氢气在螺旋管内流动冷凝传热特性研究OA

Objective:To satisfy the optimization and design requirements of small-scale hydrogen liquefaction systems cooled by cryogenic refrigerators,this study investigates the flow-condensation heat-transfer process of hydrogen in a small-diameter helical tube.The objective is to reveal the condensation behavior and key factors influencing hydrogen under cryogenic conditions.Owing to its low viscosity,surface tension,and two-phase density ratio,the heat-transfer mechanism of hydrogen differs significantly from that of conventional refrigerants.However,existing experimental data and correlations for hydrogen flow condensation remain limited.Therefore,in this study,the effects of mass flux,saturation pressure,and temperature difference on the flow condensation heat-transfer performance of hydrogen were investigated,thus providing experimental evidence and theoretical references for the structural design and performance optimization of small-scale hydrogen liquefaction systems. Methods:An experimental setup for hydrogen-flow condensation was established using a Gifford–McMahon refrigerator.The condenser comprises a stainless-steel helical coil(inner diameter:1.75 mm)welded onto a copper block in close contact with the refrigerator to ensure efficient heat transfer.Experiments were conducted at three saturation pressures(495 kPa,665 kPa,and 850 kPa)with mass fluxes ranging from 15 kg/(m2·s)to 40 kg/(m2·s).The temperature,pressure,and flow rate were measured,and the hydrogen-condensation heat-transfer coefficient was calculated using a thermal-resistance model and the heat-balance method.Prior to the hydrogen-condensation experiments,the refrigerator performance was tested,and a fitted function relating the cooling capacity to the cold-head temperature was obtained.Additionally,computational fluid dynamics(CFD)simulations were performed to verify the uniformity of the internal temperature distribution of the copper block.The results confirmed that the measured temperature represented the refrigerator cold-head temperature,thus ensuring the reliability of the heat-transfer calculations. Results and Discussions:The condensation heat-transfer coefficient increased with mass flux,with a higher growth rate at low mass fluxes and a more gradual increase at high mass fluxes.This behavior is primarily due to the thinning of the liquid film,enhanced convective heat transfer,and reduced thermal resistance as the two-phase flow velocity increased.At low mass fluxes,increasing the saturation pressure decreases the condensation heat-transfer coefficient,which is attributed to liquid-film thickening,reduced thermal conductivity,and weakened film fluctuations.However,at high mass fluxes,turbulence dominates heat transfer,thereby diminishing the effect of pressure,and the heat-transfer coefficients stabilize at different saturation pressures.The condensation heat-transfer coefficient decreased with increasing temperature difference.A larger temperature difference thickened the liquid film,increased the liquid viscosity,and increased the gas-liquid density ratio,thereby increasing the thermal resistance.This trend aligns with the data reported by Ohira et al.,thereby verifying the reliability of the experimental results. Conclusions:Within the experimental range,the hydrogen-condensation heat-transfer coefficient increased significantly with mass flux,with an overall enhancement of 74%-133%.Moreover,the increase was more pronounced under low mass fluxes.Additionally,at low mass fluxes,when the saturation pressure increased from 495 kPa to 850 kPa,the condensation heat-transfer coefficient decreased by 47%,whereas the effect of pressure weakened at high mass fluxes.Under the present experimental conditions,the condensation heat-transfer coefficient ranged from 594 W/(m2·K)to 1 388 W/(m2·K).