The Characteristics And Mechanisms Of Self-excited Oscillation Pulsating Flow On Suppressing Heat Transfer Deterioration Of Supercritical CO₂

Carbon dioxide（CO₂）is considered as a kind of non-toxic and non-combustible natural working fluid.It is widely used in various new power generation systems and low-grade waste heat recovery due to stable chemical properties and superior thermophysical properties,which not only can significantly reduce the volume of thermal system,but also can effectively improve the cycle thermal efficiency.The thermophysical properties of supercritical CO₂ change drastically with temperature near the pseudo-critical point(T_pc),resulting in a complex boundary layer structure,further causing heat transfer enhancement（HTE）and heat transfer deterioration（HTD）.HTD is always accompanied by a sudden increase in wall temperature and a sudden drop in heat transfer coefficient,making the irreversible loss of heat transfer process increase and the thermal efficiency of system decrease.Moreover,the excessive heating of surface leads to the corrosion of tube wall and even making the tube burn out.These undesirable phenomena pose a great danger to the safety of system.Self-excited oscillation pulsating flow as a passive enhanced heat transfer technology has a simple oscillator structure with small volume and without any additional energy to generate disturbance.Therefore,in this paper,we carry out experimental and numerical simulation studies to suppress the HTD of supercritical CO₂ heated in vertical upward tube with Helmholtz oscillator,and provide valuable basic data and physical model to effectively suppress the HTD of supercritical CO₂.Firstly,an experimental investigation on suppressing HTD of supercritical CO₂by self-excited oscillation pulsating flow is carried out.It is conducted at the pressure 8 MPa,the mass fluxes from 350 to 450 kg/（m²·s）,the heat fluxes from 30 to 72 k W/m²,and the inlet temperature 288.15 K.The heat transfer characteristics are compared with that of without Helmholtz oscillator at the inlet of the heated tube under the same operating conditions.The experimental results show that self-excited oscillation pulsating flow can significantly reduce the peak of wall temperature and improve the heat transfer performance of supercritical CO₂.The HTE effect is mainly concentrated in HTD region of smooth tube without Helmholtz oscillator and the ratio of HTE gradually decreases along the flow direction.The heat transfer parameters present some oscillations with small amplitude along flow direction before pseudo-critical point(T_pc).In addition,the local and average ratio of HTE can be up to 5.79 and 3.41,respectively,at the mass flux of 450 kg/（m²·s）and heat flux of 56 k W/m².Then,the SST k-ωturbulent model is used to further reveal the characteristics and mechanisms of self-excited oscillation pulsating flow on suppressing HTD of supercritical CO₂.The effects of heat flux,pulsation amplitude and pulsation period on the heat transfer performance are analyzed.The simulation results show that the effect of self-excited oscillation pulsating flow on suppressing the HTD of supercritical CO₂ is more significant at higher heat flux and the peak of wall temperature can be reduced by100 K at the average mass flux of 800 kg/（m²·s）,the period 0.008 s,the amplitude 52kg/（m²·s）,and the heat flux 200 k W/m².Under the action of pulsating flow,the velocity distribution of“M-shape”appears later and gentler,the peak point is further away from the near-wall region,and the average density of the low-density layer of the viscous bottom layer（y⁺<5）is higher,which lead to the thickness of low-density layer thinner and the thermal resistance lower.The production and diffusion of turbulent kinetic energy（TKE）are improved at log layer（30<y⁺<0.2r）and core region（0.2r<y⁺<r）.In addition,the pulsating flow parameters（period and amplitude）do not show monotonous trends on the heat transfer performance.The comprehensive calculation results found that the heat transfer performance with the period of 0.016 s and the amplitude of 100 kg/（m²·s）is the best in all calculated cases with the average mass flux of 800 kg/（m²·s）.Finally,to further reveal the formation mechanisms of self-excited oscillation pulsating flow formation at the outlet of Helmholtz oscillator,the turbulent viscosityμ_t of RNG k-εmodel is modified and the numerical simulation study of the flow characteristics of supercritical CO₂ flowing in Helmholtz oscillator is carried out.The results show that the pulsating flow formation is mainly due to the occurrence of cavitation phenomenon when the local pressure in the chamber drops to the saturation pressure,resulting in cavitation cloud.And the process of cavitation clouds variation with time leads to the accumulation and release of pulsating flow energy,which successfully converts the steady flow at the inlet of Helmholtz oscillator into pulsating flow at the outlet.The important operating and structural parameters have significant effects on the pulsating flow characteristics,with the increase of inlet pressure,the pulsating intensity increases and the main frequency moves towards high frequency.Within the range of conditions studied in this paper,the pulsating amplitude of the outlet fluid velocity can be as high as 2.093m/s and 1.1 m/s,respectively,when the dimensionless cavity diameter（D_c/d₁）and cavity length（L_c/d₁）are 11.5 and 4.0.