In food industry, heat treatment of highly viscous fluids in continuous process is more and more common and the process should perform homogenous thermal treatment in order to ensure quality and safety of the final product. To improve treatment homogeneity, geometric modifications could be used even in laminar regime inducing flow perturbation and mixing. In this work, our objectives were (i) to investigate residence time distribution (RTD) for industrial indirect Joule effect heaters (JEH) with smooth (ST) and modified (MT) tubes, (ii) to demonstrate and quantify the efficiency of geometrical modifications and (iii) to propose a general reactor model including flow regime (10