Please enable JavaScript.
Coggle requires JavaScript to display documents.
Heat Transfer and Cooling Techniques at Low Temperature - Coggle Diagram
Heat Transfer and Cooling Techniques at Low Temperature
Heat transfer at low temperature
Thermal conduction
General laws
Any surface at finite temperature absorbs, reflects, and emits electromagnetic radiation.
Spectral and directional distribution.
Shielding and multilayer insulation
To understand the philosophy of ‘passive’ thermal shielding from room temperature to low temperature
Simple evaluation using the blackbody relation q=σ (T^4warm−T^4cold )
In practice, passive thermal shielding is implemented using a multilayer insulation (MLI)
system.
Radiation exchange between two surfaces
Heat transfer by radiation between two enclosed surfaces.
To determine one view factor from knowledge of the other.
Thermal radiation
Conduction in solids
Heat transfer without mass transfer.
Flux density: q = -k(T)∇T
Determine of heat losses and heat interception between room temperature and the low temperature of the system under consideration.
Conduction in liquids
Bad thermal conductors.
Thermal conductivity usually decreases with temperature
Conduction in gas
Heat transfer between two surfaces in a gas is of interest:
-To evaluate the heat leak
-To characterize thermal switches.
λ≫ L, free molecular regime:
-Obtained at low residual pressure.
-Described by Kennard’s law.
λ ≪ L, hydrodynamic regime:
-Obtained at high residual gas pressure
-Described by a Fourier law
Convection
Forced convection
the correlations used for non-cryogenic fluids are suitable at low temperature in the forced convection regime.
Introduction + Natural convection
the heat can be transferred in the fluid by movement of matter.
The movement of matter can be created externally by a pump or a pressurization system.
When the fluid movement is created internally, by a decrease or increase of the fluid density or by the buoyancy effect, it is called ‘natural convection’.
Boiling convection
Heat is transferred between a surface and the fluid by the conjunction of a phase change and the vapor bubble movement in the vicinity of the surface.
Heat transfer combines natural convection in the liquid, latent heat to be absorbed for the bubble formation and the bubble hydrodynamics.
The heat transfer process depends on:
-bubble growth rate
-detachment frequency
-number of nucleation sites, and the surface conditions.
Condition
Before onset of boiling:
Natural convection takes place
The boiling heat transfer
is extremely efficient
When boiling is activated, the wall temperature increase is slowed down.
After onset of boiling:
The evolution of nucleate
boiling is encountered
From partially to fully
developed nucleate boiling
Vapor content and structure are
continuously increasing
At critical point, the vapor production is so high that the vapor structures coalesce and form a blanket of vapor at the heating surface
Film boiling occur.
Pool Boiling:
Several regimes can be identified
Cooling techniques at low temperature
Introduction
Cooling a device to low temperature can be achieved with or without cryogenic fluids.
Cooling method:
Indirect cooling method:
A cryogenic fluid is used without
direct contact with the device
but only through intermediate
components.
Use for the reduction of the cryogenic fluid inventory while keeping a high fluid–solid heat transfer coefficient.
Dry method:
A cryocooler is used without any fluid as coolant.
Use to avoid dealing with a cryogenic fluid
Wet method:
A cryogenic fluid is used in
contact with the device.
Use to evacuate a large heat load or maintain a uniform temperature within the system
Different methods of cooling
Forced flow
Used to reduce the amount of cryogen, especially in indirect cooling composed of a network of peripheral tubes.
High heat transfer can be achieved in different cryogens in single phase.
Advantages:
A heat transfer coefficient comparable to that of pool boiling in helium.
The lack of hydraulic instabilities in the single-phase flow compared to a two-phase flow
Disadvantage:
The pressurization system.
The implementation of the circulation pump and its maintenance at low temperature.
The implementation of the heat exchanger system to sub-cool the fluid.
Two-phases:
Advantage:
Having an almost isothermal flow
due to the high heat transfer
Cryogen-free cooling and the coupled system
Wick-based heat pipe:
developed at the nitrogen temperature range.
can transfer around 50 W between the cold source.
The system to be cooled with a temperature difference of around 10 K
Cryogen-free cooling method:
one can use a conductive thermal link between the cold source (the cryocooler) and the low-temperature device.
Oscillating Heat Pipe (OHP) :
Consists of a capillary tube, wound in a serpentine manner, connecting the ends to the inlets
Utilizes the pressure change due to volume expansion and contraction during a phase transition to excite the oscillation of liquid slugs and vapor bubbles between the evaporator and the condenser.
Can be used with different fluids and exhibit large equivalent heat transfer conductivity
Vertical Thermosyphon:
Option to consider when creating a thermal
link.
The working principle somewhat similar to the above-mentioned circulation loop based on the weight unbalance.
There is a single vertically oriented tube.
The liquid flows down the wall and the vapor flows in a counter-current manner to the liquid at the center of the tube
Capillary-pumped devices:
A flow is created by capillary pressure
in a porous medium at the liquid/vapor interface.
Simplest system is a heat pipe, with a wick inside the pipe serving a porous media
Baths
Fluid at the free surface is at saturation
(T ≈ Tsat).
It is a direct or indirect cooling method with no net liquid mass flow and where the main heat transfer process is essentially due to latent heat of vaporization.
Superfluid helium (He II) bath cooling techniques are frequently employed to maintain a low temperature in superconducting accelerator cavities or certain accelerator magnets (below 2.17 K).
Advantages:
Simplicity of the cryogenic design and operation
High heat transfer due to nucleate boiling
Almost constant surface temperature
Disadvantages:
A large quantity of cryogen has to be handled
Risk of pressure rise
Natural and two-phase
circulation loops
Circulation loops are an auto-tuned mass flow rate system in which the flow is created by the weight unbalance between the heated branch and the feeding branch of the loop due to vaporization or decreased vapor density.
The operating principle is the same if a single-phase fluid is used in the circulation.
In a vertical configuration, two-phase flow circulation loops also have high heat transfer rates.
‘Open Loop’: where the boil-off goes out of the system to be re-liquefied somewhere else before refilling the reservoir permanently to avoid a dry-out.
‘Closed Loop’, where the vapor is re-condensed in a closed reservoir with a heat exchanger.
