版10傳熱學(xué)1對(duì)流換熱b

1、傳熱學(xué)Heat transfer張能源與動(dòng)力學(xué)院Lecture 10Principle and Technology of Enhanced Convective Heat Transfer強(qiáng)化對(duì)流換熱原理與技術(shù)Focus of this lecturePrinciple of enhanced convective heat transfer as well as its developmentTechnicalapproachesofenhancedconvectiveheat transfer for laminar and turbulent flowJet imement heat

2、transfer1-1 Introduction(1) Main objectives of heat transfer researchesp Basic heat transfer disciplinarianp Technical approaches for controlling heat transfern Regarding a typical heat transfer processIgnoringthermal radiation輻射How to enhance k？Reducing heats對(duì)流輻射對(duì)流固壁導(dǎo)熱(2) Enhanced heat transfer tec

3、hnologyp Developed vigorously from 1960sp Tightly related to applicationsCompact heat exchanger Effective coolingFor instances Compact heat exchanger Efficiency element-heat pipeHeat pipe(3) Development of enhanced heat transferp Significant demand and huge challengeApplication of enhanced heat tran

4、sfer in aero-enginesT (K)l 渦輪進(jìn)口溫度發(fā)展2000l 國外高溫合金發(fā)展 Inlet Tl 國內(nèi)高溫合金發(fā)展18001600 Coolingdemond14001200100019601970198019902000年代F119-PW-100推重比101900KF22新一代T (K)l 渦輪進(jìn)口溫度發(fā)展2000l 國外高溫合金發(fā)展 Inlet Tl 國內(nèi)高溫合金發(fā)展18001600 Coolingdemond14001200100019601970198019902000年代F119-PW-100推重比101900KF22推重比202300K推重比12-152100K

5、Bottleneck technologies BreakthroughWhat ? How?p Its innovation is tightly associated with advancesin new material and manufacturing processMicro-fins heat sinkPiezoelectric-fan heat sink1-2 Principle of Enhanced Convectionn Theory of convective heat transferFor single-phase, the inherent physical m

6、echanism on convective heat transfer enhancement relies onincrease of temperature gradient near wall- l T = h(T - T ) y fy =0Discussionsn How to realize the increasing of temperaturegradient in the viciof wall ?n Are there any differences in the heat transfer enhancement techniques for the laminar f

7、low and turbulent flow ?To answer above questionsViewing from ?Basic heat transfer knowledgeThe respective features for the velocity and temperature distribution in the boundary layerLaminar flowTurbulent flow1. Laminar flow(a) Laminar flow(b) Turbulent flown What is the feature of velocity distribu

8、tion in fullydeveloped regionParabolic profileStatic mixerDiagonal rod insertsHelical screw tapesDiagonal rod insertsStatic mixern Essentially forcing fluid to form a strongly radial large-scaled mixing2. Turbulent flow(a) Laminar flow(b) Turbulent flown What is the feature of velocity distribution

9、in fullydeveloped regionLaminar viscous layerRtorsib turbulaPin finsDimplesRibsn Essentially destroying laminar viscous layer to reduce its thicknessBrief Summaryn Various kinds of techniques: Mixing main flow and/or the flow near wall Reducing boundary layer thickness Creating rotational or seconda

10、ry flow Raising turbulence intensityPassive technologyp Extended surfacesp Rough surfacesp Displaced elementsp Swirl-flow devicesp AdditivesActive technologyp Mechanical aidsp Surface vibrationp Fluid vibrationp Electrostatic fluidp Injection & suctionp Iming jetsDiscussionsn Trying to list some exa

11、mples related to the active heat transfer enhancement ?Field synergy principle場協(xié)同原理Prof. Guo (Tsinghua University) and co-workers proposed a novel concept in 1998. Trying to present a second look on the mechanism of convective heat transfer enhancement.過增元，逸，等2004場協(xié)同原理與強(qiáng)化傳熱新技術(shù)M：電力n 2-D boundary flow

12、 and heat transfer along a plateAnalog between heat convection and heat conductionThe convective term in the energy equation for the boundary flow corresponds to heat source in heatconduction equation, but as a function of flow- q= l T vy y rcu T +u T = l T p xy y y n 2-D boundary flow and heat tran

13、sfer along a plateIntegral of equation over thermal boundary layer thicknesswall heat fluxThe wall heat flux is equal to the overall strength ofheat sources inside thermal boundary layerdt rcu T +u T = -l T0p xy dyywrcu T +u T = l T p xy y y Rewritten in vector form:Vector dot productIntegrated fact

14、or is dependent on: velocity vector,temperature gradient, intersection angler rV T = V T cos bdt rc(r T )dy = -l T0p Vywdt rcu T +u T = -l T0p xy dyywRelationships between velocity vector, velocity gradient, temperature gradient and pressure gradient， 傳熱強(qiáng)，過增元2009對(duì)流換熱層流流場的物理量協(xié)同 分析M科學(xué)通報(bào)，54(12): 1779-1

15、785與化Example 1n A fully developed laminar flow in the channel of two parallel infinite flat plates. They are partially keptatdifferenttemperatures,ThandTcrespectively.Fluid temperature at inlet of thermal entrance is Tf.(ThTfTc)ThTfTcTrying to analyze why temperature distribution in developed therma

16、l region is linear ？Is itwith our previous understanding onconvective heat transfer ？UToo far the developed thermal regionThe intersection angle is 90V0=As a result, the convection term or the heat sourceterm is zero then reducing a pueat conduction.T x=V cosTbT / x= 0TExample 2n Jet imementJetNozzl

17、ePotential coreNozzle-to-plate distanceDecaying jetWall jet Wall jet TUImed targetSn rHigh local convective heat transfer rateOn the stagnation region,the intersection angle is nearly 0egiontagnatio1-3 Jet imementn Widely used in practical applications that aim to achieve intensive heating, cooling

18、or drying ratesSome typical applicationsDrying of textile and paper productsAnnealing of plastic and metal sheetsCooling of turbine blades and electronic equipment-icing of aircraft wings and engine inletsArray jets heat sinkGuide vane-icingHot airBasic knowledge on jet imementGeneral classification

19、jet/Confined jetn Single jet/Multiple jetsn Without/With initial crossflowwith crossflowmultiple jetsconfined jetjetComplex influence factorsq Effect of flow propertiesn Jet Reynolds numbern Jet turbulencen Crossflow Reynolds numbern Unsteadinessq Effect of iming jet geometryn Injection anglen Nozzl

20、e-to-target distancen Pitches of array jetsn Jet hole shapeq Effect of wall propertiesn Curvaturen Surface roughnessn Thermal conductivityn ConfinementSources of innovationUnderstandingExample: Effect of initial cross flowInitial crossflowStaggered arrangementInline arrangementArray jets(a) mc/mj=0(

21、b) mc/mj=0.28Isograms of heat transfer coefficient1-520W/(m2K)2-415W/(m2K)3-345W/(m2K)4-260W/(m2K) First row Fifth row z/d=1First rowz/d=2Fifth rowEffect of crossflow on iming jetDiscussionComparing inline and staggered arrangement, what is your opinion?Staggered arrangementInline arrangementz/d=1z/

22、d=2z/d=3Some issues for enhanced jet imementIt is still attractive to many researchers by now due toincreasing requirement of heat transfer enhancementEnhanced strategies Active strategiesGiovanni Maria Carlomagno, Andrea IaniroThermo-fluid-dynamics of submerged jets iming at shortnozzle-to-plate di

23、stance: A review. Experimental Thermal and Fluid Science 58 (2014) 15-35 Passive strategiesPassive strategiesn Previous investigations on the passive strategies were mainly concentrated on nozzle shapen Promoting turbulence directly on the im surface with surface modificationedn Advanced passive str

24、ategies are related to the use of vortex and turbulence generatorsExample 1: Swirl jetSwirl-imement-fin cooling schemeRej=8500Oblique jet (70)Normal jetsPR Parida, SV Ekkad, K Ngo. Experimental and numerical investigation of confined oblique imement configurations for high heat flux applications. In

25、t. Journal of Thermal Sciences 50 (2011) 1037-1050Example 2: Tab-excited jetTab-excited jetcircular (left)chevron (right)Azimuthally averaged NuD. Violato, A. Ianiro, G. Cardone, et al., Three-dimensional vortex dynamics and convective heat transfer in circular and chevron iming jets, International Journal of Heat and Fluid Flow 37 (2012) 22-36.chevronbaselineh = Tw - TcTj - TcActive strategiesn Active heat transfer enhancement strategies mostly comprise jet pulsation.n Active heat transfer enhanceme