Heat recovery from micro gas turbines

Decentralised energy systems are being revolutionised by micro-combined heat and power systems (µCHP), which simultaneously provide heat and electricity with reduced emissions, thus playing a crucial role in the global energy transition. A micro gas turbine (µGT) based µCHP operates mainly on the Brayton cycle, starting with compression, followed by heat addition via a heat exchanger (recuperator) and combustion, and ending with expansion to atmospheric pressure. In this system, the recuperator increases the cycle efficiency by 10 per cent, where it accounts for about 30 per cent of the machine cost. It is therefore important to optimise the design of the recuperator, in order to achieve both a higher thermal efficiency (>85%) and a lower pressure drop (<5%).This thesis investigates MITIS patented recuperators consisting in complex wire-net microchannels and collectors, by using a computational fluid dynamics (CFD) approach. Due to the presence of distributing and collecting manifolds as well as hundreds of parallel microchannels, a complete conjugate heat transfer (CHT) analysis requires a large amount of computational power. Therefore, in this study, a novel methodology was developed, based on a reduced order model (ROM) to analyse the entire heat exchanger performance, in which the microchannels are modelled as a porous medium where a compressible gas is used as a working fluid. This approach allowed for reducing the mesh size to a considerable extent (billion cells to a few million cells). As a first step, a three-dimensional CFD analysis of conjugate heat transfer for a microchannel section was performed to calculate, using the constant integration method, the inertial and viscous coefficients of the porous medium model based on the Darcy-Forchheimer law. High-temperature variations and compressibility effects were accounted for in this analysis. Besides, a detailed investigation of the wire-net flow physics was made using a higher order Reynolds stress turbulence model to obtain the full velocity gradient tensor. This could detail the effect of anisotropic flow physics in the isotropic wire-net microchannels. Furthermore, the analysis of the turbulence production terms provided a deeper insight into flow attachment and detachment near the wire-net intersections. It was shown that shifting the critical Reynolds number to lower values using perturbators decreases the pressure losses and enhances the thermal efficiency and that there is an optimum mass flow where the thermal efficiency reaches maximum.Using the ROM approach, investigation of collectors with different microchannel configurations (s-shaped, wire-net and plane channels) showed that mass flow rate deviation decreases with an increase in microchannel resistance. The recirculation zones in the cylindrical collectors also changed the maldistribution pattern. It was also shown that there is a substantial drop in thermal effectiveness at low mass flow rates due to axial wall conduction losses and high flow maldistribution, resulting in a slow µGT start-up.Experimental tests were performed for both a single microchannel with S-shape perturbators and a micro-heat exchanger with triangular collectors. From experiments conducted under adiabatic conditions, it could be confirmed that the presence of perturbators in the microchannels shifted the onset of turbulent transition to lower Reynolds number values compared to free channels. The experimental testing of the whole recuperator showed that the collector pressure losses are around 40–50% of the microchannel ones. A comparison of experimental data with the numerical results obtained from the proposed hybrid methodology showed that the ROM can predict the heat exchanger performances, both in terms of thermal efficiency and of pressure drop, within the experimental uncertainty.