Analytical scaling of DEM particles for efficient packed-bed simulations

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.Packed and Fluidized beds are commonly found in industries such as chemical processing and refining. A major advantage to these configurations lies in the large solid-surface area exposed to the flow, allowing for rapid interaction between the solid and fluid phases. While these types of flow configurations have been heavily studied over the years, computational software and hardware are only recently becoming advanced enough to allow realistic simulations of industry relevant configurations. Recent developments have allowed for the coupling of Discrete Element Modeling approaches, where conservation equations are typically solved on a particle-by-particle basis, with traditional continuum-fluid dynamics simulations. Nonetheless, when fluid-particle interaction is important, such as in packed bed analysis, modeling of the individual particles may still be computationally prohibitive except for simple applications. Methods to improve computational costs include grain coarsening or parcel-based approaches, where particle sizes may be scaled up or groups of particles are treated statistically. The present study develops and validates an analytical approach for the scaling of the Coefficient of Drag equations in a simplified packed bed simulation with scaled-up particles, using CD-adapco’s STAR-CCM+ software. Pressure drop predictions are compared against the accepted Ergun Correlation for the high density cylindrical packed bed. Container contact forces and packed bed height are also monitored as the flow rate is increased toward fluidization. It is shown that by properly scaling the Coefficient of Drag, while doubling the particle diameter (effectively reducing the total number of simulated particles by 8), a more than 15X speed-up in simulation time is achieved. This speed-up is achieved with an increase in error of only 8% maximum for the cases studied. Additionally, similar physical behavior is observed between the cases. This analytical approach proves to be a robust method of reducing computational expense without sacrificing accuracy, effectively making industrial scale simulations feasible.dc201

Comparison Of Pressure Drop And Endwall Heat Transfer Measurements To Flow Visualization Testing Of Solid And Porous Pin Fin Arrays

This paper examines the local and averaged endwall heat transfer effects of a staggered array of porous aluminum pin fins with a channel blockage ratio (blocked channel area divided by open channel area) of 50%. Two sets of pins were used with pore densities of 0 (solid) and 10 pores per inch (PPI). The pressure drop through the channel was also determined for several flow rates using each set of pins. Local heat transfer coefficients on the endwall were measured using Thermochromatic Liquid Crystal (TLC) sheets recorded with a charge-coupled device (CCD) camera. Static and total pressure measurements were taken at the entrance and exit of the test section to determine the overall pressure drop through the channel and explain the heat transfer trends through the channel. The heat transfer and pressure data was then compared to flow visualization tests that were run using a fog generator. Results are presented for the two sets of pins with Reynolds numbers between 25000 and 130000. Local HTC (heat transfer coefficient) profiles as well as spanwise and streamwise averaged HTC plots are displayed for both pin arrays. The thermal performance was calculated for each pin set and Reynolds number. All experiments were carried out in a channel with an X/D of 1.72, a Y/D of 2.0, and a Z/D of 1.72. Copyright © 2010 by ASME

The Balance Between Secondary Air System Pressure Head Requirements, Cooling Effectiveness And Turbine Efficiency: A Parametric Study

The advantage of higher turbine inlet temperatures as a way to increase cycle efficiency is potentially outweighed by the efficiency losses caused by the increased secondary air extracted from the compressor discharge to cool turbine components. Higher cooling effectiveness schemes could be used, but pressure head required to drive the coolant flow through the hot section components may be higher than those available due to combustor pressure losses. This paper looks to determine the potential effects on the overall cycle efficiency caused by an intentional pressure drop across the combustor, allowing more aggressive cooling schemes with a lower amount of cooling air, based on data of state of the art cooling schemes (coolant flow ratio, pressure head and cooling effectiveness) and a parametric analysis of a simple cycle turbine. Results suggest that coolant flow reduction can actually result in a lower pressure drop across the cooling passages, given the decreased flow velocity ending up in higher efficiency and specific work. Enhanced cooling schemes can also allow higher turbine inlet temperatures for a given coolant flow, resulting in improved performance. Copyright © 2012 by ASME

Hot-Wire Study On The Impact Of Porous Structure On Mean And Turbulent Velocity Profiles In The Near-Field Of A High Aspect Ratio Porous Filled Slot Jet

A 2-D rectangular slot jet (AR=61) with a porous blockage is experimentally tested for mean velocity and turbulence profiles in the near field. Porosities of the blockages tested are nominally 0, 0.40, 0.50, and 0.60, all crushed aluminum foam. The presence of the porous blockage can be seen in the deformed mean profile and in the lower magnitudes of turbulence intensity. The porous blockage acts to change the relevant length scale to that more on the order of the pore size rather than the slot width. Using a method meant for standardizing the turbulent length scale calculation, a length scale for each case is calculated and found to vary weakly with respect to porosity. The three length scales are calculated for each case and are compared. The length scale based on the zero frequency extrapolation of the power spectral density gave the most reasonable results. Adiabatic film cooling effectiveness values are given for three blowing ratios for the 0.60 porosity insert. Film effectiveness is seen to increase with mass injected over the entire test surface. © 2011 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved

The Effect Of Transpiration On Discrete Injection For Film Cooling

A segment of permeable wall is installed near a row of cylindrical film holes, parallel to the flow and inclined at 35 degrees. Coolant is forced through both the permeable wall and the film holes resulting in a downstream film composed of both transpired and discretely injected coolant. The permeable wall extends 1.5 cylindrical hole diameters in the flow direction. The effects on the aerodynamic performance and cooling downstream of the row of cylindrical holes in the presence of transpiration is studied numerically with a procedure validated by hot-wire anemometer and temperature sensitive paint measurements. The hydrodynamic boundary layer in the presence of film and adiabatic film cooling effectiveness downstream of single and coupled film sources are compared with numerical predictions. The performance of the coolant film is predicted in order to understand the sensitivity of cooling and aerodynamic losses on the relative positioning of the two sources at each blowing ratio. The results indicate that a coupling of the two sources allows a more efficient use of coolant by generating a more uniform initial film. With careful optimization the discrete holes can be placed farther apart laterally and operate at a lower blowing ratio with a transpiration segment making the large deficits in cooling effectiveness mid-pitch less severe, overall minimizing coolant usage. Comparisons of linear superposition predictions of the two independent sources with the corresponding coupled scenario indicate the two films positively influence one another and surpass additive predictions of cooling. All relative placements have an overall beneficial effect on the cooling seen by the protected wall. Some cases show an increase in areaaveraged film cooling effectiveness of 300% along with a 50% increase in aerodynamic loss coefficient by injecting an additional 10% coolant. In this study the downstream transpiration placement is found to perform best of the three geometries tested while considering cooling, aerodynamic losses, local uniformity and manufacturing feasibility. With further study and optimization this technique can potentially provide more effective thermal protection at a lower cost of aerodynamic losses and spent coolant. Copyright © 2011 by ASME

PIV study on the dimple mid-plane of a narrow rectangular channel with dimples applied to one wall

This paper presents an investigation of the fluid flow in the fully developed portion of a rectangular channel (Aspect Ratio of 2) with dimples applied to one wall at channel Reynolds numbers of 20,000, 30,000, and 40,000. The dimples are applied in a staggered-row, racetrack configuration. Results for three different dimple geometries are presented: a large dimple, small dimple, and double dimple. Heat transfer and aerodynamic results from preceding works are presented in Nusselt number and friction factor augmentation plots as determined experimentally. Using particle image velocimetry, the region near the dimple feature is studied in detail in the location of the entrainment and ejection of vortical packets into and out of the dimple; the downstream wake region behind each dimple is also studied to examine the effects of the local flow phenomenon that result in improved heat transfer in the areas of the channel wall not occupied by a feature. The focus of the paper is to examine the secondary flows in these dimpled channels in order to support the previously presented heat transfer trends. The flow visualization is also intended to improve the understanding of the flow disturbances in a dimpled channel; a better understanding of these effects would lead the development of more effective channel cooling designs. Copyright © 2011 by ASME

Microturbine Recuperation: Turbulators And Their Effect On Power Density And Thermal Efficiency

Microturbines have proven to be a vital part of the distributed power generation field due to their low emissions, compact size, high reliability and low maintenance. However, microturbines operate at low pressure ratios and relatively low turbine inlet temperatures that limit cycle efficiency. In order to overcome these limitations, microturbines often utilize a recuperator or regenerator to achieve the optimal balance between improved heat rates and reduced pressure ratios across the turbine. Recuperator design aims to achieve maximum effectiveness while staying reasonably compact, which creates the need to study novel heat transfer surfaces for compact heat exchanger application. In this study, experimental data of heat transfer augmentation and friction factor augmentation values for various turbulator geometries is used to determine the required heat exchanger volume to achieve 85%, 90%, and 95% effectiveness. A parametric analysis of various recuperator channel surface areas and turbulator geometry data will be utilized to determine the feasibility of increasing thermal efficiency while remaining compact to avoid large, negative effects on power density for a hypothetical gas turbine modeled after the Turbine Technologies, Ltd. SR-30 Turbo-Jet Engine. The turbulators considered in this study consist of 4 wedges, 4 ribs, and a dimpled geometry. The results will highlight the applicability of surface features in recuperator designs that can improve overall efficiency for microturbines. Results present the power density, thermal efficiency, and specific fuel consumption as functions of heat exchanger channel Reynolds number for heat exchangers implementing different turbulators. It is shown that dimples at low Reynolds numbers yield 85% effectiveness with only a 8% reduction in power density and 90% effectiveness with only a 12% reduction in power density. Ribs and wedges also perform well but suffer from high pressure losses due to their obtrusive design. Copyright © 2012 by ASME

PIV study on the dimple mid-plane of a narrow rectangular channel with dimples applied to one wall

This paper presents an investigation of the fluid flow in the fully developed portion of a rectangular channel (Aspect Ratio of 2) with dimples applied to one wall at channel Reynolds numbers of 20,000, 30,000, and 40,000. The dimples are applied in a staggered-row, racetrack configuration. Results for three different dimple geometries are presented: a large dimple, small dimple, and double dimple. Heat transfer and aerodynamic results from preceding works are presented in Nusselt number and friction factor augmentation plots as determined experimentally. Using particle image velocimetry, the region near the dimple feature is studied in detail in the location of the entrainment and ejection of vortical packets into and out of the dimple; the downstream wake region behind each dimple is also studied to examine the effects of the local flow phenomenon that result in improved heat transfer in the areas of the channel wall not occupied by a feature. The focus of the paper is to examine the secondary flows in these dimpled channels in order to support the previously presented heat transfer trends. The flow visualization is also intended to improve the understanding of the flow disturbances in a dimpled channel; a better understanding of these effects would lead the development of more effective channel cooling designs. Copyright © 2011 by ASME