This paper presents an experimental and numerical study of a pile-type circular oscillating water column device with a U-shaped duct (circular U-OWC) for wave energy conversion in long waves. In the experiments, the circular U-OWC model and a conventional OWC model of identical chamber dimension was compared in a wave flume under various wave conditions. Orifices of different sizes are used to simulate the power-takeoff (PTO). The pneumatic power, PTO damping coefficient, and the scattering of the incident waves are measured. A RANS equation based numerical model with a modified k-w SST turbulence closure is validated using the experiment. The numerical model is utilized to investigate the spatial non-uniformity of the free surface inside the chamber, and the viscous energy loss in the U-shaped duct. It is found that for long waves, the circular U-OWC device is several times more efficient than a conventional OWC device, and the sloshing effect inside the pneumatic chamber is significantly reduced compared to a conventional OWC device. It is also found that a significant amount of energy entering the U-shaped duct is dissipated in viscous loss, recommendations on U-shaped duct design are given to help reduce the viscous loss of the device.