Heat transfer and friction characteristics of air flow in microtubes

Several researches dealing with the single-phase forced convection heat transfer inside microchannels have been published in the past decades. The performance of liquid flow has been proved that agrees with the conventional correlations very well. However, owing to the low heat transfer coefficient of gaseous flow, it is more difficult to eliminate the effects of thermal shunt and heat loss than water flow while measuring its heat transfer performance. None of the heat transfer performance experimental results have been published in the literature. This study provides an experimental investigation on the pressure drop and heat transfer performance of air flow through microtubes with inside diameter of 86, 308 and 920 μm. The Liquid Crystal Thermography method was used to measure the tube surface temperature for avoiding the thermocouple wire thermal shunt effect. The experimental results show that the frictional coefficient of gas flow in microtube is the same as that in the conventional larger tubes if the effect of gaseous flow compressibility was well taken consideration. The conventional heat transfer correlation for laminar and turbulent flow can be well applied for predicting the fully developed gaseous flow heat transfer performance in microtubes. There is no significant size effect for air flow in tubes within this diameter range.