Heat Transfer & CHF Enhancement HOME > RESEARCH > Phase Change Phenomena> Heat Transfer & CHF Enhancement

As one of a key phenomenon in thermal hydraulics system, nucleate boiling has been widely studied by numerous researchers to improve efficiency and safety of the system (i.e, nuclear power plant, refrigerating system etc.). In general, the evaluation of the boiling performance mainly focus on two physical parameters: boiling heat transfer (BHT) and critical heat flux (CHF). Since both BHT and CHF contribute the thermal system efficiency and safety, respectively many new approaches to enhance the boiling performance compared to the conventional methodology have been investigated.We study the boiling phenomena and its performance enhancement using pool boiling experiments, which is the most fundamental approach of the boiling study.

Fig.1.Typical boiling curve, showing qualitatively the dependence of the wall heat flux q on the wall superheat ΔT. Schematic drawings show the boiling process in regions I-V, and transition points A-E

We focused the boiling performance (BHT and CHF) enhancement using various surface and fluid treatment, and we also tried figure out the mechanistic understanding of the enhancement through various measurement and analysis.

- CHF / BHT enhancement using nanofluid (Previous)
- CHF / BHT enhancement using wetting-controlled surface (On going)
- CHF / BHT enhancement using nano/microstructrued surface (Ongoing)
- CHF / BHT enhancement using graphene application (Ongoing)
1 Nanofluid
2000s, the nanofluid was gotten spotlight due to the enormous enhanced CHF and it was also calcifiedthat enhancement of CHF is come from the change of surface morphology due to the nanoparticle deposition on heater surface. We performed the pool boiling experiment with various nanofluids.
Fig. 2. SEM Images of (a) Bare Wire, (b) TiO2, (c) SiO2 and(d) Ag Nanoparticles Deposited on wire after Boiling
2 Wetting Controlled Surface

Recently, the wetting behavior is considered as a key physics ofliquid-vapor phase heat transfer, i.e. boiling and condensation. Especially, wettability effect on the bubble growth and CHF of the nucleate boiling has been widely reported. We controlled the heating surface by the numerous coating method.

Fig. 3. Wetting controlled surface (a) Silicon, (b) Bromo-Terminated Monolayer, (c) Octadecyltricholorosilane Monolayer and (d) Teflon coated surface
3 Nano/microstructured surface

In recent, the nano- and micro- engineering has achieved tremendous techniques to produce a challenging and innovating heat transfer surface. We modified nano/micro structured zirconium alloy surface by anodic oxidation process (Figure 4). It shows tremendous enhancement of BHT and CHF. And, we also micro-fabricated the silicon surface. (Figure 5)

Fig. 4. Nano-Microstructured Zirconium alloy surface (Anodic Oxidation Process)
Fig. 5. Microstructures of Silicon (Deep Reaction Ion Etching Process)
4 Graphene

Graphene is the 2-D structures of Carbon, showing a tremendous high thermal conductivity. Its application and potential widely has been researched and reported, now days. We also adopted the advantage of the thermal properties in boiling phenomena, and reported innovative CHF enhancement.

Fig. 6. Bare Wire Surface and Graphene Oxide (#%) coated surface

In Pool Boiling experiments, there are mainly two kinds of heating surface geometry, i.e. wire type and plate type.

- Wire Pool Boiling

The boiling experiment of the nanofluids and graphene deposited surface has been carried out, using the wire pool boiling experiments.

Fig. 7. Wire Pool Boiling Experiment
- Plate Pool Boiling

In plate pool boiling, two methods of heating has been used. The first type is the conduction method, which use the copper bar as a conducting material to the heating surface from the cartridge heater. (Figure 8). The boiling experiment of the modified zirconium surface has been carried out through the conduction experiment setup. The second type is the joule heating method, which directly input the power through the electrical resistance. (Figure 9) Usually, the silicon structured surface has been applied to the joule heating experiments setup.

Fig. 8. Plate Pool Boiling Experiments (Conduction Method)
Fig. 9. Plate Pool Boiling Experiments (Joule Heating Method): (A) test section, (B) immersion pre-heater, (C) thermocouple, (D) sample jig, (E) pool, (F) visualization window, (G) reflux condenser, (H) PID controller, (I) power supply, (J) data acquisition system, and (K) reference resistance in constant temperature pool.