In this master's thesis we analyzed the structure, working principles and calculation methods of plate heat exchanger. With the usage of IR camera we performed experimental measurements of the temperature field on the working plate heat exchanger and focused on the part where the secondary flow exits the heat exchanger. Also we created a simplified 3D model of the same type of the heat exchanger with the purpose of simulating the same working conditions as with the experiments. By using a computer program Ansys we were able to observe and analyze the thermal field while changing the values of the fluid flow. We have found out there was a qualitative match between measurement results and simulation results. At the end of the thesis we showed the results of our parametric analysis and discussed the reasons for thermal stratification in the secondary flow in the part of the exiting connection.