Heat transfer enhancement of supercritical carbon dioxide in eccentrical helical tubes
Heat transfer enhancement of supercritical carbon dioxide in eccentrical helical tubes
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超临界二氧化碳在偏心螺旋管内的强化换热
DOI：
10.1016/j.ijheatmasstransfer.2023.125041
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发表时间：
2024
影响因子：
5.2
通讯作者：
Wenguang Li;Zhibin Yu
中科院分区：
文献类型：
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作者：
Wenguang Li;Zhibin Yu
提出了一种用于制冷系统中高效紧凑的超临界二氧化碳换热器的偏心螺旋管。采用ANSYS CFX 2019 R2软件，采用三维定常雷诺平均Navier-Stokes方程、剪切应力输运湍流模型和能量方程，模拟了质量流量为200、400 kg/m2 s时，SCO 2在螺距为25、50 mm，偏心距为0.9、1.5 mm的3种偏心螺旋管内的强制对流换热。入口压力为8.9 MPa，外壁热通量为12.24 kW/m2。利用实验得到的平均换热系数和经验达西摩擦系数对光管内的流动和传热模型进行了验证。研究了螺距、偏心距和操作条件对强化传热的影响。阐明了流动形态、传热机理和旋涡运动学。比较了超临界CO2与水、空气分别在EHT、扭曲椭圆管和锥形管中的强化传热效果。当管径为6 mm时，节距为25 mm，偏心距为1.5mm时，可获得较好的热工水力性能。较高的SCO 2质量流量使Nusselt数增加，摩擦系数减小，较高的进口压力和较大的壁面热流使Nusselt数减小，摩擦系数增大。f/f0的变化范围为2.52-1.68，随进口压力和名义雷诺数的增加而减小，在拟临界点附近，Nu/Nu 0、η和Nu/Nu 0的变化曲线呈凹形，分别为1.68-1.06和1.06-1.55，1.23-0.83和0.83-1.30。在螺距为25 mm，偏心距为1.5 mm，进口压力为8 MPa，质量流量为400 kg/m2 s，壁面热流密度为12 kW/m2时，其平均热流密度分别为0.67-0.50和0.50-0.92。EHT中出现了一种带核心流的螺旋流型，局部换热系数可以与绝对螺旋度相关联。EHT脊部的高壁面切应力是强化换热的主要原因。EHT的f/f0值介于扭曲椭圆管和锥形管之间，但Nu/Nu 0和Nu值相当。
A type of eccentrical helical tube (EHT) was proposed for more efficient and compact supercritical carbon dioxide (SCO2) heat exchangers in refrigeration systems. The forced convective heat transfer of SCO2 flowing in three designed eccentrical helical tubes with pitches of 25, 50 mm and eccentricities of 0.9, 1.5 mm was simulated by using the three-dimensional steady Reynolds-averaged Navier-Stokes equations, shear stress transport turbulence model and energy equation in ANSYS CFX 2019 R2 at mass fluxes of 200, 400 kg/m 2 s, inlet pressures of 8, 9 MPa, outwards wall heat fluxes of 12, 24 kW/m 2. The flow and heat transfer models were validated in the plain tube with experimental mean heat transfer coefficient and empirical Darcy friction factor. Influences of pitch, eccentricity and operational conditions on heat transfer enhancement were identified. Flow pattern, heat transfer mechanism and vortex kinematics were clarified. Heat transfer enhancement of SCO2 in the EHT was compared with water and air in the EHT, twisted elliptical tube and conical tube, respectively. A pitch of 25 mm and eccentricity of 1.5 mm at 6 mm tube diameter can achieve a better thermal-hydraulic performance. A higher SCO2 mass flux raises Nusselt number and reduces friction factor, a higher inlet pressure and a larger wall heat flux reduces Nusselt number but increases friction factor. The ratio f/f 0 is ranged in 2.52–1.68 and drops off with increasing inlet pressure and nominal Reynolds number, the ratios N u/N u 0, ψ and η curves show a concave shape near the pseudocritical point ranged in 1.68–1.06 and 1.06–1.55, 1.23–0.83 and 0.83–1.30, 0.67–0.50 and 0.50–0.92 at the pitch of 25 mm and eccentricity of 1.5 mm, inlet pressure of 8 MPa, mass flux of 400 kg/m 2 s, and wall heat flux of 12 kW/m 2. A helical flow pattern with a core flow occurs in the EHT, the local heat transfer coefficient can be correlated to absolute helicity. The high-wall shear stress on the ridge of the EHT is responsible for the heat transfer enhancement. The f/f 0 value of the EHT is in between the twisted elliptical tube and the conical tube, but the N u/N u 0 and ψ values are comparable.