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Engineering Edge

Making Computers Cool by Design

NMB Minebea use thermal simulation to predict design flaws

By Dr.-Ing. Anton Breier NMB Minebea GmbH

PU cooling is a critical aspect of a functioning computer system, and for this reason the need for forced-air cooling is a significant factor that should be determined at an early stage in system design. Good airflow to heat-generating components, and adequate space and power for the cooling fan are a critical design requirement for any forced convection system. One of the first steps in system design should be to estimate the required airflow. This will depend on the heat generated within the enclosure, and the maximum permitted air temperature rise.

The question becomes, how can we best determine the optimal design for computer cooling, and in particular, for CPU cooling? The answer from fan experts NMB Minebea is to use thermal simulation. Using FloTHERM® from Mentor Graphics, NMB Minebea can anticipate design flaws, and evaluate the thermal behavior of the various components that will ultimately become an important part of the final cooling design. By way of example, here's a quick view of how thermal simulation can assist in predicting and improving the airflow of an average CPU application in order to produce optimal CPU cooling within a desktop computer. The structural design of a typical CPU will look similar to Figure 1.

In this example, the axial fan sits on top of the cover of the heatsink structure. Under the heatsink structure you will find the thermal slug (shown in yellow) and the contact area to the CPU surface. Between the heatsink fin tips and the cover it is necessary to provide a gap in order to allow air to exhaust from the fan. The FloTHERM results show the distribution of the surface temperature on the heatsink and CPU for this particular heatsink-fan (or 'fansink') combination. As expected the highest temperature is seen directly at the contact area between the CPU and heatsink. Heat is transported inside the heatsink material by conduction, and from the surfaces to the air by convection. The temperature profile within the heatsink, and the resulting CPU temperature, depend on both the conduction and convection within the assembly. The heatsink has to be designed to deliver the best performance for the chosen fan in order to optimize the entire fansink design. Many factors, including the material, fin design, air velocity and surface treatment all influence the thermal performance of a heatsink.

Figure 1 FloTHERM Simulation Model of CPU

As the cooling air from the fan does not provide a uniform flow pattern, there can be considerable temperature variation within the heatsink. The insight that FloTHERM provides indicates exactly what measures should be taken for optimal cooling. In general, if high temperature gradients are observed within a heatsink then conduction should be improved. This can be done by choosing another material (alloy) with a better thermal conductivity, or by increasing the local cross-sectional area to improve the heat conduction. Looking at the flow vector field in a center cross-section, above left, FloTHERM reveals a zone below the fan motor with almost no air flow. This behavior is seen for all axial fans used in this design of heatsink. As a consequence of the stagnant air below the fan motor, very little heat is removed from the heatsink fins in this area and so almost no cooling occurs. The fins heat the stagnant air close to the fin temperature as shown on the right hand side of the graphic above.

“Increasing performance gives rise to problems related to equipment cooling. During the development phase, thermal simulations provide us with crucial information about airflow distribution as well as both air and component temperatures. The use of Computational Fluid Dynamics Software not only eliminates the need for thermal redesigns, but also facilitates shorter development times and optimized equipment cooling. We offer our customers complete system solutions, combining fans, heatsinks and power supplies with layout and dimensioning, optimized precisely to the customer's equipment.”

Dr. Anton Breier, Deputy General Manager, NMB-Minebea-GmbH

At the system level, obstructions in the airflow path increase the static pressure drop within the enclosure, reducing the air flow, so airflow obstructions should be minimized. Obstructions in the form of baffles are sometimes necessary to direct the airflow over the components that need cooling. This can lead to unforeseen consequences, such as regions of flow recirculation can occur behind the baffles leading to unexpected hot spots. This is why system-level simulators ,like FloTHERM, are critical for good equipment thermal design. The final system design should show continuous airflow through all parts of the enclosure for optimum thermal management of all heat generating components.

To assist customers, NMB offers Thermal Management Consultancy as part of their design services. This has been proven to facilitate the design process of many of NMB's customers by providing key information and solutions for their thermal cooling applications.

Figure 1 CPU Surface Temperatures

Heat is transported inside the heatsink material by conduction, and from the surfaces to the air by convection. The temperature profile within the heatsink, and the resulting CPU temperature, depend on both the conduction and convection within the assembly. The heatsink has to be designed to deliver the best performance for the chosen fan in order to optimize the entire fansink design. Many factors, including the material, fin design, air velocity and surface treatment all influence the thermal performance of a heatsink.

Figure 3&4 CPU Cooler Analysis

As the cooling air from the fan does not provide a uniform flow pattern, there can be considerable temperature variation within the heatsink. The insight that FloTHERM provides indicates exactly what measures should be taken for optimal cooling. In general, if high temperature gradients are observed within a heatsink then conduction should be improved. This can be done by choosing another material (alloy) with a better thermal conductivity, or by increasing the local cross-sectional area to improve the heat conduction.

Looking at the flow vector field in a center cross-section, above left, FloTHERM reveals a zone below the fan motor with almost no air flow. This behavior is seen for all axial fans used in this design of heatsink. As a consequence of the stagnant air below the fan motor, very little heat is removed from the heatsink fins in this area and so almost no cooling occurs. The fins heat the stagnant air close to the fin temperature as shown on the right hand side of the graphic above.

At the system level, obstructions in the airflow path increase the static pressure drop within the enclosure, reducing the air flow, so for maximum airflow obstructions should be minimized. Obstructions in the form of baffles are sometimes necessary to direct the airflow over the components that need cooling. This can lead to unforeseen consequences, such as regions of flow recirculation can occur behind the baffles leading to unexpected hot spots. This is why system-level thermal simulators like FloTHERM are critical for good equipment thermal design. The final system design should show continuous airflow through all parts of the enclosure for optimum thermal management of all heat generating components.

Figure 5 : CPU Flow Animation

To assist customers, NMB offers Thermal Management Consultancy as part of their design services. This has been proven to facilitate the design process of many of NMB’s customers by providing key information and solutions for their thermal cooling applications.

 
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