Experimental Investigations On Heat Transfer Enhancement Of Flow Boiling Inside A Horizontal Tube Based On Flow Pattern Modulation

With the population increase and economy development, energy crisis and environmental problem have become the neck bottle for the development for modern society. The effective methods to solve energy crisis are improving energy efficiency recycling of waste heat and developing of new energy and renewable energy. As one of basic energy conversion equipment, heat exchangers affect the energy utilization efficiency. Efficient heat exchanger technology can reduce the volume of the heat exchanger, reduce the cost, save material, energy consumed and space occupied, which has scientific and practical significance for improving energy efficiency and saving energy consumption and reducing pollution emission.The current method of the heat transfer enhancement is to cause rotating flow and secondary flow, which makes the center fluid exchanged with the wall-fluid, and the boundary layer destroyed. Based on the relationship between flow pattern and heat transfer, Prof. Jinliang Xu proposed the scientific concept of heat transfer enhancement using the two phase flow pattern modulation. Using a metal mesh tube (MMT) cylinder insert, the concept of flow pattern modulation was successfully verified with the adiabatic air-water two phase flow in a vertical tube.An experimental platform for flow visualization and heat transfer was built up to investigate the effect of the MMT on flow pattern modulation and heat transfer characteristics of boiling heat transfer in a horizontal tube. The test section consisted of a stainless steel tube and a mesh tube cylinder insert. The stainless tube has an inner diameter14.25m and outer diameter of18.Omm.The mesh cylinder tube insert has a diameter of10.50mm, which was made from a mesh screen with the mesh number of500. During the experiments, the effect of heat flux on the flow pattern modulation and heat transfer enhancement by using the MMT was experimentally investigated under different inlet flow patterns.The visual experiments show that the MMT significantly modulates the flow pattern of flow boiling inside the horizontal tube.(1) The MMT can prohibit the vapor phase from going into the center region of the tested assembly, forming a new three-zone flow pattern i.e. liquid phase in the center/enlarged vapor phase in the annular gap/liquid film at the near wall region;(2) the MMT can change the shape of vapor-liquid interface;(3) the MMT can advance the flow pattern transitions.The heat transfer experiments show that the MMT can enhance the heat transfer for bubbly flow, plug flow and slug flow. The maximum local heat transfer coefficient is three times than that of bare tube. In general, the heat transfer enhancement at the side is biggest and the bottom is smallest. For the wavy stratified flow, heat transfer deterioration occurred at the top and side of the tube, while the heat transfer was enhanced obviously at the bottom because of the thin liquid film heat transfer mode. Besides, the MMT obviously improves the wall temperature uniformity for all the runs except the bubbly flow under low heat flux.