Both the simulation and the experimental evaluation of a thermal microfluidic flow sensor are presented in this work. The sensor was evaluated in the calorimetric principle of operation with water as the test liquid. The simulation of the sensor operation was performed into the framework of Comsol® software package. Various combinations of microchannel geometries were tested with different input flow rate values. The operational parameters of the device were simulated by 3D models. The dominant operation parameter was determined to be the distance of the upstream-downstream sensing elements to the heater, the effect of which was theoretically extracted and experimentally verified.