of Silica fume increased. Also, the wet-mix effectively reduced the local scour volumes by 34.65% and 83.05% and decreased the silting volumes by 33.35% and 89.42% according to the percentage of Silica fume. Using a bed mixture of 8% Silica fume wet-mix showed 83.05% and 89.42% decrease in scouring and silting volumes, respectively, compared to pure fine sand bed material. From practical and financial aspects, the presented composition of bed material can effectively replace the smooth apron or at least decrease its length to reduce the construction cost. The obtained results present an untraditional technique for local scour protection downstream hydraulic structures.

Simulation of violent water flows over a movable bed using smoothed particle hydrodynamics

The interaction between bed-load sediment and water flow is an important topic of great concern which should be modeled for a wide range of hydrodynamic systems such as sea, rivers and estuaries. In this paper, smoothed particle hydrodynamics (SPH) is utilized for the simulation of dam-break propagation over an erodible bed and sediment distribution beneath the steady water flow. The proposed model describes both fluid and sediment particles as weakly compressible flow where the sediment is treated as a non-Newtonian fluid using Bingham–Cross model coupled with Newtonian treat (Owen’s relation) at interface. To cope with the difficulties arisen from different densities, the continuity and momentum equations are rather modified so that the interactions between the sediment and water are accurately modeled. First, the model is used to simulate the five dam-break models with PVC and sand bed materials and then the bed-load sediment transportation is considered for both Meyer-Peter and Einstein formulations. Comparisons are then made with the available experimental data indicating that the defined SPH model provides sensible predications for all given test cases.

D213 循環流動層ガス化における低品位石炭タールの低減と組成変化におよぼす操作条件の影響(OS5 バイオマス・新燃料・環境技術(1))

This paper examined possibility of reduction and/or reforming of tar emitted during gasification of low rank coal in low-temperature gasification. During low temperature gasification, large amount of tar emitted and then interact with carbonaceous solid residue, i.e., char. This interaction is called as volatile-char interaction (VCI), and VCI results in inhibition of char gasification. Therefore, it is important to examine effect of operating conditions on VCI in continuous reactor. In the present study, we developed a circulating fluidized bed reactor that consisted of pyrolyzer and combustor. Coal was fed in the pyrolyzer and then volatiles including tar and inorganic gas etc. and char were formed. The char and silica sand bed material were transported to the combustor to be partially combusted. It means that char was recycled to the pyrolyzer and then contacted with tar in the pyrolyzer. In our previous study, we reported reduction of heavy tar and light tar when coal was fed onto the bubbling bed (top feeding). On the contrary to the previous study, we fed coal into pyrolyzer at the bottom of the bubbling bed. The heavy tar and light tar yield decreased in comparison with the top feeding condition, because tar vapor could be cracked in the bed (secondary cracking). Furthermore, we applied Kendrick mass diagram method that was frequently used in analysis of heavy oil to detailed analysis of heavy tar. According to the Kendrick mass diagram method it was found that deoxygenation was enhanced during char recycling in the bottom feeding condition.

Air gasification of rice husk in bubbling fluidized bed reactor with bed heating by conventional charcoal

An experimental study of air gasification of rice husk was conducted in a bench-scale fluidized bed gasifier (FBG) having 210mm diameter and 1600mm height. Heating of sand bed material was performed using conventional charcoal fuel. Different operating conditions like bed temperature, feeding rate and equivalence ratio (ER) varied in the range of 750–850°C, 25–31.3kg/h, and 0.3–0.38, respectively. Flow rate of air was kept constant (37m3/h) during FBG experiments. The carbon conversion efficiencies (CCE), cold gas efficiency, and thermal efficiency were evaluated, where maximum CCE was found as 91%. By increasing ER, the carbon conversion efficiency was decreased. Drastic reduction in electric consumption for initial heating of gasifier bed with charcoal compared to ceramic heater was ∼45%. Hence rice husk is found as a potential candidate to use directly (without any processing) in FBG as an alternative renewable energy source from agricultural field.

Sediment production and yield from an alluvial gully in northern Queensland, Australia

ABSTRACTSediment production, transport and yield were quantified over various timescales in response to rainfall and runoff within an alluvial gully (7 · 8 ha), which erodes into dispersible sodic soils of a small floodplain catchment (33 ha) along the Mitchell River, northern Australia. Historical air photographs and recent global positioning system (GPS) surveys and LiDAR data documented linear increases in gully area and volume, indicating that sediment supply has been relatively consistent over the historic period. Daily time lapse photography of scarp retreat rates and internal erosion processes also demonstrated that erosion from rainfall and runoff consistently supplied fine washload (< 63 µm) sediment in addition to coarse lags of sand bed material. Empirical measurements of suspended sediment concentrations (10 000 to >100 000 mg/L) and sediment yields (89 to 363 t/ha/yr) were high for both Australian and world data. Total sediment yield estimated from empirical washload and theoretical bed material load was dominated by fine washload (< 63 µm). A lack of hysteresis in suspended sediment rating curves, scarp retreat and sediment yield correlated to rainfall input, and an equilibrium channel outlet slope supported the hypothesis that partially or fully transport‐limited conditions predominated along the alluvial gully outlet channel. This is in contrast to sediment supply‐limited conditions on uneroded floodplains above gully head scarps. While empirical data presented here can support future modelling efforts to predict suspended sediment concentration and yield under the transport limiting situations, additional field data will also be needed to better quantify sediment erosion and transport rates and processes in alluvial gullies at a variety of spatial and temporal scales. Copyright © 2013 John Wiley & Sons, Ltd.

Effects of configuration of instream wood on autumn and winter habitat use by fish in a large remeandering reach

This study examined the effects of the configuration of wood structures on the use of local autumn and winter habitats by fish in a remeandering reach of a large lowland river. Fish diversity was higher at the simple wood-structure (SWS) and the log-jam (LJ) sites than at the no-wood (NW) sites during both seasons. In particular, the diversity at the LJ sites was higher than that at the SWS sites during the winter. The abundance of the four dominant fish species was generally higher at the LJ sites than at the NW sites during both seasons. The SWS and LJ sites were characterized by greater depths, finer bed materials, and more diverse flow conditions during the autumn. During the winter, the LJ sites were characterized by slower currents and finer bed materials. These habitat features satisfied the various habitat requirements of the fish observed in our study reach; during the autumn, areas with fast and slow currents were favorable for juvenile salmon, silt and sand bed materials for lamprey, and deep areas for one species of goby, while slow currents were critical in the winter for abovementioned fish species. Engineered log jams were also effective in supporting the colonization of fish during both seasons. These findings suggest that a larger and more complex wood structure, particularly log jams, plays important roles in creating local habitats suitable for various fish species during the autumn and winter and in preserving fish abundance and diversity in large lowland rivers.

Biological crust of Nostoc flagelliforme (cyanobacteria) on sand bed materials

Nostoc flagelliforme in liquid culture was cultivated on sand bed materials as inoculum and formed a biological crust. The biological crust had no common appearance of wild N. flagelliforme thallus in naked eye observation, but it had natural morphology of wild N. flagelliforme thallus in microscopic observation. By examining the photosynthetic activity, N. flagelliforme cells on sand were found to be heat resistant. The sand grain size had some influence on N. flagelliforme growth, and the growth rate on fine sand was higher than on coarse sand. It is necessary to consider the characteristics of sand particle size to prevent desertification. N. flagelliforme cells could form the algal biological crust, which indicates that the biological crust has potential application in soil desertification and was extremely important to improve desertification.

Pneumatic and Hydraulic Systems in Coal Fluidized Bed Combustor

Problem statement: This study designed the pneumatic and hydraulic systems in coal fluidized bed combustor. These are fluidization of silica sand bed material, Air distributor, centrifugal fan, electric motor power drive and surface heat exchanger. Approach: The effects of increased gas velocity on silica sand and the resultant drag force formed the basic equations in fluidization. Air distributor was introduced to achieve pressure drop across the beds. Results: The constructed centrifugal fan was driven by selected electric motor based on pressure and temperature changes in the reactor. The dimensions of the heat transfer tube were calculated from fluid flow and energy balance equations. The values obtained were as the follows: Fluidization velocity (1.54 m sec-1), gas velocity through orifice (29.52 m sec-1), the fan electric motor (2 KW, 3 ph at 1500 pm), the steam temperature obtained was 160°C from water ambient temperature of 30°C and tube length 22 m was coiled into levels in the combustor. Conclusion/Recommendation: Precise specifications of pneumatic and hydraulic systems will adequately address the environment concern of coal fired power supply as a method to address epileptic power supply in Nigeria.

Formation and Prevention of Agglomerated Deposits During the Gasification of High-Sodium Lignite

A high-sodium lignite from the Freedom mine in North Dakota was tested in the transport gasifier at the Power Systems Development Facility (PSDF). During the first use of the high-sodium lignite in October 2003, agglomerated deposits formed at various locations in the transport gasifier system. An extensive laboratory testing program was carried out to characterize the deposits, understand the mechanism of the deposit formation, and test various methods of preventing or minimizing the agglomeration. The results of the deposit analysis and initial lab studies suggested that sodium released from the lignite was deposited on the surface of the sand bed material, resulting in the formation of sticky sodium silicates. Additional laboratory tests indicated that the agglomeration could be avoided or minimized by replacing the sand with a nonreactive bed material (e.g., coarse coal ash), operating at slightly reduced temperatures and using certain types of additives. By using these procedures, we completely eliminated the deposition problems in a subsequent gasification run in August 2004.

Influence of the amount of bed material on the distribution of biomass inorganic elements in a bubbling fluidised bed combustion pilot plant

The purpose of this work is to study how the amount of silica bed material affects the distribution of the inorganic elements in a bubbling fluidised bed (BFB) combustion pilot plant (1 MW th). Combustion tests with poplar biomass at similar fluidisation velocities and other operational conditions were assayed. The degree of interaction between the sand bed material and the biomass ash is a very important factor in the distribution of the inorganic elements in a BFB combustor. The higher the amount of silica bed material, the greater the retention of elements in the bed material. Bed accumulation can be very significant and can even retain more than 40% for major elements in poplar ash, such as calcium and potassium, as well as for trace elements such as barium, manganese, nickel, strontium and vanadium. The bed accumulation of an element depends specifically on the volatility of each element.

Coating Layers on Bed Particles during Biomass Fuel Combustion in Fluidized-Bed Boilers

Adhesive coating layers on bed particles have been thought to be the cause of the agglomeration in fluidized-bed combustion. The formation of coating layers on particles of different bed materials (quartz, natural sand, and GR Granule) has been studied when the fluidized-bed combustion of various biomass fuels (bark, different types of wood fuel, peat, biosludge, chicken litter, REF, and forest residue) occurs. The bed samples have been collected during the laboratory-scale, pilot-scale, and full-scale boiler tests. The chemical composition of the coating layers were characterized by scanning electron microscopy, coupled with energy-dispersive spectroscopy (SEM-EDS), through the utilization of line scans, as well as point and domain analyses. X-ray maps were also recorded to characterize the coating layers. The coatings on the bed particles often consisted of several superimposed layers. The innermost layer mainly contained alkali silicates, whereas the outermost layer was rich in calcium or magnesium, depending on the composition of both the bed and the fuel. Coating layers that contain potassium or sodium may be adhesive and cause the formation of agglomerates, which would explain why quartz and natural sand bed materials are problematic when fuels with high alkali-metal content are combusted. The magnesium-containing coating layers on the GR Granule particles may protect the bed particles from agglomeration, and, consequently, this bed material can be used in the combustion of otherwise problematic fuels.

Stream Erosion Potential and Stormwater Management Strategies

Hydrologic and sediment transport modeling were used to examine the effectiveness of typical stormwater management policies in reducing the potential for stream-channel erosion. Two bedload functions and three total-load transport relationships were applied to 8 mm gravel and 0.5 mm sand bed materials to compare the performance of the relationships in estimating detention requirements across modes of sediment transport. The various sediment-transport relationships yielded widely diverging estimates of sediment-transport capacity and yet suggested detention volume requirements that agreed within 20%. Detention design for control of cumulative sediment load required a detention storage volume 61% greater than a peak control detention facility and resulted in an altered temporal distribution of sub-bank-full shear stress. Design of stormwater facilities based on time-integrated sediment-transport capacity may inadvertently result in channel instability and substrate changes unless the approach accounts for the frequency distribution of sub-bank-full flows, the capacity to transport heterogeneous bed and bank materials, and potential shifts in inflowing sediment loads.

Drivability and Pullout Resistance of Helical Units in Saturated Sands

The results of an experimental investigation on the drivability of helix-shaped units into submerged sand and sand subjected to upward seepage flow are presented. In this research study, the helical units are used as structural anchoring elements. A testing program is conducted to study the effect of water existence in a sand deposit on the driving moment (torque) required to install helix-shaped units, with reference to torque required to install the same in dry sands. Dense sand is used in this study with water conditions varying from hydrostatic pressure to upward seepage flow with three different velocities. Measurements are recorded for the installation torque versus depth of installation. Based on the results of this study, it is concluded that the existence of water in the sand bed material facilitates the installation and reduces the torque required to drive the helical units. It is also found that helix drivability improves with the increase of the velocity of seepage flow, however, this seepage flow negatively reflects on the resistibility of these units to pullout forces. It is recommended that the design of both the driving technique and the pullout resistance of helix-shaped foundation units be balanced in a way that the easiness of installation and the obtained pullout resistance are optimized. It is also recommended that pullout resistance of anchors should not be compromised to achieve higher drivability.

Laboratory Study of Transport of Fine Sand

Laboratory tests were conducted to define the relationships between the transport of fine sand and the flow variables. Data from 105 experiments with sand bed material having a median sieve diameter of 0.1 mm in a 100-ft (30.48 m)-long by 4-ft (1.22 m)-wide recirculating flume show that the concentration of the transported material depends in a rather complex, but consistent way on the basic flow variables. A graphical relationship between the concentration and the flow discharge per unit of channel width defines consistent curves of equal depth and readily delineates the various bed form regimes. A similitude analysis of published transport data for coarser sands suggests a single curve for concentration versus Froude number for each median sediment size. However, the data for the 0.1-mm material suggest separate curves of concentration versus Froude number for the different bed form regimes. The data for depths less than or equal to 0.7 ft (0.21 m) define a continuous relationship like that for coarser sands, but the data for deeper depths define an extended plane bed curve segment that has not been observed for coarser material.