Lecture 4 Conduction

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University of Houston *

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4364

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Mechanical Engineering

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Oct 30, 2023

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pdf

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8/29/23 1 MECE 4364 Heat Transfer Dr. Dong Liu Department of Mechanical Engineering University of Houston 1 Lecture 4 1 Last Lecture ¨ For single mode problems, you may identify the heat transfer process and apply the rate equations directly to find the answer ¨ For multi-mode problems, you may have to 1) Analyze the heat transfer processes to determine which modes must be considered 2) Define the control volume 3) Apply the energy conservation equation and correlate the respective terms to the rate equations of conduction, convection or radiation 2 ࠵? = −࠵?࠵? ࠵?࠵? ࠵?࠵? = ࠵?࠵? ∆࠵? ࠵? ∆࠵? = ࠵? ! − ࠵? " Fourier’s law Newton’s law of cooling ࠵? = ℎ࠵? ࠵? ! − ࠵? " ࠵? #$% = ࠵? & ࠵?࠵? & ࠵? & − ࠵? ( ࠵?࠵? ( ࠵? ( Radiation heat transfer Stefan-Boltzmann’s law ࠵? = ࠵?࠵?࠵?࠵? ! ̇ ࠵? ./ ̇ ࠵? 012 + ̇ ࠵? 34/ = ̇ ࠵? 52 2
8/29/23 2 Example 5: Multimode Effects ¨ A thin electrical heating element provides a uniform heat flux ࠵? 6 77 to the outer surface of a duct through which air flows. The duct wall is 10-mm thick and a thermal conductivity of 20 W/m K. At a particular location, the air temperature is 30℃ and the convection heat transfer coefficient between the air and inner surface of the duct is 100 W/m 2 K. a) What heat flux ࠵? 0 77 is required to maintain the inner surface of the duct wall at ࠵? . = 85℃ ? b) For the conditions of part (a), what is the temperature (To) of the duct surface next to the heater? 3 3 Example 5 4 4
8/29/23 3 Example 5 5 5 Problem-Solving Technique ¨ Step 1: Problem Statement ¤ Key information given ¤ Quantities to be found ¨ Step 2: Schematic ¤ Control volume ¤ Key energy and mass transfer processes ¨ Step 3: Assumptions and Approximations ¤ Simplifications ¨ Step 4: Physical Laws ¤ Single mode or multimode ¤ Conservation equations and rate equations ¨ Step 5: Properties ¤ Property tables ¨ Step 6: Calculations ¤ Solve equations and compute numbers 6 6
8/29/23 4 Conduction (Chapter 2) 7 7 Conduction q Fourier s law : discovered by Joseph Fourier in 1822 1) Heat transfer area A is always normal to the direction of heat transfer ( A does not have to be a constant ) 2) k is the thermal conductivity ( k does not have to be a constant ) 3) Rate of conduction in a given direction is proportional to the temperature gradient in that direction 4) Negative sign ensures that heat is conducted in the direction of decreasing temperature q In heat flux form q It has a magnitude and a direction … 8 ࠵? 9 = −࠵?࠵? ࠵?࠵? ࠵?࠵? Jean Fourier (1768-1830) ࠵? 9 77 = ࠵? ࠵? = −࠵? ࠵?࠵? ࠵?࠵? dT/dx is simply the slope of the temperature curve on a T-x diagram. 8
8/29/23 5 Fourier’s Law ¨ Heat flux is a vector! ¨ In 3D, Fourier’s law becomes ¤ In the Cartesian coordinates ¤ Knowing the temperature field ࠵? = ࠵? ࠵?, ࠵?, ࠵? , heat flux can be calculated 9 ⃗࠵? %% = −࠵?∇࠵? ⃗࠵? %% = −࠵?∇࠵? = −࠵? ࠵?࠵? ࠵?࠵? ࠵? + ࠵?࠵? ࠵?࠵? ࠵? + ࠵?࠵? ࠵?࠵? ࠵? ࠵? & %% = −࠵? ’( ’& ࠵? ) %% = −࠵? ’( ’) ࠵? * %% = −࠵? ’( ’* ⃗࠵? %% = ࠵? & %% ࠵? + ࠵? ) %% ࠵? + ࠵? * %% ࠵? 9 Example 1 10 10
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