Experimental Investigation Of The Effect Of A Mesh Tube Insert On The Flow Boiling Heat Transfer Characteristics Inside A Vertical Tube

In the fields of industrial production and utilization of renewable energy, there are a lot of low-grade energy systems. How to enhance the utilization efficiency of these low-grade energy systems is of great significance to alleviate energy shortages and development of renewable energy technologies, energy efficiency is important practical significance to alleviate energy shortages and development of renewable energy technologies. Efficient use of low-grade thermal energy has become domestic and international projects in the field of thermal physics and cutting-edge research focus. The common problems of using low-grade thermal energy include the small temperature difference, the large volume of heat transfer equipment and the relatively poor heat economy. Thus, the development of efficient heat transfer principles and techniques of small temperature difference, and reduction of the volume of heat transfer equipment to improve the thermal economy are of scientific significance and will provide a scientific basis and technical support for the efficient heat exchanger design, manufacture and operation.Compared with single-phase flow heat transfer, the advantage of boiling heat transfer lies in the small temperature difference and high heat transfer efficiency. The main purpose of enhancement of boiling heat transfer is to improve the heat transfer equipment performance thus lower the irreversible energy loss enhance the energy utilization efficiency and reduce the metal consumption.Flow boiling heat transfer is closely related with two phase flow patterns. It provides a new perspective for the enhancement of boiling heat transfer by the regulation of flow patterns. Our previous experimental studies with air/water at the ambient temperature have preliminarily verified the feasibility of the flow pattern regulations by using a mesh tube insert inside vertical and horizontal tubes. Moreover, the new concept of heat transfer enhancement by the flow pattern regulation has also been proved to be valid in condensation heat transfer. For the boiling heat transfer enhancement with a mesh tube insert, a preliminary experimental research has also been conducted inside a horizontal tube by our group. In this paper, we carried out a comparative study on the flow pattern and heat transfer characteristics during flow boiling inside a vertical tubes with and without the mesh tube inserts. An experimental setup to study the flow pattern and heat transfer features has been designed and built up. With pure water as the working fluid, the mass flow rate was maintained at a constant value of 350.77kg/m2s, the inlet pressure was kept at 256.99 kPa and the heat flux range is 8.69 to 85.47 kW/m2. The vertical tube was made of stainless steel with an inner diameter of 14.25mm and outer diameter of 18.0 mm. The mesh tube insert was also make of stainless steel, the diameter and the porosity of which are 10.5 mm and 200ppi, respectively. A high-speed camera was used to study flow pattern transitions for the bare tube and the new tube with mesh cylinder insert. By the analysis of heat transfer data, the heat transfer characteristics for bubbly flow, slug flow, churn flow and churn/annulus flow has been obtained. Under different inlet vapor qualities, the influence of heat flux on flow pattern transitions and heal transfer features was also revealed. In addition, the ratio of heat transfer enhancement with mesh cylinder insert was calculated and the impact of the mesh tube insert on heat transfer mechanisms was analyzed.By visualization study, it was found that the mesh tube insert can influence the flow pattern transition obviously:(1) at relatively low heat fluxes, the flow pattern inside the tube developed slowly and the vapor/water interface didn’t expand too much.. The presence of metal mesh tube prevented the vapor bubble from flowing into the mesh tube, forming a separated flow mode. Thus, the vapor bubble was confined into the annular gap between the evaporation tube and metal mesh tube. (2) With heat flux metal mesh in the vicinity of the entrance area still blocked the vapor phase in the annular gap. When the heat flux was further increased, a fast expansion of the vapor/liquid interface will overcome the surface tension force of the mesh tube and the vapor went into the mesh tube. At the same time, the water inside the mesh tube was expelled by the vapor going inside.The data analysis showed that the effect of the mesh tube insert inside the the vertical tube is summarized as follows:(1) when the subcooling is large, the main heat transfer mechanism is convective turbulent flow. The presence of the mesh tube alleviated the turbulent mixing between the outside hot and inside cool liquids,which will deteriorated heat transfer. The heat transfer deterioration ratio reached 0.5. (2)In subcooled and saturated boiling zones with relatively low vapor qualities, the steam was inhibited by the interfacial force from going into the mesh tube and was flowing into the annular gap between the mesh tube and the outer heated tube. Due to the confinement inside the annular gap, the steam was elongated thus the enlarged thin film evaporation zone at the vapor/water interface enhanced heat transfer. Along the flow direction, the maximum ratio of heat transfer coefficients(heat transfer enhancement ratio) was 3.8. The wall superheat for the tube with mesh cylinder insert was 2.5℃ lower than that for the bare tube. (3)When the vapor quality is relative high, the dryout of the thin liquid film led to heat transfer deterioration at the heating wall. It was found that a bubble oscillation phenomenon occurred at the outlet of the mesh tube when both the vapor quality and the heating flux were high enough. This phenomenon was induced by the periodical vapor expansion and intrusion into the mesh tube.