What are the factors that affect convective heat transfer

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Convective heat transfer is a complex energy mass exchange process, so there are many factors affecting the degree of convective heat exchange.

The main ones are: (1) fluid flow state: all those that rely on external forces to promote fluid flow are called forced motion; Natural convection caused by different temperatures of various parts of the fluid is called free motion.

The convective heat transfer coefficient of the forced motion is larger than that of the freely moving fluid; Fluids in turbulent flow have a greater heat transfer coefficient than fluids in laminar flow. This is because when the fluid flow rate is high, the disturbance is strong and the heat transfer is intensified.

Whether a phase change occurs in a fluid: The convective heat transfer coefficient is much greater when the liquid is vaporized or when the gas is condensed than when there is no phase change. For example, in the process of boiling water, due to the continuous generation, expansion and detachment of bubbles on the wall, the cold fluid constantly rushes to the wall, causing the fluid to disturb violently, and the heat exchange intensity is greatly improved.

3) Physical properties of fluids: different types of fluids, different physical properties, and convective heat transfer coefficients are also quite different. Generally, the convective heat transfer coefficient of fluids with large thermal conductivity is also large; Fluids with high specific heat capacity and density have a larger convective heat transfer coefficient; When the viscosity of the fluid is large, its convective heat transfer coefficient is small.

4) The flow direction of the fluid relative to the heating surface pipe: the convective heat transfer coefficient is larger when the fluid washes the pipe horizontally than the pipe when it is longitudinally washed.

The structural characteristics of the heating surface: pipe diameter, pitch and arrangement have an impact on the heat exchange coefficient. The pipe diameter is small, the pitch is small, and the convection heat transfer coefficient is large; The convection heat transfer coefficient of the pipe is larger than that of the sequential row.

Summary. Forced convection and natural convection are two different ways of heat exchange. Forced convection is a way in which heat is transferred on the surface of an object under the action of external forces (such as wind, water, or mechanical devices).

Natural convection, on the other hand, refers to a way in which heat is transferred on the surface of an object due to the uneven temperature of the surface of the object.

Forced convection and natural convection are two different ways of heat exchange. Forced terrestrial convection refers to a way in which heat is transferred on the surface of an object under the action of external forces (such as wind, water flow, mechanical devices). Natural convection, on the other hand, refers to a way in which heat is transferred on the surface of an object due to the uneven temperature of the surface of the object.

Therefore, the difference between forced convection and natural convection is that the former is caused by external forces, while the latter is caused by external heat.

The differences between the two are as follows: different characteristics of heat transfer, different influencing factors, and different forms of heat exchange. 1. Characteristics of convective heat transfer heat transfer:

It is a complex heat transfer process in which heat conduction and heat convection exist at the same time, and there must be direct contact (fluid and wall) and macroscopic movement; There must also be a temperature difference. The characteristics of thermal convection heat transfer: it can only occur in fluids (gases and liquids), and must be accompanied by the thermal conductivity caused by the slippery movement of the molecular bench of the fluid itself.

2. Influencing factors of convective heat transfer: the cause of fluid flow; whether there is a phase change in the fluid; the flow state of the fluid (single-phase flow); physical conditions of the fluid; Geometry of the heat exchange surface Factors influencing thermal convection: temperature difference, thermal conductivity and thickness and cross-sectional area of the heat conductive object.

The convective heat transfer coefficient refers to the ability to transfer heat between a fluid and a solid during heat transfer. The influencing factors mainly include the following aspects:

1.Properties of fluids: The properties of fluids include density, viscosity, thermal conductivity, etc. The properties of the fluid will affect the flow state and heat transfer capacity of the fluid, which will affect the number of convective heat transfer systems.

2.Flow rate of the fluid: The flow rate of the fluid directly affects the flow state and heat transfer capacity. When the flow velocity is larger, the convective heat transfer coefficient of the fluid will also be greater.

3.How fluids come into contact with solids: The way fluids come into contact with solids affects the efficiency of heat transfer. For example, when the fluid is in contact with a solid, the convective heat transfer coefficient will also be larger.

4.The temperature of the fluid: The temperature of the fluid affects the rate at which its heat is transferred. The higher the temperature of the fluid, the greater its convective heat transfer coefficient.

5.Flow mode of fluid: The flow mode of fluid includes two states: laminar flow and turbulent flow. When the flow state of a fluid changes to turbulence, its convective heat transfer coefficient also increases.

6.Morphology of solid surfaces: The morphology of solid surfaces also affects the convective heat transfer coefficient. For example, when a solid surface is rough, its surface area increases, increasing the convective heat transfer coefficient.

7.Physical properties of fluids: The physical properties of fluids include density, viscosity, thermal conductivity, etc. These physical properties affect the heat transfer capacity of the fluid and thus the convective heat transfer coefficient.

To sum up, there are many influencing factors of convective heat transfer coefficient, so it is necessary to analyze and determine the heat transfer coefficient according to the specific situation in practical application to obtain a more accurate heat transfer effect.

The fundamental differences between the two are: different characteristics of heat transfer, different influencing factors, and different forms of heat exchange.

1. Characteristics of convective heat transfer heat transfer: it is a complex heat transfer process that exists at the same time as heat conduction and heat convection, and there must be direct contact (fluid and wall) and macroscopic movement; There must also be a temperature difference.

The characteristics of thermal convection heat transfer: it can only occur in fluids (gases and liquids), and must be accompanied by thermal conductivity caused by the molecular movement of the fluid itself.

2. Influencing factors of convective heat transfer: the cause of the flow of the panicle body such as the flow; whether there is a phase change in the fluid; the flow state of the fluid (single-phase flow); physical conditions of the fluid; Geometrical factors of the heat exchange surface.

Factors influencing thermal convection: temperature difference, thermal conductivity, and thickness and cross-sectional area of the thermal conductive object.

3. Convective heat exchange: there are two reasons for the formation of convection: the movement of each part of the fluid due to the density difference caused by temperature; Fluid movement driven by fans, pumps, etc. The corresponding heat transfer processes are natural convective heat transfer and forced convective heat transfer.

Forms of heat transfer in the form of thermal convection: natural convection, forced convection, and turbulence, with turbulent flow having the highest heat transfer rate.

1. The cause of fluid flow. Due to the different causes of flow, convection can be divided into two categories: forced convection and natural convection heat transfer. The causes of the two flows are different, and the velocity field in the fluid is different, so the heat exchange law is also different.

2. Whether there is a phase change in the fluid. When there is no phase change in the fluid, the heat exchange in convective heat transfer is realized due to the sensible heat change of the fluid; In the heat transfer process with phase change (such as boiling or condensation), the latent heat of phase change of the fluid often plays a major role, so the heat transfer law is different from that in the absence of phase change.

3. The flow state of the fluid (single-phase flow). In laminar flow, the fluid micromass flows in layers along the direction of the main stream in a regular layer, while in turbulent flow, strong mixing occurs between the various parts of the fluid, so the heat exchange capacity is different.

The factors influencing the development of thermal convection are ().

a.Temperature differences.

b.Thermal conductivity (correct answer).

c.Cross-section delay product of a thermal conductive (correct answer).

d.Time (correct answer).