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Heat Transfer and Cooling Techniques at Low Temperature, Equation of heat…
Heat Transfer and Cooling Techniques at Low Temperature
Heat transfer at low temperature
Thermal Radiation
Any surface at finite temperature absorbs, reflects, and emits electromagnetic radiation
The emitted radiation, Φ, consists of a continuous non-uniform distribution of monochromatic components
Radiation exchange between two surfaces
The heat transfer by radiation between two enclosed surfaces which is from one at temperature T1 with an emissivity ε1 and a surface A1, to another at T2 with ε2 and A2
Determine one view factor from knowledge of the other
Relation pertains to the enclosure surface
by the study of
Shielding and multilayer insulation
By using the blackbody radiation q = σ (Twarm^4 - Tcold^4) to understand the philosophy of ‘passive’ thermal shielding from room temperature to low temperature
Shielding is implemented using a multilayer insulation (MLI) system (also called superinsulation)
An assembly of reflective films
Separated by insulating interlayers
Operated under vacuum
The reflecting layers reduce heat transfer by radiation
The insulating interlayers reduce heat transfer by conduction between reflecting layers
The high vacuum reduces convection and residual gas conduction
Thermal Conduction
Solid
Heat transfer without mass transfer
Thermal conductors possess a larger thermal conductivity integral
This is because the thermal conductivity of most materials varies strongly with temperature
Thermal conductivity integral is use in the determination of heat losses and heat interception between room temperature and the low temperature of the system
Liquid
Bad thermal conductors
Thermal conductivity usually decreases with temperature
Gas
The heat transfer between two surfaces in a gas is of interest
To evaluate the heat leak
To characterize thermal switches
Two heat transfer regimes
Hydrodynamic regime (
λ<<L
)
Obtained at high residual gas pressure
Heat transfer is independent on pressure
Described by a Fourier law
Free molecular regime (
λ>>L
)
Obtained at low residual pressure
Heat transfer depends on the residual gas pressure
Independent of distance, L
Describe by Kennard's law
Convection
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'
Forced Convection
The correlations used for non-cryogenic fluids are suitable at low temperature
Boiling Convection
Heat is transferred between a surface and the fluid
the conjunction of a phase change
the vapour bubble movement in the vicinity of the surface
The bubble growth rate
The detachment frequency
The number of nucleation sites
The surface conditions
Condition
Pool Boiling
Several regimes
can be identified
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
Vapour content and structure are
continuously increasing
At critical point, the vapour production is so high that the vapour structures coalesce and form a blanket of vapour at the heating surface
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Cooling techniques at low temperature
Cooling method
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
Dry method
A cryocooler is used without any fluid as coolant
Use to avoid dealing with a cryogenic fluid
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
Cooling a device to low temperature can be achieved with or without cryogenic fluids
Different methods of cooling
Baths
Where the fluid at the free surface is at saturation
(T ≈ Tsat)
A direct or indirect cooling method with no net liquid mass flow
Main heat transfer process is essentially due to latent heat of vaporization
Simplicity of the cryogenic design and operation
High heat transfer due to nucleate boiling
Almost constant surface temperature
A large quantity of cryogen has to be handled
Risk of pressure rise
Superfluid helium (He II) bath cooling techniques
To maintain a low temperature in superconducting accelerator cavities or certain accelerator magnets (below 2.17 K)
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
Disadvantage of single phase:
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
Cooling in the supercritical state (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
Two-phases
Advantages:
Having an almost isothermal flow due to the high heat transfer
Natural and two-phase circulation loop
Auto-tuned mass flow rate system
The flow is created by the weight unbalance between the heated branch and the feeding branch of the loop
The operating principle is the same if a single-phase fluid is used in the circulation loop
Two-phase flow circulation loops have high heat transfer rates
Cryogen-free cooling and the coupled system
Provide a finite cooling power at a prescribed temperature
A conductive thermal link between the cold source (the cryocooler) and the low temperature device
Technical issue
Thermal link limits the
cooling for transient events
Use a thermal link with a fluid
A small natural circulation loop coupled with a cryocooler can be used to serve as a self sustaining thermal link at cryogenic temperatures
Equation of heat flux density
Thermal time constant
Purpose
To characterize the radiation heat transfer
Spectral distribution
Directional distribution
Purpose
convection heat transfer
by
Saturated bath
Advantages
Disadvantages
Vertical configuration
Overcome
