scholarly journals Half-cooling time of cabbage stored in a refrigerated room

2020 ◽  
Vol 47 (No. 2) ◽  
pp. 93-99
Author(s):  
Radovan Tůma ◽  
Jan Goliáš
Keyword(s):  
Heat Transfer ◽  
Heat Dissipation ◽  
Storage Temperature ◽  
High Capacity ◽  
Ambient Air ◽  
Cooling Time ◽  
Bottom Position ◽  
Order Of Magnitude ◽  
Head Cabbage

Cabbage heads stored in containers in a high-capacity cooling room were evaluated by the half-cooling times. The temperature of the heads and the surrounding air in their immediate vicinity was measured by a puncture thermometer. The shortest half-cooling times were measured in the immediate vicinity of the evaporator; in contrast, very long half-cooling times were measured in a container on the opposite side of the cooling room. The measured values were longer by one order of magnitude. When cabbage heads are inside the container furthest from the evaporator, without being covered by a layer of stacked containers, then the heat transfer passes directly into the ambient air, thus, the half-cooling times in this container were not the longest. The head cabbage cooling, evaluated by the half-cooling time (in hours) and the end of cooling time (in hours), in the chambers to a storage temperature (0 °C) are directly proportional. The heads in a container 14 m from the evaporator, in the bottom position, but not covered by other containers had a half-cooling time of 194.7 hours, with a total time of 973.5 h, because the air circulating around the container, permanently cooled down the stored heads. The total calculated cooling time will be extended 5.0 times. For the heads in the same position, but 7 m from the evaporator, the heat dissipation at the bottom position was so slow that the half-cooling time was 225 hours, and the total time was 1 125 hours (46.88 days), which was the longest cooling time for the cabbage heads.

scholarly journals Heat Transfer Characteristics of Aluminum Sputtered Fabrics

2018 ◽  
Vol 13 (3) ◽  
pp. 155892501801300 ◽  
Author(s):  
Hye Ree Han ◽  
Yaewon Park ◽  
Changsang Yun ◽  
Chung Hee Park
Keyword(s):  
Heat Transfer ◽  
Heat Dissipation ◽  
Ambient Air ◽  
Heat Transfer Characteristics ◽  
Transfer Coefficients ◽  
Hot Plate ◽  
Multilayer Systems ◽  
Heat Transfer Coefficients ◽  
Transfer Characteristics ◽  
Shape Memory Polyurethane

Al was sputtered onto four substrates: nylon, polyester, cotton/polyester, and shape memory polyurethane nanoweb, and the heat-transfer characteristics of the resultant materials were investigated by surface temperature measurements. The thickness of the Al layer increased linearly with sputtering time. The heat-transfer mechanisms of the multilayer systems in terms of conduction, convection, and radiation were investigated under steady-state conditions using a hot plate as a heat source in contact with Al-sputtered fabrics. The Al-sputtered fabric was placed on the hot plate, which was maintained at 35°C, and exposed to open air, which was maintained at 15°C. The temperatures of the air-facing surfaces of hot plate-Al-fabric-air (i.e., Al-phase-down) and hot plate-fabric-Al-air (i.e., Al-phase-up) systems were used to investigate the heat-transfer mechanism. It was found that heat dissipation to ambient air was much higher for the Al-phase-up system than for the Al-phase-down system. Heat-transfer coefficients of the Al surfaces were calculated and found to increase with the thickness of the Al layer. Furthermore, different conductive thermal resistances were observed for different fabrics prepared with the same Al-sputtering time. Consequently, differences in their thicknesses pore sizes, and thermal conductivities were suggested to have significant effects on their heat-transfer properties.

Heat Transfer in Encased Graphene

2009 ◽  
Author(s):  
Zhen Chen ◽  
Wanyoung Jang ◽  
Wenzhong Bao ◽  
Chun Ning Lau ◽  
Chris Dames
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Room Temperature ◽  
Heat Dissipation ◽  
Thermal Contact ◽  
Graphene Sheets ◽  
A Value ◽  
Order Of Magnitude ◽  
Device Applications ◽  
Freely Suspended

Experimentally understanding the heat transfer in graphene (sheets of graphite a few atoms thick) is important for fundamental physics as well as device applications. In particular, measurements of the heat flow through graphene encased by oxide layers are essential for future graphene-based nanoelectronics, interconnects, and thermal management structures. Here we use a “heat spreader method” to study the heat dissipation performance of encased graphene. Measurements show enhanced heat spreading by a graphene layer as compared to control samples without graphene. At room temperature, the in-plane thermal conductivity of encased graphene sheets of thickness 2 nm and 5 nm is measured to be ∼150 W/m-K, more than one order of magnitude smaller than a published report for a freely-suspended graphene sheet [A. A. Balandin et al., Nano Lett. 8, 902], as well as bulk graphite. We also used a differential 3ω method to measure the thermal contact resistance between graphene and SiO2, finding a value around 10−8 m2-K/W at room temperature. Possible reasons for the unexpectedly low thermal conductivity are also discussed.

scholarly journals Statistical and Simulation Analysis on Dimple Configurations Performance of Heat Dissipation

2021 ◽  
Vol 86 (2) ◽  
pp. 74-90
Author(s):  
Mohd Shahir Kasim ◽  
Nur Husnina Najeah Husshini ◽  
Raja Izamshah ◽  
Hema Nanthini Ganesan ◽  
Muhamad Ammar Farhan Maula Mohd Azam ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Flow Structure ◽  
Heat Dissipation ◽  
Simulation Analysis ◽  
Air Flow ◽  
Orientation Angle ◽  
Cooling Time ◽  
Cooling Effect ◽  
Staggered Arrangement

This paper presents an investigation on cooling effect and flow structure of the spherical dimple configuration during air flow on the Aluminium surface. It is prominently known that applying dimples profile causes an enhancement in heat transfer over a plain surface. A three level of Box-Behnken response surface methodology was performed to find the correlation between the input and output variables. A total of 17 different combinations of these inputs were performed throughout the experiment. The variable inputs to be investigated namely: dimple diameter of 10 - 14 mm, dimple orientation angle of 60°- 90°, and airflow velocity of 16 - 18 m/s to observe the response on the cooling time. The Aluminium block was heated to 60°C and cooled down by air flow at room temperature. The ANOVA was used to identify the significant effect of each parameter. CFD software was used as a simulation tool to analyze the flow structure and Reynolds number that associate with the heat transfer rate to support the statistical findings. Based on the result, all the input parameters are found to be significantly dominated by air flow velocity. Staggered arrangement dimple profile surface improves cooling effect by 63% over the plain flat surface. The increment in Reynolds number will increase the heat transfer which then shortening the cooling time.

scholarly journals Experimental Determination of Stator Endwall Heat Transfer

1989 ◽  
Author(s):  
Robert J. Boyle ◽  
Louis M. Russell
Keyword(s):  
Heat Transfer ◽  
Liquid Crystal ◽  
Test Section ◽  
Electrical Power ◽  
Exhaust System ◽  
Ambient Air ◽  
Reynolds Numbers ◽  
Inlet Velocity ◽  
Turbine Vane

Local Stanton numbers were experimentally determined for the endwall surface of a turbine vane passage. A six vane linear cascade having vanes with an axial chord of 13.81 cm was used. Results were obtained for Reynolds numbers based on inlet velocity and axial chord between 73,000 and 495,000. The test section was connected to a low pressure exhaust system. Ambient air was drawn into the test section, inlet velocity was controlled up to a maximum of 59.4 m/sec. The effect of the inlet boundary layer thickness on the endwall heat transfer was determined for a range of test section flow rates. The liquid crystal measurement technique was used to measure heat transfer. Endwall heat transfer was determined by applying electrical power to a foil heater attached to the cascade endwall. The temperature at which the liquid crystal exhibited a specific color was known from a calibration test. Lines showing this specific color were isotherms, and because of uniform heat generation they were also lines of nearly constant heat transfer. Endwall static pressures were measured, along with surveys of total pressure and flow angles at the inlet and exit of the cascade.

scholarly journals Influence of Ambient Temperature on Radiative and Convective Heat Dissipation Ratio in Polymer Heat Sinks

Polymers ◽  
2021 ◽  
Vol 13 (14) ◽  
pp. 2286
Author(s):  
Jan Kominek ◽  
Martin Zachar ◽  
Michal Guzej ◽  
Erik Bartuli ◽  
Petr Kotrbacek
Keyword(s):  
Heat Transfer ◽  
Ambient Temperature ◽  
Convective Heat ◽  
Heat Dissipation ◽  
High Surface ◽  
Heat Sinks ◽  
Fourth Power ◽  
Molding Process ◽  
Ambient Temperatures ◽  
University Of Technology

Miniaturization of electronic devices leads to new heat dissipation challenges and traditional cooling methods need to be replaced by new better ones. Polymer heat sinks may, thanks to their unique properties, replace standardly used heat sink materials in certain applications, especially in applications with high ambient temperature. Polymers natively dispose of high surface emissivity in comparison with glossy metals. This high emissivity allows a larger amount of heat to be dissipated to the ambient with the fourth power of its absolute surface temperature. This paper shows the change in radiative and convective heat transfer from polymer heat sinks used in different ambient temperatures. Furthermore, the observed polymer heat sinks have differently oriented graphite filler caused by their molding process differences, therefore their thermal conductivity anisotropies and overall cooling efficiencies also differ. Furthermore, it is also shown that a high radiative heat transfer leads to minimizing these cooling efficiency differences between these polymer heat sinks of the same geometry. The measurements were conducted at HEATLAB, Brno University of Technology.

scholarly journals Cooling Performance Analysis of Outside Fins of the Closed Circuit Axial Piston Transmission

Machines ◽  
2021 ◽  
Vol 9 (1) ◽  
pp. 17
Author(s):  
Chen Yang ◽  
Long-jie Yu ◽  
Junhui Zhang ◽  
Jin-yuan Qian
Keyword(s):  
Heat Transfer ◽  
Heat Dissipation ◽  
Mechanical Energy ◽  
Closed Circuit ◽  
Moderate Increase ◽  
Frictional Loss ◽  
Fin Thickness ◽  
Axial Piston ◽  
Positive Effect ◽  
Peak Value

Realizing conversion between fluid power and mechanical energy, the closed circuit axial piston transmission (CCAPT) plays a vital and indispensable role in miscellaneous industries. The frictional loss and leakage loss inside the system give rise to the inevitable temperature rise. In order to prolong the life of the device, a cooling structure on the outside of the CCAPT is designed for promoting heat dissipation. Based on the relevant heat transfer law and the temperature distribution of internal machinery elements, a spiral fin structure is designed at the shell side. With the help of numerical simulation, the effects of fin height, fin pitch, and fin thickness on the thermal performance are studied. The flow field and temperature field on the outside of the fin structure are obtained as a guidance for enhancing heat dissipation effect. Results indicate that the area of rotating elements tend to accumulate heat, where more attention should be paid for a better cooling effect. In addition to this, a moderate increase of fin height, fin pitch and fin thickness has a positive effect on heat transfer enhancement. The peak value of Nusselt number is obtained with a fin height of 7.5 mm, which is about 2.09 times that of the condition without the fin structure. An increase in fin pitch improves both heat transfer performance and comprehensive performance at the same. When fin pitch is 30 mm, Nusselt numberincreases 104% over the original condition.

scholarly journals Pool Boiling of Nanofluids on Biphilic Surfaces: An Experimental and Numerical Study

Nanomaterials ◽  
2021 ◽  
Vol 11 (1) ◽  
pp. 125
Author(s):  
Eduardo Freitas ◽  
Pedro Pontes ◽  
Ricardo Cautela ◽  
Vaibhav Bahadur ◽  
João Miranda ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Numerical Model ◽  
Heat Dissipation ◽  
Numerical Study ◽  
Pool Boiling ◽  
Temperature Drop ◽  
Surface Cooling ◽  
Average Temperature ◽  
Gold Silver

This study addresses the combination of customized surface modification with the use of nanofluids, to infer on its potential to enhance pool-boiling heat transfer. Hydrophilic surfaces patterned with superhydrophobic regions were developed and used as surface interfaces with different nanofluids (water with gold, silver, aluminum and alumina nanoparticles), in order to evaluate the effect of the nature and concentration of the nanoparticles in bubble dynamics and consequently in heat transfer processes. The main qualitative and quantitative analysis was based on extensive post-processing of synchronized high-speed and thermographic images. To study the nucleation of a single bubble in pool boiling condition, a numerical model was also implemented. The results show an evident benefit of using biphilic patterns with well-established distances between the superhydrophobic regions. This can be observed in the resulting plot of the dissipated heat flux for a biphilic pattern with seven superhydrophobic spots, δ = 1/d and an imposed heat flux of 2132 w/m2. In this case, the dissipated heat flux is almost constant (except in the instant t* ≈ 0.9 when it reaches a peak of 2400 W/m2), whilst when using only a single superhydrophobic spot, where the heat flux dissipation reaches the maximum shortly after the detachment of the bubble, dropping continuously until a new necking phase starts. The biphilic patterns also allow a controlled bubble coalescence, which promotes fluid convection at the hydrophilic spacing between the superhydrophobic regions, which clearly contributes to cool down the surface. This effect is noticeable in the case of employing the Ag 1 wt% nanofluid, with an imposed heat flux of 2132 W/m2, where the coalescence of the drops promotes a surface cooling, identified by a temperature drop of 0.7 °C in the hydrophilic areas. Those areas have an average temperature of 101.8 °C, whilst the average temperature of the superhydrophobic spots at coalescence time is of 102.9 °C. For low concentrations as the ones used in this work, the effect of the nanofluids was observed to play a minor role. This can be observed on the slight discrepancy of the heat dissipation decay that occurred in the necking stage of the bubbles for nanofluids with the same kind of nanoparticles and different concentration. For the Au 0.1 wt% nanofluid, a heat dissipation decay of 350 W/m2 was reported, whilst for the Au 0.5 wt% nanofluid, the same decay was only of 280 W/m2. The results of the numerical model concerning velocity fields indicated a sudden acceleration at the bubble detachment, as can be qualitatively analyzed in the thermographic images obtained in this work. Additionally, the temperature fields of the analyzed region present the same tendency as the experimental results.

scholarly journals Study on the Convective Heat Transfer and Fluid Flow of Mini-Channel with High Aspect Ratio of Neutron Production Target

Energies ◽  
2021 ◽  
Vol 14 (13) ◽  
pp. 4020
Author(s):  
Peng Sun ◽  
Yiping Lu ◽  
Jianfei Tong ◽  
Youlian Lu ◽  
Tianjiao Liang ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Numerical Simulation ◽  
Aspect Ratio ◽  
Optimization Design ◽  
Heat Dissipation ◽  
Neutron Production ◽  
Thermal Design ◽  
Maximum Deviation ◽  
Test Channel ◽  
Production Target

In order to provide a theoretical basis for the thermal design of the neutron production target, flow and heat transfer characteristics are studied by using numerical simulations and experiments. A rectangular mini-channel experimental model consistent with the geometric shape of the heat dissipation structure of neutron production target was established, in which the aspect ratio and gap thickness of the test channel were 53.8:1 and 1.3 mm, respectively. The experimental results indicate that the critical Re of the mini-channel is between 3500 and 4000, and when Re reaches 21,000, Nu can reach 160. The simulation results are in good agreement with the experimental data, and the numerical simulation method can be used for the variable structure optimization design of the target in the later stage. The relationship between the flow pressure drop of the target mini-channel and the aspect ratio and Re is obtained by numerical simulation. The maximum deviation between the correlation and the experimental value is 6%.

scholarly journals Analysis and Comparison of Some Low-Temperature Heat Sources for Heat Pumps

Energies ◽  
2019 ◽  
Vol 12 (10) ◽  
pp. 1853 ◽  
Author(s):  
Pavel Neuberger ◽  
Radomír Adamovský
Keyword(s):  
Heat Transfer ◽  
Low Temperature ◽  
Heat Source ◽  
Heat Pump ◽  
Heat Exchangers ◽  
Ambient Air ◽  
Heat Transfer Fluid ◽  
Heat Sources ◽  
Ground Heat Exchangers ◽  
Temperature Heat

The efficiency of a heat pump energy system is significantly influenced by its low-temperature heat source. This paper presents the results of operational monitoring, analysis and comparison of heat transfer fluid temperatures, outputs and extracted energies at the most widely used low temperature heat sources within 218 days of a heating period. The monitoring involved horizontal ground heat exchangers (HGHEs) of linear and Slinky type, vertical ground heat exchangers (VGHEs) with single and double U-tube exchanger as well as the ambient air. The results of the verification indicated that it was not possible to specify clearly the most advantageous low-temperature heat source that meets the requirements of the efficiency of the heat pump operation. The highest average heat transfer fluid temperatures were achieved at linear HGHE (8.13 ± 4.50 °C) and double U-tube VGHE (8.13 ± 3.12 °C). The highest average specific heat output 59.97 ± 41.80 W/m2 and specific energy extracted from the ground mass 2723.40 ± 1785.58 kJ/m2·day were recorded at single U-tube VGHE. The lowest thermal resistance value of 0.07 K·m2/W, specifying the efficiency of the heat transfer process between the ground mass and the heat transfer fluid, was monitored at linear HGHE. The use of ambient air as a low-temperature heat pump source was considered to be the least advantageous in terms of its temperature parameters.

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