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Experimental investigation on flow patterns of RP-3 kerosene under sub-critical and supercritical pressures
Acta Astronautica 94 (2014) 834–842
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Experimental investigation on flow patterns of RP-3 kerosene under sub-critical and supercritical pressures Ning Wang, Jin Zhou n, Yu Pan, Hui Wang Science and Technology on Scramjet Laboratory, National University of Defense Technology, Hunan, Changsha 410073, China
a r t i c l e in f o
abstract
Article history: Received 1 August 2013 Received in revised form 7 October 2013 Accepted 11 October 2013 Available online 24 October 2013
Active cooling with endothermic hydrocarbon fuel is proved to be one of the most promising approaches to solve the thermal problem for hypersonic aircraft such as scramjet. The flow patterns of two-phase flow inside the cooling channels have a great influence on the heat transfer characteristics. In this study, phase transition processes of RP-3 kerosene flowing inside a square quartz-glass tube were experimentally investigated. Three distinct phase transition phenomena (liquid–gas two phase flow under subcritical pressures, critical opalescence under critical pressure, and corrugation under supercritical pressures) were identified. The conventional flow patterns of liquid–gas two phase flow, namely bubble flow, slug flow, churn flow and annular flow are observed under sub-critical pressures. Dense bubble flow and dispersed flow are recognized when pressure is increased towards the critical pressure whilst slug flow, churn flow and annular flow disappear. Under critical pressure, the opalescence phenomenon is observed. Under supercritical pressures, no conventional phase transition characteristics, such as bubbles are observed. But some kind of corrugation appears when RP-3 transfers from liquid to supercritical. The refraction index variation caused by sharp density gradient near the critical temperature is thought to be responsible for this corrugation. Crown Copyright & 2013 Published by Elsevier Ltd. on behalf of IAA. All rights reserved.
Keywords: Flow patterns Two-phase flow Kerosene Supercritical Active cooling
1. Introduction Efficient cooling is one of the main difficulties for researches of advanced hypersonic aircraft such as scramjets. The large heat release from combustion as well as high enthalpy of the inflow leads to an extremely rigorous thermal environment. Previous studies [1,2] indicate that when the supersonic aircraft travels at Mach 2.5, the heat flux in combustor could be as high as 1 MW/m2. When Mach number reaches 6, the wall temperature would exceed 3000 K. This is far beyond the limit of most materials. Active cooling with hydrocarbon fuels such as RP-3 is considered to be most promising to solve this problem. Besides physical heat sink, additional chemical
n
Corresponding author. Tel.: þ 86 15973121346. E-mail address: [email protected] (J. Zhou).
heat sink is obtained from cracking reactions at high temperatures and thus the total cooling capacity is improved [3]. A lot of demonstrations about the necessity and feasibility of hydrocarbons applied in active cooling of hypersonic aircraft have been conducted [4–6]. Researches on hydrocarbons applied in active cooling of hypersonic aircraft are mainly focusing on five aspects [7]: cooling capacity and heat sink measurement, thermal and catalytic cracking, coking suppression, heat transfer characteristics, and injection, mixing, ignition and combustion performances. Few results are reported on the flow patterns of hydrocarbons flowing inside the cooling channels. However, depending on the pressure inside the cooling channel, different phase transition processes (liquid–gas transition, liquid–supercritical transition), and different flow patterns may happen. Without knowing the flow patterns inside the cooling channel, correct hydraulic/ thermal designs for scramjet cannot be conducted. In fact,
0094-5765/$ - see front matter Crown Copyright & 2013 Published by Elsevier Ltd. on behalf of IAA. All rights reserved. http://dx.doi.org/10.1016/j.actaastro.2013.10.008
N. Wang et al. / Acta Astronautica 94 (2014) 834–842
the pressure drops and heat transfer coefficients are strongly related to the flow patterns. Thus, study on the flow patterns inside the cooling channel is of great importance for scramjet research. Extensive theoretical and experimental work has been carried out in the field of two-phase flow patterns. Based on the size of flow channels, it could be divided into twophase flow in macroscale and microscale channels. So far, the distinction of microscale channels is still in dispute. A threshold diameter of 3 mm is adopted by many researchers. According to this criterion, the two-phase flow in cooling channels of scramjet is that in microscale channels. Due to the significant difference of transport phenomena in microscale channels as compared to conventional size channels, the flow patterns may be much different from those in macroscale channels. For example, when the channel diameter is reduced, the effect of flow orientation tends to disappear. New flow patterns, such as wedge flow [8] and liquid lump flow [9], appear. Slug flow has much longer bubbles [10,11]. Based on thermal conditions, it includes adiabatic and diabatic two-phase flows. Obviously, the two-phase flow inside cooling channels of scramjet is diabatic one which absorbs heat from the combustor. As compared to adiabatic two-phase flows, many parameters, such as local heat flux, pressure and temperature, have a great influence on diabatic flow patterns. Additionally, RP-3 kerosene is a kind of mixture which consists of hundreds of species. The initial boiling temperature for each species may differ a lot. Furthermore, at high temperatures (above approximately 800 K), cracking reactions begin to happen. All of these make the diabatic two-phase flow of RP3 inside cooling channels more complicated. Cheng et al. [12] have given a comprehensive review of the flow patterns of gas–liquid two-phase flow. As discussed in their paper, although studies on microscale channels have increased greatly in recent years, it is still in its infancy stage as compared to the vast researches on two-phase flow in macroscale channels. Additionally, most of the numerous flow-pattern maps have been developed for adiabatic conditions. In principle, adiabatic two-phase flow maps are not applicable to diabatic conditions. Fig. 1 shows a schematic representation of a horizontal tubular channel heated by a uniform heat flux and fed with subcooled liquid [13,14]. Bubbly flow, plug flow, annular flow and mist flow appear in sequence along the tube. However, the flow patterns may vary a lot when the experimental conditions such as heat flux, pressure are changed. Recently, Nigmatulin et al. have developed
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a mathematical model that determines the propagation of acoustic waves in polydisperse bubble liquids [15], and established a model of transcillatory heat transfer induced by gas bubbles buoyant in liquid [16]. Work by Lahey et al. [17] shows that thermonuclear fusion has held out the promise of cheap, clean, and virtually limitless energy. And in this process, diabatic flow would be confronted. Ivashnyov et al. [18] have proposed a special methodology to describe boiling flows under hot water depressurization. The difference in phase velocities was taken into account. Another issue worth noting is that, the physical properties of fluids have a great influence on the flow patterns. Different fluids may present much different flow patterns, even at the same conditions. However, most researches on the flow patterns in microscale channels used air–water or water–nitrogen as the working fluids. Some researchers used refrigerants such as R134a and R123. Few results are reported on the flow patterns of hydrocarbon fuels inside microscale channels. In this paper, the flow patterns of a commonly used hydrocarbon fuel in China, RP-3 were experimentally investigated. The critical pressure and temperature of it are approximately 2.3 MPa, 646 K respectively. Three distinct phase transition processes were identified for RP-3 flowing inside a cooling channel under different pressures. Furthermore, some analysis was made to conclude the different phase transition phenomena. 2. Experimental setup A schematic diagram of the experimental setup is shown in Fig. 2. The fuel is stored in a tank and pressured by a nitrogen gas source with pressure of 8 MPa. The mass flow rate of the fuel is adjusted by a needle valve (Model: 33s4F, Xiongchuan) and measured by a Coriolis-force flow meter (Model: DMF-1-1-A, SiMite) with accuracy of 70.2%. An electrical heater with maximum power of 28 kW is used to heat a stainless tube (1Cr18Ni9Ti, 1.6 m length) and the kerosene flowing inside it. Then the heated kerosene flows through a square quartz glass tube with an inner geometry of 3 3 mm2 and length of 10 cm. Compared with circular glass tubes, image distortion caused by refraction at the circular surface could be avoided. The glass tube is mounted between two special designed connecting bases, see Fig. 3. Graphite gaskets are laid at both sides in order to prevent breakage caused by instantaneous pressure shock. Two “O” rings are used for sealing. More information about the connecting bases can be found in [19].
Fig. 1. Schematic of flow patterns for flow boiling in a horizontal tube [12].
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Fig. 2. Sketch of the experimental system.
Fig. 3. Sketch of the flow visualization module.
A high speed camera (Model: MotionBLITZ EoSens Cube/ mini, Mikrotron GmbH) is laid at one side of the glass tube while a lamp (Model: QH-1300, Changcheng) is laid at the other side to provide sufficient illumination. Temperatures at the inlet and outlet of the heating tube are measured by K-type armored thermocouples with accuracy of 70.75%. Pressure at the outlet of the tube is measured by a pressure gage transducer (Model: MPM480, Maike) with accuracy of 70.1%. Then the fuel is cooled by a cooling system and collected into a tank. A back-pressure valve (Model: 9134F1, Xiongchuan) is laid between the cooling system and the collecting tank to adjust the pressure inside the tube.
A typical aviation kerosene RP-3 is used in this work. It is equivalent to Jet A-1. Its density under 2.34 Mpa varies from approximately 788.773 kg/m3 to 55.907 kg/m3 when temperature changes from 295.63 K to 786.82 K [20]. And its boiling range is from 423 K to 553 K. Composition analysis [20] shows that RP-3 consists of 52.44% alkanes, 7.64% alkenes, 18.53% benzenes, 15.54% cycloalkanes, 4.39% naphthalenes, and 1.46% other; the detailed composition of RP-3 can be found in [20]. The mass flow rate of all test cases is fixed to 2 g/s. The velocity of RP3 in the square glass tube under 2.3 Mpa varies from 0.35 m/s to 2.16 m/s when temperature
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Fig. 4. Flow patterns of RP-3 at different temperatures under 0.3 MPa.
changes from 455 K to 671 K. Corresponding Reynolds number varies from approximately 3227 to 37691. 3. Results and discussion 3.1. Flow patterns under sub-critical pressures Fig. 4 shows the flow patterns of RP-3 under 0.3 MPa in the horizontal quartz tube. RP-3 flows from right side of the tube to left side. The inlet and outlet temperatures are labeled on the figure. When temperature is low, the kerosene is liquid, and the flow is transparent and homogeneous, as Fig. 4(a) shows. As the inlet temperature increases to 463 K, the initial boil begins to happen, and a number of discrete bubbles appear in the liquid mainstream. As a result of surface tension, the bubbles are spherical. And they all float at the upper side of the mainstream because of the gravity. In Fig. 4(c), the inlet temperature increases further to 501 K and slug flow is formed. The volume of the bubbles is obviously larger than that in bubble flow, and the bubbles are no longer spherical but like slugs. And the head of the slugs is cuspate while the tail of the slugs is obtuse. Besides these large slugs, there are small slugs which have a relatively cuspate tail and look like a hemisphere. In addition, there are smaller bubbles following the large slugs. When the inlet temperature increases to 512 K, the slug flow begins to transform to churn flow, see Fig. 4(d). The interface between the slug and the liquid mainstream, especially the lower interface begins to fluctuate, as marked by the red ellipse. Besides, the tail of the slug begins to break, thus
generating more small bubbles following behind. When the inlet temperature is 515 K, churn flow is formed. Fig. 4(e) shows the primary stage of churn flow. Nearly all slugs have connected into a whole, thus forming a gas core in the middle of the tube. However, the gas core is not so stable, and a rupture may appear. At the rupture point, a lot of discrete bubbles are generated. Because of the gravity, the gas core exists in the upper part of the tube. And the interface between the gas core and the liquid mainstream is relatively smooth. When churn flow has fully developed, as Fig. 4(f) shows, a consecutive gas core is formed. And neither rupture nor bubbles are observed. The interface between the gas and the liquid presents intensive fluctuation. When the inlet temperature is further increased to 531 K, annular flow is formed, as Fig. 4(g) shows. In this flow pattern, the majority of the tube is filled by gas. Liquid only exists in the near wall region. Moreover, the interface fluctuation is much smaller than that in churn flow. And liquid droplets keep falling off the liquid layer, as marked in the red circle. When the inlet temperature is 546 K, the liquid layer becomes even smaller, and this is the end stage of annular flow, as Fig. 4(h) shows. There is almost all gas in the tube. The liquid exists in a rather small region. Similarly, it can be seen that liquid droplets fall off the liquid layer, but the quantity of the droplets becomes much smaller. After this stage, the boil process would be completed, see Fig. 4(i). There is no liquid any more and the flow becomes singlephase flow again. Fig. 5 illustrates the flow patterns under 0.8 MPa and 1.3 MPa, which are far below the critical pressure of RP-3 (2.3 MPa). The typical four flow patterns of two phase flow,
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Fig. 5. Flow patterns of RP-3 under 0.8 MPa and 1.3 MPa.
namely bubble flow, slug flow, churn flow and annular flow are observed under 0.8 MPa. However, the temperature intervals from one pattern to another are much shorter, which means that, the flow pattern transforms more quickly. Under 1.3 MPa, no slug flow is observed. When temperature is increased, more bubbles are generated while no gas slug can be found, see Fig. 5(f). When temperature increases further, the flow directly transforms to churn flow, as Fig. 5(g) shows. Then the typical churn flow and annular flow appear in sequence as temperature increases, see Fig. 5(h) and (i). Fig. 6 illustrates the flow patterns under 1.8 MPa and 2.2 MPa, which are close to the critical pressure of RP-3 (2.3 MPa). The initial boil temperature is markedly elevated. Similarly, the flow transforms quickly from one pattern to another as it does under 0.8 MPa and 1.3 MPa. It can be seen from the figure that, under 1.8 MPa, bubbles firstly appear at 618 K. As the inlet temperature increases to 624 K, the tube is nearly filled with abundant bubbles. We consider it as a new flow pattern and call it dense bubble flow. It is totally different from churn flow. In dense bubble flow, the bubbles are independent and discrete. Although the tube is full of bubbles, they do not connect to each other and there is not a consecutive gas core in the tube. When temperature becomes higher, the majority region of the tube is gas, and liquid is separated into droplets, which distribute in the gas mainstream. This is somewhat similar to bubble flow, in which discrete bubbles distribute in the liquid mainstream. It is the so-called dispersed flow. It is also different from annular flow, in which liquid only exists in the near wall region and droplets fall off this annular region. Under 2.2 MPa, the
flow also undergoes these three patterns, namely bubble flow, dense bubble flow and dispersed flow, see Fig. 6(d)–(f). And the characteristics of the new patterns are even more obvious than that under 1.8 MPa. Comparing the flow patterns of RP3 under subcritical pressures with Fig. 1, it is found that bubble flow, plug flow and annular flow, which are observed in the previous literature, also appear in the boiling flow of RP3 under a low pressure (0.3 Mpa and 0.8 MPa). Mist flow was not observed. Churn flow, which was a common flow pattern but not observed in Fig. 1, appears in the boiling of RP3 at a wide pressure range (from 0.3 Mpa to 1.3 MPa). When pressure is elevated (1.8 Mpa and 2.2 MPa), plug flow and annular flow cannot be observed. Instead, dense bubble flow and dispersed flow appear. Fig. 7 shows the difference between churn flow and dense bubble flow, as well as difference between dispersedannular flow and dispersed flow. Liquid is marked by red while gas is marked by green. In churn flow, there is a consecutive gas core existing in the middle of the tube, and liquid is pressed to the near wall region. However, in dense bubble flow, gas exists as discrete dense bubbles. Although the liquid region is also small, it is not limited to the near wall region but exists everywhere. In dispersed-annular flow, the annular region of liquid is further reduced, and there are droplets falling off the liquid layer. In dispersed flow, although liquid droplets can be observed, there is not a liquid layer. The liquid droplets are formed from separation of the liquid region by growing bubbles. Fig. 8 illustrates the transition of flow pattern under sub-critical pressures. Each color stands for a kind of flow pattern. As can be seen, when pressure is increased, some
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Fig. 6. Flow patterns of RP-3 under 1.8 MPa and 2.2 MPa.
Fig. 7. Sketch of comparison of dense bubble flow and dispersed flow with churn flow and annular flow. (For interpretation of the references to color in the text, the reader is referred to the web version of this article).
One possible reason is that the pressure difference along the surface of a large consecutive gas region would be high enough to tear the gas region into smaller bubbles under higher pressures. Besides, the two phase flow region is becoming smaller as pressure increases, and the bubbles become small and dense. It can be inferred that when pressure is higher than a certain value, the bubbles would be too small to be observed and the traditional phase change would disappear. That is the critical pressure.
3.2. Flow patterns under critical pressure
Fig. 8. Flow pattern transition of RP-3 under sub-critical pressures. (For interpretation of the references to color in the text, the reader is referred to the web version of this article).
kinds of flow pattern disappear in sequence, such as slug flow, churn flow and annular flow, and some flow patterns appear, such as dense bubble flow and dispersed flow. It seems that it is difficult to form a large consecutive gas region, such as gas slug or gas core in churn flow under relatively high pressures. Thus slug flow, churn flow and annular flow are not observed under high pressures.
The critical pressure of RP-3 can be determined by many methods. Sun et al. [21] developed a short residence time flow method and determined the critical pressure of RP-3 to be 2.39 MPa. Fan et al. [22] considered it to be 2.4 MPa according to principle of the extended corresponding states (ECS). In this paper, the critical pressure of RP-3 is determined by flow pattern variations. As aforementioned, phase change would always happen under sub-critical pressures, thus bubbles could be observed. However, as pressure approaches the critical pressure, bubbles would be smaller and smaller and disappear at last. According to the disappearance of bubbles, the critical pressure of RP-3 is determined to be 2.3 MPa.
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Fig. 9. Flow patterns of RP-3 under 2.3 MPa.
Fig. 9 shows the flow pattern variations with temperature under critical pressure. A thermocouple was put inside the quartz tube. The temperature labeled on the figure was measured by this thermocouple. At a relatively low temperature (455 K), the flow looks like laminar flow and is homogeneous. As temperature increases to 624 K, some kind of corrugation appears in the flow. When temperature comes to 646 K, the corrugation is intensified. And more importantly, a unique phenomenon of opalescence is observed. It cannot be considered as bubbles because the interface between gas and liquid cannot be distinguished even when the image zooms in. At 652 K, the opalescence region spreads nearly to the whole area of the tube. At 656 K, the opalescence begins to vanish. And some area of the tube becomes transparent again. When temperature increases to 671 K, no opalescence could be observed anymore and the total area of the flow turns transparent and homogeneous again. The opalescence phenomenon seems to appear only under critical pressure and near the critical temperature. Flow pattern images under supercritical pressures showed that no opalescence is observed, although no phase change happens either. The critical opalescence phenomenon could be explained by Einstein's fluctuation theory [23]. The intensity of light scattered from volume Vcan be expressed as follows: I 4π 2 V ½ðc=μÞð∂μ=∂cÞ2 ¼ 2 4 I0 R λ c½ð∂=∂cÞðP=kTÞ
ð1Þ
Where I0 is the intensity of the primary light; λ stands for the wavelength; c represents the concentration; m is the index of refraction; P is the pressure; k is Boltzmann's constant, and T is the temperature. Debye et al. [24] modified the expression as the following formula: " # ! 2 1 T c ΔT 8π 2 l ΔT 16π 2 r 2 2θ ¼ þ þ ð2Þ sin I T c 3 λ2 2 CT T c 3 λ2 Where Tc is the critical temperature; C is a constant characteristic for the system; l is the interaction range; λ is the wavelength; r is the radius of gyration; θ is the scattering angle. ΔTis defined as follows: ΔT ¼ T T c
ð3Þ
Then we know from Eq. (2) that IA
1 ΔT
ð4Þ
Thus, the intensity of the light scattered would reach maximum when temperature approaches the critical temperature. 3.3. Flow patterns under supercritical pressures Fig. 10 illustrates the variation of flow pattern under 3 MPa, which is higher than the critical pressure. The temperature varies from 606 K to 708 K. As the critical temperature of RP-3 is approximately 646 K, the flow transforms from pressed liquid state to supercritical state. It can be seen from the figure that no bubbles are observed during the transformation. When the temperature is 606 K, the flow seems laminar and homogeneous. As the temperature increases to 633 K, corrugation appears. When the temperature further increases, the corrugation region spreads and the corrugation intensity is enhanced. At 677 K, the corrugation intensity seems to reach maximum. And then at 686 K the corrugation begins to vanish. There is little corrugation at 699 K. When temperature increases to 708 K, no corrugation could be found anymore. The flow turns laminar and homogeneous again. The corrugation cannot be considered as turbulence. Since the images are obtained by direct photography, they do not reveal variations of the velocity but differences of intensity of the scattered light. Thus it is not reasonable to identify whether the flow has been turbulent according to these images. The corrugation is actually some dark regions in the relatively bright flow, indicating that the refractive index there has changed. As the refractive index is strongly related to the local density, it can be inferred that the corrugation actually reveals density gradient in the flow. In order to verify the above presumption, variation of the density of RP-3 with temperature under 3 MPa is plotted in Fig. 11 [20]. It can be seen from the figure that the density of RP-3 decreases nearly linearly with temperature from 300 K to 640 K. Then it decreases sharply from 640 K to 700 K. When temperature is higher than 700 K, the density decreases slowly again. The temperature range in which corrugation appears and grows is just the right temperature range in which density decreases sharply. Since there is
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Fig. 10. Flow patterns of RP-3 under 3 MPa.
fast-speed camera. The results can be summarized as follows:
Fig. 11. Variation of the density of RP-3 with temperature under 3 MPa [20].
temperature gradient in the flow, large density gradient would appear in the flow when RP-3 is heated to the temperature range in which density changes sharply with temperature. Then corrugation phenomenon happens as a reflection of the refractive index change caused by large density gradient. From the above analysis, we may conclude that: unlike the phase transition process from liquid to gas under subcritical pressures, there are neither bubbles nor abundant flow patterns. The opalescence phenomenon under critical pressure is not observed either. The main characteristic of such a transition process is that corrugation would appear when RP-3 is heated to a certain temperature range. This temperature range is near the pseudo-critical temperature and varies with pressure. The corrugation is caused by large density variation near the pseudo-critical temperature. 4. Conclusions In this paper, the flow patterns of China RP-3 kerosene under sub-critical and supercritical pressures are studied experimentally. The images of phase transition processes under different pressures are obtained by employing a
(1) Experimental observations indicate eight distinct flow patterns (liquid flow, bubble flow, slug flow, churn flow, annular flow, dense bubble flow, dispersed flow and gas flow) in the phase transition process under sub-critical pressures. Some flow patterns may not appear under a high pressure, such as slug flow, churn flow and annular flow. It is more difficult to form a large consecutive gas region such as a gas slug under high pressures. Bubbles become much smaller and denser as pressure approaches the critical pressure. (2) Opalescence phenomenon is observed in the phase transition process under critical pressure. The opalescence phenomenon does not appear under sub-critical pressures nor supercritical pressures. Debye's theory is employed to explain this phenomenon. (3) When pressure is higher than the critical pressure of RP-3, no bubbles are observed in the phase transition process from sub-critical state to supercritical state. Some kind of corrugation appears in the flow when temperature approaches the pseudo-critical temperature. The corrugation is supposed to be caused by sharp change of the local refractive index, which is resulted from the large density gradient near the pseudo-critical temperature.
Acknowledgments This work was supported by the National Natural Science Fund of China (Grant nos. 10902124 and 11142010). The authors also greatly acknowledge and thank the College of Aerospace Science and Technology at National University of Defense Technology for supporting their work. References [1] F.Q. Zhong, X.J. Fan, G. Yu, Heat transfer analysis for actively cooled supersonic combustor, J. Propuls. Technol. 30 (2009) 513–518.
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Experimental investigation on flow patterns of RP-3 kerosene under sub-critical and supercritical pressures Ning Wang, Jin Zhou n, Yu Pan, Hui Wang Science and Technology on Scramjet Laboratory, National University of Defense Technology, Hunan, Changsha 410073, China
a r t i c l e in f o
abstract
Article history: Received 1 August 2013 Received in revised form 7 October 2013 Accepted 11 October 2013 Available online 24 October 2013
Active cooling with endothermic hydrocarbon fuel is proved to be one of the most promising approaches to solve the thermal problem for hypersonic aircraft such as scramjet. The flow patterns of two-phase flow inside the cooling channels have a great influence on the heat transfer characteristics. In this study, phase transition processes of RP-3 kerosene flowing inside a square quartz-glass tube were experimentally investigated. Three distinct phase transition phenomena (liquid–gas two phase flow under subcritical pressures, critical opalescence under critical pressure, and corrugation under supercritical pressures) were identified. The conventional flow patterns of liquid–gas two phase flow, namely bubble flow, slug flow, churn flow and annular flow are observed under sub-critical pressures. Dense bubble flow and dispersed flow are recognized when pressure is increased towards the critical pressure whilst slug flow, churn flow and annular flow disappear. Under critical pressure, the opalescence phenomenon is observed. Under supercritical pressures, no conventional phase transition characteristics, such as bubbles are observed. But some kind of corrugation appears when RP-3 transfers from liquid to supercritical. The refraction index variation caused by sharp density gradient near the critical temperature is thought to be responsible for this corrugation. Crown Copyright & 2013 Published by Elsevier Ltd. on behalf of IAA. All rights reserved.
Keywords: Flow patterns Two-phase flow Kerosene Supercritical Active cooling
1. Introduction Efficient cooling is one of the main difficulties for researches of advanced hypersonic aircraft such as scramjets. The large heat release from combustion as well as high enthalpy of the inflow leads to an extremely rigorous thermal environment. Previous studies [1,2] indicate that when the supersonic aircraft travels at Mach 2.5, the heat flux in combustor could be as high as 1 MW/m2. When Mach number reaches 6, the wall temperature would exceed 3000 K. This is far beyond the limit of most materials. Active cooling with hydrocarbon fuels such as RP-3 is considered to be most promising to solve this problem. Besides physical heat sink, additional chemical
n
Corresponding author. Tel.: þ 86 15973121346. E-mail address: [email protected] (J. Zhou).
heat sink is obtained from cracking reactions at high temperatures and thus the total cooling capacity is improved [3]. A lot of demonstrations about the necessity and feasibility of hydrocarbons applied in active cooling of hypersonic aircraft have been conducted [4–6]. Researches on hydrocarbons applied in active cooling of hypersonic aircraft are mainly focusing on five aspects [7]: cooling capacity and heat sink measurement, thermal and catalytic cracking, coking suppression, heat transfer characteristics, and injection, mixing, ignition and combustion performances. Few results are reported on the flow patterns of hydrocarbons flowing inside the cooling channels. However, depending on the pressure inside the cooling channel, different phase transition processes (liquid–gas transition, liquid–supercritical transition), and different flow patterns may happen. Without knowing the flow patterns inside the cooling channel, correct hydraulic/ thermal designs for scramjet cannot be conducted. In fact,
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the pressure drops and heat transfer coefficients are strongly related to the flow patterns. Thus, study on the flow patterns inside the cooling channel is of great importance for scramjet research. Extensive theoretical and experimental work has been carried out in the field of two-phase flow patterns. Based on the size of flow channels, it could be divided into twophase flow in macroscale and microscale channels. So far, the distinction of microscale channels is still in dispute. A threshold diameter of 3 mm is adopted by many researchers. According to this criterion, the two-phase flow in cooling channels of scramjet is that in microscale channels. Due to the significant difference of transport phenomena in microscale channels as compared to conventional size channels, the flow patterns may be much different from those in macroscale channels. For example, when the channel diameter is reduced, the effect of flow orientation tends to disappear. New flow patterns, such as wedge flow [8] and liquid lump flow [9], appear. Slug flow has much longer bubbles [10,11]. Based on thermal conditions, it includes adiabatic and diabatic two-phase flows. Obviously, the two-phase flow inside cooling channels of scramjet is diabatic one which absorbs heat from the combustor. As compared to adiabatic two-phase flows, many parameters, such as local heat flux, pressure and temperature, have a great influence on diabatic flow patterns. Additionally, RP-3 kerosene is a kind of mixture which consists of hundreds of species. The initial boiling temperature for each species may differ a lot. Furthermore, at high temperatures (above approximately 800 K), cracking reactions begin to happen. All of these make the diabatic two-phase flow of RP3 inside cooling channels more complicated. Cheng et al. [12] have given a comprehensive review of the flow patterns of gas–liquid two-phase flow. As discussed in their paper, although studies on microscale channels have increased greatly in recent years, it is still in its infancy stage as compared to the vast researches on two-phase flow in macroscale channels. Additionally, most of the numerous flow-pattern maps have been developed for adiabatic conditions. In principle, adiabatic two-phase flow maps are not applicable to diabatic conditions. Fig. 1 shows a schematic representation of a horizontal tubular channel heated by a uniform heat flux and fed with subcooled liquid [13,14]. Bubbly flow, plug flow, annular flow and mist flow appear in sequence along the tube. However, the flow patterns may vary a lot when the experimental conditions such as heat flux, pressure are changed. Recently, Nigmatulin et al. have developed
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a mathematical model that determines the propagation of acoustic waves in polydisperse bubble liquids [15], and established a model of transcillatory heat transfer induced by gas bubbles buoyant in liquid [16]. Work by Lahey et al. [17] shows that thermonuclear fusion has held out the promise of cheap, clean, and virtually limitless energy. And in this process, diabatic flow would be confronted. Ivashnyov et al. [18] have proposed a special methodology to describe boiling flows under hot water depressurization. The difference in phase velocities was taken into account. Another issue worth noting is that, the physical properties of fluids have a great influence on the flow patterns. Different fluids may present much different flow patterns, even at the same conditions. However, most researches on the flow patterns in microscale channels used air–water or water–nitrogen as the working fluids. Some researchers used refrigerants such as R134a and R123. Few results are reported on the flow patterns of hydrocarbon fuels inside microscale channels. In this paper, the flow patterns of a commonly used hydrocarbon fuel in China, RP-3 were experimentally investigated. The critical pressure and temperature of it are approximately 2.3 MPa, 646 K respectively. Three distinct phase transition processes were identified for RP-3 flowing inside a cooling channel under different pressures. Furthermore, some analysis was made to conclude the different phase transition phenomena. 2. Experimental setup A schematic diagram of the experimental setup is shown in Fig. 2. The fuel is stored in a tank and pressured by a nitrogen gas source with pressure of 8 MPa. The mass flow rate of the fuel is adjusted by a needle valve (Model: 33s4F, Xiongchuan) and measured by a Coriolis-force flow meter (Model: DMF-1-1-A, SiMite) with accuracy of 70.2%. An electrical heater with maximum power of 28 kW is used to heat a stainless tube (1Cr18Ni9Ti, 1.6 m length) and the kerosene flowing inside it. Then the heated kerosene flows through a square quartz glass tube with an inner geometry of 3 3 mm2 and length of 10 cm. Compared with circular glass tubes, image distortion caused by refraction at the circular surface could be avoided. The glass tube is mounted between two special designed connecting bases, see Fig. 3. Graphite gaskets are laid at both sides in order to prevent breakage caused by instantaneous pressure shock. Two “O” rings are used for sealing. More information about the connecting bases can be found in [19].
Fig. 1. Schematic of flow patterns for flow boiling in a horizontal tube [12].
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Fig. 2. Sketch of the experimental system.
Fig. 3. Sketch of the flow visualization module.
A high speed camera (Model: MotionBLITZ EoSens Cube/ mini, Mikrotron GmbH) is laid at one side of the glass tube while a lamp (Model: QH-1300, Changcheng) is laid at the other side to provide sufficient illumination. Temperatures at the inlet and outlet of the heating tube are measured by K-type armored thermocouples with accuracy of 70.75%. Pressure at the outlet of the tube is measured by a pressure gage transducer (Model: MPM480, Maike) with accuracy of 70.1%. Then the fuel is cooled by a cooling system and collected into a tank. A back-pressure valve (Model: 9134F1, Xiongchuan) is laid between the cooling system and the collecting tank to adjust the pressure inside the tube.
A typical aviation kerosene RP-3 is used in this work. It is equivalent to Jet A-1. Its density under 2.34 Mpa varies from approximately 788.773 kg/m3 to 55.907 kg/m3 when temperature changes from 295.63 K to 786.82 K [20]. And its boiling range is from 423 K to 553 K. Composition analysis [20] shows that RP-3 consists of 52.44% alkanes, 7.64% alkenes, 18.53% benzenes, 15.54% cycloalkanes, 4.39% naphthalenes, and 1.46% other; the detailed composition of RP-3 can be found in [20]. The mass flow rate of all test cases is fixed to 2 g/s. The velocity of RP3 in the square glass tube under 2.3 Mpa varies from 0.35 m/s to 2.16 m/s when temperature
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Fig. 4. Flow patterns of RP-3 at different temperatures under 0.3 MPa.
changes from 455 K to 671 K. Corresponding Reynolds number varies from approximately 3227 to 37691. 3. Results and discussion 3.1. Flow patterns under sub-critical pressures Fig. 4 shows the flow patterns of RP-3 under 0.3 MPa in the horizontal quartz tube. RP-3 flows from right side of the tube to left side. The inlet and outlet temperatures are labeled on the figure. When temperature is low, the kerosene is liquid, and the flow is transparent and homogeneous, as Fig. 4(a) shows. As the inlet temperature increases to 463 K, the initial boil begins to happen, and a number of discrete bubbles appear in the liquid mainstream. As a result of surface tension, the bubbles are spherical. And they all float at the upper side of the mainstream because of the gravity. In Fig. 4(c), the inlet temperature increases further to 501 K and slug flow is formed. The volume of the bubbles is obviously larger than that in bubble flow, and the bubbles are no longer spherical but like slugs. And the head of the slugs is cuspate while the tail of the slugs is obtuse. Besides these large slugs, there are small slugs which have a relatively cuspate tail and look like a hemisphere. In addition, there are smaller bubbles following the large slugs. When the inlet temperature increases to 512 K, the slug flow begins to transform to churn flow, see Fig. 4(d). The interface between the slug and the liquid mainstream, especially the lower interface begins to fluctuate, as marked by the red ellipse. Besides, the tail of the slug begins to break, thus
generating more small bubbles following behind. When the inlet temperature is 515 K, churn flow is formed. Fig. 4(e) shows the primary stage of churn flow. Nearly all slugs have connected into a whole, thus forming a gas core in the middle of the tube. However, the gas core is not so stable, and a rupture may appear. At the rupture point, a lot of discrete bubbles are generated. Because of the gravity, the gas core exists in the upper part of the tube. And the interface between the gas core and the liquid mainstream is relatively smooth. When churn flow has fully developed, as Fig. 4(f) shows, a consecutive gas core is formed. And neither rupture nor bubbles are observed. The interface between the gas and the liquid presents intensive fluctuation. When the inlet temperature is further increased to 531 K, annular flow is formed, as Fig. 4(g) shows. In this flow pattern, the majority of the tube is filled by gas. Liquid only exists in the near wall region. Moreover, the interface fluctuation is much smaller than that in churn flow. And liquid droplets keep falling off the liquid layer, as marked in the red circle. When the inlet temperature is 546 K, the liquid layer becomes even smaller, and this is the end stage of annular flow, as Fig. 4(h) shows. There is almost all gas in the tube. The liquid exists in a rather small region. Similarly, it can be seen that liquid droplets fall off the liquid layer, but the quantity of the droplets becomes much smaller. After this stage, the boil process would be completed, see Fig. 4(i). There is no liquid any more and the flow becomes singlephase flow again. Fig. 5 illustrates the flow patterns under 0.8 MPa and 1.3 MPa, which are far below the critical pressure of RP-3 (2.3 MPa). The typical four flow patterns of two phase flow,
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Fig. 5. Flow patterns of RP-3 under 0.8 MPa and 1.3 MPa.
namely bubble flow, slug flow, churn flow and annular flow are observed under 0.8 MPa. However, the temperature intervals from one pattern to another are much shorter, which means that, the flow pattern transforms more quickly. Under 1.3 MPa, no slug flow is observed. When temperature is increased, more bubbles are generated while no gas slug can be found, see Fig. 5(f). When temperature increases further, the flow directly transforms to churn flow, as Fig. 5(g) shows. Then the typical churn flow and annular flow appear in sequence as temperature increases, see Fig. 5(h) and (i). Fig. 6 illustrates the flow patterns under 1.8 MPa and 2.2 MPa, which are close to the critical pressure of RP-3 (2.3 MPa). The initial boil temperature is markedly elevated. Similarly, the flow transforms quickly from one pattern to another as it does under 0.8 MPa and 1.3 MPa. It can be seen from the figure that, under 1.8 MPa, bubbles firstly appear at 618 K. As the inlet temperature increases to 624 K, the tube is nearly filled with abundant bubbles. We consider it as a new flow pattern and call it dense bubble flow. It is totally different from churn flow. In dense bubble flow, the bubbles are independent and discrete. Although the tube is full of bubbles, they do not connect to each other and there is not a consecutive gas core in the tube. When temperature becomes higher, the majority region of the tube is gas, and liquid is separated into droplets, which distribute in the gas mainstream. This is somewhat similar to bubble flow, in which discrete bubbles distribute in the liquid mainstream. It is the so-called dispersed flow. It is also different from annular flow, in which liquid only exists in the near wall region and droplets fall off this annular region. Under 2.2 MPa, the
flow also undergoes these three patterns, namely bubble flow, dense bubble flow and dispersed flow, see Fig. 6(d)–(f). And the characteristics of the new patterns are even more obvious than that under 1.8 MPa. Comparing the flow patterns of RP3 under subcritical pressures with Fig. 1, it is found that bubble flow, plug flow and annular flow, which are observed in the previous literature, also appear in the boiling flow of RP3 under a low pressure (0.3 Mpa and 0.8 MPa). Mist flow was not observed. Churn flow, which was a common flow pattern but not observed in Fig. 1, appears in the boiling of RP3 at a wide pressure range (from 0.3 Mpa to 1.3 MPa). When pressure is elevated (1.8 Mpa and 2.2 MPa), plug flow and annular flow cannot be observed. Instead, dense bubble flow and dispersed flow appear. Fig. 7 shows the difference between churn flow and dense bubble flow, as well as difference between dispersedannular flow and dispersed flow. Liquid is marked by red while gas is marked by green. In churn flow, there is a consecutive gas core existing in the middle of the tube, and liquid is pressed to the near wall region. However, in dense bubble flow, gas exists as discrete dense bubbles. Although the liquid region is also small, it is not limited to the near wall region but exists everywhere. In dispersed-annular flow, the annular region of liquid is further reduced, and there are droplets falling off the liquid layer. In dispersed flow, although liquid droplets can be observed, there is not a liquid layer. The liquid droplets are formed from separation of the liquid region by growing bubbles. Fig. 8 illustrates the transition of flow pattern under sub-critical pressures. Each color stands for a kind of flow pattern. As can be seen, when pressure is increased, some
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Fig. 6. Flow patterns of RP-3 under 1.8 MPa and 2.2 MPa.
Fig. 7. Sketch of comparison of dense bubble flow and dispersed flow with churn flow and annular flow. (For interpretation of the references to color in the text, the reader is referred to the web version of this article).
One possible reason is that the pressure difference along the surface of a large consecutive gas region would be high enough to tear the gas region into smaller bubbles under higher pressures. Besides, the two phase flow region is becoming smaller as pressure increases, and the bubbles become small and dense. It can be inferred that when pressure is higher than a certain value, the bubbles would be too small to be observed and the traditional phase change would disappear. That is the critical pressure.
3.2. Flow patterns under critical pressure
Fig. 8. Flow pattern transition of RP-3 under sub-critical pressures. (For interpretation of the references to color in the text, the reader is referred to the web version of this article).
kinds of flow pattern disappear in sequence, such as slug flow, churn flow and annular flow, and some flow patterns appear, such as dense bubble flow and dispersed flow. It seems that it is difficult to form a large consecutive gas region, such as gas slug or gas core in churn flow under relatively high pressures. Thus slug flow, churn flow and annular flow are not observed under high pressures.
The critical pressure of RP-3 can be determined by many methods. Sun et al. [21] developed a short residence time flow method and determined the critical pressure of RP-3 to be 2.39 MPa. Fan et al. [22] considered it to be 2.4 MPa according to principle of the extended corresponding states (ECS). In this paper, the critical pressure of RP-3 is determined by flow pattern variations. As aforementioned, phase change would always happen under sub-critical pressures, thus bubbles could be observed. However, as pressure approaches the critical pressure, bubbles would be smaller and smaller and disappear at last. According to the disappearance of bubbles, the critical pressure of RP-3 is determined to be 2.3 MPa.
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Fig. 9. Flow patterns of RP-3 under 2.3 MPa.
Fig. 9 shows the flow pattern variations with temperature under critical pressure. A thermocouple was put inside the quartz tube. The temperature labeled on the figure was measured by this thermocouple. At a relatively low temperature (455 K), the flow looks like laminar flow and is homogeneous. As temperature increases to 624 K, some kind of corrugation appears in the flow. When temperature comes to 646 K, the corrugation is intensified. And more importantly, a unique phenomenon of opalescence is observed. It cannot be considered as bubbles because the interface between gas and liquid cannot be distinguished even when the image zooms in. At 652 K, the opalescence region spreads nearly to the whole area of the tube. At 656 K, the opalescence begins to vanish. And some area of the tube becomes transparent again. When temperature increases to 671 K, no opalescence could be observed anymore and the total area of the flow turns transparent and homogeneous again. The opalescence phenomenon seems to appear only under critical pressure and near the critical temperature. Flow pattern images under supercritical pressures showed that no opalescence is observed, although no phase change happens either. The critical opalescence phenomenon could be explained by Einstein's fluctuation theory [23]. The intensity of light scattered from volume Vcan be expressed as follows: I 4π 2 V ½ðc=μÞð∂μ=∂cÞ2 ¼ 2 4 I0 R λ c½ð∂=∂cÞðP=kTÞ
ð1Þ
Where I0 is the intensity of the primary light; λ stands for the wavelength; c represents the concentration; m is the index of refraction; P is the pressure; k is Boltzmann's constant, and T is the temperature. Debye et al. [24] modified the expression as the following formula: " # ! 2 1 T c ΔT 8π 2 l ΔT 16π 2 r 2 2θ ¼ þ þ ð2Þ sin I T c 3 λ2 2 CT T c 3 λ2 Where Tc is the critical temperature; C is a constant characteristic for the system; l is the interaction range; λ is the wavelength; r is the radius of gyration; θ is the scattering angle. ΔTis defined as follows: ΔT ¼ T T c
ð3Þ
Then we know from Eq. (2) that IA
1 ΔT
ð4Þ
Thus, the intensity of the light scattered would reach maximum when temperature approaches the critical temperature. 3.3. Flow patterns under supercritical pressures Fig. 10 illustrates the variation of flow pattern under 3 MPa, which is higher than the critical pressure. The temperature varies from 606 K to 708 K. As the critical temperature of RP-3 is approximately 646 K, the flow transforms from pressed liquid state to supercritical state. It can be seen from the figure that no bubbles are observed during the transformation. When the temperature is 606 K, the flow seems laminar and homogeneous. As the temperature increases to 633 K, corrugation appears. When the temperature further increases, the corrugation region spreads and the corrugation intensity is enhanced. At 677 K, the corrugation intensity seems to reach maximum. And then at 686 K the corrugation begins to vanish. There is little corrugation at 699 K. When temperature increases to 708 K, no corrugation could be found anymore. The flow turns laminar and homogeneous again. The corrugation cannot be considered as turbulence. Since the images are obtained by direct photography, they do not reveal variations of the velocity but differences of intensity of the scattered light. Thus it is not reasonable to identify whether the flow has been turbulent according to these images. The corrugation is actually some dark regions in the relatively bright flow, indicating that the refractive index there has changed. As the refractive index is strongly related to the local density, it can be inferred that the corrugation actually reveals density gradient in the flow. In order to verify the above presumption, variation of the density of RP-3 with temperature under 3 MPa is plotted in Fig. 11 [20]. It can be seen from the figure that the density of RP-3 decreases nearly linearly with temperature from 300 K to 640 K. Then it decreases sharply from 640 K to 700 K. When temperature is higher than 700 K, the density decreases slowly again. The temperature range in which corrugation appears and grows is just the right temperature range in which density decreases sharply. Since there is
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Fig. 10. Flow patterns of RP-3 under 3 MPa.
fast-speed camera. The results can be summarized as follows:
Fig. 11. Variation of the density of RP-3 with temperature under 3 MPa [20].
temperature gradient in the flow, large density gradient would appear in the flow when RP-3 is heated to the temperature range in which density changes sharply with temperature. Then corrugation phenomenon happens as a reflection of the refractive index change caused by large density gradient. From the above analysis, we may conclude that: unlike the phase transition process from liquid to gas under subcritical pressures, there are neither bubbles nor abundant flow patterns. The opalescence phenomenon under critical pressure is not observed either. The main characteristic of such a transition process is that corrugation would appear when RP-3 is heated to a certain temperature range. This temperature range is near the pseudo-critical temperature and varies with pressure. The corrugation is caused by large density variation near the pseudo-critical temperature. 4. Conclusions In this paper, the flow patterns of China RP-3 kerosene under sub-critical and supercritical pressures are studied experimentally. The images of phase transition processes under different pressures are obtained by employing a
(1) Experimental observations indicate eight distinct flow patterns (liquid flow, bubble flow, slug flow, churn flow, annular flow, dense bubble flow, dispersed flow and gas flow) in the phase transition process under sub-critical pressures. Some flow patterns may not appear under a high pressure, such as slug flow, churn flow and annular flow. It is more difficult to form a large consecutive gas region such as a gas slug under high pressures. Bubbles become much smaller and denser as pressure approaches the critical pressure. (2) Opalescence phenomenon is observed in the phase transition process under critical pressure. The opalescence phenomenon does not appear under sub-critical pressures nor supercritical pressures. Debye's theory is employed to explain this phenomenon. (3) When pressure is higher than the critical pressure of RP-3, no bubbles are observed in the phase transition process from sub-critical state to supercritical state. Some kind of corrugation appears in the flow when temperature approaches the pseudo-critical temperature. The corrugation is supposed to be caused by sharp change of the local refractive index, which is resulted from the large density gradient near the pseudo-critical temperature.
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