The mixed convection flow and heat transfer characteristics of non-Newtonian (shear-thinning) water-based nanofluids past confined circular cylinders are numerically investigated. The governing continuity, momentum, and energy equations are simultaneously solved using ANSYS Fluent in two dimensional steady vertically upward flow regime. The influence of the volume fraction of nanoparticles (0.005 ≤ ϕ ≤ 0.045), Reynolds number (1 ≤ Re ≤ 40), and blockage ratio (0.1 ≤ D/H ≤ 0.5) on local and global characteristics are delineated. The effect of aiding and opposing buoyancy is incorporated by varying the Richardson number in the range -2 ≤ Ri ≤ 2. The local and average Nusselt numbers variation with respect to Richardson numbers, Reynolds numbers and volume fraction has also been studied. The local Nusselt number plots show mixed trends and asymmetric in the present range of Reynolds number and blockage ratio. The average Nusselt number increases with increase in Reynolds number, volume fraction and with the decreasing blockage ratio.