Experimental study on influence of oscillatory heat flux on heat transfer of R-134a time-periodic subcooled flow boiling in annular duct

The flow rate can fluctuate in offshore nuclear power systems which are exposed to wind and waves, as well as in loops where flow instabilities occur, resulting in different thermal-hydraulic characteristics compared with that under steady flow. Among the thermal-hydraulic characteristics, onset of nucleate boiling (ONB) model determines whether the fluid is boiling, and boiling heat transfer is crucial to equipment performance and safety, both being key issues in subcooled flow boiling. Therefore, an experimental study was conducted to investigate how an imposed periodic flow oscillation affects the boiling inception and heat transfer of subcooled flow boiling of water in a vertical tube. The experiments were conducted under atmospheric pressure with the average flow rate ranging from 96kg/m2s to 287kg/m2s and heat flux ranging from 10kW/m2 to 197kW/m2. The relative pulsatile amplitude range is 0.1–0.3 and pulsatile period range is 10s-30s. Photographic images and thermal parameters such as temperatures and flow rate were recorded. The lack of nucleation site on the heated surface of the test section results in high wall superheat at ONB. The effects of pulsatile amplitude and period on superheat at boiling onset and average heat transfer were analyzed. The results show that the superheat at boiling inception is decreased when the average heat flux is lower than the heat flux at boiling inception of the corresponding steady flow, and the superheat at boiling onset is increased when the average heat flux is higher than the heat flux at boiling onset of the corresponding steady flow. The above effect of flow rate pulsation on superheat increases with increasing amplitude and decreasing period, and the mechanism can be explained by boiling nucleation theory. The lack of large active nucleation site also affects the boiling heat transfer. By comparing the contribution of nucleate boiling to heat transfer with the widely used Cooper’s pool boiling correlation, the subcooled flow boiling was found suppressed by convection. The average heat transfer of both the intermittent flow boiling and the single phase flow is influenced by flow oscillation.

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Alumina Nanoparticle Pre-Coated Tubing Enhancing Subcooled Flow Boiling Critical Heat Flux

ASME 2009 Second International Conference on Micro/Nanoscale Heat and Mass Transfer, Volume 1 ◽

10.1115/mnhmt2009-18364 ◽

2009 ◽

Cited By ~ 2

Author(s):

Bao Truong ◽

Lin-wen Hu ◽

Jacopo Buongiorno ◽

Thomas McKrell

Keyword(s):

Heat Transfer ◽

Heat Flux ◽

Critical Heat Flux ◽

Mass Flux ◽

Boiling Heat Transfer ◽

Flow Boiling ◽

Subcooled Flow Boiling ◽

Nanoparticle Deposition ◽

Subcooled Flow ◽

Flow Boiling Heat Transfer

Nanofluids are engineered colloidal dispersions of nano-sized particle in common base fluids. Previous pool boiling studies have shown that nanofluids can improve critical heat flux (CHF) up to 200% for pool boiling and up to 50% for subcooled flow boiling due to the boiling induced nanoparticle deposition on the heated surface. Motivated by the significant CHF enhancement of nanoparticle deposited surface, this study investigated experimentally the subcooled flow boiling heat transfer of pre-coated test sections in water. Using a separate coating loop, stainless steel test sections were treated via flow boiling of alumina nanofluids at constant heat flux and mass flow rate. The pre-coated test sections were then used in another loop to measure subcooled flow boiling heat transfer coefficient and CHF with water. The CHF values for the pre-coated tubing were found on average to be 28% higher than bare tubing at high mass flux G = 2500 kg/m2 s. However, no enhancement was found at lower mass flux G = 1500 kg/m2 s. The heat transfer coefficients did not differ much between experiments when the bare or coated tubes were used. SEM images of the test sections confirm the presence of a nanoparticle coating layer. The nanoparticle deposition is sporadic and no relationship between the coating pattern and the amount of CHF enhancement is observed.

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