CHAPTER 11 TEMPERATURE AND TEMPERATURE-RELATED PARAMETERS

Introduction In this chapter, we will Explain what temperature, thermal energy, and heat transfer mean. take a close look at the role of temperature and heat transfer in engineering design and examine how they play hidden roles in our everyday lives. discuss temperature and its various scales, including Celsius, Fahrenheit, Rankine, and Kelvin. explain a number of temperature-related properties of materials, such as specific heat, thermal expansion, and thermal conductivity.

Temperature as a Fundamental Dimension In order to describe how cold or hot something is, humans needed a physical quantity, or physical dimension, which we now refer to as temperature. Regardless of which engineering discipline you are planning to pursue, you need to develop a good understanding of what is meant by temperature and how it is quantified. Electronic and computer engineers, when designing computers, televisions, or any electronic equipment, are concerned with keeping the temperature of various electronic components at a reasonable operating level so that the electronic components will function properly. Civil engineers need to have a good understanding of temperature when they design pavement, bridges, and other structures. They must design the structures in such a way as to allow for expansion and contraction of materials, such as concrete and steel. Mechanical engineers design heating, ventilating, and air-conditioning equipment to create the comfortable environment in which we rest, work, and play. They need to understand heat transfer processes and the properties of air, including its temperature and moisture content, when designing this equipment.

Material properties are a function of temperature Material properties are a function of temperature. Physical and thermal properties of solids, liquids, and gases vary with temperature. For example, as you know, cold air is denser than warm air. Temperature represents the level of molecular activity of a substance. The molecules of a substance at a high temperature are more active than at a lower temperature.

Measurement of Temperature and Its Units Early humans relied on the sense of touch or vision to measure how cold or how warm something was. In fact, we still rely on touch today This need led to the development of thermometers, which are based on thermal expansion or contraction of a fluid, such as alcohol, or a liquid metal, such as mercury. On the Celsius scale, under standard atmospheric conditions, the value zero was arbitrarily assigned to the temperature at which water freezes, and the value of 100 was assigned to the temperature at which water boils. on a Fahrenheit temperature scale, under standard atmospheric conditions, the temperature at which water freezes is assigned a value of 32, and the temperature at which the water boils is assigned a value of 212. The relationship between the two temperature scales is given by

Today we also use other changes in properties of matter, such as electrical resistance or optical or emf (electromotive force) changes to measure temperature. These property changes occur within matter when we change its temperature. Thermocouples and thermistors, shown in Figure as examples of temperature-measuring devices

Absolute Zero Temperature Because both the Celsius and the Fahrenheit scales are arbitrarily defined, as we have explained, scientists recognized a need for a better temperature scale. This need led to the definition of an absolute scale, the Kelvin and Rankine scales, which are based on the behavior of an ideal gas. The ideal gas law is given by Consider the following experiment, a rigid container is filled with a gas. The container is connected to a pressure gauge that reads the absolute pressure of the gas inside the container, as shown

and thus:

The above equation establishes a relationship between the temperature of the gas, its pressure, and the reference pressure and temperature. If we were to proceed by lowering the surrounding temperature, a lower gas pressure would result, and if we were to extrapolate the results of our experiments, we would find that we eventually reach zero pressure at zero temperature.

Temperature Difference and Heat Transfer Thermal energy transfer occurs whenever there exists a temperature difference within an object, or whenever there is a temperature difference between two bodies, or a temperature difference between a body and its surroundings. This form of energy transfer that occurs between bodies of different temperatures is called heat transfer. Additionally, heat always flows from a high temperature region to a low-temperature region There are three units that are commonly used to quantify thermal energy: (1) the British thermal unit (Btu), (2) the calorie, and (3) the joule

Modes of Heat Transfer Conduction There are three different mechanisms by which energy is transferred from a high-temperature region to a low-temperature region. These are conduction, convection, and radiation. Conduction The energy is transported from the high temperature region to the low-temperature region by molecular activity. The rate of heat transfer by conduction is given by Fourier’s law, which states: 1- The rate of heat transfer through a material is proportional to the temperature difference, normal area A of through which heat transfer occurs, 2-The heat transfer rate is inversely proportional to the material thickness over which the temperature difference exists. see next page

Convection Convection heat transfer occurs when a fluid (a gas or a liquid) in motion comes into contact with a solid surface whose temperature differs from the moving fluid There are two broad areas of convection heat transfer: forced convection and free (natural) convection. Forced convection refers to situations where the flow of fluid is forced by a fan. Free convection refers to situations where the flow of fluid occurs naturally due to density variation in the fluid. For both the forced and the free convection situations, the overall heat transfer rate is governed by

Radiation All matter emits thermal radiation. The higher the temperature of the surface of the object, the more thermal energy is emitted by the object. A good example of thermal radiation is the heat you can literally feel radiated by a fire in a fireplace. The amount of radiant energy emitted by a surface is given by the equation

Specific Heat Have you noticed that some materials get hotter than others when exposed to the same amount of thermal energy? For example, if we were to expose 1 kg of water and 1 kg of concrete to a heat source, you would see that the concrete would experience a higher temperature rise. The reason for this material behavior is that when compared to water, concrete has a lower heat capacity.