The present work is a numerical study of the transient mixed convection flow in a ventilated enclosure made up of air-filled inclined walls with discrete time periodic heat sources. The lower and upper walls are inclined at different angles with horizontal axis and their respective length is considered as of the order of vertical walls. Using co-ordinate transformation, the physical domain is transformed to computational domain and the governing equations are solved by using FVM based SIMPLE algorithm. An inlet port with cold fresh air from the outside atmosphere is considered along the left vertical wall and flush out through the outlet port along the right vertical wall. A two dimensional projection analysis is made by computing the temperature fields, thermal and fluid irreversibilities and flow lines for various values of Richardson number and inclination angle with Reynolds number Re = 50, 100, 300, 500 and Prandtl number Pr = 0.71. The impact of inclination angle and Richardson number on average heat transfer rate, mass transfer rate, entropy generation, Bejan number and cooling efficiency inside the enclosure is considered to find the effective performance of the system. We have observed a significant temperature variation by changing the inlet fluid velocity and temperature with the inclination angle.