Optimization of Finned-Tube Heat Exchanger in a Gravity-Assisted Separated Heat Pipe

Finned-tube heat exchanger(FTHE)is often used as an evaporator in commercial products of separated heat pipe(SHP).The working conditions of FTHE in gravity-assisted SHP are significantly different from those working in refrigerators and air conditioners.Although FTHE is widely used in commercial products of SHP,previous research on its characteristics is very limited.In this paper,a mathematical model for a SHP with FTHE as the evaporator and plate heat exchanger as the condenser is established and verified with experiments.Parametric analyses are carried out to investigate the influences of evaporator design parameters:air inlet velocity,number of tube rows,tube diameter,and fin pitch.With the increasing of air velocity,number of tube rows and tube diameter,and the decreasing of fin pitch,the heat transfer rate increases,while the energy efficiency ratio(EER)decreases monotonically.Using the total cost of the ten-year life cycle as the performance index,the structure parameters of the evaporator with a given heat transfer rate are optimized by the method of orthogonal experimental design.It is found that the total cost can differ as large as nearly ten times between groups.Among the three factors investigated,the number of tube rows has a significant impact on the total cost of the evaporator.With more tube rows,the total cost will be less.The impacts of fin pitch and tube diameter are insignificant.These results are of practical importance for the engineering design of FTHE in gravity-assisted SHP.

Impacts of wind conditions on hydrogen leakage during refilling hydrogen-powered vehicles

Although hydrogen leakage at hydrogen refueling stations has been a concern,less efforts have been devoted to hydrogen leakage during the refueling of hydrogen-powered vehicles.In this study,hydrogen leakage and dilution from the hydrogen dispenser during the refueling of hydrogen-powered vehicles were numerically investigated under different wind configurations.The shape,size,and distribution of flammable gas clouds(FGC)during the leakage and dilution processes were analyzed.The results showed that the presence of hydrogen-powered vehicles resulted in irregular FGC shapes.Greater wind speeds(v wv)were associated with longer FGC propagation distances.At v_(wv)=2 m·s^(−1)and 10 m·s^(−1),the FGC lengths at the end of the leakage were 7.9 m and 20.4 m,respectively.Under downwind conditions,higher wind speeds corresponded to lower FGC heights.The FGC height was larger under upwind conditions and was slightly affected by the magnitude of the wind speed.In the dilution process,the existence of a region with a high hydrogen concentration led to the FGC volume first increasing and then gradually decreasing.Wind promoted the mixing of hydrogen and air,accelerated FGC dilution,inhibited hydrogen uplifting,and augmented the horizontal movement of the FGC.At higher wind speeds,the low-altitude FGC movements could induce potential safety hazards.