What is the role of the guide vane in an expander? Can it generate cold? What is the difference between it and isenthalpic throttling? Please give me some advice!

It mainly involves changing the direction of flow. The most commonly used turbine expanders at present are of the radial-axial type, meaning that the airflow flows radially from the outer edge of the impeller toward the axis before turning axial for exhaust. The direction in which the airflow enters the impeller should preferably be along the direction of the blades. In this way, the impact loss of the airflow on the blades is minimized, which is known as shock-free intake. However, since the impeller itself is in rotational motion – that is, the blades have a rotational circumferential velocity – if the airflow exiting the diffuser flows toward the axis, the actual direction of the airflow entering the impeller is inclined backward with respect to the impeller. Just as falling raindrops on the windows of a moving train do not fall vertically downward but at an angle backward. Therefore, the actual direction of the airflow exiting the deflector should be inclined in the direction of the impeller’s rotation, that is, it should form a certain angle with the circumferential velocity of the rotating impeller. In this way, when the impeller rotates, as long as the circumferential velocity and the absolute velocity are properly matched, it is possible to ensure that the airflow velocity relative to the blades flows radially along the blades.

As the gas flows through the guide, the increase in its kinetic energy is due to a decrease in its internal enthalpy. Is this a conversion between two different forms of energy within the gas? Is there no energy output? The total energy of the gas does not decrease. Therefore, it cannot be assumed that as the airflow passes through the deflector, a decrease in its internal enthalpy means that cold energy is generated. Because this high-speed airflow is not used behind the deflector to drive the impeller to perform work externally, aside from being used to overcome friction and impacts and thereby causing a decrease in pressure, the rest of this kinetic energy is converted into an increase in the gas’s enthalpy. This situation is the same as when air flow passes through a valve for throttling. At the valve, the flow velocity increases due to the narrow flow channel; after passing through the valve, the flow velocity decreases as the flow channel widens. The kinetic energy is then converted into an increase in enthalpy. Therefore, the enthalpy value remains constant before and after throttling. It can be seen that the deflector is merely intended to do external work on the gas in order to prepare for the generation of cold energy; it does not produce cold energy on its own. (How to understand the part in red?) ? ? ）

I don’t think it’s absolute; there is some energy loss in the process of gas flow. Energy loss represents a form of conversion, only this type of conversion accounts for a very small proportion of the total energy in the system, so it can be ignored in practical applications. That’s what I think

What is the difference between gas passing through a guide and gas passing through a throttle valve?

The effect of flow redirection on velocity is much smaller than that of throttling

A flow director is used to change the direction of fluid flow without altering its velocity. When gas passes through a throttle valve, its pressure increases and its velocity rises; after passing through the throttle valve, the fluid expands and vaporizes, absorbing heat in the process!

As the gas passes through the guide, which is essentially a nozzle (you can search for ‘Laval nozzle’ on Baidu), its velocity increases while its temperature and pressure decrease. The internal energy is converted into kinetic energy, resulting in a decrease in enthalpy. If it does not drive the impeller to perform work, its velocity will gradually decrease as kinetic energy turns back into internal energy, just like in a throttling process. In expanders, there is a concept of reaction degree, which is the enthalpy drop in the impeller divided by the total enthalpy drop, and it is roughly 0.5. This shows that the enthalpy drop is 50% at the diffuser stations and 100% at the impulse stations; it’s merely a difference in perspective between academic and practical approaches. After all, the gas behind the diffuser must exert some force on the impeller