The objective of this study is to understand the flow physics and resulting heat transfer behind the mixing of highly viscous solid propellant in a vertical three blade mixer. The mixer comprises a four-winged central agitator rotating in the counter-clockwise direction and two other two-winged agitators rotating in clockwise direction. The temperature rises due to the shearing of the solid propellant. Uncontrolled temperature rise may result in the self-ignition of the propellant and other fire hazards. Thus it becomes important to quantify the heat generated due to viscous dissipation to attain a controlled atmosphere for mixing. A detailed CFD analysis is carried out, and two-dimensional energy equation with viscous dissipation term is solved to quantify the temperature rise due to viscous dissipation. The effect of angular velocity of the agitator and viscosity of the propellant over temperature rise is studied quantitatively using the overset method in OpenFOAM. The maximum velocity of the propellant is observed at the tip of agitators, whereas maximum temperature rise is found around the vicinity of the blade profile. A correlation is proposed to predict the temperature rise with time due to the viscous effect for the given range of angular velocity. © The Authors, published by EDP Sciences, 2019.