Numerical simulation of fuel flow and heat transfer in a serpentine tube considering the fuel variable properties
[Objective]Air-fuel heat exchangers consisting of serpentine tubes are used to cool aero-engines,whose turbine blades and other hot parts are generally cooled by the air taken from the compressor outlet,which may have temperatures as high as 900 K.A practical approach uses engine fuel,usually at a temperature close to the normal atmospheric temperature,to precool the air taken from the compressor outlet to improve its cooling capacity.This process can be realized in air-fuel heat exchangers.This work aims to analyze the fuel flow and heat transfer in a serpentine tube and explore the influence of fuel inlet velocity,tube wall temperature,and straight section length of the serpentine tube on the flow and heat transfer characteristics,with an emphasis on the differences in such characteristics between the straight and curved tube segments of the serpentine tube and between the constant and variable fuel properties.[Methods]Considering heat exchanger compactness and the possible coking that may take place during the flow of fuel through the heat exchanger,the inside diameter of the serpentine tube is set to be 2.0 mm.To arrange the heat exchanger in the annular space between the combustion chamber wall and the main shaft of the aero-engine,the straight section length of the serpentine tube is set to be 65.000 mm.The serpentine tube is composed of 14 straight and 13 curved tube segments,constituting approximately 7 cycles,provided that 1 cycle is defined to include 2 straight and 2 curved tube segments.When the fuel flows through the serpentine tube,its temperature may increase by several hundred degrees Kelvin.This aspect promptes the consideration of fuel variable properties in the simulation model.The uniqueness of this work lies in the fact that it deals with variable property fuel flow and heat transfer in a thin serpentine tube.The low Reynolds number k-ω flow model is employed in the simulations,and calculations are implemented for fuel entering velocities of 1,2,3,4,and5m/s,tube wall temperatures of 450,600,750,and 900 K,and serpentine tube straight section lengths of 65.000,32.500,and 0 mm for both constant and variable fuel properties.The inlet fuel temperature is 350 K,whereas the pressure is 5 MPa.[Results]The calculation results revealed that the fuel temperature increased along the serpentine tube for both constant and variable fuel properties but increased more rapidly for variable properties than for constant properties.The fuel velocity remained constant for the constant property but varied nonlinearly for variable properties.The convective heat transfer coefficient remained almost constant for the constant property but exhibited a remarkable increase along the tube for variable properties.Moreover,the curved tube section exhibited a markedly larger convective heat transfer coefficient than the straight tube section.The variable property experienced a noticeably smaller pressure drop than the constant property,similar to the pressure drop in the curved tube section compared to the straight tube section;nevertheless,the pressure drop per unit tube length was considerably larger for the curved tube section than for the straight tube section.The tube wall temperature had a remarkable impact on fuel flow and heat transfer characteristics for the variable properties,whereas the straight tube segment length had a relatively weak influence on such characteristics.[Conclusions]The findings of this study support the fact that neglecting the variable fuel properties leads to an underestimation of the convective heat transfer coefficient and an overestimation of the pressure loss in the serpentine tubes used in air-fuel heat exchangers.
serpentine tubejet fuelvariable propertyflow and heat transfernumerical simulation
万方数据
维普
