Li Xinfang Wang Xianju Zhu Dongsheng
(1Zhongshan Torch Polytechnic,Zhongshan 528436,China) (2College of Science,South China Agriculture University,Guangzhou 510642,China) (3Guangzhou Institute of Energy Conversion,Chinese Academy of Sciences,Guangzhou 510640,China)
Numerical analysis on cold storage characteristic of nanoparticle-enhanced phase change material for energy saving
Li Xinfang1Wang Xianju2Zhu Dongsheng3
(1Zhongshan Torch Polytechnic,Zhongshan 528436,China) (2College of Science,South China Agriculture University,Guangzhou 510642,China) (3Guangzhou Institute of Energy Conversion,Chinese Academy of Sciences,Guangzhou 510640,China)
The cold storage characteristic of nanoparticle-enhanced phase change material(NEPCM)was investigated by using numerical simulation method with Fluent software. The influence of Grashof number and particle concentration on the cold storage performance was discussed . The numerical results indicate that the cold storage characteristic of the NEPCM largely depends on the volume fraction of nanoparticles,while exhibits little sensitivity to Grashof number. As the volume fraction increases,the freezing time of the NEPCM is lowered for a given initial Grashof number. The total freezing time of the NEPCM can be lowered by 16.3% with Cu nanoparticles volume fraction is 1.0%. The reduction of the freezing time is attributed to the higher thermal conductivity of the NEPCM. At the same time,less energy per unit mass of the NEPCM is needed for freezing the NEPCM because of the lower latent heat of fusion.
nanoparticle-enhanced phase change material;cold storage;freezing time;numerical simulation
Because the conventional energy sources are quickly depleted and the demand of energy is growing,more and more researchers pay attention to renewable energy sources and energy storage systems. Solid-liquid phase change provides considerable advantages such as high storage capacity and nearly isothermal behavior during the melting/freezing processes. During these years,researchers have tried to find new way to develop energy storage system. Using nano technology to enhance the heat transfer indicates great opportunity in storage system. Because the thermal conductivity of conventional heat transfer fluids is low,nanotechnology is considered to enhance thermal characteristics with substantially higher conductivities. The presence of the nanoparticles in the fluids increases appreciably the effective thermal conductivity of the fluid and consequently enhances the heat transfer characteristics. Masuda et al.[1]reported that the thermal conductivity was enhanced by dispersing ultra-fine(nanosize)particles in liquids. Soon thereafter,Choi[2]first designated the new fluids with higher thermal conductivity as "naofluids". Khanafer et al.[3]simulated heat transfer characteristic of the nanofluids in a two-dimensional enclosure for various pertinent parameters. Khodadadi et al.[4]were the first to report the improved functionality of phase change material (PCM) with nanoparticles. They found that the nanoparticle-enhanced phase change materials(NEPCM)exhibit higher thermal conductivity than the base fluids. Zhu et al.[5]simulated the thermal energy storage behavior of SiC-H2O nanofluids in a two-dimensional enclosure. Ranjbar et al.[6]studied the solidification behavior of the NEPCM and the relevant parameters . They found the nanofluid heat transfer rate increases with the increase in the nanoparticles volume fraction. Also,it was found that the temperature gradient in the liquid is too small to cause a significant natural convection in the liquid. Thus,heat conduction is dominant in heat transfer of both solid and liquid[7-8]. In this study the freezing of the NEPCM was studied.
A physical model of two-dimensional enclosure is shown in Figure 1,in which the sideHis filled with nanofluid. The physical dimension of the enclosureHis chosen to be 10 mm. The horizontal walls are assumed to be adiabatic,not conductive,and impervious to mass transfer. The NEPCM in the enclosure is incompressible and the flow is laminar. The particle shape and particle size distribution are uniform,and both the nanoparticles and the base fluid are in a thermal equilibrium,which is consistent with the flow velocity The nanoparticles are assumed to have a uniform shape and size. Moreover,it is assumed that both the fluid phase and nanoparticles are in thermal equilibrium state and they flow at the same velocity. The left vertical wall is maintained at a high temperature(TH),while the right vertical wall is kept at a low temperature(TC). The thermophysical properties of the NEPCM are assumed to be constant except for the density variation in the buoyancy force,which is based on the Boussinesq approximation.
Fig.1 Physical model of two-dimensional enclosure
Under these assumption,the governing equations are:
Continuity:
避嫌,怕人说闲话,也是古往今来廉政官员的“标配”,他们爱惜羽毛,珍重名誉,谨言慎行,有所不为。于谦有《入京诗》一首:“绢帕蘑菇与线香, 本资民用反为殃; 清风两袖朝天去, 免得闾阎话短长。”代表了许多廉洁官员的心声,正是靠了“怕人说闲话”的念想,他们拒绝诱惑,不谋私利,守身如玉,初心不改。这也就是《菜根谭》说的那个意思:“一点不忍的念头,是生民生物之根芽;一段不为的气节,是撑天撑地之柱石。”
(1)
X-momentum equation:
The instantaneous streamlines within the NEPCM for the initial 15s during the freezing of the NEPCM for an initialGr=104and a solid particle volume fraction of 1% are shown in Figure 4. The streamlines att=0 correspond to a similar case studied by Khanafer et al.[3]and Khodadadi et al[4],and a CW rotating vortex is clearly observed. As a result of the sudden lowering of the temperatures of the two active walls att>0,the CW rotating vortex diminishes in strength and spatial coverage due to formation of a CCW rotating vortex next to the left wall. The creation of the dual-vortex flow pattern was examined in greater detail by lowering the time step to 0.1 s for this case. Note that the formation,growth and equilibration of the CCW vortex during the initial 15 s involves a dynamic interaction with the initially strong CW vortex. At thet=10-15 s instant,two vortices rotating in opposite directions and nearly equal in size are observed squeezed between the left wall and a thin frozen layer next to the right wall. For the remainder of the freezing process,the dual-vortex structure will persist however due to the leftward movement of the freezing front,the vortices will shrink in coverage space and their strength will decay. It should be noted that the actual Grashof number for this unsteady freezing problem decreases with time due to the continuous shrinking of the distance between the left wall and the liquid-solid interface.
(2)
Y-momentum equation:
(3)
Energy equation:
(4)
In addition,the thermal conductivity of the NEPCM was measured by a Hot Disk Thermal Constants Anlyser,and the viscosity of the NEPCM was measured using capillary viscometers.
(7)
3 Khanafer K,Vafai K,Lightstone M L. Buoyancy-driven heat transfer enhancement in a two-dimentional enclosure utilizing nanofluids[J]. Int. J. Heat Mass Transfer,2003,19:3639-3653.
(8)
Whereas the heat capacitance,latent heat of fusion for the NEPCM and part of the Boussinesq term are:
(9)
(10)
(11)
Withφis the volume fraction of the nanoparticles and subscripts f,nf and p stand for base fluid,NEPCM and nanoparticles,respectively.
4 Khodadadi J,Hosseinizadeh S F. Nanoparticle-enhanced phase change materials(NEPCM)with great potential for improved thermal energy storage[J]. Int. J. Comm Heat Mass Transfer,2007,34:534-543.
The above equations can be cast in non-dimensional form by incorporating the following dimensionless parameters:
The pertinent thermophysical properties are given in Table 1. The present processing method can be found elsewhere[3-4].
根据构图检查是否满足所有条件,由CD=4,可得到CD的距离EK=KD=2;以E为圆心为半径画圆,作AB⊥CD,垂足为B,AB=3,满足条件;最后,基于问题进行思考,如图11,因为DB=BK-KD,作EG⊥AB垂足为G,由构图可知,可求弦心距因为四边形EGBK为矩形,则所以
Table 1 Thermophysical properties of the nanoparticle,water and NEPCM
The initial and boundary conditions for the present investigation are presented as Table 2.
Table 2 Initial and boundary conditions
Starting at timet=0,the temperatures of both active left and right walls were lowered by the same amount such that the cold right wall was held 10 ℃ below the freezing temperature of the base fluid(TH=273.15 andTC=263.15 K). Consequently,the NEPCM will start freezing on the right wall and the solid front travels to the left. The remaining boundary conditions were unchanged in comparison to the conditions prior tot=0.
Starting with steady natural convection within the Cu-H2O NEPCM that is inside a differentially-heated square cavity,freezing of the Cu-H2O NEPCM was investigated. The temperatures of the left and right walls were lowered by 10 ℃. In effect,the cold right wall was held 10℃ lower than the freezing temperature of the base fluid(273.15 K). Consequently,the NEPCM will start freezing on the right wall and the solid front travels to the left. The other boundary conditions remained unchanged. Solid particle volume fractions of 0,0.1%,1.0% and 5.0% were considered for two initial Grashof numbers of 104and 105. The pertinent properties are given in Table 1. Contours of the volume fraction of the NEPCM during freezing at various time instants are shown in Figure 2 and Figure 3 for an initial Grashof number of 104. The time instants in Figure 2 are 100 and 600 s,the time instants in Figure 3 are 1 200 and 2 400 s. Color gray is used to identify the liquid phase,whereas color black is indicative of the frozen solid phase. In general,the sharp liquid-solid interface is nearly vertical with a mild misalignment toward the colder wall early on,thus favoring a longer wetted length on the top insulated wall. This can be attributed to the buoyancy-driven convection in the cavity that was already at full strength att=0 in the form of a clockwise(CW)rotating vortex. Fort>0,the strength of this vortex diminishes whereas a second counter-clockwise(CCW)rotating vortex is created next to the left wall(see Figure 4).
In order to verify the numerical code,comparison of the average Nusselt number along the hot wall with previous works[9-11]for different Rayleigh numbers is shown in Table 3. This table shows an excellent agreement between our results and those of other benchmark solutions,which suggests that our calculation method is adequate to describe the phase change process of the NEPCM correctly.
《普通高中生物学课程标准(2017版)》延续了2003版和2011版课程标准注重“探究能力”养成的思路,将“科学探究”列为生物学科四大核心素养之一。可见,培养学生的科学探究能力历来是生物学教学的重要目标与要求。如何在教学实践中更好地促进学生科学探究能力的提高,教师除了充分挖掘教材,在常规教学中力求实现教学目标,还应该适应学生的个性发展,因材施教,通过各种途径,为对生物学科有浓厚兴趣的学生创造更多自主探究的机会。
Table 3 Comparison of present numerical simulation with previous works
The SIMPLE method within version 6.2 of the commercial code Fluent was utilized to solve the governing equations. For all the cases reported here,uniform grid spacings for bothxandydirections were utilized. The calculating time step was 1s. The Quick differential algorithm was used to deal with the momentum and energy equations,whereas the PRESTO algorithm was used to deal with the pressure correction equation. The under-relaxation factors for the velocity components,pressure correction,thermal energy and liquid fraction were 0.5,0.3,1 and 0.9 respectively. In order to satisfy convergence criteria(10-7for continuity and momentum,and 10-9for thermal energy),the number of iterations for every time step was set to 1 000.
For this Grashof number(Gr=104),it is observed that as the solid particle volume fraction is raised,the Cu-H2O NEPCM will freeze more rapidly. There are two possible reasons to explain the behavior of the quicker freezing rate. One is the higher thermal conductivity for the NEPCM,because the crystal growth mainly depends on heat transfer. At the process of freezing,a large amount of heat will be discharged. If the heat can’t be released in time,the freezing process will be hindered. After adding the nanoparticles to the base fluid,the fluid has higher thermal conductivity. Therefore,the freezing speed of the NEPCM is able to be accelerated. Another reason may be the Cu nanoparticles acting as a nucleating agent. It is known that the freezing process of pure water has a supercooling degree about 4 ℃. When the nanoparticles are added into water,the supercooling degree of water is decreased according to the mechanism of heterogeneous nucleation. The beginning of freezing time is ahead. This is also helpful to save the freezing time.
2.2.2 Decomposition of vanillin to small carboxylic acids
Fig.2 Contours of volume fraction of NEPCM at various time instants during freezing course(Gr=104)
Fig.3 Contours of volume fraction of NEPCM at various time instants during freezing course(Gr=104)
Fig.4 Streamline patterns of Cu-H2O NEPCM at various time instants for initial 15 s during freezing course(Gr=104,φ=1.0%)
哥白尼所担心的灾难终于降临到布鲁诺的头上。在阴森的宗教法庭上,红衣大主教罗伯特·贝拉赫曼(三十年后他还审判了伽利略)主持对布鲁诺的审判。空荡荡的教堂,一张长桌子,几枝残烛。罗伯特和几个陪审隐在桌后,几乎看不清他们的身形。 烛光中那几只蓝绿的眼睛,令人想起半夜里在田野上遇见的恶狼。
The freezing times for pure water and Cu-H2O NEPCM for initial Grashof numbers of 104and 105are summarized in Table 4.The liquid volume that continuously decreases from the start of the freezing exhibits little sensitivity to the value of the initial Grashof number. On the other hand,the volume of the NEPCM is strongly dependent on the solid particle volume fraction of the dispersed nanoparticles. For a given initial Grashof number(Gr=104),The whole freezing time of distilled water is 3 and 130 s,0.1% NEPCM is 2 and 820 s,1.0% NEPCM is 2 and 620 s,and 5.0% NEPCM is 2 and 210 s,and the whole freezing time of the three NEPCM can be lowered by 9.9%,16.3%,and 29.4%,respectively. It reveals that the higher the nanoparticle volume fraction is,the faster the decrease of the liquid volume fraction,and the shorter the whole freezing time. The reduction of the freezing time is attributed to the higher thermal conductivity of the NEPCM. At the same time,less energy per unit mass of the NEPCM is needed for freezing the NEPCM because of the lower latent heat of fusion. This phenomenon is in agreement with the study of Liu[12],who experimentally investigated TiO2-BaCl2-H2O suspensions for thermal energy storage,respectively. Thus,the application of NEPCM in the cooling industry can improve the performance of refrigeration systems and save the running time for refrigeration systems.
Table 4 Freezing time of Cu-H2O NEPCM
In order to solve the imbalance of electrical demand in summer and save energy,using the thermal energy storage of phase change material is one of the effective ideas. The potential of Cu-H2O NEPCM as a new PCM was investigated numerically in this study. Probably due to the enhancemant of thermal conductivity,the freezing rate of fluids is enhanced. The whole freezing time can be saved by 9.9%,16.3%,and 29.4% at the 0.1%,1.0%,and 5.0% NEPCM. The reasons for this phenomenon may be the higher thermal conductivity for the NEPCM and the Cu nanoparticles acting as a nucleating agent. In summary,the calculated quicker freezing rate of the NEPCM is a clear indicator of its great potential for thermal energy storage applications.
Reference
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2 Choi S U S. Enhancing Thermal conductivity of fluids with nanoparticles. In:Siginer D A,Wang H P. Developments and application of non-newtonian flows[C]. ASTM Spec.Tech. Publ.,1995:99-105.
The density of the NEPCM is given by:
2.教师要引导学生学习几何知识的现实意义。“几何”的原意是“测量土地的技术”,可通过资料让学生了解几何在古代的一些作用。在生产实践和建设现代化祖国的过程中,几何这门科学也有广泛的应用。教师可引导学生观察周围的图形,并提出一些启发思索的问题,如怎样画美丽的图形,怎样测量建筑物的高度等,使学生感知到几何知识无处不在,几何原理无处不用,从而激发学生的学习兴趣,培养学生审美情趣。
中国物流业潮起潮涌,在资本的助推下强者更强,行业集中度在上升,企业规模在壮大,中国物流业将迎来并购整合的黄金时代。
1.1 TOFD(衍射时差法)超声技术是利用缺陷部位的衍射波信号来检测和测定缺陷尺寸的一种超声波检测方法,通常使用纵波斜探头,采用一发一收模式。
5 Zhu D S,Wu S Y,Yang S. Numerical simulation on thermal energy storage behavior of SiC-H2O nanofluids[J]. Energy Sources,Part A,2011,33:1317-1325.
6 Ranjbar A A,Kashani S,Hosseinizadeh S F,et al. Numerical heat transfer studies of a latent heat storage containing nano-enhanced phase change material[J]. Thermal Science,2011,15:169-181.
7 Kashani S,Ranjbar A A,Abdollahzadeh M,et al. Solidification of nano-enhanced phase change material(NEPCM)in a wavy cavity[J]. Heat and Mass Transfer,2012,48(7):1155-1166.
8 Hosseini S M J,Ranjbar A A,Sedighi K,et al. Melting of nanoprticle-enhanced phase change material inside shell and tube heat exchanger[J]. Journal of Engineering,2013:8.
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参与本次调查的964例患者中,入院时评分高于3分的患者有770例,评分高于5分的患者有196例,排除接诊即立刻进入ICU接受抢救的患者以外,接诊后病情发生变化,后续转入ICU的患者有10例,接诊至普通病房进行治疗者有134例。患者入院后,具使用NEWS评分进行评价,通过对病情的详细观察,并针对实际情况给予护理措施以及治疗措施后,964例患者中NEWS评分达到0~4分的患者全部治愈,其病情好转率达到了100%;其中196例5~8分的患者入院后出现了14例死亡,所占比例为7.14%,其病情好转率达到了92.96%。
12 刘玉东. 纳米复合低温相变蓄冷材料的制备及热物性研究[D]. 重庆:重庆大学,2005.
Liu Y D. Study on preparation and thermal properties of phase change nanocomposites for cool storage[D]. Chongqing:Chongqing University,China,2005.
2016-03-15;
2016-07-07
广东省科技计划项目(2016A010104002)、广东省高等学校优秀青年教师培养计划项目(Yq2013197)。Funded by Science and Technology Planning Project of Guangdong Province of China(Grant No.2016A010104002),Program for Excellent Young Teachers of Higher Education Institutions of Guangdong Province of China(Grant No.Yq2013197).
李新芳,女,37岁,博士、副教授。Li Xinfang,femal,37 years old,doctor and associate professor of Zhongshan Torch Polytechnic in China.
TB611
A
1000-6516(2016)06-0054-08
纳米颗粒强化相变蓄冷特性的数值模拟
李新芳1王先菊2朱冬生3
(1中山火炬职业技术学院 中山 528436) (2华南农业大学理学院 广州 510641) (3中国科学院广州能源研究所 广州 510640)
采用Fluen软件对纳米颗粒强化相变蓄冷特性进行了数值模拟,重点分析纳米粒子添加量和Gr数对蓄冷性能的影响,并解释其换热机理。研究结果表明:纳米颗粒的体积分数是影响纳米颗粒强化相变材料结冰时间的一个主要因素,但Gr数对其结冰时间影响不大。对于一给定的Gr数,随着纳米粒子体积分数的增加,结冰时间减少,纳米粒子体积分数为1.0%时,纳米颗粒强化相变材料的结冰时间降低了16.3%。这是由于纳米颗粒强化相变材料具有较高的导热系数。另一方面,由于纳米颗粒强化相变材料融解潜热降低,则纳米颗粒强化相变材料结冰时,每单位质量的纳米颗粒强化相变材料需要的能量较少,所以纳米颗粒强化相变材料具有较高的热释放率,在相变储能应用中具有巨大优势。
纳米颗粒强化相变材料 蓄冷 结冰时间 数值模拟