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Chapter 2 Principles of Heat

Chapter 2 Principles of Heat

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Chapter 2 Principles of Heat

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  1. Chapter 2 Principles of Heat 建筑外环境 人体对热湿环境的反应 A good thermal environment is a major aspect in the successful performance of a building. Both human beings and their buildings interact with the heat that surrounds them and they also contribute to this heat. 热湿环境 空气质量环境 光环境 声环境

  2. the requirements of human comfort, the types of heat loss and heat gain by buildings,and the nature of moisture in the air. Topics that are relevant to thermal design include: 建筑外环境 热湿环境 空气质量环境 声环境 光环境

  3. Before these topics are considered, this chapter describes: 2.1 Nature of heat, its measurement and its effect 热的性质、量度和作用 2.2 Heat transfer 热传递 2.3 Gases and vapours, and their effects 气体和水蒸气,及其作用 2.4 Refrigerators and heat pumps 制冷机及热泵 Let’s move on to the text. The fist part:

  4. 2.1 Nature of heat 热的性质 2.1.1 Heat energy 热能 Heat is a form of energy 热是能量的一种形式 The joule is the standard SI unit of energy used for measuring any other form of energy. Other units of energy still found in use include the following units. Unit: calorie, where 1 cal 4.187 J kilowatt hour, where 1kwh=3.6MJ British Thermal Unit, where 1BTU=1.055kJ 英制热量单位

  5. Heat energy is an internal molecular property of a material. Other forms of energy including mechanical energy, electrical energy and chemical energy can all be converted to thermal energy. Can you give some examples ? Thermal energy often forms an intermediate stage in the production of other forms of energy. now, I want someone to read the examples given in the book

  6. Power is a measure of the rate at which work is done, or at which energy is converted from one form to another. Unit : watt (W) By definition, 1watt = 1 joule / second. The watt is often used in the measurement of thermal properties and it is useful to remember that it already contains information about time and there is no need to divided by seconds. 2.1.2 Power 功率能力, 力量, 动力, 权力,

  7. Temperature is not the same thing as heat. A red-hot spark火花is at a much higher temperature than a pot of boiling water; yet the water has a much higher heat “content” than the spark and is more damaging. Temperature is the condition of a body that determines whether heat shall flow from it. Unit: kelvin (K) 2.1.3 Temperature温度

  8. Heat flows from objects at high temperature to objects at low temperature. When there is no net heat transfer between two objects they are at the same temperature.

  9. The human body is sensitive to temperature but it is unreliable for measuring temperature. a metal surface and a plastic surface at the same temperature, which one feels colder? The brain tends to judge temperature by the rate of heat flow in or out of the skin. A thermometers is an instrument that measures temperature by making use of some property of a material that changes in a regular manner with changes in temperature. 2.1.4 Thermometers 温度计

  10. Some of the more common types of thermometers are described below: • Mercury-in-glass thermometer 玻璃水银温度计 • Alcohol-in-glass thermometer玻璃酒精温度计 • Thermoelectric thermometer 热电温度计 • Resistance thermometer 电阻温度计 • Optical pyrometer 光学高温计 How do these thermometers measure temperature?

  11. Thermoelectric thermometer 热电温度计 • Use the electric current generated in a thermocouple,热电偶which is made by joining two different metals such as iron and constantan alloy铁-康铜合金. • The current quickly varies with temperature and can be incorporated in remote or automatic control systems 热电偶:用于准确测量温度的热电子元件,由两种连在一起的不同金属组成,这样连结点间产生的电压变化就是两点间温度差异的量度

  12. Resistance thermometer 电阻温度计 • Use the change in electrical resistance which occurs when a metal changes temperature. • Pure platinum is commonly used and the changes in its resistance can be measured very accurately by including the thermometer in an electrical circuit 电路

  13. Optical pyrometer 光学高温计 • Measure high temperature by examining the brightness and colour of the light emitted from objects at high temperatures. • The light varies with temperature and is compared with a light from a filament灯丝at a known temperature.

  14. Celsius scale 摄氏温标 The Celsius temperature scale numbers the temperature of the melting point as 0, and the boiling point of water as 100 CELSIUS TEMPETATURE ( ) is a point on a temperature scale defined by reference to the melting point of ice and the boiling point of water. Unit : degree Celsius (℃) 2.1.5 Temperature scales 温标 This is a proper non. Notice that the first letter must be capitalized

  15. THERMODYNAMIC TEMPERATURE (T) is a point on a temperature scale defined by reference to absolute zero and to the triple point of water Unit : degree kelvin (K) The general relationship between the two temperature scales is given by the following formula. T= +273 Thermodynamic scale 热力学温标

  16. The same mass of different materials can hold different quantities of heat. To produce the same rise in temperature , water and oil, which one should be supplied more heat ? Water has a greater heat capacity than oil. 2.1.6 Heat capacity 热容

  17. The SPECIFIC HEAT CAPACITY (C) 比热of a material is the quantity of heat energy required to raise the temperature of 1 kg of that material by 1 degree kelvin ( or 1 degree Celsius) Unit: J/ kg. K (or J/kg℃) Table 1.1 gives values for a variety of materials What does 4190 J/ kg. K mean? Water is a good medium for storing heat. How to use the high heat capacity of water?

  18. The same mass of different materials may occupy different volumes of space, depending on their densities. Unit: kilogram per cubic metre ( kg/m3) Heavyweight masonry materials石材, such as brick, concrete and stone, have high densities. This means that relatively small volumes of these materials have a large mass and therefore provide a relatively high heat capacity within in a small volume. Next conception 2.1.7 Density 密度

  19. There are three possible states of matter Solid state Liquid state Gas state 2.1.8 Change of state 相态变化

  20. The state of a substance depends on the conditions of temperature and pressure which act on the substance. the absorption of heat by a solid or a liquid can produce the following changes of state Solid → liquid → gas the release of heat from a gas or a liquid can produce the following changes of state Gas → liquid → solid

  21. How most substance behave with heating being’s useful to consider Changes of state for water Sensible heat is the heat energy absorbed or released from a substance during a change in temperature. Latent heat is the heat energy absorbed or released from a substance during a change of state, with no change in temperature. A liquid may change to a gas without heating being supplied, by evaporation for example. The latent heat required for this change is taken from the surroundings and produces an important cooling effect. 2.1.9 Sensible and latent heat 显热和潜热 Figure 1.1

  22. Enthalpy can be described an the total heat content of a sample, with reference to 0℃. 焓可以描述为相对于0℃所包含的总热能。 物理意义:⑴H=U+pV   焓=流动内能+推动功 ⑵焓表示流动工质所具有的能量中,取决于热力状态的那部分能量 2.1.10 Enthalpy 焓

  23. For the particular example of water shown in figure 1.1 , the steam at 100 ℃ has a much higher total heat content than liquid water at 100 ℃. Steam at high temperature and pressure has a very high enthalpy, which makes it useful for transferring large amounts of energy such as from a boiler to a turbine. This steam is also very dangerous if it escapes.

  24. Both sensible and latent heat are forms of heat energy that are measured in joules, although they are calculated in different ways. 2.1.11 Calculation of heat quantities 热量计算

  25. Sensible heat: When a substance changes temperature the amount of sensible heat absorbed or released is given by the following formula Worked example 1.1 ,page 13 H= quantity of sensible heat (J) m = mass of substance (kg) c = specific heat capacity of that substance (J/kg K)

  26. Latent heat: During a change of state in substance the amount of latent heat absorbed or released is given by the following formula. H= quantity of latent heat (J) m= mass of substance (kg) l = specific latent heat for that change of state (j/kg)

  27. Specific latent heat (l) 比潜热is a measure of the latent heat absorbed or released from a particular material for a given change of state. Unit : J/kg Specific latent heat of ice=335kJ/kg Specific latent heat of steam=2260kJ/kg Worked example 1.2

  28. 2.1.12 Expansion 膨胀 Most substance expand on heating and contract on cooling. If the natural expansion and contraction of a body is restricted then very large forces may occur. Different substances expand by different amounts • The coefficient of linear thermal expansion 线性热膨胀系数 is a measure of the relative change of length.

  29. The coefficient of linear expansion for steel is about 12x10-6/ ℃. What does this mean? Concrete expands at a similar rate to steel The expansion of aluminium is about twice that of steel The expansion of plastic is up to ten times that of steel Allowance must be made in construction for the effects of expansion. Particularly for concrete, metals and plastics. Sometimes we can use the expansion of solid figure 1.2 simple thermostat 简易式温度调节器 1) Solids

  30. 2) Liquids Liquids tend to expand more than solids, for the same temperature rise. The expansion rates of different liquids vary and the expansion of alcohol is about 5 times that of water. It is useful to notice the following questions. Does any liquid contract upon cooling? How to use the expansion of liquids? ——thermometers What is the bad effect of liquids expand.

  31. 3) Gases The expansion of gases is hundreds of times greater than the liquids .This behaviour will be described in the later section on gases and vapours . The concept of an absolute zero of temperature绝对 零度was a result of imagining the effect of cooling an ideal gas. Starting at 0 ℃, this ideal gas would shrink in size by 1/273 for each drop in temperature of 1 ℃. At -273 ℃ the volume of the gas would therefore be zero, and matter would have Disappeared.

  32. Now let’s move on to the second topic2.2 Heat transfer 传热 • Heat energy always tends to transfer from high temperature to low temperature regions. via three different modes of heat transfer. 1 Conduction 导热 2 Convection 对流 3 Radiation 辐射

  33. 2.2.1 Conduction 导热 Conduction is the transfer of heat energy through a material without the molecules of the material changing their basic positions. Conduction can occur in solids, liquids and gases although the speed at which it occurs will vary.

  34. Different materials conduct heat at different rates and the measurement of thermal conductivity导热率is described in the next chapter. Metals are the best conductors of heat, because of the free electrons that they possess. Poor conductors are called insulators隔热体.

  35. Since there is no Chinese, I will ask some questions to see if you’ve understood. • Please answer in English, using your own words 1) When conduction happens, whether the molecules of materials change their positions. 2) Can conduction occur in gases? 3) Can you give an example of heat transfer by conduction?

  36. Conduction depends on molecular activity and requires the presence of matter . Example is heat transfer through wall. Heat conduction heat transfer rate (Qk) through a body, as shown in Figure at steady state is governed by Fourier's Law which is expressed as Here is a picture of heat conduction Translate this phrase into Chinese

  37. Convection 对流 Heated air rises, cools, thenfalls.  Air near heater isreplaced by cooler air, and the cycle repeats.

  38. Convection is the transfer of heat energy through a material by the bodily movement of particles Convection can occur in fluids ( liquids and gases) ,but never in solids. Convection heat transfer is of two types: (a) Natural / Free Convection (b) Forced Convection uses a mechanical pump to achieve a faster flow of fluid. The basic equation for convection heat transfer isNewton's Law of Cooling The second method of heat transfer 2.2.2 Convection 对流

  39. Stack effect 烟囱效应 Stack effect describes the natural convection that occurs in buildings causing warm air to flow from the lower to the upper stories 主要作用是拔火拔烟,排走烟气,改善燃烧条件。

  40. There are some pictures showing the damage of stack effect during a fire

  41. Radiation is the transfer of heat energy by electromagnetic waves. Heat radiation occurs when the thermal energy of surface atoms in a material generates electromagnetic waves in the infra-red range of wave length. The last method of heat transfer 2.2.3 Radiation 辐射

  42. The governing equation for radiation is Stenfan-Boltzmann Law

  43. Dull black surfaces阴暗的黑色表面have the highest absorption and emission of radiation heat. Shiny silver surfaces光亮的银色表面have the lowest absorption and emission of radiation heat. The rate at which a body emits heat increases with the temperature of the body.

  44. The greenhouse effectIs one result of the differing properties of heat radiation when it is generated by bodies at different temperatures. As illustrated in figure 2.2.4 The greenhouse effect 温室效应 玻璃对不同波长的辐射有选择性太阳短波辐射进如玻璃,室内吸收后温度升高发出长波辐射透不过玻璃

  45. That atmosphere surrounding the Earth also behaves as a large “green house” around our world. This particular green house effect therefore contributes to global warming.

  46. After 2.2 heat transfer, we will talk about2.3 Gases and vapours 2.3.1 Gases Kinetic Theory:动力学理论the molecules of a gas are always in motion and their velocity increase with temperature. The collisions of many molecules acting on a particular area will be detected as pressure.

  47. unit: pascal (Pa) Is it a base unit in SI system? By definition: 1 pascal = 1newton/ metre2 (1N/m2) Other units which may be found in use include : millibars (mb) , where 1 mb = 101 Pa mm of mercury , where 1mm = 133Pa