Study On Heat Transfer Enhancement With Viscoelastic Fluid Pulsating Flow In Microchannels

TheⅣgeneration nuclear reactor with lead bismuth alloy as coolant adopts microchannel heat sink as one of the power conversion scheme to meet the power generation demand under high heat flux environment.While it is urgent to develop more efficient micro scale heat transfer technology to cover the increasing demand for the heat dissipation since the traditional microchannel heat sink can hardly maintain it.The main thesis of this paper is to realize the flow instability in the microchannel by introducing flow nonlinearity which should be viscoelasticity and pulsating flow.By coupling two heat transfer technologies with characteristic time scale,the resonance effect of them is realized and greater disturbance is generated,finally resulting in heat transfer enhancement.Therefore,numerical simulation and experimental research are carried out in this paper to determine the influence of various factors on its enhanced heat transfer effect and reveal its enhanced heat transfer mechanism.The details are as follows:First of all,the results and conclusions of the resonant effect by viscoelastic fluid pulsating flow in this paper are not only the verification of the existing flow theory,but also the development of the application of this flow in the field of heat transfer.In the experimental study,the overflow tank with periodic changing liquid level height is adopted as the flow supply to realize the pulsating pressure inlet.The relationship between its flow and heat transfer performance with the research parameters is obtained by multi physical field measurement.In the numerical simulation research,the open source software Open FOAM is used to calculate the flow and heat transfer of viscoelastic fluid pulsating flow in microchannel,to further expand and improve the laws found in the experiment,establish the empirical relationship of working condition parameter laws at the maximum enhanced heat transfer,and supplement the details of flow and temperature distribution that are difficult to capture in the experimental research.And the heat transfer augmentation mechanism by viscoelastic fluid pulsating flow is revealed by these two researches.In the research of rectangular microchannel.We obtained an obvious enhancement on both flow and heat transfer characteristics.The heat transfer enhancement ratioηincreases with larger elasticity of flow but lower Reynolds number(Re).There exists an optimal frequency Wo_owhereηreaches maximum for certain Re and Weissenberg number(Wi).The Wo_o trends to be lower value as the growth of Re and Wi,which implies the occurrence of resonance effects by viscoelastic fluid pulsating flow.It is found an increasement on flow rate which shares the same law of heat transfer enhancement by viscoelastic fluid pulsating flow.In addition,reversal flow is found in both experimental and numerical study when the pulsating pressure inlet is zero,which should be attributed to the relaxation of viscoelastic macromolecule.The relaxation causes the secondary flow of surrounding macromolecule and promote the heat exchange between fluid and heating wall.Further heat transfer enhancement is achieved by resonance effect which destroys the thermal boundary,keeping it an underdeveloped state,reducing the thermal resistance and finally enhancing the thermal performance.Finally,the law is extended to the application of parallel multi-channel,and similar heat transfer enhancement laws are obtained,which proves the feasibility of using viscoelastic fluid pulsating flow to enhance heat transfer.The research of heat transfer enhancement by viscoelastic fluid pulsating flow is conducted in this paper.The parametrical effects and mechanism on heat transfer enhancement is studied.We assume that the balance between the relaxation time of viscoelastic fluid molecules and the pulsation period is the most fundamental control factor.The results of this paper lay a foundation and has guiding significance for the application of viscoelastic fluid pulsating flow in the field of heat transfer in the future.