The plate fin heat exchangers usually have either rectangular or triangular shaped flow passage. In comparison to triangular flow passage, rectangular flow passage gives comparatively higher heat transfer at the cost of higher pumping power. In the present investigation, flow passage is modified by rounding the corner of triangular passage to investigate the heat and flow characteristics of air flowing through it. Comparison of performance between modified and rectangular flow passage has also been presented and discussed. The radius of curvature of the rounded corner has been kept constant with value of 0.49 times duct height (H). The dimple was also fabricated at the inner side of the flow passage and arranged in rectangular array. Distance between them was defined by two different dimensionless parameters, relative transverse width (x/h), and relative streamwise length (z/h), whereas, dimensionless height of the protrusion is defined by relative dimple height (h/D). Noticeable increment in both heat transfer and friction factor has been observed by modifying the duct corners and 2.98 times increment in Nusselt number resulted due to dimples in modified duct for h/D, x/h, and y/h value of 0.44, 10, and 10, respectively, in comparison to smooth duct at Reynolds number of 19,500. For similar combination of roughness parameters, highest frictional penalty was estimated with value of 4.46 times that of the smooth duct at Reynolds number of 4400. Additionally, the comparative assessment of heat transfer enhancement (Nuenh), frictional penalty (fpenalty), and thermohydraulic performance index (THPi) has also been carried out to understand the suitability of round cornered duct. In comparison to protruded rectangular duct, 28% higher THPi is obtained in modified duct under similar conditions.