[Hyunwoo]

Hello everyone,

Firstly, please excuse my poor English.

I encountered a problem while conducting heat transfer calculations in a circular tube: The heat absorbed by the fluid, calculated manually using q=m*cp*ΔT, does not match the heat (q=q"*A) obtained from Fluent's calculations. I have tried refining the mesh and adjusting the boundary conditions at the inlet, but the results remain inconsistent. I am eager to resolve this issue and would greatly appreciate any assistance you can provide.

Here is a summary of the simulation model and boundary conditions:

1.png

Firstly, the model consists of a tube nested within a solid cylinder, as depicted in the figure 1. The tube has a height of 0.4 m and a diameter of 0.01 m, while the surrounding solid cylinder has a diameter of 0.04 m. The fluid domain is located within the central 0.01 m diameter tube, while the region between the 0.01 m diameter tube and the 0.04 m diameter tube constitutes the solid domain.

The outer wall of the 0.04 m diameter tube is set as 300 K, while the wall of the 0.01 m diameter tube is modeled as a coupled wall. The top and bottom sections of the tube are subjected to adiabatic conditions. The inlet is set as a mass flow inlet, and the outlet is a pressure outlet.

The fluid domain utilizes air provided by Fluent, while the solid domain uses aluminum with a modified thermal conductivity of 1e-5 W/m K.

As the mass flow rate at the inlet is reduced from 0.004025768290112 kg/s to 4.025768290112E-10 kg/s, the flow regime inside the tube transitions from turbulent to laminar. I have employed both the k-omega SST model and the laminar model accordingly.

When I calculate the heat absorbed by the fluid using the inlet fluid temperature (26.80403 K) and outlet temperature with the m*cp*ΔT method and export the heat entering the fluid domain through the tube wall in Fluent, I noticed that initially, the two calculated heats are roughly the same. However, as the mass flow rate decreases, the discrepancy between them begins to significantly increase. The calculated mass flow rate and the error are shown in the figure 2.

2.png

I hope this clarifies the situation. Thank you very much for your help.