Thermal Convection: A Pillar of Heat Transfer
In a majority of thermal management solutions, we use thermal convection as a means to remove heat away from our sensitive components and devices. In the rare case we don't use convection, it's because we have little to no fluid to work with. Particular applications, like ones in the aerospace industry, are devoid of fluid and cannot utilize thermal convection. Otherwise, it's the most popular way to get heat out of products.
But in the industry, you've probably heard the terms "forced convection" and "natural convection" thrown about. While it may not seem a big deal between the two, it has a large impact on how your thermal management solution is designed. So let's take a look at natural convection versus forced convection and get into the differences.
Convection: The Tale of Two Processes
The process of convection as we refer to it in thermal management is a actually a combination of two processes. The first process is technically conduction, where the heat from the heat sink surface transfers to any fluid that contacts that surface. The second process is considered advection, which is bulk flow of fluid warmed by the device away from the heat source. What we do instead of referring to both individually, we lump them together as one single term: convection.
It's important to understand the two portions of convection when we're trying to improve our thermal performance of our solutions. When we comprehend the parts of convection, we're more able to break down and improve each of these parts to better our overall heat transfer.
Natural Convection versus Forced Convection
We classify the type of convective flow as either natural or forced. We make this designation since each has it's own implications for the application and product as a whole. These different types of flow have different design constraints and concerns that need to be individually addressed.
It's Natural (Flow)
Natural convection is when the natural buoyancy drives the advective flow. You've probably heard the terms "plume" or "chimney effect" to describe natural convection. Essentially, as the fluid inside or near the heat source and heat sink gets hotter than ambient temperature, it has less pressure. Here on Earth, we have gravity, so less pressure means more buoyancy. This pressure differential generates movement of the hotter air upwards, away from the source of gravity. The cooler surrounding fluid then fills the place the hot air is leaving from, thus generating a flow inwards and then upwards.
Go with the Flow: Natural Thermal Convection Design Considerations
Super Reliability for Natural Thermal Convection Solutions
In applications where reliability is critical, natural convection is the preferred type of flow within a thermal management solution. By relying on natural forces to apply movement to your fluid, key components like fans or pumps aren't required. These components, while heavily engineered and tested, will still wear down over time. As long as you have frictional parts, like the motors in fans or pumps, you'll be concerned about the reliability of your fluid movers.
Fluid Options for Natural Thermal Convection
Natural convection tends to be easier in air cooled applications as opposed to liquid cooled systems. Liquid needs to be contained and unless the system is submerged, and most electronics don't go well with liquids, the whole route of the liquid needs to be planned out and contained. This implies more engineering time especially during the design and validation portions of product development. On the other hand, we're surrounded by air and any movement of air away from a system will be quickly replaced by other ambient air.
Natural Thermal Convection Fin Spacing
When you're talking about natural convection versus forced convection heat sinks, you'll see a difference in the overall structure of the heat sink. No matter the fluid, we want to optimize our heat sink to maximize the chimney effect. This means there is enough room between heat sink fins for them to "breathe". You need enough room to heat up next to the fins within their boundary layer on each side of the gap, as well as some extra room in the middle for air to flow upwards. You'll see the looser fin spacing on the thermal contours below on the left allow cooler air to get much further up the fin gaps than the heat sink on the right. That's why you'll notice some heat sinks have much larger fin gaps than others. The ones with fin gaps of about 1/4" and larger are generally designed for natural convection.
Forcing the Subject of Forced Thermal Convection
When a mechanism besides natural buoyancy generates this advective flow, we call it forced convection. In these cases, we're typically using something like a fan or pump to drive the flow of fluid. Forced convection can also be generated by things such as someone blowing on their skin to cool down a burn, or palm frond wielding servants. The point is that there is some sort of mechanism besides physics driving the flow, it's considered forced convection.
Design Implications for Forced Convection
Force Out the Heat!
The big positive attribute of forced convection versus natural convection is the increased amount of heat transfer. By being able to move more fluid through a system in the same period of time, more heat absorbed by the fluid can be forced away from your heat source. This keeps the heat from lingering and building up and in thermal management, that is the last thing we want.
What Forced Convection Means for Reliability
Unfortunately, the drawback of having something force flow through your system is that it might give out. Frictional parts in our pumps and fans wear out, the minor burn victim gets light headed from all that blowing, or the servants go to eat or sleep. These things cannot run indefinitely. That's where design engineers need to consider the reliability of their components and make sure that the end product is serviceable enough to replace broken parts or the parts are able to live longer than the expected lifetime of the final product. This is especially true for critical devices that support life or safety.
Moving Parts and Noise
Since forced convection requires moving parts to make fluid flow faster, it also produces sound. Fan or pump motors generate more noise compared to natural convection. For some applications, this can be a real drawback. I mean, it really takes you out of your immersive experience with video games or a movie when a fan jumps into high gear and starts humming loudly. You still need the fan, since you want to play games and watch movies for years to come. But that fan might kick in during those intense moments of your audio/visual experience.
Design Implications for Forced Convection
When it comes to your design and your final product needs, you and your end customer are the experts. You should be able to determine your preferred flow type based off your reliability and end user requirements. But remember, you're not alone. Aavid Genie can help walk through the process of looking at natural convection versus forced convection for your application. If you find you need more help, Boyd Design Engineers have developed solutions for tough, high power natural convection situations or made forced convection solutions meet hard reliability requirements. Either way your application goes, whether it be natural or forced, Boyd can help you out with what you need.
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