Heat Transfer (Essay Sample)

Heat Transfer
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INTRODUCTION
In physics, heat is defined as a form of energy that flows from hotter systems to cold systems (Lienhard, 2008). On the other hand, heat transfer is a branch of thermal engineering that deals with generation, conversion, exchange and the use of thermal energy. Heat transfer deeply explains the exchange of thermal energy between systems. The exchange of energy between these physical systems depends on the temperature and pressure difference that exists between the systems (Lienhard, 2008).
When the system is at a different temperature from another system or its surroundings, there is a net flow of heat so that the systems are at the same temperature (Sieder, 1995). However, when two bodies or systems are at the same temperature, there is no exchange of heat between them. according to Lienhard (2008),the systems are said to be in thermal equilibrium that is reached when all the bodies or systems involved are attain the same temperature .
There are various modes of heat transfer. The most common mechanisms of heat transfer are conduction, radiation, and convection.
CONDUCTION
In heat transfer, heat conduction means transfer of thermal energy between the adjacent molecules or particles in a substance because of a temperature difference between the molecules (Lienhard, 2008). On a microscopic scale, this mode of heat transfer takes place as hot vibrating molecules and atoms interact with the neighboring molecules and atoms. In the process of that interaction, the molecules exchange some heat.
Heat conduction takes place in all forms of matter; liquids, gases, and solids. In solids, heat conduction is due tom the vibration and interaction of molecules in a lattice (Sieder, 1995). Conduction in liquids and gases heat conduction is caused by the diffusion and collisions of molecules during their random motion (Brownian motion) (Sieder, 1995).
Steady state conduction is a form of heat conduction that takes place when the temperature difference causing the conduction is constant. In this case, the spatial distribution of temperatures does not change anymore.
Another example of conduction is called transient conduction, also termed as non-steady state. Transient conduction occurs when temperature within an object changes as a function of time.
FACTORS AFFECTING HEAT CONDUCTION
There are various factors that determine the rate at which heat flows through conduction especially in solids. One of the factors that affect heat conduction is the temperature difference. The greater the temperature gradient between the two ends of the conductor, the faster the rate of conduction. Therefore, the rate of heat conduction in solids is directly proportional to the temperature gradient/difference (Yunus, 2011).
Another major factor that affects conduction in metals is the cross-sectional area of the conductor. The rate conduction is directly proportional to the cross-sectional area of the solid. A wide metal bar conducts heat better than a thin one, both of the same material.
The nature of the material on which conduction takes place also influences the rate of conduction. Different materials have different conductivities. The best ordinary metallic conductors are gold, copper, silver, and aluminum (Yunus, 2011). These metals have higher conductivities as compared to others.
Finally, the other factor that affects the rate of heat conduction through conduction is the length of the material that translates to the distance heat must travel. It has established experimentally that the conducted heat is inversely proportional to the length of the material (Lienhard, 2008). Therefore, long materials conduct heat at a slower rate than shorter ones (Yunus, 2011). The reason is that less heat that will makes it through long materials.
CONVECTION
Also known as convective heat, transfer, convection is the transfer of heat from one point to another through fluids (Lienhard, 2008). In Physics, the term fluid is understood to mean a substance that distorts under shear stress. Fluids include the gases and liquids. Lienhard (2008) affirms that the convection is a mode of heat transfer that is dominant in liquids and gases only.
A practical example of heat transfer through convection is water boiling in a pot on a hot plate. The pot heats up at the bottom via conduction. The water that is in direct contact with the hot bottom of the pot heats up. It is at this point that the convection starts: the hotter water at the bottom rises as cooler water on the surface of the pot sinks. Yunus (2011) claims tha the reason is that the hot water is more buoyant that the cold one. This process continues on thus creating vertical circulation convection current.
There are two types of convective heat transfer which are forced and natural convection. Natural convection takes place when the motion of the fluid is caused by buoyant forces resulting from the difference in density (Yunus, 2011). The cause of the variation in density of the fluid is the variation of temperature of the fluid. For example in the absence of external influence, when the fluid is in contact with a hot surface, molecules scatter, making the fluid less dense. Consequently, the fluid is displaced upwards to the cooler parts and the cooler fluid sinks to the surface to replace it (Yunus, 2011). As a result, convection currents are formed. It clearly explains the scenario that is observed when water heats in a pot from below.
Forced convection takes place in fluids when a fluid flow is stimulated by external forces as by stirring and use of pumps. Application of these forces creates convectional currents that are artificially induced.
APPLICATIONS OF HEAT CONVECTION
Convection currents have several important applications in everyday life. One of the most crucial applications of convection currents is the circulation of air in the room. The air that the occupant of a room breath out is warmer compared to the air in the room. The warmer air moves up in the room and gets out through the ventilators as the colder fresh air comes into the room through the door and windows.
Sea and land breeze are convection currents. During the daytime, the land absorbs heat faster than the water in the sea. As a result, the air above the land becomes warmer than the air above the sea, and consequently, convectional currents form. The phenomenon is called a sea breeze. During the night, the sea water is warmer than the land. The air above the sea, therefore, is wa...