Page 127 - Chapter 3 - Fundamentals of Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
Appendix 2 – Thermal Diffusivity
A more appropriate unit for “α” frequently used is mm2/s. The thermal diffusivity for "soft" human tissue is approximately 0.1 - 0.15 mm2/s.
α = k /(ρ * CP) - thermal diffusivity
where
k = thermal conductivity [W/(m·K)] ρ = density [kg/m3]
CP = specific heat capacity [J/(kg·K)]
The relation between heat conduction distance ‘D’, heating time ‘t’ and thermal diffusivity ‘α’ is:
𝐷 = √𝛼𝑡 𝑜𝑟 𝑡 = 𝐷" 𝛼
What is important to notice is that the speed of the heat wave is independent of the temperature which implicates that you can't speed up the cooking process by increasing the temperature. This explains why you can't force the cooking process of the ham in the example above by increasing the temperature!!
Another important thing to notice is that the cooking time is four times longer if the size of the food object is doubled i.e. the cooking times increase with the square of the dimension.
In this video you can see how a heat wave moves through a solid object.
Heat or increased temperature is simply an amplified level of movement/vibration of atoms and molecules and the speed of the heat wave depends on the type of atoms and molecules that the object consists of.
Let’s look at some cooking examples. Assume that you want to fry a steak with a thickness of 10 mm. Let’s assume that you want it well done.
The thermal diffusivity for meat is around 0.15 mm2/s. We can calculate the time it takes for the heat wave to reach the midpoint (5 mm into the beef) by applying the equation above.
________________________________________________________________________ 127 Chapter 3 Laser/IPL Hair Removal
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