Twin-entry turbine losses: An analysis using CFD data

Abstract

The current paper presents a computational fluid dynamics (CFD) flow behaviour and losses analysis of twin-entry radial turbines in terms of its Mass Flow Ratio ( MFR, the ratio between the flow passing through one of its intake ports and its total mass flow), focusing on the mixing phenomena in the unequal admission conditions cases. The CFD simulations are first validated with experimental data. Then, the losses mechanisms are analysed and quantified in the different parts of the twin-entry turbine in terms of the MFR value. A sudden expansion is found at the junction of both branches in the interspace between volutes and rotor for unequal and partial admission cases. Tracking the flow coming from each of the turbine intake ports, it has been observed that both flow branches do not fully mix with each other within the rotor. Another source of losses has been identified in the contact between both flow branches due to their momentum exchange that depends on the difference between both flow branches velocities. These losses have not been considered before, and they should be included in mean line loss-based models for twin-entry turbine since they are very significant for unequal admission conditions.

Experimental assessment of the rotor outlet flow in a twin-entry radial turbine by means of Laser Doppler Anemometry

“…As described in Figure 6, the LDA measurement points are 3 mm downstream of the rotor nut. There is a sudden expansion just at the rotor nut, as explained in Galindo et al 20 This sudden expansion reduces the axial Mach number near the rotor nut, and it explains the axial Mach number distribution along the radial coordinate.…”

“…The experimental results agree with the CFD simulations results, as shown in Figure15. The CFD results were globally validated in previous works 19,20. However, they have been locally validated in the outlet section now with these experimental measurements.Moreover, these results corroborate the hypothesis of linear outlet area variation with the MFR value made in the one-dimensional model used 19.…”

“…The regions with more complex phenomena arise have a more refined mesh. After a mesh independence study, as detailed in Galindo et al, 20 a 5.5 million cells mesh is chosen, as shown in Figure A1. The global convergence index is <0.1% for the total pressure at the rotor inlet, the efficiency, the torque and the reduced mass flow.…”

“…The CFD results were globally validated in previous works. 19,20 However, they have been locally validated in the outlet section now with these experimental measurements.…”

“…They help to extrapolate and predict twin-entry turbines performance. The authors of this work are also developing a one-dimensional loss-based model described in Galindo et al 19,20 It takes into account the losses due to the momentum exchange between flow branches. These losses produce a strong effect on the performance prediction in unequal admission conditions.…”