Heating and Cooling Cycles, Stirling and Thermoacoustic engine understanding

Have you ever heard about Heating and Cooling Cycles, do you know about Stirling and Thermoacoustic engine ? In this article will cover the stuffs.

The technologies used in most of the world’s cooling machinery are based on cycles (called thermodynamic cycles) of heating and cooling what is called a “working fluid.” In each cycle, the fluid (which may be a gas or a liquid) undergoes several steps:(1) compression, which raises its pressure and temperament;(2) heat rejection (expulsion);(3) expansion, which lowers its pressure and temperature; and (4) heat uptake from the material to be cooled. Work is done on the fluid to make it move heat energy from a region at a lower temperature to one at a higher temperature.

How do the stirling-engines work
How do the stirling-engines work

When the system operates, the working fluid may either remain in the gaseous form or undergo phase changes (“vapor cycles”) from liquid to vapor to liquid again. In the latter case, the fluid is referred to as a refrigerant. During evaporation, the refrigerant absorbs heat; during condensation, it loses heat. An advantage of using refrigerants is that during phase changes, heat transfer occurs at constant temperature. Moreover, the energy transferred per unit mass of liquid is high. Such vapor cycles have long been employed in most domestic refrigeration and air-conditioning systems.

Conventional systems using gas as the working fluid include those that operate on what are called Brayton and Stirling cycles. (Modern Stirling systems include pulse-tube systems.) Thermoacoustic engines also use gas. Brayton cycles, which often use air as the working fluid, are employed in airplanes and modern European trains, where reliability and weight are more critical than cost and efficiency. Stirling system have been around for a long time and offer high efficiency by a high cost, and they have some reliability problems. They are widely used in low-temperature applications, such as gas liquefaction.

Thermo-acoustic engine
A Thermo-acoustic engine

Stirling and thermoacoustic engines both use sound energy. But while the former use sound in the form of traveling waves (see main text) and require large piston displacements, the latter use standing waves and need only small (or no) piston displacements. The compression and expansion processes occur at near-constant temperature in Stirling engines but not in thermoacoustic engines. Brayton systems circulate all of the working fluid from cold to hot temperatures, so the fluid experiences large pressure changes, and expensive equipment is needed to achieve efficient compression and expansion processes. In thermoacoustic engines, each fluid element operates between temperature and pressure ratios that are much lower, making compression and expansion processes simpler and more efficient.


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