Study On Heat Transfer And Pressure Drop Characteristics Of Three-dimensional Internally Finned Elliptical Tubes

Enhanced heat exchanger tubes are widely used in petroleum,chemical,metallurgical,power and other fields.Every improvement of heat transfer performance means greater economic and social benefits.Three-dimensional inner-fin heat exchanger tube is a kind of high-efficiency heat transfer enhancement tube which is widely used at present.Because of the disturbance of fins,the heat transfer effect of three-dimensional inner-fin circular tube is much better than that of smooth tube Compared with circular tube,the elliptical tube has many advantages,such as smaller windward area outside the tube,compact structure,smaller vortices formed at the tail,and certain heat transfer effect in the tubeConsidering the advantages of three-dimensional inner-ribbed circular tube and elliptical tube,a three-dimensional inner-ribbed elliptical tube is proposed in this paper.By fitting the relationship between Nusselt number and friction coefficient in three-dimensional inner-ribbed elliptical tube,the influence of five factors(length-to-length axis ratio,rib height,rib width,rib spacing and rib density)on heat transfer and pressure drop characteristics is analyzed,and the mechanism of heat transfer enhancement in tube is obtained.For the purpose,the heat transfer and pressure drop characteristics of three-dimensional inner fin heat exchanger tubes were experimentally and numerically simulated.The main contents and conclusions are as follows:The heat transfer and pressure drop characteristics of three-dimensional inner-ribbed elliptical tube and three-dimensional inner-ribbed circular tube are compared and analyzed by experiments.Under the same rib parameters,the average thermal performance coefficient of three-dimensional inner-ribbed elliptical tube is 1.428,and that of three-dimensional inner-ribbed circular tube is 1.409,which are higher than that of smooth tube.It shows that the heat transfer effect of three-dimensional inner-ribbed elliptical tube is better than that of three-dimensional inner-ribbed circular tube.Because the three-dimensional elliptical tubes with inner ribs adopt the form of cross-row,the influence of different rib parameters on heat transfer is quite different.In this paper,the optimal geometric parameters of three-dimensional elliptical tubes with inner ribs are selected by means of orthogonal experiment.The results show that:when the rib parameter range for 1.0?a/b? 2.0,0.05385 ?h/d ?0.09217,0.01923 ? b/d ?0.03687,0.06731 ? Pt/d ?0.1152,Pr=0.7,32 ? z ?56,15000<Re<40000,the order of five factors affecting the thermal performance coefficient is:rib height H>rib density Z>rib width B>rib width B>long-short axis ratio a/b>rib spacing Pt,and the optimal combination scheme of The ratio of long axis to short axis is 2,the height of ribs is 4 mm,the width of ribs is 1.4 mm,the spacing of ribs is 3.5 mm,and the density of ribs is 56.The rib height and density are sensitive to the performance of heat exchanger tubes.The influence of single factor change of geometrical structure on heat transfer effect is analyzed by numerical calculation,and the relationship between Nusselt number and friction coefficient in three-dimensional elliptical tube with inner ribs is fitted,which provides a reference for practical engineering.The purpose of three-dimensional inner-fin heat exchanger tube is to enhance heat transfer by enhancing turbulence of fluid and enlarging heat transfer area in tube.By analyzing temperature field and velocity field in tube,the mechanism of enhanced heat transfer in three-dimensional inner-fin elliptical tube is expounded.The secondary flow caused by fins and elliptical tubes in three-dimensional inner-fin tube changes the distribution of velocity field and temperature field in tube,and also changes the distribution of temperature field in tube.The distribution of local convective heat transfer coefficient reduces the angle between the velocity field and the heat flow gradient field in the tube,and achieves the effect of heat transfer enhancement.The effects of cos 0,dT/dy and dT/dy ·cos ? on heat transfer are analyzed by means of field synergy theory.It is found that the distribution of dT/dy· cos ? on the isovelocity line is basically consistent with the distribution of local convective heat transfer coefficient.It shows that the heat transfer effect is controlled by the synergistic effect of heat transfer temperature difference,temperature boundary layer thickness and the angle between velocity vector and temperature gradient.This paper tentatively puts forward a formula for calculating the area expansion ratio of heat exchanger tubes,and briefly analyses the relationship between the area expansion ratio and the thermal performance coefficient.