Numerical Simulation And Experimental Study Of Flow Around Tube Bundles With Micro-rib Structure

At present,China has become a major country of energy production and consumption.With the increasing shortage of power and the process of modern industrialization,energy saving has become the requirements of the times.With the contradiction between people’s pursuit of more efficient heat transfer and the limitation of traditional heat transfer working medium and regular heat exchanger tube type,heat transfer enhancement technology has become urgent needs.In the field of passive heat transfer enhancement technology,improving the thermal properties of heat transfer working medium and adjusting the structural parameter of heat exchanger are the two main means of enhancing heat transfer.Therefore,the following two aspects are considered in this thesis:one is to replace deionized water with Ti O₂-H₂O nanofluids;the other is to adjust the structural parameters of the tube bundle,including the micro ribs and the tube spacing.The flow and heat transfer characteristics of fluid in the tube bundle were studied by experiment and numerical simulation from two aspects of heat transfer working medium and tube bundle structure.In this paper,the tube bundle is selected,and the influence of micro rib,tube spacing,nanofluids and Reynolds number on the flow and heat transfer characteristics was investigate by quantitative analysis.The main conclusions from the study are as follows.（1）Ti O₂-H₂O nanofluids are prepared by two-step method.Stability and thermal physical properties of nanofluids with different mass fractions are analyzed.Stability of nanofluids is found to be satisfactory with the method of static observation,and nanofluids can be used in the experiment.Compared with the deionized water which is the base fluid of nanofluids,it was found by measurement that the thermal conductivity and viscosity of the nanofluids are slightly increased.（2）Effects of rib number（N=4,6,8）and rib height（H=2mm,3mm,4mm）on flow and heat transfer characteristics in the tube bundle were investigated considering the influence of heat transfer working mediums（nanofluids and deionized water）and rib types（vertical ribs and ring ribs）,and heat transfer enhancement effect was comprehensively evaluated.The results showed that in the scope of this study,the tube bundles with vertical rib and ring rib show better heat transfer effect when the number of rib is N=8 and N=4 respectively.The heat transfer effect of the tube bundle is enhanced with the increase of the rib height.The influence of nanofluids on the heat transfer performance increases at first then decreases with the increase of mass fraction,and the best mass fraction is 0.4%.For the change of Nusselt number,the tube bundles with vertical rib are more sensitive to the change of rib height,and the tube bundles with ring rib are more sensitive to the change of rib number.The resistance coefficient of tube bundle is enhanced significantly with the application of the micro-rib structure and nanofluids.In the scope of this study,the tube bundles with vertical rib and ring rib show better heat transfer effect when the number of rib is N=4 and N=8respectively.The resistance coefficient of the tube bundle increases with the rib height.For the change of resistance coefficient,the tube bundles with vertical rib and ring rib are more sensitive to the change of rib height.The trend of comprehensive performance index is similar to that of the Nusselt number.The application of the micro-rib structure and nanofluids increases the Nusselt number in the tube bundle by70.6%,and the resistance coefficient is increased by 81.37%.In the scope of this study,comprehensive performance index of the tube bundle with the application of the micro-rib structure and nanofluids can be greater than 1,and the maximum can reach 1.54.（3）On the base of the research of micro-rib structure,the effect of other tube bundle structure parameters was studied.Flow and heat transfer characteristics in the tube bundles with different transverse tube pitch ratio（S_T/D=1.5,2,2.5）were compared under the same conditions,the influence of nanofluids was considered,and heat transfer enhancement effect was comprehensively evaluated by the comprehensive performance index.The results show that the vertical rib and ring rib have better heat transfer performance when the transverse tube pitch ratio is S_T/D=2 and S_T/D=1.5 respectively,in the scope of this study.The change of heat transfer effect with the increase of mass fraction of nanofluid in the tube bundle has a similar trend as before.The application of the different types of micro-rib structure,transverse tube pitch ratios and nanofluids increases the Nusselt number of the tube bundle by 65.98%,and the resistance coefficient is increased by 70.0%.In the scope of this study,comprehensive performance index of the tube bundle with the application of the micro-rib structure and nanofluids can be greater than 1,and the maximum can reach 1.51.There are 4 tables,43 figures and 90 references in the thesis.