An experimental study of CHF and power transients in flow boiling at low-pressure low-flow conditions

ABSTRACT

Flow boiling experiments were conducted in an annular channel at very low flow rates and atmospheric pressure varying the inlet subcooling from 20 to 40${ }^{\circ }\mathrm{C}$. critical heat flux occurred during the oscillatory flow pattern transition from slug to churn regime. Comparison of low-pressure and low-flow critical heat flux correlations with the present data has shown that these correlations are not applicable for prediction of critical heat flux under oscillatory flow pattern transition from slug to churn. Power transient experiments were also performed at low-pressure and low-flow conditions. Flow regimes and the sequence of events leading to critical heat flux were visually observed using the high-speed camera.

KEYWORDS:

• Flow boiling
• critical heat flux
• low-pressure and low-flow conditions
• power transients

Nomenclature

$A_f$	=	flow area $\left({\mathrm{m}}^2\right)$
$A_f$	=	heated area $\left({\mathrm{m}}^2\right)$
C	=	constant in Wallis flooding correlation
CHF	=	critical heat flux ($\mathrm{k}\mathrm{W}/{\mathrm{m}}^2$)
$D_h$	=	hydraulic diameter $\left(\mathrm{m}\right)$
$f$	=	oscillation frequency (${\mathrm{s}}^{-1}$)
$G$	=	mass flux ($\mathrm{k}\mathrm{g}/{\mathrm{m}}^2\mathrm{s}$)
g	=	acceleration due to gravity $\left({\mathrm{m}}^2/\mathrm{s}\right)$
${\mathrm{h}}_{\mathrm{f}\mathrm{g}}$	=	latent heat of vaporization $\left(\frac{\mathrm{k}\mathrm{J}}{\mathrm{k}\mathrm{g}}\right)$
$q$	=	applied wall heat flux ($\mathrm{k}\mathrm{W}/{\mathrm{m}}^2$)
$q_{CHF}^{{ }^{\prime \prime }}$	=	critical heat flux ($\mathrm{k}\mathrm{W}/{\mathrm{m}}^2$)
t	=	time ($\mathrm{s}$)
T	=	temperature (${ }^{\circ }\mathrm{C}$)
$\mathrm{\Delta }{T}_{sub,in}$	=	inlet degree of subcooling (${ }^{\circ }\mathrm{C}$)
$T_{in}$	=	inlet temperature (${ }^{\circ }\mathrm{C}$)
$x_c$	=	critical quality

Greek symbols

$\delta$	=	gap width
${\rho }_f$	=	density of the saturated liquid $\left(\mathrm{k}\mathrm{g}{\mathrm{m}}^{-3}\right)$
${\rho }_g$	=	density of the saturated vapor $\left(\mathrm{k}\mathrm{g}{\mathrm{m}}^{-3}\right)$
$\mathrm{\Delta }\rho$	=	density difference between the saturated liquid and vapor $\left(\mathrm{k}\mathrm{g}{\mathrm{m}}^{-3}\right)$