Research On High-Altitude Flow And Heat Transfer Characteristics And Enhancement Of Air-Cooled Heat Exchangers For Aviation Piston Engines

With the accelerated development of China’s unmanned intelligent combat forces,it is imperative to strongly promote the pre-research and batch production process of military high-altitude long-endurance piston UAVs,of which,achieving a cruising altitude of more than 10,000meters is one of the primary development directions for the high-altitude performance of aviation piston engines.Multi-stage turbocharging technology is the key technical means to improve the high-altitude output and lifting limits for all types of aviation piston aircraft,but it brings a significant increase in the thermal load of the engine system,and the harsh high-altitude environment further exacerbates the difficulty of thermal management of aviation piston engines.It is urgent to develop efficient thermal management technology for aviation piston engines at cruising altitudes above 10,000 meters.Among them,the heat exchanger is the main energy deployment carrier in the thermal management system,and the investigation of its high-altitude performance is one of the basic scientific problems of the thermal management system,and the enhancement of its performance in low-temperature and low-pressure environment is the key technology of the aero-piston engine that needs to be broken through.In order to promote the development of heat exchanger and even thermal management system of aero-piston engine,this paper mainly focuses on the current situation that the mechanism of high-altitude flow and heat transfer and the change of heat exchanger performance with altitude are unknown,and carries out the research on the mechanism of influence of altitude increase on its flow heat transfer characteristics,and develops the performance enhancement means for single heat exchanger and tandem heat transfer module with the purpose of improving the performance at high altitude,and carries out the research on the effect of different structuring parameters on their strengthening effect with altitude variation was studied.The main research contents and conclusions of this paper include:（1）Study on key influencing parameters of high-altitude convective heat transfer and numerical simulation methods of heat exchangers.The theoretical analysis reveals that turbulent-laminar transitions occur in the flow boundary layer during the elevation process,thus increasing the thickness of the flow boundary layer and reducing the heat transfer effect,and the parameters in the thermal boundary layer are also directly influenced by the atmospheric physical parameters that change with altitude.Subsequently,a multi-scale coupling simulation method of heat exchanger is proposed,which does not rely on large amounts of high-altitude test data.Based on this method,the corresponding simulation model is constructed.The simulation results are in good agreement with the existing test data,and the errors are within a reasonable range,which makes up for the shortcomings of the existing methods such as inapplicability at high altitude,low precision and long calculation time.（2）Study on the influence mechanism and high-altitude performance prediction of an air-cooled intercooler.Simulation calculations were carried out for an air-cooled intercooler under two cruise states of constant and iso-thrust correction speed,and the change law and interaction mechanism of each performance index with the increase of altitude were compared and analyzed.It was found that no matter what the cruise state was,the overall heat transfer of the intercooler would first increase and then decrease with the increase of altitude,and the turning point occurred at the altitude of 11km.The maximum heat transfer rate under constant speed and iso-thrust correction speed are 36.39%and 50.11%higher than that on the ground condition,respectively.Further exploring the key factors for the power recovery of the heat exchanger at high altitude,it can be seen that only under iso-thrust correction speed,the maximum heat transfer at any altitude is higher than that on the ground,and the high-altitude effectiveness recovery coefficientk_εndecreases first and then increases with altitude.The local effectiveness ε_n only needs to reach 56.6%of the ground effectivenessε₀ at 11～16km and 74.9%of the ground effectivenessε₀ at 20km to complete the power restoration.（3）Study on the performance enhancement of a single heat exchanger based on intake enhancement.It is found that the presence of the front convergent guiding component in the finite space nacelle obviously strengthens the heat transfer performance of the heat exchanger unit,and the enhancement of heat transfer rate increases with the altitude,up to 34.9%at 20km above sea level,and the optimal included angles between convergent guiding and flow normal direction is 30° below 11km and 45° above 11km,and its optimal clearance height range from the upper surface of the heat exchanger is 7～9mm with the altitude,and the interval is gradually reduced.（4）Study on interaction mechanism and strengthening of high-altitude flow and heat transfer performence of double heat exchangers.It is found that the rear heat exchanger unit has no significant effect on the front unit,but the enhancement of the heat transfer process of the front heat exchange unit will directly reduce the heat transfer rate of the latter,and the degree of influence expands with the elevation.The mid-connected guiding component can increase the internal flow velocity of each unit,but reduce the overall effective cooling flow rate.Only when the altitude is lower than 13km,the strengthening effect can increase the overall heat transfer of the series module by 7.15%at most,and the front convergent guiding component can increase the overall heat transfer of the series module by 20.7%with the gap height of 4mm.And under this arrangement,there is still an optimal gap height range determined by both the front and rear units,which is 0～13mm.The simulation data applied to the altitude of 0～20km show that the performance enhancement method of the series heat exchange module using the mid-connected and the front rear guiding component respectively is more suitable when the altitude of less than 13km and 15km.