Thermal resistance

Summary

Thermal resistance is a heat property and a measurement of a temperature difference by which an object or material resists a heat flow. Thermal resistance is the reciprocal of thermal conductance.

(Absolute) thermal resistance R in kelvins per watt (K/W) is a property of a particular component. For example, a characteristic of a heat sink.
Specific thermal resistance or thermal resistivity Rλ in kelvin–metres per watt (K⋅m/W), is a material constant.
Thermal insulance has the units square metre kelvin per watt (m2⋅K/W) in SI units or square foot degree Fahrenheit–hours per British thermal unit (ft2⋅°F⋅h/Btu) in imperial units. It is the thermal resistance of unit area of a material. In terms of insulation, it is measured by the R-value.

Absolute thermal resistance Absolute thermal resistance is the temperature difference across a structure when a unit of heat energy flows through it in unit time. It is the reciprocal of thermal conductance. The SI unit of absolute t

Official source

https://en.wikipedia.org/wiki/Thermal_resistance

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The LHCb VELO Upgrade I, currently being installed for the 2022 start of LHC Run 3, uses silicon microchannel coolers with internally circulating bi-phase CO2 for thermal control of hybrid pixel modules operating in vacuum. This is the largest scale application of this technology to date. Production of the microchannel coolers was completed in July 2019 and the assembly into cooling structures was completed in September 2021. This article describes the R & D path supporting the microchannel production and assembly and the motivation for the design choices, together with the achieved fluidic and thermal performance. The Thermal Figure of Merit of the microchannel coolers is measured on the final modules to be between 1.5 and 3.5 K cm(2) W-1, depending on glue thickness. The microchannel coolers constitute 18% of the total radiation length of the VELO and less than 2% of the material seen before the second measured point on the tracks. Microchannel cooling is well suited to the VELO implementation due to the uniform mass distribution, close thermal expansion match with the module components and resistance to radiation.

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In the present paper, a closed-loop thermosyphon cooling system for power electronics has been developed and tested. The evaporator is a multi-microchannel heat sink designed for high heat fluxes. The power electronics module is emulated by four independent electrical heaters, which can generate a non-uniform heat flux on the evaporator. The condenser is air-cooled and is tested in both natural convection and forced convection modes. R1234ze has been selected as the working fluid. Although in natural convection mode the cooling system experiences a maximum thermal resistance of approximately 0.6 K/W, the complete passiveness makes such a system very attractive. With values of heat fluxes of 107 W/cm(2) and 25 W/cm(2) equally distributed on the four heaters, the maximum heater temperature measured is 53 degrees C at a saturation temperature of 45 degrees C. In forced convection mode, two different values of air flow have been tested. A reduction of 50% on the overall thermal resistance has been measured with an imposed air flow rate of 87.8 m(3)/h, consuming only 5.29 W to drive the fan for dissipating 70.2 W. At this condition, the maximum heater temperature is reduced to 36 degrees C. In summary, it has been demonstrated that a thermosyphon coupled with air cooling can dissipate high heat fluxes of power electronics with no or little energy consumption. (C) 2017 Elsevier Ltd. All rights reserved.

Pergamon-Elsevier Science Ltd2017

Novel pulsating heat pipe for high density power data centres: performance and flow regime analysis

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With the advancement of technologies such as Artificial Intelligence and 5G telecommunications, where computational rate is constantly pushing its limits, highly efficient and reliable cooling devices are starting to become an overly critical factor. Not only good performance and reliability are needed, but final users are also aiming to minimize maintenance and costs. These are the reasons why a device that uses inherent forces of two-phase flow is so attractive. In this paper, a novel pulsating heat pipe is presented, and its performance is tested with a low-GWP working fluid. The novel pulsating heat pipe has a footprint area of 75 x 65 mm(2) and a height of 18.5 mm. The secondary side of the device is water-cooled. The experimental results highlight how the flow regimes detected via the temperature's readings affect the thermal performance. The performance of the pulsating heat pipe is defined as the maximum heat load dissipated from the source, while maintaining its base temperature to a value of 90 degrees C. Among all the conditions tested, the best performance was achieved with the refrigerant R1233zd(E), able to dissipate 850 W (heat flux of 30 W/cm(2)), corresponding to an overall thermal resistance of 0.065 K/W.

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