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2 2010

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Analiza niestabilności przemian fazowych czynników energetycznych. Część II - Badania eksperymentalne własne

Bohdal T.

Rocznik Ochrona Środowiska

2010

Tom 12

95-107

W praktyce eksploatacyjnej mają często miejsce przypadki zaburzeń typu jednostkowego spowodowane wystąpieniem gwałtownej zmiany parametrów układu [3]. Jako przyczyny tego typu zaburzeń można wymienić, między innymi, wystąpienie niedrożności zaworu lub kanału przepływowego, powstanie awarii układu zasilania parownika lub skraplacza, zamknięcie lub otwarcie zaworu odcinającego, gwałtowna zmiana obciążenia cieplnego, uszkodzenie wentylatora chłodnicy lub skraplacza chłodzonego powietrzem itd. Oprócz zaburzeń jednostkowych mogą występować w obiegu chłodniczym zaburzenia generowane w sposób periodyczny, na przykład wskutek okresowego działania elementów automatyki chłodniczej, czy dynamicznej zmiany parametrów zasilania parownika, wynikających ze zjawiska migotania termostatycznego zaworu rozprężnego [4]. W części pierwszej [6] opracowania przedstawiono wyniki analizy danych literaturowych w zakresie niestabilności przemian fazowych czynników energetycznych. W niniejszej pracy przedstawiono wyniki własnych badań eksperymentalnych procesu wrzenia proekologicznego czynnika chłodniczego podczas przepływu w kanałach rurowych w warunkach zaburzeń generowanych jednostkowo. 2010

Experimental tests were conducted of bubble boiling under the conditions of impulse disturbances. The chief goal of the examinations was the recognition, registration and description of those phenomena which occur in the unstable states of the system inside a coil pipe during boiling in the flow of the refrigerant. The experimental tests were carried out on two stages. On the first stage, the development was produced of the refrigerant's boiling in a coil pipe by opening the cut-off valve on the refrigerant's inlet to the coil pipe. The opening of the valve enabled the flow of the refrigerant through a thermostatic expansion valve (which feeds the coil pipe) and the commencement of boiling in the coil pipe. The boiling process commenced at the start of the coil pipe and displaced along its length until the desired vapour overheating was achieved on the outflow. The flow of the refrigerant in the coil pipe was preceded by the transition of the signal of an increased pressure in the form of a wave which was displacing with vp velocity. Next, together with the transition of the front of the boiling refrigerant (the boiling front) the temperature of the wall along the coil pipe decreased. Therefore, a transition of the temperature wave occurred but with VT velocity. On the second stage, a decay was produced of the boiling of the refrigerant in the coil pipe by means of closing of the cut-off valve on the inlet of the refrigerant to the coil pipe. During this process, a transition of the signal of a reduced pressure occurred as first, in the form of a wave with vp velocity in the opposite direction to the flow of the agent. At the same time, a withdrawal was observed of the "boiling area" as a result of the compressor constantly sucking off the vapour of the refrigerant. Thus, the length of the "overheating area" increased. The wall temperature increased alongside with the decay of boiling, which was manifested in the transition of the wave temperature with VT velocity. The research results demonstrated that the processes of the development and decay of boiling in a coil pipe possess similar wave natures, while some of their physical quantities are different. The wave nature of the transfer of impulse disturbances in a diphase medium with a boiling refrigerant is to be considered as a joint feature. In each case when an impulse disturbance was produced (when the valve was being opened or closed), a transition of the pressure wave occurred with vp velocity, and then the refrigerant flew with mass flux density (wp), which was manifested in a change of the temperature of the refrigerant and the channel wall. Also, the transition of the temperature wave with VT velocity was registered each time, as well. It was established on the basis of the research conducted that the velocity values of the pressure wave vp were different in the case of the development and decay of boiling (opening and closing of the cut-off valve). Thus, higher values of vp corresponded to the values of the pressure change signal L1p during the development of boiling in relation to the case of a decay of boiling (Fig. 5). The fundamental reason for this is different values of the void fraction qJ of the refrigerant in the coil pipe. As concerns the development of boiling, the signal of pressure change L1p displaced inside the coil pipe which was filled practically with the dry saturated vapour of the refrigerant (possibly with a small admixture of liquid drops: fog qJ ~ 0); at the same time, when boiling decayed, the signal of pressure change L1p was transferred in a diphase system with a variable liquid content on the way from the expansion valve to the end of the coil pipe (quality x 0.15-7-1). Data from the literature confirm a strong dependence of velocity vp from the void fraction qJ. In the cases under exami!lation, the transition velocities of the pressure wave were over two times as high for the development of boiling in comparison with the decay of boiling for the same values of pressure change L1p. The process conditions mentioned above exert an influence on the value of the substitute Reynold's number Re and on the course of the experimental dependence which describes the development and decay of boiling (3).

Analiza niestabilności przemian fazowych czynników energetycznych. Część I- Ocena stanu wiedzy

Bohdal T.

Rocznik Ochrona Środowiska

2010

Tom 12

61-93

W opracowaniu ograniczono zakres analizy do dwóch charakterystycznych przemian fazowych, to znaczy wrzenia i skraplania. Brak pełnej odwracalności między tymi przemianami nie pozwala na deterministyczny sposób ich analizy, bowiem ich realizacji towarzyszą, niekiedy jakościowo różne zjawiska. Bez względu na to, czy przemiana fazowa zachodzi w objętości, czy też w przepływie, występowanie stanów niestabilnych tłumaczy się powstawaniem warunków nierównowagi termodynamicznej podczas ich realizacji. Wspólna przyczyna wywołania niestabilności skutkuje bardzo wieloma ich odmianami. Według obecnego stanu wiedzy klasyfikacja typów niestabilności jest bardzo utrudniona, z uwagi na znaczne rozproszenie źródeł bibliograficznych oraz stosowaną terminologię. Podkreślić należy, że zdecydowana większość publikacji w literaturze dotyczy prezentacji wyników badań eksperymentalnych, natomiast od kilku lat obserwuje się systematyczny wzrost liczby publikacji zawierających analizy teoretyczne niestabilności przemian fazowych. Istnieje jednak wiele obszarów, które wymagają dalszych badań.

A principle of operation of some machines and electrical equipment consists in making use of the phase transition of energy media in a thermodynamic cycle. Under the notion of an energy medium we understand both the energy carrier and also the thermodynamic factor being subject to the transitions and taking part in the conversion of energy, being directly or indirectly involved in it. Water, refrigerants, water solutions of salt, etc are rated among energy media. The fact that the phase transitions of energy media occurring in evaporators and condensers of machines and equipment are very 'sensitive' to all the instabilities, both external and internal in character, appearing in the course of operational use could be considered as well-substantiated. In general, the instabilities in a two-phase flow could be divided into two categories. The flow is considered as static stable if the source of instabilities is inseparably tied up with parameters of the steady-state system. Due to the fact, that the instability follows from a change in value of the steady-state system parameters one can expect that it is possible to predict the onset of the instability merely knowing this steady state. The static instability mostly leads to the other working point of this system in a steady state or the periodic oscillations in its behaviour. As an example of static instabilities the instability of the first boiling crisis or a so-called Leddinegg instability could be mentioned. The instability of the first boiling crisis takes place in case of changing the heat-exchange mechanism during the process of boiling in volume. When the heat flux on the heated surface reaches the critical value the bubble boiling is replaced by the film boiling. If the thermal or hydrodynamic reactions, giving the distinct inertial effects, are the main reason for the system instabilities, then the flow is unstable, dependent on so-called dynamic instabilities. Such the instabilities in flow through the two-phase medium of a fluid-gas type could be transferred by means of two mechanisms, i.e. the acoustic waves (pressure instabilities) and the waves of mass flux density change (as an effect of the filling ratio fluctuations). These phenomena are wavy in character, but the velocities of wave propagation are very different. The acoustic waves are marked by high frequencies, whereas the wave oscillations of mass flux density change usually have much lower frequency. The acoustic instabilities result from the pressure-wave propagation in a two-phase flow. The acoustic oscillations may occur during the subcooled boiling and at the developed boiling in flow providing the critical heat flux was reached and the system was converted into the film boiling. According to Ber-gles, the acoustic oscillations may affect the course of flow. An amplitude of the acoustic pressure oscillations may reach the high value, compared the average value of the fluctuation frequency transition in two-phase media. The frequency of oscillations of this type recorded during the experimental investigations was within the scope 10 -10 000 Hz. The wave of the mass flux density change velocity is relatively low, due to the time required for the fluid particle to flow through a coil pipe. Waves of this type are observed mostly in the course of boiling in flow, when a coil pipe is supplied with the fluid heated below the temperature of saturation (subcooled boiling). These oscillations follow directly from the relation between the process of boiling and the properties of a two-phase flow. An instantaneous drop in a flow rate at the intake results in an increase of the specific enthalpy in this region. The higher enthalpy at a part of a subcooled flow leads to a local rise in temperature of the medium. It reduces the value of fluid underheating to the saturation temperature and shifts the initial point of boiling inversely to a flow of the medium. From the onset of boiling in flow, a local filling ratio and a coefficient of vapour dryness in a coil pipe were increasing. A local increase in the vapour dryness and the filling ratio led to the instability in a thickness of a thin film of fluid on a coil pipe wall. It can produce a change in the flow structure from a bubble an annular flow, which consequently causes the flow to be re-accelerated. There is an increase in a local gradient of pressure leading to the further drop in a total pressure during the two-phase flow in a coil pipe. Small fluctuations in a flow rate could be intensified until the specific amplitude of the wave of the mass flux density change was obtained. It has been confirmed by experimental investigations, which reveal the characteristic oscillation features of the wave of the mass flux density change. They pointed out that the oscillations of the wave of the mass flux density change are strongly dependent of changes in a heat flux density, a degree of cross-section reduction at the intake and the outlet of the medium in a coil pipe, a single- and two-phase frictional pressure drop in a coil pipe, super-cooling, a flow rate of the medium and changes in the system pressure.