A gas is allowed to expand from a high pressure to low pressure adiabatically through a narrow opening, the cooling effect of gas takes place.  It’s called ____?

When a gas is subjected to unrestricted expansion adiabatically, it undergoes cooling due to overcoming of attractive forces which were prevalent under high pressure. This is called Joule-Thompson effect. Gases like hydrogen and helium undergo heating as little or almost no inter molecular forces are present in them.