Study of the spray characteristics of a diesel surrogate for diesel engines under sub/supercritical states injected into atmospheric environment

•Liquid and vapor phase spray of heptane from sub- to supercritical states were compared.•Images of both overall and near-field spray were studied and quantitatively analyzed.•Fuel temperature leads to diverse jet paths, resulting in different spray structure.•Fuel pressure and orifice rather than temperature affect shock at supercritical conditions.•A new coefficient was proposed of dimensionless Mach disk diameter on pressure ratio.