Study On The Heat Transfer Characteristics Of Spraying Falling Film In Abandoned Laneway

To better utilize rich geothermal resources in coal mine area,spraying and falling films for waste-heat utilization towards the abandoned mines is put forward.The water films generated by spray system exchange for roadway walls,after which it will be collected as the cold and heat source,solving the lack of water inflow in water source heat pump of coal mines.The core of this method is the process of the heat exchange for falling water film over a vertical wall,which belongs to the heat exchange of large-scale rough surface.Based on the shortage of large-scale rough surface of heat exchange process in previous studies,velocity distribution of falling water film over a vertical wall and the similarity of Reynolds are adopted to derive the temperature field distribution and Stanton-Stm formula theoretically.In the guidance of heat transfer and the similarity theory,the simulation of abandoned mines’ and the design for the large-scale rough surface of heat exchange system are performed with different experimental operating conditions.Therefore,the thesis focused on flow and heat exchange for turbulent flowing of falling water film over a vertical wall depending on different working water jet capacity,inlet temperature,rough surface.Also,combined with the validation of formula,the research and analysis of this topic can provide theoretical guidance on the heat exchange for spraying and falling films for waste-heat utilization.Main findings can be listed as follows:(1)Theoretical research aimed at the flow and heat exchange of falling water film over a vertical wall.The similarity of Reynolds are adopted to derive the Stanton-Stm formula based on smoothness and roughness,respectively.By comparison,the main factors of heat exchange for turbulent flowing of falling water film are heat transmission resistance derived from molecular conduction process of rough elements in the potholes,Pr,roughness and the changes of average thickness and flow velocity of falling water film caused by rate of flow.(2)Experimental platform of falling water film over a vertical wall and the heat exchange was designed under the condition of roughness.With the simulation of the real abandoned mines,the key experimental problems of length at entry,heat flow control,temperature field distribution,roughness of different walls and spray water volume control can be solved directly,contributing to the experimental system of falling water film over a vertical wall and the heat exchange.This system,consist of water spray system,vertical rough wall,data measurement and acquisition system,can control and monitor spray water,flow velocity,film thickness,temperature at wall surface and entry,promoting the development of falling water film over a vertical wall and the heat exchange properties.(3)The experiments of falling water film and the heat exchange properties were carried out under the same condition of roughness,focusing on the variation of average thickness and flow velocity of falling water film.Then the coefficient of heat transfer under different flow volume and temperature was analysed.The results show that in the control of flow within the scope of this study,coefficient of convective heat transfer increased by flow volume,and increased slowly with the decrease of Stanton-Stm number.Under the same water quantity,the coefficient increased with the decrease of Pr.That is to say,the increase of flow volume is not meant to the increase of the coefficient of convective heat transfer without limit.Therefore,when flow rate increases to a certain value,other methods such as the increase of entry temperature need to facilitate the coefficient.(4)The influence of falling water film and the heat exchange properties by roughness has been studied.Three different rough walls were used to the experiments,and average film thickness and coefficient of heat transfer were analysed through the condition of flow volume,temperature and roughness.The research exhibited that no obvious change of flow properties was observed in the falling water film with the increase of roughness.Also,the fluctuation of falling water film increased with its roughness,inhibiting the average film thickness.On the other hand,the increase of roughness promoted the backflow disturbance inside of independent wave,facilitating exchange efficiency inside of falling films.Therefore,with the ensurance for breakage or damage of turbulent flowing of falling water film in abandoned mines,we can appropriately promote the roughness of walls to increase the coefficient of heat transfer for falling films.