D54 Analysis of thermal flows

Topic

This comprehensive analysis of the insect's heat fluxes—based on a model established elsewhere in this set—breaks down its final energy balance into convective, radiative, and evaporative components, thereby making it possible to determine which of these three mechanisms dominates under any specific environmental condition.

Convection is the dominant term for an insect at rest in a ventilated environment; radiation dominates in environments with high radiant temperatures, such as under direct sunlight; and evaporation becomes significant only for species that produce specific secretions or under conditions of extreme heat. Exposure to even higher radiant temperatures causes the radiative component to dominate even more strongly over the other two elements of the energy balance.

The insect's convection coefficient is calculated using the Nusselt number (derived from another model in this set), which in turn depends on the Reynolds number of the airflow around the insect's body; the Ranz-Marshall correlation for a sphere serves as a first approximation for any rounded body equivalent to the insect. Consequently, a higher Reynolds number for the airflow around the insect results in a higher convection coefficient, according to the Ranz-Marshall correlation. This same laser flash method measures the thermal diffusivity of the insect's cuticle—specifically the same cuticle section addressed by another model in this set regarding mechanosensory zones—by recording the temperature response on the rear face of a cuticle sample following a pulse of heat applied to its front face; the sample's half-response time is, consequently, inversely proportional to the cuticle's thermal diffusivity. A shorter half-response time for the cuticle sample results in a correspondingly higher thermal diffusivity value.

Thus, this model—incorporating the energy balance broken down into convective, radiative, and evaporative components—allows the thermography (from another model in this set), the anemometry (introduced at the start of this paragraph), and the heat-flow calorimetry (from a separate model regarding metabolic heat production) to collectively achieve a complete closure of the insect's energy balance. Any improvement in the precision of these three methods—thermography, anemometry, or heat-flow calorimetry—results in a correspondingly more accurate final closure of the energy balance.

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