Ethanol drop impingement on ultracold surfaces under low-temperature cold-start conditions of engines

Abstract

Spray impingement on ultracold surfaces is a key factor that affects the cold start performance of an internal combustion engine in some high latitudes and high altitudes regions, whose environment temperature could reach −40 °C. Ethanol is selected as the test liquid considering the real fuel properties in engines, and the specific drop splashing and the film spreading process on ultracold surfaces are investigated comprehensively in this work. Splashing process becomes more severe on the surfaces at −30 °C, accompanied by more and faster secondary droplets produced during the impingement. The film spreading process does not differ significantly on the surfaces of different temperatures during the early spreading phase (τ≤1). An improved dimensionless spreading factor βn≡D/D0/Re0.2 is proposed to describe the spreading dynamics in this stage, and a better convergence is achieved among the comprehensive experiments regarding different kinds of liquids and surface temperatures. In the later spreading phase, liquid film spreads faster on warmer surfaces and retracts slightly after reaching its maximum spreading factor βmax on a room-temperature surface. However, the liquid film on cold surfaces will keep stretching for a much longer time and finally obtains a larger βmax. An improved model is established to predict the βmax under the comprehensive conditions. Specific assumption and treatment are applied in the model considering the operating conditions of the internal combustion engines. The model provides a good prediction of the maximum spreading factors under different surface temperatures.