3D coupled CFD/FEM modelling and experimental validation of a planar type air pre-heater used in SOFC technology

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Abstract

In this present paper, a coupled 3D thermo fluid–thermo mechanical modelling approach of a plate type air pre-heater is introduced. The conjugate heat transfer within the pre-heater is numerically solved using 3D computational fluid dynamics. The model considers the discrete structure of the whole pre-heater assembly including the air channels, plates and manifold ports. Wall surface and gas temperature measurements are performed using thermocouples to validate the numerically predicted calculations. The results show good agreement, implying that the proposed model can be used in the design and process optimisation of the air pre-heater. Moreover, a submodel created from the thermofluid analysis has been used to investigate the thermomechanical behaviour of the air pre-heater manifold region using the nonlinear finite element method. The nonlinear elastic–plastic behaviour of the material has been considered. The component locations susceptible to stress could be determined. The approach results in reduced prototype costs and product development time in the design and optimisation of efficient air pre-heater designs.

Introduction

Plate heat exchangers (PHEs) have been extensively used for heating, cooling and heat generation purposes in various fields such as mechanical engineering, chemical, food and pharmacy industries due to their superior thermal efficiency, easy handling, compactness and flexibility. Basically, the heat exchanger assembly consists of a stack of metal plates pressed together in a frame. Manifold ports on the corners supply flow to the plates in which the fluid flows through thin channels. Heat is exchanged via the plate separating the fluids flowing on each side of the plate. The number of plates, their perforation, location of the inlet and outlet connections at the corners characterise the PHE configuration, which further defines the flow distribution inside the plate pack [1]. Because of the large variety of heat exchanger plates, the design of the PHE is highly specialised [2]. Due to high optimisation costs, computational modelling has been used to aid in the design and optimisation of PHEs. Many model examples have been introduced in the literature to investigate the thermal efficiency and flow distribution of PHEs. Such examples include numerical approaches applied by Kandlikar and Shah [3] that have been used to compare different configurations of heat exchangers. A design methodology has been proposed by Shah and Focke [4] to determine the proper plate number, size and corrugation type. Their approach was applied to a single pass counter current flow. A study has been introduced by Gut and Pinto [2], investigating the configuration influence on the parameter estimation of sizing PHEs. The model considered pure counter current flow.

The fast developing high temperature fuel cell technology, in particular solid oxide fuel cells is also involved in using PHEs. The integrated module developed and tested in Jülich is a fuel cell system in which all components are scaled to a nominal SOFC stack power of 5 kW. The design and allocation of each component plays a crucial role in the overall efficiency of the entire multi-component system [5]. Fig. 1 illustrates the integrated module setup for a 5 kW module. For details it is referred to [6], [7].

The optimal proportioning and structural design of each component within the integrated module depends upon the process conditions it is subjected to. Each component is subjected to different thermal load ranging from combustion to chemically reacting species transport. This yields in different thermal and mechanical behaviour, which has to be considered when investigating each component. To investigate experimentally the effects of the design and operating parameters on each system component, or on the overall integrated module performance, is prohibitive. This effort increases especially at elevated temperatures. The use of a trio approach for a systematic component optimisation has been introduced in a previous study [8]. The use of computational modelling is an attempt to bring mathematical optimisation into the product development chain. However, the design of the individual components combined with the multiphysics, is very complicated. The special designed plate type heat exchanger (air pre-heater) is one of the major components within this integrated system. It is used to heat up the air fed into the cathode side of the fuel cell system. For this purpose the exhaust gas flowing out of the afterburner component is used to heat up the air via heat exchanging. As the component is subjected to high thermal gradients, it is exceedingly important to consider the thermomechanically induced stress of the component during the design process as well [7]. Moreover, as the velocity and the heat transfer coefficient depends on the component configuration, single heat transfer correlations such as 1D plug-flow models are not adequate; especially, as the component is asymmetrical configured. Therefore, a fully 3D analysis is required. Fig. 2 illustrates the flow sheet of the integrated module reflecting the central role of the heat exchanger.

The literature review reveals that not much attention has been given to the development and testing of high temperature heat exchangers used in fuel cells. The research has been focused on performance analyses within systems [9], [10], [11], [12]. Moreover, the thermomechanical behaviour has not been considered yet. Zhang introduced a performance study on a compact heat exchanger reformer considering a one-dimensional flow along the flow passage [13]. Lasbet et al. [14] introduced a plate heat exchanger within a bipolar plate used in PEMFCs, which operates at 80°C–90 °C and concentrates on size reduction. Most of the work published in the literature has been applied to symmetrical configurations, investigating flow arrangements for series and parallel PHEs. The high thermal efficiency of this type PHEs is well known; however, due to system configurations asymmetrical configurations are also used, as it is also the configuration used in this present study. This paper focuses on a synergistic blend between measurements and numerical simulations into developing a plate type heat exchanger that minimises prototype costs while increasing the knowledge of the thermomechanical behaviour of the air pre-heater.

Section snippets

Wall surface and gas temperature measurements

To test and investigate the thermal behaviour of the air pre-heater, two air pre-heater flat plates were manufactured each for the exhaust gas side and the air flow side, reflecting the accurate geometric representation made from the material Crofer® 22 APU. The plates show two distinct configurations such that the air side plate has one slightly lateral to the centre located inlet region, and two side located outlets, thus an asymmetrical construction is present. This is due to the flow

Modelling and experimental validation

This section comprises the coupled thermofluid/thermomechanical analyses of the heat exchanger, calculated numerically. The mathematical formulation of the numerical approaches is presented and followed by the computational modelling proposed for both computational fluid and solid mechanics.

Thermofluid flow analysis

The sensitivity of the analyses to turbulence has been tested using enhanced wall treatments and ensured that the flow inside the heat exchanger is laminar and similar results are obtained. Hence, the laminar solver has been used for the investigations. The predicted CFD results enable a detailed view of the temperature distribution of the PHE. High temperature regions could be determined, which may indicate the regions susceptible to thermally induced stress within the component. Fig. 7

Conclusions

A coupled CFD/FEM approach has been used to investigate the thermofluid/thermomechanical behaviour of a high temperature PHE used in SOFC technology. The virtual prototype of the PHE, representing the exact configuration including the flat and cover plates, channels, and the manifold ports has been developed. Wall surface and gas temperature measurements of the heat exchanger are performed so as to provide validation data to evaluate the feasibility of the model to be employed as a design tool.

Acknowledgements

The technical stuff of the Research Centre Jülich is gratefully acknowledged for their support.

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