Numerical study of axial wall conduction in fully heated microtubes for cryogenic fluid

Abstract

In this rapidly progressing era everything is going in small size or which we called miniaturization of technology. This is the time of miniature things due to compactness. There is a huge industry growing on parallel to the high temperature which we called cryogenic world. In cryogenic field there are a lot of gases which we use for different purposes. Helium is one of these cryogenic gases which liquefy at 4.2K. Helium is very costly gas so we use this in any industry in closed cycle manner. Helium goes down to the 4.2K so we can use it in different processes. The thermo-physical properties of Helium change with temperature appreciably, so we cannot treat it as a constant property fluid. In this age the micro-tube heat exchanger are used for heating or cooling of cryogenic gases. In this work we tried to find out the most suitable material, suitable thickness of microtube with the help of change in different parameters. In this work, a two dimension numerical study is carried out to study the effect of axial wall conduction in fully heated circular microtube (in conjugate heat transfer mode) subjected to constant wall heat flux at the outer surface. A microtube of inner diameter of 0.4 mm and total length of 60 mm is considered in the numerical modeling. The cross sectional surfaces of the microtube are keeps adiabatic. Simulation have been done for the change in parameter like flow rate (Re =100-500), wall to fluid conductivity ratio (ksf =1.71-2822.3684) and wall thickness to inner radius ratio (äsf =1-5). The result shows that conductivity ratio and wall thickness play dominant role in conjugate heat transfer process. It is found that there exist an optimum ksf at which Nuavg is maximum when other parameters are kept constant. Nuavg is found to be lower for higher wall thickness (äsf). When Helium flow rate is increased, it is found that Nuavg increases.