What are the influencing factors of heat transfer coefficient?

ImpactConvective heat transfer coefficientfactors

The convective heat transfer process is the heat transfer process between the fluid and the wall, so all the factors related to the fluid flow and the wall surface will inevitably affect the value of the convective heat transfer coefficient. The flow form of the fluid, the physical properties of the fluid, whether the fluid has phase transition, and the geometry, size, relative position and other factors of the heating surface.

Generally speaking, for fluids of similar properties, the heat transfer coefficient of forced convection is greater than that of natural convection. The convective heat transfer coefficient of turbulent flow is greater than that of laminar flow.

The convective heat transfer coefficient of the flow, for the same flow form, the greater the flow velocity, the greater the convective heat transfer coefficient.

The physical properties of fluids include the viscosity and thermal conductivity of the fluid.

Density, specific heat capacity.

coefficient of volume expansion, etc. For the heat transfer with phase change, as well as the influence of phase change heat, except for viscosity, the rest of the physical properties, with the increase of its convective heat transfer coefficient increases correspondingly. For the same fluid, the heat transfer coefficient with phase change is greater than that without phase change.

The shape, size, and relative position of heat transfer affect heat transfer, such as heating in winter, the heating wall should be placed in the lower part of the space, and conversely, the cooling device in summer should be placed in the upper part of the space.

The heat transfer coefficient is a process quantity, and its magnitude depends on the physical properties and flow rate of the fluid on both sides of the wall, the shape of the solid surface, and the thermal conductivity of the material.

coefficient and other factors. In the calculation of building heat loss, it is a parameter that characterizes the total heat transfer performance of the envelope, and its value depends on the materials used in the envelope, the construction and the environmental factors on both sides of the envelope.

The larger the heat transfer coefficient, the worse the thermal insulation effect of the envelope, such as the heat transfer coefficient of the metal window of a single layer of 3mm thick glass.

370mm thick plastered on both sides.

The heat transfer coefficient of the brick wall is.

The higher the k-value, the more intense the heat transfer process becomes. The heat transfer coefficient depends not only mainly on heat and cold.

The physical properties of the fluids and the average flow of each.

speed, but also with the thickness of the solid wall surface and its material.

The thermal conductivity and many other factors are generally related to specific experiments and according to the heat transfer equation.

calculations are determined, or by calculating the heat transfer process.

The total thermal resistance rt per unit area is obtained.

Extended information: For the heat exchanger commonly used in air conditioning engineering, if other additional thermal resistance is not considered, the heat transfer coefficient k value of the single-layer envelope structure can be calculated as follows:

k=1/(1/h1+δ/λ+1/h2)

w/(㎡·c)

Among them, h1, h2 - heat exchange coefficient of the two surfaces of the envelope, w (·c);

Pipe wall thickness, m;

Thermal conductivity of the tube wall, w (m·°C).

Calculation of the average heat transfer coefficient of an exterior wall under the influence of a peripheral thermal bridge.

km=(kpfp+kb1fb1+kb2fb2+

kb3fb3 )/(

fp +fb1+fb2+fb3)

where: km—the average heat transfer coefficient of the exterior wall [w (;

kp—heat transfer coefficient of the main part of the exterior wall [w (;

KB1, KB2, KB3—Heat transfer coefficient of thermal bridge around the perimeter of the exterior wall [W (;

fp—the area of the main part of the exterior wall;

FB1, FB2, FB3—The area of the thermal bridge around the perimeter of the exterior wall.

The heat transfer coefficient is a process quantity, and its magnitude depends on the physical properties and flow rate of the fluid on both sides of the wall, the shape of the car hole on the solid surface, the thermal conductivity of the material and other factors. In the calculation of heat loss in buildings, it is the characterization of the envelope.

The parameters of the total heat transfer performance, the value of which depends on the materials used in the envelope, the structure of the silver cavern and the environmental factors on both sides.

The heat transfer coefficient used to be called the total heat transfer coefficient. The current national standards and specifications are uniformly named as heat transfer coefficient. Heat transfer coefficient k

The value refers to the temperature difference of the air on both sides of the envelope structure under the condition of stable heat transfer is 1 degree (k, the heat transferred through 1 square meter area within 1 hour, the unit is watt square meter degree (w ·k, where k can be replaced).

Convective heat transfer coefficientIt refers to the heat exchange per unit area between the solid surface and the fluid in a unit time, which reflects the heat exchange capacity between the fluid and the surface of the solid Shenxiao Cong.

It reflects the heat transfer capacity between the fluid and the solid surface, the value of the convection heat transfer coefficient of the surface and the physical properties of the fluid in the heat transfer process.

The shape, position, temperature difference between the surface and the fluid, and the flow velocity of the fluid are closely related, and the change range is very large, the greater the flow velocity of the fluid near the surface of the object, the greater the surface convective heat transfer coefficient.

Theoretical development. The physical significance of the convective heat transfer coefficient h is that when the temperature difference between the fluid and the solid surface is 1k, the magnitude of the heat h that can be transferred by the wall area of 1m*1m per second reflects the strength of convective heat transfer.

As mentioned above, h is related to the factors that affect the heat transfer process and can vary over a large range, so Newton.

The formula can only be seen as the heat transfer coefficient.

It does not reveal the intrinsic connection between the factors affecting convective heat transfer and h, nor does it bring any substantial simplification to the engineering calculation, but only transfers the complexity of the problem to the determination of the heat transfer coefficient, so the main task in the engineering heat transfer calculation is to calculate h.

1. Different meanings: The physical meaning of convective heat transfer coefficient refers to the heat that can be transferred between the fluid and the wall surface in a unit temperature difference and unit time per unit area, and its size expresses the strength of the convective heat transfer process. The heat transfer coefficient is a scale that characterizes the intensity of the heat transfer process, which is numerically equal to the body temperature of the hot and cold flow.

2. Different objects: the heat conduction and fuel coefficient is generally for heat conduction, and the heat transfer coefficient is generally for convective heat transfer.

The relationship between the convective heat transfer coefficient and the heat transfer coefficient: The heat transfer coefficient affects the magnitude of the convective heat transfer coefficient.

The heat transfer coefficient is a process quantity, and its magnitude depends on the physical properties of the fluid on both sides of the wall, the flow rate, the shape of the solid surface, the thermal conductivity of the material and other factors. In the calculation of building heat loss, it is a parameter that characterizes the total heat transfer performance of the envelope structure, and its value depends on the car cavity material used in the envelope, the structure and the environmental factors on both sides of the envelope.

The heat transfer coefficient used to be called the total heat transfer coefficient. The current national standards and specifications are uniformly named as heat transfer coefficient. The heat transfer coefficient k value refers to the temperature difference of the air on both sides of the envelope structure is 1 degree (k, the heat transferred through 1 square meter area within 1 hour, the unit is watt square meter degree (w ·k, where k can be replaced) under stable heat transfer conditions.

The velocity of fluid flow: The main contradiction between heat transfer and heat transfer in the heat transfer boundary layer is convection. Obviously, increasing the flow velocity can reduce the heat transfer boundary layer, thereby increasing the heat supply coefficient and strengthening the convective heat transfer process.

Convection of the fluid: natural convection or forced convection. Obviously, the velocity of the fluid is higher than that of natural convection when forced convection.

Other influencing factors of convective heat transfer coefficient1. Types of fluids: liquids, gases, vapors.

2. The properties of the fluid: the specific heat, thermal conductivity, density, viscosity and so on of the fluid are greatly affected. For example, if the thermal conductivity of the fluid is large, the thermal resistance of the heat transfer boundary layer is small, and the heat supply coefficient is larger.

For fluids with high viscosity, the number is small at the same flow rate, and the heat transfer boundary layer is correspondingly thicker, and the heat coefficient is smaller.

3. The shape, position and size of heat transfer, and the heat transfer surface of different shapes, such as round tubes or flat plates or tube bundles; Whether it is inside or outside the tube, whether it is placed vertically or horizontally, and regardless of the diameter and length of the pipe, it all have an impact on the heating coefficient.

Warm-hu elevation differenceConvective heat transfer coefficient per acreIt is related to wall temperature.

If convective heat transfer is known, then the temperature difference is inversely proportional to the convective heat transfer coefficient.

relationship, the convective heat transfer coefficient is obtained by dividing the temperature difference by convection heat transfer area and then dividing the heat exchange area. If the convective heat transfer is not known, the convective heat transfer coefficient should be solved according to the experimental correlation between the characteristic numbers. In fact, the convective heat transfer coefficient is not mechanically related to the temperature difference.

Compute. Surface convective heat transfer coefficient.

and the physical properties of the fluid during heat exchange.

The shape and location of the heat exchange surface and the flow rate of the fluid are closely related. The greater the velocity of the fluid near the surface of an object, the greater the convective heat transfer coefficient of its surface. If people are in an environment with large wind speed, due to the large convective heat transfer coefficient of the first surface, its heat dissipation (or heat absorption) is also larger.

The convective heat transfer coefficient can be calculated using an empirical formula, usually using the Buzz formula.

The above content refers to: Encyclopedia - Convection heat transfer coefficient.

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.