Study On Enhancing Heat Transfer For Singlephase And Two-phase Flow Boiling In Micro/minichannel Using Turbulence Elements

With the continuous advance of micro-machining technology,the development and application of micro-scale devices or equipments are greatly improved.However,with the further increase in heat load of the devices or equipments,the traditional micro/mini-channel heat sink has been unable to meet its stringent heat transfer requirements.Therefore,it is an urgent problem to further enhance the heat transfer performance of micro/mini-channel.Given that,the single-phase convective heat transfer enhancement of micro-channel using straight pin-fins or longitudinal vortex generators turbulence elements is studied numerically,as well as the two-phase flow boiling heat transfer enhancement and instability suppression of mini-channel heat sink having twisted tape inserts is investigated experimentally.This research work is mainly divided into three parts:（1）Effects of straight pin-fin relative height(ratio of pin-fin height to micro-channel height H_rc=0～1)and relative offset value(ratio of adjacent pin-fin offset value to micro-channel width W_sc=0～0.7)on convective heat transfer of rectangular micro-channel heat sink with variable fluid properties are numerically studied.Results show that,increasing pin-fin height usually results in higher heat transfer enhancement,but renders larger flow resistance.Increasing offset value of pin-fins properly can enhance heat transfer,but extremely larger offset value leads to the decrease in heat transfer enhancement.In this work,the micro-channel with H_rc=0.75 and W_sc=0.6 yields the greatest overall thermal performance with the maximum overall thermal performance factor of 1.43.In addition,the viscosity plays a key role in the flow and heat transfer performances,but others present unobvious effect.It suggests that the temperature dependency of fluid viscosity should be taken into consideration in numerical simulation.（2）An interrupted micro-channel heat sink with longitudinal vortex generators（LVG）pairs in transverse micro-chambers is designed.The effects of LVG height（H=0.025 mm～0.175 mm）,transverse spacing（S=0.01 mm～0.07 mm）and setting angle（A=30°～75°）on flow,heat transfer and overall thermal perfornmances are studied.Results indicate that,LVG pairs introducing in the interrupted micro-channel with transverse micro-chamber presents better heat transfer enhancement in comparison with interrupted micro-channel.LVG pairs generate strong longitudinal vortexes to intensify flow disturbance,resulting in thinning the flow and thermal boundary layers as well as accelerating the mixing effect of hot and cool fluids,thus improving heat transfer performance and decreasing entropy generation.In this work,the geometric parameters for the micro-channel with the best overall thermal perfornmance is the group including H=0.075 mm,S=0.01 mm and A=75°,this micro-channel yields the greatest overall thermal performance with the maximum overall thermal performance factor of 1.45.（3）Effects of twisted tape insert length（L_t=50 mm,100 mm and 150 mm）and twist ratio（R=3,4 and 5）on flow boiling heat transfer and instability in mini-channel heat sink are experimentally examined using the distilled water.Results show that,twisted tape inserts not only generate swirl flow with centrifugal force,but also provide more seed vapor cores by contacting mini-channel walls.Therefore,twisted tape insert improves the fluid mixing and accelerates the bubble nucleation,growth and detachment in mini-channel,which can significantly enhance flow boiling heat transfer.In this work,the optimal mini-channel heat sinks with twisted tape inserts（L_t=150 mm and R=3）performs the highest heat transfer coefficient,and achieves a maximum of 80.9%increase in heat transfer coefficient compared with plain mini-channel heat sink.Additionally,mini-channel heat sinks with twisted tape inserts can effectively suppress the boiling instabilities characterized by inlet pressure fluctuations.