Influence of Heating Rate on Subcooled Flow Boiling Critical Heat Flux in a Short Vertical Tube

Abstract

The subcooled flow boiling critical heat flux (CHF) for the flow velocities (u = 4.0 to 13.3 m/s), the inlet subcoolings (∆T sub, in = 130 to 161 K), the inlet pressure (P in = 812 to 1 315 kPa), the dissolved oxygen concentration (O 2 = 5.88 and 7.34 ppm) and the increasing heat input (Q 0 exp(t/τ), τ = 38.1 ms to 8.3 s) are systematically measured by the experimental water loop installed the pressurizer. The SUS304 tube of test tube inner diameter (d = 6 mm), heated length (L = 60 mm), L/d = 10 and wall thickness (δ = 0.5 mm) with the rough finished inner surface (Surface roughness, Ra = 3.18 µm) is used in this work. The CHF data for high heating rate were compared with the quasi steady state ones previously obtained and the values calculated by the steady state CHF correlations against outlet and inlet subcoolings. Transient CHF correlation against inlet subcooling has been given based on the experimental data for wide exponentially increasing heat input (Q 0 exp(t/τ), τ = 38.1 ms to 8.3 s). The influence of heating rate on CHF was investigated into details and the dominant mechanism of subcooled flow boiling critical heat flux for high heating rate was discussed.

Transient Critical Heat Fluxes of Subcooled Water Flow Boiling in a SUS304-CIRCULAR Tube with Twisted-Tape Insert

“…The transient CHFs, q cr,sub , are almost constant for the exponential period, τ, higher than around 800 ms and they become higher with the decrease in the τ. Those show nearly the same trend of dependence on transient CHF data for empty SUS304-tubes of d=3, 6, 9 and 12 mm with the exponentially increasing heat input (13)(14)(15) , although the SUS304 twisted-tape of width (w=5.6 mm), thickness (δ T =0.6 mm), total length (l=372 mm) and twist ratio (1) and (2). The transient CHF experiments with rapidly increasing temperature are very severe for test tube and these SUS304 tubes were not the same serial number but the same manufacturer.…”

“…Equation (23) can also describe these ratios for the p* ranging from 48.21 to 5.196×10 3 (49 points) within ±50 % difference. Equation (23) was derived based on the experimental data on an empty SUS304-tube for the inner diameters of 3, 6, 9 and 12 mm and the heated lengths of 33.15 to 132.9 mm with L/d=5.48 to 11.075 at the outlet pressures of around 800 and 1100 kPa with the exponentially increasing heat input (13)(14)(15) . The transient CHF correlation against inlet subcooling for the test tubes with various twisted-tape inserts is derived due to the effect of boiling number based on swirl velocity, Bo cr,sw , and Weber number based on swirl velocity, We sw , on the basis of Eq.…”

“…(1) (10,11) and the transient CHF correlation against inlet subcooling for the empty test tubes, Eq. (5) (13)(14)(15) as follows: …”

“…And the steady-state CHF correlations against inlet and outlet subcoolings for the test tubes with various twisted-tape inserts are derived as follows due to the effect of boiling number based on swirl velocity, Bo cr,sw , and Weber number based on swirl velocity, We sw . (5) and (6) (13)(14)(15) .…”

“…First is to measure the transient CHFs for SUS304-tube with a twisted-tape of twist ratio (y) of 3.37 for the wide ranges of heating rates (τ), swirl velocities (u sw ) and inlet subcoolings (ΔT sub,in ). Second is to compare the above results with the steady-state CHF data for the SUS304-circular tubes with various twisted-tape inserts (10)(11)(12) and the transient CHF data for the empty SUS304-tubes previously obtained (13)(14)(15) . Third is to clarify the influence of twisted-tape insert, heating rate and swirl velocity on the transient CHF.…”