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Low Temperature - Coggle Diagram

- - - - Single Phase Flow - disadvantages in the pressurization system, implementation of the circulation pump and its maintanance, implementation of heat exchanger system, temperature range limitation
      - Two Phase Flow - advantages in having an almost isothermal flow due to high heat transfer even at high vapour quality
  - - - Types of Circulation loops - Open Loop : boil off goes out of the system to be reliquefies before refilling the reservoir - Closed Loop : the vapour re-condensed in a closed reservoir wih a heat exchanger
      - Advantages : no need for a circulating system
        Disadvantages : flow instabilities at low heat flux
  - - - Advantages : easy implementation, accurate as provide finite cooling power at a prescribed temp
        Disadvantages: Working temperature higher, distribution of cooling power over the entire system, thermal diffusion in thermal link limits the cooling for transient events
      - Method to use a thermal link with a fluid
        
        Capillary - pumped devices
        
        A flow is created by capillary pressure in porous medium at liquid/vapor interface
        
        Heat pipe with a wick inside the pipe serving as porous media
        
        Transfer 50 W between the cold source at 10 K
        
        Cryogenic Loop Heat pipe
        
        Heat pipe in a loop configuration to create mass flow rate
        
        Transmit 40 W between cold source at 6 K
        
        Oscillating Heat Pipe (OHP)
        
        Consist of capillary tube, wound in serpentine manner, connecting the ends of the inlets
        
        Advantages : utilized the pressure change due to volume expansion and contraction during a phase transition
        
        Vertical Thermosyphon
        
        Used circulation loop and based on the weight unbalance but with sigle vertically oriented tube - The liquids flow down the wll and the vapour flows in a counter-current manner to the liquid at the centre of the tube - Transmit 20 W at 10 K
        
        Coupled Circulation Loop
        
        Small natural circulation loop coupled with a cryocooler serve as a self sustaining thermal link
        
        Composed of condenser separator cooled by second stage of cryocooler
        
        Liquid helium flow through this loop and re-condensed in the condenser
        
        Transfer 5000 W/m.m.K at 4.2 K
- - - - Conductors possess a larger thermal conductivity integral.
      - thermal conductivity integral determine heat losses and heat interception
        
        heat interception depends on thermalization temp, material properties, geometry system
      - Thermal design at low T
        
        thermal resistance of the structural material
        
        transient conduction process
    - - Thermal Conductivity decreases with temp
        
        mostly negligible
    - - Two different heat transfer regimes
        
        Free molecular regimes
        
        Hydrodynamic regime
  - - - simplest correlation :
        
        n=1/4 for laminar regime
        
        n=1/3 for turbulent regime
    - - Heat is transferred between a surface and the fluid
        
        phase change
        
        vapour bubble movement in vicinity
      - Heat transfer depends on
        
        bubble growth rate
        
        detachment frequency
        
        number of nucleation sites
        
        surface condition
  - - - Ideal Surface: Blackbody
        
        absorbs incident radiation regardless wavelength and direction
      - Emissivity decreses withtemperater but increases with oxidation, impurities, dirt
    - - The reflecting layers reduce heat transfer by radiation
        
        the insulating interlayers reduce heat transfer by conduction between reflecting layers
        
        high vacuum reduces convection and residual gas conduction
      - An assembly of reflective films separated by insulating nterlayers operated under vacuum