流体力学与传热课件Heat Transfer and Its Applications
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Experiment does confirm the independence of k for a wide range of temperature gradients except for porous solids.
On the other hand, k is a function of temperature, but not a strong one.
If , however, matter appears in its path, the radiation will be transmitted, reflected, or absorbed. It is only the absorbed energy that appears as heat.
The negative sign reflects the physical fact that heat flow occurs from hot to cold and the sign of the gradient is opposite that of the heat flow.
In using equation it must be clearly understood that the area A is that of a surface perpendicular to the flow of heat and distance n is the length of path measured perpendicularly to area A.
The net flow is always in the direction of the temperature decrease.
Steady-State Heat Transfer
The heat transfer occurs in the control volume where the rate of accumulation of heat is zero and the temperatures at various points in the system do not change with time it is called as steady-state heat transfer.
qห้องสมุดไป่ตู้A
h(tw
t
f
)
(4.1-2)
Note that the linear dependence on the temperature driving force tw-tf is the same as that for pure conduction in a solid of constant thermal conductivity.
Chapter 4 Heat Transfer and Its Applications
INTRODUCTION AND MECHANISMS OF HEAT TRANSFER
4.1.1 Nature of heat flow
When two objects at different temperature are brought into thermal contact , heat flow from the object at the higher temperature to that at the lower temperature.
Thermal Conductivity
The proportionality constant k is a physical property of the substance. It , like the viscosity µ, is one of the socalled transport properties of material.
For small ranges of temperature, k may be considered constant.
For larger temperature ranges, the thermal conductivity can usually be approximated by an equation of the form
The basic relation for heat flow by conduction is the proportionality between heat flux and the temperature gradient.
It can be written dq k T dA n
The partial derivative calls attention to the fact that the temperature may vary with both location and time.
In gases, conduction occurs by the random motion of molecules.
4.1.3 Convection
When a current or macroscopic particle of fluid crosses a specific surface, such as the boundary of a control volume, it carries with it a definite quantity of enthalpy.
For steady one-dimensional flow. Equation may be written
dq k dt dA dx
Where q= rate of heat flow in direction normal to surface x=distance measured normal to surface k= thermal conductivity
4.1.4 Radiation
Radiation is a term given to the transfer of energy through space by electromagnetic waves.
If radiating is passing through empty space, it is not transformed to heat or any other form of energy, nor is it diverted from its path.
4.2 Heat Transfer by Conduction
Conduction is most easily understood by considering heat flow in homogeneous isotropic solids because in these there is no convection and the effect of radiation is negligible.
• k vary over a wide range. They are highest for metals and lowest for finely powdered materials from which air has been evacuated.
Fourie’s law states that k is independent of the temperature gradient.
4.2-6
Gases have the smallest thermal conductivities, with values as low as 0.007 W/m.ºC.
The heat-transfer coefficient is not an intrinsic property of the fluid,
but depends on the flow patterns determined by fluid mechanics as well as on the thermal properties of the fluid.
forced convection
If the currents are set in motion by the action of a mechanical device such as a pump or agitator, the flow is independent of density gradients, and is called forced convection.
The forces used to create convection currents in fluids are of two types:
natural convection and force convection
Natural convection
If the currents are the result of buoyancy forces generated by differences in density and the differences in density are in turn caused by temperature gradients in the fluid mass, the action is called natural convection.
The energy emitted by a black body is proportional to the fourth power of the absolute temperature.
Wb T 4
σ=stefan-boltzmann constant T=absolute temperature
k a bT
4.2-5
1. Gases
Theories to predict thermal conductivities of gases are reasonably accurate and are given elsewhere. The thermal conductivity increases approximately as the square root of the absolute temperature and is independent of pressure up to a few atmospheres.
Such a flow of enthalpy is called a convective flow of heat.
The convection flux is usually proportional to the difference between the surface temperature and temperature of the fluid, as stated in Newton’s law of cooling
According to Fourier’s law, the heat flux is proportional to the temperature gradient and opposite to it in sign. For one-dimensional heat flow
dq k dt dA dx
Thermal conduction for various materials
• In metals, thermal conduction results from the motion of free electrons.
In solids that are poor conductor of electricity and in most liquids, thermal conduction results from momentum transfer between adjacent vibrating molecules or atoms.
On the other hand, k is a function of temperature, but not a strong one.
If , however, matter appears in its path, the radiation will be transmitted, reflected, or absorbed. It is only the absorbed energy that appears as heat.
The negative sign reflects the physical fact that heat flow occurs from hot to cold and the sign of the gradient is opposite that of the heat flow.
In using equation it must be clearly understood that the area A is that of a surface perpendicular to the flow of heat and distance n is the length of path measured perpendicularly to area A.
The net flow is always in the direction of the temperature decrease.
Steady-State Heat Transfer
The heat transfer occurs in the control volume where the rate of accumulation of heat is zero and the temperatures at various points in the system do not change with time it is called as steady-state heat transfer.
qห้องสมุดไป่ตู้A
h(tw
t
f
)
(4.1-2)
Note that the linear dependence on the temperature driving force tw-tf is the same as that for pure conduction in a solid of constant thermal conductivity.
Chapter 4 Heat Transfer and Its Applications
INTRODUCTION AND MECHANISMS OF HEAT TRANSFER
4.1.1 Nature of heat flow
When two objects at different temperature are brought into thermal contact , heat flow from the object at the higher temperature to that at the lower temperature.
Thermal Conductivity
The proportionality constant k is a physical property of the substance. It , like the viscosity µ, is one of the socalled transport properties of material.
For small ranges of temperature, k may be considered constant.
For larger temperature ranges, the thermal conductivity can usually be approximated by an equation of the form
The basic relation for heat flow by conduction is the proportionality between heat flux and the temperature gradient.
It can be written dq k T dA n
The partial derivative calls attention to the fact that the temperature may vary with both location and time.
In gases, conduction occurs by the random motion of molecules.
4.1.3 Convection
When a current or macroscopic particle of fluid crosses a specific surface, such as the boundary of a control volume, it carries with it a definite quantity of enthalpy.
For steady one-dimensional flow. Equation may be written
dq k dt dA dx
Where q= rate of heat flow in direction normal to surface x=distance measured normal to surface k= thermal conductivity
4.1.4 Radiation
Radiation is a term given to the transfer of energy through space by electromagnetic waves.
If radiating is passing through empty space, it is not transformed to heat or any other form of energy, nor is it diverted from its path.
4.2 Heat Transfer by Conduction
Conduction is most easily understood by considering heat flow in homogeneous isotropic solids because in these there is no convection and the effect of radiation is negligible.
• k vary over a wide range. They are highest for metals and lowest for finely powdered materials from which air has been evacuated.
Fourie’s law states that k is independent of the temperature gradient.
4.2-6
Gases have the smallest thermal conductivities, with values as low as 0.007 W/m.ºC.
The heat-transfer coefficient is not an intrinsic property of the fluid,
but depends on the flow patterns determined by fluid mechanics as well as on the thermal properties of the fluid.
forced convection
If the currents are set in motion by the action of a mechanical device such as a pump or agitator, the flow is independent of density gradients, and is called forced convection.
The forces used to create convection currents in fluids are of two types:
natural convection and force convection
Natural convection
If the currents are the result of buoyancy forces generated by differences in density and the differences in density are in turn caused by temperature gradients in the fluid mass, the action is called natural convection.
The energy emitted by a black body is proportional to the fourth power of the absolute temperature.
Wb T 4
σ=stefan-boltzmann constant T=absolute temperature
k a bT
4.2-5
1. Gases
Theories to predict thermal conductivities of gases are reasonably accurate and are given elsewhere. The thermal conductivity increases approximately as the square root of the absolute temperature and is independent of pressure up to a few atmospheres.
Such a flow of enthalpy is called a convective flow of heat.
The convection flux is usually proportional to the difference between the surface temperature and temperature of the fluid, as stated in Newton’s law of cooling
According to Fourier’s law, the heat flux is proportional to the temperature gradient and opposite to it in sign. For one-dimensional heat flow
dq k dt dA dx
Thermal conduction for various materials
• In metals, thermal conduction results from the motion of free electrons.
In solids that are poor conductor of electricity and in most liquids, thermal conduction results from momentum transfer between adjacent vibrating molecules or atoms.