The decrease of vapor flux in a direct contact membrane distillation (DCMD) system is usually triggered by temperature polarization (TP) and concentration polarization (CP). Optimal spacer design is therefore essential to minimize vapor flux decrease. In this study, a numerical DCMD model was developed using computational fluid dynamics. Using a simultaneous evaluation of the effects of TP, CP, and spacer shape and configuration on vapor flux, shear stress and hydraulic pressure drop, the model seeks an optimal manufacturable spacer design. Fifty-one different spacer designs were comprehensively compared after model validation. Based on the simulation results, a symmetric circular-zigzag configuration of spacers gives the best performance for vapor flux, 26% higher than an empty channel. Spacer design optimization was conducted with reference to various sizes and number of spacer filaments. In conclusion, with relatively expensive heat sources, a symmetric circular zigzag spacer design with a larger diameter and more filaments can be theoretically recommended to maximize vapor flux; in contrast, with cost-free heat sources, a smaller diameter size and fewer filaments can be theoretically recommended to minimize hydraulic pressure drop. In addition, the model outlined in this study could be used to evaluate the performance of other membrane processes associated with spacers.