Flow field of a triple-walled gas-sampling probe with sub-cooled boiling effect

A numerical investigation has been carried out on the flow field inside and around a water-cooled gas-sampling probe, located downstream of a gas turbine combustor. The probe is of triple-walled stainless steel, where the gas sample is transported through a centre tube, while preheated and pressurized cooling water flows through two surrounding annuli. Complex conjugate heat transfers between the exhaust gas mixture, cooling water and probe walls are modeled. The numerical results provide detailed information for understanding the complex flow field and heat transfers within and around the gas-sampling probe. At a typical practical operating condition, the forced-convection subcooled boiling phenomenon occurs in the cooling water circuit of the probe. A simplified approach is used to account for this sub-cooled boiling effect, and the vaporization rate of water is estimated. More importantly, the predicted temperature increase of cooling water is consistent with the measured value from combustor testing. The present study has proved that numerical simulation is a valuable tool for probe design and operation, and some recommendations are included. In addition, the laminar sub-layer in the near-wall region should be properly resolved in order to accurately predict conjugate heat transfer in such flows.