Novel Axisymmetric Diffusion Bonded Recuperator for Gas Turbines

The small gas turbines systems, arbitrary categorized as microturbines (5-200 kW) and miniturbines (200-500 kW) are the current most economical solution for the distributed power generation market. The thermal efficiency of such microturbines without and with a recuperator is about 20 and 40% respectively, thus a recuperator is mandatory to reach higher cycle efficiencies. However, the recuperator accounts for about 25-30% of the turbine total cost and its temperature and pressure are constrained depending on the material and construction method, being the bottleneck of the improvement and advancement of this kind of power generation plant. Thus, the actual focus is to develop high performance recuperators able to withstand high temperatures and pressure at minimum cost. There are several different recuperators present on the market, each with their own heat transfer surface and manufacturing method, but all present drawbacks and are relatively old compared to the actual manufacturing methods. For instance, the rectangular offset strip fin geometry, which is one of the highest performance surfaces, is expensive to manufacture and weak to withstand temperature and pressure due to brazing requirements. Hence, in this thesis, a completely novel modular axisymmetric recuperator concept is proposed, joined by diffusion bonding technique, one of the current most advanced heat exchanger manufacturing methods. For the recuperator core, a novel heat transfer surface is proposed based in the rectangular offset strip fins, the thermal and hydraulic characteristics of which were determined experimentally. The devised heat transfer and pressure drop correlations show 85% agreement with the experimental data in the range of 500<3000. A code for the recuperator design, using entropy generation minimization, was developed to predict the recuperator performance and size the optimum recuperator core dimensions. The design code was validated with CFD which in turn was validated with experimental data. The heat transfer and pressure drop CFD results agreed the experimental data with deviation within 3.2% and 27.7%, respectively, and the design code agreed the CFD results with deviation within 0.9% and 11.9%, respectively. Four recuperator study cases for different turbine sizes, 100kW, 100kW_beta, 1250kW and 5000kW, were designed using the design code. The results show the proposed concept can achieve high effectiveness (~90%) with low pressure drop (<4%) with a volume compatible with the current recuperators. Furthermore, the novel recuperator concept has a list of advantages, which makes attractive its application on the future gas turbines, encouraging the research continuity of the proposed concept.