convergence problem

caesarch · January 26, 2016, 04:21

Hello everyone. Now I am using CFX to simulate the flow and heat transfer in a heat exchanger. It is a liquid-solid conjugate heat transfer problem. Geometry model of heat exchanger is composed of three domains: gas 1 domain, gas 2 domain and solid domain. In this simulation, RGP (real gas property) file is used to specify gas properties.
Now I encounter a problem: solution can be converged only when relative pressure of outlet is set as zero. However, relative pressures of two outlets of gas 1 domain and gas 2 domain can’t be both set as zero.
In the simulation, a reference pressure must be specified. When I specify the reference pressure of gas domain 1, for example 2MPa, the reference pressure of entire simulation is set as 2MPa, i.e., the reference of gas domain 2 is 2MPa as well. When relative pressures of two outlets of gas domain 1 and gas domain 2 are set as zero, the solutions of gas 1 domain and gas 2 domain can be both converged. But relative pressures of two outlets can’t be both set as zero. Outlet relative pressure of gas 1 domain is set as zero, and the solutions of gas 1 domain are converged. Outlet relative pressure of gas 2 domain is set as non-zero value, for example 5MPa, and the solutions of gas 2 domain are not converged.
Please help me to solve this problem! Thanks!

ghorrocks · January 26, 2016, 06:50

This suggests you are reaching a numerical round-off limit. Make sure you are using double precision numerics - this may fix the problem. If this does not fix the problem you have a tricky problem. You will have to try things which improve general numerical robustness, such as better mesh quality, smaller time steps and better initial conditions.

January 26, 2016, 04:21	convergence problem	#1
caesarch New Member Join Date: Jan 2016 Posts: 1 Rep Power: 0	Hello everyone. Now I am using CFX to simulate the flow and heat transfer in a heat exchanger. It is a liquid-solid conjugate heat transfer problem. Geometry model of heat exchanger is composed of three domains: gas 1 domain, gas 2 domain and solid domain. In this simulation, RGP (real gas property) file is used to specify gas properties. Now I encounter a problem: solution can be converged only when relative pressure of outlet is set as zero. However, relative pressures of two outlets of gas 1 domain and gas 2 domain can’t be both set as zero. In the simulation, a reference pressure must be specified. When I specify the reference pressure of gas domain 1, for example 2MPa, the reference pressure of entire simulation is set as 2MPa, i.e., the reference of gas domain 2 is 2MPa as well. When relative pressures of two outlets of gas domain 1 and gas domain 2 are set as zero, the solutions of gas 1 domain and gas 2 domain can be both converged. But relative pressures of two outlets can’t be both set as zero. Outlet relative pressure of gas 1 domain is set as zero, and the solutions of gas 1 domain are converged. Outlet relative pressure of gas 2 domain is set as non-zero value, for example 5MPa, and the solutions of gas 2 domain are not converged. Please help me to solve this problem! Thanks!

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January 26, 2016, 06:50		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	This suggests you are reaching a numerical round-off limit. Make sure you are using double precision numerics - this may fix the problem. If this does not fix the problem you have a tricky problem. You will have to try things which improve general numerical robustness, such as better mesh quality, smaller time steps and better initial conditions.