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Heat Transfer, Solid, Fourier Law, 4, Liquid, Gas, λ ≪ L, it corresponds…
Heat Transfer
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- λ ≫ L, it corresponds to the free molecular regime, obtained at low residual pressure, and the
heat transfer depends on the residual gas pressure and is independent of L. Describe by Kennerd's Law.
Any surface at finite temperature absorbs, reflects, and emits electromagnetic radiation.
The heat transfer between two enclosed surface can be written as: where q12 is the heat transfer rate from surface 1 to surface 2. F12 is the ‘view factor’, which is the fraction of the heat leaving surface 1 that is intercepted by surface 2.
To understand the philosophy of ‘passive’ thermal shielding from room temperature to low temperature, simple evaluation using the blackbody relation without knowledge of the emissivities and view factors.
The heat flux at the wall is computed with correlations of the type Nu = f(Gr, Pr, L)
The correlations used for non-cryogenic fluids are suitable at low temperature in the forced convection regime.
Heat is transferred between a surface and the fluid by the conjunction of a phase change and the vapour bubble movement in the vicinity of the surface.
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- λ ≪ L, it corresponds to the hydrodynamic regime, obtained at high residual gas pressure and the heat transfer is independent of pressure and described by a Fourier law.
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The emitted radiation, Φ, consists of a continuous non-uniform distribution of monochromatic components
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An intermediate surface held at intermediate temperature is required to reduce the heat load at low temperature.
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To optimize the use of MLI, the maximum number of layers per centimetre should be between 20 and 30, and isothermal contact points are required.
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Used for cryogenic fluids, since two-phase phenomena take over even at low heat fluxes.
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- natural convection in the liquid
- latent heat to be absorbed for the bubble formation
- the bubble hydrodynamics.
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