Presentation on theme: "Heat Transfer/Heat Exchanger"— Presentation transcript:
1Heat Transfer/Heat Exchanger How is the heat transfer?Mechanism of ConvectionApplications .Mean fluid Velocity and Boundary and their effect on the rate of heat transfer.Fundamental equation of heat transferLogarithmic-mean temperature difference.Heat transfer Coefficients.Heat flux and Nusselt correlationSimulation program for Heat Exchanger
2How is the heat transfer? Heat can transfer between the surface of a solid conductor and the surrounding medium whenever temperature gradient exists.ConductionConvectionNatural convectionForced Convection
3As a result of heat exchange Natural and forced ConvectionNatural convection occurs whenever heat flows between a solid and fluid, or between fluid layers.As a result of heat exchangeChange in density of effective fluid layers taken place, which causes upward flow of heated fluid.If this motion is associated with heat transfer mechanism only, then it is called Natural Convection
4Forced ConvectionIf this motion is associated by mechanical means such as pumps, gravity or fans, the movement of the fluid is enforced.And in this case, we then speak of Forced convection.
5Heat ExchangersA device whose primary purpose is the transfer of energy between two fluids is named a Heat Exchanger.
6Applications of Heat Exchangers Heat Exchangers prevent car engine overheating and increase efficiencyHeat exchangers are used in Industry for heat transferCar—prevent overheating with “high performance cooling applications”Maintain fluids at optimal temperature such that engine efficiency is increased and component wear minimizedBoilers and condensers are opposite processes used in industry-Boilers used to create steam—sometimes to drive steam driven turbines-condenseros bring steam back to liquid for reuseDistillation set-ups typically use condensers to condense distillate vapors back into liquid.Ultimately, condensers-cool refrigerant vapor back to liquid-- sub cooled liquid flows out of the condenser and into the evaporator where it absorbs heat from air in the surrounding pipe and generates cold air.Air conditioners utilize a closed heat exchange system that separates the hot and the cold fluids. Energy is transported via convection between the fluid and inner pipe surface, conduction through the pipe and then convection from the outer pipe surface into the second fluid. This closed system is very similar to the one examined in this lab experiment, and its widespread applicability is why this lab was designed.Heat exchangers are used in AC and furnaces
7The closed-type exchanger is the most popular one. One example of this type is the Double pipe exchanger.In this type, the hot and cold fluid streams do not come into direct contact with each other. They are separated by a tube wall or flat plate.
8Principle of Heat Exchanger First Law of Thermodynamic: “Energy is conserved.”Control VolumeQhCross Section AreaHOTCOLDThermal Boundary Layer
9Region II : Conduction Across Copper Wall THERMALBOUNDARY LAYERQ hotQ coldRegion III: Solid – Cold Liquid ConvectionNEWTON’S LAW OF CCOLINGEnergy moves from hot fluid to a surface by convection, through the wall by conduction, and then by convection from the surface to the cold fluid.ThTi,wallTo,wallTcRegion I : Hot Liquid-Solid ConvectionNEWTON’S LAW OF CCOLINGRegion II : Conduction Across Copper WallFOURIER’S LAW
10Velocity distribution and boundary layer When fluid flow through a circular tube of uniform cross-suction and fully developed,The velocity distribution depend on the type of the flow.In laminar flow the volumetric flowrate is a function of the radius.V = volumetric flowrateu = average mean velocity
11In turbulent flow, there is no such distribution. The molecule of the flowing fluid which adjacent to the surface have zero velocity because of mass-attractive forces. Other fluid particles in the vicinity of this layer, when attempting to slid over it, are slow down by viscous forces.Boundary layerr
121) Heat must transfer through the boundary layer by conduction. Accordingly the temperature gradient is larger at the wall and through the viscous sub-layer, and small in the turbulent core.The reason for this is1) Heat must transfer through the boundary layer by conduction.2) Most of the fluid have a low thermal conductivity (k)3) While in the turbulent core there are a rapid moving eddies, which they are equalizing the temperature.Tube wallheatinghcooling
13Region I : Hot Liquid – Solid Convection U = The Overall Heat Transfer Coefficient [W/m.K]Region I : Hot Liquid – Solid ConvectionRegion II : Conduction Across Copper WallRegion III : Solid – Cold Liquid Convection+rori
14Calculating U using Log Mean Temperature Hot Stream :Cold Stream:Log Mean Temperature
15A A Log Mean Temperature evaluation COUNTER CURRENT FLOW CON CURRENT FLOW∆T12∆ AAT1A12T2T3T6T4T7T8T9T10WallT12456378910ParallelFlowT12453789106Counter- CurenFlow
17h DIMENSIONLESS ANALYSIS TO CHARACTERIZE A HEAT EXCHANGER Further Simplification:Can Be Obtained from 2 set of experimentsOne set, run for constant PrAnd second set, run for constant Reh
18Empirical Correlation For laminar flowNu = 1.62 (Re*Pr*L/D)For turbulent flowGood To Predict within 20%Conditions: L/D > 100.6 < Pr < 16,700Re > 20,000
19Experimental Apparatus Two copper concentric pipes Switch for concurrent and countercurrent flowTemperatureIndicatorHot Flow RotametersCold Flow rotameterHeat ControllerTemperature ControllerTwo copper concentric pipesInner pipe (ID = 7.9 mm, OD = 9.5 mm, L = 1.05 m)Outer pipe (ID = 11.1 mm, OD = 12.7 mm)Thermocouples placed at 10 locations along exchanger, T1 through T10
21Examples of Exp. Results Theoretical trendy = x –Theoretical trendy = 0.026xExperimental trendy = x – 4.049Experimental trendy = x –Theoretical trendy = xExperimental Nu = Re0.7966Pr0.4622Theoretical Nu = 0.026Re0.8Pr0.33Experimental trendy = x –
22Effect of core tube velocity on the local and over all Heat Transfer coefficients