Thermophysical Properties and Supercritical Heat Transfer Characteristics of R515A

The heat transfer of supercritical fluids is a vastly growing field, specifically to find suitable alternative to replace conventional R134a, which can be beneficial for climate change. A considerable suggestion is R515A which possesses considerably lower global warming potential. The present simulations are designed to study supercritical fluid R515A under cooling conditions in horizontal position. The effect of pressure, mass flux, heat flux and tube diameter were considered for horizontal tube in the vicinity of pseudo critical temperature. Numerical investigations on heat transfer characteristics of supercritical fluid R515A were performed using widely used shear-stress transport (SST) model. Moreover, heat transfer correlations were developed and suggested to accurately predict Nusselt number within 10% accuracy. The simulation results showed about 3.98% average absolute deviation.

Numerical Investigations on Supercritical Heat Transfer Characteristics of Environmental Friendly Refrigerants

The heat transfer of supercritical fluids is a vastly growing field, specifically to find suitable alternatives to replace conventional R134a, which can be beneficial for climate change. Most of the experimental and numerical investigations have been conducted to explore supercritical water, carbon dioxide and R134a as heat transfer working fluids. Hydrofluoroolefin (HFO) and refrigerants blends have been considered the most environment-friendly refrigerants to replace Chlorofluorocarbons (CFCs), Hydrochlorofluoro-carbons (HCFCs) and Hydrofluorocarbons (HFCs). Their main advantage of zero Ozone Depletion Potential (ODP) and comparatively lower Global Warming Potential (GWP) have attracted growing amount of attention to mitigate environmental issues. This work adopts the computational method and takes the environmentally friendly refrigerants to investigate the heat transfer characteristics under widely used shear-stress transport (SST) model. A comprehensive comparison was performed at reduced pressure of 1.10 for supercritical fluids R515A, R1234ze(E) and R134a. The peaks of heat transfer coefficient occurred in the vicinity of pseudo critical temperature for all of these considered fluids;however, R134a resulted in higher heat transfer coefficient, Reynolds number and Prandtl number in comparison with R515A and R1234ze(E). The higher heat transfer coefficient of supercritical fluid R134a is owing to its thermophysical properties and the specific heat plays crucial role in the heat transfer of supercritical fluids. Owing to environmental issues, R515A can be a considerable replacement of R134a. R1234ze(E) is also promising alternative to R134a;however, safety issues should thoroughly concern its mild flammable characteristics.

异丁烷绝热毛细管流动均相流和分离流模型对比

毛细管作为一种膨胀装置在小型制冷系统和热泵系统中得到了广泛的应用。尽管毛细管具有简单的几何形状,其中的绝热流动却是极其复杂的。本文对模拟异丁烷(R600a)在毛细管中绝热流动的均相流动模型和分离流模型进行了对比研究。研究了不同的摩擦系数和两相黏度关系式的组合以及亚稳态流动对流动特性的影响。结果表明,采用分离流模型时预测的质量流量较低。分离流模型在预测超过2 kg·h^(−1)的质量流量时表现较好,Colebrook摩擦系数关系式结合Dukler或McAdams黏度关系式得到的偏差较小,在忽略和考虑亚稳态流动的情况下分别为5.43%,5.49%和5.44%,5.43%。此外,采用Bittle和Pate摩擦系数和Dukler黏度关系式的均相流模型在低于2 kg·h^(−1)的质量流量范围内具有最高的精确度,不考虑亚稳态流动情况下的平均误差为4.12%,考虑亚稳态流动情况下的平均误差为3.37%。

Impacts of Nanoparticle Shape on Al₂O₃-Water Nanofluid Flow and Heat Transfer over a Non-Linear Radically Stretching Sheet

The results of this article can be useful in science and technology advancement, such as nanofluidics, micro mixing and energy conversion. The purpose of this article is to examine the impacts of nanoparticle shape on Al2O3-water nanofluid and heat transfer over a non-linear radically stretching sheet in the existence of magnetic field and thermal radiation. The different shapes of Al2O3 nanoparticles that have under contemplation are column, sphere, hexahedron, tetrahedron, and lamina. The governing partial differential equations (PDEs) of the problem are regenerated into set of non-linear ordinary differential equations (ODEs) by using appropriate similarity transformation. The bvp4c program has used to solve the obtained non-linear ordinary differential equation (ODEs). The Nusselt number for all shapes of Al2O3 nanoparticle shapes in pure water with is presented in graphical form. It has reported that the heat transfer augmentation in lamina shapes nanoparticles is more than other shapes of nanoparticle. The relation of thermal boundary layer with shapes of nanoparticles, solid volume fraction, magnetic field and thermal radiation has also presented with the help of graphical representation. It is also demonstrated that lamina shape nanoparticles have showed large temperature distribution than other shapes of nanoparticles.

Convection Heat Transfer from Heated Thin Cylinders Inside a Ventilated Enclosure

Experimental study was conducted to determine the effect of velocity of axial fan,outlet vent height,position,area,and aspect ratio(h/w)of ventilated enclosure on convection heat transfer.Rectangular wooden ventilated enclosure having top and front transparent wall was made up of Perspex for visualization,and internal physical dimensions of box were 200 mm×200 mm×400 mm.Inlet vent was at bottom while outlet vents were at the side and top wall.Electrically heated cylindrical heat source having 6.1 slenderness ratio was fabricated and hanged at the centre of the enclosure.To calculate heat transfer rates,thermocouples were attached to the inner surface of heat source with silica gel.Heat source was operated at constant heat flux in order to quantify the effect of velocity of air on heat transfer.It was observed that average Nusselt number was increased from 68 to 216 by changing velocity from 0 to 3.34 m/s at constant modified Grashof number i.e.5.67E+09.While variation in outlet height at the front wall did not affect heat transfer in forced convection region.However,Nusselt number decreased to 5%by changing the outlet position from top to the front wall or by 50%reduction in outlet area during forced convection.Mean rise in temperature of enclosure increased from 8.19 K to 9.40 K by increasing aspect ratio of enclosure from 1.5 to 2 by operating heat source at constant heat flux i.e.541.20 w/m^(2).

Experimentation on Optimal Configuration and Size of Thin Cylinders in Natural Convection

In this paper,an experimental study of laminar,steady state natural convection heat transfer from heated thin cylinders in an infinite air medium has been reported.Two electrically heated cylinders having the same slenderness ratio(L/D)i.e.6.1 but different diameters i.e.3.8 cm and 5.08 cm were used.105 experiments were carried out to study the effect of diameter and inclination angle of thin cylinder on natural convection heat transfer.After mandatory corrections of radiation and endcap heat losses,convective heat transfer results were presented in the form of local and average dimensionless numbers.For vertical configuration of thin cylinder,Nusselt number was varied from 52.99 to 95.10 corresponding to 1.28×10^(8)≤Ra*L≤1.08×10^(10).While for horizontal configuration,Nusselt number was varied from 10.74 to 17.78 corresponding to 9.42×10^(4)≤Ra*D≤8.17×10^(6).Results were compared with the published data and found satisfactory as the maximum percentage difference was only 3.09%.The essence of research is that the heat transfer coefficient increases with decrease in diameter and increase in inclination angle.Smoke flow visualization was done to capture patterns of fluid flow.Finally,comparison was made to quantify increase in Nusselt number from slender cylinder as compared to the flat plate.