Haimin Wang

Insulating oil possesses high specific heat capacity and desirable non-flammability. In this study, thermal runaway was triggered by overheating a Cr20Ni80 resistance wire wrapped around the surface of cylindrical high-nickel batteries, which were completely immersed in insulating oil. Two battery orientations were investigated: horizontal and vertical. Compared with natural convection cooling, immersion cooling in insulating oil exhibited significant advantages. During thermal runaway, the positive electrode temperature, negative electrode temperature, and voltage underwent four distinct stages. Notably, relative to the vertical orientation, the horizontal orientation delayed thermal runaway onset time by 30.46% and prolonged the interval from the first voltage drop to thermal runaway initiation by 20%. Furthermore, the oil temperature difference between the positive and negative electrodes under horizontal orientation was substantially smaller than that under vertical orientation, with a maximum difference of 104.2ºC. Under both immersion cooling configurations, no smoke or flame was observed during Stages I and II, whereas mild gas release occurred in Stage III. However, during Stage IV, a striking difference emerged: the vertical orientation exhibited violent flame ejection at a 68º angle, while no such ejection was observed under the horizontal orientation. In addition, the vertical orientation caused the most severe structural damage, characterized by complete carbonization. Therefore, the horizontal orientation combined with complete immersion in insulating oil is considered the most suitable cooling configuration for high-nickel batteries.

Vapor injection (VPI) can significantly enhance the heating performance of electric vehicle (EV) heat pump systems under low ambient temperatures, making the integrated design and control of the VPI loop essential. This study uses R290 as the working fluid and investigates the gas–liquid flow characteristics of the vapor-injection electronic expansion valve (VPI-EXV) in the VPI loop. The evaporation coefficient in the Lee model is calibrated using four typical operating conditions, keeping the relative errors of both total mass flow rate and injection ratio predictions within 10%. Results show that valve opening is the dominant factor: as the opening increases from 10% to 100%, the injection ratio rises from 0.24 to 0.83, while increasing outlet pressure from 0.58 MPa to 0.78 MPa and inlet subcooling from 0 °C to 10 °C reduces it by about 18% and 9%, respectively. The 90° turning structure inside the VPI-EXV induces recirculation and high turbulent kinetic energy downstream of the throttling region, modifying the outlet gas-liquid distribution, based on which an injection ratio control strategy with valve opening as the primary variable is proposed.

A novel distributed electric resistance model is used to describe the heat generation characteristics of a cell with the electric resistance of a cell is divided into three parts: the main body, two current collectors, and two electrode tabs. This model was used to simulate the effects of natural convection cooling and forced air convection cooling on tabs of a lithium-ion pouch battery, and the effects of dry air and wet air on the cooling effect. The parameter of the module cooling coefficient (MCC) was introduced to evaluate the effect of thermal management. The results show that wet air has a better cooling effect than dry air, but the cooling effect of wet air with 15% and 75% relative humidity is almost the same, and increasing the inlet air velocity can significantly improve the cooling effect. In addition, the index MCC of positive and negative tabs and the heat flux during cooling process are quantified, so as to provide better service for thermal management design.