This study aims to investigate the reversible operation of a bladeless air expander prototype operated reversibly in compressor mode to understand the performance by numerical method and compare its results experimentally. A bladeless machine can reverse its operation by simply inverting the rotational speed. However, expander and compressor performance may differ significantly since losses are exacerbated in the compressor mode. The prototype was previously tested as an expander (experimental highest isentropic efficiency of 36.5%). In this work, the reverse mode is discussed, when the prototype is actuated as a compressor, with and without diffuser at variable rotational speeds. In compressor mode, the fluid enters through the center axially, passes radially outwards through disk gaps, and exits throughout the diffuser. The momentum transfer and pressure gain are carried out by the shear force produced on the surface of the rotating disk. An experimental/theoretical analysis focused on the pressure ratio, mass flow, and efficiency of bladeless compressor is conducted. High losses (main leakage across the rotor) were noticed during the experiments, affecting the overall Tesla compressor performance. Numerical calculations are carried out to estimate leakage losses by comparison with experimental results. It is shown that the original expander design would require specific modifications to reduce end disk leakages, which are higher in compressor mode than in expansion mode, significantly affecting the elaborated net mass flow. Improved diffuser, scroll, disk end gaps, and sealing mechanisms are discussed in order to augment overall performance of the bladeless prototype in compressor mode.