| In order to build a clean,low-carbon,safe,and effective contemporary energy system in China,it is vital to realize the efficient utilization of medium-low temperature waste heat resources,which make up about 54% of industrial waste heat.At present,trans-critical organic Rankine cycle(T-ORC)is widely used in medium and low temperature waste heat recovery systems with high efficiency.The heat exchangers in T-ORC are horizontally arranged because of the drastic thermal property changes of the supercritical fluids,resulting in complex heat transfer situations that seriously affect the safety and efficiency of the system’s operation.In this paper,a new heat transfer enhancement technique,Discrete Double Inclined Ribs(DDIR),is applied to supercritical horizontal flow,and the effect of DDIR on the horizontal flow and heat transfer characteristics of supercritical R134 a is investigated by numerical simulation.The results of this research have significance for improving the thermal economy of heat exchangers to optimize the efficiency of T-ORC systems.Main conclusions drawn are as follows:(1)The DDIR horizontal tube is highly adaptable to changes in operating conditions because the spiral effect,which is guided by the discrete double inclined ribs,suppresses the formation and development of the non-uniform temperature field in the main stream crosssection and maintains the dominance of forced convection in the tube.The PEC of the horizontal ST is improved by at least 70% as a result of DDIR intervention,and circumferential thermal inhomogeneity can be decreased by up to 90%.The rib pitch has the greatest effect on heat transfer,followed by the rib height and rib inclination angle,respectively.The optimum values of the above three rib structure parameters are 20 mm,0.8 mm,and 60°,respectively.(2)While buoyancy dominates the formation and development of vortices in the horizontal ST,but the effect on the flow field in the horizontal tube of DDIR is not significant.Instead of suppressing the negative impact of significant buoyancy on heat transfer,as is the case with the vertical DDIR tube,the inclined discrete ribs boost heat transfer by enhancing the disturbance of the working substance within the tube to encourage mixing between hot and cold fluids.Additionally,compared to subcritical conditions,the cumulative impact of the supercritical conditions’ drastic change of specific heat capacity results in a stronger heat transfer enhancement effect.(3)The difference in flow field is the main reason for the different thermal and hydraulic characteristics of different supercritical DDIR horizontal tubes,and the arrangement of discrete ribs is closely related to the pattern of flow field.The unequally spaced arrangement of the ribs will develop two vortices in the main stream’s radial direction,which promotes mixed heat transfer between the hot and cold fluids in the tube and has the greatest obvious impact on the enhancement of heat transfer.The symmetrical arrangement of the ribs along the gravity direction has little effect on the formation and development of the flow field,so the enhancement of heat transfer is not obvious.Among the four arrangements,the unequally spaced unsymmetrical rib arrangement has the most significant improvement on the comprehensive heat transfer performance.The PEC can be improved by 1.6 to 2.5 times in the studied working condition range.The contribution of the unequally spaced arrangement to the heat transfer enhancement is dominant.(4)The DDIR arrangement along the mirror image will result in a corresponding symmetry of the flow field and other physical fields of supercritical R134 a.The rotation angle affects the circumferential inwall temperature and the inwall heat flux distribution but does not have a significant effect on the overall heat transfer characteristics. |
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