Page 270 - Chapter 3 - Laser/IPL Hair Removal

where
k = thermal conductivity [W/(m·K)] ρ = density [kg/m3]
CP = specific heat capacity [J/(kg·K)]
Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
Appendix 4 – Thermal Diffusivity/Conductivity
Heat conducts or diffuses through any medium it is in, from hot regions to colder regions. This is simply the Zeroth Law of Thermodynamics. When discussing thermal diffusivity, which is usually labelled “α”, we can work out how quickly a hot object cools.
The 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
The relation between heat conduction distance ‘D’, heating time ‘t’ and thermal diffusivity ‘α’ is:
𝐷 = √𝛼𝑡 𝑜𝑟 𝑡 = 𝐷2 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.
________________________________________________________________________ 270 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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