Characteristics Of Oscillating Flows And Effects On Heat And Mass Transfer Of Key Components In Stirling Cycles

Stirling engines can be applied in many fields,such as renewable energy,distributed energy system and submarines.Oscillating flows are the basic characteristics of Stirling cycle.Due to the lack of understanding of oscillating flows,the existing analysis models of Stirling cycle are based on the flow and heat transfer characteristics of steady flow.It is very difficult to describe the flow and heat transfer characteristics in the Stirling cycle and the mechanism of work-heat conversion accurately by those analysis models and the deviations of those analysis models are usually vey large and the applicability is limited.It is necessary to have a systematic study of the characteristics of oscillating flows and theirs effects on heat and mass transfer of key components in Stirling cycles,which can be used to provide guidance for the improvement of those analysis models.Many of the studies about the oscillating flows in the past are based on the assumption of incompressible flows,which are different from that in real Stirling engines,whose flow and heat transfer characteristics are affected by the expansion and compression of gas.A combined experimental and simulation study has been carried out and the effects of the expansion and compression of gas on the axial and radial flow field have been discussed.For the axial flow field,phase differences exist between the velocities of different cross sections and also between the velocities and pressures at the same cross section,which cause the variation of the amplitude and crank angle of mass flow rates.The parameter,cycle rate S is first proposed,which is definded as the ratio of the cycled gas mass to the swept gas mass by the pistons.It is found that the cycle rate varies with the volume Vmon(between the measured cross section and the inlet of expansion cylinder)as a parabola.The similarity parameters for the incompressible oscillating flow have been modified based on this parameter.For the radial flow field,the time when the "velocity annular" occurs,duration and intensity of the "velocity annular" are all affected by the expansion and compression of gas.The maximum dimensionless velocity,which is defined as the ratio of the maximum point velocity to the average velocity of the cross section,is proposed as a characteristic parameter to analyze the effect of expansion and compression of gas on the time when the "velocity annular" occurs at different cross sections.The intensity and duration of the "velocity annular" are larger than that of incompressible oscillating flow by 2.00-85.38%and 4?-49.7%under the same average kinetic Reynolds number(738.6?754.8).The effect of pressure drop of regenerator on the oscillating flows has been investigated.It is found that the deviation of friction factor is 7.11?25.84%between the steady and oscillating flows.The relative deviations of maximum and cycle averaged pressure drop are found to be less than 7.3%and 9.6%when the correlation equation of steady flow for friction factor is applied in the model of oscillating flows.Nonetheless,it should be noted that the pressure drop of regenerator can affect the mass flow rates in oscillating flows significantly,which is always neglected in many analysis models of Stirling cycle.The results show that the pressure drop can be neglected only when the dimensionless pressure drop of oscillating flow is less than4.22×10-2.The pressure drop of regenerator can cause the decrease of amplitude of mass flow rates and the cycle rate through regenerator rapidly.A correlation equation of friction factor of regenerator for compressible oscillating flow has been proposedbased on the modified dimensionless fluid displacement and kinectic Reynolds number.There is phase difference between the flow in the regenerator and the movement of piston with the effect of pressur drop,and a correlation equation for the dimensionless phase difference has been proposed based on the modified dimensionless fluid displacement,the ratio of the length of the regenerator to the hydraulic diameter and kinectic Reynolds number.In addition,the pressure drop of regenerator would also affect the flow inside other heat exchangers between the cylinder and regenerator,the mass flow rates at different cross sections can be estimated by the proposed equations.Above knowable,the characteristics of oscillating flows are all related with cycle rate,kinectic Reynolds number and dimensionless fluid displacement,thus a quantitative method can be proposed to describe the flow characteristics of oscillating flows.The characteristics of heat and mass transfer of the heater and cooler in a Stirling cycle have been investigated.It is found that the transition of the heat transfer process(heating process-cooling process or cooling process-heating process)and the phase difference of the heat transfer along the tube are the main reason for the occurrence of"temperature annular".The unsteady heat transfer characteristics have significant effect on the heat transfer of the cycle.For the cooler,the cycle averaged Nu of reverse flow in the deceleration period is higher than that of the steady flow by 12.79?20.47%,while the cycle averaged Nu in the acceleration period is lower than that of the steady flow by 1.18?6.17%.In general,the cycle averaged Nu of the oscillating flow are higher than that of the steady flow by-1.62?5.71%and 0.31?6.17%in the heating process and cooling process,respectively.The Simple analysis method has been built based on the data and correlation equations obtained in this study.The data of GPU-3 Stilring engine has been used to verify this model.The result shows that the accuracy of this model is improved and its applicability is extended greatly.Under the measured working condition(He,2.76MPa,4.41MPa,5.52MPa,6.9MPa),the maximum relative error of the indicated power is decreased from 59%to 23.15-30.85%and the maximum relative error of thermal efficiency is decreased from 38%to 15.52?16.05%.