沸騰二重管에 있어서의 流動特性과 Burnout = A Study on the Characteristic of Flow and Burnout in Bolling Annulus

다국어 초록 (Multilingual Abstract)

The purposes of the present study are concerned with the boiling flow characteristic in annulus with different outer diameters, and the effect of annular gap size on the burnout behavior.
The experiment constitutes two parts: (1) natural and (2) forced convections. For the experiment of natural convection, two concentric glass tubes were used to make annulus with an inner diameter fixed at 12mm and with three different outer diameters, 16, 20, 24mm, respectively.
The inner tube was wound with a nichrome wire as an electric resistance heating element, making the heated length of 250mm.
The experiment was conducted at stsyem pressure of 1kg/㎠ and with an inlet subcooling 0-20℃ in the full range of throttling ratio.
For the experiment of forced convection, investigation was carried out, under the same condition, for 16, 18, 20, 22, 26mm at system pressure 1kg/㎠ and coolant mass velocity 2×10 exp (5)-3×10 exp (6)kg/m^2h.
The experimental results could be summerized as follows:
Ⅰ. The experiment of Natural Convection.
1) With the increase of ΔT_sub, D_2 and A/A_o, q_bo, on the whole, increases; with the decrease of ΔT_sub, D_2 and A/A_o, hydrodynamic instavility is accelerated prematually.
2) With the increase of ΔT_sub, D_2 and A/A_o, the burnout characteristic shows the high velocity-low quality burnout; with the decrease, the burnout characteristic shows low velocity-high quality burnout.
3. With the decrease of A/A_o, hydrodynamic instability is significantly restrained; and the difference of q in each D_2, under the same condition, is gradually reduced, finally converging into 1.9×10 exp (5)kcal/m^2-hr.
Ⅱ. The experiment of Forced Convection.
1) With the decrease of inlet subcooling ΔT_sub and outer diameter D_2, hydrodynamic instability is accelerated prematurely; with the increase of D_2, coalecence of bubbles is restrained, and, at the same time, a tendency toward spray annular flow becomes dominant.
2) The beneficial effects to contribute to making the spray annular flow are partly due to turbulent diffusion by the increase of mass flow rate G, and partly to the entrainment of water droplet by the increase of heat flux q.
3) When the both conditions of hydrodynamic instability occurrence and spray annular flow formation are matched to coexist, oscilatory spray annular flow takes place in the test section.
4) With the increase of ΔT_sub, G, and D_2, the burnout heat flux q, on the whole, increases.
5) With the increase of diameter D_2, the burnout characteristic shows the high velocity-low quality burnout; with the increase of D_2, the burnout characteristic shows low velocity-high quality.
6) Experimental values of heat flux can be considerably well correlated to the pressure drop between inlet and outlet of the test section.
Nomenclature
B =Constant.
C_P =Specific heat at constant pressure.
D_1 =Inner diameter of annulus.
D_2 =Outer diameter of annulus.
G =Mass flow rate.
h_fg =Latent heat of vaporization.
h_f =Saturation enthalpy.
h_1 =Inlet enthalpy.
K_2 =Friction coefficient of fluid.
L =Heated length.
L_BO =Heated length up to burnout location.
P =pressure.
ΔP =pressure drop between inlet and outlet.
q =Heat flux.
q_BO =Burnout heat flux.
r_1 =Radious of inner tube.
r_2 =Radious of outer tube.
r_max =Radious up to maximum velocity layer.
T =Temperature.
ΔT_sub =Inlet subcooling.
U_m =Average velocity of fluid.
U_mo =Average velocity at inlet.
X_BO =Burnout quality.
α_vm =Steam void.
τ_1 =Shearing stress at inner wall.
τ_2 =Shearing stress at outer wall.
γ =Specific weight.
μ =Coefficient of absolute velocity.
ρ_L =Density of liquid.
d =Density of fluid.

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