Experimental investigation of bladeless expander with an incompressible fluid

For small-medium heat pumps and small-scale energy storage (e.g., hydro) and other small-scale applications (up to 10kWe), an economical, reliable, durable, robust, and acceptable performing bladeless expander is an attractive technology. In this article, experiments are performed on a bladeless expander of 1kW design power with water as a working fluid. The complete expansion is in the subcooled liquid phase with an overall pressure drop across the expander in the 2-14 bar range. The water expander is designed as a similitude case study for a butane heat pump, where such a bladeless expander could replace the expansion valve recovering untapped energy from isenthalpic to isentropic expansion. Unlike conventional bladed expanders, the present bladeless expander consists of several co-rotating compact disks, closely spaced and parallelly mounted on the shaft which transmits torque using wall shear forces. The present expander design is an improved version resulting from the detailed loss characterisation done on earlier air expanders. The article begins with the definition of design conditions for water expander starting from the expected butane expansion in the heat pump, fully inside the liquid region. The rotor design of a bladeless expander is outlined using dimensionless parameters that dictate the performance features. The turbine is designed for 1kW of power output and 2kg/s mass flow with an overall pressure drop of 14 bar with a rotational speed of 8000 rpm. The resulting turbine rotor consists of an 80mm disk outer diameter, 120 disks and a 0.1mm gap between them. An experimental test rig employing water as a working fluid is described. Experiments are conducted for overall pressure drop ranging in the 2-14 bar interval, with a maximum rotational speed of 6500 rpm. The performance is recorded with two different stator configurations, having two different throat dimensions for varying mass flow at maximum inlet pressure. Peak total to static efficiency of 30% is obtained with a net power of 670 W at ∼3000 rpm. An experimental ventilation loss (end wall viscous disk friction) is performed with both water and air as working fluids to estimate the power loss. It is found that ventilation loss is the major source of loss in the present turbine prototype with a power loss of 250W@3000 and 1100W@8000 rpm, varying quadratically with rotational speed. It is finally concluded that the expander performance is promising because ventilation losses can be potentially reduced with established strategies used in conventional expanders.