Twoshell-and-tubeheatexchangers(STHXs)usingcontinuoushelicalbafflesinsteadofsegmentalbafflesusedinconventionalSTHXswereproposed,designed,andtestedinthisstudy.ThetwoproposedSTHXshavethesametubebundlebutdifferentshellconfigura-tions.Theflowpatternintheshellsideoftheheatexchangerwithcontinuoushelicalbaffleswasforcedtoberotationalandhelicalduetothegeometryofthecontinuoushelicalbaffles,whichresultsinasignificantincreaseinheattransfercoefficientperunitpressuredropintheheatexchanger.Properlydesignedcontinuoushelicalbafflescanreducefoulingintheshellsideandpreventtheflow-inducedvibrationaswell.TheperformanceoftheproposedSTHXswasstudiedexperimentallyinthiswork.TheheattransfercoefficientandpressuredropinthenewSTHXswerecomparedwiththoseintheSTHXwithsegmentalbaffles.Theresultsindicatethattheuseofcontinuoushelicalbafflesresultsinnearly10%increaseinheattransfercoefficientcomparedwiththatofconventionalsegmentalbafflesforthesameshell-sidepressuredrop.Basedontheex-perimentaldata,thenondimensionalcorrelationsforheattransfercoefficientandpres-suredropweredevelopedfortheproposedcontinuoushelicalbaffleheatexchangerswithdifferentshellconfigurations,whichmightbeusefulforindustrialapplicationsandfur-therstudyofcontinuoushelicalbaffleheatexchangers.ThispaperalsopresentsasimpleandfeasiblemethodtofabricatecontinuoushelicalbafflesusedforSTHXs.

Effective boundary condition is one of the most critical problems in the computation of micro-channel flows with direct simulation Monte-Carlo (DSMC) method. In the present work, the implementation of DSMC with specified pressure boundary condition (PBC) was discussed in detail. The variations of gaseous local pressure, temperature and number density of the particles caused by the temperature difference between channel walls and gas were presented. It was found that with the increase of both gaseous compressibility and rarefaction, the pressure distribution along micro-channel became more nonlinear. Heat transfer occurred almost only at channel inlet and outlet, and the average wall heat flux increased almost linearly to the inlet-to-outlet pressure ratio. The computational results also showed that PBC was more suitable for the simulation of micro-channel flow problems than the conventional velocity boundary condition (VBC).

在介绍螺旋折流板管壳式换热器的结构及原理的基础上，对壳程传热强化及阻力特性的研究现状进行了总结，分析了壳侧流体的流动和换热机理，表明螺旋折流板结构是改善壳侧流动换热性能的有效措施与弓型折流板换热器相比，螺旋折流板换热器的最大特点是单位压降下的壳侧换热系数高。结合具体实例介绍了其在石油化工、能源动力及核能应用等行业中的应用前景。关于螺旋折流板换热器，还有许多问题需要进一步研究，如流动换热的机理以及影响流动换热机理的几何因素、相变情形、介质物性等。

Three-dimensionalturbulentflowandheattransferinaninternallyfinnedtubewithablockedcore-tubehavebeennumericallystudiedbytherealizablek−εturbulencemodelwiththewall-functionmethod.Thenumericalmethodisvalidatedbycomparingthecalculatedresultswithexperimentaldata.Therangeofratioofblockedcore-tubeoutsidediametertoouter-tubeinsidediameter(d0/Di)isfrom0.25to0.75.Thecomputationalresultsdemonstratedthatthereexistsanoptimalratioof(d0/Di)underbothidenticalmassflowrateandidenticalpressuredrop.Theoptimalratioof(d0/Di),whichisreducedwiththeincreaseofmassflowrate,isapproximately0.5to0.625atgivenmassflowrateforbothconstantwalltemperatureanduniformwallheatflux.Theoptimalratioof(d0/Di)atagivenpressuredropisfrom0.44to0.50,whichisalsoslightlyreducedwiththeincreaseofpressuredrop.Furthermore,theoptimalratioof(d0/Di)isnotsensitivetothenumberofcross-sectionwavyfinsofaninternallylongitudinalfinnedtube,intherangeofafinwavenumberof15–25.

Forflowsassociatedwithmicroelectromechanicalsystems(MEMS),theheatfluxspeci-fied(HFS)boundaryconditionexistsbroadly.However,problemswiththeHFSboundaryconditionhavenotbeenwellrealizedinthesimulationsofmicrochannelflowsusingthedirect-simulationMonteCarlo(DSMC)method.Inthepresentwork,inversetemperaturesampling(ITS)isusedtodealwithdiatomicgaseousflowandheattransferinamicrochan-nel.Thistechniqueprovidesanapproachtocalculatethemolecularreflectivecharacteristictemperaturefromthemolecularincidentenergyandtheheatfluxatthewallboundary.CoupledwiththeDSMCmethod,thisdiatomicmoleculeITStechniquecanbeusedtotreattheHFSboundaryconditionsintheDSMCmethod.Verificationindicatesthatthepro-poseddiatomicmoleculeITSmethodcanaccuratelysimulatethegaseousflowandheattransfer.InPartIIofthiswork,theproposedmethodisappliedtodemonstrategeneralmicrochannelgaseousflowpropertiesunderuniformheatfluxboundaryconditions.atthesametime,thenewmethodisadoptedtonumericallyinvestigatetheeffectsofwallheatfluxongaseousflowandheattransferproperties.

Inthefirstpartofthiswork(PartI),wepresentedandvalidatedtheDSMC-HFSmethod,whichcanbeusedtodealwithheatfluxspecifiedboundaryconditionsinDSMCsimula-tions.Inthisarticle,themethodisappliedtodemonstratethegeneralpropertiesofrarefieddiatomicgaseousflowinamicrochannelunderuniformheatfluxboundaryconditions.Theeffectsofwallheatfluxongaseousflowandheattransfercharacteristicsareinvestigatednumericallyanddiscussedindetail.Itcanbeconcludedfromthepresentresearchthatgaseousrareficationandcompressibilityincreasewiththeincreaseofthewallheatflux.Gasaccelerationathigherwallheatfluxismoreobviousthanthatatlowerwallheatflux.Thehighwallheatfluxreducesthemassflowrateandelevatestheheattransferabilityexceptatthechannelinlet.

In this paper, a SIMPLE-like algorithm on collocated grid system has been developed. The ability to suppress the spurious pressure field is achieved via introducing the pressure difference between adjacent two grid points into the convection-diffusion finite difference scheme, and the interfacial velocity is obtained by simple linear interpolation. The differencing scheme, discretization of governing equations and solution procedure of the algorithm are described in detail. In order to check the validity of the algorithm, several test cases which have analytical or benchmark solutions are presented. Good agreements are obtained between the numerical and the corresponding analytical or benchmark solutions. The ability of the improved algorithm to suppress the spurious pressure field is demonstrated via a 3-D example.