Study On Boiling Heat Transfer Characteristics And Critical Heat Flux Of Heated Rod

In the petrochemical,food,pharmaceutical and other industries,boiling heat transferation is widely used because of its high heat transfer capacity,in which rod-like electric heated element is very common.The mechanism of boiling is very complex.And it is a violent evaporation process through a large number of bubbles formation,growth and movement of the refrigerant chang from the liquid to the gaseous state.Boiling heat transfer occurs on the boiling wall,of which the geometric characteristics and heat transfer characteristics have a great impact on the heat transfer capacity and are attentioned by many researchers.The local location of the boiling surface also has a great influence on the boiling heat transfer.Pool boiling heat transfer has a very important application value in the industrial field,because it relates to how to maximize the potential of phase change heat exchanger in order to achieve the purpose of saving energy.Therefore,it is of great scientific and economic significance to improve the boiling heat transfer capacity through studying how the boiling heat transfer characteristics change with the geometric structure of heated surface and geometric position.At the same time,the critical heat flux of the heated surface is studied to improve the boiling heat transfer efficiency and equipment safety,which is importantly practical significant.In this paper,the experiment and numerical simulation of the pool boiling of the large height-diameter-ratio electric heated element are carried out.The variation of the boiling heat transfer characteristics of the heated surface is studied by changing the heating power of the heated rod and placed modes,which includes the following three aspects:Visual experimental study and temperature measurement of pool boiling with a bundle of heated rods.During the experiment,high-speed photography was used to collect the video files of the nucleation,growth and detachment of vapor bubbles.And the flow patterns of the fluid medium near the heating wall were taken,too.Through the processing of the collected video files,the growth rate of the heated surface bubbles,the growth period,the separation diameter,the separation frequency and the nucleation density are obtained.And then the phase transition rate and heat flux density of the heated surface are gained by calculation.The comparison of how the boiling heat transfer characteristics changes with the different heating power of the heated rod and different axial height is taken to get the change rule.In the experiment,the temperature data of the surface of the heated rod is to be collected and compared.The variation of the surface temperature of the heated rod along the axial height was analyzed.Numerical simulation of pool boiling with single heated rod vertically placed in is taken.Based on the ANSYS FLUENT platform,this paper constructs the hexahedral structured grid,and simulates the boiling phenomenon of the pool by using the wall boiling model.Firstly,the numerical simulation of the experimental conditions is carried out.The numerical results and experimental results are compared to verify the reliability of the numerical model.In order to save the computational cost,the pool boiling of a single heated rod is numerically simulated under the premise of verifying the reliability of the numerical model.The numerical results show that the heat flux and boiling heat transfer coefficient of the heated surface increase with the heating power and the axial height of the heated rod when the heated rod is placed vertically.And changing law of the nucleation density,the bubble separation diameter and the bubble separation frequency are obtained with heating power is gained through analysis.The mechanism of how the boiling heat transfer characteristics change with geometric position is further analyzed by local vapor volume fraction cloud diagram,gas velocity vector diagram and gas flow line diagram.The heat flux and the boiling heat transfer coefficient of the heated surface were changed with the circumferential angle when the heating rod was placed horizontally.By analyzing the liquid temperature cloud diagram,the gas volume fraction cloud diagram and the gas velocity vector diagram,the advantages and disadvantages of the two kinds of placement modes are obtained.