pressure drop and two phase problem

omaralyahia · July 14, 2015, 03:59

I have narrow Vertical Rectangular channel, heated from both side. the flow direction is downward flow, normal flow velocity (1 m/s) at the inlet B.C and average static pressure at the outlet, the total length of the flow channel is 0.3 m so the static pressure at the outlet will be ( rho*g*h=2935 Pa). The first problem is regarding to the pressure and pressure drop, whereas the total pressure at the outlet is lower than the total pressure at the inlet, but in reality at the flow velocity (1 m/s) the total pressure at the outlet should be the higher than the inlet due to the static pressure term (gravity effect). I make the flow in downward direction but I couldn't find how to specify the gravity direction except by actuating the buoyancy option On, but even if I used this option, the result not changing.

The other problem that at the same power level and boundary condition, if I applied the wall boiling model, the wall temperature would decrease much. For example at 10 kW the maximum wall temperature is 115 C, and at that power there is boiling and bubbles. So if in need to see the void fraction, I shall apply the wall boiling model, and after that the wall temperature decrease to around 85 C, however at this temperature there is no boiling anymore. So my question is how to match the wall temperature before and after applied the two-phase model (Wall boiling model)?

Your time response is highly appreciated
Thank you so much
Best Regards
Omar Al-Yahia

Steffen595 · July 16, 2015, 19:01

did you define a value for gravity after switching buoyancy on? And in the right direction?
Maybe check, if total pressure changes?
Maybe Ansys is not concerned with the static pressure, only dynamic. Switch off anything else (boiling, velocity). Just put a column of liquid in and gravity. Then see, what happens. Total Pressure should be static+dynamic. Pressure is just dynamic.

Edit: seems you are right. For 1 m water I get 38Pa pressure, up or down, depending on gravity direction. Should be 98kPa. Both Pressure or Total Pressure.

July 14, 2015, 03:59	pressure drop and two phase problem	#1
omaralyahia New Member Omar Sharief Join Date: Jun 2015 Posts: 7 Rep Power: 11	I have narrow Vertical Rectangular channel, heated from both side. the flow direction is downward flow, normal flow velocity (1 m/s) at the inlet B.C and average static pressure at the outlet, the total length of the flow channel is 0.3 m so the static pressure at the outlet will be ( rhogh=2935 Pa). The first problem is regarding to the pressure and pressure drop, whereas the total pressure at the outlet is lower than the total pressure at the inlet, but in reality at the flow velocity (1 m/s) the total pressure at the outlet should be the higher than the inlet due to the static pressure term (gravity effect). I make the flow in downward direction but I couldn't find how to specify the gravity direction except by actuating the buoyancy option On, but even if I used this option, the result not changing. The other problem that at the same power level and boundary condition, if I applied the wall boiling model, the wall temperature would decrease much. For example at 10 kW the maximum wall temperature is 115 C, and at that power there is boiling and bubbles. So if in need to see the void fraction, I shall apply the wall boiling model, and after that the wall temperature decrease to around 85 C, however at this temperature there is no boiling anymore. So my question is how to match the wall temperature before and after applied the two-phase model (Wall boiling model)? Your time response is highly appreciated Thank you so much Best Regards Omar Al-Yahia

July 16, 2015, 19:01		#2
Steffen595 Senior Member Join Date: May 2012 Location: Melbourne Posts: 161 Rep Power: 14	did you define a value for gravity after switching buoyancy on? And in the right direction? Maybe check, if total pressure changes? Maybe Ansys is not concerned with the static pressure, only dynamic. Switch off anything else (boiling, velocity). Just put a column of liquid in and gravity. Then see, what happens. Total Pressure should be static+dynamic. Pressure is just dynamic. Edit: seems you are right. For 1 m water I get 38Pa pressure, up or down, depending on gravity direction. Should be 98kPa. Both Pressure or Total Pressure. Last edited by Steffen595; July 17, 2015 at 09:51.