It is widely held in industry that the use of vitiated heaters is to be avoided in combustion tests. The present paper reviews the situation and gives results which suggest the magnitude of the problem with respect to the various combustion test measurements. Vitiation temperatures up to 800 K are considered and the effects are assessed against: (a) gas temperatures within the combustor, (b) combustor wall temperatures, (c) flame radiation effects, (d) air/fuel distribution within primary zone, and (e) pollutant effects. It is demonstrated that some of the above effects may be minimized by using oxygen replenishment. The paper concludes that in no circumstances should vitiated preheating be used. Oxygen replenishment, whilst largely overcoming the deleterious effects of items (a) to (d) above, is not sufficient to overcome the effects upon the formation of pollutants, particularly oxides of nitrogen.

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