30 Jan 2019 05:50

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By John Booher In Blog, TotalTempComments

What are the trade offs of heat transfer via conduction versus convection in regards to thermal testing?

Heat transfer via conduction is generally faster and more efficient. Depending on your purposes, conduction has a some clear performance advantages and some limitations which we will talk about. In general however, when appropriate conduction is often the best choice for performance.

More to the point, in this discussion, we are talking about trade offs between a temperature chamber and thermal platforms. Temperature chambers are an example of forced convection. Forced convection in a temperature chambers is achieved by the circulation of air using a fan.

Alternatively, natural convection is simply warmer air expanding and becoming less dense, naturally rising creating upward air flow with the colder air sinking to replace the heated air. As a practical note, some chambers have very little forced air flow and thus are limited in convection heat transfer performance. On the other hand increasing airflow excessively can produce unwanted heating by air friction and wasted money and energy spinning large blowers.

For completeness, briefly here, Radiation as the third main method of heat transfer is typically only used for thermal test where conduction or convection are not so practical such as thermal vacuum testing of products with non flat surfaces.

The popular example of comparison between conduction and convection is the pot on the stove.

Courtesy NASA / Machine Design

Mostly radiation transfers heat from the burner to the pan, convection transfers the heat within the water with hotter, less dense water rising and cooler water sinking until temperatures equalize. small thermal chamber in this example will cause your finger to feel the heat and maybe prevent you from picking up the pan.

In the example of a temperature chamber, a fan augments the natural convection flow with a stirring action, greatly increasing heat transfer, to be more precise, this is process is often known as advection. The convective heat transfer in the air of a chamber is far less effective due to the relative density of air compared to water. This shows up as a much lower heat transfer coefficient in heat transfer equations.

For those who think more clearly in terms of equations, here are the simplified comparison of the math behind the heat transfer.

For Conduction: Q = [k ∙ A ∙ (Thot – Tcold)]/d

Conduction is the direct diffusion of heat through a solid material

Q is the amount of heat transferred per unit time

k is the thermal conductivity of the barrier

A is the heat transfer area

T-hot and T-cold are the temperatures of the regions for heat transfer

d is s the thickness of the barrier

With Convection, the Equation looks like this:

Convection being the heat transfer between two items via moving groups of molecules

Q = hc ∙ A ∙ (Ts – Tf)

Where Q is again the amount of heat transferred per time unit

Hc is the convective heat transfer coefficient

A is again the heat transfer area

Ts and Tf are the temperatures of the surface and the temperature of the fluid (air)

These equations describe natural convection heat transfer, not assisted by fan or stirrer. The equations become considerably more complex and generally subject to hueristics and detailed modeling in order to achieve at least moderate accuracy. Variables such as calculating the barrier layers at surfaces, determination of laminar v. turbulent flow and general calculations based on the geometry of the air space fit into the equations. All that boils down to the generality that the more you can stir the air the better the heat transfer – however it will never be as good as conduction. The kitchen example here is that you may be able to keep your hand in a hot oven for a couple seconds without getting burned but a just a few milliseconds of conduction will cause ‘cutaneous damage’ to your finger so with this you can see the relative rate of heat transfer with conduction and convection.

Besides burning your finger, conduction has its issues as well. While the Heat transfer coefficient, hc is the main limiting factor for heat transfer by convection, The area (A) is the main limiting factor for heat transfer by conduction on thermal platforms. Extra care must be taken as well to ensure that the area is really what you think it is(link). Often voids or irregular sufaces limit the area of actual contact.

TotalTemp’s solution to optimal heat transfer is with the intelligent combination both conduction and convection (advection) in our new Hybrid Benchtop Chamber.

Feel free to contact us with any questions concerns or interest in thermal platforms Hybrid benchtop chambers or any thermal testing equipment. We are always available with quality before and after sales support.