Introduction to Heat and Mass Transfer. Week 1

Transcription

1 Introduction to Heat and Mass Transfer Week 1

2 This Lecture Course Organization Introduction Basic Modes of Heat Transfer Application Areas

3 Instructor Information Professor Chang-Da Wen 温昌達» Office: (7F)» Phone: ext » Course Website:

4 Lecture Information Lecture Hours:» Tuesdays, 11:10 am 12 pm, Room 91207» Thursdays, 3:10 5 pm, Room TA: 張嘉元» Office: 91C02 (12F)» st940001@gmail.com» Phone: ext ext. 43» Office hours: Wednesdays, 7-9 pm

5 Textbook(Reference) Principles of Heat and Mass Transfer John Wiley, 7 th Edition (2012) Incropera, DeWitt, Bergmann, and Lavine Read the assigned sections from the textbook before coming to class Lectures are meant for further insights and discussion

6 Prerequisites Many concepts in heat transfer require background in physics and fluid mechanics you must be familiar with such material (or review on your own) Prerequisites:» Fluid Mechanics» Thermodynamics» Heat Transfer

7 Homework HW problems may be assigned every week HW problems will be collected and graded Late HW will not be accepted Solutions will be available with Course TA and post in TA s office (go check it during the office hours) Solve HW problems and additional problems from the textbook to get solid practice for examinations

8 Examinations One comprehensive final exam No make-up exams Exams are open-book/open-notes Academic dishonesty of any sort (homework or exams) will be treated in accordance with university regulations

9 Grading Homeworks 30% Take-home Exam/Projects 30% One final exam 30% Lecture attendance 10%

10 No make-up exam!! Your final grade is not negotiable!!

11 Sources of Help Instructor TA Office Hours Attend and participate in class (attendance sheet will be passed) Read textbook and review class notes everyday Complete and study homework assignments

12 Course Goals In this course, we will:» learn how to recognize relevant transport processes given an engineering problem» understand three modes of heat transfer and the related equations and correlations» apply the above knowledge to practical thermal and mass exchange systems» develop a basic understanding for the challenging yet very exciting engineering frontiers of the new millennium» appreciate the beauty and power of the science of heat transfer

13 What is Temperature? Temperature is a measurement of the amount of heat a substance contains. There are three common temperature scales: Fahrenheit, Celsius, and Kelvin. Our intuitive notion is that two systems in thermal contact should exchange no heat, on average, if and only if they are at the same temperature.

14 Thermometers Thermometers-Bimetal: Two different metals are bonded together. Since metal expand at different rates, movement occurs depending on the temperature fluctuation. Glass bulb: Mercury or other liquid will expand. Galileo thermometer: These interesting models operate based on principles of specific gravity. As the temperature of the fluid increases, the relationship of relative weight between the fluid and the floats changes.

15 Thermometers Electronic digital resistance (RTD and Thermistor): These are based on the change in resistance of a conductor when the temperature of the wire changes. Infrared thermometer/pyrometer: It s reasonable to expect that the temperature of a material affects the color. As it turns out, the infrared light spectrum works very well for this and is the basis for the infrared thermometer or pyrometer.

16 Thermometers Thermocouple: These operate based on the temperature change that occurs at the junction of two dissimilar wires. When the temperature changes a small current is generated by the junction.

17 Thermocouple From Wikipedia, the free encyclopedia A thermocouple or thermocouple thermometer is a junction between two different metals that produces a voltage related to a temperature difference. Thermocouples are a widely used type of temperature sensor and can also be used to convert heat into electric power. They are inexpensive and interchangeable, have standard connectors, and can measure a wide range of temperatures. The main limitation is accuracy: system errors of less than one kelvin (K) can be difficult to achieve. Any circuit made of dissimilar metals will produce a temperaturerelated potential. Thermocouples for practical measurement of temperature are made of specific alloys, which in combination have a predictable and repeatable relationship between temperature and voltage. Particular alloys are used for different temperature ranges. Other properties, such as resistance to corrosion, may also be important when choosing which type of thermocouple is most appropriate for a given application.

18 Thermocouple From Wikipedia, the free encyclopedia Where the measurement point is far from the measuring instrument, the intermediate connection can be made by extension wires, which are less costly than the materials used to make the sensor. Thermocouples are standardized against a reference temperature of 0 degrees Celsius; practical instruments use electronic methods of cold-junction compensation to adjust for varying temperature at the instrument terminals. Electronic instruments can also compensate for the varying characteristics of the thermocouple, and so improve the precision and accuracy of measurements. Thermocouples are widely used in science and industry; a few applications would include temperature measurement for kilns, measurement of exhaust temperature of gas turbines or diesel engines, and many other industrial processes.

19 Thermocouple

20 Thermocouple Thermocouple Application (see complementary materials) Thermocouple Fabrication (see complementary materials) Thermocouple Calibration (see complementary materials)

21 Project #1 Project topic: What is Thermocouple? Individual project report (no page limit) is due on Thursday(10/12) before the class Late project report is not accepted Students have to make a thermocouple and do the calibration in front of TA in 91X02 in order to pass the fabrication test and get the credit (schedule will be announced later)

22 What is Heat Transfer? Thermodynamics studies how systems change from one state to another due to work and heat interactions Classical thermodynamics deals only with equilibrium states of system Heat transfer is an inherently non-equilibrium process i.e. we deal with heat (or energy) transfer owing to temperature difference In heat transfer, we study principles necessary to quantify the different ways by which thermal energy can flow

23 Applications Electronics Cooling for High Heat Flux Compressor Design Refrigeration/Air Conditioning Systems

24 Applications (contd.) Engine Design Understanding trajectory of football Design of Clothing for Human Comfort

25 Applications (contd.) Development of Ultra-strong Materials Boiling Heat Transfer

26 Pioneers of Heat Transfer Joseph Fourier Josef Stefan Ludwig Boltzmann Isaac Newton Max Planck Albert Einstein

27 Pioneers of Heat Transfer (contd.) Reynolds Rayleigh Stanton Biot Peclet Grashof Prandtl Nusselt Schmidt Sherwood

28 Basic Modes Conduction: thermal energy in transient due to temperature gradient across stationary solid or fluid Convection: thermal energy in transient due to temperature difference between a moving fluid and usually a solid bounding surface Conduction and convection require physical medium Radiation: thermal energy in transient due to the emission of electromagnetic waves without presence of physical medium

29 Conduction Conduction random atomic or molecular motion at microscopic level (diffusion) Energy consists of microscopic energy (translational, vibrational, rotational, spin as well as chemical and nuclear) For fluids:» higher temperature associated with higher energy molecules while lower temperature associated with lower energy molecules» collisions cause thermal energy transfer from higher energy molecules to lower energy molecules

30 Conduction (contd.) For solids:» lattice waves due to atomic activity cause thermal energy transfer for non-conductors» motion of free electrons through lattice structure (in addition to lattice waves) responsible for thermal energy transfer in conductors

31 Conduction (contd.) Fourier s Law» Conductive heat flux: where, q cond = Heat flux in the direction of temperature gradient (W/m 2 ) k = Thermal conductivity in the direction of heat transfer (W/m-K) dt/dx = Temperature gradient (K/m)» Conductive heat transfer rate: where, T " q k cond x T " q q A ka cond cond x q cond = Heat transfer rate due to conduction (W) A = Area normal to temperature gradient (m 2 ) T 1 q cond x T 2

32 Convection Convection bulk motion at macroscopic level (advection) in addition to diffusion Velocity and thermal boundary layer development when fluid moves across a stationary surface No slip at the surface i.e. only conduction (diffusion) at the surface With increase in velocity away from the surface, contribution due to advection increases