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"Heat transfer"
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A comprehensive review on microchannel heat sinks for electronics cooling
The heat generation of electronic devices is increasing dramatically, which causes a serious bottleneck in the thermal management of electronics, and overheating will result in performance deterioration and even device damage. With the development of micro-machining technologies, the microchannel heat sink (MCHS) has become one of the best ways to remove the considerable amount of heat generated by high-power electronics. It has the advantages of large specific surface area, small size, coolant saving and high heat transfer coefficient. This paper comprehensively takes an overview of the research progress in MCHSs and generalizes the hotspots and bottlenecks of this area. The heat transfer mechanisms and performances of different channel structures, coolants, channel materials and some other influencing factors are reviewed. Additionally, this paper classifies the heat transfer enhancement technology and reviews the related studies on both the single-phase and phase-change flow and heat transfer. The comprehensive review is expected to provide a theoretical reference and technical guidance for further research and application of MCHSs in the future. The studies on microchannel heat sinks for electronics cooling are reviewed comprehensively. The main research areas of interest for microchannel heat sinks are classified. The studies on both single-phase and phase-change flow cooling are reviewed. The characteristics, application conditions and shortcomings of microchannel heat sinks with different structures, working fluids, materials and some other influencing factors are introduced. The prospects for and development trends of microchannel heat sinks are revealed based on the overall review and analysis.
Journal Article
Convective and evaporative heat transfer coefficients during drying of ivy gourd under natural and forced convection solar dryer
In the present work, a study on convective heat, mass transfer coefficients and evaporative heat transfer coefficient of the thin layer drying process of ivy gourd is performed. The experiment was conducted in three drying modes such as natural, forced convection solar dryer and open sun drying. The hourly data for the rate of moisture removal, sample temperature, relative humidity inside and outside the solar and ambient air temperature for complete drying have been recorded. The drying air temperature varied from 55, 65, 70 and 75 °C, and the air velocity was 1, 1.5 and 2 m/s. All the drying experiments had shown a falling rate period. The data obtained from experimentation have been used to evaluate the experimental constant values of
C
and
n
by simple regression analysis. Based on the values of “
C
” and “
n
”, convective and evaporative heat transfer coefficients for ivy gourd were determined. The average convective heat and mass transfer coefficients varied between 2.64 and 8.30 W/m
2
°C and 0.0025 to 0.0076 m/s for temperature ranges, at the different air velocities, respectively. The average evaporative heat transfer coefficient for ivy gourd varied from 181.89 to 421.84 W/m
2
°C. It was observed that convective and evaporative heat transfer coefficients increase with the increase in drying air temperature. The rate of increment of evaporative heat transfer coefficient is higher than the convective heat transfer coefficient. The intensity of heat and mass transfer during solar drying depends on the drying air temperature and velocity.
Graphical abstract
Journal Article
A comprehensive review of methods of heat transfer enhancement in shell and tube heat exchangers
A wide range of studies was conducted to increase the heat transfer rate and reduce the size and cost of shell and tube heat exchangers (STHE). The paper’s contributions lie in its ability to provide a comprehensive, up-to-date, and systematic overview of the various methods available for heat transfer enhancement in STHEs, making it an essential resource for researchers, engineers, and practitioners in the field of heat transfer. The studies that researched the overall heat transfer coefficient (
U
), number of transfer units, exergy efficiency, pressure drop, and thermal–hydraulic performance were reviewed. There are some advantages of the passive method such as no external needed power and lower operating cost compared to the active methods. The studies broadly support the view that heat transfer enhancement in STHE is heading toward considerable progress. A total of 47.8% of studies have focused on the passive approach, the air injection method, enhancing heat transfer utilizing nanofluids, and compound methods have percentages of studies 20.2, 22.3, and 9.7%, respectively. The air bubble injection causes the rise of the
U
ratio where the maximum value was indicated at 452% compared to only water flow. Swirl vane, corrugated tube, and wire coil insert have U ratio values of 130, 161, and 264%, respectively. Nanofluid results in a growth in the heat transfer where the TiO
2
has the maximum
U
ratio (175.9%) compared to traditional fluid. The combination of air injection and passive heat augmentation methods, which was shown to be a substantial solution to several issues, needs to be the focus of more work in the future. Geometrical changes in tube surfaces in STHE are too required in the future with the use of materials coating to enhance heat transfer. The theoretical analysis of heat transfer techniques still needs to be improved, especially for pertinent empirical formulations. Also, since there aren’t many relevant numerical simulations, more attention is required.
Journal Article
Experimental assessment of convective and radiative heat transfer coefficients for various clothing ensembles
The convective and radiative heat transfer coefficients of clothing are important parameters for human thermoregulation and comfort models. Many researchers have studied convective and radiative heat transfer coefficients of the naked human body. However, there is limited information on convective and radiative heat transfer coefficients for the clothed human body. Therefore, this study aims to confirm whether the convective and radiative heat transfer coefficients vary with different clothing ensembles in addition to clarifying how the difference in clothing heat transfer coefficients affects the prediction of thermal comfort index, such as the predicted mean vote (PMV) index. The convective and radiative heat transfer coefficients for eight sets of clothing ensembles were measured through a manikin experiment. The results demonstrated that (1) the largest difference between convective heat transfer coefficients for various clothing ensembles was 32%, and (2) PMV values differed between the clothing ensembles with the largest value being approximately 0.2, which corresponds nearly 1 °C change in the indoor temperature. Therefore, it is necessary to consider the actual clothing convective heat transfer coefficient for the precise prediction of thermal comfort.
Journal Article
Characterizing the Relationship between Temperature and Soil Moisture Extremes and Their Role in the Exacerbation of Heat Waves over the Contiguous United States
Increased heat-wave frequency across the United States has led to the need for improved predictability of heat-wave events. A detailed understanding of land–atmosphere interactions and the relationship between soil moisture and temperature extremes could provide useful information for prediction. This study identifies, for many locations, a threshold of soil moisture below which there is an increase in the sensitivity of atmospheric temperature to declining soil moisture. This shift to a hypersensitive regime causes the atmosphere to be more susceptible to atmospherically driven heat-wave conditions. The soil moisture breakpoint where the regime shift occurs is estimated using segmented regression applied to observations and reanalysis data. It is shown that as the soil gets drier, there is a concomitant change in the rate of decrease in latent heat flux and increase in sensible heat flux leading to a strong positive feedback of increased air temperature near the surface, which further dries out the soil. Central, southwestern, and southeastern parts of the United States seem to have regions of clear regime shifts, while the eastern part of the United States generally does not get dry enough to reveal significant breakpoints. Sensible heat flux is seen to be a primary driver of this increased temperature sensitivity aided by the drop in latent heat flux. An investigation of flux tower sites verifies the breakpoint–flux relationships found in reanalysis data. Accurate estimation of these breakpoints can contribute to improved heat-wave prediction.
Journal Article
Comparative Experimental Study on Heat Transfer Characteristics of Building Exterior Surface at High and Low Altitudes
The external surface heat transfer coefficient of building envelope is one of the important parameters necessary for building energy saving design, but the basic data in high-altitude area are scarce. Therefore, the authors propose a modified measurement method based on the heat balance of a model building, and use the same model building to measure its external surface heat transfer coefficient under outdoor conditions in Chengdu city, China at an altitude of 520 m and Daocheng city at an altitude of 3750 m respectively. The results show that the total heat transfer coefficient (
h
t
) of building surface in high-altitude area is reduced by 34.48%. The influence of outdoor wind speed on the convective heat transfer coefficient (
h
c
) in high-altitude area is not as significant as that in low-altitude area. The fitting relation between convection heat transfer coefficient and outdoor wind speed is also obtained. Under the same heating power, the average temperature rise of indoor and outdoor air at high-altitude is 41.9% higher than that at low altitude, and the average temperature rise of inner wall is 25.8% higher than that at low altitude. It shows that high-altitude area can create a more comfortable indoor thermal environment than low-altitude area under the same energy consumption condition. It is not appropriate to use the heat transfer characteristics of the exterior surface of buildings in low-altitude area for building energy saving design and related heating equipment selection and system terminal matching design in high-altitude area.
Journal Article
A near-field radiative heat transfer device
Recently, several reports have experimentally shown near-field radiative heat transfer (NFRHT) exceeding the far-field blackbody limit between planar surfaces1–5. However, owing to the difficulties associated with maintaining the nanosized gap required for measuring a near-field enhancement, these demonstrations have been limited to experiments that cannot be implemented in large-scale devices. This poses a bottleneck to the deployment of NFRHT concepts in practical applications. Here, we describe a device bridging laboratory-scale measurements and potential NFRHT engineering applications in energy conversion6,7 and thermal management8–10. We report a maximum NFRHT enhancement of approximately 28.5 over the blackbody limit with devices made of millimetre-sized doped Si surfaces separated by vacuum gap spacings down to approximately 110 nm. The devices use micropillars, separating the high-temperature emitter and low-temperature receiver, manufactured within micrometre-deep pits. These micropillars, which are about 4.5 to 45 times longer than the nanosize vacuum spacing at which radiation transfer takes place, minimize parasitic heat conduction without sacrificing the structural integrity of the device. The robustness of our devices enables gap spacing visualization by scanning electron microscopy (SEM) before performing NFRHT measurements.
Journal Article
On an averaged energy-balance method for the analysis of wavy microchannels
This study presents numerical simulations of the convective heat transfer on wavy microchannels to investigate heat transfer enhancement in these systems. The objective is to propose a methodology based on local and global energy balances in the device, instead of the commonly used Nusselt number, as an alternative for the thermal analysis. This investigation is carried out on a single-wave microchannel model of size 0.5 mm by 0.5 mm by 20 mm length, with water flowing inside the channel, exposed to a heat influx of 47 W/cm
2
at the bottom. The governing equations for an incompressible laminar flow and conjugate heat transfer are first built, and then solved, for representative models, with copper as the solid-block material under a number of operating conditions (cold-water flowrates of
R
e
=
50
, 100, and 150), by the finite element technique. From computed velocity, pressure and temperature fields, local and global energy balances based on cross-section-averaged velocities and temperatures enable calculating the heat rate at each section of the corresponding device. Results from this study for two different designs, namely, serpentine and divergent-convergent layouts, show that this so-called averaged energy-balance methodology enables higher accuracy than that based on Nusselt numbers since neither transfer coefficients nor characteristic temperatures are needed.
Journal Article