Important thing to remember:

The fluid aspect, not the thermal aspect, owns the temperature of the fluid.

There are many mechanisms by which heat is transferred through fluids and between fluids and the environment. GUNNS can model all of them:

Advection is the transport of heat in the bulk flow of a fluid.

• All fluid mass has heat content (its internal energy, which we can approximate as its specific enthalpy).
• As the fluid mass moves around it carries that heat around with it. This is how ATCS coolant loops transport the heat from sources to sinks.
• This heat transport is equivalent to a heat flux q = ṁ·h, where ṁ is mass flow rate (kg/s) and h is specific enthalpy (J/kg).
• Advection happens automatically in all fluid flows in GUNNS so you don’t have to do anything to enable it.
• Keep in mind that heat is always moving around as it is carried inside the fluid flow in your network.

Remember the definition of temperature: a measure of the average kinetic energy of the molecules in a region.

The random motion of molecules is like advection but on the molecular scale; molecules with different energies mix together, causing the average kinetic energy of the molecules, and therefore the temperature in a volume of fluid to become uniform over time.

In GUNNS, the fluid contents of a node are ALWAYS assumed to be uniform (we say homogenous), and that goes for temperature as well as mixture. So, when fluids of different temperatures flow into a node from different locations, the mixing occurs instantaneously, regardless of how fast or slow the actual mixing & diffusion process would actually take. This is a very important limitation to keep in mind.

We can model this heat diffusion over realistic time scales between the fluids of two nodes connected by the GunnsFluidHatch link. GunnsFluidHatch is just a valve that also models diffusion. Hatches tend to diffuse more across them than valves because they have big cross-sectional areas — hence the link name.

Heat flows from hot to cold through any material, and this includes fluids. How well the heat conducts from point A to B through a material is governed by the material’s thermal conductivity. The thermal conductivity has units of (W/m/K) and in the fluid aspect, this is a property of the GUNNS fluid type.

Strictly speaking, fluid heat conduction is only the transfer of kinetic energy between molecules when they collide, and is different from diffusion. This distinction doesn’t matter for solids since the molecules don’t mix freely but tend to stay in place and vibrate against each other (conduction) and there isn’t diffusion.

Fluid conductance is very slow compared to the other heat transport mechanisms, so we usually don’t model it. In GUNNS, the only link type that models it is the GunnsFluidTank — useful for conducting heat between a tank wall and the fluid stored inside. This is an interface between the fluid & thermal (wall) aspects.

This is the big one: by far the dominant fluid heat transfer effect in most systems. You know it intuitively as the “wind-chill” effect. It might also explain “brain freeze¹”. This happens any time there is fluid flowing past a surface with a different temperature. This typically happens in heat exchangers and coldplates, and it’s the chief mechanism for transferring heat to & from the fluid in flow loops.

Forced convection is basically conduction combined with advection. The conduction effect between the wall and a moving fluid is greatly increased because the movement of the fluid creates shear effects near the wall that increases the heat transfer coefficient of the wall/fluid boundary. Also, the fluid that received or transmitted the heat transfer is continuously replaced by fresh fluid by the bulk flow (advection), and this keeps the delta-temperature between the wall and fluid high, which also increases heat transfer.

In GUNNS, forced convection can be modeled in most conductor-type links. This includes most valves, pipes, pumps/fans and heat exchangers. The effect is always optional and configurable. There are two main ways to configure it, and each link type will use one of these ways:

• Configuring HTC by pipe goemetry: rather than having to guess or find the device’s heat transfer coefficient, you can specify the pipe goemetry and GUNNS will derive the HTC for you. It also automatically models the difference in laminar vs. turbulent flow on convection (turbulent flow greatly increases heat transfer). However note that GUNNS can only assume it’s a long, straight pipe with circular cross-section, which convective devices typically are not. For this reason, the real device goemetry is only a starting point and you typically have the tweak the numbers to get the right performance anyway.
• Configuring HTC directly

Refer to the GunnsDraw link help pages for details on how to configure a link. GunnsFluidHeatExchanger has more detail on how forced convection works in GUNNS and how to use it.

In forced convection, there is some external source causing the bulk fluid flow, like the wind or the pumps & fans in your systems.

Natural (also called “free”) convection is like forced convection except the heat transfer itself causes the bulk fluid motion. Heat transfer causes the fluid temperature to change, and this also changes the fluid’s density. This creates a density gradient across the fluid as you move away from the wall. In the presence of an acceleration field like gravity, the denser fluid wants to fall “downward” and be replaced by less dense fluid. This creates a circulation of fluid in the vicinity of the wall, and thus convection.

GUNNS doesn’t automatically model this, but you can fake it by using forced convection and creating your own circulation flow. You can use a GunnsFluidPotential link to create a flow loop in the circulation pattern that the natural convection would cause, and then drive the GunnsFluidPotential with some external model logic (a Spotter perhaps) to create the flow rate as a function of acceleration field strength and heat transfer amount, etc.

All matter emits and absorbs radiation, and this includes fluids as well. Typically fluids don’t radiate much until they get very hot (like a glowing hot plasma), so we usually ignore this effect.

GUNNS does not model this effect so we won’t go into detail here, but you can fake it. If you know how much radiation heat flux should be absorbed or emitted by a fluid, there are a few ways you can add or remove that heat yourself:

• A GunnsFluidHeater link can add or remove heat directly to the fluid contents of the connected node.
• Tank links like GunnsFluidTank and GunnsFluidAccum have interfaces to add or remove heat directly from their contained fluid.

However, you must be very careful not to overheat or overcool the fluid, since the methods described above are brute-force methods that force the heat exchange regardless of the fluid’s thermal capacity. If you are not careful, you can cause extreme temperatures that can feedback into pressure and blow up the network.

¹ Research needed. Very cold beer would be a good working fluid for research.

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