Thermal Analysis is a key part of a thermal design. These days, almost anything can be modelled with state-of-the-art analysis and modeling tools available in the market. When used correctly by a seasoned thermal engineer or expert thermal analysis consultant, these tools can give you accurate results quickly and cost-effectively. Gone are the days when thermal engineers labored for weeks, or even months, designing thermal experiments to get limited information on a given thermal layout. Now, one can build a model with a few clicks and change a variable or two and see results almost instantly. Many of these tools have made great progress over the years in accuracy, ease of use and sophistication.
When conducting thermal modelling in any thermal design, there are a few steps that thermal engineers must go through carefully. These include domain sizing, boundary conditions, model details, thermal properties, power loading, meshing, and post-processing.
A thermal model must have a domain within which the analysis must be conducted. The domain will include the key elements of the system that needs to be modeled. The domain also determines how the system being modeled interacts with the environment. Therefore, it is critical that the thermal engineer or thermal consultant use the right domain size and shape for our model. In general, domain boundaries are chosen so that either a given variable has a fixed value at the boundary, or the spatial change of a variable is close to zero at the boundary. Therefore, when we establish a domain, we need to be careful, so such assumptions are not violated, at least not by a whole lot.
In domain sizing, we must also strike a balance between unnecessarily large domain size and model accuracy. Often, larger domain size means larger mesh count and longer run time. The seasoned thermal engineer would know the appropriate domain size by experience, typically from prior models. Otherwise, one may conduct sensitivity analysis with 2-3 variations of domain size to determine the effect of domain size on key variables.
By model details, we mean the physical details of the system being analyzed. If a laptop computer is being modeled, for example, the model will have the key components, such as the enclosure, display, PCB with components, Hard Drive and/or SSD, wireless components, power supply, interface materials, any cooling solutions, etc. Today’s thermal analysis tools can import the physical model in its entirety. However, it important that we exclude minor details from the model so the mesh size is not too large. In many cases, small pins, protrusions and curves do not matter much in thermal analysis, especially when they are far away from the main heat sources inside the system and/or when they are away from main airflow paths.
Thermal properties include variables like thermal conductivity, specific heat and density. In steady state analysis, thermal conductivity is the main variable to consider. In transient analysis, density and specific heat will also be important, in addition to thermal conductivity. All components in the thermal model must be assigned the right thermal properties. These properties govern how heat is transferred from one component to the other, and ultimately to the environment. Poor choice of thermal properties will, therefore, be reflected in the end results as well.
Electronic devices become hot because heat is generated within some components. This heat comes from the power each component draws from the power supply. Almost all power consumed by a given component within an electronic system is converted into heat. Therefore, it is very important that we know the exact power load on every component in our system. Our solution is as good as the accuracy of these inputs. In case of uncertainty, it is customary to err on the side of more conservative inputs. In some cases, a plot can be established for a range of power loads to see what if scenarios for different loads.
Thermal analysis is conducted by digitizing the entire model domain into small areas called mesh elements or cells. Essentially, we break up the entire model domain into thousands of small volumetric cells. Within each cell, typically, we assume an average value for each variable such as temperature. The variables are supposed to vary between neighbouring cells according to certain assumed function and governed by partial differential equations. The partial differential equations are, in turn, influenced by the thermal properties we discussed above, such as thermal conductivity, density and specific heat of each cell.
The last step in thermal modeling is what is called post-processing. When a model finishes its run, it is time to check the solution. With today’s state-of-the-art thermal analysis tools, there are numerous ways to display results. One can display temperature or any other variable contours on a point, plane, or surface. We may also build derivatives or functions of variables and display them on points, planes and surfaces. There is no limit to how much we can slice and dice the thermal solution.
Thermal Design Solutions is a consulting firm specializing in thermal solutions for electronic systems. For more information about its services or to discuss your project with one of our professionals, please visit us at ThermalDS.com, or send us email to info@thermalDS.com.
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