Study On Convective Heat Transfer Enhancement In Tubes And Exergy Destruction Analysis

With the rapid development of Chinese economy,energy and environmental problems are becoming more and more serious.It is crucial to the sustainable economic development to improve energy efficiency and implement energy conservation and emission reduction.Thermal energy is one of the forms of energy,the transfer and conversion of which widely exist in various industrial fields.Convective heat transfer mainly occurs in heat exchangers,so it is of great significance to develop heat transfer enhancement technology to realize high efficiency of heat exchanger.In this paper,the researches focus on convective heat transfer in a single tube and developing new turbulence technology.Numerical simulation is conducted to investigate the flow structure and mechanism of heat transfer enhancement,and the irreversibility of a heat transfer process is analyzed.According to the guidance of the heat transfer enhancement theory,the optimization calculation is conducted to achieve the active design for the techniques as well.The concept of available potential is introduced,the balance equation of which is derived.The expressions for local exergy destruction rates are gained,and the relationship between the available potential and exergy destruction is analyzed.This paper further elaborates the specific meaning of thermal dissipation and power consumption as representing the irreversibility of heat transfer and fluid flow,respectively.The scientific nature and feasibility of applying the principle of exergy destruction minimization to active design for heat transfer techniques are analyzed.Based on the optimal flow structure of longitudinal swirling flow with multiple vortexes,a novel tube insert denoted as center-connected deflectors is proposed,and a numerical simulation with Re ranging from 300 to 1500 is conducted to analyze the thermo-hydraulic performance.The effects of Re and geometrical parameters including pitch（P）,inclined angle（α）and deflector diameter（d）are analyzed.The results indicate that the insert enhanced heat transfer with an acceptable increase of pressure drop by guiding the fluid from the core to boundary region and generating longitudinal swirl flow with multiple vortexes in the tube.Meanwhile,the analysis of exergy destruction based on the second law of thermodynamics are conducted.According to the calculation results,when compared to a smooth tube,the Nusselt number is increased by 2.51-9.46 times with the friction factor increasing to 2.48-10.77 times.Efficiency evaluation coefficient（EEC）was in range of 0.92-1.56.The thermal dissipation is reduced by 4.82-10.56 times with power consumption increasing to 3.78-12.50 times.A design method is proposed by applying the principle of exergy destruction minimization to determine the optimal geometric parameters of the insert,which is significant for the active design of the tube insert.The multi-objective optimization is performed by genetic algorithm along with artificial neural network.Thermal dissipation and power consumption ratios are chosen as optimization objectives.Methods for determining suitable parameters with different criteria are discussed.Among them,a compromised solution on the Pareto front is obtained by TOPSIS method.The result indicated that the EEC of the optimized structure（d=3.63 mm,α=35.85°,and P=67.89 mm）is 1.32 and that the optimal result based on the principle of exergy destruction minimization has considerable overall performance.The internal rib tube is applied to strengthen the convection heat transfer of supercritical CO₂ in the tube.Under the cooling boundary conditions,the effects of different arrangement of P-type and V-type ribs on the velocity and temperature fields in the tube are studied.The results indicate that the flow structure in the smooth tube is mainly affected by the buoyancy force driven by gravity,while the heat transfer and flow pattern in tube with ribs is affected by both the effect of turbulence and buoyancy force.Compared with a smooth tube,longitudinal swirl flow is generated in the tube with the ribs.With the mixing of the fluid effectively promoted in the tube,the wall temperature improved,and thus the heat transfer temperature difference between the wall and the fluid reduced,the local heat transfer coefficient in the tube under the condition of constant heat flux is increased intensively.The V-shaped ribs form a multi-vortex longitudinal swirling flow in the tube,while the P-shaped ribs mainly form a single vortex field.The spiral direction is varied with positions of ribs.At the same time,the distribution of exergy destruction of working medium under supercritical state is analyzed.As the physical properties of the working medium in supercritical state vary greatly along the path,under the cooling boundary conditions,the thermal dissipation in the tube increases along the path while the power consumption decreases,which is obviously different from the law presented by the working medium in state of atmospheric pressure.