The term heat transfer describes the movement of thermal energy from one entity to another at a lower temperature. Such objects could be in the solid state, gaseous state, and liquid state. There are three modes of heat transfer. According to Welty, Rorrer, & Foster (2015), heat may be conveyed from one thing to another object through three modes: radiation, conduction, and convection. If there exists a variation in temperature, heat will constantly move from a greater to lower temperature. Heat transfer thus occurs when objects have different temperatures and refer to how the difference is changed to an equilibrium state. Subsequently, thermodynamics deals with objects that are in the equilibrium state.
Thermal conduction or heat conduction refers to a movement of heat energy from one solid to another that has a variance in temperature when the solids are in contact (Ozisik, 2018). Heat energy flows from the hotter body to the cooler one until both attain thermal equilibrium. According to Welty, Rorrer, & Foster (2015), conduction occurs through the collision of particles and atoms caused by vibration resulting from thermal energy. Fundamentally, an atom received thermal energy from a heat source and starts vibrating, they then collide with adjacent atoms and transmit thermal energy to them, i.e. as the atoms collide, and faster atoms give up thermal energy to relaxed particles. Slower atoms gain heat energy, start vibration and strike with other atoms in the cool solid. This procedure remains till heat from the warmer solid spreads equally through the cooler object. From Murshed & de Castro (2016), some substances are better conductors of heat than others, for instance, solids are better heat conductors that liquids and on the other hand, liquids are better heat conductors than gases. The difference in their ability to conduct heat can be attributed to the distance between particles and atoms, i.e. atoms in solids are pacts and close together, atoms in liquids are further apart and those in gases furthest apart. Conduction is an everyday phenomenon and people can experience it whenever they touch something that is hotter than them.
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In solids, thermal conductivity varies between different objects. According to Ozisik (2018), thermal conductivity refers to the rate at which heat passes through given objects, which is expressed as the quantity of heat which flows through a unit area with a temperature difference of one degree unit per distance. Objects with the best thermal conductivity are metals whereas nonmetals such as plastics, glass, and wood have poor thermal conductivity. Solids which are poor conductors of heat are called insulators. Thermal conduction has everyday applications in our lives. Some applications and examples of conduction in our daily lives include warming your back with a heating pad, a metallic spoon heating when placed inside boiling water and heat transfer from a hot burner on a stove to a pan. Insulators are used to cover metallic appliances such as iron boxed and pans to prevent the people from using them from getting burnt as a result of thermal conduction.
Convection is a mode of heat transfer by mass movement of fluids such as water and gases. Convection is a form of heat transfer that can only happen in fluids because it involves these gases and liquids physically moving. From Murshed & de Castro (2016), convention occurs when there is a temperature gradient between two parts of a gas or liquid, the warmer part of the fluid rises while the cooler part sinks. This sequence usually results in a constant pattern and heat is transported to colder regions. In fluids, convection is the most operational way of thermal transfer. Convection happens when heat from a source heats up gases and fluids till they expand and they become less dense, this section of fluid rises and the cooler part, being denser sinks. This is because hot gases and liquids are less dense than cooler fluids, this creates a convection current which transport energy. According to Ozisik (2018), convection usually results in circulation in a fluid, as the fluid gets heated, it expands and becomes less dense and buoyant, cooler fluid which is denser descends and this creates a pattern of circulation may be formed.
Convection can be witnessed in our day to day situations, for instance, boiling water in a pan, the bubbles of the boiling water are the hotter parts of while the water on top of the pan is the cooler area. The best illustration of convection in gases is hot air ascends and cold air descends to replace it in the atmosphere, in essence, the movement of the air transports thermal energy. According to Murshed & de Castro, (2016), convection is believed to play a significant role in conveying thermal energy from the epicenter of the Sun to the surface and also the movement of magma in the earth's core. In typical heat transfer, it is hard to quantify the impacts of convection because it intrinsically hinges on small non-uniformities in an otherwise fairly uniform medium.
Unlike other forms of heat transfer such as convection and conduction which require a medium to transfer heat, radiation does not require a medium. From Howell, Menguc, & Siegel (2015), radiation is a mode of thermal transfer which does not need any connection between a source of heat and an object. Radiation can occur in a vacuum, the prime example is the heat from the sun, we cannot touch the sun but its heat reaches the earth through radiation. According to Howell, Menguc, & Siegel (2015), heat can be transferred through empty space through thermal radiation. Heat radiation is a process by which a heated object releases energy from the sun in every direction and move to points of absorption at the speed of light. Thermal radiation is a form of energy transference containing electromagnetic waves. In this mode of heat transfer, no medium is needed and no mass is exchanged. Usually, objects emit radiation when high energy electrons in high orbitals fall to low orbitals (lower energy levels). The energy which is lost during this transition is lost as electromagnetic radiations or light. According to Sparrow (2018), the energy which is gained by an atom results in atoms jumping to higher energy levels. It is necessary to note that all objects emit and absorb radiations. In an instance when the absorption of thermal energy is equivalent to the production of energy, the temperature of an item will remain the same. However, if absorption of thermal energy is higher than the release of energy, the temperature of the item increases. If the absorption of energy in a body is lower than the release of energy, the temperature of the object drops.
Radiation has numerous application in our day to day lives. Some of the significant applications of heat radiations include heat radiators in air conditioners and vehicles are painted black in order to attain a cooling effect by radiating a lot of heat, electric heaters have brightly polished surfaces which helps to reflect radiation heat (these surfaces remain cool despite reflecting heat over long periods), the base of cooking utensil are dull so as to absorb more heat from a heat source and finally, greenhouses which are used in cold climates to help plants grow by trapping heat, infrared radiation from the sun enters the greenhouse and warms the plants, nonetheless the infrared red emitted from this is weak and cannot penetrate out of the greenhouse which raises the temperature of the greenhouse (Sparrow, 2018).
This paper has explored heat transfer and looked at the different modes of heat transfer. Heat transfer exists whenever there is a temperature gradient since heat spontaneously flows from a body with a high temperature to one with a colder temperature. Notwithstanding, this is not always the case since some forms of heat transfers such as radiation do not require any medium to transport heat from one point to another. This is because energy is transmitted as electromagnetic waves which travel at the speed of light. Other forms of heat transfer such as convection and radiation require a medium to transmit thermal energy from one object to another. The dissimilarity between the three modes of heat transfer is the medium they use since, radiation travels through a vacuum, convection is heat movement over fluids (gases and liquids) and conduction is a movement of through solids. Heat transfer has numerous applications in our daily lives and has helped improve the quality of life by harnessing energy.
References
Howell, J. R., Menguc, M. P., & Siegel, R. (2015). Thermal radiation heat transfer . CRC press.
Murshed, S. S., & de Castro, C. N. (2016). Conduction and convection heat transfer characteristics of ethylene glycol-based nanofluids–a review. Applied energy , 184 , 681-695.
Ozisik, M. N. (2018). Inverse heat transfer: fundamentals and applications . Routledge.
Sparrow, E. M. (2018). Radiation heat transfer . Routledge.
Welty, J. R., Rorrer, G. L., & Foster, D. G. (2015). Fundamentals of Momentum, Heat, and Mass Transfer: International Student Version . Wiley.
