Introduction of Heat Transfer Prepared by: Nimesh Gajjar GIT-MED
Difference between heat and temperature Temperature is a measure of the amount of energy possessed by the molecules of a substance. It manifests itself as a degree of hotness, and can be used to predict the direction of heat transfer. The usual symbol for temperature is T. The scales for measuring temperature in SI units are the Celsius and Kelvin temperature scales. Heat, on the other hand, is energy in transit. Spontaneously, heat flows from a hotter body to a colder one. The usual symbol for heat is Q. In the SI system, common units for measuring heat are the Joule and calorie.
Difference between thermodynamics and heat transfer Thermodynamics tells us: how much heat is transferred (dq) how much work is done (dw) final state of the system Heat transfer tells us: how (with what modes) dq is transferred at what rate dq is transferred temperature distribution inside the body
Definition of Heat Transfer Heat transfer is energy in transit due to temperature difference. Whenever there exists a temperature difference in a medium or between media, heat transfer must occur. Basic requirement for heat transfer : difference. presence of temperature Note: There can be no net heat transfer between two mediums that are at the same temperature. temperature Heat flow occurs only in the direction of decreasing The temperature difference is the driving force for heat transfer, just as the voltage difference is the driving force for electric current flow and pressure difference is the driving force for fluid flow.
Modes of Heat Transfer Conduction: An energy transfer across a system boundary due to a temperature difference by the mechanism of intermolecular interactions. Conduction needs matter and does not require any bulk motion of matter. Convection: An energy transfer across a system boundary due to a temperature difference by the combined mechanisms of intermolecular interactions and bulk transport. Convection needs fluid matter. Radiation: Radiation heat transfer involves the transfer of heat by electromagnetic radiation that arises due to the temperature of the body. Radiation does not need matter.
The heat transfer characteristics of a solid material are measured by a property called the thermal conductivity (k) measured in W/m.K. It is a measure of a substance s ability to transfer heat through a solid by conduction. The thermal conductivity of most liquids and solids varies with temperature. For vapors, it depends upon pressure. Thermal Conductivity Convective Heat Transfer Coefficient The convective heat transfer coefficient (h), defines, in part, the heat transfer due to convection. Common units used to measure the convective heat transfer coefficient are W/m 2.K
Heat Quantities Quantity Text Notation SI Unit English Unit heat Q Joule (J) Btu (heat transfer) heat rate q Watt (W) Btu/hr (heat transfer rate) (heat energy rate) (rate of heat flow) heat flux q W/m 2 Btu/hr-ft 2 (heat rate per unit area) heat rate per unit length q W/m Btu/hr-ft volumetric heat generation q W/m 3 Btu/hr-ft 3
Conduction When the handle of a spoon stirring a cup of hot chocolate gets hot, it s because of conduction. Conduction is the transfer of heat through a solid or from one solid to another. When the particles of a solid are heated they gain energy and vibrate more quickly. They bump into neighboring particles and transfer the energy to them. When a stove is hot, the particles in the element speed up. They bump against the particles in the pot so the pot particles gain energy and heat up!
When you heat a metal strip at one end, the heat travels to the other end. As you heat the metal, the particles vibrate, these vibrations make the adjacent particles vibrate, and so on and so on, the vibrations are passed along the metal and so is the heat. We call this? Conduction
Conduction HOT (lots of vibration) COLD (not much vibration) Heat travels along the rod 11
Conduction Conduction is the process whereby heat is transferred directly through a material, any bulk motion of the material playing no role in the transfer. Those materials that conduct heat well are called thermal conductors, while those that conduct heat poorly are known as thermal insulators. Most metals are excellent thermal conductors, while wood, glass, and most plastics are common thermal insulators. The free electrons in metals are responsible for the excellent thermal conductivity of metals. 12
Convection The soup particles that are heated by conduction have increased energy and spread out. They rise and push the cooler particles near the top toward the sides. These cold particles then sink to take the place of the rising hot particles. When these particles come in contact with the bottom of the pot they heat up and the cycle continues. Eventually all of the soup is warm.
What happens to the particles in a liquid or a gas when you heat them? The particles spread out and become less dense. A liquid or gas. This effects fluid movement.
Fluid movement Cooler, more dense, fluids sink through warmer, less dense fluids. In effect, warmer liquids and gases rise up. Cooler liquids and gases sink.
Water movement Cools at the surface Convection current Cooler water sinks Hot water rises
Cold air sinks Where is the freezer compartment put in a fridge? Freezer compartment It is put at the top, because cool air sinks, so it cools the food on the way down. It is warmer at the bottom, so this warmer air rises and a convection current is set up.
The third method of heat transfer How does heat energy get from the Sun to the Earth? There are no particles between the Sun and the Earth so it CANNOT travel by conduction or by convection.? RADIATION
Through the process of radiation, heat flows in the form of waves. The radiation waves flow from hot objects and are absorbed by cooler objects. The cooler objects heat up as they absorb the waves.
Radiation Radiation travels in straight lines True/False Radiation can travel through a vacuum True/False Radiation requires particles to travel True/False Radiation travels at the speed of light True/False
1. Which of the following is not a method of heat transfer? A. Radiation B. Insulation C. Conduction D. Convection
2. In which of the following are the particles closest together? A. Solid B. Liquid C. Gas D. Fluid
3. How does heat energy reach the Earth from the Sun? A. Radiation B. Conduction C. Convection D. Insulation
Why Do We Sweat? Sweating is important for our body to stay cool in hot weather. When a liquid changes to a gas, energy is absorbed from the liquid s surroundings. Heat removed from your body as your perspiration evaporates, makes you cooler.
Mechanisms of Heat Transfer Latent heat Convection Conductio n Radiation R. Shanthini 17 May 2010 25
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