The reduction of oxygen consumption is a key factor to improve the energy density of underwater Stirling engine. A series of fundamental experiments are carried out to elucidate the spray characteristics of soybean oil/2,5-dimethylfuran (DMF) blended fuel in an underwater Stirling engine. Spray characteristics such as spray penetration, spray angle, spray area, and light intensity level under low injection and ambient pressures are obtained using image post-processing method. The results show that the effects of injection pressure, ambient pressure, and nozzle diameter on the transient spray characteristics of underwater Stirling engine are similar to those of diesel engine. However, in the steady spray process, the injection pressure has little effect on spray near angle, and the spray far angle increases with the increase of the injection pressure. Compared with the spray far angle at injection pressure of 3 MPa, the spray far angle at 5 MPa and 7 MPa increased by 11.38% and 18.14% respectively. The addition of DMF can obviously improve the atomization of soybean oil/DMF blended fuel. The spray angle of blended fuel in transient process increases with the increase of the DMF concentration. The spray near angle has exceeded that of diesel (46.21◦) when the DMF volume fraction exceeds 25%. The spray far angle is equivalent to that of diesel when the DMF volume fraction reaches 75%. Moreover, the spray with gas ejection no longer keeps conical, the droplet diameter distribution is more dispersed, and the droplet diameter is smaller.
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