There exist three states of matter, i.e. gas, liquid and solid and a flow in which two or three of these states are mixed is called a multi-phase flow. We analyze the flow and heat transfer of multiphase flows using numerical simulations and visualization systems. The thermal flow in the garbage incinerator and the dough mixing in the noodle dough kneader are analyzed as a flow of mixed solids and gases. Bubbles generated in the molten solder bath are analyzed as a flow of mixed liquid and gas. For such problems, we use open source code e.g. OpenFOAM for simulation. In order to visualize heat and flow, PIV, large-diameter Mach-Zehnder interferometer, system schlieren, and infrared thermal image analyzer are utilized.