Abstract
A significant improvement in the economy-of-scale of small-scale organic Rankine cycle (ORC) systems can arise from the appropriate design of components that can be manufactured in large volumes and implemented flexibly into a wide range of systems and potential applications. This, in turn, requires accurate predictions of component performance that can capture variations in the cycle conditions, parameters or changes to the working fluid. In this paper previous work investigating a modified similitude theory used to predict the performance of subsonic ORC turbines is extended to analyse the supersonic flow of organic fluids within 2D converging-diverging nozzles. Two nozzles are developed using a minimum length method of characteristics design model coupled to REFPROP. These are designed for R245fa and Toluene as working fluids with nozzle exit Mach numbers of 1.4 and 1.7 respectively. First, the nozzle performance is confirmed using CFD simulations, and then further CFD simulations are performed to evaluate the performance of the same nozzles over a range of different inlet conditions and with different working fluids. The CFD simulations are compared to predictions made using the original and modified similitude theories, and also to predictions made by conserving the Prandtl-Meyer function for the different operating conditions. The results indicate that whilst the modified similitude model does not accurately predict nozzle performance, conserving the Prandtl-Meyer function allows to predict the nozzle outlet Mach number to within 2% providing there is not a significant change in the polytropic index. Finally, the effect of working fluid replacement on the ORC system is discussed, and preliminary results demonstrate the possibility of matching a particular turbine to a heat source through optimal working fluid selection.
Highlights
The organic Rankine cycle (ORC) is a promising technology for the conversion of low temperature heat sources into mechanical work across a range of power outputs ranging from a few kW to the MW scale
Since the head coefficient is not the same, these results indicate that the modified similitude model cannot fully predict the supersonic nozzle performance following a change in the inlet conditions
To investigate nozzle performance following a change in the operating conditions, 9 operating cases were defined for each nozzle covering a range of nozzle Reynolds numbers and values for the polytropic index k (P/ρk = constant)
Summary
The organic Rankine cycle (ORC) is a promising technology for the conversion of low temperature heat sources into mechanical work across a range of power outputs ranging from a few kW to the MW scale. Despite successful commercialization for large-scale systems [1], implementation of smaller systems has not followed the same progress, primarily due to their high capital costs. Among other improvements such as in performance and reliability, improvements in their economy-of-scale can act as an important enabling factor in their widespread implementation. One approach to achieving this is to develop ORC systems that operate efficiently over a range of operating conditions, whilst using different working fluids. Robust techniques do exist for the design of components that best match targeted operating conditions, the development of ORC components that can operate flexibly over a range of conditions requires methods to predict the performance of these components at design and off-design conditions.
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