Experimental Investigation On Characteristics Of Pulsating Heat Pipe By Ultrasonic Wave And With Nanofluids

As heat transfer device with two-phase flow,pulsating heat pipe(PHP)has a widely application prospect in the fields of improving energy efficiency and thermal management.Based on the excellent heat transfer performance of nanofluids and the characteristics of assisted ultrasonic strengthening technology,a series of experiments were accomplished to investigate the performance of PHP with nanofluid.Subsequently,the oscillation and start-up characteristics of the PHP assisted with ultrasonic were experimentally tested.The main work and conclusion are summarized as follows:(1)A visualization experimental system for PHP was designed and built.The visualization tests of PHP with deionized water(DI water)and different concentrations of nanofluid were carried out.Results show that the addition of nanoparticles will on the one hand promote the phase transition of the working fluid in the PHP and on the other hand increase the instantaneous velocity and the driving force of working fluid.These are benefit for the reflux of condensed liquid and effectively avoid the phenomenon of ′dry out′.(2)In order to analyzing the influence of nanofluid concentrations on the heat transfer performance of PHP,the wall temperature variation with time was measured and combined with the visualization fluid oscillation frequency and velocity,the results were analyzed and discussed.It can be found that the PHP filled with nano-fluid has the gradual start-up process at low heating power,while it has the sudden start-up process at high heating power.Besides,there is an optimum concentration of nanofluids for enhanced effect.In detail,the maximum heat transfer enhancement efficiency with nanofluid concentration of 1.0 wt % can reach 40.1% at the heating power of 50 W.However,with the increase of the nanoparticle concentration,the collisions become more intense between different particles and the thermal resistance increases continuously.As a result,the start-up process and the thermal performance deteriorate in the tube.(3)The influence of assisted ultrasonic on the performance of PHP was discussed,and the variation of operating condition and thermal performance of PHP with or without ultrasonic ultrasound was compared and investigated.The results show that the ultrasonic supplies additional disturbance to the PHP,and consequently promotes the phase transformation and oscillation of the working fluid.This can effectively solve the problems that the PHP hardly reaches start-up process at low heating power and is more inclined to enter the ′dry out′ state at high heating power.Therefore,the added ultrasonic could broaden the working range of the PHP.(4)The performance of nano-fluid PHP with ultrasonic were tested and analyzed.The results show that the collapse of cavitation bubble induced by ultrasonic wave can produce some microjets.The combined effects of acoustic energy disturbance and cavitation jets could work together to promote the dispersion and energy transfer of nanoparticles.This can improve heat transfer performance of PHP with highconcentration nanofluids.In addition,the ultrasonic has more obvious enhancement effect at low heating power.For PHP with nanofluid concentration of 1.0 wt %,the heat transfer enhancement ratio of ultrasonic wave and nanofluids can reach 1.09 at the heating power of 10 W,but it is only 0.16 when the heating power is 40 W.