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**Heat Transfer/Heat Exchanger**

How is the heat transfer? Mechanism of Convection Applications . Mean fluid Velocity and Boundary and their effect on the rate of heat transfer. Fundamental equation of heat transfer Logarithmic-mean temperature difference. Heat transfer Coefficients. Heat flux and Nusselt correlation Simulation program for Heat Exchanger

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**How is the heat transfer?**

Heat can transfer between the surface of a solid conductor and the surrounding medium whenever temperature gradient exists. Conduction Convection Natural convection Forced Convection

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**As a result of heat exchange **

Natural and forced Convection Natural convection occurs whenever heat flows between a solid and fluid, or between fluid layers. As a result of heat exchange Change 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

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Forced Convection If 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.

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Heat Exchangers A device whose primary purpose is the transfer of energy between two fluids is named a Heat Exchanger.

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**Applications of Heat Exchangers**

Heat Exchangers prevent car engine overheating and increase efficiency Heat exchangers are used in Industry for heat transfer Car—prevent overheating with “high performance cooling applications” Maintain fluids at optimal temperature such that engine efficiency is increased and component wear minimized Boilers 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 reuse Distillation 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

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**The 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.

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**Principle of Heat Exchanger**

First Law of Thermodynamic: “Energy is conserved.” Control Volume Qh Cross Section Area HOT COLD Thermal Boundary Layer

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**Region II : Conduction Across Copper Wall**

THERMAL BOUNDARY LAYER Q hot Q cold Region III: Solid – Cold Liquid Convection NEWTON’S LAW OF CCOLING Energy 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. Th Ti,wall To,wall Tc Region I : Hot Liquid-Solid Convection NEWTON’S LAW OF CCOLING Region II : Conduction Across Copper Wall FOURIER’S LAW

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**Velocity 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 flowrate u = average mean velocity

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**In 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 layer r

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**1) 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 is 1) 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 wall heating h cooling

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**Region I : Hot Liquid – Solid Convection**

U = The Overall Heat Transfer Coefficient [W/m.K] Region I : Hot Liquid – Solid Convection Region II : Conduction Across Copper Wall Region III : Solid – Cold Liquid Convection + ro ri

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**Calculating U using Log Mean Temperature**

Hot Stream : Cold Stream: Log Mean Temperature

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**A A Log Mean Temperature evaluation COUNTER CURRENT FLOW**

CON CURRENT FLOW ∆ T 1 2 ∆ A A T1 A 1 2 T2 T3 T6 T4 T7 T8 T9 T10 Wall T 1 2 4 5 6 3 7 8 9 10 P ara ll e l Fl ow T 1 2 4 5 3 7 8 9 10 6 Co un t e r - C u re n F l ow

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T1 A 1 2 T2 T3 T6 T4 T7 T8 T9 T10 Wall

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**h DIMENSIONLESS ANALYSIS TO CHARACTERIZE A HEAT EXCHANGER**

Further Simplification: Can Be Obtained from 2 set of experiments One set, run for constant Pr And second set, run for constant Re h

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**Empirical Correlation**

For laminar flow Nu = 1.62 (Re*Pr*L/D) For turbulent flow Good To Predict within 20% Conditions: L/D > 10 0.6 < Pr < 16,700 Re > 20,000

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**Experimental Apparatus Two copper concentric pipes**

Switch for concurrent and countercurrent flow Temperature Indicator Hot Flow Rotameters Cold Flow rotameter Heat Controller Temperature Controller Two copper concentric pipes Inner 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

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**Examples of Exp. Results**

Theoretical trend y = x – Theoretical trend y = 0.026x Experimental trend y = x – 4.049 Experimental trend y = x – Theoretical trend y = x Experimental Nu = Re0.7966Pr0.4622 Theoretical Nu = 0.026Re0.8Pr0.33 Experimental trend y = x –

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**Effect of core tube velocity on the local and over all Heat Transfer coefficients**

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