流体力学与传热 ：Chapter 4 Heat Transfer

Basic Mechanisms of Heat Transfer
The mechanisms by which the heat flows are three:
❖Conduction
❖Convection
❖ Radiation
4.1.2 Conduction
If a temperature gradient exists in a continuous substance, heat can flow unaccompanied by any observable motion of matter.
Chapter 4 Heat Transfer and Its Applications
INTRODUCTION AND MECHANISMS OF HEAT TRANSFER
4.来自百度文库.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.
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.
q A
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.
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
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.
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.
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.
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.
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
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.