Publications by Mohammad Alwazzan
Thin liquid film evaporation Convective heat and mass transfer Dry cooling condensers a b s t r a... more Thin liquid film evaporation Convective heat and mass transfer Dry cooling condensers a b s t r a c t Low heat transfer coefficient (HTC) in air/fin-side is the bottleneck of dry cooling strategies for thermal power plants. Inspired by the phase change heat transfer during the perspiration of mammals, a sweating-boosted air cooling strategy with on-demand water dripping is proposed. The testing samples are featured with macroscale grooves for global liquid delivery, and with nanoscale hydrophilic copper oxide (CuO) wick structures for local liquid spreading. The experiments of sweating-boosted air cooling are conducted in a wind tunnel system. There are three wetting conditions with increasing dripping rates: dry, partially wetted, and flooded conditions. In the partially wetted conditions, the surface temperatures reduce and HTCs increase with increasing dripping rates. For a given dripping rate of water, HTCs are enhanced and surface temperatures are reduced with increasing air velocities. High air velocity and low surface temperature have a trade-off effect on the evaporation process. This effect results in almost constant saturated dripping rates for a given thermal load. A linear relationship between the saturated dripping rates and the thermal loads confirms that the evaporation dominates the heat transfer process of sweating-boosted air cooling. Complete surface wetting is obtained on the designed surfaces, but no obvious effect of groove width on HTCs is observed. Sweating-boosted air cooling can significantly increase air-fin side HTC in air cooled condenser (ACC), and dramatically reduce the water consumption compared to current water evaporative condenser (WEC). This research provides a fundamental understanding on the sweating-boosted effects on the air cooling.
Condensation heat transfer performance can be significantly enhanced by patterning the condenser ... more Condensation heat transfer performance can be significantly enhanced by patterning the condenser surface with different wettability regions as shown by numerous studies, including part I of this study. In part I of this study, some patterned surfaces with alternative parallel straight stripes consist of hydropho-bic (b) and less-hydrophobic (a) regions at different ratios exhibited higher heat transfer rate than others. In this Part II of the study, our objective is to analyse the droplet dynamics during water vapor condensation on hybrid-wettability patterned horizontal tube surfaces under saturation conditions near the atmospheric pressure. Three major outlines were found in the course of the droplets dynamic investigation. First, the existence of an optimum (b/a) ratio that maximized the condensation heat transfer rate, as demonstrated in part I of a sample carrying b and a-regions widths of 0.6 mm and 0.3 mm, respectively is justified. This is because the optimum ratio exhibits the maximum droplet departure frequency and the minimum droplet area coverage rate among other samples. Second, the reduction in the heat transfer rate resulting from any deviation from the optimum ratio is also identified. We observed that by increasing the a-regions width on the hybrid patterned surface, the condensation was dominated by the filmwise mode, thus reducing the condensation rate. In contrast, decreasing the width of a-regions less than the optimum ratio was found to be unfavourable due to the increase in the bridging droplets observed and discussed herein. Lastly, the undesirable observed bridging phenomenon found to occur on all tested hybrid patterned surfaces, can significantly influence the condensation heat transfer performance. A bridging droplet can be referred to a droplet joined (bridged) by two, three, or four neighboring a-stripes. Increasing these unwanted droplets formation frequency can induce additional thermal resistance which can reduce the condensation rate. The most dominant and frequent bridging droplet type observed herein was found to be for droplets that were bridged by two a-regions, followed by those between three and four a-regions. A quantitative method (i.e. Bridging coverage area rate) was adapted herein to quantify the influence of the velocity, frequency, and size of the three types of bridging droplets on the condensation rate of the hybrid patterned surfaces.
Condensation heat transfer performance can be improved by increasing the condensate removal rate.... more Condensation heat transfer performance can be improved by increasing the condensate removal rate. Commonly, this can be achieved by promoting dropwise condensation mode in which super/hydrophobic coatings applied on the entire condenser surface. Herein, alternative mini-scale straight patterns consisted of hydrophobic (b) and less-hydrophobic (a) regions were formed on the condenser tubes. The existence of the two adjacent regions generates wettability gradient which can mitigate condensate and increase its removal rates. A parametric study was conducted to experimentally determine the influence of (b/a) ratios on the heat transfer performance and droplet dynamic under saturation condition near the atmosphere pressure with the presence of non-condensable gases (air). The results reveal that all patterned surfaces exhibited a drastic enhancement in terms of condensation heat transfer coefficient and heat flux compared to those of filmwise condensation. More interestingly, some (b/a) ratios significantly outperformed a surface with a complete dropwise condensation. In addition, an optimum (b/a) ratio of (2/1) exists with b and a-regions widths of 0.6 mm and 0.3 mm, respectively. The heat transfer coefficient of the optimum ratio is peaked at a value of 85 kW/m 2 K at a subcooling of 9 °C, which is 4.8 and 1.8 times that of a complete filmwise and dropwise condensation, respectively. Our study also reveals that the b-regions served mainly as droplet nucleation sites with rapid droplets mobility; whereas the a-regions promoted droplet removal from the neighboring b-regions, and served as drainage paths where condensate can be drained quickly under gravitational force. Furthermore, the existence of both a and b-regions on the condensing surface controls the droplets maximum diameters of the growing dro-plets on the b-regions. The maximum diameter is approximately 0.56 ± 3% mm, which is 26% the size of the droplets maximum diameter on a full b-region surface. In summary, this wettability-driven mechanism allows droplets to be removed from the condensing surface at higher rates, leading to a substantial enhancement in the condensation heat transfer coefficient.
Improving mixing is an effective method to enhance flow boiling in microchannels. However, it is ... more Improving mixing is an effective method to enhance flow boiling in microchannels. However, it is challenging to induce since the flow in microchannels is laminar under typical operating conditions. We report that flow boiling of 1methoxyheptafluoropropane (HFE 7000) in a parallel microchannel array was significantly enhanced by chaotic mixers patterned on the bottom walls. The microchannel array consists of five parallel channels (height, width, length: 250 µm x 220 µm x 10 mm). The chaotic mixers consist of seven cycles with 12 staggered herringbone grooves (50 µm depth and width with 90˚ between two asymmetric arms) in each cycle. Its asymmetry is defined by the off center position of the apex of the herringbone groove. Compared with a smooth-wall microchannel array with identical channel dimensions, heat transfer coefficient and critical heat flux of flow boiling on HFE 7000 were enhanced up to 45 % and 61 % using chaotic mixer pairs in microchannels. Mass fluxes range from 1000 to 2200 kg/m 2 -s and wall heat fluxes from 10 to 198 W/cm 2 .
Improving mixing is an effective means to enhance single-and two-phase heat transfer in microchan... more Improving mixing is an effective means to enhance single-and two-phase heat transfer in microchannels. However, it is challenging to induce since the flow in microchannels is laminar in the most working conditions. We report that heat transfer rate and critical heat flux (CHF) on 1-methoxyheptafluoropropane (HFE-7000) can be significantly enhanced by patterning embedded micromixers on the bottom walls in a parallel silicon microchannel array, which consists of five parallel channels (height, width, length: 250 µm × 220 µm × 10 mm). Compared with a plain-wall microchannel array at a mass flux range of 1018 to 2206 kg/m 2 · s and a heat flux range of 10 to 198 W/cm 2 , singlephase heat transfer rate, two-phase heat transfer rate, and CHF are enhanced up to 221%, 160%, and 61% using microscale staggered herringbone mixers in microchannels, respectively. These mixers consist of 7 or 3.5 Hz with 12 staggered herringbone grooves (50 µm in depth and width) with 90°between two asymmetric arms in each cycle. Its asymmetry is defined in accordance with the off center position of the apex of the herringbone groove. Finally, experimental results suggest that the locations and coverage of the micromixers have significant impacts on both single and two-phase heat transfer in microchannels. Index Terms-Electronics cooling, dielectric fluid, micromixer, microchannel, flow boiling.
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Publications by Mohammad Alwazzan