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Thermal convection: natural versus forced convection
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. Space applications have no access to fluid and cannot utilize thermal convection. Otherwise, it’s the most popular way to get heat out of products.
The selection between natural convection and forced convection has a large impact on how your thermal management solution is designed. Let's compare natural convection and forced convection.
In thermal management, a convection solution actually relies both on convection and conduction. The first heat transfer process is technically conduction, where the heat from the heat sink or liquid cold plate 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. Once we grasp the parts of convection, we’re able to break down and improve each of these parts to maximize 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. Heat plume and chimney effect are both terms that describe natural convection. As fluid near the heat source and heat sink gets hotter than ambient temperature, it has less pressure. 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. Air is readily available and requires minimal routing and design overhead to develop a suitable solution. Liquid natural convection solutions, known as immersion cooling, requires additional planning and consideration. Liquid needs adequate containment, sealing and leak protection and may potentially need specialized dielectric fluids to protect electronics from shorting. This implies more engineering time especially during the design and validation portions of product development.
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. 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.
When a mechanism besides natural buoyancy generates this advective flow, we call it forced convection. In these cases, we’re typically using a fan or pump to drive the flow of fluid.
By increasing the flow rate, forced convection is more effective at heat transfer versus natural convection. 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 heat from accumulating.
Frictional parts in pumps and fans wear out. Active air mover components cannot run indefinitely. 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.
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. Noise can detract from user experience, generate excessive vibration, or even generate safety issues. When adding in a fan, thermal designers must consider the end application and select a fan or blower that meets performance requires, but not exceed any noise or vibration thresholds.
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. Eaton 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, Eaton can help you out with what you need.
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