Numerical And Experimental Study On A 3-stage Looped Traveling-wave Thermoacoustic Engine

With the rapid development of distributed energy systems and the utilization of renewable energy,efficient cascade utilization technology has become a heated topic.Especially,there is a huge potential for the energy recovery of intermediate temperature region.The traveling-wave thermoacoustic engine has many advantages such as high efficiency,simple structure,and low cost.Particularly,the multi-stage looped structure has lower onset temperature difference and high energy density,which is a very promising technology in renewable energy recovery of a wide temperature range.However,the non-linear characteristics of the thermoacoustic engine and the mechanisms of thermoacoustic conversion still remain obscure.The commonly used linear thermoacoustic theory lacks the features of the time-domain dynamic process and non-linear characteristics.Methods like CFD are also limited due to its high demand for computing space and time.To better reveal the self-excited oscillation mechanism and non-linear phenomena in the thennoacoustic engine,numerical and experimental studies are carried out to investigate the operating characteristics of the 3-stage looped traveling-wave thermoacoustic engine in this paper.The details are as follows:1.Based on the basic governing equations of fluids,a one-dimensional unsteady model(1DUM)of the 3-stage looped traveling-wave thermoacoustic engine is developed by using MATLAB programming.Simulation result shows that 1DUM is capable of predicting the complete evolution of pressure wave from a static initial condition because the axial conduction of gas is considered in this model.Comparisons of different initial disturbance are made and it's proved that different disturbance do not affect the growth curves and steady-state results.The high-order harmonics in pressure wave and the gas temperature fluctuations in the regenerator are also obtained in the simulation,which is verified by experiment and publications;2.A 3-stage looped traveling-wave thermoacoustic engine is designed and built.The comparisons of the oscillation frequency and peak-to-peak of pressure between the experiment,1DUM simulation,and DeltaEC simulation,later are made under different charging pressures and temperature differences.The results show that 1DUM provides a more accurate frequency prediction with a maximum error of 0.4 Hz in the tested range.Strong jet streaming appears in the experiment,causing the difference between the simulation and experiment.In addition,the correlations of flow and heat exchange are responsible for the large difference between peak-to-peak amplitudes of pressure calculated by DeltaEC and 1DUM,for example,when the engine is charged with 1 MPa Nitrogen gas with a temperature difference of 400 K,the error between simulations and the experiment can reach 62%for DeltaEC and 118%for 1DUM.Last,the distribution of the acoustic field characteristics calculated by 1DUM is consistent with the simulation results by DeltaEC with a maximum error of phase angle less than 1°,despite there are some differences of the peak-to-peak amplitudes of pressure and volume flow rate.More amendments of the correlations for the oscillating flow and heat transfer are needed to achieve higher accuracy.Since 1DUM doesn't require the empirical setting of initials,the process of modeling is greatly simplified and it's suitable for complex thermoacoustic system modeling;3.Based on the experimental system,the research of onset and damping characteristics is carried out.With charging pressure of 4.0 MPa and nitrogen as the working gas,the temperature difference of onset is about 44 K.Also,noticeable thermal hysteresis phenomena appear in the experiment.By using acoustic RC load,the primary investigation on the output characteristics of the 3-stage looped traveling-wave thermoacoustic engine was carried out.The result shows that optimization of load impedance is crucial for the output performance.When the engine is initially charged with Nitrogen gas of 3 MPa and the heat source temperature is 543 K,the maximum sound power gained by experiment is 78.3 W and the maximum relative Carnot efficiency is about 2.2%.Strong nonlinear streaming is found in experiments,causing huge energy loss and low efficiency.Further work will be focused on streaming inhibition and reducing heat loss.