| Tube outside Nusselt number is a very important parameter during design and examination in tubular heat exchangers(such as Economizer, air preheater in power plant, some kinds of heat exchange equipment in metallurgy industry, water heater in daily life and so on). It is generally obtained by the experimental method, namely using Newton cooling formula to get tube outside heat transfer coefficient, then get the tube outside Nusselt number.The experimental method which studies the heat exchangers is various, using the electricity direct heater to simulate the second boundary condition is more common in all the method, the tube interior was heated by electricity, and the air across the tube outside. This method is based on regarding the entire heat exchanger as equal heat flux, but the actual situation is not so, the temperature must change when interior hot fluid pass through the interchanger, and the heat flux changes also, the electric heating way is surely different with the real heat transfer situation. Discussing the corresponding relations and the difference between outside heat transfer coefficient and Nusselt number under these two kinds of boundary condition, it has vital significance for the heat exchange equipment design.This paper studies the pin-fin tube, smooth tube and finned tube on the same wind channel test apparatus through experimental method. Using the method of electric heating to simulate the second boundary condition, and using the method of hot fluid flowing in the interior tube and the air across the tube outside to simulate the third boundary condition. The range of the experimental Reynolds number is 5000~35000. The experimental result indicated that, the Nusselt number of smooth tube in the third boundary condition is 1.048 times of its in the second boundary condition. The Nusselt number of Finned tube in the third boundary condition is 1.073 times of its in the second boundary condition and the Nusselt number of Pin-fin tube in the third boundary condition is 1.1 times of its in the second boundary condition. And the ratio has nothing to do with the Reynold number change. Under the same boundary condition, integral pin-fin tube has stronger heat transfer characteristic compared with smooth tube and finned tube. In order to make further studies on the reason of this kind of difference, FLUENT software is used to carry on the numerical simulation to the following field: temperature field and pressure field of smooth tube and pin-fin tube exchanger. The... |
