The Effect of Plugged Tubes on Gas Mixing in AGR Boilers

Researcher: Alastair West

Supervisor(s): Profs. H. Iacovides, D. Laurence and B.E. Launder
Sponsor: British Energy / EPSRC
Start Date: October 2008 End Date: October 2012
Keywords: tube banks, gas mixing, heat transfer

Overall Research Aim

To provide a computer-based predictive model of the effects of tube plugging for the incorporation into an existing boiler code.

Research Progress

1 - Introduction

As the UK's fleet of Advanced Gas-cooled Reactors (AGRs) approach the end of their operational lives, ageing effects must be remediated for a number of reactor components. Within the boiler this has lead to the plugging of certain tubes in order to prevent steam leaking into the primary gas circuit. With these blocked-off tubes not able to remove heat from the neighbouring gas, an adverse effect is seen on the spread of steam temperatures within the boiler tubes and subsequently on the stability of the boilers at low loads. This effect will only deteriorate with time, and current thermo-fluid models are unable to calculate boiler conditions accurately which could potentially reduce the scheduled lifetime of the AGRs. The boiler is located within the reactor pressure vessel and accessibility is severely limited therefore computational simulations will be used to predict gas mixing around these tubes. Initial Reynolds Averaged Navier-Stokes (RANS) simulations of generic tube banks are conducted and compared against experimental data in order to determine the most effective turbulence models to apply to the real-life problem. The mesh will then be extended to incorporate more complex geometries including the boiler casing. This will ultimately contribute to defining new and more precise operating safety limits so that the boilers can once again operate with optimum efficiency.

2 - Square In-line Tube Bank

This geometry is of special interest as there is reliable LES (Afgan, 2007) and experimental data (Aiba et al. 1982) of several varying pitches for a detailed comparison against RANS turbulence models. A 2 x 2 periodic tube section is chosen to represent a tube bank deep within the boiler (Figure 1). A constant mass flow rate is imposed to obtain the desired bulk velocity by specifying a self-correcting mean pressure gradient at every time step. A similar method is used to extract the thermal energy being fed into the system through the heat flux conditions at the walls.
Figure 1: Computational Domain

Figure 2: Instantaneous velocity field and pressure field respectively showing vortex shedding using SSG model.

2 - In-line Tube Bank with Alternating Longitudinal Pitch

This geometry is found in the superheater section of an AGR boiler. Experimental data shows this geometry experiences a cross-flow secondary motion. The URANS simulation was conducted at Reynolds number 66,000 based on tube diameter and bulk velocity at minimum flow area. Instantaneous flow features are shown in the animations below. The overall thermal diffusion is increased by the cross-flow convective motion due to the vortex shedding behind the tubes.
Figure 3: Instantaneous velocity magnitude showing vortex shedding pattern using k-epsilon LP model.
Figure 3: Instantaneous velocity magnitude and temperature field respectively using k-epsilon LP model.
Figure 3: Instantaneous velocity magnitude and temperature field respectively using RSM SSG model.

Last Modification: r20 - 2012-02-19 - 21:21:05 - AlastairWest

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gifgif KEPSLP_Umag_scal.gif manage 2337.7 K 2012-02-19 - 14:04 AlastairWest  
gifgif KEPS_Umag_100_2.gif manage 6729.3 K 2012-02-19 - 17:32 AlastairWest  
gifgif SSG_Umag_scal.gif manage 2560.9 K 2012-02-19 - 15:50 AlastairWest  
elseEXT batchy_velocity manage 134.0 K 2010-04-14 - 16:42 AlastairWest velocity
pngPNG batchy_velocity.PNG manage 134.0 K 2010-04-14 - 16:57 AlastairWest velocity
pngpng path2383.png manage 1314.5 K 2010-04-14 - 16:39 AlastairWest computational domain
gifgif ssg_period.gif manage 6014.4 K 2010-10-24 - 12:09 AlastairWest vortex_shedding_video_ssg
jpgjpg tempshedding.jpg manage 93.4 K 2010-04-14 - 16:43 AlastairWest temperature
gifgif test3.avi.gif manage 430.8 K 2012-02-18 - 18:41 AlastairWest  
Topic revision: r20 - 2012-02-19 - 21:21:05 - AlastairWest

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