Experimental Study On Effect Of Axial Heat Conduction In The Wall On Thermal Convection In Mini-tube

With the rapid development of micro-fabrication technique and MEMS technology, so micro/mini tubes with an equivalent hydraulic diameter of micron/millimeter are widely used in the current industry and study of convection in such tubes are becoming more and more important. Because of the limitation with the micro-fabrication technique, a micro/mini tube usually has a great wall thickness relative to the tube inner diameter. So the effects of axial heat conduction in the tube wall, which can be neglected for the normal-sized tube case, must be considered when studying the convective heat transfer in a micro/mini tube.The effects of heat conduction in tube wall on single-phase laminar convection of deionized water in a mini-tube having a comparatively great wall thickness were investigated experimentally and numerically in this research. The two tubes were used in the experiments. One had a 0.924 mm inner diameter and a 2.1 outer-to-inner diameter radio; the other had a 0.952 mm inner diameter and a 2.4 outer-to-inner diameter radio. In the tests both tubes were made of stainless steel. The tube was heated by applying a D.C. voltage directly to the two tube ends. The overall heat transfer coefficients (K) and Nusselt number (Nu) based on the arithmetic mean temperature difference between the fluid and wall were obtained. The results of K and Nu are consistent with that for normal-sized tubes. However, discrepancies exist between the Nusselt number based on the one-dimensional model that neglects the axial heat conduction and the standard value given by conventional theory, where the temperature difference between the wall temperature and local bulk fluid temperature is used to calculate the heat transfer coefficient.Numerical calculations for the conjugate problem of in-tube convection and tube wall conduction were conducted on the basis of the experimental measurements using the FLUENT code. The results showed that the flow were fully developed and the Nusselt number based on the classic theory was close to 4.36 while the Nusselt number based on one-dimensional model was greater than 4.36 and increasing with the Re increasing.The effects of tube wall heat conduction on convection in mini-tube were the same as that in a normal-sized thick wall tube. The Nusselt numbers obtained from the one-dimensional model based on the experimental measurements may be either greater the standard value given by the conventional theory (uniform heat source in the tube wall) or smaller than it (iso-thermal or iso-flux at the outer tube wall), depending on the conditions. This was not a fresh phenomenon in a micro/mini scale.