Enthalpy Change during Throttling

As long as kinetic and gravitational potential energy effects are negligible and the flow is at steady state, the enthalpy change will be zero. Your equation already told you that. If there are constricted passages, or even if the pipe is of constant diameter (so that there is a viscous pressure loss), a liquid will typically heat up. This is because enthalpy is a function of temperature and pressure, so if the pressure decreases (say for an incompressible liquid), the temperature must increase.

In the case of a gas, the situation is a little different. When you have a nozzle, there can be changes in kinetic energy of the gas. So, if the kinetic energy increases, the enthalpy can decrease. But in the case of a highly restricted flow, such as a porous plug or a constricted valve, the kinetic energy changes are typically going to be negligible, and the enthalpy will be constant through the device. But here is where non-ideal gas effects come into play. For an ideal gas, if the enthalpy is constant, the temperature is constant. For a real gas, the enthalpy is a function both of temperature and pressure. So even at constant enthalpy, the temperature can change. Depending on the deviation of the gas from ideal behavior and the specific operating pressures, the gas temperature can either increase of decrease. This is the Joule-Thomson effect.