Heat flow in refractories: how does the thermal conductivity of refractory bricks generally vary with porosity (all else being equal within a composition family)?

Introduction / Context:
Thermal conductivity is critical for lining design, influencing heat losses, shell temperatures, and thermal gradients. Porosity disrupts solid–solid heat paths; therefore, understanding the conductivity–porosity trend helps engineers choose between dense and insulating refractories for different furnace zones.

Given Data / Assumptions:

• Comparison within a similar chemistry (e.g., alumino-silicate family).
• Air (or gas) in pores has much lower thermal conductivity than the solid phase.
• Microstructure is the main variable changing with porosity.

Concept / Approach:
Heat conduction in ceramics proceeds primarily through the solid network. Introducing pores inserts low-conductivity regions and breaks conduction chains. Consequently, as porosity increases, bulk conductivity drops. Dense bricks with fewer pores provide continuous conduction paths and thus higher thermal conductivity. Insulating bricks purposefully introduce high porosity to reduce conductivity.

Step-by-Step Solution:
Recognize air-filled pores act as thermal barriers.As porosity ↑ → more barriers → overall conductivity ↓.Therefore, conductivity increases when porosity decreases.Select the statement reflecting this inverse relationship.

Verification / Alternative check:
Data sheets for dense vs insulating grades within the same chemistry show a clear conductivity drop with increasing porosity, validating the trend.

Why Other Options Are Wrong:

• Decrease with decreasing porosity: opposite of observed behavior.
• Independence from porosity: incorrect; porosity is a primary control.
• Increases with entrapped air: air lowers, not raises, conductivity.
• Random/unrelated: contradicted by extensive measurements.

Common Pitfalls:
Confusing radiation heat transfer effects at very high temperatures; within common ranges and similar microstructures, porosity still dominates conduction trends.

Final Answer:
It increases as porosity decreases (denser bricks conduct better).