Modelling long term pressure and temperature oscillations in the flow boiling in microchannels

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.Heat exchangers equipped with microchannels have many industrial applications. This is due to their capability for transfer of high heat fluxes. In flow boiling in microchannels three different flow regimes: steady bubbly/slug flow regime, bubbly/annular alternating flow and annular/mist alternating flow have been observed. In the present paper the model of heat and mass transfer in microchannels system has been proposed. Oscillations between quasi steady state of boiling in microchannels have been considered. Changes of two-phase flow in microchannels have been described by the relations between: heating surface temperature, vapour quality, liquid mass flux, pressure drop in microchannels and heat flux absorbed by boiling liquid. Simulation results indicate that shape of curve pressure-drop-versus-flow-rate is responsible for appearance or disappearance of heating surface temperature and pressure oscillations.dc201

Identification of flow boiling patterns in mini channel with the use of analysis of pressure oscillations

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.The dynamics of pressure fluctuations occurring in flow boiling in a minichannel with diameter of 1 mm, which consists of the heated section and glass condenser has been experimentally investigated and analysed. The recurrence plot and PCA of recurrence plot quantities have been applied. High speed digital video camera has been used to identify two-phase flow patterns in the glass condenser. The low frequency pressure fluctuations with relatively high amplitude which are caused by presence of compressible volume in the system have been observed. These fluctuations are accompanied by high frequency pressure oscillations. It has been found that during those fluctuations the dominant frequency of pressure fluctuations changes chaotically in time. Such pressure oscillations are accompanied by the flow patterns changes in the condenser. In the paper the new method of two-phase flow identification has been presented. The method is based on the analysis of dynamics of signal recorded from the pressure sensor. It has been shown that in the plots of two first components the points are grouped in the six separated clusters, which correspond to different flow patterns.dc201

New method for the identification of two-phase flow patterns in a minichannel

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.In the paper the new method for the identification of two-phase flow patterns in a minichannel has been proposed. The two-phase flow (water-air) occurring in rectangular minichannel (3x3 mm) has been analysed. It was tested whether the change of the two-phase flow pattern was accompanied by changing the dynamics of two-phase flow. The signal from laser - phototransistor sensor, recorded during the experiment, was analysed. The proposed method was based on the recurrence plot and the principal component analysis. The thirteen coefficients describing the dynamics of considered data (results of recurrence quantification analysis) have been treated as a set of variables describing the dynamics of two phase flow patterns. The principal component analysis (PCA) has been used to obtain the set of independent variables, which characterise the dynamics of system under consideration. Obtained results show that the first two components explain the 91% of input data variation. It has been shown that in the plots of two first components the points are grouped in the four separated clusters, which correspond to different flow patterns (two types of bubbly flow, and two types of slug flow - small and long slugs).cf201

Non-periodic liquid movement inside the glass nozzle during air bubbles departures in water

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.In the paper the dynamics of liquid movement inside the glass nozzle submerged in the glass tank filled by distilled water has been analyzed. Considered liquid movement accompanied the process of bubble departures from the nozzle. There has been investigated the influence of air volume flow rate on the dynamics of liquid movement. The position of liquid-air interface inside the glass nozzle has been measured using the high speed camera. Recorded videos have been analyzed using the special software allowing to obtain the time series of position of air-liquid interface inside the glass nozzle. The dynamics of such obtained time series has been analyzed using the non-linear methods. The attractor reconstruction, correlation dimension and reoccurrence plot have been applied.dc201

Chaotic air pressure fluctuations during departure of air bubbles from two neighbouring nozzles

In the experiment, bubbles were generated from two brass nozzles with inner diameters of 1.1 mm. They were submerged in the glass tank filled with distilled water. There have been measured the air pressure fluctuations and the signal from the laser-phototransistor sensor. For analysis of the pressure signal the correlation (the normalized cross - correlation exponent) and non-linear analyses have been used. It has been shown that hydrodynamic interactions between bubbles can lead to bubble departure synchronization. In this case the bubble departures become periodic. The results of calculation of correlation dimension and the largest Lyapunov exponent confirm that hydrodynamic bubble interactions observed for 4 mm spacing between nozzels cause the periodic bubble departures from two neighbouring nozzles

The chaotic bubble departures as Aresult of nonlinearity of gas compression in the plenum

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.In the paper it has been analysed the influence of: nozzle diameter and height of liquid over the nozzle on the chaotic character of bubble departures. The analysis has been conducted based on the numerical model of chaotic bubble departures proposed in the paper [3]. The following forces in the model of bubble growth have been considered: drag force, Boussinesq–Basset force, added-mass forces and the changes in the gas momentum. The influence of Boussinesq–Basset and added-mass forces on the frequency of bubble departures has been discussed. It has been shown that the intensity of appearance of the chaotic bubble departures is caused by increase in the nozzle diameter and height of liquid over the nozzle. The increase of the nozzle diameter causes the decrease of the bubble departure frequency and the depth of liquid penetration into the nozzle. It has also been shown that the increase of liquid height over the nozzle outlet modify the character of bubble departure. The increase of liquid height over the nozzle modify the frequency of bubble departure and depth of liquid penetration into the nozzle. Both forces: Boussinesq–Basset and added mass promote the bubble departures and decrease the depth of liquid penetration into the nozzle. The added mass force is greater than the Boussinesq–Basset force.dc201

Modelling of Pressure-Drop Instability in Single and Multi Microchannels System

In the paper the model of pressure-drop oscillations has been proposed. The model was based on the iterative solution to equations. The dynamics of pressure-drop oscillations in a single channel and in two neighbouring channels have been analyzed. There has been assumed that the pressure-drop oscillations in the system are caused by interactions between the heat supply system and liquid supply system. These interactions influence the heat and mass transfer inside the microchannel. Obtained results indicate that the shape of pressure drop curve has a significant influence on the system stability. When the slope of curve ? = () in the region between function extremes increases then the pressure oscillations become chaotic. In case of multichannel system the thermal interactions (occurring through the channel walls) and hydrodynamic interactions (occurring inside the common channels outlet) have been considered. Four types of two-phase flow behaviours in parallel channels have been observed depending on the intensity of interactions: alternate oscillations, consistent oscillations, periodic oscillations and completely synchronized oscillations. Obtained qualitative results have been compared with conclusions of experimental results reported by other researches. The good qualitative agreement with experimental results has been obtained

Wykrywanie aktywności użytkownika przy użyciu analizy Recurrence Plot

As computers are getting faster and disks are growing bigger more data describing user behaviour can be gathered. These data can be analysed to gain insight into user behaviour and then to detect user traits. Currently many different methods are used to analyse data — and there is still no one best method for analysing different parameters of computer systems. Computer systems behave non-linearly because they run many programs on multi-user operating systems; this causes inter-program dependencies requiring nonlinear methods to analyse gathered data. The aim of the article is to present how non-linear methods are able to detect subtle changes introduced into system by user’s actions. Number of interrupts per second was chosen as variable describing system’s behaviour. Analysis presented in this article focuses on idle system and system busy accessing hardware. Article shows that using recurrence plot can reveal similarities in behaviour of the system running different programs, and therefore can be used to detect similarities and differences in users behaviour. This article presents analysis of system activity through usage of series of recurrence plots to detect changes introduced by user actions. Analysis of lengths of horizontal and vertical lines on recurrence plots allows for describing periodicity of the system. This allows for gaining insight into behaviour of entire computing environment. Article shows that different tasks (such as network transmission, writing or reading from CD-ROM, compressing data) result in different recurrence plots; at the same time changes introduced by those tasks are hard to detect without analysis data. This means that usage of recurrence plot is crucial in detecting changes introduced in system by user’s actions.Dzięki nieustannemu wzrostowi wydajności systemów komputerowych możemy gromadzić coraz więcej danych opisujących aktywność systemów. Dane te mogą być analizowane aby zyskać wgląd w zachowanie użytkowników. Uważamy że systemy komputerowe, z racji działania w nich wielu programów które wpływają wzajemnie na siebie, mają charakter nieliniowy. Dlatego też spośród wielu istniejących metod analizy dużych zbiorów danych zdecydowaliśmy się na użycie nieliniowych metod analizy. Artykuł przedstawia wykorzystanie nieliniowych metod w celu wykrycia subtelnych zmian wprowadzonych do systemu poprzez działanie użytkownika. Analiza skupia się na porównaniu systemu bezczynnego i takiego w który działają programy uruchomione przez użytkownika. Jako zmienna najlepiej charakteryzująca system została wybrana liczba przerwań na sekundę. Artykuł przedstawia użycie wykresu recurrence plot w celu wykrycia podobieństw w zachowaniu systemu, a przez to w działaniu użytkownika. Badanie systemu wykorzystuje serię wykresów aby wykryć charakter zmian wprowadzonych przez użytkownika. Analiza długości pionowych i ukośnych linii pozwala na wykrycie okresowych zachowań komputera, a tym samym na lepsze zrozumienie procesów zachodzących w całym systemie. Pokazane zostało że różne zadania (transmisja danych przy użyciu sieci komputerowej, nagrywanie plików na dysk CD, odczyt plików z dysku DVD, kompresja danych) generuja˛ różne wykresy recurrence plot. Ponieważ zmiany stanu systemu nie znajdują odzwierciedlenia w sygnale przedstawiającym liczbę przerwań na sekundę, użycie recurrence plot jest kluczowe do wykrycia zmian spowodowanych przez użytkownika

Mini bubbles generator in mini channel

W pracy przeprowadzono analizę porównawczą rozwiązań konstrukcyjnych mieszalników fazy ciekłej z fazą gazową. Pokazano, że w zaproponowanych rozwiązaniach trudno jest sterować ilością pęcherzy w podłączonym do nich minikanale, a tym samym niemożliwe jest uzyskanie wszystkich rodzajów przepływu dwufazowego. W pracy przedstawiono własną koncepcję generatora mini i mikropęcherzy, który pozwala na formowanie oczekiwanej struktury przepływu dwufazowego w minikanale. Generator wykorzystano do wytworzenia minipęcherzy w minikanale. Urządzenie testowano dla czterech różnych przewodów o średnicach wewnętrznych: 1, 2, 4, 5 mm. We wszystkich przypadkach generator tworzył pęcherze o różnych średnicach.The gas-liquid mixers construction were compared in this paper. New conception of mini and micro bubbles generator is presented. The generator is used to form of mini bubbles in mini channel. The testing was conducted in tubes with the internal diameters of 1, 2, 4 and 5 mm. The different diameters of bubbles were formed for each generator constructions