Introduction to combined heat transfer mechanism

Combined Heat Transfer Mechanism

As interesting as the effect of heat transfer on the system is how it occurs. Heat transfer occurs whenever there is a temperature difference. It may happen as quickly as through a cooking pot or as slowly as through a picnic ice chest wall. It is difficult to imagine a situation where heat transfer does not occur because so many processes involve heat transfer. However, all heat transfer is done in only three ways.

Conduction is the transfer of heat through a stationary substance through physical contact. (Matter is stationary on a macroscopic scale. It is known that the thermal motion of atoms and molecules occurs at any temperature above absolute zero.) From the burner of the stove through the bottom of the pot to the food in the pot. The heat transferred to is transferred to. By conduction.

Convection is the heat transfer due to the macroscopic movement of fluid. This type of transfer is done, for example, in forced-air furnaces or meteorological systems.

Radiative heat transfer occurs when microwaves, infrared radiation, visible light, or another form of electromagnetic radiation is emitted or absorbed. A clear example is global warming caused by the sun. A less obvious example is heat radiation from the human body.

In the figure at the beginning of this chapter, fire warms the snowshoe's face primarily by radiation. Convection carries some heat to them, but most of the airflow from the fire is upward (forming the familiar flame shape) and carries heat to the food and sky being cooked. Snowshoes are dressed in a low-conductivity design to prevent heat from escaping outside the body.

This section details these methods. Each method has its own unique and interesting characteristics, but all three have two things in common.

In fireplaces, heat transfer occurs in all three ways: conduction, convection, and radiation. Radiation is the source of most of the heat transferred to the room. Heat transfer, which is also caused by conduction to the room, is much slower. Convective heat transfer is also caused by the cold air entering the room around the window and the hot air coming out of the room up the chimney.

Give examples of everyday life (different from the text) for each mechanism of heat transfer that confirms comprehension.

 

Conduction: If you have a hot cup of coffee, the heat will be transferred to your hand. Convection: Heat is transferred when the barista "steams" cold milk to make hot chocolate. Radiation: Heat is transferred from the sun to a water bottle containing tea leaves to make "Santi". There are numerous other answers available.

 

transmission

When you barefoot across the carpet in the living room of a cold house and step into the tile floor of the kitchen, your feet feel cold on the tiles. This result is interesting because both the carpet and tile floors are at the same temperature. The difference in sensation is explained by the difference in heat transfer coefficient. The heat loss of the skin in contact with the tile is faster than that of the carpet, so the sensation of cold is stronger.

 

Some materials conduct thermal energy faster than others. It indicates a material that conducts heat slowly (good insulation, or inadequate heat conductor), which is used to reduce the flow of heat in and out of the house.

 

Insulation is used to limit heat conduction from the inside to the outside (winter) and from the outside to the inside (summer). (Credit: Giles Douglas)

 

 

The molecular image of heat conduction helps to justify the equations that explain it. Shows the molecules of two objects at different temperatures. {T} _ {\ text {h}} and {T} _ {\ text {c}} "and" cold ". The average kinetic energy of the molecules of a hot object is higher than that of a cold object. When two molecules collide, energy moves from high-energy to low-energy molecules. In metals, images also contain free valence electrons that collide with each other, collide with atoms, and transfer energy as well. The cumulative effect of all collisions is the net heat flux from hot to cold objects. Therefore, the heat transfer coefficient increases with increasing temperature difference \ text {Δ} T = {T} _ {\ text {h}}-{T} _ {\ text {c}}. At the same temperature, the net heat transfer coefficient is zero. Heat conduction is proportional to the cross-sectional area because the number of collisions increases with increasing area.

This is the eq of the second element.

 

The molecules of two objects at different temperatures have different average kinetic energies. Collisions that occur on the contact surface tend to transfer energy from the hot to cold regions. In this figure, the molecules in the cold region (right side) have low energy before the collision, but the energy increases when they collide with high-energy molecules on the contact surface. In contrast, molecules in the hot region (on the left) have high energy before the collision, but lose energy when they collide with low-energy molecules at the contact surface.

The third amount that affects conductivity is the thickness of the material through which heat transfer passes, indicating a slab of material that is hotter on the left side than on the right side. A series of molecular collisions causes heat to move from left to right. The greater the distance between hot and cold, the longer it takes for the material to transfer the same amount of heat.

Heat conduction occurs through any material represented here by a rectangular bar, whether it is glazing or walrus fat.

All four of these quantities are estimated from the experiment and appear in simple equations confirmed by the experiment. The conductive heat transfer coefficient through the slab of the following material is given by the following equation.

 

 

Where P is the power or heat transfer coefficient in watts or kilolocaries / second, A and d are their surface area and thickness as shown. The typical value of thermal conductivity.

More generally, we can write

 

Where x is the coordinates in the direction of the heat flow. Since the power and area are constant, dT / dx is constant and the temperature is from

Thermal conductivity values ​​for common materials are shown at temperatures close to.

 

Thermal conductivity of material k

 

Diamond 2000

Silver 420

Copper 390

Gold 318

Aluminum 220

Steel 80

Steel (stainless steel) 14

Ice 2.2

Glass (average) 0.84

Concrete brick 0.84

Water 0.6

Adipose tissue (without blood) 0.2

Asbestos 0.16

Gypsum board 0.16

Wood 0.08–0.16

Snow (dry) 0.10

Cork 0.042

Glass wool 0.042

Wool 0.04

Down feather 0.025

Air 0.023

Styrofoam 0.010