Heat Transfer Mechanisms
HEAT TRANSFER MECHANISMS
Heat energy is being transferred from one location to another because of a temperature difference. The three mechanisms for heat transfer are:
• Conduction.
When you grip the hot handle of a pan on a stove, you feel conduction in action. Heat flows from the pan and along the length of the handle to its cooler free end. This is because one end of the rod is held at the high temperature, and the other end stays at the lower temperature. Although the rod itself doesn’t move, heat flows down it like a current because points along the rod have different temperatures. Just as a change in voltage produces a current in an electrical circuit, the change in temperature causes heat to be conducted along the rod.
• Convection.
In addition to conduction, heat can also be transferred by a fluid that is in motion; that process is known as convection. The cooling system of an automobile engine, for instance, operates by pumping a mixture of water and antifreeze through passageways inside the engine’s block. Excess heat is removed from the engine, transferred temporarily to the coolant by convection, and ultimately released into the air by the vehicle’s radiator. Because a pump circulates the coolant, heat transfer is said to occur by forced convection. Some kitchen ovens have a forced convection feature that circulates the heated air to heat food more quickly and evenly. In other circumstances, a liquid or gas can circulate on its own, without the help of a pump or fan, because of the buoyancy forces created by temperature variations within the fluid. As air is heated, it becomes less dense, and buoyancy forces cause it to rise and circulate. The rising flow of warm fluid (and the falling flow of cooler fluid to fill its place) is called natural convection. Thermals develop near mountain ridges and bodies of water; these are natural convection currents in the atmosphere that sailplanes, hang gliders, and birds take advantage of to stay aloft.
Heat energy is being transferred from one location to another because of a temperature difference. The three mechanisms for heat transfer are:
• Conduction.
When you grip the hot handle of a pan on a stove, you feel conduction in action. Heat flows from the pan and along the length of the handle to its cooler free end. This is because one end of the rod is held at the high temperature, and the other end stays at the lower temperature. Although the rod itself doesn’t move, heat flows down it like a current because points along the rod have different temperatures. Just as a change in voltage produces a current in an electrical circuit, the change in temperature causes heat to be conducted along the rod.
• Convection.
In addition to conduction, heat can also be transferred by a fluid that is in motion; that process is known as convection. The cooling system of an automobile engine, for instance, operates by pumping a mixture of water and antifreeze through passageways inside the engine’s block. Excess heat is removed from the engine, transferred temporarily to the coolant by convection, and ultimately released into the air by the vehicle’s radiator. Because a pump circulates the coolant, heat transfer is said to occur by forced convection. Some kitchen ovens have a forced convection feature that circulates the heated air to heat food more quickly and evenly. In other circumstances, a liquid or gas can circulate on its own, without the help of a pump or fan, because of the buoyancy forces created by temperature variations within the fluid. As air is heated, it becomes less dense, and buoyancy forces cause it to rise and circulate. The rising flow of warm fluid (and the falling flow of cooler fluid to fill its place) is called natural convection. Thermals develop near mountain ridges and bodies of water; these are natural convection currents in the atmosphere that sailplanes, hang gliders, and birds take advantage of to stay aloft.