Influence factors and numerical investigations of the pressure loss of oscillating fluid flow

Regenerative gas cycles, including the Stirling engine, are sealed machines using pistons within cylinders to transfer energy from a heat source to a colder reservoir by using a gas as working substance. In this connection the fluid flow inside the heat exchanger pipes is oscillating. Therefore, the heat transfer and pressure loss normally is not steady. In this contribution the velocity and the pressure loss of oscillating pipe flow are investigated theoretically and numerically. Circular as well as rectangular canals with Reynolds numbers in the area of laminar and turbulent flow are regarded. The numerical simulation was implemented in ANSYS-CFX using moved grids to describe the volume change of the pistons. The accordance between analytic solution and numerical simulation for the case of low Reynolds numbers is very good. As expected clear differences appeared between the analytic solution and the numerical simulation with rising Reynolds numbers (turbulent flow) which could not be validated according to missing analytic equations. Dimensionless parameters and a half-empiric equation by using complex numbers which approximates the time dependency of the pressure loss were developed by analysis and evaluation the influencing quantities. The included model coefficients can be determined by numerical simulations. They facilitate an estimation of the pressure loss without usage of time-consuming CFD calculations.