Heat Exchanger is a device that transfers heat from one medium to another, without allowing them to mix. This transfer of heat is used to cool gases.
Example: The condenser is a heat exchanger that is used in refrigeration process, which removes heat from the hot gas and releases it to a heat sink. The removal of heat from the hot gas causes it to condense to a liquid.
Dufour Effect is best defined as the transport of heat due to the gradients in concentrations of species.
Joule-Thomson Effect is defined as the decrease in temperature which takes place when a gas expands through a throttling device such as a nozzle.
Example: Here in the animation, the gas, which is at a higher temperature is compressed by the piston such that the gas is allowed to expand through a throttle valve. This expansion results in the decrease in the temperature of the gas, that is, the cooling of the gas takes place.
Ranque Effect states that, whenever any mass of gas, whatever its initial density and temperature, is caused to rotate axially, via exogenous and/or endogenous effects, it will gradually cool in the area near the axis of rotation and will warm in the external areas. This was applied in Vortex tube refrigeration. This was discovered by French physicist Georges Ranque.
Example: The animation is of a Vortex tube. The basic working of a vortex tube is that the compressed air is supplied to the vortex tube and the air passes through nozzles that are tangent to an internal counterbore. These nozzles set the air in a vortex motion. This spinning stream of air turns 90° and passes down the hot tube in the form of a spinning shell, similar to a tornado. A valve at one end of the tube allows some of the warmed air to escape. What does not escape, heads back down the tube as a second vortex inside the low-pressure area of the larger vortex. This inner vortex loses heat and exhausts through the other end as cold air.
Conduction is defined as the communication of heat from one body to another, when they are in contact, or through a homogenous body, from particle to particle. This transfer of heat is used in cooling of gas.
Example: In order to reduce the temperature inside a room, an air-conditioner is used. This results in cooling of the entire room by transferring the coolness to the air near it, which in turn transfers to the molecules next to it. This transfer takes place till the entire room reaches the same temperature. This idea has been illustrated with an animation.
Rarefaction can be defined as the decrease in density and pressure in a medium, such as air, caused by the passage of a sound wave.
Example: The expansion tube built at Stanford is designed to accelerate a reactive test gas mixture to Mach 4-7. The test gas is initially shocked to an intermediate velocity and temperature (limited to about 1000 K to avoid autoignition) in the driven section. When the primary shock reaches the expansion section, it initiates an unsteady rarefaction which simultaneously accelerates and cools the test gas.
Thermoacoustics is the study of the thermoacoustic effect which states that, a sound wave heats and cools small parcels of gas along the length of its propagation. This effect is used in refrigeration.
Example: The animation shows that as the sound wave travels back and forth in the tube, the gas compresses and expands. When the gas compresses it heats up and when it expands it cools off.