Refrigerator is a device that absorbs entropy at the low temperature end and ejects it at the high temperature end. Transporting mechanisms of entropy from cold to hot temperature by fluid flow are discussed. Thermal effects due to fluid oscillations, which are called thermoacoustic effects, are described and their applications to refrigerators are discussed. Systematic discussions based on thermoacoustics involve, in particular, the pumping loss in regenerator, the shuttle loss of displacer, the dream pipe, the basic pulse-tube, the Stirling engine, the Stirling refrigerator, the Gifford-McMahon refrigerator and the modified pulse-tube. Long history of investigating theroacoustic phenomena, including those of combustion system and Wheatley's natural engines, are reviewed briefly. Better understanding of regenerator would result in further development of more efficient regenerator and refrigerator. A pulse tube refrigerator of progressive wave type would be one of small cryocoolers in future.
