•Sinusoidal wavy plate fins with phase shift of 0°, 90° and 180° are investigated.•Thermal performance of SW-PFHS and CCSW-PFHS are compared.•The phase shift has significant effect on the thermal ...performance.•The proper working conditions of sinusoidal wavy fins heat sinks are reported.
This article presents new experimental data on the influence of phase shift, air velocity, heat sink base surface temperature on heat transfer coefficient, and pressure drop of airflow through sinusoidal wavy plate fin heat sinks (SW-PFHS) and crosscut sinusoidal wavy plate fin heat sinks (CCSW-PFHS). Sinusoidal wavy plate fins with a wave length of 18.68 mm, amplitude of 2.0 mm, and phase shift of 0°, 90°, and 180° are used. For CCSW-PFHS, the sinusoidal wavy plate fin is cut transversely at crests and troughs with a 2 mm length. The test runs are performed at an air velocity ranging between 1 and 5 m/s and a heat sink base surface temperature of 70 °C, 90 °C, and 110 °C. The results show that the higher phase shift and air velocity lead to the enhancement of the heat transfer coefficient and pressure drop. Conversely, the heat sink base surface temperature has a slight effect on the heat transfer coefficient and pressure drop. The heat transfer coefficient of SW-PFHS is enhanced when increasing the phase shift compared with a phase shift of 0°. Under the same phase shift, the Nusselt number of CCSW-PFHS is higher than that of SW-PFHS by about 5.9–19.1%. The CCSW-PFHS with a phase shift of 180° yields the highest TPF.
•A state of art of boiling heat transfer on micro/nano structured surface was introduced.•The technique of micro/nano structures fabrication was introduced.•The enhanced boiling heat transfer was ...reviewed by the physical mechanism.•The enhanced critical heat flux was reviewed by the physical mechanism and model.•The further research and new insight was proposed.
In the recent decades, the rapid growth of surface modification and fabrication technologies has facilitated the achievement of boiling heat transfer enhancement on micro/nanostructured surfaces. In this paper, several researches on the micro/nanostructured surfaces that have been designed to enhance boiling heat transfer are introduced and closely reviewed. Firstly, theoretical and experimental researches on nucleate boiling heat transfer (NBHT) and critical heat flux (CHF) are introduced in the outline. The fabrication techniques for achieving these engineered surfaces, which are technically classified into machining, coating, chemical process, and micro/nanoelectromechanical systems, are described in detail in the paper. Explanations and analysis of the results of boiling heat transfer enhancement tests are presented in view of NBHT and CHF. Finally, the special features of the existing surfaces capable of enhancing boiling heat transfer are summarized, and the need for future research is also presented.
•The Venturi nozzle design with entry and exit angles, which leads the self-sucked air flow.•Microbubbles generation regarding to entry and exit angles.•High speed visualization study to track the ...self-sucked air flow stream broken into microbubbles.
We studied the effect of varying the entry and exit angles of Venturi nozzles on the formation of microbubbles in Venturi nozzle-type microbubble generators. We 3D-printed nozzles with five entry angles (15, 22, 30, 38 and 45°) and five exit angles (15, 22, 30, 38 and 45°). For the visualization experiment, we inserted the nozzles into a cover case made of aluminum and transparent acrylic. We measured the pressure drop and the air flow rate with respect to the entry and exit angles, determined the diameters of the bubbles using a digital camera, and analyzed bubble breakage by observing the behavior of the bubbles using a high-speed camera. We confirmed that the exit angle (not the entry angle) is dependent on the pressure drop and found that the air flow rate did not vary linearly with the fluid flow rate, as expected according to Bernoulli's theorem. Instead, it tended to remain constant or decrease as the fluid flow rate increased due to the abnormal flow. The sizes of the bubbles decreased as the exit angle increased, except in cases where the outlet angle was greater than 30° at high flow rates (260–300 LPM). We observed a change in bubble size with respect to exit angle. According to our visualization, the bubbles were broken by the flow separation at the beginning of the divergence at the exit.
•Air natural convection in vertical parallel plates is investigated experimentally for wall temperature conditions up to 500 °C.•When both side walls are heated, the reverse flow does not occur at ...the outlet.•Flow behavior is considered by the thermal behaviors based on the measured wall and air temperatures.•Flow pattern in the channel would be determined by the geometry rather than thermal boundary condition.•Two physical correlations related to the heat transfer and induced flow rate are presented
In this study, we investigated the turbulent air natural convection phenomena between open-ended vertical parallel plates under asymmetric heating conditions. The rectangular air channel consisted of two insulating walls and two heated walls: one of the vertical walls is heated to maintain uniform wall temperature conditions at various temperatures from 50 to 500 °C, and the other wall is indirectly heated by thermal radiation from the directly heated wall. We measured both wall temperature distributions in the vertical direction and the air temperature profiles with the horizontal direction at certain heights.
The results showed that when both walls are heated asymmetrically, reverse flow was absent at the outlet of the channel, as confirmed by a simple visualization method. In addition, we confirmed the practical result of a deviation between the mean air temperature and the measured air temperature at the center of the channel. From the results of the wall temperature distributions and the air temperature profiles, we suggest a reasonable flow behavior, which is the generation of two different recirculation flow patterns that are not affected by changes in the thermal boundary. In addition, we compared the ratio between the convective and radiative heat rates on the directly heated wall. Based on the experimental results, we analyzed the natural convection heat transfer between vertical parallel plates and we proposed two correlations. The heat transfer correlation agreed well with the correlation for turbulent natural convection on a single infinite vertical plate. The correlating equation related to the induced flow rate was presented as a modified Rayleigh number to estimate the flow rate.
In this study, the condensation heat transfer on a vertical tube in a steam-air‑helium mixture was investigated. The effect of helium on condensation heat transfer was analyzed using a heat and mass ...transfer analogy model and a new effective diffusivity which was proposed based on the Maxwell-Stefan diffusion model. The condensation heat transfer was increased at a low helium content and decreased at a high helium content, which could be explained by the competition induced by the increasing diffusivity due to helium and reduction in density difference between the bulk conditions and wall. In addition, the vapor content and wall subcooling influenced the effect of helium. An experimental investigation was conducted to analyze the helium effect on condensation heat transfer. The helium content was measured using a gas analyzer, while the vapor content was obtained under the assumption that the gas was saturated. The results of the experiment were consistent with the theoretical analyses, although the helium effect at a low helium content was stronger than that predicted by the theoretical model. An improved helium effect factor was proposed after fitting the theoretical model to the experimental data, and the model predicted the effect of helium reasonably well.
•The stainless steel modified surface was fabricated for chevron plate heat exchanger.•The anti-fouling performance would be estimated on bare, modified, PMMA and BN coated.•The dynamic wetting ...characteristics could be possible to explain the anti-fouling performance.
In this study, we aimed to develop a method for modifying SUS304 with micro/nanoscale holes using an electrical etching technique, and evaluated its impact on the anti-fouling performance of chevron plate heat exchangers. The modified SUS304 specimen exhibited hydrophobic wetting characteristics; however, water droplets stuck to the wall under inclined orientations at angles of 30° and 60°. We coated the modified SUS304 specimen with polymer (polymethyl methacrylate; PMMA) and hexagonal boron nitride (BN) particles to realize the hydrophobic and superhydrophobic wetting characteristics. To evaluate the anti-fouling performance of the heat exchanger, we carried out fouling acceleration experiments in a 2000 ppm CaCO3 solution for 2 h. The anti-fouling performance was determined by measuring the overall heat transfer coefficients during fouling acceleration experiments. As the operation time increased, the CaCO3 particles were deposited on the surface of the heat exchanger, the overall heat transfer coefficient decreased, and the fouling factor ultimately increased. Out of bare, modified, PMMA- and BN-coated heat exchangers, the PMMA-coated heat exchanger exhibited the strongest anti-fouling performance, with the lowest increment in the fouling factor. Based on analysis of the contact angle hysteresis (CAH) in each case, we suggest that CaCO3 particles in solution would be deposited better in the case of the highest CAH. This is possible in cases without the triple contact line, and is caused by filling the polymer into micro/nano holes on the modified surface.
As novel technologies have been developed, emissions of gases of volatile organic compounds (VOCs) have increased. These affect human health and are destructive to the environment, contributing to ...global warming. Hence, regulations on the use of volatile organic compounds have been strengthened. Therefore, powerful adsorbents are required for volatile organic compounds gases. In this study, we used graphene powder with a mesoporous structure to adsorb aromatic compounds such as toluene and xylene at various concentrations (30, 50, 100 ppm). The configuration and chemical composition of the adsorbents were characterized using scanning electron microscopy (SEM), N
adsorption-desorption isotherm measurements, and X-ray photoelectron spectroscopy (XPS). The adsorption test was carried out using a polypropylene filter, which contained the adsorbents (0.25 g), with analysis performed using a gas detector. Compared to graphite oxide (GO) powder, the specific surface area of thermally expanded graphene powder (TEGP800) increased significantly, to 542 m
g
, and its chemical properties transformed from polar to non-polar. Thermally expanded graphene powder exhibits high adsorption efficiency for toluene (92.7-98.3%) and xylene (96.7-98%) and its reusability is remarkable, being at least 91%.
•Air natural convection in vertical parallel plates is investigated based on the previously reported experimental results with controlling the damper installed at the end of the channel.•The form ...loss coefficient of the damper estimated in the natural convection has little difference from the coefficient estimated in the forced convection.•In a lab-scale experiment, the thermal stratification of the ambient air is normally positive, and it reduces the flow rate and heat transfer performance.•When the film temperature is higher than 100 °C, the Rayleigh number trend changes from increase to decrease as the wall temperature increases in air.•In the natural convection condition with controlling damper, it cannot apply conventional heat transfer correlation, which consists of the Rayleigh number, Prandtl number, and Nusselt number.
The natural convection of air between vertical parallel plates under asymmetrical heating was investigated focusing on the pressure behavior and the heat transfer performance as varying two experimentally controlled parameters, namely, the damper closing angle (0 ∼ 45 °) and the thermal operating condition (1.7 ∼ 6.4 kW). By varying the damper closing angle, the form loss coefficient of the damper was estimated from the measured pressure and the calculated pressure difference relating to the thermal gravity effect. It was verified that the form loss coefficient estimated for natural convection was close to the reported form loss coefficient estimated for forced flow. The thermal stratification of the ambient air was additionally considered in the pressure analysis. In addition, the heat transfer was analyzed in terms of the heat transfer correlation. As the damper angle increased, the heat transfer performance decreased owing the reduced flow rate. However, as the thermal operating condition intensified, the heat transfer performance was increased by the improved flow rate. Interestingly, the trend of the Rayleigh number was the same regardless of the two control parameters, even though an intensifying thermal boundary condition improved the heat transfer performance and an increasing damper closing angle weakened the heat transfer performance, conflictingly. Therefore, the correlation conventionally used for natural convection cannot be applied under the conditions of a high wall temperature and control using a damper because it does not consider the flow rate effect. To consider the flow rate effect additionally in the heat transfer correlation, a correlation based on Jackson's mixed convection correlation is suggested. The suggested correlation predicts the heat transfer performance on each wall by considering the effects of the damper angle and operating condition, simultaneously, within the error of less than 30%.
In this study, the effects of wall subcooling on the condensation heat transfer in steam-air mixture on a vertical tube was investigated. The effect of wall subcooling on condensation heat transfer ...was analyzed experimentally and using heat and mass transfer model. The experimental analysis indicated that wall subcooling effect varied depending on the degree of subcooling, i.e., the decrement of condensation heat transfer became less in low subcooling. In addition, the noncondensable gas fraction affected the wall subcooling effect. Such phenomenon was also found in the analysis using heat and mass transfer model. Through analysis using heat and mass transfer model, it was found that the accumulation of noncondensable gas and steam diffusion have competing effects. The former deteriorates and the latter enhances the condensation heat transfer with increasing wall subcooling. In low subcooling condition, both effects have higher rate of change, resulting in the different subcooling effect with that of high subcooling condition. We then developed an empirical correlation of condensation heat transfer over a wide range of wall subcooling; this predicted the experimental results reasonably well, particularly at low subcooling.
•Air natural convection in vertical parallel plates is investigated experimentally with controlling the damper installed at the end of the channel.•Two different viewpoints of the transition ...phenomena in the natural convection are suggested based on the flow regimen change and flow pattern change.•The change in the wall temperature distribution cannot be the basis to judge the flow regime change.•In the entrance region, abnormal flow, such as recirculation flow and reverse flow, can appear below the middle height of the channel.•In the fully developed region, the abnormal flow disappears, and the heat transfer performance can increase by the well-developed upward flow.
It was investigated the phenomenon of the air natural convection between vertical parallel plates with a damper installed at the end of the channel under asymmetrical heating conditions. The thermal behavior in the channel analyzed based on the vertically measured wall temperature distributions and bulk temperature distribution and the horizontal profiles of the air temperature measured at four heights. As the operating condition intensified, there was no change in either wall temperature distribution trend whereas the bulk temperature distribution trend changed only near the inlet through a change in the flow regime. This change in the flow regime affected the bulk temperature profile only near the wall. However, the overall temperature distributions and trends of the temperature profiles did not change drastically except near the inlet and they were similar to each other regardless of the operating condition and damper closing angle. On the basis of these results, it was focused on the transition phenomenon of natural convection between vertical parallel plates. It was considered the transition phenomenon from two viewpoints, that is, a change in flow regime and a change in flow pattern. In terms of a change in the flow regime, a distinguishable difference in the overall heat transfer performance was confirmed between laminar and turbulent regimes, but it does not affect the trends of the wall temperature distributions. Regarding the flow pattern change, criteria were suggested for distinguishing flow patterns between the entrance region and fully developed region at the mid-height of the channel according to the vertical temperature distributions and horizontal air temperature profiles. To verify the validity of the proposed concept, we made comparisons with the results of previous research and confirmed that our concept can be well applied.
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