Experimental Study On Two Phase Flow Visualization And Heat Transfer Enhancement Of Flat-plate Single-loop Pulsating Heat Pipes

Pulsating heat pipe(PHPs)is a new type of passive cooling device which can transfer heat efficiently.It is getting a great deal of attention due to its simple structure,low cost,wide application field.Pulsating heat pipe heat transfer performance is affected by many factors together,which lead to complex internal working fluid flow mode and show the characteristics of nonlinear pulse.In addition,the enhancement of heat transfer in the pulsating heat pipe,to reduce the temperature difference between evaporator and condenser,is also an important aspect of the engineering application.Therefore,in this paper,we design a visualization test bench,use refrigerant R245 fa as the working fluid to explore the flow pattern and thermal performance simultaneously by changing the different factors respectively.Experimental results show that it may be oscillating flow in PHPs at first,then translate to the circulation with flow reversals when the heat input increases and finally become unidirectional flow.During this progress,thermal performance of the PHPs will gradually increase until it reaches the heat transfer limit.The higher the filling ratio of liquid,the lower the heat input to a directional flow cycle and the worse thermal performance it has.Orientation of the PHPs has a great influence on the thermal performance,the PHPs under the arrangement of 45° not only deteriorates the thermal performance,but also bring the heat transfer limit forward.When the groove is 2 mm×0.5 mm,it promotes the flow inside the PHPs to become unidirectional but deteriorates the heat transfer.The comparison was made between normal loop and an improved loop with interconnecting channel(IC)between the two vertical parallel channels.The incorporation of IC provides an additional short-cut for fluid flow and prolongs the oscillation regime.The IC located in evaporator outlet suppresses the liquid from the condenser to flow to the evaporator and make the dry-out in evaporator move forward,but it can cause the flow in the upper loop circulating in the anti-clockwise direction.The IC located in condenser improves the ratio of vapor in lower loop,but suppresses the circulation,with the fixed direction circulation only occurring at high filling ratio and high working load.Compared with normal loop,the IC arranged in the center of the condenser(1IC-1/2),changes its heat transfer: at low filling rate(40,50%),the thermal performance of PHPs only improves at the low heat input;at medium filling rate(60%),the whole is optimized,and when the heat input is 6 W,thermal resistance optimizes about 30%;At high filling rate(70,80%),there are only weak deterioration in some heat input.Compared with 1IC-1/2,the IC arranged at the exit of the evaporator at the same time(2IC)can increase thermal resistance sharply,only a local optimization in heat input for 3 W;when IC arranges in the condenser pipe only,the higher position it is,the weaker enhancement of the thermal performance it has.In a word,at a suitable liquid rate and position arrangement,IC not only decrease thermal resistance but also delay the deterioration of heat transfer.