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

Processes ◽  
2021 ◽  
Vol 9 (4) ◽  
pp. 689
Author(s):  
Thomas Eppinger ◽  
Nico Jurtz ◽  
Matthias Kraume
Keyword(s):  
Heat Transfer ◽  
Fixed Bed ◽  
Particle Diameter ◽  
Diameter Ratio ◽  
Spherical Particles ◽  
Reactor Wall ◽  
Entry Length ◽  
Fixed Bed Reactors ◽  
Wall Structures ◽  
Small Tube

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.

Evaluation of Structural Properties of Cylindrical Packed Beds Using Numerical Simulations and Tomographic Experiments

2009 ◽  
Vol 7 (1) ◽  
Author(s):  
Nestor J Mariani ◽  
Wilson I Salvat ◽  
Agustina Campesi ◽  
Guillermo F Barreto ◽  
Osvaldo M Martínez
Keyword(s):  
Fixed Bed ◽  
Diameter Ratio ◽  
The Other ◽  
Spherical Particles ◽  
Packed Beds ◽  
Particle Center ◽  
Aspect Ratios ◽  
Fixed Bed Reactors ◽  
Local Properties ◽  
Ct Data

This contribution is focused on the analysis of the structure of packed beds of spherical particles at relatively low aspect ratios (i.e., particle to tube diameter ratio) as those arising in multitubular fixed bed reactors. On one hand, the computed tomography (CT) technique is employed to evaluate the position of each particle in the packing and from this information local properties such as particle center distribution and radial porosity profile were obtained. On the other hand, results from a previously developed algorithm to simulate packings were compared with those from our CT data and from literature sources. The agreement was very satisfactory.

scholarly journals A Simplified Method for Modeling of Pressure Losses and Heat Transfer in Fixed-Bed Reactors with Low Tube-to-Particle Diameter Ratio

Energies ◽  
2021 ◽  
Vol 14 (3) ◽  
pp. 784
Author(s):  
Tymoteusz Świeboda ◽  
Renata Krzyżyńska ◽  
Anna Bryszewska-Mazurek ◽  
Wojciech Mazurek ◽  
Alicja Wysocka
Keyword(s):  
Porous Medium ◽  
Cross Section ◽  
Fixed Bed ◽  
Particle Diameter ◽  
Weighted Average ◽  
Diameter Ratio ◽  
Fixed Bed Reactor ◽  
Fixed Bed Reactors ◽  
Simplified Method ◽  
Medium Method

This manuscript presents a simplified method of modeling fixed-bed reactors based on the porous medium. The proposed method primarily allows the necessity of precisely mapping the internal structure of the bed—which usually is done using real object imaging techniques (like X-ray tomography) or numerical methods (like discrete element method (DEM))—to be avoided. As a result, problems with generating a good quality numerical mesh at the particles’ contact points using special techniques, such as by flattening spheres or the caps method, are also eliminated. The simplified method presented in the manuscript is based on the porous medium method. Preliminary research has shown that the porous medium method needs modifications. This is because of channeling, wall effects, and local backflows, which are substantial factors in reactors with small values of tube-to-particle-diameter ratio. The anisotropic thermal conductivity coefficient was introduced to properly reproduce heat transfer in the direction perpendicular to the general fluid flow. Since the commonly used fixed-bed reactor models validation method based on comparing the velocity and temperature profiles in the selected bed cross-section is not justified in the case of the porous medium method, an alternative method was proposed. The validation method used in this work is based on the mass-weighted average temperature increase and area-weighted average pressure drop between two control cross-section of the reactor. Thanks to the use of the described method, it is possible to obtain satisfactorily accurate results of the fixed-bed reactor model with no cumbersome mesh preparation and long-term calculations.

Radial heat transfer in fixed-bed packing with small tube/particle diameter ratios

2008 ◽  
Vol 45 (4) ◽  
pp. 417-425 ◽  
Author(s):  
A. Grah ◽  
U. Nowak ◽  
M. Schreier ◽  
R. Adler
Keyword(s):  
Heat Transfer ◽  
Fixed Bed ◽  
Particle Diameter ◽  
Small Tube ◽  
Radial Heat

scholarly journals Enhancing the Thermal Performance of Slender Packed Beds through Internal Heat Fins

Processes ◽  
2020 ◽  
Vol 8 (12) ◽  
pp. 1528
Author(s):  
Nico Jurtz ◽  
Steffen Flaischlen ◽  
Sören C. Scherf ◽  
Matthias Kraume ◽  
Gregor D. Wehinger
Keyword(s):  
Heat Transfer ◽  
Heat Transport ◽  
Thermal Performance ◽  
Internal Heat ◽  
Spherical Particles ◽  
Reactor Wall ◽  
Packed Beds ◽  
Radial Heat ◽  
Simulation Results ◽  
The Impact

Slender packed beds are widely used in the chemical and process industry for heterogeneous catalytic reactions in tube-bundle reactors. Under safety and reaction engineering aspects, good radial heat transfer is of outstanding importance. However, because of local wall effects, the radial heat transport in the vicinity of the reactor wall is hindered. Particle-resolved computational fluid dynamics (CFD) is used to investigate the impact of internal heat fins on the near wall radial heat transport in slender packed beds filled with spherical particles. The simulation results are validated against experimental measurements in terms of particle count and pressure drop. The simulation results show that internal heat fins increase the conductive portion of the radial heat transport close to the reactor wall, leading to an overall increased thermal performance of the system. In a wide flow range (100<Rep<1000), an increase of up to 35% in wall heat transfer coefficient and almost 90% in effective radial thermal conductivity is observed, respectively.

An Experimental Investigation on Forced Convection Heat Transfer From a Cylinder Embedded in a Packed Bed

1994 ◽  
Vol 116 (1) ◽  
pp. 73-80 ◽  
Author(s):  
K. Nasr ◽  
S. Ramadhyani ◽  
R. Viskanta
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Forced Convection ◽  
Convection Heat Transfer ◽  
Particle Diameter ◽  
Packed Bed ◽  
Spherical Particles ◽  
Theoretical Equation ◽  
Convection Heat ◽  
Forced Convection Heat Transfer

Forced convection heat transfer from a cylinder embedded in a packed bed of spherical particles was studied experimentally. With air as the working fluid, the effects of particle diameter and particle thermal conductivity were examined for a wide range of thermal conductivities (from 200 W/m K for aluminum to 0.23 W/m K for nylon) and three nominal particle sizes (3 mm, 6 mm, and 13 mm). In the presence of particles, the measured convective heat transfer coefficient was up to seven times higher than that for a bare tube in crossflow. It was found that higher heat transfer coefficients were obtained with smaller particles and higher thermal conductivity packing materials. The experimental data were compared against the predictions of a theory based on Darcy’s law and the boundary layer approximations. While the theoretical equation was moderately successful at predicting the data, improved correlating equations were developed by modifying the form of the theoretical equation to account better for particle diameter and conductivity variations.

Sign in / Sign up

Export Citation Format

Share Document