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.
但在行业中，你可能听到了“强迫对流”的条款和“自然对流”抛出。虽然两者之间可能似乎似乎似乎似乎不大，但它对您的热管理解决方案设计了很大影响。所以让我们来看看自然对流与强迫对流并进入差异。
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.
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.
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.
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.
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.
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.
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