Experimental Study On Suppressing Heat Transfer Deterioration Of Supercritical CO₂ In Vertical Tubes Based On Helmholtz Oscillator

Improving the utilization rate of waste heat recovery is one of the important means to attain the goal of energy step utilization.The trans-critical CO₂ Rankine system shows a great potential for this application.As one of the important components of the trans-critical CO₂ Rankine system,the gas heater faces serious challenges in practical applications.When the CO₂ mass is heated near the pseudo-critical point in gas heaters,it is likely to occur the severe heat transfer deterioration（HTD）under high thermal load conditions due to the drastic changes of thermal physical properties as well as the buoyancy and thermal acceleration effects.The occurrence of HTD can lead to a reduction in system thermal efficiency and an increase in pipeline thermal corrosion,jeopardizing system operational safety.Therefore,the design of one structure with better heat transfer performance is the key to improving the thermal efficiency and ensuring the safety of system operation.Helmholtz oscillator is a cylindrical cavity with two open ends and a closed perimeter.It is a fluid mechanical component with passive enhanced heat transfer because its internal flow field can generate pulsating flow without additional excitation.In this manuscript,the pulsating flow generated by the Helmholtz oscillator is used to suppress the HTD of supercritical CO₂（S-CO₂）heated in a vertical circular tube,and the related experimental research and theoretical analysis are carried out with the smooth tube（ST）and the smooth tube with Helmholtz oscillators（THOs）as the research objects.The main contents are as follows.（1）The Helmholtz oscillator for suppressing the HTD of S-CO₂ heated in tubes is designed and processed based on the theory of fluidic networks.An experimental bench for S-CO₂ flow and heat transfer based on Helmholtz oscillator is built and its reliability is verified.The convective heat transfer characteristics of S-CO₂ in ST and THOs are investigated experimentally at mass fluxes from 144 to 750 kg/（m²·s）,heat fluxes from23.0 to 128.1 k W/m²,inlet temperatures from 15.0 to 25.0°C,inlet Reynolds number from 6.6×10³～4.5×10⁴,and a fixed pressure of 8.0 MPa.（2）Based on the above study,it is found that the mass flux has a greater effect on THOs in suppressing the HTD of S-CO₂.When the mass flux is greater than 350 kg/（m²·s）,THOs can effectively improve the convective heat transfer intensity of S-CO₂,and the locally enhanced heat transfer ratio(Nu_x,THO/Nu_x,ST)can reach 9.86.In addition,within the range of structure parameters in this manuscript,the outlet diameter of Helmholtz oscillator has a relatively large effect on the heat transfer characteristics of S-CO₂ in the tube.At a mass flux of 400 kg/（m²·s）,the thermal performance evaluation factor of Helmholtz oscillator with upstream and downstream nozzle diameter of 2.0 and 1.8 mm,respectively,is better,with values up to 3.09.（3）The buoyancy and thermal acceleration effects of S-CO₂ in THOs and ST are compared and analyzed.The results show that the buoyancy effect in THOs is significantly weakened,and the mechanism of THOs to inhibit the severe HTD of S-CO₂in the heating tube is revealed.The heat transfer correlation of S-CO₂ in the heated tube with Helmholtz oscillators with high accuracy is proposed by considering the thermo-physical properties of CO₂ and the oscillator’s structure parameters,in which 99.1%of the data points deviate from the correlation equation within±20%.