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Subatmospheric pool boiling of water at very low liquid levels

Hałon Tomasz, Kaczmarek Dominika, Lada Wiktoria, Zajączkowski Bartosz

Archives of Thermodynamics

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2023

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Vol. 44, no 4

447--461

EN

The paper discusses how the vapour bubbles growing during boiling under the near-triple point pressure influence the heat transfer coefficient when the refrigerant level is lower than the bubble departure diameter. The experiments were carried out for liquid levels of 0.57 to 1.89 cm, saturated pressure range between 0.9 and 4 kPa (saturation temperatures between 5.5 and 29◦C). Boiling occurred on a plain surface with wall heat flux densities between 0.43 and 5.93 Wcm−2. We determined boiling curves for the low-pressure process and analyzed the changes in wall superheat for different filling levels. The experimentally obtained heat transfer coefficient (HTC) was compared with the theoretical values produced by the most popular mathematical expressions used at higher pressures. We also prepared the boiling map, where we specified two boiling regimes: the regime of convection or small popping bubbles and the regime of isolated bubbles. The results indicate that the level of liquid can be neglected within the heat flux range analyzed in this study. The main mechanism of heat transfer under measured conditions is heat convection and conduction, rather than evaporation. The experimentally determined difference between the heat transfer coefficients for different levels of liquid is under 100 Wm−2K−1 (for the same heat flux and pressure at the wall).

2

Czynniki chłodnicze – ograniczenia napełnienia i bezpieczeństwo użytkowników według dyrektywy Urządzenia ciśnieniowe, ISO 817 i PN-EN 378

Zajączkowski Bartosz, Hałon Tomasz, Reszewski Stefan

Instal

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2020

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nr 7

13--18

PL

W artykule przedstawiono metodologię obliczania maksymalnych napełnień instalacji chłodniczych czynnikami chłodniczymi w zależności od ich palności i toksyczności. Metodologia została zaczerpnięta z ISO 817:2014 i EN 378-1:2016. Pokazano również przykłady obliczeń oraz przykłady rozwiązań technicznych, dzięki którym można rozwiązać problemy związane z projektowaniem wieloparowaczowych instalacji chłodniczych, postępując zgodnie z normami i praktyką inżynierską.

EN

The article presents the methodology for calculating the maximum refrigerant charge in refrigeration installations depending on refrigerants flammability and toxicity. The methodology is taken from ISO 817: 2014 and EN 378-1: 2016. Examples of calculations and examples of technical solutions are also shown, thanks to which it is possible to solve problems related to the design of multi-evaporator refrigeration installations by following the standards and engineering practice.

3

Pressure drop and temperature uniformity during flow boiling of refrigerant R245fa in microchannels

Sandler Stanisław, Zajączkowski Bartosz, Białko Bogusław

Zeszyty Energetyczne

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2017

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T. 4

1--12

EN

Cooling computer processors (CPUs) requires dissipating heat from small heat transfer areas. This results in high heat flux densities to be rejected from the microprocessor. Flow boiling in microchannels receives much attention as a potential solution for CPU cooling. It is characterized by high heat transfer coefficients and requires less working fluid inventory than air-based solutions. However, large pressure drop occurs during phase transition. Moreover, CPU cooling system should provide wall temperature uniformity of the cooled component. Heat transfer coefficient, pressure drop and microprocessor wall temperature depend on microchannel geometry, thermophysical properties of refrigerant, and saturation temperature at which the process is held. This paper focuses on studying pressure drop and temperature uniformity of 40 X 40 mm microchannel evaporator with R245fa as a working fluid. The analysed heat flux density is 80 kW/m2 and the vapor quality change along the heat exchanger is 0.2. The study covers saturation temperatures ranging from 30 to 70°С and microchannel diameters varying between 0.35 and 2 mm. Results of the analysis show that the heat transfer coefficient and wall temperature uniformity increase with increasing saturation temperature and decreasing hydraulic diameter. The maximum and minimum observed non-uniformities were 2.58 and 0.69 K, respectively. Decreasing hydraulic diameter increases pressure losses in the micro-evaporator. The observed pressure drop ranged from 38 to 3753 Pa. Saturation temperature has negligible impact on pressure drop.