Full Coverage Impingement Heat Transfer: Cooling Effectiveness

Husain

Journal of Turbomachinery

et al. 2014

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Conjugate heat transfer (CHT) computational fluid dynamic (CFD) predictions were carried out for a 10 × 10 square array of impingement holes, for a range of pitch to diameter ratio X/D from 1.9 to 11.0 at a constant impingement gap Z of 10 mm and pitch X of 15.24 mm. The variation of X/D changes the impingement wall pressure loss for the same coolant mass flow rate and also changes the interaction with the impingement gap cross-flow. The experimental technique to determine the surface averaged heat transfer used the lumped capacity method with Nimonic-75 metal walls with imbedded thermocouples and a step change in the hot wall cooling to determine the heat transfer coefficient h from the transient cooling of the metal wall. The test wall was electrically heated to about 80 °C and then transiently cooled by the impingement flow and the lumped capacitance method was used to measure the locally surface average heat transfer coefficient. The predictions and measurements were carried out at an impingement jet mass flux of 1.93 kg/s m2 bar, which is a typical coolant flow rate for regenerative impingement cooling of low NOx gas turbine combustor walls. The computations were conducted for a fixed hot side temperature of 353 K that was imposed at the hot face of the target wall. The wall temperatures as a function of distance along the gap were computed together with the impingement gap aerodynamics. Surface average heat transfer coefficient h and pressure loss predictions were in good agreement with the experimental measurements. However, there was less good agreement for the axial variation of the local surface averaged h for lower values of X/D. The surface averaged heat transfer to the impingement jet wall was also computed and shown to be roughly 70% of target wall impingement heat transfer.

Conjugate Heat Transfer Computational Fluid Dynamic Predictions of Impingement Heat Transfer: The Influence of Hole Pitch to Diameter Ratio X/D at Constant Impingement Gap Z

Husain

Journal of Turbomachinery

et al. 2014

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Conjugate Heat Transfer CFD Predictions of Impingement Jet Array Flat Wall Cooling Aerodynamics With Single Sided Flow Exit

Volume 3B: Heat Transfer

Staggs

2013

Conjugate heat transfer CFD studies were undertaken on impingement square jet arrays with self induced crossflow in the impingement gap with a single sided exit. The aim was to understand the aerodynamic interactions that result in the deterioration of heat transfer with axial distance, whereas the addition of duct flow heat transfer would be expected to lead to an increase in heat transfer with axial distance. A square array of impingement holes was investigated for a common geometry investigated experimentally, pitch to diameter ratio X/D of 5 and impingement gap to diameter ratio Z/D of 3.3 for 11 rows of holes in the crossflow direction. A metal duct wall was used as the impingement surface with an applied heat flux of 100kW/m2, which for a gas turbine combustor cooling application operating at steady state with a temperature difference of ∼450K corresponds to a convective heat transfer coefficient of ∼200 W/m2K. A key feature of the predicted aerodynamics was recirculation in the plane of the impingement jets normal to the cross-flow, which produced heating of the impingement jet wall. This reverse flow jet was deflected by the cross flow which had its peak velocity in the plane between the high velocity impingement jets. The cross-flow interaction with the impingement jets reduced the interaction between the jets on the surface, with lower surface turbulence as a result and this reduced the surface convective heat transfer. A significant feature of the predictions was the interaction of the cross-flow aerodynamics with the impingement jet wall and associated heat transfer to that wall. The results showed that the deterioration in heat transfer with axial distance was well predicted, together with predictions of the impingement wall surface temperature gradients.

Conjugate Heat Transfer CFD Predictions of Impingement Heat Transfer: Influence of the Number of Holes for a Constant Pitch-to-Diameter Ratio X/D

Hussain

Volume 5A: Heat Transfer

et al. 2014

Conjugate heat transfer CFD studies were undertaken on the influence of the number of impingement holes/unit surface area or hole density n (m−2) for n from 1076 to 26910m−2 at a constant X/D of 4.7, with n varied by varying the hole diameter D from 1.31 to 6.54mm and pitch X varied from 6.1mm to 30.5mm. Square array impingement cooling geometries for the jet holes were used with a 152.4 × 152.4mm experimental wall area. The impingement gap had a single sided exit which generated a cross-flow in the gap. The number of impingement holes N in the cross-flow direction was 5, 10, 15 and 25. A coolant mass flux G of 1.93kg/sm2bar was investigated at a constant impingement gap Z of 10mm (Z/D 1.53–7.65 as n was varied). This high coolant mass flow simulated the coolant flow for regeneratively cooled combustors using all the combustor air flow to cool the combustor wall prior to entering the low NOx flame stabiliser. The predictions were compared with experimental results for the heat transfer coefficient h, that used the lumped capacitance method. The predictions of the surface averaged h and pressure loss ΔP/P were in good agreement with the measured results. The predictions showed that increasing the number of impingement jet holes resulted in lower h, due to the impact of cross-flow for large numbers of holes. At the other extreme, a very small number of holes were predicted to have high thermal gradients. The maximum heat transfer was found experimentally and computationally to be 4306 holes per m2 for an X/D of 4.7, with acceptable thermal gradients.