This work is to investigate the internal flow pattern and macroscopic spray characteristics of R134a discharged through twin-orifice atomizers. Two inlet orifices with the diameters of 0.6 mm and 3.0 mm were subsequently implemented and connected to the supply line of R134a to find the influence of the inlet orifice geometry on flow and spray characteristics. Transparent atomizers were manufactured to study the effects of different diameters of discharge nozzles on the internal flow and atomization. An injection pressure of 6.0 MPa was operated to produce "stable" flash-boiling sprays vertically, and the ambient condition was atmospheric. Internal flow in the expansion chamber and external liquid atomization were observed simultaneously during the actuation. The relationship between the internal boiling flow in the expansion chamber and external flash-boiling spray were analyzed. A "sudden" gas-liquid transition was observed when the inlet/discharge orifice ratio is one. This work provides a guide and reference for the volatile or liquefied material discharged through a twin-orifice atomizer. (C) 2015 Elsevier Ltd. All rights reserved.