Optimal micro channel recuperators for small-sized gas turbines

Thermodynamic efficiencies based on the first and second law of thermodynamics are assessed for their use in the design of recuperators in small-sized gas turbines. It is shown that the first and second law cycle efficiency as well as the Witte-Shamsundar efficiency for heat exchangers show a similar dependency on the heat exchanger pressure drops. This results in design relations for hydraulic diameter ratios and their related number of channels ratio. Further, it is shown that the Witte-Shamsundar efficiency focusing on heat exchanger inlet temperatures only can be used as a good approximation for the overall optimal recuperator design when also pressure drops over the recuperator are accounted for. However the approximation deteriorates with higher heat exchanger mass flow density and lower compressor ratio as well as with decreasing turbine inlet temperature ratio. Deviations from the full cycle efficiency considerations are assessed. It is concluded that the use of the Witte-Shamsundar efficiency for optimization of recuperators in small-sized applications like the 1.5 kW PowerMems gas turbine needs careful assessment.