1-way FSI Analysis of an Heat Exchanger for Energy Recovery
Low-carbon biomass is a critical renewable energy resource. To keep this bio-energy sustainable it is critical to develop more efficient heat exchangers for the boilers that produce this type of energy. So when Uniconfort, a manufacturer of biomass boilers, decided to make use of CAE tools to design a new concept heat exchanger they turned to EnginSoft, experts in the use of CFD tools and analysis, for specific assistance. The scope of the project was to design a heat exchanger that would optimize heat transfer performance and improve the general lifespan of the heat exchanger through the use of a one-way fluid-structure interaction analysis. The challenge was to do this without increasing the overall cost of the heat exchanger.
The object of this study was a heat exchanger, consisting of a sequence of four large vessels (chambers) each with a bundle of tubes inside, which is used for heat recovery from biomass combustion. The modeling of the problem required a multi-physics approach where the mechanical stresses are strictly related to the temperature distribution on the structure which is actually a function of the flow field.
The CFD and FEA Methodologies
The first step was to perform a 1-way fluid structure interaction (FSI) analysis. The flow conditions inside the system were evaluated through a 3D thermo-fluid dynamic simulation to gain an insight into the flow and to obtain the temperature distribution in the tube bundles. The model we built included the combustion by-products and cold air. The combustion products derived from biomass combustion, flowed from the main entrance into a series of passages through the four chambers, transferring energy to the cold air which entered the domain from the opposite side into the tube bundle. The analysis also considered:
- conjugate heat transfer taking into account the conductive solid material of the tubes
- turbulence which was solved using the Shear Stress Transport model
- thermal radiation effects
The results of the CFD simulation were used as a boundary condition for the second step, a thermo-structural simulation executed via a finite element analysis (FEA). The analysis was multi-step with material and geometric non-linearity, large deflections and contacts. The material used was steel, considered as isotropic and elastic-perfectly plastic. Young modulus and yield stress are functions of the material temperature. The purpose of the analysis was to verify the stress levels in the internal structure, the maximum displacements and the creep behavior. The CAE tools used for this project were ANSYS ICEM CFD, CFX and Mechanical. The combustion products were characterized with specific properties provided by Uniconfort.
Both the CFD and the FE analyses confirmed that the general behavior was in line with Uniconfort’s expectations. The results showed that the heat exchanger could be substantially improved by executing some minor modifications. Critical areas for thermal stress were identified (high temperature gradients) and local structural reinforcements were proposed and verified. The average stress level on the structure was actually reduced by 30%. With this project EnginSoft gave Uniconfort insight into the heat transfer phenomena taking place in the heat exchanger providing substantial assistance in the definition of an optimized design for the heat exchanger.
"With EnginSoft's help and virtual prototyping we increased the lifespan of our product, reducing the average stress level in critical areas of the structure by 30%."