What is Thermal Analysis?
Thermal analysis is a study of material properties as they are subjected to change in temperature. In a constructed manner, thermal analysis is also used to study the heat transfer through the structures where parameters such as but not limited to heat capacity and thermal conductivity are considered.
In metal production, thermal analysis is used as a production technique to aid the production process. Techniques used are as follows:
Annealing.
Annealing is used for reducing strength (so as to aid in processing), improve ductility and toughness, refine grain size. Here materials are heated above their re-crystallization temperature and are furnace cooled at a controlled rate up to certain temperature (where all process changes are complete) and then it can be air cooled.
Normalizing
Annealing is time consuming and when maximum softness and ductility is not required normalizing can be used. Here materials are heated above their re-crystallization temperature and then are cooled in still air (no controlled uniform cooling like annealing). Annealing produces material with uniform properties but with normalizing cooling will be different at different locations and hence properties.
Quenching
It is rapid cooling of the heated metals. Different quench rates were possible using different quenchants (water, Brine, oil, moth balls etc.). Unlike normalizing and annealing, quenching always doesn’t give same properties. For e.g. in Precipitation hardening (for increasing strength of non-metals) quench produces a structure which is soft and ductile and a solution which is non equilibrium supersaturated single phase solid solution. And also crystal structure is not changed. But in Steel quench produces non equilibrium supersaturated single phase solid solution but is hard and brittle (martensitic).
Tempering
Done after quenching. A controlled reheat in the stable phase region (no re-crystallization and we avoid it) to increase ductility and toughness at the expense of hardness and strength. In steels, quench and temper are used to produce tempered martensitic having the superior combination of strength and toughness.
A fundamental approach would also require the understating of the types of a computational analytical solution [applies to both Finite Element Analysis (FEA) and Computational Fluid Dynamics (CFD)] pertaining to the thermal analysis:
Linear solutions.
In a linear solution, the material thermal properties do not change with time or temperature and there is no radiation. It can be confusing for some solvers to define a linear solution, as the default solution strategy is nonlinear, and will only revert to a linear solution in the absence of radiation and other nonlinear effects.
Nonlinear solutions.
In a nonlinear solution, the material thermal properties can vary with temperature. Thermal boundary conditions and loading can also vary with temperature. As mentioned, the presence of radiation will force a nonlinear solution. Examples of nonlinearity include thermal conductivity, convective heat transfer coefficient and applied heat flux from the source as a function of temperature. Nonlinear analysis requires a load incrementing strategy with the total thermal loading broken down into successive steps. This is exactly analogous to structural nonlinearity.
Steady-state analysis.
The steady-state in a thermal event occurs when the temperature distribution and all thermal flows stabilize and remain constant through time. The steady-state can be calculated directly by performing an energy balance assuming this stabilized condition. Steady-state conditions are often of interest to derive a temperature distribution over component, which is then used in a subsequent structural analysis.
Transient analysis.
In this type of analysis, the initial conditions are defined and then time stepping solutions carried out in response to the thermal loading and boundary conditions. The calculation can be carried out through to a steady-state condition, or to evaluate initial thermal shock loadings, for example where the steady state condition is not important. One of the considerations in the transient thermal analysis, somewhat analogous to a transient dynamic analysis, is an accurate calculation of the time step required. In the thermal case, there is a stability criterion that is often used to establish what this should be. It may be that the solver uses this time step automatically, or the user has control to set up one’s own time step. Time steps coarser than the stability limit is not advised. In some analyses, the rate of change of thermal effects is dominant at the beginning of the analysis, so time steps here will be critical. In other analyses, a nonlinear effect may occur well into the time history and require fine attend steps. Many solvers have adaptive time stepping that can, to some extent, deal with this variation in optimum time step.
Having any thermal analysis challenges you wish to resolve? Drop us a mail at sales@nac.com.sg to discuss how Finite Element Analysis (FEA) and Computational Fluid Dynamics (CFD) is able to assist in overcoming them!
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Johnny Wong Senior Consultant, Partner NAC Consultancy Pte Ltd |
Relevant article(s):
Introduction to Finite Element Analysis (FEA)
How accurate is Finite Element Analysis (FEA) Simulation?
A Brief introduction to Computational Fluid Dynamics (CFD)