Lecture 1 Introduction

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4364

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Oct 30, 2023

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8/21/23 1 MECE 4364 Heat Transfer Prof. Dong Liu Department of Mechanical Engineering University of Houston 1 Lecture 1 1 General Information 2 ¨ Lectures: MW 2:30-4:00 PM Office hours: MW 4:10-5:30 PM ¨ Email: dongliu@uh.edu ¨ Textbook ¤ Bergman, Lavine, Incropera, and DeWitt, Fundamentals of Heat and Mass Transfer, John Wiley & Sons, 7 th or 8 th edition, 2011 ¨ Reference book ¤ Cengel and Ghajar, Heat and Mass Transfer: fundamentals & Applications, 6 th edition, McGraw Hill Education, 2020. (ISBN: 978-0-07-339819-8) 2

8/21/23 2 Class Coverage 3 ¨ Introduction ¤ Scope and mechanics of course. ¤ Introduction to the physics of three heat transfer modes. ¨ Conduction ¤ Introduction to heat conduction. ¤ One-dimensional, steady-state conduction. ¤ Fins and fin characterization. ¤ Two-dimensional heat conduction. ¤ Transient heat conduction. ¨ Convection ¤ Introduction to convection. Equations of motion and energy. ¤ Boundary layer equations. ¤ Forced convection in external flows. ¤ Forced convection in internal flows. ¤ Natural convection. ¨ Radiation ¤ Basic concepts of thermal radiation. ¤ View factor. ¤ Radiation heat transfer. ¤ Radiation shields. 3 Important Dates 4 ¨ First day of classes – Aug 21, 2023 ¨ Last day to add a class – Aug 28, 2023 ¨ Academic Holiday – Labor Day Holiday Sep 4, 2023, Thanksgiving Holiday Nov 22-25, 2023 ¨ ORD/Last day to drop a course or withdraw without receiving a grade – Sep 6, 2023 ¨ Last day to drop a course or withdraw with a “W” – Nov 15, 2023 ¨ Last day of class – Dec 2, 2023 Class Date Topic 1 8/21 Introduction 2 8/23 Applications of Heat Transfer Laws 3 8/28 Conduction 4 8/30 Heat Conduction Equation 9/3 Labor Day Holiday 5 9/6 1D Steady-State Conduction 6 9/11 1D Steady-State Conduction for Radial Systems 7 9/13 Conduction with Internal Heat Generation 8 9/10 Fin Analysis 9 9/20 Fin Analysis Examples 10 9/25 Midterm Exam 1 (2:30-4:00 PM) 11 9/27 2D Steady-State Conduction 12 10/2 Transient Conduction Part 1 13 10/4 Transient Conduction Part 2 14 10/9 Introduction to Convection 15 10/11 External Flow Part 1 16 10/16 External Flow Part 2 17 10/18 External Flow Part 3 18 10/23 External Flow Part 4 19 10/25 Internal Flow Part 1 20 10/30 Midterm Exam 2 (2:30-4:00 PM) 21 11/01 Internal Flow Part 2 22 11/06 Internal Flow Part 3 23 11/8 Internal Flow Part 4 24 11/13 Convection Heat Transfer Examples 25 11/15 Natural Convection 26 11/21 Natural Convection Nov 22-25 Thanksgiving Holiday 27 11/27 Radiation Part 1 28 11/29 Radiation Part 2/ Final Exam?? 29 12/11 Final Exam (2:00 - 5:00 PM) 4

8/21/23 3 Grading ¨ Grading ¤ Homework 10% ¤ Midterm Exam 1 30% Coverage: Lectures 1-9 ¤ Midterm Exam 2 30% Coverage: Lectures 11-19 ¤ Final Exam 30% Coverage: Coverage: Lectures 21-28 ¨ Homework ¤ Roughly one assignment every two lectures ¤ Posted and collected on Blackboard ¤ Due at 11:59 PM on the due date ¤ Completion grade , but crucial for self-learning and exams ¨ Exams ¤ Midterm Exam 1 – Sep 25, 2023 (2:30 - 4:00 PM) ¤ Midterm Exam 2 – Oct 30, 2023 (2:30 - 4:00 PM) ¤ Final exam – Nov 29 (2:30 - 4:00 PM) or Dec 9, 2022 (2:00 - 5:00 PM) 5 5 ¨ A bottle of cold Coke left in a room warms up ¤ Energy transfer starts from the warm (higher temperature) medium to the cold (lower temperature) one ¤ Energy transfer stops when the two mediums reach the same temperature ¨ Heat transfer ¤ The process of energy transfer from one system to another as a result of temperature difference – transfer of thermal energy ¤ The science that deals with the determination of the rates of such energy transfers What is Heat Transfer? 6 6

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8/21/23 4 ¨ Thermodynamics ¤ 1 st law: Energy conservation - the amount of energy transfer into (out of) a system must be equal to the amount of increase (decrease) of the energy of that system ¤ 2 nd law: Entropy never decreases - heat must be transferred in the direction of decreasing temperature ¨ Thermodynamics determine the direction and amount of heat transfer for any system undergoing any process from one equilibrium state to another Relation of Heat Transfer to Thermodynamics 7 Initial state Ambient Final state Ambient System System Q W System Ambient 7 ¨ In practice, we are more concerned about the rate of energy transfer (heat transfer per unit time) ¤ “Heat transfer” determines the rates of heat transfer to (or from) a system, the time of heating or cooling as well as the variation of the temperature Moreover ¨ Thermodynamics deals with equilibrium states and changes from one equilibrium state to another ¨ Heat transfer deals with systems that lack thermal equilibrium, and thus it is a nonequilibrium phenomenon Relation of Heat Transfer to Thermodynamics 8 8

8/21/23 5 ¨ Heat transfer is ubiquitous in everyday life and engineering systems ¤ Human comfort ¤ Household appliances ¤ Industrial heat transfer equipment Applications of Heat Transfer 9 9 Historical Background ¨ What is heat ? ¤ Is heat the warmth, hotness or coldness? ¤ Does a body store a definite amount of heat in it? ¨ Caloric theory of heat – until the middle of 19th century 10 ¨ Heat is a fluid-like substance called the caloric ¤ Massless, colorless, odorless, and tasteless ¤ Can be poured from one body into another ¨ When caloric is added to a body, its temperature increases; and when caloric is removed, its temperature decreases ¨ When a body could not contain any more caloric, the body is said to be saturated with caloric ¨ Caloric is a substance that could not be created or destroyed 10

8/21/23 6 Historical Background ¨ Key drawback of the caloric theory ¤ Heat can be generated indefinitely by friction ¨ Kinetic theory of heat ¤ Treats molecules as tiny balls that are in motion and possess kinetic energy ¤ Heat is defined as the exchange of energy associated with the random motion of atoms and molecules ¤ Different temperature – different levels of motion ¨ Heat is the transfer of thermal energy when there is a temperature difference ¨ When the transfer process stops, no more heat 11 Benjamin Thompson (1753-1814) Grinding the barrel of a cannon produces heat James Joule (1818-1889) 11 Different Forms of Energy ¨ Total energy E (or e on a unit mass basis) – the sum of mechanical, kinetic, potential, electrical, magnetic, chemical, and nuclear energy of a system ¨ Internal energy U (or u on a unit mass basis) – the sum of energy related to the molecular structure of a system and the degree of the molecular activity ¤ U = kinetic energy + potential energy ¤ A system has a higher U at higher temperatures ¤ For incompressible substance , i.e., a substance whose density does not change with temperature or pressure 12 ࠵?࠵? ≡ ࠵? % ࠵?࠵? where c is the specific heat (J/(kg K)) and m is the mass of the system ∆࠵? ≡ ࠵?࠵?∆࠵? = ࠵?࠵? ࠵? ! − ࠵? " 12

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8/21/23 7 Different Forms of Energy ¨ Enthalpy h ¤ For systems involving fluid flow where P is pressure and V is the volume of the system, PV is the flow energy (i.e., energy needed to push a a fluid and to maintain flow) ¨ Unit of energy ¤ SI: Joule (J) ¤ English system: British thermal unit (BTU) n Energy needed to raise the temperature of 1 lbm of water at 60 ℉ by 1 ℉ n 1 BTU = 1055.056 J ¤ Calorie (cal) n Energy needed to raise the temperature of 1 gram of water at 14.5 ℃ by 1 ℃ n 1 cal = 4.1868 J 13 ℎ ≡ ࠵? + ࠵?࠵? 13 Energy Transfer ¨ Energy can be transferred by two mechanisms: heat transfer and work ¤ An energy transfer is heat transfer if its driving force is a temperature difference. Otherwise, it is work. ¨ Work done per unit time is called power W ¤ Unit: Watt (W) or horse power ( 1 ℎ࠵? = 746 ࠵?) ¤ The energy of a system decreases as it does work, and increases as work is done on it q Heat transfer per unit time is heat transfer rate q q Unit: # $ = ࠵? q Heat flux : the rate of heat transfer per unit area normal to the direction of heat transfer (Unit: W/m 2 ) q Total amount of thermal energy transferred during a time interval ∆࠵? 14 work ࠵? = # ! ∆# ࠵? % ࠵?࠵? ࠵?′′ ≡ ࠵? ࠵? ࠵? = 24 ࠵? ࠵? !! ≡ ࠵? ࠵? = 24 ࠵? 6 ࠵? " = 4 ࠵?/ ࠵? " 14

8/21/23 8 Conservation of Energy ¨ First law of thermodynamics ¤ Total energy of a system is conserved ¤ Energy can neither be created nor destroyed during a process; it can only change forms ¤ Note: n Energy can be transferred to/from a system by heat , work , and mass flow ¨ Energy balance equation ¨ In the rate form ¨ Steady state with no generation, the rate form becomes 15 ࠵? 01 − ࠵? 234 + ࠵? 561 = ∆࠵? $4 ̇ ࠵? 01 − ̇ ࠵? 234 + ̇ ࠵? 561 = ̇ ࠵? $4 15 Conservation of Energy ¨ Energy balance equation ¨ In heat transfer analysis, we only care energy transfer due to a temperature difference: heat balance ¤ Note: the conversion of nuclear, chemical, mechanical, and electrical energies into thermal energy as heat generation ¤ The general rate form can be very useful 16 ̇ ࠵? 01 − ̇ ࠵? 234 + ̇ ࠵? 561 = ̇ ࠵? $4 ࠵? 01 − ࠵? 234 + ࠵? 561 = ∆࠵? $4 16

8/21/23 9 ¨ Define the control volume and the control surface ¨ Energy conservation equation ¨ Note: ¤ Energy inflow and outflow occur at the control surface ¤ Energy generation and energy storage occur inside the control volume Conservation of Energy 17 Heat radiated to the inner surface of the wall of an oven is convected to the cold surroundings We want to keep track of temperature change in the wall ̇ ࠵? 01 − ̇ ࠵? 234 + ̇ ࠵? 561 = ̇ ࠵? $4 Wall q 1 q 2 Convection Radiation T = ? mass: m specific heat: c ̇ ࠵? 01 =࠵? " ̇ ࠵? 234 =࠵? ! ̇ ࠵? 561 = 0 ̇ ࠵? $4 = ࠵? ࠵?࠵? ࠵?࠵?࠵? = ࠵?࠵? ࠵?࠵? ࠵?࠵? ࠵? " − ࠵? ! = ࠵?࠵? ࠵?࠵? ࠵?࠵? 17 ¨ Energy conservation ¨ Define a special control volume: the surface (zero thickness, zero volume, zero mass) ¨ Surface energy balance Surface Energy Balance 18 Heat conducted to the outer surface of the wall of a house in winter is convected away by the cold outdoor air while being radiated to the cold surroundings We want to keep track of energy transfer at the surface ̇ ࠵? 01 − ̇ ࠵? 234 + ̇ ࠵? 561 = ̇ ࠵? $4 ࠵? 7218 = ࠵? 7219 + ࠵? :;8 ̇ ࠵? 561 = ̇ ࠵? $4 = 0 ̇ ࠵? 01 = ̇ ࠵? 234 ࠵? " = ࠵? ! + ࠵? < 18

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8/21/23 10 Modes of Heat Transfer 19 Fire – driving force Water – thermal energy People – energy carrier No carrier Carrier not moving Carrier moving 19 Modes of Heat Transfer 20 ¨ Heat transfer is the form of energy that can be transferred from one system to another as a result of temperature difference ¨ Three modes of heat transfer ¤ Conduction through a solid or a stationary fluid – no bulk motion ¤ Convection from a surface to a moving fluid – bulk motion ¤ Radiation exchange between two surfaces – no media '' q 1 T 2 T 1 2 T T > '' q s T Moving fluid, T ∞ s T T ¥ > Surface T 1 Surface T 2 1 '' q 2 '' q Conduction Convection Radiation 20