Any form of electrical lighting product produces a negative product: heat. From incandescent sources to fluorescent lighting, generations of engineers are developing ways to minimize heat or separate heat from light sources or equipment. However, led lighting is currently bringing new and different challenges with ever-increasing quality and increasing forms.
The development of heat will reduce the LED light output, causing a change in color and, at the same time, shortening the life of the component. It is said that thermal management is by far the most critical aspect of LED system design. From an engineer's point of view, this means learning tools that are common across structural and electronic design fields, and familiar with processes outside the scope of structural and electronic design. Fortunately, there are already thermal design solutions that can help simplify the engineer's design path in terms of thermal verification and testing challenges.
Verification design concept
When developing a new light source system, it is necessary to verify the most basic product concepts so that the structural and aesthetic aspects of the idea can be unified with the actual needs of the thermal performance.
The key to successful LED system design is to efficiently transfer the heat of the active device from its PN junction to the environment. Both the PCB board and the housing that solder the LEDs participate in the heat flow path. The design engineer must determine that the enclosure and shroud are efficient at transferring heat. Manufacturing and testing a series of physical samples to verify this is expensive and takes a lot of time, so recent design engineers typically use a software-based approach in the early stages of design.
A more popular approach is to use computational fluid dynamics (CFD=Computational Fluid Dynamics) analysis to virtually simulate a planned device. For early design concepts, this method is much more flexible and effective than building a physical sample. After the virtual model has been well characterized, the physical sample can be used to determine what works and what does not. CFD simulations have traditionally been performed by analysts with advanced data and fluid mechanics backgrounds who require analysts to perform complex CFD modeling tools.
The latest development brings CFD technology to the desktop of structural engineers, greatly simplifying and speeding up analysis. The new process, Synchronous CFD, automates most of the work steps required to prepare and perform simulation analysis. Seamless integration into a common MCAD environment, such as Pro/ENGINEER®, enables structural engineers to create virtual simulation models of a new light source design and detect thermal performance.
A seamlessly integrated synchronous CFD application such as Mentor Graphcis's FloEFDTM software:
–– Using the dimensions and physical characteristics of the design saved in the MCAD application by directly using the MCAD model
––Detect fluid and solid areas and mesh them to create an advanced automatic grid
––Help engineers set boundary conditions
–– automatically provides solution control settings to help determine convergence during solver calculations
The bottom line of design verification, whether the product is a new light source or a modification of the existing design, should be a detailed understanding of the thermal performance of the LED device. If the existing equipment is placed in the new LED luminaires developed, it is also important to understand the thermal performance in detail, because it is the key to match the thermal performance of the original device with the luminaire.
Thermal data should be obtained from LED suppliers, and although this information is lacking in current industry standards, the information provided in printed product specifications is often not comprehensive enough. For example, thermal performance data is usually provided, but it may not include the operating temperature range and the temperature at which the end user's system actually works. However, design engineers must rely on data or simple models provided by LED manufacturers to perform light source evaluations, or in-house testing, or to begin early light source designs.
Figure 1: MCAD model of LED light source (click on image to enlarge)
The process begins with the structural design of the light source. Figure 1 shows the original design steps. The system described contains an integral joint (yellow part in the figure) that connects the outer cover of the luminaire and the fins of the luminaire cover act as a heat sink. The connector is inserted into a socket that is sometimes designed to allow it to conduct heat further as part of the cooling system. However, in this particular system, the socket is just a tool to support and connect the fixture. The light source is a power LED that is mounted on a metal core PCB. In Figure 1, the length of the luminaire is omitted, which is to better show the details of the LED.
With the Synchronous CFD tool, the job of preparing for thermal analysis can be performed very efficiently. Synchronous CFD applications are seamlessly integrated into the MCAD environment, so the size and physical characteristics of the light source already exist in CFD applications. Specifically, as shown in Figure 1.
The CFD tool also automatically simulates the presence of internal voids in the area of the fluid that needs to be treated, if needed. This need is more common when dealing with fluid problems in pipes. For the airflow through the light source and the airflow around the light source, we need to pay attention to this application.
Create a calculation grid
(click on the image to enlarge)
Figure 2: Computational grid of the center section of the luminaire assembly
Internal temperatures and their distribution are the focus of thermal analysis. Synchronous CFD applications (also using MCAD data) can display any two-dimensional surface, providing a cross-sectional view showing the internals of the light source. However, the first step is to create a computational grid, as shown in Figure 2. Synchronous CFD performs this step automatically.
The grid here is just a concept, however, the grid is at the heart of complex CFD calculations. The surface of the device is distributed with small rectangular elements, each spanning between the solid and the fluid, and the separate areas are calculated separately. The program then generates a mixed result that includes all the meshes.
Note that in Figure 2, the grid cells are of different sizes. The mesh size clustered around the LED is smaller than the mesh around the outer cover. This is because advanced synchronous CFD technology requires higher mesh accuracy
The area automatically provides a finer mesh.
Next, the project must define boundary conditions, which are the operating parameters and limits of the equipment to be used in the calculation. The external ambient temperature must be set, as well as the power of the LED device, which will be used in multiple iterations of CFD analysis.
Figure 3: Temperature through the two-dimensional surface of the center of the luminaire, and vector, showing the flow caused by natural convection outside the luminaire. The vectors in this and other related cloud maps were calculated using Mentor Graphics' FloEFD software. FloEFD is a market-proven synchronous CFD product that integrates seamlessly into the MCAD environment and directly uses structural model CFD tools.
Figure 3 shows the CFD calculation results for the cross section. It not only shows the distribution of heat in the physical parts of the luminaire, but also shows the airflow vector convecting outside the luminaire. The 3D map in this case is suitable for visualizing internal conditions, but the flow vector fits into a 2D plane. In this view, the color ranges from red (hottest) to blue (coolest), with orange and green between the two extremes.
Transfer heat
(click on the image to enlarge)
Figure 4: Particle traces as the entire luminaire faces down, showing how natural convection causes air to flow outside the luminaire enclosure. As the flow in the upper part of the luminaire accelerates, the "hot smoke" decreases. (Note: For the sake of reading, the temperature legend in this figure is enlarged.)
Of course, the purpose of the analysis is to determine that the planned design will transfer heat from the LED source and safely bring the heat to the environment. Figure 4 is another view of a synchronous CFD that provides an important answer to this question. In this picture, the particle traces the airflow path, like a small dust collector. Again, the color here represents the heat distribution, but in this picture, the color corresponds to the value. Note this flow pattern: blue (cold) air enters from below and is heated to blue-green as it passes through the source. Convection brings the heated air up and takes away the heated air. Is this enough for the luminaire and is there any other cover in the final design? This problem can only be answered by the design engineer, but synchronous CFD analysis has provided data to help make judgments.
Figure 5: Surface temperature prediction on the MCAD model
Figure 5 provides additional information. This is a thermal gradient of the LED and the overall assembly of the housing in the solid model of Figure 1. The legend provides detailed information on the temperature. Based on this information, it is easy to make a judgment, such as whether the touch temperature is within a safe range.
to sum up
Synchronous CFD simulation/analysis processes such as Mentor Graphics' FloEFD software are indispensable for improving design concepts. It is much cheaper than creating physical samples and test samples; the automated process created in synchronous CFD means that the cycle preparation for the first evaluation is simple, and for each subsequent simulation, the operation is faster. It is an environmental trend to promote the establishment of physical samples after the design is completely optimized. Synchronous CFD can be used to quickly determine the optimal number of slots for the bell housing and the thickness of the metal between the slots to maximize the amount of heat that can be dissipated into the environment.
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