Axial Back Conduction in Cryogenic Fluid Microtube

Abstract

Cryogenic technology is now a rapidly progressing system which is used in different cooling processes because the behaviour of many physical materials changes beyond our expectations. For example copper behave normally as other materials for electrical conductivity but at the cryogenic temperature it behaves as superconductor. Actually there is no certain temperature from which the cryogenic temperature starts but according to the scientist below -1500C or 123 K cryogenic temperature starts. Also the time is to use products of compactness which is known as miniaturization. In the engineering background there are many researchers who have studied and developed the micro channels as the cooling process is very efficient because the surface area to volume ratio is very less. So it is now a keen interest to use cryogenic temperature in the micro channels. There are different gases present in our atmosphere which are used as cryogenic fluids, example Helium, Nitrogen, Oxygen, etc., as boiling points of these gases are below cryogenic temperature. The boiling point of liquid Nitrogen is 77.2 K and the freezing point is 63 K. In this present work cryogenic gas is intended to flow through a circular micro channel and a two dimensional numerical simulation is carried out for an internal convective laminar flow through the channel, subjected to constant wall heat flux to see the axial back conduction in the solid substrate of the tube which leads to conjugate heat transfer. Nitrogen gas is used as working fluid to flow through the microtube. Thermo-physical properties (e.g. density, viscosity, specific heat and thermal conductivity) of nitrogen gas change appreciably with the temperature, thus thermophysical properties function of temperature are used as UDF as described in numerical simulation chapter. The micro channel of 0.4 mm diameter and 60 mm length are kept constant and δsf (i.e. ratio of wall thickness (δs) to inner radius (δf)) is varied such as 1, 2, 3, 4 & 5 throughout the simulation. Other variable parameters are Reynold’s number varies as 100 & 500 and ksf (i.e. solid conductivity ratio to fluid conductivity ratio) varies from 22.07931 to 45980.71. In this work it is tried to find out most suitable material i.e. ks value as well as suitable wall thickness of the microtube i.e. δs value with the help of change in different parameters. After the completion of the numerical analysis the conclusions found are, (i) wall conductivity ratio and wall thickness ratio play dominant role in the effect of axial back conduction, (ii) there exist an optimum ksf value at which average Nusselt number (Nuavg) is maximum while other parameters are kept constant, (iii) at higher value of δsf, average Nusselt number becomes lower, (iv) Nuavg increases with increase in flow rate i.e. increasing value of Reynolds number.