This document presents a simulation-based study to understand flow boiling of water at low mass flux in a high aspect ratio microchannel subjected to constant wall temperature. In addition, numerical investigations are done to understand the influence of mass flux, at low mass fluxes, on flow boiling in high aspect ratio microchannels subjected to constant wall temperature. The dimensions of the microchannel under consideration are 1 mm (width) and 70 mm (length) while the mass flux is 30 kg/m2 s. Studies help identify the flow regimes associated with flow boiling in high aspect ratio microchannel as well as the associated flow boiling parameters such as vapor fraction, vapor quality, temperature, heat flux, heat transfer coefficient, velocity, and pressure. The flow regimes are bubbly, bubble-slug, slug and semi-annular. The findings indicate that bubbles form and coalesce slowly, resulting in the production of low-quality vapors in bubbly flow. As the boiling progresses, this flow transforms into slug flow to annular flow characterized by higher vapor quality with reduction in heat transfer rate, drop in pressure and increase in velocity along the length of the microchannel. The increase in mass flux, from 30 to 90 kg/m2s, delayed the occurrence of flow regimes as well as significantly affected the flow boiling parameters.

This document presents a heat sink having microgap integrating ridges as the flow passage for purposes of thermal management of microelectronics. Studies are carried out for Reynolds number ranging from 250 to 1600. Increase in Reynolds number, irrespective of the dimensions of the heat sink, is observed to decrease and increase the thermal resistance and pressure drop, respectively. Regarding the influence of the height of the ridges, there exists a threshold for the same for influencing the thermal resistance of the heat sink. When the height of the ridges is increased by 3 times, the thermal resistance at the largest height is reduced by as much as 70 % relative to the thermal resistance at the smallest height; the pressure drop increased by as much as 3 times with increase in the height of ridges. When the number of ridges is increased by 16 times, the thermal resistance for the largest number of ridges is 75 % lower than that for the smallest number of ridges; the pressure drop increased by as much as 2 times for the same increase in the number of ridges. Regarding the length of the ridges, it must be below a threshold for positively influencing the thermal resistance of the heat sink. With increase in the length of the ridges by 4 times, the thermal resistance at the largest length is observed to be as much as 85 % lower than that at the smallest length and over the same increase in the length of the ridges, the pressure drop increased by 1.14 times. Increase in the height of the microgap from 150 μm to 250 μm reduced the thermal resistance of the heat sink by as much as 45 %; the pressure drop increased by as much as 7 times for this change in height of the microgap. The Nusselt number and friction factor associated with different designs of the microgap integrating ridges that are considered as part of this study are provided in this document.

In this study, experimental investigation is carried out to examine flow boiling heat transfer and hydrodynamic instability in boiler employing structured and unstructured minichannels for deionized water. The study compares boilers with unstructured surface and structured surface, consisting of circular pin fins, to assess their thermal and hydrodynamic performance. This study evaluates both local and average heat transfer coefficients, along with pressure drops, across periments were conducted in lab-scale boilers consisting of a rectangular aluminum minichannel with a hydraulic diameter of 2.85 mm, dimensions of 270 mm in length, 30 mm in width and 1.5 mm in height. An inline circular pin fin configuration is used for the setup. The results demonstrate that the circular pin fin surface significantly enhances the boiling heat transfer coefficient, achieving about 70% higher than that on unstructured surface. At a mass flux of 50 kg/m2 center dot s, the local heat transfer coefficient reaches 78.19 kW/m2 center dot K for the structured surface compared to just 9.77 kW/m2 center dot K for the unstructured surface. In addition, on contrary to the unstructured surface, the structured surface exhibits a more stable and longer homogeneous nucleation regime at lower mass flux, which promotes uniform bubble formation and delay in transition to the slug flow regime. Circular pin fins surface also showed more steady pressure fluctuations, especially close to dry-out phase.

Thermal control is essential for spacecraft operation, ensuring functionality in extreme space environments. This study presents a comprehensive bibliometric analysis of global research on spacecraft thermal control from 2010 to 2025. A total of 293 documents were retrieved from the Scopus database with an emphasis on passive thermal control technologies. A detailed examination of publication trends, key contributors, influential authors, keyword dynamics, and highly cited documents was presented. The findings showed a fluctuation in publications, peaking in 2022, with the United States, China, and Canada leading in citations. Conference papers accounted for 57% of the publications, highlighting the accelerated pace of knowledge in this field. Acta Astronautica has received 473 citations, whereas Solar Energy Materials and Solar Cells have 367 citations. Notably, advanced radiator technologies, especially adaptive radiator solutions, dominate highly cited documents, indicating a shift toward innovative thermal management approaches. This trend highlights a growing focus on thermal control solutions for limited radiator space and extreme temperature fluctuations, enabling longer, more complex missions. This study provides foundational insights into research trajectories and emerging priorities in passive spacecraft thermal control, guiding future advancements to enhance resilience in diverse and challenging environments. Future research is expected to focus on optimizing the performance of variable-emissivity radiators and validating these technologies through flight demonstrations to gain heritage and increase their technology readiness level.

This study investigates the use of ultrasonic atomization in the humification and dehumidification (HDH) desalination system to capture thermal energy directly from solar radiation at low energy consumption. This use of ultrasonic atomization within the HDH desalination system aims to increase evaporation rate, solar energy capturing and freshwater production yield. The desalination setup consists of solar humidifier, ultrasonic atomizer, dehumidifier, fresh water and sea water tanks, and auxiliary components. The atomized water droplets in the solar humidifier absorb directly the incident solar energy enabling direct evaporation to humidify the airvapor mixture. The humidified mixture flows to the dehumidifier which consists of foam packing insertions in a chamber enabling counter-current flow arrangement leading to condensation. The daily freshwater production is monitored for 6 h, and the maximum production yield obtained is 5.79 l/day/m2 of direct solar capturing area. The maximum GOR obtained is 0.49. The desalination system achieved a recovery ratio 68.4 %. The findings of this study suggest to use of ultrasonic atomizer in the direct solar HDH desalination systems to improve the system performance.

This study quantifies the heat transfer coefficient, pressure drop, Nusselt number and friction factor associated with flow of water in zig-zag microchannels, with square cross-section and sharp edges, when subjected to constant heat flux boundary condition on two sidewalls and bottom wall. Studies are done for Reynolds number ranging from 50 to 500. The heat transfer coefficient, pressure drop, Nusselt number, and friction factor of zig-zag microchannels are higher than that of straight microchannel of similar dimensions irrespective of the Reynolds number. The thermal parameters of the zig-zag microchannel are higher than that of the straight microchannel by ∼ 1.03, at Reynolds number of 50, and ∼ 1.57, at Reynolds number of 500. The hydraulic parameters of the zig-zag microchannel are higher than that of the straight microchannel by ∼ 1.3 and ∼ 1.49 at the lowest and highest Reynolds number, respectively. The thermal enhancement factor of the zig-zag microchannel is greater than unity irrespective of the Reynolds number and it increased with increase in Reynolds number which indicates that the benefits, i.e. enhancement of thermal parameters, outweigh the cost, i.e. increase in hydraulic parameters. The thermal enhancement factor is as high as ∼ 1.38 at the highest Reynolds number. Increase in hydraulic diameter, for a specific Reynolds number, decreases and increases heat transfer coefficient and pressure drop of the zig-zag microchannel, respectively. Increase in hydraulic diameter increases the Nusselt number and decreases friction factor for a specific Reynolds number. For a specific Reynolds number, all thermohydraulic parameters increased with increase in orientation and number of units. Irrespective of the dimensions, increase in Reynolds number leads to increase in heat transfer coefficient, pressure drop, and Nusselt number as well as decrease in friction factor. In addition, this work presents Nusselt number and friction factor correlations associated with flow of water in the zig-zag microchannel when subjected to constant heat flux on three walls; the correlations are unique to this study and beneficial for practising engineers.

By 2050, global desalination capacity is expected to reach 192 million m3/day which is twice the current capacity. This expectation is due to the increase in demand from increasing population, industrial activities and urbanization. The potential of solar energy in arid regions fosters the application of solar thermal desalination systems. The novelty of this study is highlighted by harnessing direct solar energy within the humidifier unit and utilizing forced convection to enhance evaporation. Unlike convention solar humidification and dehumidification (HDH) systems, the presented system operates capturing direct solar energy, coupled with ultrasonic atomization of droplets, and its solar still state-of-art design. The humidification process is driven via two primary mechanisms, natural convection and evaporation of atomized droplets capturing direct solar energy. The dehumidification process is achieved through direct contact condensation. This solar HDH systems operating in the closed-loop operation produces a daily freshwater yield of 10.2 L on average with 6 h of operation. The maximum GOR achieved is 0.57. Findings of this study indicate that coupling ultrasonic atomizer within a direct solar humidifier and its hybrid solar still design enhances humidification process, increases efficiency endorsing dual freshwater production channels, and implementation of this technology suitable for regions with abundant solar energy.

In this study, flow boiling heat transfer in a minichannel heat sink on smooth and pin-fins surfaces with open channel configuration was investigated experimentally. This study focuses on examining the pressure drop characteristics associated with boiling heat transfer on smooth surfaces, as well as surfaces featuring mini pinfins of square and circular geometries, heat transfer coefficient (HTC) and Nusselt number. The experiments were done over different ranges of liquid mass fluxes from 36 to 144 kg.m- 2s- 1 and a constant heat flux of 49 kW.m- 2. The minichannel has a rectangular shape with a hydraulic diameter of 2.85 mm. The experimental testing is carried out on aluminum surface in a minichannel that has a length of 270 mm, width of 30 mm, and height of 1.5 mm. The experimental results show that circular pin-fin surfaces achieved the highest HTC (8.91 kW.m-2K- 1) compared to square pin-fin (5.75 kW.m-2K- 1) and smooth surfaces (4.11 kW.m-2K- 1) at a mass flux of 144 kg/m2s. The circular pin-fin surface exhibited a higher pressure drop due to the dense fin arrangement under boiling conditions, but it significantly dampened backflow fluctuations. In contrast, the square pin-fins surface had the lowest pressure drop while smooth surface experienced the highest backflow fluctuations, reducing its stability under boiling conditions.

As the global demand for freshwater continues to increase, the critical need for efficient desalination methods has prevailed. Targeting the arid regions with high solar irradiance, solar-powered thermal desalination techniques have opportunities. This study introduces an innovative approach to improve the performance of Humidification-Dehumidification (HDH) desalination system using direct solar energy and integrating a low-energy ultrasonic atomizer device. In this system, thermal energy is supplied to the humidifier via direct solar energy capturing and the ultrasonic atomizer generates fine droplets suspended within the humidifier, where evaporation occurs. The atomized droplets are subjected to direct solar irradiance and forced convection in a closed-loop operation, facilitating efficient evaporation. Over multiple days, the freshwater production was monitored and an average daily output of 4.6 L with 6 h of operation was achieved with a given direct solar capture area of 1.54 m2. The maximum freshwater production rate is 8.92 L per day. The results reveal a notable increase in freshwater production between 11:00 and 14:30, coinciding with a significant rise in the relative humidity difference between the inlet and outlet of the humidifier. The system achieved a maximum water recovery at 14:30, highlighting its efficiency during peak operational hours. Compared to existing studies, these findings demonstrate the effectiveness of ultrasonic atomization in enhancing the humidification process, reinforcing its potential for improving solar-driven HDH desalination systems. This study proposes the potential application of ultrasonic atomization as a viable method to enhance solar desalination performance and encourages its adoption for sustainable water production in regions with abundant solar energy.

This work details a silicon-based MEMS heat sink having straight microchannels integrating rectangular sidewall cavities in in-line pattern and employing water for thermal management of microelectronic chips. Simulation-based studies are done for Reynolds number (Re) between 100 and 750 and the model is validated. The thermal resistance (R-th,R-total) and pumping power (PPf), of the proposed MEMS heat sink, are lower than that of the conventional MEMS heat sink. At the largest Re, the R-th,R-total of the proposed MEMS heat sink is only similar to 78 % of the R-th,R-total of the conventional MEMS heat sink and PPf of the former is only similar to 91 % of the latter. Moreover, the Nusselt number (Nu) and Poiseuille number (Po) of the straight microchannel integrating rectangular sidewall cavities is higher and lower than that of the straight microchannel, respectively. With the increase in Re, the Nu of the proposed MEMS heat sink in comparison with that of a conventional MEMS heat sink varied from similar to 103 % to similar to 147 % while the Po of the former in comparison with the latter varied from similar to 81 % to similar to 91 %. There exists a threshold for the length of the sidewall cavities below which the performance of the proposed MEMS heat sink is not influenced by the same. Above this threshold, increase in length decreases, for a specific Re, both R-th,R-total and PPc as well as increases and decreases the Nu and Po, respectively. Increase in Re leads to reduction of R-th,R-total and increase of Nu with decrease in the width of the sidewall cavities; both PPf and Po increase with increase in Re though the influence of width of sidewall cavities on them is negligible. The increase in the number of sidewall cavities decreases R-th,R-total and PPf while increasing and decreasing the Nu and Po, respectively. The increase in hydraulic diameter decreases both R-th,R-total and PPf while increasing both Nu and Po.

Passive enhanced heat exchangers offer the benefits of improved heat transfer and can be easily modified. Heat transfer coefficient, pressure drop, thermal performance factor, and economic analysis in annular flow heat exchangers having corrugated rods were experimentally and numerically investigated in this study. Based on the analysis conducted by thermal performance factor and an economic study, the five-start corrugated rod was found to possess both functional and economic advantages. The thermal performance factor value of the five-start rod ranged from 1.46 to 1.92, indicating its superior performance. Additionally, utilizing the five-start rod resulted in cost savings of approximately 5.95 percent.

This research evaluates heat transfer and pumping power associated with heat sinks having straight microchannels with triangular ribs on sidewalls using Ansys Workbench. The performance of the heat sinks is evaluated in terms of thermal resistance, pumping power, and a figure of merit (FOM). The heat sink with triangular ribs exhibits lower thermal resistance and higher pumping power than the straight MCHS. In addition, this study evaluates the influence of geometric parameters of the ribs on thermal resistance and pumping power. This study finds that increasing rib dimensions reduces thermal resistance by ∼ 57% while increasing pumping power; however, the FOM improves with reduction in dimensions of the ribs. Reducing the pitch of the fins reduces thermal resistance by ∼ 71% along with increase in the pumping power. Reducing microchannel spacing lowers thermal resistance by ∼68% without affecting pumping power, leading to improved FOM. The reduction in the dimensions of the microchannels improved thermal resistance by ∼ 52% while increasing pumping power; the FOM of MCHSs increased with an increase in channel dimensions.

This study investigates the design and thermal performance of straight microchannel heat sinks featuring sidewall fins, using the Fluent module in Ansys Workbench. The performance of these heat sinks is evaluated using the average Nusselt number and required pumping power. Results indicate that the incorporation of square-shaped fins enhances thermal performance but increases pressure drop. Enlarging the pin fin size significantly enhances thermal efficiency, yielding a 150

This study examines the use of interactive simulation to improve the teaching and education of students in fluid mechanics course. The improved teaching method investigates the efficacy of using interactive simulation such as computational fluid dynamics (CFD) on students’ performance in fluid mechanics course. There is an increasing interest to teach fluid mechanics with interactive simulation and using CFD method, particularly to mechanical engineering students. Among the very important advantages of using CFD in the teaching of fluid mechanics are a thorough understanding of fluid motion, fluid interaction with surfaces and its effect on pressure, and velocity distribution in the studied flow problem. It is found that students' attention and performance in fluid mechanics course is improved by employing simulation-based project in teaching. The study demonstrates an enhanced teaching and educational method in teaching fluid mechanics course.

A simulation-based study is carried out on microchannel heat sink with smooth surface and square ribs embedded surface. The heat sink is made of microchannels where each channel is 150 μm in width, and 1 cm in length. Water is used to study the heat sink. The study is conducted for a range of Reynolds numbers, from 100 to 500. The fluids' inlet and outlet temperatures, and the surface temperatures are used to calculate the thermal performance. According to the investigation, a higher Reynolds number raises the heat transfer coefficient and increasing Nusselt number. Additionally, it has been noted that raising the Reynolds number lowers the friction factor. It was evident that the square ribs microchannel had a higher heat transfer rate than the smooth channel. Additionally, it is observed that the heat sink with square ribs microchannels overall friction factor is higher than the heat sink with smooth channels. It is also found that the pressure drop increases with increasing Reynolds number.

This paper studies the use of computational fluid dynamics (CFD) to enhance students' understanding as an effective educational and learning method. The improved educational method studies the impact of CFD implementation in course project on students' comprehension and performance in fluid mechanics course. Implementing the CFD method is increasingly essential specifically for Mechanical Engineering students, and it can also be applicable to variety of fields, including Science, Technology, Engineering, and Mathematics (STEM). One of the most important improvements of using CFD in course teaching is the strong comprehension and improved students’ performance related to the fluid movements, frictional affect, pressure and velocity variation. This was evident through the strong interactive teaching in classroom. The use of simulation related to the studied fluid flow case has proven to be effective in enhancing student attention and improving their understanding of fluid mechanics.

A MEMS heat sink having straight microchannels integrating sidewall cavities in staggered pattern is conceptualized in this work and its thermal resistance and pumping power characterized for Reynolds number ranging from 100 to 750. Over this Reynolds number range, the thermal resistance and pumping power of this MEMS heat sink are lower than that of a MEMS heat sink having straight microchannels. With increase in Reynolds number, the relative reduction of thermal resistance varied from 1.8% to 23.4% while the relative reduction of pumping power varied between 18.6% and 6.5%. The proposed MEMS heat sink achieves reduction in thermal resistance with reduced pumping power and this is its novelty. The Nusselt and Poiseuille numbers of the straight microchannel integrating sidewall cavities are higher and lower than that of a straight microchannel without sidewall cavities, respectively. With increase in Reynolds number, the relative increase of Nusselt number ranged between 6.6% and 46.5% while the relative reduction of Poiseuille number ranged from 18.6% to 6.5%. The influence of the geometric features of the MEMS heat sink having straight microchannels integrating sidewall cavities is examined in this study. It is identified that there is a threshold value for the length of the sidewall cavities below which sidewall cavities do not influence thermal resistance and pumping power; above this threshold value, increase in the length of the sidewall cavities lead to reduction in thermal resistance and pumping power as well as increase and decrease in Nusselt and Poiseuille numbers, respectively. Increase in length of sidewall cavities led to the relative reduction of thermal resistance and pumping power varied from 1% and 19% at the lowest Reynolds number to 23.2% and 7.3% at the highest Reynolds number. With increase in the length of sidewall cavities, the relative increase of Nusselt number and relative reduction of Poiseuille number ranged from 2.3% and 19%, for Reynolds number of 100, to 38.9% and 7.3%, for Reynolds number of 750. It is identified that for every Reynolds number there exists an optimal value for the width of the sidewall cavities at which thermal resistance and Nusselt number are minimum and maximum, respectively; similar observation is made regarding the relationship between hydraulic diameter and thermal resistance while Nusselt and Poiseuille numbers decrease and increase with hydraulic diameter, respectively. Also, increase in the number of sidewall cavities lead to reduction in thermal resistance and pumping power; increase in the number sidewall cavities leads to increase and decrease in both Nusselt and Poiseuille numbers, respectively. With increase in the number of sidewall cavities, the relative reduction of thermal resistance at the lowest and highest Reynolds numbers are 1.4% and 1.8%, respectively; the relative reduction of pumping power at the lowest and highest Reynolds numbers are 12.6% and 7.1%, respectively. The relative increase of Nusselt number at the lowest and highest Reynolds numbers are 29.2% and 23.6%, respectively; the relative reduction of Poiseuille number at the lowest and highest Reynolds numbers are 12.6% and 7.1%, respectively. The model is validated using data from literature.

This paper investigates pool-boiling heat transfer using surface modification, examining smooth and rough pin-finned surfaces on copper and aluminum. The study visualizes bubble formation and departure diameter, aiming to predict heat transfer coefficients (HTC) under different heat fluxes (HF). Four surfaces were tested: smooth copper, pin-fin copper, smooth aluminum, and rough pin-finned aluminum. Results reveal differences in bubble characteristics and formation/departure times between smooth and rough surfaces. For example, on smooth copper, bubble diameter sizes (low, medium, and CHF) were 0.2803 cm, 0.437 cm, and 0.67 cm, with corresponding formation and departure times of 0.2859 s, 0.305 s, and 0.4025 s. Pin-finned copper surfaces exhibited larger diameters (0.745 cm, 1.38 cm, 1.95 cm) and shorter times (0.184 s, 0.207 s, 0.417 s). Smooth aluminum surface had diameters (0.63 cm, 0.96 cm, 1.27 cm) and times (0.27s, 0.36s, 0.40s), while pin-finned aluminum showed sizes (0.24 cm, 0.30 cm, 0.83 cm) and times (0.11 s, 0.13 s, 0.21 s). The study concludes that rough pin-finned copper enhances boiling HTC and HF at lower wall superheat temperatures compared to the smooth surface. Keywords: Pool boiling, heat transfer, bubble dynamics, critical heat flux.

In this investigation, flow boiling heat transfer and hydrodynamic instability using deionized water in a minichannel with smooth and square pin fin surfaces was experimentally examined. The study is aimed to assess local and overall heat transfer coefficients, heat transfer coefficients, pressure drop and Nusselt number across various liquid mass flux ranges (50-93 kg m- 2 s-1) and a constant heat flux of 46.91 kW/m2. The minichannel, with a hydraulic diameter of 2.85 mm, features a rectangular shape. Experimental testing was conducted on aluminum surface within a minichannel of dimensions 270 mm in length, 30 mm in width, and 1.5 mm height. The square pin fins are integrated at the base of the boiling surfaces, arranged in staggered form. The results revealed that the boiling heat transfer coefficient on square pin fin surfaces was approximately 88 % higher than that on smooth surfaces. Additionally, a notable increase in local heat transfer coefficient was observed for square pin fin surfaces (i.e., 34.64 kW/m2.K) compared to smooth surfaces (8.09 kW/m2.K) at the same mass flux of 50 kg/m2. s. Besides, the square pin fins surface exhibited a lower and stable pressure drop, particularly near annular flow regimes, compared to the smooth surface. Notably, the highest pressure drop observed was 4.8 kPa for the square pin fins surface, while the smooth surface experienced a higher pressure drop of 6.36 kPa, accompanied by more pronounced pressure fluctuations during annular flow.