What drives in-furnace gas circulation? Circulation and mixing of gases inside a furnace are caused primarily by which of the following?

Introduction / Context:
Uniform temperature and rapid heating depend on vigorous internal gas circulation. Understanding the physical drivers of this circulation helps in optimizing burner placement, dampers, and baffles for better heat transfer and product quality.

Given Data / Assumptions:

• Standard fuel-fired furnace with stack/ducting.
• Natural convection and forced flow both present.
• No unusual flow constraints beyond typical practice.

Concept / Approach:
Three effects drive circulation: (1) thermal expansion/contraction changes volume and creates motion; (2) buoyancy from density differences promotes natural convection (hot gases rise, cold gases sink); and (3) imposed pressure gradients from draft fans or stack effect move gases through the enclosure and flues. All three contribute materially to in-furnace flow patterns and mixing.

Step-by-Step Solution:

Verification / Alternative check:
CFD simulations and flow-visualisation experiments in furnaces show combined buoyancy-driven and pressure-driven recirculation loops, confirming the synergy of mechanisms.

Why Other Options Are Wrong:

• Any single effect alone: In real furnaces, multiple mechanisms act simultaneously.
• Chemical composition changes only: While reaction heat release affects temperature, composition alone does not drive flow without associated temperature/pressure differences.

Common Pitfalls:
Ignoring buoyancy in high-temperature zones; attributing all flow to fans when natural convection can be dominant locally.

Final Answer:
All (a), (b) and (c).