Modes of Heat Transfer-1 P M V Subbarao Professor Mechanical Engineering Department Various Possibilities of a Natural Happening ….. Change in Class Room : LH 519

General Conduction Heat Transfer in a System Conduction Heat Transfer from/to a System: Heat flux due to conduction

Heat transfer through a plane slab

HEAT TRANSFE IN TURNING Cooling of a workpiece on a lathe is affected not only by the usual factors such as exposure of its surfaces and their velocities, but also by the presence of the chuck and the jet the cutting fluid splashing on the workpiece while moving with the tool. The cooling effect of the surface area under the impinging coolant jet is not uniform and therefore the heat transfer coefficient in this zone should be treated as a distribution. The initial conditions of cooling at the beginning of the pass, when the jet flashes also on the side face of the workpiece, poses additional challenge in determination of heat transfer coefficients.

Heat Convection Convection uses the motion of fluids to transfer heat. In a typical convective heat transfer, a hot surface heats the surrounding fluid, which is then carried away by fluid movement such as wind. The warm fluid is replaced by cooler fluid, which can draw more heat away from the surface. Since the heated fluid is constantly replaced by cooler fluid, the rate of heat transfer is enhanced.

Natural Convection Natural convection (or free convection) refers to a case where the fluid movement is created by the warm fluid itself. The density of fluid decrease as it is heated; thus, hot fluids are lighter than cool fluids. Warm fluid surrounding a hot object rises, and is replaced by cooler fluid. The result is a circulation of air above the warm surface

Forced Convection Forced convection uses external means of producing fluid movement. Forced convection is what makes a windy, winter day feel much colder than a calm day with same temperature. The heat loss from your body is increased due to the constant replenishment of cold air by the wind. Natural wind and fans are the two most common sources of forced convection.

Newton's Law of Cooling The rate at which a hot body cools to the temperature of its surroundings is given by an empirical formula first discovered by Sir ISAAC NEWTON Newton's law of cooling states, "For a body cooling in a draft, the rate of heat loss is proportional to the difference in temperatures between the body and its surroundings." Since the temperature change is proportional to the heat transfer. Energy transport due to two combined effects: diffusion: random molecular energy advection: bulk (macroscopic) fluid motion The process of convection occurs between a moving fluid and a surface at different temperatures.

Convection Heat Transfer where h is the local heat transfer coefficient. Both the flux and transfer coefficient vary along the surface.

The local heat transfer rate is Local and Average Heat Transfer Rate The Total heat transfer rate is Where h is average heat transfer coefficient:

The total heat transfer rate Q is Where, h avg is the average convection heat transfer coefficient for the entire surface. where Therefore

Thermal Radiation Radiation does not require a medium to pass through; thus, it is the only form of thermal energy travel present in vacuum. It uses electromagnetic (photons) means of transportation, which travels at the speed of light. Thermal radiation is emitted by any matter with temperature above 0 degree Kelvin (-273 °C). Radiative heat transfer occurs when the emitted radiation strikes another body and is absorbed. The electromagnetic spectrum classifies radiation according to wavelengths of the radiation. Main types of radiation are (from short to long wavelengths): gamma rays, x-rays, ultraviolet (UV), visible light, infrared (IR), microwaves, and radio waves. Radiation with shorter wavelengths are more energetic and contains more heat.