Modes of heat transfer in fluids: In liquids and gases, the dominant mechanism of heat transfer under most engineering conditions is:

Introduction / Context:
Heat can move by conduction, convection, or radiation. In fluid systems (liquids and gases), bulk motion often accompanies heat transfer, greatly increasing the rate compared with pure molecular conduction.

Given Data / Assumptions:

• Fluid is free to move (not locked in a porous solid matrix).
• Boundary layers develop along heated/cooled surfaces.
• Temperatures are moderate; radiation is not dominant unless temperatures are very high or surfaces are highly radiative.

Concept / Approach:
In fluids, conduction always exists microscopically, but macroscopic heat transfer is typically governed by convection (forced or natural). Forced convection arises from external pumping or fans; natural convection arises from buoyancy due to density differences in a gravitational field. Radiation can dominate at very high temperatures or in gases with strong participating media effects, but that is not the general case.

Step-by-Step Solution:

Verification / Alternative check:
Compare orders of magnitude: thermal conductivities of air and many liquids are small; without convection, purely conductive heat transfer across macroscopic fluid layers is very weak.

Why Other Options Are Wrong:

• Conduction: present but generally secondary in mobile fluids over engineering length scales.
• Radiation: can be significant at high temperatures or in furnaces, but not the default dominant mechanism.
• None of these: incorrect because convection is well established.

Common Pitfalls:
Assuming conduction dominates in all contexts; in fluids, boundary-layer convection is the design focus except in microgaps or stagnant layers.

Final Answer:
Convection