BIOL 3211 Heat Transfer Objectives

Evaporation

Heat energy carried away by water as it evaporates

Radiation

Transfer of heat energy through electromagnetic waves 9mainly infarred frequencies)

No direct contact between surfaces

Evaporation and radiation

Direct contact between surfaces

Conduction and convection

Conduction

Direct transfer og heat energy through contact

Convection

Conduction to a fluid (liquid or gas)

Convection types

Free and forced

Free convection

Fluid is stationary, movement is due to differences in heat energy ONLY

Forced convection

Fluid is moving because of other forces (wind, currents)

Conduction vs. Convection vs. Radiation

Physical contact vs. density of molecules vs. electromagnetic waves

Thermal budget

ΔH_total= H_Metabolic+H_Convection+H_Conduction+H_Radiation+ H_Evaporation

Heat gain

Metabolic, convection, conduction, radiation

Heat gain only

Metabolic

Heat loss

Convection, conduction and radiation

Heat loss

Evaporation

Heat energy loss or gain is measured in

Watts (joules/second)

Fourier’s Law equation

Q = ΔT/R

Fourier’s law

A flux equation that describes the rate of flow of heat energy through a substance in CONDUCTION

Biological substances

Include different types of tissues (like muscle and fat) and insulating structures (fur and feathers)

Smaller animals have

Smaller total mass, smaller volume, and larger SA:V ratio

Smaller total mass + Smaller volume + Larger SA:V ratio =

Better exchange of vital substances

Increasing surface area

Are better at exchanging vital substances

Heat is related to

Composition, temperature gradient

Composition

Some tissues are better at retaining heat, and others are better at dissipating heat

Temperature gradient

Larger differences between body temperature and ambient temperature will result in large heat loss

Q = ΔTK/L (Heat transfer) and Q_T = ΔTKA/L

Q = amount of heat transferred per unit area

ΔT = change in temp (K)

K (kappa) = thermal conductivity (Watts/mK)

A  = surface area (m²)

L = thickness of substance (m)

Heat transfer equation is used to

Calculate the amount of heat loss through a layer of a substance PER UNIT area of the substance, useful for comparing animals of DIFFERENT surface areas in WATTS/m²

Fourier’s Law equation is used to

Used to calculate the TOTAL amount of heat lost through a substance, useful for comparing animals of the SAME surface areas in WATTS

Thermal conductivity (K)

The amount of heat energy that can be conducted by (transfer through) a substance

High thermal conductivity are

Conductors

Low thermal conductivity are

Insulators

Basking

• Maximizes heat through radiation, conduction an/or convection

• Increases body temp

• Increases Q_T, 𝝙T, and A

• Increases 𝝙H, H_C, H_C, H_R

Countercurrent

• Minimizes heat loss through conduction to ice and convection to air and water by minimizing 𝝙T

• Decreases Q_T, 𝝙T

• Decreases H_MET, H_C, H_C

Fat/blubber

• ↓H_Conductive and H_Convective

• ↓ H_Met (less metabolic heat is needed)

• Low K value, ↑L

Hair/Fur/Wool/Feather

• ↓H_Conductive, H_Convective 

• ↑H_R (if color of fur affects radiation)

• ↑L, ↓K

• Some students say: ↓ΔT (because the trapped air is kept warm)

Postural changes

• Warm up:

  • ↓A

  • ↓H_Convective,↓H_Convective, ↑H_R

• Cool down:

  • ↑A

  • ↑H_Convective,↑H_Convective, ↓H_R

Sweating

• ↑evaporation

• ↑ H_Met (but less than heat lost through evaporation)

• (ΔT not changed. Will change ΔH as a result of evaporation)

Huddling

• ↑H_Conductive gain

• ↓H_Convective loss

• ↓ H_Met (less metabolic heat is needed)

• ↓A, ↓ΔT (microclimate)

Nest

• Prevents heat loss or heat gain:

  • ↓H_Conductive, H_Convective, H_R

  • ↑L, ↓K, ↓ΔT, ↓A

Vasodilation larger vascular radii

• Q_T, A, ΔH, H_Cond, H_Conv increases

• L decreases

Vasoconstriction

• Q_T, A, ΔH, H_Cond, H_Conv decreases

• L increases

Thermal window

• Anatomical region specialized to have lots of capillaries near the surface to facilitate heat heat exchange with the environment

• Q_T, A, ΔH, H_Cond, H_Conv increases

• L decreases

Hibernation and Torpor

• Q_T, ΔT, H_Met, H_Cond, H_Conv decreases

• Use less energy for thermoregulation = decrease metabolic heat production

• Lowered body temp set point = minimizing ΔT between body and environment

• Less heat loss through conduction or convection

Hibernation

Endotherm, still thermoregulates, but at lower body temperature set point range

Torpor

Thermoregulation is suspended (to a point); can be either in an ectotherm ro endotherm