Empirical modeling of a Lysholm helical screw expander

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Testing of a Lysholm helical screw expander having a rotor diameter of 5.12 inches and an expansion ratio of 5.3 was performed with simulated geothermal fluid at an inlet pressure of 120 psia. The inlet quality, engine speed and pressure ratio were independently varied to produce a three-dimensional data matrix of 104 data points. Statistical curve fitting methods were adapted to produce equations for mass flow rate and power output in terms of the three variables associated with the data matrix. These explicit equations were combined with a steam table subroutine to produce a computer model for prediction of mass flow rate, power, efficiency and exhaust quality at a given set of operating conditions. The predictive ability observed during validation of the model is adequate for use in modeling of hybrid geothermal energy conversion systems. Mass flow rates werre predicted to within 2.7% of the measured values. For most conditions, predictions of power were within 3.3% of the measured values. Predictions of efficiency reflected the combined errors in prediction of power and mass flow rate. Efficiency predictions were in error by as much as 5% of the measured values. The predictions for exhaust quality were within 0.4% of the measured values. Isentropic engine efficiencies of over 43% were measured during the tests. An observed peak in efficiency represented an optimum pressure ratio and a trade-off between lower leakage fractions at high speeds and low inlet qualities, increasing frictional losses at high speeds, and increasing fluid and power densities at low qualities.