A boiler's overall efficiency is computed from a simple energy balance and usually agrees well with measurement. What sets the boiler's life, however, is not the total but the local heat flux.
Why local distribution matters
- Overheating. If a tube wall locally exceeds the design temperature, creep damage begins; the overall balance does not show this.
- Fouling and slagging. Ash sticks to surfaces within a particular temperature range. Where it accumulates is set by the local temperature distribution.
- Corrosion. In reducing zones the near-wall gas composition accelerates corrosion, and that too is local.
- Thermal stress. Stress arises between regions that expand differently. See the structural mapping note.
Radiation is the governing mechanism
At boiler temperatures most of the heat reaching the wall is transported by radiation, and the combustion products are a participating medium. The accuracy of the wall heat flux distribution depends directly on the accuracy of the radiation model and the gas properties [1]. See the radiation note.
With solid fuels, particle radiation must also be included; soot and ash particles change the radiative balance markedly.
An extra dimension in reactors
In chemical reactors the temperature distribution is not only a strength question but a selectivity question: unwanted side reactions accelerate at hot spots and product quality falls. In systems with runaway potential the local peak temperature is directly a safety criterion.
In packed beds the flow distribution is distorted by the wall effect — porosity is higher near the wall and the flow escapes there. That affects both conversion and heat transfer and should be captured in the porous media model.
References
- Modest, M. F. & Mazumder, S. (2021). Radiative Heat Transfer, 4th ed. Academic Press. ISBN 978-0-323-98406-5.
- Ergun, S. (1952). Fluid flow through packed columns. Chemical Engineering Progress, 48(2), 89–94.