Abstract
The forced-convection flow boiling heat transfer in narrow-annular flow passages was experimentally examined by using R-113. The gap clearance and the mass flux were δ = 0.5 ∼ 10.0 mm and G = 102 ∼ 103 kg/m2s, respectively. When the clearance was narrow (δ = 0.5 and 1.0 mm), the critical heat flux (CHF) condition which was defined as the deviation from the fully developed nucleate boiling state occurred in the first place, then the wall temperature excursion (EXC) was initiated after a further increase in the heat flux. At the CHF condition, the wall temperature did not rise sharply. The difference between those heat fluxes, qCHF and qEXC, became small as the flow rate was increased. When the clearance became wide (δ ≥ 2.0 mm), the CHF condition and the EXC occurred simultaneously. The EXC heat flux qEXC increased sharply with an increase in δ until δ = 2.0 mm. In δ > 2.0 mm, the increasing rate calmed down to show tendency reaching some asymptotical value, qEXC of the co-current flow was considerably higher, approximately ten times, than qEXC of the counter-current flow. The difference decreased as δ became wide. The critical heat flux, qCHF, correlation, which was developed by considering that whether the dried area which was created because of wearing-out of the liquid layer under a bubble could be rewetted or not decide the critical condition, predicted well the present experimental results of qCHF. However, when δ was extremely narrow such as δ = 0.5 and 1.0 mm, the heat transfer deterioration due to the appearance of a coalescent bubble occurred before the condition defined above, thus the correlation overpredicted qCHF.