The temperature change in a throttling process

[riquelme8]

Hi,

I'm writing a small student-paper on heat pumps, and I'm not at the point where i explain the physics behind it. This is probably a silly question, but there's something I can't seem to understand about the throttling process.

First off I get that the enthalpy is conserved through the throttling process, and that U_potential increases, causing the U_kinetic to decrease, which cause the liquid to cool. But this is my problem with the theory:

Let's say that the throttle is 2 cm long. If you look at the liquid as it goes into the first cm of the throttle, you decrease the volume of the liquid, causing the temperature to increase. And if you then look at the process where it moves on through the last part of the throttle, it'll increase it's volume, thereby decrease it's temperature. So why aren't the liquid at the same temperature as when it went into the throttle?

 

[andrien]

Ever heard of joule-thomson effect.which describes the temprature variation during throttling process.the reason for getting cooled or heated depends on joule- thomson coefficient and whether the gas temperature is above or below the inversion temperature.As for the physical mechanism I am quoting from wiki

As a gas expands, the average distance between molecules grows. Because of intermolecular attractive forces (see Van der Waals force), expansion causes an increase in the potential energy of the gas. If no external work is extracted in the process and no heat is transferred, the total energy of the gas remains the same because of the conservation of energy. The increase in potential energy thus implies a decrease in kinetic energy and therefore in temperature.
A second mechanism has the opposite effect. During gas molecule collisions, kinetic energy is temporarily converted into potential energy. As the average intermolecular distance increases, there is a drop in the number of collisions per time unit, which causes a decrease in average potential energy. Again, total energy is conserved, so this leads to an increase in kinetic energy (temperature). Below the Joule–Thomson inversion temperature, the former effect (work done internally against intermolecular attractive forces) dominates, and free expansion causes a decrease in temperature. Above the inversion temperature, gas molecules move faster and so collide more often, and the latter effect (reduced collisions causing a decrease in the average potential energy) dominates: Joule–Thomson expansion causes a temperature increase.

moreover the temperature variation is measured with respect to pressure to determine joule- thomson coefficient at constant enthalpy.