As it is a modern market requirement that hydroelectric power plants become more flexible, hydraulic turbine manufacturers are requested to extend the operating range of their machines. For single regulated Francis turbines, consequent off-design conditions involve more complex flows than the ones of the classical continuous operating range. During partial load operation, several hydraulic phenomena are known to occur in the machine. One of them is the development of inter-blade vortices in the runner, and concerns especially low head turbines. Due to these complex flows, the machines are operated with higher levels of dynamic stresses in the runner that can potentially lead to a premature damage of existing units. Therefore, a better understanding of partial load flows in Francis turbines is required to provide new appropriate designs. To describe the full-size machine behavior, numerical simulations and test rig measurements are used in a complementary way during the design stage. Therefore, new CFD modelisation and experimental techniques are required to address off-design operation. In this study, a reduced scale model of a low head Francis turbine was tested. It was equipped with on-board strain gauges and pressure sensors, in addition to the classical instrumentation. High-speed visualizations of the flow at the inlet of the runner channels were performed by means of three special guide vanes. The experimental data were used to validate numerical simulations of the flow inside the runner. URANS calculations were performed using a commercial code. At partial load conditions, the location of the inter-blade vortices was successfully simulated and the comparison between numerical and experimental results showed a good agreement. The present test rig measurements campaign and numerical simulations are providing further insights about partial load flows inside low head Francis turbine runners.