Simulation Study On Heat Transfer Performance Of Venturi-like Pulsating Heat Pipe

In recent years,the arrival of 5G era has provided a lot of convenience for people,meanwhile,higher requirements has been put forword for the performance of electronic equipment.Heat dissipation efficiency of electronic equipment has a significant impact on working performance,the pulsating heat pipe,as one of the heat sink elements of electronic equipment,which plays an important role.The most studied structure is the pulsating heat pipe with uniform diameter but non-uniform diameters pulsating heat pipe.In order to optimize the performance of pulsating heat pipe,the evaporation section of uniform diameter pulsating heat pipe is transformed into a Venturi-like structure.VOF model in CFD simulation was used to simulate the uniform diameter pulsating heat pipe in the 1-3 loop,the 80 mm Venturi-like pulsating heat pipe in the 1-5 loop,and the single loop Venturi-like pulsating heat pipe with 60 mm and 40 mm Venturi-like structure in the insulation section.1?3 loop uniform diameter pulsed heat pipe and insulation section Venturi-like single loop Venturi pulsed heat pipe with 60 mm and 40 mm length Filling Height of 75 mm.The filling height of single loop 80 mm Venturi-like pulsating heat pipe is 60?105mm.The filling height of the 2?5 loop 80 mm Venturi-like pulsating heat pipe is75?105mm.The gas-liquid distribution,temperature,gas-liquid volume fraction,pressure,flow rate,effective heat transfer coefficient and thermal resistance were analyzed.Firstly,the pressure distribution of the left and right structures is completely symmetrical when a pulsating heat pipe with uniform diameter is started.Meanwhile,the rise of liquid phase is also fully symmetrical.However,the pressure distribution in the left and right structures is not uniform when the Venturi-like pulsating heat pipe is started.The pressure on the right side is large and the liquid phase rises greatly.The flow rate of the working fluid passing through the Venturi structure is completely in line with the Venturi effect when the working fluid in the tube forms a unidirectional circulation flow.Secondly,the Venturi-like pulsating heat pipe has a higher boiling degree of starting liquid phase and a larger amount of bubbles compared with uniform diameter pulsating heat pipe.The time for reaching the top of the condensation section and realizing reflux is reduced.The liquid plug in the condensation section has small volume and uniform distribution.Starting temperature of evaporation section rises slightly.After stable operation,the amplitude of temperature oscillation is stable.The pressure at the contact interface between the condensing section and the adiabatic section varies greatly.The average pressure is small.The pressure difference between evaporator and conderser is large.The velocity of working fluid increases.Thirdly,compared with the Single Loop Venturi-like pulsating heat pipe with the diameter of 80 mm,the liquid volume with the diameter of 1 mm decreases to 60 mm increases by 32.4%.The boiling degree of working fluid is reduced and the amount of bubbles is reduced.The time required for the working medium to rise to the top of the condensation section and return is increased by 1.4s.The liquid volume increases by 64.8% when the diameter of 1 mm is reduced to 40 mm.The filling volume is the same as that of pulsating heat pipe with uniform diameter.The time required for the liquid plug to rise to the top of the condensation section and return is increased by 2.0s.The left liquid phase first rises to the condensing section,and then the right liquid phase begins to rise.The the pressure difference and effective heat transfer coefficient between evaporater and condener of the 80 mm single loop Venturi-like pulsating heat pipe increase with the increase of liquid filling height.Finally,the pressure rise of the middle loop is the largest when the multi loop pulsating heat pipe starts up.The pressure rise of the outermost loop is small.When the liquid plug passes through the expanding section of the multi loop Venturi-like pulsating heat pipe,the gas plug is broken.The gas impinges on the liquid plug,and the flow is disordered.Then the small liquid plug is formed again.When the multi loop Venturi-like pulsating heat pipe starts up,the liquid height presents cosine distribution.The height of liquid phase of pulsating heat pipe with uniform diameter presents parabola distribution of upper opening.The temperature jump degree,oscillation amplitude and thermal resistance of evaporation section decrease with the increase of loop number or filling height.The pressure difference between evaporation and condensation increases with the increase of loop number or filling height.The simulation result shows that the Venturi-like pulsating heat pipe can not only operate normally,but also the flow rate and pressure change in the operation process are in line with the Venturi effect perfactly.Not only the uneven pressure distribution of the Venturi-like pulsating heat pipe optimizes the start-up performance,it achieves better heat transfer performance under low liquid filling and heating power as well.The uniform liquid phase distribution,stable temperature oscillation,large pressure fluctuation,large pressure difference and high flow rate change show that the Venturi-like pulsating heat pipe provides sufficient circulation power to keep the working medium in good operation.Moreover,it shows the heat transfer efficiency of working fluid is improved,the reflux power is enough,the evaporation section is not easy to dry up and the heat transfer limit is optimized.The increasing effective heat transfer coefficient and decreasing thermal resistance indicate that the effective heat transfer rate of the Venturi-like pulsating heat pipe plug has been increasing and the heat transfer preformance has been improving.By contrast,the Venturi-like pulsating heat pipe in terms of start-up performance,operation power,heat transfer limit and heat transfer performance aspects is going to be more advantage.Figure 36 table 2 reference 65...