Heat Transfer

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Heat Transfer

Heat transfer is the exchange of thermal energy between physical objects. Heat will naturally flow from a hotter to a colder object (2nd Law of Thermodynamics). Heat transfer is a discipline in thermal engineering that deals with the production, use, conversion and exchange of thermal energy (heat) between physical systems. Heat transfer is divided into various mechanisms, e.g. heat conduction, heat convection, heat radiation and energy transfer through a phase change. Engineers are also considering transferring masses of various chemical species, both cold and hot, to achieve heat transfer.

Conduction

Heat is transferred from hot to cold water until both samples reach the same temperature. In this case, the heat transfer from the hot water through a metal can to the cold water is sometimes referred to as conduction. With conductive heat flow, heat is transferred from one place to another without material flow. The transfer of heat from the air to a surface and from the surface to air is called surface conductivity or film conductivity or film coefficient. Next to the surface, there is an airfoil, which has the same effect as other building material with its own conductivity and resistance to heat flow through a structural (thermal) barrier.

Convection

Convection is a type of heat exchange that mainly occurs in liquids and gases. With this method, heat transfer occurs with the real movement of matter from one place in the body to another. When we boil water, we see bubbles and currents in the water. Hot water on the floor becomes lighter and moves upwards, causing cold and denser water to fall from the top and heat up. 

The net result of the growing hot fluid is the transfer of heat from one place to another. The convective heat transfer method always relies on the transfer of heat through the movement of matter. 

Such spontaneous heat transfer always occurs from one high-temperature area to another low-temperature area, as described in the Second Law of Thermodynamics. Heat convection occurs when a mass flow of a fluid (gas or liquid) transfers heat along with the flow of matter in the fluid. The flow of fluid can be forced by external processes or sometimes (in gravitational fields) by buoyant forces that arise when heat energy expands the fluid (for example, in a streak of fire), which affects its own transmission.

Convective heat transfer between fluid and wall is more intense than heat conduction itself due to the additional convective heat transfer, which is the most intense in the turbulent fluid flow regime, in which the effective thermal conductivity of the fluid increases due to turbulent mixing. Heat transfer deep into the fluid stream can be artificially enhanced using various heat transfer enhancement techniques, e.g. by roughing the wall, making secondary flows and including promoters and porous elements. Convective heat transfer occurs in both the external flow through the body and the internal flow of fluid.

Convective heat transfer is the heat transfer that is caused by the combined effect of convective and molecular transfer. For practical purposes, the most important case of heat exchange is convective heat exchange between a flowing medium and its interface with another medium, which interface is called a heat exchange surface or wall when the other medium is a solid. Total heat transfer, i.e. heat exchange between two flowing media via a fixed partition, plays an important role in heat exchangers.

Thermal Insulators

Thermal insulators are materials specially designed to reduce heat transfer by reducing conduction, convection or both. Thermal resistance is a thermal property and measurement in which an object or material withstands heat flow (heat per unit of time or thermal resistance) in relation to a temperature difference.

Radiation

Radiation is the transfer of heat by electromagnetic radiation through space from one body to another without affecting the space between them. Radiation or spectral radiation is a measure of the amount of radiation that is emitted or emitted. Radiation barriers are materials that reflect radiation and therefore reduce heat transfer from radiation sources.

Thermal Conductivity

Thermal conductivity is the heat flow through the fabric due to the temperature difference on both sides of the fabric. Thermal conductivity is usually associated with heat transfer through solids, but can also be done through liquids and gases. Convection is therefore not present in the solid because it depends on the free movement of the substance due to temperature differences. 

Heat usually does not flow through liquids and gases through thermal conductivity. The model for explaining heat transfer by mass of liquids and gases involves convection. 

Heat transfer can differ distinctly at the nanoscale from that at the macroscale. Recent advancement in computational and experimental techniques has enabled a large number of interesting observations and understanding of heat transfer processes at the nanoscale.