The gas-liquid mixing in a mixing tank is an important process in many industrial applications, such as chemical and biochemical processing which mostly deal with the non-Newtonian fluids. The design and optimization of the aerated mixing tank with such characteristic is a challenging task. Most of these challenges are due to the non-Newtonian behavior of the fluid, which can lead to compartmentalization of the mixing tank, and formation of oxygen segregation zones. These issues become more pronounced at larger scales. These problems can be addressed by selecting the appropriate mixing equipment. The coaxial mixing systems consisting of the inner and outer impellers exhibited better performance in gas dispersion in the non-Newtonian fluids compared to that of the conventional mixing systems. Therefore, the primary objective of this study was to identify the mixing dead zones and determine their impact on the overall mixing process for the coaxial mixing system at two different scales. These dead zones are regions of low mixing intensity. Hence, this research focused on the evaluation of the hydrodynamics attained by a coaxial gas-liquid mixing tank through the numerical and experimental methods. The study was conducted using computational fluid dynamics (CFD) and electrical resistance tomography (ERT) methods. The effect of the aeration rate, inner impeller speed, and rotating mode on the creation of dead zones was investigated. The location and the size of dead zones were greatly dependent on the rotating mode and the size of the mixing vessel. Furthermore, the research outlined the relationship between the gas phase retention and the hydrodynamics of the coaxial mixing tank. This study highlights the importance of considering both the hydrodynamics and gas hold-up when designing the coaxial mixers containing a non-Newtonian fluid.