Study On Heat Transfer In Tube With Expanded Enhanced Surface Of Herringbone Rib And Composite Structure

In this article,the extended surface enhanced heat transfer technology is mainly studied by using two typical heat forms with or without phase transformation:convective condensation heat transfer and single phase flow heat transfer.In the convective condensation heat transfer experiment,smooth tube（ST）,ellipsoid pit and threaded interstitied-rib composite tube（Dimple&HX）,ripple pattern tube（1EHT）and herringbone tooth tube（HB）were studied.The effects of tube,saturation temperature,mass flow rate,average vapor quality,enhanced expanded surface structure on the condensation heat transfer coefficient and friction pressure drop were studied.It is found that the higher the thermal conductivity of the tube,the better the condensation heat transfer coefficient and the difference of friction pressure drop in the same working condition is not large.Compared with the refrigerant fluid at high saturation temperature,the refrigerant at low saturation temperature has stronger heat transfer performance due to the influence of saturation temperature on the physical parameters of the refrigerant fluid density,viscosity,Prandtl number,tension coefficient and specific heat at constant pressure.The higher the average vapor quality,the better the heat transfer performance of all heat exchange tubes,and the greater the pressure drop.For the three kinds of heat exchange tubes with extended surfaces,ST,1EHT and HB,the condensation heat transfer coefficient and friction pressure drop also increase with the increase of mass flow rate,but the condensation heat transfer coefficient of Dimple&HX shows low sensitivity to mass flow rate,and the friction pressure drop also increases with the increase of mass flow rate.However,the condensation heat transfer coefficient has little change on the whole,which is mainly due to the influence of its special tube type on its flow state.In addition,the convective condensation heat transfer coefficient of Dimple&HX with 1EHT,HB and composite structure increases more than that of the smooth tube.Compared with smooth tube,the heat transfer coefficients of Dimple&HX,1ETHT and HB increased by 75.2%～112.3%,27.0%～49.0%and81.6%～103.4%,respectively.The friction pressure drop increased by 42.3%～103.6%,66.5%～101.7%and 36.8%～56.2%,respectively.In the study on the comprehensive heat transfer performance PEF of convective condensation heat transfer,it is found that the overall PEF of HB is relatively good,around 1.3.The PEF of 1EHT increased with the increase of flow velocity,and the overall PEF varied in the range of 0.91～1.1.The PEF of Dimple&HX decreases with the increase of mass flow velocity,ranging from 0.85 to1.45.The optimal PEF appears at 130kg m^-2 s^-1 mass flow velocity.In the study of single-phase flow heat transfer,a combination of numerical simulation and experiment is adopted.The smooth tube and the herringbone tooth tube with the extended surface of herringbone rib are taken as the research objects.The influences of the height of the ribs,the spacing between the ribs and the Angle of the herringbone tooth tube on the flow heat transfer are mainly studied.The results show that the herringbone toothed tube with herringbone rib extended surface has better heat transfer performance than the smooth tube.Higher rib height and larger herringbone Angle are beneficial to the synergistic effect of velocity field and temperature field in the herringbone tube.The distribution of low synergy Angle is wider and the average synergistic Angle is smaller.Although smaller rib spacing can not make the overall average synergy Angle smaller,more dense herringbone ribs will make the fluid velocity field and temperature field near the wall surface of the herringbone better synergy.The higher the ribs of the herringbone canal,the larger the Nu and f.In the study and analysis of comprehensive heat transfer performance PEC,the higher the rib,the better the overall comprehensive heat transfer performance.Rib spacing also has an important effect on heat transfer of herringbone tube.At the same Reynolds number,the herringbone canal with smaller intercostal spacing,whether Nu,f or PEC,is larger.In the range of 45°～120°herringbone Angle,the larger the herringbone Angle will promote the radial convective mixing in the heat exchange tube,so the larger the herringbone Angle,the larger the Nu and f will increase.However,when the herringbone Angle continued to increase,Nu changed little.In the range of 60°～180°herringbone Angle,the comprehensive heat transfer performance factor PEC of herringbone toothed tube also decreased,and only the herringbone toothed tube with 45°herringbone Angle showed a trend of increasing with the increase of Re.