Throttling Process:

A throttling process is an adiabatic and irreversible process in which a fluid undergoes a rapid expansion or contraction through a small opening or valve without any heat transfer occurring. This process is also known as Joule-Thomson expansion or throttling expansion.

Constant Enthalpy:

In a throttling process, the parameter that remains constant is the enthalpy (option C). Enthalpy is defined as the sum of the internal energy and the product of pressure and volume of a system. It is represented by the symbol H.

Explanation:

During a throttling process, the fluid passes through a small constriction or valve, causing a sudden drop in pressure. As the fluid expands, its temperature also decreases due to the decrease in pressure. However, the enthalpy of the fluid remains constant throughout the process.

This can be understood by considering the Joule-Thomson coefficient (μ) of the fluid. The Joule-Thomson coefficient is a measure of how the temperature of a fluid changes when it undergoes a throttling process. It is given by the equation:

μ = (∂T/∂P)H

where ∂T/∂P represents the partial derivative of temperature with respect to pressure at constant enthalpy.

For an ideal gas, the Joule-Thomson coefficient is given by:

μ = 1/Cp

where Cp is the specific heat capacity at constant pressure.

Since enthalpy remains constant during a throttling process, the Joule-Thomson coefficient is directly related to the specific heat capacity at constant pressure. As a result, the temperature change during throttling depends on the specific heat capacity rather than the enthalpy.

Therefore, in a throttling process, while temperature and pressure may change, the enthalpy of the fluid remains constant.