Abstarct: Vortex Tube (VT) has been used in industries due to simplicity, low cost, high reliability in the production of hot and cold streams. The potential application of VT in natural gas industry and also the lack of studies on the use of natural gas as the working fluid make VT as a subject with significant importance. The main purpose of this thesis is to develop a high efficiency VT with natural gas as working fluid. The effects of main parameters such as dimensions of the various components, flow and thermal fields inside VT and thermophysical parameters of fluid flow on VT performance are investigated. This thesis could divided into three sections as: Experimental investigation, numerical study and application of VT in Natural Gas industry. Experimental investigation has been carried out to increase the efficiency of Vortex Tube. In experimental investigation, the effects of importance parameters such as: inlet pressure, cold orifice area, divergence angle of orifice and nozzle area have been investigated. Increase in pressure causes increase in momentum of inflow, resulting in better rotation of flow inside the vortex chamber and consequently better thermal separation. In addition, reduction in optimum cold mass fraction occurs. The orifice area as well as hot outlet area are balancing the mass flow rate. Increase in orifice area causes a higher amount of flow moves out from the cold outlet. Conversely, increasing the orifice area causes that the return cold flow from hot side mixes with inlet flow. Therefore, the determination of the optimum diameter for cold orifice has significant importance. Cold orifice angle has a similar effect as orifice area, but increasing this parameter cannot continually increase optimum cold mass fraction. By increasing orifice angle to optimum value of 4.1º, the cold mass fraction increases. This increase represents a further reduction of pressure in the cold side and therefore more flow passes from the cold side. But increase in angle more than the optimum value decreases the cold flow. It seems that diffuser shaped configuration of orifice is the reason of this phenomenon. The optimum experimental values for cold orifice diameter, divergence angle of the orifice and Nozzle Area Ratio are obtained 0.64, 4.1º and 0.14 respectively. Furthermore, to improve the performance of a conventional Vortex Tube, Annular Vortex Tube is introduced for the first time in this thesis. The flow after passing through the valve cone is not allowed to flow out from the hot exit, but again hot stream passes over the hot tube. The new design of VT improves cold and hot temperature differences around 5% and 2% respectively. In numerical analysis, the governing equations of momentum and energy in VT have been solved for compressible turbulent flow. The walls are insulated and no-slip condition is considered for the velocity on the walls. Boundary conditions for k and ɛ equations are turbulence intensity and hydraulic diameter. Discretization method for flow is second order upwind and discretization method for k and ɛ , is Quick scheme. Methane is considered as a real and ideal gas in numerical analysis. To compute Methane properties as a real gas, Redlich-Kwong equation of state is used as a User Defined Function in fluent. Comparing numerical values of axisymmetric and three-dimensional, it could be concluded that three-dimensional solution is approaching the axisymmetric solution as the number of nozzles is increasing. It was also observed that the cold mass fraction is a function of the output pressure of warm and cold sides and the ratio of the hot and cold output area. However, inlet pressure does not affect this cold mass fraction. Increase in cold outlet pressure causes cold mass fraction decreases. In addition, increase in hot outlet pressure causes the cold mass fraction increases. At the final part of the thesis, the application of VT in Natural Gas industry has been investigated. In one case, replacement of VT instead of expansion valve is investigated. This replacement decreases the energy consumption of City Gate Stations and increases LNG production of LNG liquefaction plants. To reduce energy demand of City Gate Stations in cold weather, VT is used with a geothermal system. This proposed system will save more than 90% of the consumed fuel. In addition, in production of LNG through a mini liquefaction plant, replacement of VT with expansion valve will increase the percentage of LNG by around three times.