混合制冷工质R1234yf/R32 PVTx性质的实验研究

利用基于Burnett法搭建的PVTx性质测试系统测量了混合制冷工质R1234yf/R32在温度为253~313 K范围内、R1234yf的质量分数为20%时的气相PVTx性质.详细介绍了该测量系统基于Burnett法的测量原理和PVTx性质测试系统的实验步骤,并对各种测量误差及不确定度进行了分析.根据实验数据拟合出混合制冷工质的气态维里方程,并比较了第二、第三维里系数与温度的关系.结果表明:实验数据与拟合得到的方程具有较好的一致性,为基础热物性研究和进一步研究替代制冷剂提供了详实的数据.     

二氧化碳/二甲醚混合工质热泵系统循环性能分析

在相同的工况条件下,对二氧化碳(R744)/二甲醚(RE170)混合工质与四种常见的热泵工质R22、R134a、R410A和R407C的亚临界循环性能进行了分析计算。结果发现,在R744/RE170质量配比为30/70下,系统的制热循环性能系数最大,其值为4.922,分别比R22、R134a、R410A和R407C系统提高了17.53%、30.52%、19.09%和16.52%;此时,系统的冷凝压力为2.276MPa仅高于R134a系统,压缩机压比为3.708,压缩机出口工质排气温度为92.6℃。     

高压壬酸乙酯比定压热容的实验测量与关联

壬酸乙酯是生物柴油的主要组分之一,其热物理性质是生物柴油制备及应用过程设计的基础数据。利用流动型量热法高压流体比定压热容实验测量系统对壬酸乙酯在温度为303.00～393.00 K、压力为0.10～25.00 MPa时的比定压热容进行了测量,该系统比定压热容的测量不确定度为0.98%。通过实验数据研究了壬酸乙酯比定压热容的变化规律,并对壬酸乙酯比定压热容的实验数据进行了关联,其计算值与实验值之间的最大偏差为0.54%。     

R410A/R404A/R407C在T型翅内螺纹水平管外的降膜蒸发换热研究

为分析R410A、R404A、R407C在T型翅内螺纹水平强化管外降膜蒸发的换热特性,分别在变喷淋密度(0. 047～0. 113 kg/(m·s))、变蒸发温度(0～16℃)以及变热流密度(10～40 k W/m~2)条件下进行了实验,采用"Wilson"图解法以及热阻分离法进行实验数据处理,得到了3种制冷工质在管外降膜蒸发时的换热特性。结果表明:随着喷淋密度的增加,R410A、R404A和R407C 3种制冷工质的管外降膜蒸发传热系数先增加后减少,存在最佳喷淋密度,分别为0. 092、0. 088和0. 095 kg/(m·s);随着蒸发温度的升高,R410A和R404A的管外降膜蒸发传热系数先减小后增大,而R407C的管外降膜蒸发传热系数则一直在增大,但均小于R410A和R404A;随着热流密度的增加,3种制冷工质的管外降膜蒸发传热系数也随之增大,其中,R410A的换热性能最好,R404A次之,R407C最差。通过传热分析以及实验数据拟合,得到了3种制冷剂的降膜蒸发传热关联式。     

低熔点熔融盐管内强迫对流换热的实验研究

利用课题组自行配制的二元混合熔融盐(KNO3-Ca(NO3)2)开展导热油与低熔点熔融盐的管内强制对流实验研究。通过实验得到导热油与低熔点熔融盐的总换热系数,并通过最小二乘法和Wilson分离法得到了管内低熔点熔融盐侧的对流换热系数及其准则关联式。与不同的经典传热关联式对比,最大偏差为+23%。考虑高温熔融盐的变物性特征,利用黏度项对Dittus-Boelter方程关联式进行修正。经过修正后的Dittus-Boelter方程与实验测试结果最大偏差为-15%,偏差值明显减小。过渡流实验数据和Hausen方程及Gnielinski方程的最大偏差均为10%,实验结果验证了传热关系式仍适用于高温熔融盐的结论。     

流动量热法测量超临界流体定压比热实验研究

超临界流体的比定压热容数据的准确测量对于众多科学研究和工程应用至关重要。基于流动型量热法建立了一套适用于测量在25.00~80.00 ℃ 的温度、0.10~12.00 MPa 压力范围内的比定压热容在线测量实验系统,其测量的扩展相对不确定度为1.66%~2.46%（置信因子k=2）。在使用纯水（工况压力为7.00~12.00 MPa,温度25.00~60.00 ℃）、环己烷（工况压力为2.00~6.00 MPa,温度30.00~80.00 ℃）和CO2（工况压力为6.70~12.00 MPa,温度25.00~70.00 ℃）进行测量的基础上,得到平均绝对偏差分别为0.14%、0.99%和0.78%,最大绝对偏差分别为0.26%、1.32%和2.28%。验证了实验系统的实用性和可靠性,能够为科研和工程设计提供准确的数据测量支持。     

O2在庚酸乙酯中的溶解度测量

利用等体积饱和法气体溶解度实验系统首次测量了温度为293.00~333.00K、压力为0.70~4.00MPa内O2在庚酸乙酯中的溶解度数据,溶解度的相对扩展不确定度小于4%。实验结果表明:O2在庚酸乙酯中的溶解度随着压力升高而增大,随温度升高而减小。利用NRTL方程对溶解度实验数据进行了关联,计算结果与实验数据之间的平均绝对偏差为1.6%。     

R-1234yf在[HMIM][Tf2N]中的溶解度

基于等体积饱和法测量了温度在293.0~353.0K、压力在99.0~925.0kPa时R1234yf在离子液体1-己基-3-甲基咪唑双三氟甲磺酰亚胺盐([HMIM][Tf_2N])中的溶解度,温度、压力、溶解度的测量不确定度分别为0.02K、0.3kPa、3%。利用改进的Krichevsky-Kasarnovsky方程和NRTL方程对R1234yf在[HMIM][Tf_2N]中的溶解度进行了关联,计算值与实验值之间的相对偏差绝对平均值均为0.7%,最大相对偏差均小于1.9%。     

二氧化碳/丙烷用于热泵系统循环性能分析

在热泵热水器名义工况条件下,对二氧化碳(R744)/丙烷(R290)自然混合工质与四种常见的热泵工质R22、R134a、R410A和R407C的亚临界循环性能进行了分析对比。结果表明:R744/R290的最优质量配比为21/79,系统的制热循环性能系数(COPh)比R22系统提高了10.46%、比R134a系统提高了22.67%、比R410A系统提高了11.93%、比R407C系统提高了9.52%;在10%~35%、0%~35%、10%~35%和10%~35%的R744质量配比内,R744/R290可分别实现对四种常见热泵工质的替代;在最优质量配比下,R744/R290系统的冷凝压力低于R410A系统,压比仅为3.271,排气温度为80.9℃。     

折返式鼠笼弹性支承刚度试验技术研究

为准确测量折返式鼠笼弹性支承的刚度,在对其进行数值分析的基础上,设计了自动控制刚度试验系统,根据试验数据分析不同的位移传感器、不同的施力位置、测点位置和力施加速度对支承刚度试验值的影响。研究表明:数值仿真得到的鼠笼弹性支承变形规律与试验测试结果一致,折返套尾端位移比折返处大,且位移与施加的力呈线性关系;不同直径的电涡流传感器测得的刚度与电子千分表相比,最大偏差为10.17%;折返处的刚度测量值稳定性好,力施加位置改变时刚度测量值的最大变化幅度仅为2%,而折返套尾部刚度的最大变化幅度达到20.6%;力施加速度小于60 N/s时其对刚度试验值影响很小。     

Experimental study and empirical correlation development of fuel properties of waste cooking palm biodiesel and its diesel blends at elevated temperatures

In this experimental work, the density, dynamic viscosity and higher heating value of methyl ester based waste cooking palm-biodiesel oil (WMEPB) was investigated under varying temperature and blend ratio condition with No. 2 diesel fuel. The transesterified fatty acid methyl ester of palm vegetable oil collected from local food and beverage shops was used as neat biodiesel. Four different fuel blends (20%, 40%, 60% and 80% by volume mixing with base diesel) were studied along with base No. 2 diesel fuel and pure biodiesel. Tests for dynamic viscosity and density were performed in the temperature range 0–130 °C for each fuel sample whereas the higher heating values were determined at 25 °C room temperature condition. It is found that pure biodiesel has the highest density and dynamic viscosity at a given temperature whereas it exhibits lowest combustion heating value among the six fuels. Moreover, the density for each fuel sample decreases linearly with the increase in temperature. On the other hand, the dynamic viscosity decreases exponentially with the temperature for each fuel sample. In addition, based on the experimental results, regression correlations have been proposed for the density, dynamic viscosity, and higher heating value of the fuels. Subsequently, comprehensive error analyses of these proposed correlations were performed. In particular, the correlation for density and dynamic viscosity were respectively compared with Kay's mixing rule and Grunberg-Nissan mixing rule theory in order to validate their applicability. It is found that density correlations predicted within ±0.3% average error band. And, as high as 72.2% of the dynamic viscosity data were in the range of ±5% average error while the remaining data fell within ±10% error range. And finally, through a comparative study with the available fuel property results of fresh methyl ester palm biodiesel, it is found that available existing correlations derived from fresh palm biodiesel studies can not accurately predict the fuel properties of same waste biodiesel and its blends with diesel.     

多流体模型在燃气轮机燃烧室三维反应流中的应用

对一种模型燃气轮机燃烧室中的三维反应流进行了数值模拟，模型燃烧室的燃料是CH4，燃烧类型是预混燃烧，在数值模拟过程中，采用了Spalding于1995年提出来的多流体模型来对燃烧室中的湍流预混燃烧进行了数值模拟，在数值模拟过程中考虑了辐射问题，采用了六通量辐射模型。通过数值模拟给出了速度，压力，湍流脉动动能，湍流动能耗散率，焓值，湍流粘度，温度，密度，燃烧产物质量分数，氧的质量分数，燃料/空气混合比，燃料质量分数，空间三个方向的辐射热通量以及各种流体的质量分数等变量的分布情况，此外，还采用传统的旋涡破碎模型对此燃烧室进行了数值模拟，并对两种方法的结果进行了分析比较，由分析可以看出多流体模型的结果接近于实际情况，对模型燃烧室进行三维反应流数值模拟的工作为今后对实际燃烧室反应流的数值模拟打下了一定的基础。     

Thermophysical properties of fly ash–Cu hybrid nanofluid for heat transfer applications

The effect of temperature and concentration on the thermophysical properties of fly ash–copper (80% fly ash and 20% Cu by volume) water-based stable hybrid nanofluid is studied. The experiments are conducted for the volume concentration range of 0 to 0.5% in the temperature range of 30 to 60°C. The nanoparticles have been characterized by transmission electron microscopy and dynamic light scattering to determine an average nanoparticle diameter of 15 nm. The stability of nanofluid in the presence of surfactant Triton X-100 is examined with the help of zeta potential. The maximum enhancement in thermal conductivity and viscosity is 19% and 22%, respectively. The outcome of the present study showed that density, thermal conductivity, and viscosity of the hybrid nanofluid increased, whereas specific heat decreased with an increase in the nanofluid concentration. In addition, the specific heat and thermal conductivity increase, there is a decrease in density and viscosity of the hybrid nanofluid with an increase in temperature.     

Experimental exploration and theoretical certainty of thermal conductivity and viscosity of MgO-therminol 55 nanofluid

In this study, a combination of thermal conductivity, viscosity, and density characteristics are experimentally probed for attaining maximum heat transfer using MgO-Therminol 55 as nanofluid is reported. Recent studies proved that nanofluids have miserable properties that make them feasibly useful in many applications in heat transfer compared to base fluid.MgO-Therminol 55 nanofluid is synthesized by diffusion of MgO nanoparticles of size 160–190 nm in Therminol 55 at different concentrations (0.05%–0.3%). Thermal conductivity and viscosity are calculated at a temperature range of 30–60°C using kd₂ analyzer and Fenske viscometer. Data obtained from the experimental results reveals that when volume concentration is increased with respect to that thermal conductivity increases, viscosity decreases and density decreases at different temperatures. The proposed models were supportive to the experimental data.     

The effects of Ohmic heating and viscous dissipation on unsteady MHD and slip flow over a porous rotating disk with variable properties in the presence of Hall and ion-slip currents

The effects of Ohmic heating and viscous dissipation on unsteady laminar magneto-hydrodynamics (MHD) flow of a viscous Newtonian and electrically conducting fluid over a rotating disk taken into account the variable fluid properties (density, (ρ), viscosity, (μ) and thermal conductivity, (κ)) in the presence of Hall and ion-slip currents effects have been examined. These fluid properties are taken to be dependent on temperature. The unsteady Navier–Stokes equations along with the energy equation are reduced to a system of ordinary differential equations by using similarity transformations and the resulting equation system is solved numerically by using a shooting method. Results for the details of the velocity as well as temperature are shown graphically and the numerical values of the skin friction and the rate of heat transfer are entered in tables.     

Prediction models for density and viscosity of biodiesel and their effects on fuel supply system in CI engines

Biodiesel is a promising non-toxic and biodegradable alternative fuel used in the transport sector. Nevertheless, the higher viscosity and density of biodiesel poses some acute problems when it is used it in unmodified engine. Taking this into consideration, this study has been focused towards two objectives. The first objective is to identify the effect of temperature on density and viscosity for a variety of biodiesels and also to develop a correlation between density and viscosity for these biodiesels. The second objective is to investigate and quantify the effects of density and viscosity of the biodiesels and their blends on various components of the engine fuel supply system such as fuel pump, fuel filters and fuel injector. To achieve first objective density and viscosity of rapeseed oil biodiesel, corn oil biodiesel and waste oil biodiesel blends (0B, 5B, 10B, 20B, 50B, 75B, and 100B) were tested at different temperatures using EN ISO 3675:1998 and EN ISO 3104:1996 standards. For both density and viscosity new correlations were developed and compared with published literature. A new correlation between biodiesel density and biodiesel viscosity was also developed. The second objective was achieved by using analytical models showing the effects of density and viscosity on the performance of fuel supply system. These effects were quantified over a wide range of engine operating conditions. It can be seen that the higher density and viscosity of biodiesel have a significant impact on the performance of fuel pumps and fuel filters as well as on air-fuel mixing behaviour of compression ignition (CI) engine.     

An experimental study on viscosity of alumina-engine oil: Effects of temperature and nanoparticles concentration

In the present study, the dynamic viscosity of alumina-engine oil nanofluid in different solid volume fractions and temperatures was experimentally investigated. The nanofluid samples were prepared in the solid volume fractions of 0.25%, 0.5%, 0.75%, 1%, 1.5% and 2% under the temperature range of 5 to 65 °C. The measurements were carried out by CAP 2000 + Viscometer, supplied by Brookfield of the USA. Using the experimental data, new correlations for predicting the dynamic viscosity of alumina-engine oil at different temperatures were proposed. The experiment results at different shear rates showed that all nanofluid samples exhibit the Newtonian behavior. The results also revealed that the viscosity of the nanofluid increases with the solid volume fraction. Moreover, it has been found that with increasing temperature, the viscosity of nanofluids decreases, and it was more tangible at the lower temperatures. The comparison between experimental findings and theoretical models showed that these models failed to predict the correct values of the viscosity of the nanofluids at all solid volume fractions. The experimental data also indicated that the maximum viscosity enhancement of nanofluid was 132% compared with that of base fluid.     

Study on wall temperature distribution of oscillating tube

The wall temperature distribution and heat transfer process of the oscillating tube have been investigated in this paper using both numerical simulation and experimental method. The wall temperature of oscillating tube increases rapidly in the inlet and then decreases slowly, moreover, the rally phenomenon of wall temperature near the closed end is observed. With the increase of jet flow frequency, the highest wall temperature increases and the location of that moves towards the inlet. The velocity of pressure wave in the oscillating tube almost remains constant even its intensity changes. The quantity of heat transfer between the gas and inner wall of the oscillating tube determines the wall temperature of every location, and the pressure wave disturbance can cause the heat transfer quantity change. Each pressure wave has its own disturbance range. The wall temperature distribution can be explained by the change of pressure wave intensity and its disturbance time. Besides, the step and rally of wall temperature are discussed, which shows that the conditions of heat transfer can be improved due to intersection or reflection of pressure waves.     

Effect of heat transfer on stability and transition characteristics of boundary-layers

Stability and transition problems of two dimensional boundary-layers with heated walls have been studied numerically using the linear stability theory. Incompressible stability equations have been modified to account for the variation of temperature dependent fluid properties across the layer. The equations obtained have been solved with an efficient shoot-search technique. Low speed flows of air and water have been analyzed with a wide range of heat transfer rates. In addition to the mean velocity profile characteristics, variable viscosity and density terms in the stability equations also have considerable influence on the results of the stability and transition analysis.     

Unsteady nanofluid flow between two spinning expanding disks with continuous vertical motion under the influence of modified Hall effect

The principle aim of this article is to detect the effects of externally applied magnetic force in the nanofluid flow, flowing between two co-axially rotating and expanding disks where the upper disk is continuously moving vertically upward and downward. Also, the modified Hall Effect has been considered as an effective factor of the flow. The lower disk is vertically static. The rotation and vertical motion of the disks create a three-dimensional flow of nanofluid. Heat and mass density along with the motion of the flow has been analyzed under the variation in magnetic and Hall parameters. The findings have been compared with the results in Von Karman flow of nanofluid between two rotating and stretching disks. The velocity components have been largely influenced by magnetic and Hall parameters in case of downward movement of the upper disk. The fluid temperature is detected higher in case of upward velocity and lower in case of the downward motion of the upper disk. The heat transferability of the disks is effected differently at two different disks with the influence of magnetic force and Hall effect.