E12 Heat convection

Topic

The circulation of a fluid—whether water or air—around the food makes heat convection the dominant mechanism of heat transfer at the food's surface. In the cooking process, this convection manifests either as boiling water surrounding the food (natural convection driven by density gradients resulting from the water heating up) or as circulating air in a convection oven (forced convection driven by an external fan). When the surrounding fluid circulates more intensely around the food, heat convection transfers heat to the food's surface more effectively.

Newton's law quantifies the heat flux transferred to the food's surface via a convective heat transfer coefficient. This coefficient depends heavily on both the fluid's properties and the flow regime as the fluid circulates around the food. Water—characterized by high thermal conductivity and a Prandtl number of approximately seven—achieves much higher convective heat transfer coefficient values ​​than air; this difference explains why cooking the food in water is significantly faster than cooking it in an air-based oven. Because the surrounding fluid is water rather than air, the convective heat transfer coefficient is much higher, resulting in a much faster transfer of heat to the food's surface. Therefore, this model—based on the specific heat convection and convective coefficient initially established—calculates that same convective coefficient for the geometry and culinary fluid in question using a set of dimensionless numbers (Nusselt, Grashof, Reynolds, and Prandtl). These numbers universally capture the physics of fluid motion and heat transfer through empirical correlations that relate the Nusselt number to the Reynolds and Prandtl numbers (for forced convection) or to the Grashof and Prandtl numbers (for natural convection). In natural convection—such as in a pot of hot water—the density difference resulting from the temperature gap between the fluid and the food surface drives the fluid flow; conversely, in forced convection—such as in a fan-assisted oven or with a blowtorch—it is the externally imposed fluid velocity that determines the convective coefficient. Consequently, if the fluid (whether natural or forced) circulates more vigorously around the food, the convective coefficient increases, leading to faster and more uniform cooking before the heat delivered to the surface propagates into the interior via the heat conduction process described in greater detail by another model in this set.

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