Experimental Study On Thermoacoustic Engine Under The Coupling Effect Of Sound And Gravity Fields

Thermoacoustic engine is a new type of device which can convert thermal energy intosound energy. It has many advantages, such as simple construction, high reliability andnon-polution. Since thermoacoustic prime mover can work at a low temperature, it has broadapplication prospect on waste heat recovery and solar energy utilization. Solar poweredthermoacoustic engine takes the solar radiation as heat source. Its tilt angle is constantlychanging when it rotates with the sun. During the raotating process, thermoacoustic engineworks under the coupling effect of sound and gravity fields. And the flow and heat transferprocess in thermoacoustic core is complex. In this context, experimental study aboutthermoacoustic prime mover and thermoacoustic engine driven by loudspeaker were carriedout in this paper under multi-tilt angles, the flow and heat transfer characteristic inthermoacoustic engine under the coupling effect of sound and gravity fields was discussed.Firstly, a standing-wave thermoacoustic prime mover was built. Its basic operatecharacteristics such as onset and damping temperatures, frequency and pressure amplitudewere experimentally studied, and the onset process of thermoacoustic self-excited oscillationwas shown by frequency analysis. The temperature distribution within stacks during the onsetand damping processes were observed by thermal infrared imager. The experiment resultsshowed that thermal energy is not only converted into sound energy after onset, but also betaken to the cold end by oscillatory flow. Theroforce, the heat loss in thermoacoustic engineinduced by sound oscillation after onset is inevitable.Secondly, it was found that sound disturbance can effectively decrease the onsettemperature of thermoacoustic prime mover, which is benefical for low grade heat utilization.To provide experiment basis for the selection of adaptable disturbance signal in practice, theeffect of different disturbance signals on thermoacoustic forced oscillation was discussed.The experiment results showed that thermoacoustic forced oscillation has selectivity for thedisturbance frequency, the optimal disturbance frequency is the fundamental frequency ofthermoacoustic prime mover. Onset temperature of thermoacoustic forced oscillationdecreases with the increase of disturbance power. However it does not decrease any morewhen the lowest onset temperature reached. Onset temperature is the key factor to influence the pressure amplitude of thermoacoustic forced oscillation. The lower the onset temperatureis, the smaller the pressure amplitude is.Thirdly, to study the effect of tilt angle on the performance of thermoacoustic primemover, the onset temperature, damping temperature and temperature disturbation withinstacks under five tilt angles (45°,90°,0°,-45°and-90°) were compared. The influence ofthermal convection induced by gravity on onset process was analyzed. It was found thatstanding-wave thermoacoustic prime mover has the lowest onset and damping temperatureswhen it is installed horizontally. During the process of rotation, its onset temperature rises tosome extent. When the heater is above the cooler(45°and90°), thermals induced by gravityflows upward which is not benefical for the heat transfer between gas and solid in stacks, sothe onset temperature increases. After onset, thermoacoustic oscillation forceses thermals toflow to the cold end and makes the temperature of stacks increase. When the heater is underthe cooler(45°and90°), thermals from hot end to cold end is serious which induce lots ofheat loss and the onset temperature of primes mover increase obviously. After onset, strongthermoacoustic oscillation suppresses the thermal convection in stacks and makes thetemperature of stacks decrease.Fourthly, to better study the alternating flow and heat transfer rule in thermoacousticengine under the coupling effect of sound and gravity fields, loudspeaker was used tosimulate the thermoacoustic oscillation. And the temperature and velocity distributions withinthermoacoustic core under five tilt angles (45°,90°,0°,-45°and-90°) were observed bythermal infrared imager and particle image velocimetry (PIV). The experimental resultsshowed that the flow and heat transfer characteristic of thermoacoustic engine is affected bothby thermal convection and thermoacoustic oscillation. When thermoacoustic engine isinstalled horizontally, natural convection in thermoacoustic core shows as cycle flow and thetemperature distributes as ladder pattern. After the addition of sound oscillation, the heattransfer mode in thermoacoustic core changes into forced convection and the temperaturefield redistributes as parabolic shape. When the temperature difference between heater andcooler increases, the cycle thermal convection becomes stronger which makes the averageradial speed in thermoacoustic core gradually decrease from top to bottom. When the hot endis above the cold end (45°and90°), thermal convection in thermoacoustic core is very weak and the velocity changes following the normal sine rule. When the hot end is under the coldend (-45°and-90°), the upward thermals in thermoacoustic core is strong, which makesthermoacoustic core have high temperature. Also, the periodic velocity in thermoacousticcore has an obviously upward shift due to the effect of thermals. It is just becaust of thecombined effect of thermoacoustic coscillation and thermal convection that thermoacousticengine shows different operating characteristics under different tilt angles.