Study of Boiling in Short Narrow Channels and Its Application to Design of Magnets Cooled by Liquid H2 and N2

A study has been made of the critical heat input, Q̇c, to liquid H₂ and N₂ boiling in an annular gap, of cross‐section area, A, between a vertical cylindrical heater of height, Z, and various concentric plugs. The data are in approximate agreement with calculations based on an assumption of homogeneous and frictionless two phase flow. The pressure drop across the channel is spent primarily to accelerate the vapor being formed and the liquid carried with it. The derived relation does not contain any empirical coefficients or exponents. For ρv,2≪ρL the general equation reduces to $${\mathrm{Q̇=AL(\rho }}_V{\text{,2 ρ}}_L{Z}_e{\mathrm{gf}}_2{)}^{\frac{1}{2}}$$ , where L is the latent heat of vaporization, ρv,2 the exit vapor density, ρL the liquid density, g the gravitational acceleration, Z_e the hydrostatic head equivalent to the pressure drop across the heater, and f₂ the mass fraction of vapor in the fluid leaving the heater. Forced flow experiments give f₂. At Q̇c equal to that found in natural convection boiling, f₂=̇0.5 for both liquids. For Z=3 or 6 in. and a channel thickness of 0.022 in. the ratio of observed to calculated Q̇ is 0.95 for N₂ and 0.70 for H₂. The calculation is equally good for critical natural convection experiments, Z_e=Z, and for subcritical as well as critical forced flow measurements, which extend up to Z_e=70 Z. The optimum operating vapor pressure is discussed for magnets cooled by ideal natural convection boiling. A simple roll‐type of coil is discussed and is shown to be superior in some respects to more conventional windings.