strange problem....
 

Hello Everybody,

I am solving to simulate the flow inside the microchannel. Cross section of the channel is 55umx300um and length is 10mm. All the walls are adiabatic and fluid enters at 300K at velocity of 1m/s. The working fluid is air. The temperature is initialized to 300K everywhere. The solution for this problem should be isothermal temperature distribution. But when I solve it, temperature starts to increase though there is no heat source anywhere. But if I change the working fluid to water the solution converges (not at 300K but 303K, Strange!!) . Even if I take air as the fluid and increase its density by a factor of 1000, solution converges. Does anyone have any idea why this is happening? Since the velocity is low viscous dissipation should not be important. Other than that, I can't think of any heat source that would increase the temperature of the fluid.

Any idea or suggestions would be greatly appreciated.

Thanks Vivek