Numerical simulations of heat transfer characteristics of gas–liquid two phase flow in microtubes

• Flow patterns affect the two phase thermal performance.
• Modified Nusselt number is proposed for multiphase flow.
• Redistribution of velocities around gas phase enhances the thermal performance.
• Pressure loss has been accounted for the evaluation of thermal performance.
• Optimal bubble size for heat transfer in microtubes has been identified.

Numerical simulations of the heat transfer characteristics in axisymmetric air–water two phase flow have been carried out in microtubes of inner diameter 300 and 500 μm. The two phase flow is achieved by injecting nitrogen gas coaxially through a centrally positioned tube to the continuous liquid phase flow. This arrangement can produce a series of bubbles encapsulated by the continuous water phase. Uniform heat flux is applied on the outer surface of the outer tube. Comparison of the simulated bubbly flow and flow visualization bubbly flow results obtained from experiments show that the difference is within 10%. Subsequent simulation results show that the Nusselt number enhancement can be as high as 200% while the two phase frictional pressure loss for the bubbly flow is about 20% higher than that of the liquid flow alone. The results also show that the heat transfer performance varies with the bubble size, frictional pressure drop and Reynolds number. Analysis of the velocity and temperature profiles near a bubble shows that the bubble obstructs the path of the liquid flow, forcing the redistribution of the axial and radial velocities around the bubble. This redistribution enhances the thermal mixing and is found to be the main reason that enhances the heat transfer performance.