| Kerosene has been widely used as coolant to effectively cool airborne equipment and aero-engine to ensure that the aircraft can safely work.In cooling channels,kerosene absorbs lots of heat and the temperature increases gradually.Consequently,dissolved oxygen and other trace components can format deposition via chain radical reactions,i.e.thermal oxidation deposition.Thermal oxidation deposition process is coupled with fuel flow and heat transfer,which shows great influence on flight safety.Therefore,more attention should be paid to the complex process to study its dynamic behavior.In addition,the influence mechanism of many factors should be analyzed.Moreover,the effect of deposition on heat transfer of kerosene in cooling channels can be studied.Therefore,the following studies have been conducted in this work.The dynamic deposition behavior is captured by in-situ heating experimental system in the temperature range of 623-723 K.In thermal oxidation deposition process,dissolved oxygen reacts with trace components to form insoluble substances.Rough metal surface has many dents,which provides catalytic active sites and lengthens residence time.Hence,the initial surface deposition is formed.Deposition mainly develops via accumulation rather than covering the plate.In addition,the long-term deposition process can consume surface metal.The macromolecular substance of deposition triggers a series dehydrogenation and deoxidation reactions.Thereby,deposition become loose,and some deposition exfoliates.Thus,the height decreases in this long term process.In cooling channel,fuel temperature increases from room temperature to 750 K.Consequently,deposition growth proceeded orderly in five morphological steps of small particle,large particle,cluster,block and dented block deposition.Then,the pseudo-detailed mechanism is validated based on the thermophysical properties of kerosene and the dynamic deposition behavior.The formation of deposition mainly depends on space insoluble,indicating the reliability pseudo-detailed chemical kinetic mechanism.In this process,the decomposition of hydroperoxide is crucial.The higher decomposition rate can promote deposition formation and enlarge deposition region.Based on the pseudo-detailed mechanism,the influencing mechanism of dominant factors to thermal oxidation deposition in cooling channel is analyzed.In cooling channels,deposition precursor is firstly formed and consumed in sublayer near the wall.Fuel temperature dominates thermal oxidation deposition process,and the higher temperature can significantly improve deposition rate.Complex physical factors primarily affect the following three sub-process: chemical reaction,reactant supply,and reactant diffusion.Hence,the deposition process can be divided into three types: chemical reaction controlled,reactant supply controlled,and reactant diffusion controlled.When the fuel temperature is lower,the processed is controlled by chemical reaction.If there is a deposition rate peak in cooling channel or the consumption proportion of precursor exceeds(1-1/e),the deposition process is controlled by reactant supply.Otherwise,the process is controlled by reactant diffusion.For the reactant supply controlled process,total deposition amount increases linearly with mass flow rate,and there is a function relationship between deposition rate and inlet Reynolds number.When the process is controlled by reactant diffusion,total deposition amount increases slightly.The effect of thermal oxidation deposition on heat transfer is studied in practically used serpentine cooling channels.In serpentine cooling channels,the heat transfer coefficients of kerosene increase 3.5-56.7%,and the deposition amounts decrease 4.8-24.8%.Moreover,the heat transfer performance is further enhanced and the deposition amount is further reduced with increase in number of bending section.The strong secondary flow,triggered by bending section,can effectively enhance heat transfer.In addition,serpentine tube can reduce the heat transfer deterioration caused by entrance effect in tube inlet and drastic thermophysical properties variations in supercritical temperature range.Thermophysical properties variations and centrifugal force affect the heat transfer behavior and deposition characteristics in essence.In deposition process,liquid-space coking particle can influence heat transfer by affecting the thermophysical properties.Under higher particle concentration,the heat transfer can be improved.In practical,the concentration of liquid-space particle is lower,having a slight negative effect on heat transfer.On the contrary,surface deposition shoes greater effect on fuel heat transfer.At larger mass flow rate or higher system pressure,the heat transfer performance of kerosene decreases significantly after deposition formation.A lower system pressure can slightly suppress the surface deposition formation,but the deposition structure was denser which shows a greater negative effect on heat transfer.Therefore,considering the heat transfer performance before and after surface deposition formation,a relatively lower supercritical pressure is more preferable for the effective long-term thermal protection. |
