University of Twente Student Theses


Adjoint-based optimization study of a heat exchanger : single and multi-objective shape optimization

Iordache, C. (2017) Adjoint-based optimization study of a heat exchanger : single and multi-objective shape optimization.

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Abstract:An adjoint gradient-based optimization method is used for optimizing a pin-fin heat exchanger mounted in a domestic boiler. The advantage of this method is that an improved shape of the pins can be reached with respect to the operational conditions at the same cost as the CFD simulation. The optimum shape is computed within the boundary that encloses the cross-section of the pins. A modification of this boundary is realized via the adjoint optimization method and a new shape is obtained. This process of shape optimization and its functionality is investigated by optimizing a simplified model of a pin-fin heat exchanger which represents the main goal of this thesis. This is done via a CFD numerical simulation tool ANSYS Fluent. The study is carried on by optimizing heat transfer rate and pressure drop. The optimization of these objectives is done by analyzing its changes due to a modification in the shape. The changes in flow field are analyzed as well to understand its effects on the variation of the objectives. This study used a two dimensional simplified model for the optimization of each objective. The results show that an extremely accurate converged flow solution as well as using constant thermophysical properties improve the predictability of the optimized objectives. However, the predictability is decreasing as the degree of shape change increases. The predictability is also affected by the choice of discretization scheme used in the flow solver. It was concluded that the pressure drop is more sensitive to changes in shape than the heat transfer rate for the same percentage of optimization. Furthermore, modifying the shape in the direction of heat transfer rate improvement is done at the cost of higher pressure drop. A sufficiently high change in the objective leads to a computation of a different improvement direction. A new optimum geometry with a different improved shape is obtained. Further study investigates the multi-objective optimization. Different combinations of pressure drop and heat transfer rates have been used as a multi-objective optimization. The difference in magnitude of each objective optimization shows that heat transfer rate improvement is higher in magnitude near the heat exchanger`s entrance. The pressure drop direction of improvement is lower in magnitude and is more uniformly distributed along the heat exchanger domain. The total direction of improvement for multi-objective optimization is determined by the summation of each objective`s direction of improvement. Multi-objective optimization is possible and a higher optimization rate is achieved for a certain combination of the objectives. Also, due to large changes in shape after a high number of optimization cycles (>30) highly distorted cells appear. An improvement in mesh quality is required. Moreover, the results obtained from the two dimensional domain allow for a successful approximation of the three dimensional domain. The optimization of the three dimensional geometry results in improved heat transfer rate and pressure drop. An influence of the area chosen as an enclosed boundary for optimizing the pins gives different results. A larger area provides more degree of freedom for the movement of the shape boundary, leading to a higher optimization of the objectives. Finally, the end geometry is obtained for the optimized solutions.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:52 mechanical engineering
Programme:Mechanical Engineering MSc (60439)
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