Chapter Basics
2.1 Symbols and units
Project
International unit
Imperial Units
ANSYS code
Length
m
ft
Time
s
s
Quality
Kg
lbm
Temperature
℃
oF
TEMP
Force
N
lbf
Energy (calories)
J
BTU
Power (heat rate)
W
BTU / sec
HEAT
Heat flux
W/m2
BTU/sec-ft2
HFLUX
Heat rate
W/m3
BTU/sec-ft3
HGEN
Thermal conductivity
W/m- ℃
BTU / sec-ft-oF
KXX
Convection coefficient
W/m2- ℃
BTU/sec-ft2-oF
HF
Density
Kg/m3
lbm/ft3
DENS
Specific heat
J/Kg- ℃
BTU / lbm-oF
C
Enthalpy
J/m3
BTU/ft3
ENTH
2.2 Recalling the classical theory of heat transfer
Thermal analysis to follow the first law of thermodynamics, that energy conservation law.
For a closed system (no inflow or outflow of mass):
Where: - heat
- Do work
- The system can
- The system kinetic energy
- System potential energy
For most engineering heat transfer problems:;
Usually do not consider doing work: then;
For the steady-state thermal analysis: that the heat flow is equal to the heat flow;
For the transient thermal analysis: that the inflow and outflow of heat transfer rate is equal to the system to change.
2.3 The heat transfer means
2.3.1 Heat conduction
Thermal conductivity can be defined as complete contact between two objects or between different parts of an object caused due to temperature gradient within the energy exchange. Fourier law of heat conduction follows:, the formula for the heat flux (W/m2), the thermal conductivity (W/m- ℃), the negative sign indicates the direction of heat flow to lower the temperature.
2.3.2 Convection
Thermal convection is the solid surface in contact with the fluid around it between, due to the existence of temperature difference due to heat exchange. Thermal convection can be divided into two categories: natural convection and forced convection. Thermal convection described by Newton cooling equation:, the formula for the convective heat transfer coefficient (or film coefficient of heat transfer, heat transfer coefficient, film coefficients, etc.); for the solid surface temperature, the temperature of the surrounding fluid.
2.3.3 Thermal radiation
Emission of electromagnetic radiation that objects can be, and was absorbed into other objects, the heat exchange process heat. Objects, the higher the temperature, the heat radiation per unit time the more. Heat conduction and convection heat transfer medium are required, and heat radiation without any medium. In essence, the thermal radiation in a vacuum the highest efficiency.
In engineering is usually considered between two or more objects, the radiation, the system and each object also absorb heat radiation. The net heat transfer between them can Stephen - Boltzmann equation to calculate:, the formula for the heat flow rate, for the radiation rate (black degrees), for the Stefan - Boltzmann constant, is about about 5.67 × 10 -8W/m2.K4, is radiating an area, the ground radiating to radiating 2 1 form factor for radiating an absolute temperature absolute temperature for the radiating 2, can be seen from the above formula, Thermal analysis of thermal radiation contains highly nonlinear.
2.4 steady-state heat transfer
If the net flow filter to 0, that is, the heat flow of decency with the heat generated by the system itself is equal to the heat out of the system:, the system thermal steady state. In the steady state thermal analysis, the temperature of any node does not change over time. Steady-state thermal analysis of the energy balance equation (in matrix form):
Where: for the conduction matrix, contains the thermal coefficient, convection and radiation coefficient and shape factor;
The node temperature vector;
Heat flux vector for the node, including thermal generation;
ANSYS difference using model geometry, material thermal properties and boundary conditions imposed, generation, and.
2.5 Transient heat transfer
Transient heat transfer process is a system of heating or cooling process. In this process, the system temperature, heat flux, thermal boundary conditions and the system can have a significant change over time. According to the energy conservation principle, the transient heat balance can be expressed as (in matrix form):
Where: for the conduction matrix, contains the thermal coefficient, convection and radiation coefficient and shape factor;
For the specific heat matrix, consider the system will increase;
The node temperature vector;
The temperature on the time derivative;
Heat flux vector for the node, including thermal generation;
2.6 Linear and Nonlinear
If any of the following generation, was non-linear thermal analysis:
Thermal properties with temperature changes, such as K (T), C (T), etc.;
Boundary conditions with temperature changes, such as h (T), etc.;
Nonlinear element;
Considering radiation heat transfer;
Non-linear thermal analysis of the heat balance equation is:
2.7 Boundary conditions and initial conditions
ANSYS Thermal Analysis of boundary conditions or initial conditions can be divided into seven: temperature, heat flux, heat flux, convection, radiation, heat insulation, heat. In this guide, you will see the relevant menu ANSYS commands and their equivalent path. The only reference to the commands, including command names, because it does not always need to specify all the parameters, and that the combination of different parameters have different effects. The ANSYS command more narrative, refer to the "ANSYS Commands Reference".
Menu path too close to be a complete list,
2.8 Thermal analysis of error estimates
Only for the assessment of the error caused by inadequate grid density;
Only, or SOLID SHELL thermal unit (only one temperature degree of freedom);
Based on cell boundary heat flux is not continuous;
Only one kind of material, linear, steady-state thermal analysis and effective;
Using the adaptive mesh can be controlled error.