Study On The Turbulent Flowand Heat Transfer Characteristics Of Viscoelastic Fluid Based Nanofluid

The viscoelastic fluid such as aqueous solution of surfactant or polymer caninduce a great reduction of flow resistance at turbulent flow state. However, avery serious heat transfer deterioration always occurs simultaneously in a systemcontains heat transfer units. The nanofluids prepared by suspending somenanoscaled particles with high thermal conductivity in water and adding auxiliarystabilizer can enhance the heat transfer performance of a heat transfer system.Nevertheless, the extra flow resistance will be introduced caused by thesuspended soild particles. To solve the issues of heat transfer deterioration inviscoelastic fluid turbulent flow and increase of resistance in nanofluid flow is animportant subject in practical engineering application for these two kinds offluids. From this viewpoint, the present thesis will combine the advantage ofviscoelastic fluid with that of nanofluid, hoping to prepare a kind of stablenanofluid using viscoelastic fluid as base fluid successfully and realize the effectsof fluid resistance reduction and heat transfer enhancement.Firstly, the required experiment equipments and self-made pipe flowconvective heat transfer and flow resistant test experiment system are introduced.Then, by considering the thermal dispersion effect of nanoparticles, the thermaldispersion model and energy equation which used to simulate heat transfer effectof nanofluid is deduced. And then, the direct numerical simulation method isintroduced at last.In the experiment study aspects, the nanofluid which named viscoelasticfluid based nanofluid (VFBN) has been successfully prepared by using aqueoussolution of cetyltrimethylammonium chloride (CTAC) and sodium salicylate(NaSal) with1:1in mass ratio as viscoelastic base fluid and copper (Cu) particleswith high thermal conductivity as nanopaticles. The detailed preparation methodof VFBN is introduced. Subsequently, the studies of suspension stability, thermalconductivity, rheology and surface tension for VFBN are carried out in thepresent thesis. A theoretical prediction model for thermal conductivity of VFBNwith spherical copper nanoperticles or long cylindrical carbon nano-tubes is proposed by modifying Li-Qu-Feng’s thermal conductivity model. Themeasurement results of thermal conductivity of VFBN show that this kind offluid has a larger thermal conductivity than water, and its thermal conducticity isincreased with the increase of volume fraction of copper nanoparticle and fluidtemperature. The rheology study results show that the VFBN retains the shearthinning behavior in its viscosity, which implies a potential flow drag reducingability.Based on the studies of thermal conductivity and rheology of tested fluid, theflow resistance and heat transfer performance of water, viscoelastic base fluid,water based nanofluid and VFBN are measured by using pipe flow convectiveheat transfer and flow resistance test experiment system. The experiment resultsshow that the water based nanofluid flows own a singnificant heat transferenhancement than water flows and its heat transfer enhancement is increased withthe increase of nanoparticle volume fraction and Peclet (Pe) number, however,the increase of flow resistant is not obvious. As for viscoelastic fluid flows, theheat transfer is deteriorated and flow resistance is reduced, besides, the heattransfer reduction rate is always larger than flow drag reduction rate. Whenadding copper nanoparticles in viscoelastic fluid and forms a VFBN, for this kindof fluid flows, the added nanoparticles weaken the viscoelasticity of base fluid,while enhance the heat transfer performance. By ploting and analyzing the flowcomprehensive performance phase diagram of different fluid flows, theviscoelastic fluid based nanofluid flows show a flow drag reduction and heattransfer enhancement than viscolelastic fluid flows, and the intended purposes ofthe present study are achived.In the theoretical study aspects, the constant thermal dispersion coefficientof copper nanofluid in experiment is got by comparisons of the simulated resultsand experiment results. Direct numerical simulations (DNS) of nanofluid flowsare carried out by using this method. From the fluid flow comprehensiveperformance phase diagram got by DNS results, it shows the similar trend withexperiment results. Meanwhile, through analyzing the simulated velocity field,temperature field and conformation filed of different fluid cases, deducing andcalculating the flow resistance with viscous, turbulent and viscoelasticcontributions and heat transfer rate with conductive, turbulent and thermal dispersion contributions of nanoparitcles from momentum equation and energyequation respectively, the mechanisms of heat transfer enhancement and flowdrag reduction of VFBN is studied further. Finaly, based on analogy theory, theratio of Chilton-Colburn factor to flow friction coefficient of different fluid flowcases is calculated. The results show that nanofluid which use water andviscoelastic fluid as base fluids show significant enhancement of heat transferperformance again.