Wyniki wyszukiwania - Biblioteka Nauki

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Ekologiczno-energetyczne aspekty badań eksperymentalnych procesu skraplania czynników chłodniczych w minikanałach rurowych

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Bohdal T. , Charun H. , Kuczyński W.

Rocznik Ochrona Środowiska

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2009

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tom Tom 11

143-162

EN

In the era of miniaturization of power equipment, application of compact systems is a very advantageous solution. Compact heat exchangers which are used in refrigerating systems do not only transfer a large density of the heat flux but also perform environment-friendly functions; a refrigerant occupies a small volume, and when there is no leak tightness, there is little threat to the natural environment. Application of heat exchangers with flux of single-phase mediums (water or air) is not sufficient any more. That is the possibility of utilization of phase changes of refrigerants (boiling and condensation) realized during flux in channels with small diameter. Conception of constructing so-called compact heat exchangers (evaporators and condensers) which are a part of cooling installation was created that way. However the designers of compact heat exchangers have to face today difficult problem of choice sufficient computational procedures in the range of the heat exchange and flow resistances in channels with hydraulic diameter smaller than 3 mm [13]. Wellknown in literature procedures, tested for conventional channels (d > 3 mm) theoretically and experimentally, do not have to and in most cases are not suitable for micro- and mini-channels. This causes need to find and point the best computational formulae to be used in compact exchangers. It should be underlined, that number publications, presenting description of heat exchange and flow resistances during boiling in the mini- and micro-channels is much larger, than for the case of condensation in such channels. Up to date knowledge about conventional channel proves that processes of boiling and condensation in flow cannot be treated as "symmetrical" phenomena. Also mechanism of energy transport and momentum in processes of boiling and condensation in flow both in conventional channels and in mini-channels. The basic part of the paper is constituted by the presentation of the author's own experimental research on heat exchange in the condensation process of R134 a refrigerant in pipe mini-channels with an internal diameter of 1.6÷3.3 mm. On the basis of a comparative analysis, the usefulness of Akers and Shah's correlations was stated for the determination of the heat transfer coefficient in this process. The range of divergence of experiment results and calculations according to Ackers correlation was significantly smaller and that is why this correlation may be recommended for designers. Experiments were conducted in the following range of parameters: saturation temperature tk = 35÷40°C, density of mass stream (wp) = 200÷600 kg/(m2źs) and density of heat stream q = 5÷50 kW/m2

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A conductometric method for determining of the upward gas-liquid-liquid three-phase flow

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Płaczek M. , Pietrzak M. , Witczak S.

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tom Vol. 39, no. 3

97--110

EN

This paper reports the results of research involving observations of flow patterns during air-oil-water three-phase flow through a vertical pipe with an internal diameter of 0.03 m and a length of 3 m. The conductometric method based on the measurement of electrical conductivity of the gas-liquid-liquid system was used to evaluate the flow patterns. In the studies, a set of eight probes spaced concentrically in two tube sections (four probes per each) with a spacing of 0.015 m were used. The paper presents a theoretical description of the test method and the analysis of the measurement results for air-oil-water multiphase flow system. Results of this study indicate that the developed method of characterizing the voltage of the gas-liquid-liquid system can be an important tool supporting other methods to identify flow patterns, including visual observation.

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Badanie niestabilności skraplania proekologicznego czynnika chłodniczego w minikanałach rurowych

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Bohdal T. , Charun H. , Kuczyński W.

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tom Tom 13

377-391

PL

Celem niniejszych badań było rozpoznanie tego problemu i stwierdzenie, czy zjawiska rozprzestrzeniania się niestabilności w mini-kanałach podczas skraplania mogą być porównywalne do tych, które są znane dla kanałów konwencjonalnych. Należy wskazać, że w skraplaczu chłodniczym pracującym w warunkach eksploatacyjnych zaburzenia mogą być generowane impulsowo (skokowo) [5] lub periodycznie [6]. Przykładowo, skokowe zamknięcie lub otwarcie zaworu na zasilaniu wężownicy skraplacza wywołuje typowe zaburzenia impulsowe, które mogą wywołać niestabilności związane np. z rozwojem lub zanikiem skraplania czynnika chłodniczego. W dalszej części opracowania przedstawiono wyniki badań eksperymentalnych wpływu zaburzeń generowanych periodycznie na proces skraplania czynnika chłodniczego w mini-kanałach rurowych skraplaczy kompaktowych.

EN

Under real conditions, the work of conventional and compact refrigerating machines and devices (with exchangers which construction is based on the use of mini-channels or micro-channels) is subject to a lot of disturbances which are the result of both external and internal impacts. Such phenomena occur very frequently in the operational practice. The cause of disturbances may be the start-up or the stoppage of the installation, a change of the heat load of the exchangers, opening or closing of the valves, the work of automatic control elements etc. Considering the above, investigations on the condensation of the refrigerant in the flow in pipe mini-channels in the conditions of external disturbances were conducted. The development or decay of condensation was produced through a unitary change of the parameters of the two-phase system, e.g. the pressure and the mass flux density in the pipe channel. It was confirmed that two-phase media exhibit wave properties, while the disturbances produced displaced with a finite velocity. It was demonstrated that in contrast with conventional channels, the value of the boundary velocity of the displacement of pressure disturbances is obtained in mini-channels with substantially lower frequencies of their generation. Investigations were conducted with the use of a compressor refrigerating cycle with the environmentally friendly R404A refrigerant.