High fidelity heat transfer analysis is a method to estimate thermal performance of heat exchangers by considering non-uniformity effects of flow and temperature in the coming flue gas via CFD simulations. Boiler heat balance tools and other heat exchanger performance estimators often assume a uniform flow at the inlet and outlet while the flue gas flow is always uneven in terms of temperature and velocity.
Non-uniformity of Flue Gas Flow in Terms of Temperature and Velocity
Uniform Flow Assumption in Heat Exchanger Software
Due to the complex geometry of finned tubes, it is impossible to solve total heat exchanger configurations in a conjugate heat transfer analysis (see figure below) to extract the exact distribution of temperature in steam, flue gas and metal.
Conjugate Heat Transfer Analysis for Header and Tubes in HRSG
Boiler makers use the uniform-flow assumption to perform their calculations and specify some design margins in addition with an allowable degree of non-uniformity in flue gas flow to be sure that the final product is in the safe side. However, this method will come with some weaknesses:
- Margins are always conservative.
- Does not consider finned tube arrangements.
- Results in unnecessarily high duct length.
- Requires high value of draft loss.
- Fin density is not considered.
- Does not consider steam temperature.
- Accurate stress analysis is impossible.
WHAT IS OUR METHOD?
We implement the concept of heat exchanger for every computational cell in CFD simulations to calculate the heat transfer rate (between gas and steam) based on the local properties of steam and flue gas. In this way, an accurate estimation of thermal performance of finned tubes will be performed. Then, the temperature distribution in fins and tubes can be calculated without producing complex mesh for them. Indeed, the steam flow and heat exchanger effects are solved virtually by implementing a mathematical heat sink model in flue gas flow.
Concept of High-Fidelity Heat Transfer Analysis
SAMPLES OF RESULTS
Results of High-Fidelity Heat Transfer Analysis on Superheater of HRSG