Tong quan ve truyen nhiet - DH CN.PDF

Ly Ngoc Minh, Senior Lecturer 1

TRÖÔØNG ÑAÏI HOÏC COÂNG NGHIEÄP TPHCMKHOA COÂNG NGHEÄ HOAÙ HOÏC VAØ MOÂI TRÖÔØNG

CHƯƠNG ITỔNG QUAN VỀ TRUYỀN NHIỆT


VAI TROØ CUÛA TRUYEÀN NHIEÄT

MÁY HƠI NƯỚC THỜI CỔ ĐẠI

Ñoäng cô hôi nö ôùcthôøi Hy-laïp coåñaïi


Một hình ảnh TB Nhiệt trong thực tế


Câu hỏi thảo luận: cho biết những hình thứctrao đổi nhiệt xảy ra trong hình ảnh này?


Hãy mô tả các quá trình TĐN xảy ra trong hìnhảnh dưới đây?


NOÀI HÔI OÁNG LOØ OÁNG LÖÛA NAÈM


TRUYỀN NHIỆT

NHIỆT BAO GIỜ CŨNG ĐƯỢCTRUYỀN TỪ NƠI CÓ NHIỆT ĐỘ CAOĐẾN NƠI CÓ NHIỆT ĐỘ THẤP.


3 DẠNG TRAO ĐỔI NHIỆT CƠ BẢN

DẪN NHIỆTDẪN NHIỆT

BỨC XẠ NHIỆTBỨC XẠ NHIỆT

ĐỐI LƯU NHIỆTĐỐI LƯU NHIỆT


The Nature of Heat

• Definition: Heat is the internal kinetic energy ofthe atoms and molecules that make up asubstance.– 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 1degree 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 raiseone pound of water by 1 degree Fahrenheit. 1 BTU = 252calories = 1,054 Joules.


Properties of Heat

• Two liters of boiling water has more energythan one one liter of boiling water.

• Heat will not flow between two objects of thesame temperature.

• Heat is energy in the process of beingtransferred from one object to another becauseof the temperature difference between them


Heat Capacity

• Definition: Heat capacity is the amount of heatrequired 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 ofheat.

• Example: water

– Materials with small heat capacities do not hold heatwell.

• Their temperatures will rise quickly for a given amount ofheat

• Example: air


Mechanisms of Heat Transfer

• Conduction• Convection• Advection• Radiation


Conduction

• Conduction is the transfer of heat within asubstance, molecule by molecule


TRUYỀN NHIỆT DẪN NHIỆT

T1

T2

d


TRUYỀN NHIỆT DẪN NHIỆT

DT1 DT2 DT3


Conductivity of Various Substances

427Silver80Iron2.7Granite2.1Ice2.1Wet Soil0.63Snow0.60Water at 20 °C0.25Dry Soil0.023Still air at 20 °CHeat ConductivitySubstance


Convection

• Convection is heat transfer by the movement ofa fluid in the vertical direction


Advection

• Advection is the movement of heat in thehorizontal (north/south/east/west) direction.

• Any measurable property of air can beadvected.


TRUYỀN NHIỆT ĐỐI LƯU


Khái niệm về tuần hòan

HÔI RA

NÖÔÙC CAÁP

OÁ NGNÖ

ÔÙC

X UOÁ NG

OÁNG NÖÔÙC LEÂNOÁNG NÖÔÙC XUOÁNG

Sự luân chuyển củanước trong nồi hơi


Radiation

• Radiation allows heat to be transferred throughwave energy

• These waves are called electromagnetic wavesbecause the energy travels in a combination ofelectric and magnetic waves

• The energy a wave carries is related to itswavelength (distance from crest to crest)


Electromagnetic Spectrum


Stephan-Boltzmann Law

• All things with a temperature above absolute zero emitradiation.

• As the temperature of an object increases, moreradiation is emitted each second

E = σT4

where σ is a constant, T is the temperature of an objectin Kelvin and E is the maximum rate of radiationemitted per square meter.


Wein’s Law

• Wein’s law states that the wavelength of maximumemission of an object is related to the object’stemperature.

λmax = θ/T

where λmax is the wavelength in micrometers (μm) atwhich the maximum radiation emission occurs, θ is aconstant equal to 2897 μm K, and T is the temperaturein Kelvin.


Emission Spectrum ofan Object with

Constant Temperature


Summary


Wein’s Law

• Wein’s law states that the wavelength of maximumemission of an object is related to the object’stemperature.

λmax = θ/T

where λmax is the wavelength in micrometers (μm) atwhich the maximum radiation emission occurs, θ is aconstant equal to 2897 μm K, and T is the temperaturein Kelvin.

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Tong quan ve truyen nhiet - DH CN.PDF