The effect of non-zero thermal radiation transmissivity of the aluminized mylar film on the thermal performance of super-insulation

When certain fractal geometries are used in the design of fins or heat sinks the surface area available for heat transfer can be increased while system mass can be simultaneously decreased. In order to assess the thermal performance of fractal fins for application in the thermal management of electronic devices an experimental investigation was performed. The experimental investigation assessed the efficiency, effectiveness, and effectiveness per unit mass of straight rectangular fins inspired by the first four iterations of the Sierpinski carpet fractal pattern. The thermal performance of the fractal fins was investigated in a natural convection environment. While fin efficiency was found to decrease with fractal iteration fin effectiveness per unit mass increased with fractal iteration. In addition, a fractal fin inspired by the fourth iteration of the Sierpinski carpet fractal pattern was found to be more effective than a traditional straight rectangular fin of equal width, height, and thickness. When compared to a traditional straight rectangular fin, or the zeroth fractal iteration, a fin inspired by the fourth fractal iteration of the Sierpinski carpet fractal pattern was found to be 4.87% more effective, 15.19% less efficient, and 67.98% more effective per unit mass. The amount of the total heat transfer attributed to thermal radiation was also dependent on fractal iteration. Thermal radiation accounted for 45.52% of the total heat transfer for the baseline case, or zeroth fractal iteration. Thermal radiation accounted for 51.94%, 50.17%, 52.77%, and 66.62% of the total heat transfer for the first, second, third, and fourth fractal iteration respectively.

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Free Convection Heat Transfer From Sierpinski Carpet Fractal Fins of Varying Size

Volume 8: Heat Transfer and Thermal Engineering ◽

10.1115/imece2017-71697 ◽

2017 ◽

Author(s):

David Calamas ◽

Daniel Dannelley ◽

Jennifer Shaffer ◽

Valentin Soloiu

Keyword(s):

Heat Transfer ◽

Free Convection ◽

Thermal Radiation ◽

Thermal Performance ◽

Unit Mass ◽

Sierpinski Carpet ◽

Fractal Pattern ◽

Sierpiński Carpet ◽

The Impact ◽

Equal Height

This works experimentally investigates the thermal performance of extended surfaces inspired by the first four fractal iterations of the Sierpinski carpet fractal pattern in a free convection environment. Fractal fins inspired by the Sierpinski carpet fractal pattern can result in an increase in surface area for convective heat transfer coupled with a simultaneous decrease in mass and are thus desirable in aerospace applications. The thermal performance of the Sierpinski carpet fractal fins was quantified based on fin efficiency, fin effectiveness, and perforated fin effectiveness. When compared with a solid rectangular fin, without perforations, and of an equal base area and package volume a fin inspired by the fourth iteration of the Sierpinski carpet fractal pattern was found to be more effective at dissipating heat by convection. The impact of fin size on the thermal performance of the fractal fins was investigated for a range of power inputs applied at the base (2.5 W, 5 W, and 10 W). A 5.08 cm × 5.08 cm (2 in × 2 in × 1/16 in) fractal fin inspired by the fourth iteration of the Sierpinski carpet fractal was found to have a convective effectiveness, convective efficiency, and convective effectiveness per unit mass, 10.91% more, 10.31% less, and 77.65% more, than a traditional solid (non-perforated) rectangular fin of equal height, width, and thickness. Similarly, a 10.16 cm × 10.16 cm (4 in × 4 in × 1/8 in) fin inspired by the fourth fractal iteration was found to have a convective effectiveness, convective efficiency, and convective effectiveness per unit mass, 3.97% more, 15.91% less, and 66.54% more, than a traditional solid (non-perforated) rectangular fin of equal height, width, and thickness. Thus, the thermal performance of the fractal fins increased as the size of the fins decreased. Regardless of size, the contribution of thermal radiation was significant (often greater than 50%) and should not be neglected. In general, for a fin with a uniform cross-section, intersurface thermal radiation accounts for a significant percentage of thermal radiation heat transfer, particularly as the size of the perforations decreases.

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Generalized Equation for Thermal Conductivity of MLI at Temperatures From 20K to 300K

Heat Transfer, Volume 1 ◽

10.1115/imece2003-41830 ◽

2003 ◽

Author(s):

Lixing Gu

Keyword(s):

Heat Transfer ◽

Thermal Conductivity ◽

Thermal Radiation ◽

Thermal Performance ◽

Ambient Air ◽

Liquid Hydrogen ◽

Heat Fluxes ◽

Layer By Layer ◽

Storage Tanks ◽

Thermal Conductivities

Multilayer insulation (MLI) has the lowest thermal conductivity of any currently used insulation in high vacuum environments and is used in cryogenic insulation system to minimize heat leaks in liquid hydrogen storage tanks. MLI consists of highly reflective radiation shields separated by spacers or insulation. The thermal conductivity of MLI varies with both temperature and vacuum level. Most published apparent thermal conductivities were measured for temperatures between 80K and 300K; some of the published data were for temperatures between 20K and 80K. Since the temperature of liquid hydrogen is 20K and the storage tanks are exposed to ambient air, it is essential to know the thermal performance of MLI for the temperature range of 20K to 300K. In addition, in order to provide a detailed temperature distribution and to optimize insulation systems with respect to the number of layers of MLI, layer density, insulation weight, and separator configuration, the layer-by-layer thermal performance of MLI has to be established for efficient storage tank design. A general equation for thermal conductivity was developed based on heat transfer principles for a wide range of temperature differences and vacuum levels. The equation consists of four heat transfer modes: 1) thermal radiation between two adjacent reflectors, 2) thermal radiation absorbed by spacers 3) gas conduction, and 4) solid spacer conduction. The equation can be applied for the temperature ranges of liquid hydrogen up to ambient, and for pressure ranges between 1.33 mPa to 1.33 kPa (0.01 millitorr and 10 torr). The predicted layer-by-layer temperatures, heat fluxes and apparent thermal conductivities using the developed thermal conductivity equation show very good agreement with measured data between the temperatures of 80K and 300K at the various pressure levels. When the equation was applied for a temperature of 20K, heat fluxes increased due to the larger temperature difference, while apparent thermal conductivities decreased due to the lower cold side temperature.

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Experimental study of thermohydraulic characteristics and irreversibility analysis of novel axial corrugated tube with spring tape inserts

The European Physical Journal Applied Physics ◽

10.1051/epjap/2020200192 ◽

2020 ◽

Vol 92 (3) ◽

pp. 30901

Author(s):

Suvanjan Bhattacharyya ◽

Debraj Sarkar ◽

Ulavathi Shettar Mahabaleshwar ◽

Manoj K. Soni ◽

M. Mohanraj

Keyword(s):

Heat Transfer ◽

Experimental Study ◽

Thermal Performance ◽

Working Fluid ◽

The Novel ◽

Heat Exchanger Tube ◽

Heat Transfer Augmentation ◽

Corrugated Tube ◽

The Impact ◽

Exchanger Tube

The current study experimentally investigates the heat transfer augmentation on the novel axial corrugated heat exchanger tube in which the spring tape is introduced. Air (Pr = 0.707) is used as a working fluid. In order to augment the thermohydraulic performance, a corrugated tube with inserts is offered. The experimental study is further extended by varying the important parameters like spring ratio (y = 1.5, 2.0, 2.5) and Reynolds number (Re = 10 000–52 000). The angular pitch between the two neighboring corrugations and the angle of the corrugation is kept constant through the experiments at β = 1200 and α = 600 respectively, while two different corrugations heights (h) are analyzed. While increasing the corrugation height and decreasing the spring ratio, the impact of the swirling effect improves the thermal performance of the system. The maximum thermal performance is obtained when the corrugation height is h = 0.2 and spring ratio y = 1.5. Eventually, correlations for predicting friction factor (f) and Nusselt number (Nu) are developed.

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Characterizing the convective heat exchange with plastic shading nets under natural arid conditions

International Journal of Energy Technology ◽

10.32438//ijet.11415 ◽

2019 ◽

pp. 11-20

Author(s):

Ahmed M Abdel-Ghanya ◽

Ibrahim M Al-Helal

Keyword(s):

Wind Speed ◽

Thermal Radiation ◽

Convective Heat ◽

Convective Heat Transfer Coefficient ◽

Convective Heat Exchange ◽

Radiative Properties ◽

Wooden Frame ◽

Arid Conditions ◽

Air Temperatures ◽

Radiation Fluxes

Plastic nets are extensively used for shading purposes in arid regions such as in the Arabian Peninsula. Quantifying the convection exchange with shading net and understanding the mechanisms (free, mixed and forced) of convection are essential for analyzing energy exchange with shading nets. Unlike solar and thermal radiation, the convective energy, convective heat transfer coefficient and the nature of convection have never been theoretically estimated or experimentally measured for plastic nets under arid conditions. In this study, the convected heat exchanges with different plastic nets were quantified based on an energy balance applied to the nets under outdoor natural conditions. Therefore, each net was tacked onto a wooden frame, fixed horizontally at 1.5-m height over the floor. The downward and upward solar and thermal radiation fluxes were measured below and above each net on sunny days; also the wind speed over the net, and the net and air temperatures were measured, simultaneously. Nets with different porosities, colors and texture structures were used for the study. The short and long wave’s radiative properties of the nets were pre-determined in previous studies to be used. Re and Gr numbers were determined and used to characterize the convection mechanism over each net. The results showed that forced and mixed convection are the dominant modes existing over the nets during most of the day and night times. The nature of convection over nets depends mainly on the wind speed, net-air temperature difference and texture shape of the net rather than its color and its porosity.

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