Research On The Flow And Heat Transfer Mechanism Of Cryogenic Fluid In The Micro/mini-channel Of Combined Cycle Engine Precooler

The paper carries out researches with precooled combined cycle engine and its precooler.The validation of engine scheme has been completed and the effect of precooler is evaluated.Then the flow and heat transfer characteristics of coolant in mini-tube of precooler is carried out,which is helpful for the design.The paper carries out modeling and performance calculation for the deeply precooled combined cycle engine.The selected engine is Synergistic Air-Breathing Rocket Engine(SABRE).A steady component-level model of the engine was constructed.The rule of performance variation of engine in the air-breathing mode along the flight trajectory was obtained,and the effect of precooler on engine performance was studied.It is indicated that the calculations are of high reliability with thrust error less than 6%.The model can accurately calculate performance of SABRE under the air-breathing mode;SABRE is characterized by both large thrust as rocket engines and high specific impulse as aircraft engines,and its specific impulse is between 28000~44000m/s;the helium loop needs to be equipped with a pressure reducing valve(PRV)in order to maintain a pressure balance;by reducing temperature of inlet air through the precooler,the engine working range can be widened to 25 km and Ma5 to achieve hypersonic flight.Exergy efficiency analysis based on the model was carried out,and equivalence ratio was determined to be the main influencing factor of cycle,then the optimization method was proposed.The results indicate cycle efficiency of SABRE is between 29.7%~41.7% along the flight trajectory,and most of the wasted exergy is occupied by the unburned hydrogen in exit gas.Exergy loss exists in each engine component,and the sum losses of main combustion chamber(CC),pre-burner(PB),precooler(PC)and 3# heat exchanger(HX3)are greater than 71.3% of the total loss.The performance of PC is very important for the engine,and two cycle optimization solutions have been proposed.The helium loop adjustment is to increase the outlet temperature of PC,while the helium recycling scheme is to increase local coolant flow rate of PC.Both the schemes can improve the heat exchange performance of PC,and increase the cycle efficiency by 3% and 5.6%,respectively.The structural parameters and performance calculation models of PC have an impact on the engine.In particular,the flow and heat transfer model in the tubes affect the boundary of working range.An accurate flow and heat transfer model is needed and it is necessary to carry out the research of cryogenic fluid in mini-channels.The flow and heat transfer characteristics of cryogenic fluid in mini-tube of precooler is investigated with experiment,numerical simulation and theory analysis.The experimental system for flow and heat transfer characteristics of cryogenic fluid is founded.The system decreases the atmosphere heat flux through vacuum and decreasing temperature.The single-phase flow and heat transfer characteristics of liquid nitrogen in minichannel are affected by heat flux,mass flow,and tube diameter.When the wall roughness is small,the conventional channel prediction can be basically used.When the roughness is large,the Colebrook and Gnielinski correlations have a high prediction accuracy in which the roughness is taken into account.The pressure drop characteristics of two-phase flow of nitrogen can be predicted by the homogeneous model.With the increase of heat flux,the boiling heat transfer coefficient increases first and then decreases.The increase of mass flux and inlet pressure is beneficial to improve the heat transfer coefficient;and with the smaller tube comes the larger heat transfer coefficient.The boiling heat transfer of liquid nitrogen can be divided into two regions: saturated boiling region and heat transfer deteriorating region,with critical mass quality of 0.15.In the saturated boiling region,the heat transfer coefficient gradually decreases with the increase of the mass quality,and the nucleate boiling is the dominant heat transfer mode at low mass quality.With the increase of the mass quality,the liquid film gradually evaporates and The dominant heat transfer mode is membrane boiling.The Tran predictive correlation is more suitable for prediction of nitrogen flow boiling heat transfer coefficient in mini-channel,and the average prediction accuracy of boiling heat transfer coefficient for 0.5mm and 1.2mm channels is 35% and 45% respectively.The growth of bubble is controlled by heating effect mainly,it mains sphere in the prophase and the diameter increases linearly with time until the channel size play roles.The confinement of wall makes the bubble expand upstream and downstream.The bubble growth velocity will be increased by increasing the superheated degree,decreasing the system pressure,inlet velocity or tube diameter.The higher bubble growth frequency,the easier two adjacent bubble coalescent,and the elongated bubble is effect of bubble growth and coalescent.When the bubble growth frequency is low,the bubble coalescent disappears and the bubble grows independently.