Cfd as a Design Tool for a Concentric Heat Exchanger

A concentric gas-to-gas heat exchanger is designed for application as a recuperator in the domestic boiler industry. The recuperator recovers heat from the exhaust gases of a combustion process to preheat the ingoing gaseous fuel mixture resulting in increased fuel efficiency. This applied study shows the use of computational fluid dynamics (CFD) simulations as an efficient design tool for heat exchanger design. An experimental setup is developed and the simulation results are validated. INTRODUCTION To increase the fuel efficiency of gas-fired domestic boiler appliances, the principle of recuperation is investigated. By recovering heat from the exhaust gases of the combustion process, the ingoing fuel mixture is preheated. This preheating results in an increased adiabatic flame temperature and hence in an increase in fuel efficiency [1]. The heat transfer from the exhaust gases to the ingoing fuel mixture is usually achieved by the application of a compact gas-to-gas heat exchanger (recuperator). Implementing recuperation in practical applications was for long a niche because of high costs in relation to the gain in efficiency which made them not economically feasible [2]. However, rising energy prices and emissions legislation have changed this fact, resulting in numerous recuperator research activities being carried out in recent years. In this study a recuperator design is proposed for a specific type of gas-fired domestic boiler appliance. Based on the results of preliminary design iterations, in combination with manufacturability and the current system lay-out, it was decided to focus on a counter-flow concentric tube heat exchanger design. In this heat exchanger type the exhaust gases flow through a tube in the center, while the fuel mixture counter-flows through the surrounding ring shown in the geometry in Figure 1. In this way the fuel mixture acts as an insulator to thermally insulate the exhaust gases from the interior of the appliance. A common approach to design a heat exchanger is the use of a one-dimensional model such as the P-NTU, ε-NTU or LMTD method [3][4]. However, these methods do not take into account additional effects such as axial conduction and nonuniform heat transfer coefficients. Although extension of these 1D models to include such effects is generally possible, detailed temperature and velocity profiles in flow cross section cannot be obtained. In cases where this information is required to make proper design decisions, computational fluid dynamics (CFD) comes into focus as a relevant design tool. In the recuperator design presented in this study, good flow temperature uniformity in the radial direction is of high importance in order to prevent undesired auto-ignition of the fuel mixture. Therefore a CFD model is proposed which can be used as a realistic design tool capturing detailed information about heat exchange and flow temperature uniformity. Figure 1 Core profile of designed concentric heat exchanger, Lcore = 200 mm, Dout = Ø 88 mm. NOMENCLATURE α [°] Angular location around core Ac [m ] Flow cross-sectional area Cp [kJ/kg∙K] Specific heat capacity at constant pressure Dh [m] Hydraulic diameter, Dh = 4Ac P Dout [m] Core outside diameter εrel [-] Relative solution error of numerical simulation k [W/m∙K] Thermal conductivity