## Fluids

#### Monitoring of Wall Heat Flux in CFX

• Oliver_Paulick
Subscriber

I want to simulate a flow in a cavity and evaluate the heat transfer coefficient at the wall (steady state simulation, fixed temperature at the wall). I am using ANSYS CFX 19.2

To make sure, that i have reached a steady state, i want to monitor the local wall heat flux durig the runtime on a few selected points. Unfortunally, the wall heat flux is not available in CFX Pre, so i cant use it as monitor point.

I assume, that CFX Post calculates the wall heat flux out of the basic items from the solver of the last iteration.

My idea was to create in CFX Pre an expression to replicate the wall heat flux from CFX Post, so that i can choose this expression as a monitor point in the solver.

I used the ANSYS documentation "CFX Solver Theory Guide - 2.8.1 Mathematical Formulation". In the Setup, I selected automatic wall treatment, i used the formulas shown below to calculate the wall heat flux manually. Unfortunally, my solution deviates from the evaluation of the wall heat flux in CFX Post.

My questions ist: Why can I not calculate the "correct" wall heat flux q_w?

Do I use the wrong formulas, or do i evaluate the needed items (such as heat capacity, conductivity, turb. kinetic energy, viscosity, density) at the wrong locations?

My current approach is:

I evaluate heat capacity, conductivity, turb. kinetic energy, viscosity, density at the center of the wall adjacent element, at Wall Distance = s/2 (thickness of first element layer s is known)

I evaluate T_f directly at the Wall (Wall distance = 0)

T_W is the fixed Temperature of the Boundary (Also Wall distance = 0)

Delta U = ([email protected] Distance = s) - ([email protected] Distance = 0)

I calculate q_w with the formulas shown below. Next, i compare it to the evaluation of q_w in the same wall adjacent cell in CFX Post

Do you have any clues or hibts to solve my problem?