| 1. |
- Cao, Zhen, et al.
(author)
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Nanoparticle-Assisted Pool Boiling Heat Transfer on Micro-Pin-Fin Surfaces
- 2021
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In: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 37:3, s. 1089-1101
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Journal article (peer-reviewed)abstract
- Boiling heat transfer intensification is of significant relevance to energy conversion and various cooling processes. This study aimed to enhance the saturated pool boiling of FC-72 (a dielectric liquid) by surface modifications and explore mechanisms of the enhancement. Specifically, circular and square micro pin fins were fabricated on silicon surfaces by dry etching and then copper nanoparticles were deposited on the micro-pin-fin surfaces by electrostatic deposition. Experimental results indicated that compared with a smooth surface, the micro pin fins increased the heat transfer coefficient and the critical heat flux by more than 200 and 65–83%, respectively, which were further enhanced by the nanoparticles up to 24% and more than 20%, respectively. Correspondingly, the enhancement mechanism was carefully explored by high-speed bubble visualizations, surface wickability measurements, and model analysis. It was quantitatively found that small bubble departure diameters with high bubble departure frequencies promoted high heat transfer coefficients. The wickability, which characterizes the ability of a liquid to rewet a surface, played an important role in determining the critical heat flux, but further analyses indicated that evaporation beneath bubbles was also essential and competition between the wicking and the evaporation finally triggered the critical heat flux.
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| 2. |
- Cao, Zhen, et al.
(author)
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Pool boiling heat transfer of FC-72 on pin-fin silicon surfaces with nanoparticle deposition
- 2018
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In: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 126, s. 1019-1033
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Journal article (peer-reviewed)abstract
- In the present study, two types of micro-pin–fin configurations were fabricated on silicon surfaces by a dry etching method, i.e., staggered pin fins (#1) and aligned pin fins with empty areas (#2). The micro-pin–fin surfaces were then further modified by depositing FeMn oxide nanoparticles (∼35 nm) electrostatically for 8 h and 16 h, respectively, namely #1-8h, #1-16h, #2-8h and #2-16h. Subcooled pool boiling heat transfer was experimentally studied on these surfaces at atmospheric pressure, using FC-72 as the working fluid. The results showed that in comparison to the smooth surface, pool boiling heat transfer was significantly enhanced by the micro-pin-fin surfaces and the maximum superheat was considerably decreased. Additionally, critical heat fluxes were also greatly improved, e.g., the critical heat flux on #1 was almost twice of that on the smooth surface. Generally, the nanoparticle deposition could further enhance pool boiling heat transfer, including the heat transfer coefficient and critical heat flux (CHF). High speed visualizations were taken to explore the mechanisms behind the heat transfer performance. The bubble behavior on the micro-pin–fin surfaces with and without nanoparticles was compared at low, moderate and high heat fluxes, respectively. The wickability of FC-72 on the test surfaces was measured, based on which, a modified CHF model was proposed to predict the experimental CHFs. Accordingly, a possible mechanism of CHF enhancement was described.
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| 3. |
- Ji, Xinyu, et al.
(author)
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Jet array impingement boiling in compact space for high heat flux cooling
- 2023
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In: Applied Thermal Engineering. - : Elsevier BV. - 1359-4311. ; 219
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Journal article (peer-reviewed)abstract
- To achieve high heat flux cooling, a distributed confined jet array impingement boiling device was designed and tested by using HFE-7100 as working fluid. The experimental study on the heat transfer characteristics was conducted on smooth silicon surface and micro-pin-finned surfaces with mass flux ranging from 760 ∼ 3040 kg/m2·s under atmospheric pressure and an inlet subcooling of 40 K. The results indicated that with the increase of the jet velocity, nucleate boiling was suppressed, and the forced convection heat transfer was enhanced. The heat transfer was greatly intensified on micro-pin-finned surfaces with a maximum increase of the heat transfer coefficient of 220 % due to the increase in specific surface area and the number of nucleation sites. Moreover, the critical heat flux (CHF) can reach 280 W/cm2. The mechanism of CHF improvement was analyzed. The two-phase flow structure within the confinement space and capillary wicking effect of the micro-pin-finned surface are superimposed, resulting in two distinct CHF mechanisms. A new correlation with a mean absolute error of 3.5 % for predicting the heat transfer coefficient of the jet impingement boiling was proposed by considering the effect of micro-pin–fin structure on heat transfer.
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| 4. |
- Ji, Xinyu, et al.
(author)
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Two-phase flow characteristics and visualization of distributed confined array jet boiling
- 2024
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In: Case Studies in Thermal Engineering. - 2214-157X. ; 57
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Journal article (peer-reviewed)abstract
- Confined array jet boiling can achieve high heat flux in a compact space and its flow resistance characteristics are critical to the design of cooling systems. The boiling images of distributed confined jet of HFE-7100 is recorded by a high-speed camera in this study. The effects of jet mass flux, jet height and surface structure of two-phase jet flow characteristics on micro-pin-finned surfaces are studied. A benefit from the distributed configuration of the jet array, in contrast to previous studies, is that the jet boiling pressure drop is independent of the heat flux, but only related to the jet mass flux. The effect of the surface structure on the pressure drop is negligible. Jet flow instability will be triggered by intermittently blockage of the jet inlet and outlet by large vapor masses in certain heated surfaces. Reducing the jet height can suppress two-phase flow instabilities while keeping the pressure drop almost constant and the CHF to slightly increase. The COP of distributed jet impingement boiling cooler proposed in this work can be up to 6 times higher than that of the conventional jet boiling cooler, and more than 2 times higher that of the microchannel heat sink.
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| 5. |
- Liu, Bin, et al.
(author)
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Pool boiling heat transfer of N-pentane on micro/nanostructured surfaces
- 2018
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In: International Journal of Thermal Sciences. - : Elsevier BV. - 1290-0729. ; 130, s. 386-394
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Journal article (peer-reviewed)abstract
- In the present study, one type of uniformly nanostructured surface (NPDS) was modified by electrophoretic deposition. Two kinds of micro/nanostructured surfaces (FLS1 and FLS2) were fabricated on copper surfaces by femtosecond laser processing. The micro/nanostructured surfaces were further modified by electrophoretic deposition. Afterwards, composite micro/nanostructured surfaces (CS1 and CS2) were developed. Saturated pool boiling heat transfer of the modified surfaces was investigated experimentally. An organic fluid, n-pentane was chosen as the working liquid. Heat transfer coefficient and critical heat flux (CHF) of smooth and micro/nanostructured surfaces were studied. The results showed that the heat transfer coefficient (HTC) of all structured surfaces increased obviously with a notable decrease of wall superheat at CHF compared to the smooth surface, which was attributed to increments in nucleation site density and heat transfer area. The CHF of femtosecond laser processed surfaces was also increased compared with the smooth surface due to a much higher liquid spreading ability, while a uniformly nanostructured surface has no augmentation in CHF. Composite micro/nanostructured surfaces show the best heat transfer performance among all tested surfaces, and the critical heat flux and heat transfer coefficient were increased by more than 60% and 300% over the smooth surface, respectively. The liquid spreading ability of n-pentane on the tested surfaces was measured. For the well wetting liquid, the liquid spreading ability of the heated surface, instead of the wettability, is the main factor for CHF enhancement. It is suggested that a surface with multiscale structures can be an efficient way for boiling heat transfer enhancement.
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| 6. |
- Ma, Xiang, et al.
(author)
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Flow boiling frictional pressure drop inside micro/mini-channels : A new general model and experimental investigation
- 2024
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In: Applied Thermal Engineering. - 1359-4311. ; 247
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Journal article (peer-reviewed)abstract
- In this study, a novel general model for flow boiling frictional pressure drop inside micro/mini-channels was proposed based on theoretical analysis and experimental evaluation. Experiments were conducted to obtained flow boiling pressure drop of deionized water, HFE7100 and R134a in micro-channels under various experimental conditions. Then, a wide database from 33 previous literatures consists 3854 experimental data points covering 11 different working fluids, e.g., carbon dioxide, new electronic fluorinated solutions, refrigerants and deionized water, among others, and the operation conditions were as following: system temperature of −40–90 ℃, saturated pressure of 101–3970 kPa, hydraulic diameter of 0.1–2.6 mm, liquid subcooling of 5–75 K, mass flux of 50–3000 kg/(m2·s), heat flux of 0–4000 kW/m2, liquid-only Reynolds number of 40–12,000, vapor quality of 0–1, and reduced pressure of 0.0045–0.5380 in the database. Both the Reynolds numbers of vapor and liquid were calculated using the hydraulic diameter and vapor quality. The present data points were evaluated by 20 existing classical models (including the homogeneous and separated flow ones) for the flow boiling frictional pressure drop. However, the predictions of these models for the present data points had low accuracy, especially for the subcooled points at low vapor quality. Therefore, a more accurate prediction model was developed based on the present database by distinguishing the subcooled and saturated boiling. This novel prediction model can predict 75.4 % and 89.7 % of data points within ±30 % and ±50 % error bands, its mean absolute percent error (MAPE) is 19.23 %, which shows good predictive ability. Besides, the reliability of the new model was also further verified with our experimental results.
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| 7. |
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| 8. |
- Wang, Xueli, et al.
(author)
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Correlations for prediction of the bubble departure radius on smooth flat surface during nucleate pool boiling
- 2019
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In: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 132, s. 699-714
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Journal article (peer-reviewed)abstract
- Based on a modified force balance model, new correlations were proposed for the prediction of vapor bubble departure radius in saturated and subcooled pool boiling under atmospheric pressure. To predict the departure radius, the wall temperature and contact angle are two important input parameters. Instead of the static contact angle, the present correlations use the dynamic advancing contact angle at root of the bubble base at the moment before bubble detachment (i.e., the maximum dynamic advancing contact angle) to calculate the bubble departure radius. The results show that for the bubble departure radius obtained in this study, the developed correlation can predict all the data points within a maximum error of 3.8% in both normal earth gravity and 0.01ge reduced gravity. Moreover, for data sets in the literature including 1g saturated boiling, 1g subcooled boiling, saturated boiling in reduced gravity, and subcooled boiling in reduced gravity, it is also demonstrated that compared with the thirteen existing correlations, the proposed correlations exhibit a big improvement in predicting bubble departure radius.
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| 9. |
- Wang, Xueli, et al.
(author)
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Enhancement of loop heat pipe performance with the application of micro/nano hybrid structures
- 2018
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In: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 127, s. 1248-1263
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Journal article (peer-reviewed)abstract
- To further improve the flat-type loop heat pipe (LHP) performance, this study evaluates the practical potential of use of highly enhanced boiling structures. It is found that in our proposed new heat pipe (NHP) system, the working fluid from the evaporator outlet to the condenser inlet is in a liquid–vapor two phase flow, which is different from the classical LHP theory. A new P-T diagram is developed to better understand the thermal and hydraulic process during the NHP steady operation. In this study, by using the laser ablation technique two different types of micro- and nanoscale hybrid structures are synthesized on the boiling pool substrate. It is indicated that the formed valleys with a larger opening width play an important role in more effectively improving the bubble nucleation and bubble growth at the micrometer sites, which can subsequently lead to an increased number of active nucleation sites. The best loop performance is obtained with the micro-cone structured substrate at a heat load of 140 W, at which the maximum boiling pool heat transfer coefficient of 42.17 kW/m2·K is achieved. Compared with the polishing Cu substrate, it is enhanced by 110%. When maintaining the boiling pool temperature lower than 85 °C, the proposed new heat pipe system can tolerate a maximum heat flux of 35.12 W/cm2, which is larger than that of the most conventional LHPs with methanol as the working fluid.
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| 10. |
- Wang, Xueli, et al.
(author)
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Investigation of bubble departure radius in subcooled pool boiling under microgravity condition
- 2018
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In: Heat Transfer and Thermal Engineering. - 9780791852118 ; 8A-2018
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Conference paper (peer-reviewed)abstract
- The bubble departure radius is a very important parameter for bubble dynamics during boiling heat transfer. In this study, experiments of highly subcooled nucleate pool boiling of FC-72 were conducted on two different sized silicon chips (chip S 2×2 and chip S 1×1) in short-term microgravity and normal gravity conditions by utilizing the drop tower in Beijing. During the experimental study, bubble dynamics were captured by a high-speed digital camera. From the images at the bubble departure moment, the bubble departure radius was obtained. Although the traditional force balance model is modified through the addition of a Marangoni force, it still cannot precisely predict the bubble departure radius in the microgravity condition, especially in the low heat flux regime. By using the advancing contact angle measured from the bubble departure moment instead of the static contact angle, and considering the bubble asymmetry due to the small bubble coalescence and the surrounding liquid motion, a revised force balance model is proposed. It can predict the experimental bubble departure radius within a deviation of ±3.8% for both silicon chips in the whole heat flux range.
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| 11. |
- Zhang, Yonghai, et al.
(author)
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Heat transfer correlations for jet impingement boiling over micro-pin-finned surface
- 2018
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In: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 126, s. 401-413
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Journal article (peer-reviewed)abstract
- Heat transfer performance of submerged jet impingement boiling over staggered micro-pin-finned surfaces was investigated using air-dissolved FC-72. The dimension of the silicon chips is 10 × 10 × 0.5 mm3 (length × width × thickness) on staggered micro-pin-fins with four dimensions of 30 × 30 × 60 μm3, 50 × 50 × 60 μm3, 30 × 30 × 120 μm3 and 50 × 50 × 120 μm3 (width × thickness × height, named S-PF30-60, S-PF50-60, S-PF30-120, and S-PF50-120) were fabricated by using the dry etching technique. The effects of micro-pin-fins, jet-to-target distance (H = 3, 6, and 9 mm), and jet Reynolds number (Re = 2853, 5707, and 8560) on jet impingement boiling heat transfer performance were explored. For comparison, experiments with jet impinging on a smooth surface were also conducted. The results showed that all micro-pin-finned surfaces show better heat transfer performance than that of a smooth surface. The largest Nusselt number is 1367, corresponding to a heat transfer coefficient of 26387 W·m−2·K−1 with S-PF30-120 at Re = 8560, H/d = 2, and q = 151 W·cm−2, which is approximately twice the largest Nusselt number of Chip S. In the single-phase heat-transfer-dominant region, the Nusselt number (Nu) is mainly influenced by several dimensionless numbers, including Reynolds number (Re), boiling number (Bo), the ratio of jet-to-target distance to jet diameter (H/d), the ratio of micro-pin-finned surface area to smooth surface area A/AS, and a dimensionless number corresponding to flow resistance Dh/Lh. Correlations to predict Nu in both single-phase heat-transfer-dominant region and two-phase heat-transfer-dominant region for smooth and micro-pin-finned surfaces were proposed. The results show that most data (96%) in the single-phase heat-transfer-dominant region and most data (96%) in the two-phase heat-transfer-dominant region were predicted within ±13% and ±15%, respectively. In addition, CHF correlations for smooth and micro-pin-finned surfaces were also proposed, and most data (95%) are predicted within ±20% for a smooth surface and all the data within ±5% for the micro-pin-finned surfaces.
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