Abstract
Diaphragm pumps often experience cavitation and subsequent fluid flow oscillation when delivering an organic fluid in small/micro scale organic Rankine cycle (ORC). The cavitation behaviour of diaphragm pumps has rarely been investigated for organic fluids so far. Three-dimensional, unsteady cavitating flows of organic fluid R245fa in a diaphragm pump were simulated with ANSYS 2019R2 CFX in suction stroke in terms of the k -ω turbulence model, the ZGB cavitation model, rigid body motion model for one-dimensional motion of valve and moving mesh technique for the first time. The thermodynamic effect in cavitation of R245fa was considered. The vapour volume fraction threshold for cavitation inception was determined, and the cavitation inception and cavitation developed states were identified, and vortex production and entropy generation rate during cavitation were clarified. Cavitation inception emerges at the edge of the valve seat, then on the valve surface. With cavitating development, the pressure and force on the valve, valve opening, and velocity oscillate violently due to vapour bubble collapse cycles. Expansion cavitation and flow induced cavitation happen in sequence at different crank rotational angles. The maximum temperature depression is 0.549 K in the cases studied. The volume-integrated entropy generation rate in the valve chamber correlates to cavitation states.
Highlights
The organic Rankine cycle (ORC) is a Rankine cycle by using an organic fluid with low boiling point as working medium to generate power from lower temperature sources such as biomass combustion, industrial waste heat and geothermal heat, etc [1e5]
Six transient computational fluid dynamics (CFD) simulations were performed at inlet pressures p1 1⁄4 141, 89, 88, 87, 86, and 85.2 kPa by employing the computational models stated in Section 3 and fine mesh in terms of 100 of number of time-steps to identify cavitation behaviour of the valve
Cavitation inception happens on the chamfer of the valve seat based on 0.10 vapour volume fraction threshold, on the valve surface along with significant oscillation in pressure and force on the valve, and valve opening, and velocity
Summary
The organic Rankine cycle (ORC) is a Rankine cycle by using an organic fluid with low boiling point as working medium to generate power from lower temperature sources such as biomass combustion, industrial waste heat and geothermal heat, etc [1e5]. Mechanical feed pumps are commonly employed to deliver an organic fluid to the evaporator in an ORC system. The feed pump in ORC systems is featured with high head (20e4000) kPa and low mass flow rate (0.01e10) kg/s [6]. The rotodynamic pump has one or more rotating impellers to raise liquid pressure and velocity continuously, but the positive displacement pump delivers a liquid with a periodical increasing and decreasing volume intermittently. At the same head and flow rate, the positive displacement pump is subject to a better efficiency but higher maintenance costing and more complicated pipe systems than the rotodynamic pump
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