In combustion engineering, oxygen enrichment of the combustion air changes several performance indicators. Which of the following does <em>not</em> increase when the oxygen content of air is enriched under otherwise comparable firing conditions?

Introduction / Context:
Oxygen enrichment means supplying air with a higher percentage of O2 than ambient air. It is widely used to intensify combustion, raise flame temperatures, and boost heat transfer. However, not all performance metrics increase. This question probes your ability to predict directional effects on flame temperature, heat transfer, flue-gas composition, and stack losses.

Given Data / Assumptions:

• Comparisons are made at similar firing rates and furnace conditions, except for increased O2 in the oxidant.
• Enrichment reduces accompanying nitrogen, thus reducing total flue-gas flow for a given fuel input.
• Excess O2 (residual oxygen) control remains reasonable and safe.

Concept / Approach:
Increasing O2 concentration in the oxidant enhances adiabatic flame temperature because less inert N2 dilutes the flame. Higher flame temperature typically increases radiative and convective heat transfer to the load (higher temperature difference and emissive power). Flue gas often shows a higher measured O2 percentage unless excess O2 setpoints are actively lowered; at constant excess, the concentration reading can be higher because of reduced N2 dilution. Stack loss primarily depends on the mass and temperature of exhaust gases. Because enrichment reduces N2, the mass of flue gas for a given heat input usually drops—tending to reduce, not increase, stack losses for comparable stack temperature.

Step-by-Step Solution:
Note enrichment effect: less N2, hotter flames.Infer greater heat transfer rate (hotter gases, more radiation).Recognize flue-gas O2 reading can rise unless excess trim compensates.Evaluate stack loss = m_dot(exhaust) * cp * (T_stack - T_ref); m_dot falls with less N2, so stack loss does not increase.

Verification / Alternative check:
Plant trials commonly report fuel savings and lower stack losses after enrichment due to reduced flue-gas mass flow, provided draft and temperature are held similar.

Why Other Options Are Wrong:
Flame temperature: Increases because of less inert dilution.Oxygen in flue gas: Often increases at constant excess-air control points.Heat transfer rate: Increases via higher radiative heat flux.

Common Pitfalls:

• Assuming higher O2 always raises every metric; stack loss depends on mass flow and temperature.
• Confusing O2 enrichment with “more excess air,” which can have opposite effects.

Final Answer:
stack loss.