Numerical Investigation On The Nucleate Pool Boiling Heat Transfer Of R134a And R1234ze(E) Outside Tubes

R134a is limited in its application due to high Global Warming Potential（GWP）,and R1234ze（E）has similar thermal properties with R134a and low GWP value and is of great interest.The structural parameters of the enhanced tube have different effects on the heat transfer by boiling outside the tube under different working conditions,and the selection of the best tube type of the enhanced tube for specific working conditions is of great significance to improve the heat transfer efficiency.In this paper,numerical simulations and experimental studies on the nucleation boiling heat transfer performance of R134a and R1234ze（E）under different working conditions outside the horizontal plain tube and enhanced tube are carried out,and the numerical calculation results are compared with the experimental results,and the following conclusions are drawn.（1）By comparing the numerical simulation results of boiling heat transfer of R1234ze（E）outside the horizontal plain tube and B5060-3D type tube with the experimental results,it can be seen that the deviation of the plain tube is within±27%and that of the B5060-3D type tube is within±25%,and the simulation results are in good agreement with the experimental results.（2）The boiling heat transfer coefficients outside the tube of R134a are both greater than those of R1234ze（E）under the same working conditions,and the boiling heat transfer coefficients of R1234ze and R134a outside the tube of the plain tube are close,while outside the tube of the B5060-3D enhanced tube,the difference between the boiling heat transfer coefficients outside the tube of R1234ze（E）and R134a is obvious at 6℃working conditions.R1234ze（E）and R134a outside the plain tube heat transfer coefficient difference is 27.8%of the average value of B5060-3D type tube.（3）The boiling heat transfer coefficient of R1234ze（E）outside the tube on B5060-3D type tube is increasing with the increase of heat flow density,but its growth rate is smaller compared to that of plain tube.The relationship between the heat transfer coefficient outside the tube and heat flow density of plain tube and B5060-3D type tube are:h_o=1.23q^0.64,h_o=2.51q^0.22.（4）In the range of fin density from 38 to 56 fpi,the boiling heat transfer coefficient outside the tube of R1234ze（E）has the characteristic of increasing first and then decreasing with the increase of fin density,and the heat transfer coefficient is the largest when the fin density is 50fpi.（5）In the range of fin density of 50 fpi and fin height of 0.4～0.8 mm,the boiling heat transfer coefficient outside the tube of R1234ze（E）has the characteristic of increasing and then decreasing with the increase of fin height,and the maximum heat transfer coefficient is at the fin height of 0.6 mm.