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Список литературы
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УДК 629.76
ИССЛЕДОВАНИЕ ВЛИЯНИЯ ПАРАМЕТРОВ АКУСТИЧЕСКОГО ВОЗДЕЙСТВИЯ НА ПРОЦЕССЫ ТЕПЛО- И МАССООБМЕНА В ЁМКОСТИ, ЧАСТИЧНО ЗАПОЛНЕННОЙ ЖИДКОСТЬЮ
INVESTIGATION OF THE INFLUENCE OF ACOUSTIC PARAMETERS ON THE PROCESSES OF HEAT AND MASS TRANSFER IN A TANK PARTIALLY FILLED WITH LIQUID
В. И. Трушляков1, А. А. Новиков1, И. Ю. Лесняк1, E. Chillemi2
'Омский государственный технический университет, г. Омск, Россия 2Миланский технический университет, г. Милан, Италия
V. I. Trushlyakov1, A. A. Novikov1, I. Y. Lesnyak1, E. Chillemi2
'Omsk state technical university, Omsk, Russia 2Milan technical university, Milan, Italy
Abstract. The analysis of the existing results of studies on the process of heat and mass transfer, during the evaporation of different model liquids under conditions of ultrasonic exposure and reduced pressure, was carried out. Main results state that the best performances are obtained from kerosene, in terms of evaporation rate, but it is necessary more investigation in order to show a clear dependence on the characteristics of both the ultrasonic waves, namely frequency and amplitude, and the model liquids. The results from the mathematical modeling of the process of evaporation of a model liquid were obtained in the form of temperature variation of the liquid, at different frequencies of the acoustic waves (22 kHz, 30 kHz, 37 kHz) and at an amplitude of the oscillations of the bottom of the bath of 2 цт. Both program and methodology for conducting experiments on the evaporation of a liquid, under reduced pressure and at different frequencies of acoustic waves, have been developed. The initial data, variable parameters, assumptions and limitations for conducting experiments were determined. Piezoceramic emitters were used as emitters of ultrasonic waves, with the same frequencies of the acoustic wave and amplitude of the oscillations of the bottom of the bath used in the mathematical model.
Keywords: mathematical model, acoustic effects, heat and mass transfer, program of experiments.
DOI: 10.25206/2310-9793-2018-6-2-114-120
I. Introduction
Ultrasounds are widely used in different fields from medical to sonochemistry and food industry: in this study, attention is focused on engineering applications to check the present state of the art and to lay the foundations to exploit such knowledge for future removal of space debris from Earth orbit. Thanks to studies in weightlessness, it is known that position of liquid propellant in fuel tanks of launcher stages or satellites at end-of-life condition is random. A number of researches on evaporation of model liquids in a closed volume, simulating the allocated volume of a real fuel tank, have shown that a considerable amount of energy is necessary to evaporate a given mass of fuel. Ultrasounds and reduced pressure are thought in order to decrease the amount of energy needed. Ultrasounds are acoustic waves of which frequencies are higher than the upper limit of the human hearing range, around 16 or 20 kHz. These waves are often classified according to their frequency or power (inversely proportional). Between 20 and 100 kHz, waves are defined as "low frequency ultrasound". At high power, ultrasounds are able to modify the medium in which they are passing through. Moreover, power ultrasounds can also disrupt a fluid bulk to create cavitation or acoustic streaming, two phenomena with powerful macroscopic effects for heat transfer enhancement. Further in the frequency spectrum, above 1 MHz, is found "low power ultrasound", at a "very high frequency" that does not affect the medium of propagation. Consequently, these are specifically used for medical diagnosis or non-destructive material control. In fact, ultrasound waves diffusion into fluids is widely used in different fields like surgery, food industry and sonochemistry (the application of ultrasound to chemical reactions and processes, which mainly exploit the phenomenon of acoustic cavitation).
In the last years, different studies have been developed in order to apply the effects of ultrasonic waves on the engineering field. Lot of publications dealing with fundamental studies can be found in the literature. Most of these works are performed at the laboratory scale involving academic setups and usually using classical low frequency ultrasound, with the final goal of determining the mass of evaporated liquid, its evaporation rate and the correlation with the characteristics of the ultrasonic waves, namely amplitude and frequency. Some of them also consider the effect of decreased pressure, instead of the atmospheric one, together with the analysis of the characteristics of model liquids [1 - 4]. Usually, the most studied factor for the evaporation enhancement is the intensification of convection. Convection, like boiling, is one of the most studied modes of heat transfer under the influence of ultrasonic vibrations. Increases in heat transfer coefficients up to 25 times have been reported. Furthermore, it is evident that acoustic cavitation is the predominant phenomena for heat transfer enhancement. [5] Detailed researches, aimed to other fields of application, have been performed in order to quantify the effectiveness of material drying through ultrasounds. Final results show that the drying of the material depend on different aspects like the distribution, intensity, rate and quality of ultrasounds [6] and it is believed that the nonlinear effects in the acoustic field are the result of changes in the physical properties of the medium [7]. Computational fluid dynamics analysis has been also performed. One of them deals with the propagation of power ultrasounds through a liquid inside a cylindrical reactor. This flowing liquid not only initiates acoustic cavitation, but also fluid dynamics phenomena such as convection and acoustic streaming. Calculations were undertaken using CFD codes and a clear link between the acoustic pressure and the cavitation field has been established. [8] By the way, most of literature demonstrated to exploit piezo electric transducers in order to induce vibration in model liquids through ultrasounds. These devices are powered by a generator working at ultrasonic frequency that, considering the phenomenon from an energetic point of view, represent the best choice.
II. PROBLEM STATEMENT
The most common technique used to influence liquids with ultrasonic waves is the use of transducers. These devices consist of a vibrating surface open to the air, which exploit the so-called solid surface excitation in order to transfer vibrations from itself to the droplets through piezoelectric effect, which transform the electrical energy provided to the transducer into mechanical one to obtain the desired vibration at ultrasonic frequency. The solid surface excitation method is the most used: it allows the direct passage from the vibration surface to the model liquid of the ultrasonic waves. It is preferred because, for example, during the use of levitation based methods results very difficult to transfer enough energy from air to the droplets. For this reason, direct contact appears to be the best one to achieve resonant disintegration, which is one of the most efficient ways to enhance the rate of evaporation of a liquid. When a liquid is subjected to ultrasonic waves, it is broken up into droplets through ultrasonic atomization by means of pressure perturbations produced by the transducer. The first observed effect in this process is the micro-droplets excitation, which at their resonant frequency (and through a secondary stage of ultrasound vibration thank to is possible to reduce their size through disintegration in smaller droplets), will end up imploding. This procedure follows the D2 law, which affirms that the time for a single droplet to evaporate is proportional to the square of its diameter. For this reason, the smaller the droplets, the shorter the time to evaporate and to reduce their size.
III. THEORY
A number of researches investigated this phenomenon. Application of ultrasounds is demonstrated to break up bonds of the liquid, leading to the formation of liquid droplets, which represents the first step in the enhancement of evaporation using acoustic waves. The most relevant results dealt with the possibility that the breaking up of capillary waves on the liquid surface would generate the droplets. Otherwise, the formation is suggested to be caused by cavitation effect too. In the end, later researches suggested that the most likely mechanism that leads to nebulization was the conjunction of these two effects through the so-called Conjunction theory. These two factors are described hereafter:
1. Capillary waves occur at and travel along the interface of two fluids, where the form and propagation of the wave is mainly due to surface tension effects. Application of ultrasounds causes the capillary waves to form at the liquid-air interface. If the intensity of the ultrasound is high enough, the wave peaks will disintegrate forming small droplets. The dimension of such droplets can be determined through relations involving their characteristics and knowing the average bubble radius, it is possible to calculate the velocity of collapsing of such bubbles and so the cavitation energy.
2. Cavitation is the formation and subsequent collapse of a vapor bubble, formed by the forces acting on the liquid. Cavitation usually occurs when the liquid is subjected to rapid variations of pressure, forming cavities in the liquid into which the pressure is lower than into the fluid. These voids, when subjected to higher pressure that is transferred into them for pressure difference, implode releasing an important quantity of energy, which may lead to shock waves. Considering ultrasonic atomization, the pressure variation is caused by the oscillations generated by the acoustic field produced by the ultrasonic waves. Depending on the intensity of such oscillations, cavitation occurs, even if it has not been well understood yet which is its role in the droplets formation. It likely both helps the formation of capillary waves and is the maker of a wide range of droplet sizes. The parameters characterizing the ultrasonic cavitation are several: collapsing speed of the bubbles, pressure into them at the time of collapse, number of bubbles in the cavitation area, energy released during the explosion (Cavitation energy), etc.
Simplified droplet evaporation theories are based on the D2 law, which affirms that the time for a single droplet to evaporate is proportional to the square of its diameter: the smaller the droplets, the shorter the time to reduce their size
and to evaporate, so the higher the evaporation enhancement. Moreover, also the y ratio will be higher for smaller
droplets, leading to greater rates of heat and mass transfer during evaporation: that's why atomization process allows to accelerate also normal evaporation, using very smaller droplets. Many studies, investigating the evaporation of micro-droplets into air, assumed that the heat transfer between droplet and air is conductive rather than convective, and the mass transfer is governed by Fick's law of diffusion rather than convective diffusion. If it were possible to generate sufficient relative movement between the air and droplets, it would be possible to promote both convective heat transfer and convective mass transfer, accelerating as a consequence the evaporation process. This may be possible through Ultrasound vibrations. The use of ultrasounds would allow a further enhancement of normal heat and mass transfer during evaporation, because it would generate great relative movement between the air and the model liquid droplets. Anyway, it is necessary to consider also that on different droplet sizes the effect of ultrasound will be different. In fact, convec-tive enhancement is lower for smaller droplets, since they tend to move with air oscillations and Reynold's number is very low. So, convective effect is larger for larger droplets, together with the fact that lower frequencies are probably better for the larger droplets as the amplitude of air molecule vibration is larger. The phenomena that ultrasound vibrations induce and that allow the heat and mass (and consequent evaporation) enhancement are the followings:
1. Acoustic Resonance, or droplets disintegration through acoustic resonance, is a phenomenon that consist in the excitement of the droplets at one of their resonant frequencies with subsequent break-up of the them into air. The effectiveness of Acoustic Resonance will depend on a number of factors:
a.Droplet size and size range. Droplet size range depend on the type of device used to generate the droplets, because depending on the nebulizer the generated droplets will be of different sizes. Specifically for this reason, it is not possible to excite all droplets, because at different sizes correspond different resonant frequency.
b. Method of excitation. The use of a suitable ultrasound generator that will operate close to the resonant frequency and powerful enough to induce large oscillations in the droplet. Like a piezoelectric generator, which is able to excite just a small portion of the droplets at their resonant frequency, having each transducer its working range.
c.Droplet resonant frequencies and transmission energy. Droplets resonant frequencies strongly vary with droplet diameter D3, so the smaller the droplet, the higher the frequency.
2. Acoustic Squeezing, is the trend of a droplet to deform in shape, to flatten out and at some critical point to self-disintegrate into many smaller droplets. This has been studied to happen just for mm sized droplets, acoustically levitated in air. Using an ultrasound levitator an ultrasound standing wave is produced: it generates an upward (or levitation) force on the droplet if it is placed close to one of the pressure nodes of the standing wave. The levitation force is balanced from the weight force of the droplet that will be kept stationary, in air, in the device. The action of the acoustic levitation force in conjunction with the opposing weight force generates stresses that cause the droplet to deform in shape and if the amplitude of the levitation signal is increased the droplet is observed to change shape, flatten out and then self-disintegrate.
3. Acoustic Streaming flow (forced air current), can be created above a surface, considering levitating spherical particles and making them spin around themselves, generating localized gas flows around the particles. This occurs as a result of non-linear interactions between oscillating gas and particle, usually when Strouhal number is small, which means that there are large droplets and small air particles amplitude. This effect may increase the convection heat transfer and mass transfer process for the droplet proportionally to the stream velocity. This is another interesting example of how acoustic streaming can modify heat transfer coefficients. The typical order of magnitude of acoustic Streaming velocity is usually a few cm per second (1 to 100), but it also appears to vary slightly with ultrasonic power and frequency.
4. Acoustic Cavitation, is the major phenomenon for heat transfer enhancement that may arise from the propagation of ultrasonic waves into a liquid. As already stated it is the formation, growth, oscillation and powerful collapse of gas bubbles into a liquid. Defining acoustic cavitation, one must also describe precisely the experimental conditions at which it occurs (gas dissolution, temperature, pressure, etc.), because it depends on several parameters. When the local pressure is decreased sufficiently below the vapor pressure during the rarefaction period of the sound wave, the static pressure and the cohesive forces are overcome and gas bubbles are formed and later they will collapse violently. Two types of acoustic cavitation exist: stable and transient. Stable Cavitation occurs when bubbles oscillate around an equilibrium size. Transient one when they exist for less than one cycle. Important facts to underline are the fact that the implosion of a vaporous cavity is more violent than a gas-filled one, because when vapor is turned into liquid, there is no residual gas damping the collapse of the bubble. A bubble implosion near a solid-liquid interface disrupts thermal and velocity boundary layers, reducing thermal resistance and creating microturbulence. Also for this reason, acoustic cavitation is believed to be the major effect of ultrasonic heat transfer enhancement.
IV. RESULTS
In the following the preliminary results of a new mathematical model, dealing with the evaporation of different model liquids (distilled water, alcohol mixture, kerosene) with an oscillation amplitude of 2 ^m of the bottom of the bath under conditions of a constantly decreasing pressure in the VC from 101 kPa to 0.8 kPa, are presented. These results represent the first step in the development of a new mathematical model. Some assumptions and limitations have been considered in order to implement it: liquid in the form of mirror, evaporated mass as summation of time integrals, no presence of bubble or fluid motion otherwise a computational fluid dynamic study should be performed, but this is not the aim of this study. In Figures 1-3, the trends of temperatures are shown in function of time: the considered time is not the total time of the experiment, because the model presented some problems in the moment the temperatures becomes lower than zero and instability phenomena present themselves. This is probably due to the further assumption of initially constant vapor pressure: the fact this parameter does not change probably leads to alterations in the model, not only in terms of instability, but also in the modification of the results obtained in previous models. In fact, the trend of kerosene temperature was the one with best performances, whereas now it is the worst one. The next version of the model will aim to the resolution of these problems. In Figure 5 the trend of the decreased pressure during the experiment is shown.
f = 22 kHz, A = 2 Lim
30 -1-1-г--т^-г
,10 -1-1-1-:-1-1-
0 50 100 150 200 250 300 350
Time [s]
Fig. 1. Temperature variation for amplitude of 2 ^m and frequency of 22 kHz
f = 37 kHz, А = 2 jjm
40 -1-1-1---г1—
.10 -1-1-1-1-1-1-
0 50 100 150 200 250 300 350
Time [s]
Fig. 3. Temperature variation for amplitude of 2 ^m and frequency of 37 kHz
хЮ4 Pressure iri VC
o-1-1-1-1-1-' ~ —- —1
0 50 100 150 200 250 300 350
Time [s]
Fig. 4. Decreased pressure trend in the VC
V. CONCLUSIONS
The possible developments that future researches may try to focus on are different. The most remarkable are briefly outlined hereafter:
1. Repeat the experimental campaign at different frequencies, thus obtaining more complete data and a wider view of the dependence between the characteristics of the ultrasound waves and the evaporation rate.
2. The use of a bigger bath, in order to perform studies for greater quantities of model liquids and with amplitudes higher than 3 ^m.
3. Verify that the proposed model is optimal even under conditions of increasing pressure and not only at reduced and atmospheric pressure.
4. Improvement of the quality of measurements, especially temperature using infrared sensors that, unlike thermocouples, are not influenced by oscillations induced by ultrasounds as there is no contact between elements.
5. A CFD model will be very interesting and useful to develop, because it will help on the study by making parametric simulation considering also the presence of bubbles and the fluid motion.
6. Positioning a camera inside a tank in micro gravity to study the patterns of the liquid motion when in a weightlessness condition, thus allowing to obtain key information for the optimization of the gasification system against the known positioning of the residues.
Acknowledgments
This work was supported by the Russian Federation Ministry of Education and Science within the public contract with subordinate educational organizations, the project "Improvement of environmental safety and economic efficiency of launch-vehicles with cruising liquid rocket engines", application No. 9.1023.2017/PCh.
References
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3. Trushlyakov V. I., Lesnyak I. Y. , Galfetti L. An experimental investigation of convective heat transfer at evaporation of kerosene and water in the closed volume // Thermophysics and Aeromechanics. 2017. Vol. 24, no. 5. P. 771-781.
4. Semenov A. A., Feoktistov D. V., Zaitsev D. V., Kuznetsov G. V. , Kabov O. A. Experimental investigation of liquid drop evaporation on a heated solid surface // Thermophysics and Aeromechanics. 2015. Vol. 22. P. 771 -774.
5. Legay M., Gondrexon N., Le Person S. , Boldo P., Botemps A. Enhancement of Heat Transfer by Ultrasound: review and recent advances // Hindawi Publishing Corporation International Journal of Chemical Engineering. 2011. P. 1-17.
6. Khmelev V., Shalunov A. V., Barsukov R. V. , Tsyganok S. N., Lebedev A. N. N. Research on ultrasonic drying efficiency // Journal of Zhejiang University-Science A. 2011. Vol. 12. P. 247-254.
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УДК 629.76
ИССЛЕДОВАНИЕ ПРОЦЕССА ИСПАРЕНИЯ ЖИДКОСТИ В ВАКУУМНОЙ КАМЕРЕ ПРИ УЛЬТРАЗВУКОВОМ ВОЗДЕЙСТВИИ
INVESTIGATION OF THE LIQUID EVAPORATION PROCESS IN A VACUUM CHAMBER
WITH ULTRASOUND IMPACT
В. И. Трушляков1, А. А. Новиков1, И. Ю. Лесняк1, C. Spada2
'Омский государственный технический университет, г. Омск, Россия 2Миланский технический университет, г. Милан, Италия
V. I. Trushlyakov1, A. A. Novikov1, I. Y. Lesnyak1, C. Spada2 'Omsk state technical university, Omsk, Russia 2Milan technical university, Milan, Italy
Аннотация. Проведены экспериментальные исследования процесса испарения модельных жидкостей (дистиллированная вода, спиртовая смесь, керосин ТС-1) в замкнутом объём при параметрическом ультразвуковом воздействии (УЗВ) на жидкость в условиях пониженного давления с целью использования полученных результатов для разработки методики проектирования системы испарения неиспользуемых жидких остатков ракетного топлива в баках ракет и магистралях отработавших ступеней ракет -носителей (РН). Разработана программа и методика проведения экспериментов, создан экспериментальный стенд, реализована программа экспериментов. Определены исходные данные, варьируемые параметры, допущения и ограничения. В качестве излучателя ультразвуковых волн использовался пьезоке-рамический излучатель (ПКИ) с частотой 25 кГц и с возможностью регулирования амплитуды колебаний дна ванны, на которой располагалась модельная жидкость от 1 до 3 мкм. Получены экспериментальные зависимости изменения температур модельных жидкостей и газа в вакуумной камере (ВК) при УЗВ в условиях пониженного давления (до 0.2 кПа). Определены массы испарившихся модельных жидкостей и скорость испарения. Проведен сравнительный анализ полученных экспериментальных данных для различных модельных жидкостей, который показал, что скорость испарения увеличивается при увеличении амплитуды колебаний дна ванны, при этом наибольшая скорость испарения при одних и тех же условиях у керосина марки ТС-1.
Ключевые слова: вакуумное воздействие, ультразвук, тепло- и массообмен, эксперимент.
DOI: 10.25206/2310-9793-2018-6-2-120-126
I. Введение
В настоящее время актуальной является задача уменьшения техногенного воздействия РН на окружающую среду. Это связано с тем, что в топливных баках РН после выключения маршевого жидкостного ракетного двигателя остатки топлива могут составлять до 3 % от начальных запасов топлива [1, 2], что приводит к риску взрыва отработавших ступеней на орбитах и к пожарам в результате удара отработавшей ступени о поверхность Земли в районе падения.
