Heat Transfer Topics

• convective heat transfer
• heat transfer coefficient
• system approximation
• heat transfer analysis
• transfer of thermal energy
• transient conduction
• accurate approximation
• conduction system
• biot number
• second law of thermodynamics
• law of thermodynamics
• thermal equilibrium
• heat resistance
• thermal resistance
• conductive heat
• mathematical solution
• constant heat
• physical situation
• fluid motion
• uniform temperature

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Heat Transfer, Heat Transfer

When an object or fluid is at a different temperature than its surroundings or another object, transfer of thermal energy , also known as heat transfer, or heat exchange , occurs in such a way that the body and the surroundings reach thermal equilibrium. Heat transfer always occurs from a higher-temperature object to a cooler temperature one as described by the second law of thermodynamics or the Clausius statement. Where there is a temperature difference between objects in proximity, heat transfer between them can never be stopped; it can only be slowed.

This is a method of approximation that suitably reduces one aspect of the transient conduction system (that within the object) to an equivalent steady state system (that is, it is assumed that the temperature within the object is completely uniform, although its value may be changing in time). In this method, a term known as the Biot number is calculated, which is defined as the ratio of resistance to heat transfer across the object's boundary with a uniform bath of different temperature, to the conductive heat resistance within the object. When the thermal resistance to heat transferred into the object is less than the resistance to heat being diffused completely within the object, the Biot number is small, and the approximation of spatially uniform temperature within the object can be used. As this is a mode of approximation, the Biot number must be less than 0.1 for accurate approximation and heat transfer analysis. The mathematical solution to the lumped system approximation gives Newton's law of cooling, discussed below.

As fluid motion goes faster the convective heat transfer increases. The presence of bulk motion of fluid enhances the heat transfer between the solid surface and the fluid.

The h is the constant heat transfer coefficient which depends upon physical properties of the fluid such as temperature and the physical situation in which convection occurs. Therefore, the heat transfer coefficient must be derived or found experimentally for every system analyzed. Formulae and correlations are available in many references to calculate heat transfer coefficients for typical configurations and fluids. For laminar flows the heat transfer coefficient is rather low compared to the turbulent flows, this is due to turbulent flows having a thinner stagnant fluid film layer on heat transfer surface.

A thermal circuit is the representation of the resistance to heat flow as though it were an electric resistor. The heat transferred is analogous to the current and the thermal resistance is analogous to the electric resistor. The value of the thermal resistance for the different modes of heat transfer are calculated as the denominators of the developed equations. The thermal resistances of the different modes of heat transfer are used in analyzing combined modes of heat transfer.The equations describing the three heat transfer modes and their thermal resistances, as discussed previously are summarized in the table below: In cases where there is heat transfer through different media (for example through a composite), the equivalent resistance is the sum of the resistances of the components that make up the composite.

Using the thermal circuit concept, the amount of heat transferred through any medium is the quotient of the temperature change and the total thermal resistance of the medium.As an example, consider a composite wall of cross- sectional area A. The composite is made of an L 1 long cement plaster with a thermal coefficient k 1 and L 2 long paper faced fiber glass, with thermal coefficient k 2 . The left surface of the wall is at T i and exposed to air with a convective coefficient of h i . The Right surface of the wall is at T o and exposed to air with convective coefficient h o.

In space vacuum, satellites use multi-layer insulation which consists of many layers of aluminized (shiny) mylar to greatly reduce radiation heat transfer and control satellite temperature.

Condensation heat transfer, like boiling, is of great significance in industry. During condensation, the latent heat of vaporization must be released. The amount of the heat is the same as that absorbed during vaporization at the same fluid pressure.

Typically, thermodynamics is a prerequisite to undertaking a course in heat transfer, as the laws of thermodynamics are essential in understanding the mechanism of heat transfer. Other courses related to heat transfer include energy conversion, thermofluids and mass transfer.

Source: Wikipedia > Heat Transfer