The work of this paper represents the first stage of a whole modeling and computational solution procedure for an on-going research on agricultural fires at the laboratory of environmental research, Assiut University. The paper presents an accurate and efficient semi-implicit pressure-based algorithm developed for solving numerically the conservation equations governing weakly compressible single-fluid flows. The algorithm uses a finite-volume technique applied to structured non-orthogonal multi-block curvilinear meshes with co-located grid arrangement. An in-house CFD code has been developed to implement the numerical solution of the present algorithm. The code is an integrated one consisting of the main elements, the pre-processor, the solver and the post-processor. This in-house CFD code undergoes a standard verification and validation process using a variety of standard test cases covering broad range of CFD applications. The results of the present code for all test cases are in good agreement with those of the well-established reference CFD codes. This insures the present code verification and validation.