Bài giảng Tổng quan về truyền nhiệt

Ly Ngoc Minh, Senior Lecturer 1 TRÖÔØNG ÑAÏI HOÏC COÂNG NGHIEÄP TPHCM KHOA COÂNG NGHEÄ HOAÙ HOÏC VAØ MOÂI TRÖÔØNG CHƯƠNG I TỔNG QUAN VỀ TRUYỀN NHIỆT Ly Ngoc Minh, Senior Lecturer 2 VAI TROØ CUÛA TRUYEÀN NHIEÄT MÁY HƠI NƯỚC THỜI CỔ ĐẠI Ñoäng cô hôi nö ôùc thôøi Hy-laïp coåñaïi Ly Ngoc Minh, Senior Lecturer 3 Một hình ảnh TB Nhiệt trong thực tế Ly Ngoc Minh, Senior Lecturer 4 Câu hỏi thảo luận: cho biết những hình thức trao đổi nhiệt xảy ra trong hình ảnh này? Ly Ngoc Minh, Senior Lecturer 5 Hãy mô tả các quá trình TĐN xảy ra trong hình ảnh dưới đây? Ly Ngoc Minh, Senior Lecturer 6 NOÀI HÔI OÁNG LOØ OÁNG LÖÛA NAÈM Ly Ngoc Minh, Senior Lecturer 7 TRUYỀN NHIỆT NHIỆT BAO GIỜ CŨNG ĐƯỢC TRUYỀN TỪ NƠI CÓ NHIỆT ĐỘ CAO ĐẾN NƠI CÓ NHIỆT ĐỘ THẤP. Ly Ngoc Minh, Senior Lecturer 8 3 DẠNG TRAO ĐỔI NHIỆT CƠ BẢN DẪN NHIỆT I T BỨC XẠ NHIỆT IỆT ĐỐI LƯU NHIỆTI L IỆT Ly Ngoc Minh, Senior Lecturer 9 The Nature of Heat • Definition: Heat is the internal kinetic energy of the atoms and molecules that make up a substance. – Since it is a form of energy, it is measure in the standard unit of a Joule. – More commonly, it is measured in the following units: • calorie – heat energy needed to raise 1 gram of water by 1 degree Celsius. 1 calorie = 4.186 Joules. • Calorie – commonly used to measure energy content of food. 1 Calorie = 1000 calories. • British Thermal Unit (BTU) – heat energy needed to raise one pound of water by 1 degree Fahrenheit. 1 BTU = 252 calories = 1,054 Joules. Ly Ngoc Minh, Senior Lecturer 10 Properties of Heat • Two liters of boiling water has more energy than one one liter of boiling water. • Heat will not flow between two objects of the same temperature. • Heat is energy in the process of being transferred from one object to another because of the temperature difference between them Ly Ngoc Minh, Senior Lecturer 11 Heat Capacity • Definition: Heat capacity is the amount of heat required per unit increase in temperature. – It is a measure of how well the substance stores heat. – Heat added = heat capacity x (change in temperature) – Materials with large heat capacities hold heat well. • Their temperatures will not rise much for a given amount of heat. • Example: water – Materials with small heat capacities do not hold heat well. • Their temperatures will rise quickly for a given amount of heat • Example: air Ly Ngoc Minh, Senior Lecturer 12 Mechanisms of Heat Transfer • Conduction • Convection • Advection • Radiation Ly Ngoc Minh, Senior Lecturer 13 Conduction • Conduction is the transfer of heat within a substance, molecule by molecule Ly Ngoc Minh, Senior Lecturer 14 TRUYỀN NHIỆT DẪN NHIỆT T1 T2 d Ly Ngoc Minh, Senior Lecturer 15 TRUYỀN NHIỆT DẪN NHIỆT DT1 DT2 DT3 Ly Ngoc Minh, Senior Lecturer 16 Conductivity of Various Substances 427Silver 80Iron 2.7Granite 2.1Ice 2.1Wet Soil 0.63Snow 0.60Water at 20 °C 0.25Dry Soil 0.023Still air at 20 °C Heat ConductivitySubstance Ly Ngoc Minh, Senior Lecturer 17 Convection • Convection is heat transfer by the movement of a fluid in the vertical direction Ly Ngoc Minh, Senior Lecturer 18 Advection • Advection is the movement of heat in the horizontal (north/south/east/west) direction. • Any measurable property of air can be advected. Ly Ngoc Minh, Senior Lecturer 19 TRUYỀN NHIỆT ĐỐI LƯU Ly Ngoc Minh, Senior Lecturer 20 Khái niệm về tuần hòan HÔI RA NÖÔÙC CAÁP OÁ NG NÖÔ ÙC X U OÁ NG OÁNG ÖÔÙCLEÂ OÁNG ÖÔÙCXU Sự luân chuyển của nước trong nồi hơi Ly Ngoc Minh, Senior Lecturer 21 Radiation • Radiation allows heat to be transferred through wave energy • These waves are called electromagnetic waves because the energy travels in a combination of electric and magnetic waves • The energy a wave carries is related to its wavelength (distance from crest to crest) Ly Ngoc Minh, Senior Lecturer 22 Electromagnetic Spectrum Ly Ngoc Minh, Senior Lecturer 23 Stephan-Boltzmann Law • All things with a temperature above absolute zero emit radiation. • As the temperature of an object increases, more radiation is emitted each second E = σT4 where σ is a constant, T is the temperature of an object in Kelvin and E is the maximum rate of radiation emitted per square meter. Ly Ngoc Minh, Senior Lecturer 24 Wein’s Law • Wein’s law states that the wavelength of maximum emission of an object is related to the object’s temperature. λmax = θ/T where λmax is the wavelength in micrometers (μm) atwhich the maximum radiation emission occurs, θ is a constant equal to 2897 μm K, and T is the temperature in Kelvin. Ly Ngoc Minh, Senior Lecturer 25 Emission Spectrum of an Object with Constant Temperature Ly Ngoc Minh, Senior Lecturer 26 Summary Ly Ngoc Minh, Senior Lecturer 27 Wein’s Law • Wein’s law states that the wavelength of maximum emission of an object is related to the object’s temperature. λmax = θ/T where λmax is the wavelength in micrometers (μm) atwhich the maximum radiation emission occurs, θ is a constant equal to 2897 μm K, and T is the temperature in Kelvin.