Study On Heat Transfer Enhancement With Typical Non-newtonian Fluid In Microchannel

Miniaturization,integration and portability have become the leading direction of modern industry.Highly miniaturized and integrated electronic devices often release a large amount of heat within their micro scale inter-components areas,which under certain circumstances,may lead to the micro-device’s functional failure.This calls for techno-scientific efforts for efficient heat removal from the mentioned devices.The core idea of this work is the introduction of non-Newtonian fluid in a microchannel for unique flow characteristic to promote microscale heat transfer.Through selecting suitable non-Newtonian fluid as working medium,coupling other methods,such as complex microstructure,pulsating flow and acoustic surface wave,explore the heat transfer performance and mechanism to obtain efficient heat transfer methods for heat dissipation of miniaturized instruments.Based on the passive and active heat transfer methods,this thesis fouces on four types methods,details are as follows:First,based on passive heat transfer enhancement technical,the characteristics of convective heat transfer and fluid flow within a square cross-section serpentine microchannel are experimentally studied for three groups of viscoelastic fluids polyethylene oxide(PEO)solutions,power-law fluid sodium carboxymethyl cellulose(CMC)solutions and Newtonian fluid sucrose solutions.The results show that the measurements of friction factor increase significantly for PEO solutions compared with CMC solutions and sucrose solutions over a range of Weissenberg number(Wi)from 2.4 to 53.69.The convective heat transfer enhances due to elastic turbulence in comparison to that achieved by the equivalent Newtonian fluid flow at the same Graetz number(Gz).The mechanism responsible for heat transfer enhancement can be attributed to the occurrence of the elastic instability and turbulence,which are created by the non-linear interaction between elastic stresses generated within the flowing high-molecular-weight polymer solutions and the streamline curvature of serpentine microchannel.Next,modifying geometrical structures has been proven to be very effective for the development of efficient heat transfer enhancement in microscale.A numerical simulation for the flow characteristics and heat transfer performance of non-Newtonian fluid flow in a manifold microchannel(MMC)heat sink and traditional microchannel(TMC)heat sink are studied.The non-Newtonian fluid was described by the power-law model.Comparing with Newtonian fluid flow,the introduction of pseudo-plastic fluid flow greatly improved the heat transfer efficiency owing to the generation of secondary flow due to the shear-thinning property.Besides,the temperature distribution in MMC was more uniform by using pseudo-plastic fluid.Furthermore,based on active heat transfer enhancement technical,a numerical investigation on the pulsating effect on the heat transfer performance of non-Newtonian fluid pseudo-plastic fluid flow in a manifold microchannel heat sink.The pulsating flows are respectively selected as square-wave,sinusoidal-wave and semi-sinusoidal wave pulsating inlets with time.The results show that introducing pulsating flow can significantly improve the overall thermal performance comparing with that of steady flow.The two scenarios existing in pulsating flow are responsible for the heat transfer enhancement,which are the continuously developing characteristics of the flow and the induction of secondary flows or reversal flows in the pulsating flow.In addition,the pulsating inlet with sinusoidal wave lead to the highest heat transfer coefficient among the three types of pulsating inlet conditions in consideration.In the last,the heat transfer performance in a straight microchannel heat sink which applies standing surface acoustic waves(SSAW)to disturb the flow is investigated.It is found that the introduction of SSAW can greatly enhance the overall heat transfer.This is the key mechanism of heat transfer enhancement: more full SSAWs are imposed on the bottom wall,which can induce more acoustic vortices.To sum up,the smaller area of the channel or the more SAW energy of additional input can improve the heat transfer performance in a straight microchannel.Finally,we elucidate the underlying mechanisms of heat transfer enhancement peculiar to the excitation of SSAW in microchannel,which is attributed to the appearance of acoustic vortices and the disruption of thermal boundary layer induced by acoustic streaming.And coupling with pseudoplastic fluid,its shear thinning characteristic gradually makes the fluid viscosity decrease after the acoustic vortices generating.And the fluid resistance also decreases.Under the condition of the same SSAW energy,the acoustic vortices in the pseudoplastic fluid flow can obtain a higher flow rate,which brings a stronger convective heat transfer efficiency.In summary,the present study can give deep insight into the heat transfer characteristics for the typical non-Newtonian fluid in the complex microchannel.Moreover,the introduction of pulsating inlet flow and acoustic surface wave on the heat transfer process has a deeper understanding.And this work explores the mechanism of the different technical means on the heat transfer enhancement.It will lay a foundation and provide important guidance for the future practical application of non-Newtonian fluid in heat transfer.