Slag pits hall thermal simulation

Thermal analysis of a steel hall considered to be used as slag pit. Main aim of the project was to determine temperature distribution on construction beams of the hall. Temperature of the joist beams is crucial considering stability of the building's construction.

Slag pits hall thermal simulation

The aim of the project was to perform a thermal analysis of steel hall. It was considered by the local steel mill that inside the unused building new slag pits would be located. Purpose of such slag pits is to cool and store hot slag from blast furnace. Since the fresh slag temperature can reach 1200°C the concern was that joist beams of the hall would receive major amounts of heat and could possibly bend. Additionally, lot amounts of steam are produced during slag cooling, which would possibly affect thermal conditions inside the hall. The hall contains several openings, which lets fresh air to come in and hot air to come out. You can see the overview of the hall geometry on Image.

Slag pits hall thermal simulation
Geometry overview of analysed hall

The project was divided into three stages. First was to consider the share of radiation in overall heat transfer process. Moreover, at this point,  partial disassembly of the roof was considered to improve thermal conditions inside the building. Therefore, another objective of the 1st stage was to determine how much would the roof modification improve the thermal conditions. Second stage of the analysis included steam generation from slag cooling. Steam is a medium that participates in radiation heat transfer, therefore it was expected that it would isolate the beams from heat source. Final stage considered conceptual modification of the building. If the temperatures obtained within first and second stage of simulation would exceed the critical values, a radiation blocking screen would be considered. The simulation was performed using ANSYS Fluent software.

Slag pits hall thermal simulation
Temperature distribution on the exterior walls of the hall. Top view - with roof partially deassembled; bottom view - with roof closed.

One of the challenges of this simulation was to include all factors that could possibly affect thermal conditions of the building. Solar load is one of them. With the use of Solar Ray Tracing model solar heat flux has been estimated on each wall of the building, and included as heat generation inside the walls. Furthermore, the worst case scenario was considered in the analysis – boundary conditions on the wall correspond to hot day with wind absence (30°C temperature, 5 W/m2K heat transfer coefficient). First stage used Surface to Surface model, since no radiation participating fluid was present in the hall. Second and third stage on the other hand, used Discrete Ordinates model which can include fluids optically thick.

Slag pits hall thermal simulation
Mesh comparison

First stage showed that radiation plays major role in heat transfer for this particular case. Obviously, lack of steam increases radiation significance, but still, the difference of temperature for case with and without radiation is considerable. Moreover, idea of opening the roof turned out to be very efficient. Average temperatures of joist beams could be reduced for up to halve the value over the most heated region. Unfortunately, modification of the roof was declined after the first stage, as the stability of the building was endangered in case of strong winds occurring.

Slag pits hall thermal simulation
Maximum temperatures comparison of case with and without conceptual radiation screen

During the first stage two different approaches to mesh generation were considered. First method was based on Sweep algorithm. The geometry was first decomposed into simple, sweepable bodies and meshed using ANSYS Meshing program. Some regions were too complicated to use Sweep method and were discretized with tetra mesh. Second approach used Cut-Cell algorithm. No decomposition was required since this method creates mesh for the whole assembly. Results from both methods were then compared. No significant difference has been noticed, and so for further calculations Cut-Cell method has been chosen. It has proven to be less time consuming for generation and produced half the count of the cells in the domain.

Slag pits hall thermal simulation
Steam streamlines with and without additional screen

Second part of the analysis included steam produced from slag cooling. The amount of water used for cooling is significant (34 m3/hour) and so it mustn’t be ignored in the thermal analysis. The simulation showed, that from a thermal point of view, steam gives positive outcome. With density much lower than dry air, steam quickly moves towards the roof and becomes an isolator from radiation of the slag. Therefore, temperatures reached for the 2nd stage were considerably lower.

Although the steam reduced the temperatures on joist beams, maximum temperatures were still too high to accept. Hence, a radiation screen was proposed over the high temperature slag pit, that would partially block the radiative heat transfer from slag to hall beams. The screen is a form of a shutter so the conventional flow of the steam wouldn’t be blocked completely. Simulation showed considerable temperature reduction in the area of highest beam temperatures (as shown on Image 4).

In this project the influence of the hot slag stored inside the steel hall on its joist beams has been estimated. Eventually, we’ve succeeded to reduce the temperatures reached on the crucial construction parts of the hall by including a radiation screen over the slag pits. The crucial part of the simulation was to check and include all the factors that could influence the thermal conditions of the hall.

Slag pits hall thermal simulation

MESco

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