This paper proposes a new predictive model for flow distribution in dual-zone packed bed reactors, which consist of two concentric regions, an inner core and an outer ring, each containing pellets with the same shape but different sizes. This configuration aims to manipulate flow paths for further improving reactor performance. The proposed model, derived from the Ergun equation, captures how the combination of different pellet diameters, relative core sizes and inlet conditions influence the flow distribution between the two regions of the bed. The model's predictions are compared against reactor-scale CFD data across a wide range of Reynolds numbers and bed geometry combinations, showing strong agreement for all the tested conditions. According to the results, different terms of the Ergun equation dominate in different regions of the bed, influencing the local flow distribution and pressure drop. This insight highlights the importance of considering these varying effects when optimizing the packed bed reactor design. This study provides a theoretical framework to better understand and control flow distribution in dual-zone packed beds, with potential applications in catalytic chemical processes. The proposed model offers a useful engineering tool for novel packed reactor configurations.