Influence of Macroscopic Wall Structures on the Fluid Flow and Heat Transfer in Fixed Bed Reactors with Small Tube to Particle Diameter Ratio

Fixed bed reactors are widely used in the chemical, nuclear and process industry. Due to the solid particle arrangement and its resulting non-homogeneous radial void fraction distribution, the heat transfer of this reactor type is inhibited, especially for fixed bed reactors with a small tube to particle diameter ratio. This work shows that, based on three-dimensional particle-resolved discrete element method (DEM) computational fluid dynamics (CFD) simulations, it is possible to reduce the maldistribution of mono-dispersed spherical particles near the reactor wall by the use of macroscopic wall structures. As a result, the lateral convection is significantly increased leading to a better radial heat transfer. This is investigated for different macroscopic wall structures, different air flow rates (Reynolds number Re = 16 ...16,000) and a variation of tube to particle diameter ratios (2.8, 4.8, 6.8, 8.8). An increase of the radial velocity of up to 40%, a reduction of the thermal entry length of 66% and an overall heat transfer increase of up to 120% are found.

Numerical Investigation of Thermally Developing and Fully Developed Electro-Osmotic Flow in Channels with Rounded Corners

Electro-osmotic flow, that is, the motion of a polar fluid in microducts induced by an external electric field, is one micro-effect which allows fluid circulation without the use of mechanical pumping. This is of interest in the thermal management of electronic devices, as microchannels with cross sections of almost arbitrary shape can easily be integrated on the chips. It is therefore important to assess how the geometry of the channel influences the heat transfer performance. In this paper, the thermal entry region and the fully developed electro-osmotic flow in a microchannel of rectangular cross section with smoothed corners is investigated for uniform wall temperature. For the fully developed region, correlations for the Poiseuille and Nusselt numbers considering the aspect ratio and nondimensional smoothing radius are given, which can be used for practical design purposes. For thermally developing flow, it is highlighted how smoothing the corners increases the value of the local Nusselt number, with increases up to 18% over sharp corners, but that it also shortens the thermal entry length. It is also found that Joule heating in the fluid may cause a reversal of the heat flux, and that the thermal entry length has a linear dependence on the Reynolds number and the hydraulic diameter and on the logarithm of the nondimensional Joule heating.

Predictors of Malreduction in Zone II and III 5th Metatarsal Fractures Fixed with an Intramedullary Screw: A Retrospective Analysis

Category: Midfoot/Forefoot; Sports Introduction/Purpose: Percutaneous fixation of 5th metatarsal fractures may lead to malreduction due to improper implant selection and placement. Our aim was to test the effects of screw entry, length, and diameter on malreduction, delayed union, non-union, or refracture. Methods: We retrospectively reviewed zone II and proximal zone III 5th metatarsal fractures managed with intramedullary screw fixation. Comparisons were made between plantar cortex distraction/lateral cortex distraction and ratios of screw length, diameter, and entry point using multiple regression analysis. A further analysis was carried out between time to union and distraction in the lateral and plantar cortices. Results: Plantar and lateral gap were both correlated with entry point ratio on lateral and AP view respectively (p<0.001 for both views). We did not see an association between plantar and lateral gap with screw diameter ratio (p=0.393 for AP and p=0.981 for lateral) or screw length ratio (p=0.966 for AP and p=0.740 for Lateral). Ratio of postop/preop apex height on AP and lateral showed correlation to presence of lateral and plantar fracture gap respectively (p<0.0001). Presence of a plantar gap did have a slight influence on time to union (p=0.044). Most fractures showed radiographic union at 12 weeks (38/44 that were followed until union). There were no refractures or nonunions as per available records. Conclusion: Our study shows that screw length and diameter did not lead to significant plantar or lateral fracture site distraction. However, entry point had a significant effect on plantar and lateral gap on post-operative x-ray. Patients with a plantar gap did have an increased risk of delayed union. Entry point should be given more significance rather than screw diameter and length in managing zone 2/3 fifth metatarsal base fractures. This is contradictory to existing radiologic studies. [Table: see text]

Significance of Rarefaction, Streamwise Conduction, and Viscous Dissipation on the Extended Graetz–Nusselt Problem: The Case of Finite-Length Microchannels with Prescribed Wall Heat Flux

The article addresses the extended Graetz–Nusselt problem in finite-length microchannels for prescribed wall heat flux boundary conditions, including the effects of rarefaction, streamwise conduction, and viscous dissipation. The analytical solution proposed, valid for low-intermediate Peclet values, takes into account the presence of the thermal development region. The influence of all transport parameters (Peclet Pe, Knudsen Kn, and Brinkman Br) and geometrical parameters (entry length and microchannel aspect ratio) is investigated. Performances of different wall heat flux functions have been analyzed in terms of the averaged Nusselt number. In the absence of viscous dissipation Br=0, the best heating protocol is a decreasing wall heat flux function. In the presence of dissipation Br>0, the best heating protocol is a uniform wall heat flux.

Flow Structures and Unsteady Behaviors of Film Cooling from Discrete Holes Fed by Internal Crossflow

The flow structures and unsteady behaviors of a flat plate film cooling flow behind a single row of circular holes fed by internal crossflow were extensively investigated. The investigation was achieved experimentally using fast-response pressure-sensitive paint (PSP) at a high frame rate and numerically using large-eddy simulation (LES). During the experiment, the coolant flow was discharged from discrete holes (i.e., a row of circular holes with 3D spacing, 6.5D entry length, and 35 deg incline angle) via a crossflow channel. Two blowing ratios (M = 0.4 and 0.8) were tested at a density ratio of DR = 0.97. The measured unsteadiness caused by the predicted flow structure over the coolant surface was identified by spatial correlation. The unsteady signatures were decomposed and demonstrated by proper orthogonal decomposition (POD). The results reveal that the flow structure plays the main role in cooling performance and its instability. The internal flow produced a vortex tube structure that was responsible for the shear vortex (i.e., Kelvin–Helmholtz instabilities) between the coolant and the mainstream at the hole exit. The internal crossflow forced the legs of the counter-rotating vortex pair (CRVP) to spread laterally, and the coolant to fluctuate asymmetrically around the discrete holes. This unsteady behavior may potentially cause high thermal stress and leads to blade cracking over a long time.

1006. Do Antibiotic Choices Made in the ED Influence Inpatient Therapy?

Background Inappropriate antibiotic use (AU) is common among inpatients and may begin in the emergency department (ED). ED clinicians often make the first antibiotic decisions in patient care, but it is unknown whether or not these decisions influence inpatient AU. Understanding prescribing practices at transitions of care is critical for implementing effective stewardship initiatives. Methods We performed a retrospective cohort study of AU in patients admitted to Duke University Hospital through the ED between July and December 2018. Included encounters had a minimum 2-day length of stay and received an antibiotic in both the ED and inpatient setting. Individual encounter IDs were used to link ED and inpatient AU reports generated from the DASON Antimicrobial Stewardship Assessment Portal. We compared the last ED administration date/time to the first inpatient unit administration for each agent. An antibiotic started in the ED was considered continued upon admission if the first inpatient administration occurred within 30 hours following the last ED administration. Demographic, clinical indication on order entry, length of therapy, and prescriber data were also collected. Results We included 3,336 encounters and 2,940 unique patients in the analysis. The median (IQR) patient age was 60 (42–72) years, and the most common indications for AU in the ED were sepsis (23.1%), pneumonia (17.8%), ABSSSI (15.5%), and intra-abdominal infection (12.8%). At least one antibiotic initiated in the ED was continued upon admission within 30 hours in 2,495 (74.8%) encounters. The most common antibiotics continued upon admission were piperacillin/tazobactam (32.8%), vancomycin (24.9%), and ceftriaxone (13.7%). The most common indications for agents continued upon admission were pneumonia (18%), intra-abdominal infection (15%), and ABSSSI (15%). Two or more antibiotics were continued upon admission in 916 (27.4%) encounters. Conclusion In our retrospective review of ED antibiotic encounters resulting in admission for at least 2 days, three out of four encounters had at least one antibiotic continued upon admission. This finding highlights the importance of initial appropriate antibiotic selection and suggests stewardship interventions should target EDs as well as inpatient prescribing. Disclosures All authors: No reported disclosures.

Entry Length Requirements for Two- and Three-Dimensional Laminar Couette–Poiseuille Flows

In this study, we examine the development length requirements for laminar Couette–Poiseuille flows in a two-dimensional (2D) channel as well as in the three-dimensional (3D) case of flow through a square duct, using a combination of numerical and experimental approaches. The parameter space investigated covers wall to bulk velocity ratios, r, spanning from 0 (purely pressure-driven flow) to 2 (purely wall driven-flow; 4 in the case of a square duct) and a wide range of Reynolds numbers (Re). The results indicate an increase in the development length (L) with r. Consistent with the findings of Durst et al. (2005, “The Development Lengths of Laminar Pipe and Channel Flows,” ASME J. Fluids Eng., 127(6), pp. 1154–1160), L was observed to be of the order of the channel height in the limit as Re→0, irrespective of the condition at the inlet. This, however, changes at high Reynolds numbers, with L increasing linearly with Re. In all the cases considered, a uniform velocity profile at the inlet was found to result in longer entry lengths than in a flow developing from a parabolic inlet profile. We show that this inlet effect becomes less important as the limit of purely wall-driven flow is approached. Finally, we develop correlations for predicting L in these flows and, for the first time, also present laser Doppler velocimetry (LDV) measurements of the developing as well as fully-developed velocity profiles, and observe good agreement between experiment, analytical solution, and numerical simulation results in the 3D case.

Unsteady Analysis of Adiabatic Film Cooling Effectiveness Behind a Row of Circular Holes Fed by Internal Crossflow

The adiabatic film cooling effectiveness behind a single row of circular holes fed by internal crossflow was measured by fast-response pressure-sensitive paint technique. During the experiment, the coolant flow was discharged from the coolant holes via either plenum or crossflow channel. The test model has a row of circular holes with 3D spacing, 6.5D entry length, and 35° inclination angle. Two blowing ratios (M = 0.40 and 0.80) were tested with a density ratio of 0.97. A numerical steady-state RANS simulation, using SST k-ω and Realizable k-ε turbulence models, was conducted to understand the internal crossflow behaviors. The unsteadiness caused by the flow structures (counter-rotating vortex pair (CRVP) and horseshoe vortex) was quantified by the root mean square and the cross-correlations. In addition, the proper orthogonal decomposition was used to identify the large-scale unsteady coherent structures and their contributions. The fluctuations of the crossflow feed were asymmetric, which were significantly weaker compared with the plenum case. The CRVP, as the most significant coherent structures, were demonstrated to play the main role in the unsteadiness of the crossflow feed.